线程与线程池的那些事之线程池篇（万字长文）

{"type":"doc","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"本文关键字：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"线程","attrs":{}}],"attrs":{}},{"type":"text","text":"，","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"线程池","attrs":{}}],"attrs":{}},{"type":"text","text":"，","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"单线程","attrs":{}}],"attrs":{}},{"type":"text","text":"，","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"多线程","attrs":{}}],"attrs":{}},{"type":"text","text":"，","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"线程池的好处","attrs":{}}],"attrs":{}},{"type":"text","text":"，","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"线程回收","attrs":{}}],"attrs":{}},{"type":"text","text":"，","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"创建方式","attrs":{}}],"attrs":{}},{"type":"text","text":"，","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"核心参数","attrs":{}}],"attrs":{}},{"type":"text","text":"，","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"底层机制","attrs":{}}],"attrs":{}},{"type":"text","text":"，","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"拒绝策略","attrs":{}}],"attrs":{}},{"type":"text","text":",","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"参数设置","attrs":{}}],"attrs":{}},{"type":"text","text":",","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"动态监控","attrs":{}}],"attrs":{}},{"type":"text","text":"，","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"线程隔离","attrs":{}}],"attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"线程和线程池相关的知识，是Java学习或者面试中一定会遇到的知识点，本篇我们会从线程和进程，并行与并发，单线程和多线程等，一直讲解到线程池，线程池的好处，创建方式，重要的核心参数，几个重要的方法，底层实现，拒绝策略，参数设置，动态调整，线程隔离等等。主要的大纲如下：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/c5/c591ff5f6c93bc0dce66a8103082e5d7.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"线程池的好处","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"线程池，使用了池化思想来管理线程，池化技术就是为了最大化效益，最小化用户风险，将资源统一放在一起管理的思想。这种思想在很多地方都有使用到，不仅仅是计算机，比如金融，企业管理，设备管理等。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"为什么要线程池？如果在并发的场景，编码人员根据需求来创建线程池，可能会有以下的问题：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我们很难确定系统有多少线程在运行，如果使用就创建，不使用就销毁，那么创建和销毁线程的消耗也是比较大的","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"假设来了很多请求，可能是爬虫，疯狂创建线程，可能把系统资源耗尽。","attrs":{}}]}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"实现线程池有什么好处呢？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"降低资源消耗：池化技术可以重复利用已经创建的线程，降低线程创建和销毁的损耗。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"提高响应速度：利用已经存在的线程进行处理，少去了创建线程的时间","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"管理线程可控：线程是稀缺资源，不能无限创建，线程池可以做到统一分配和监控","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"拓展其他功能：比如定时线程池，可以定时执行任务","attrs":{}}]}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"其实池化技术，用在比较多地方，比如：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"数据库连接池：数据库连接是稀缺资源，先创建好，提高响应速度，重复利用已有的连接","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"实例池：先创建好对象放到池子里面，循环利用，减少来回创建和销毁的消耗","attrs":{}}]}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"线程池相关的类","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"下面是与线程池相关的类的继承关系：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/91/91dbb496ba05a930cb4347729d88ddc9.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"Executor","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"Executor","attrs":{}}],"attrs":{}},{"type":"text","text":" 是顶级接口，里面只有一个方法","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"execute(Runnable command)","attrs":{}}],"attrs":{}},{"type":"text","text":"，定义的是调度线程池来执行任务，它定义了线程池的基本规范，执行任务是它的天职。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"ExecutorService","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"ExecutorService","attrs":{}}],"attrs":{}},{"type":"text","text":" 继承了","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"Executor","attrs":{}}],"attrs":{}},{"type":"text","text":"，但是它仍然是一个接口，它多了一些方法：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/9a/9a6b75f7dddfe76eaa8288184dcb4067.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"void shutdown()","attrs":{}}],"attrs":{}},{"type":"text","text":":关闭线程池，会等待任务执行完。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"List shutdownNow()","attrs":{}}],"attrs":{}},{"type":"text","text":":立刻关闭线程池，尝试停止所有正在积极执行的任务，停止等待任务的处理，并","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"返回一个正在等待执行的任务列表（还没有执行的）","attrs":{}},{"type":"text","text":"。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"boolean isShutdown()","attrs":{}}],"attrs":{}},{"type":"text","text":":判断线程池是不是已经关闭，但是可能线程还在执行。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"boolean isTerminated()","attrs":{}}],"attrs":{}},{"type":"text","text":":在执行shutdown/shutdownNow之后，所有的任务已经完成，这个状态就是true。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"boolean awaitTermination(long timeout, TimeUnit unit)","attrs":{}}],"attrs":{}},{"type":"text","text":":执行shutdown之后，阻塞等到terminated状态，除非超时或者被打断。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":" Future submit(Callable task)","attrs":{}}],"attrs":{}},{"type":"text","text":": 提交一个有返回值的任务，并且返回该任务尚未有结果的Future，调用future.get()方法，可以返回任务完成的时候的结果。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":" Future submit(Runnable task, T result)","attrs":{}}],"attrs":{}},{"type":"text","text":":提交一个任务，传入返回结果，这个result没有什么作用，只是指定类型和一个返回的结果。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"Future> submit(Runnable task)","attrs":{}}],"attrs":{}},{"type":"text","text":": 提交任务，返回Future","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":" List> invokeAll(Collection extends Callable> tasks)","attrs":{}}],"attrs":{}},{"type":"text","text":":批量执行tasks，获取Future的list，可以批量提交任务。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":" List> invokeAll(Collection extends Callable> tasks,long timeout, TimeUnit unit)","attrs":{}}],"attrs":{}},{"type":"text","text":":批量提交任务，并指定超时时间","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":" T invokeAny(Collection extends Callable> tasks)","attrs":{}}],"attrs":{}},{"type":"text","text":": 阻塞，获取第一个完成任务的结果值，","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":" T invokeAny(Collection extends Callable> tasks,long timeout, TimeUnit unit)","attrs":{}}],"attrs":{}},{"type":"text","text":":阻塞，获取第一个完成结果的值，指定超时时间","attrs":{}}]}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"可能有同学对前面的","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":" Future submit(Runnable task, T result)","attrs":{}}],"attrs":{}},{"type":"text","text":"有疑问，这个reuslt有什么作用？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"其实它没有什么作用，只是持有它，任务完成后，还是调用 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"future.get（）","attrs":{}}],"attrs":{}},{"type":"text","text":"返回这个结果，用","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"result","attrs":{}}],"attrs":{}},{"type":"text","text":" new 了一个 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"ftask","attrs":{}}],"attrs":{}},{"type":"text","text":"，其内部其实是使用了Runnable的包装类 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"RunnableAdapter","attrs":{}}],"attrs":{}},{"type":"text","text":",没有对result做特殊的处理，调用 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"call()","attrs":{}}],"attrs":{}},{"type":"text","text":" 方法的时候，直接返回这个结果。（Executors 中具体的实现）","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":" public Future submit(Runnable task, T result) {\n if (task == null) throw new NullPointerException();\n RunnableFuture ftask = newTaskFor(task, result);\n execute(ftask);\n return ftask;\n }\n\n static final class RunnableAdapter implements Callable {\n final Runnable task;\n final T result;\n RunnableAdapter(Runnable task, T result) {\n this.task = task;\n this.result = result;\n }\n public T call() {\n task.run();\n // 返回传入的结果\n return result;\n }\n }\n","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"还有一个方法值得一提：","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"invokeAny()","attrs":{}}],"attrs":{}},{"type":"text","text":": 在 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"ThreadPoolExecutor","attrs":{}}],"attrs":{}},{"type":"text","text":"中使用","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"ExecutorService","attrs":{}}],"attrs":{}},{"type":"text","text":" 中的方法 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"invokeAny()","attrs":{}}],"attrs":{}},{"type":"text","text":" 取得第一个完成的任务的结果，当第一个任务执行完成后，会调用 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"interrupt()","attrs":{}}],"attrs":{}},{"type":"text","text":" 方法将其他任务中断。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"注意，","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"ExecutorService","attrs":{}}],"attrs":{}},{"type":"text","text":"是接口，里面都是定义，并没有涉及实现，而前面的讲解都是基于它的名字（规定的规范）以及它的普遍实现来说的。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"可以看到 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"ExecutorService","attrs":{}}],"attrs":{}},{"type":"text","text":" 定义的是线程池的一些操作，包括关闭，判断是否关闭，是否停止，提交任务，批量提交任务等等。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"AbstractExecutorService","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"AbstractExecutorService","attrs":{}}],"attrs":{}},{"type":"text","text":" 是一个抽象类，实现了 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"ExecutorService","attrs":{}}],"attrs":{}},{"type":"text","text":"接口，这是大部分线程池的基本实现，定时的线程池先不关注，主要的方法如下：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/90/90125abb1e14439db0cdd3e0634a3535.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"不仅实现了","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"submit","attrs":{}}],"attrs":{}},{"type":"text","text":"，","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"invokeAll","attrs":{}}],"attrs":{}},{"type":"text","text":"，","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"invokeAny","attrs":{}}],"attrs":{}},{"type":"text","text":" 等方法，而且提供了一个 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"newTaskFor","attrs":{}}],"attrs":{}},{"type":"text","text":" 方法用于构建 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"RunnableFuture","attrs":{}}],"attrs":{}},{"type":"text","text":" 对象，那些能够获取到任务返回结果的对象都是通过 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"newTaskFor","attrs":{}}],"attrs":{}},{"type":"text","text":" 来获取的。不展开里面所有的源码的介绍，仅以submit()方法为例：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":" public Future> submit(Runnable task) {\n if (task == null) throw new NullPointerException();\n // 封装任务\n RunnableFuture ftask = newTaskFor(task, null);\n // 执行任务\n execute(ftask);\n // 返回 RunnableFuture 对象\n return ftask;\n }\n","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"但是在 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"AbstractExecutorService","attrs":{}}],"attrs":{}},{"type":"text","text":" 是没有对最最重要的方法进行实现的，也就是 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"execute()","attrs":{}}],"attrs":{}},{"type":"text","text":" 方法。线程池具体是怎么执行的，这个不同的线程池可以有不同的实现，一般都是继承 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"AbstractExecutorService","attrs":{}}],"attrs":{}},{"type":"text","text":" (定时任务有其他的接口)，我们最最常用的就是","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"ThreadPoolExecutor","attrs":{}}],"attrs":{}},{"type":"text","text":"。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/44/4482409d96ebfd2488372d3f1fba0587.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"ThreadPoolExecutor","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"重点来了!!!","attrs":{}},{"type":"text","text":" ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"ThreadPoolExecutor","attrs":{}}],"attrs":{}},{"type":"text","text":" 一般就是我们平时常用到的线程池类，所谓创建线程池，如果不是定时线程池，就是使用它。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"先看","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"ThreadPoolExecutor","attrs":{}}],"attrs":{}},{"type":"text","text":"的内部结构(属性)：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public class ThreadPoolExecutor extends AbstractExecutorService {\n // 状态控制，主要用来控制线程池的状态，是核心的遍历，使用的是原子类\n private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));\n // 用来表示线程数量的位数（使用的是位运算，一部分表示线程的数量，一部分表示线程池的状态）\n // SIZE = 32 表示32位，那么COUNT_BITS就是29位\n private static final int COUNT_BITS = Integer.SIZE - 3;\n // 线程池的容量，也就是27位表示的最大值\n private static final int CAPACITY = (1 << COUNT_BITS) - 1;\n\n // 状态量，存储在高位，32位中的前3位\n // 111（第一位是符号位，1表示负数），线程池运行中\n private static final int RUNNING = -1 << COUNT_BITS; \n // 000\n private static final int SHUTDOWN = 0 << COUNT_BITS;\n // 001\n private static final int STOP = 1 << COUNT_BITS;\n // 010\n private static final int TIDYING = 2 << COUNT_BITS;\n // 011\n private static final int TERMINATED = 3 << COUNT_BITS;\n\n // 取出运行状态\n private static int runStateOf(int c) { return c & ~CAPACITY; }\n // 取出线程数量\n private static int workerCountOf(int c) { return c & CAPACITY; }\n // 用运行状态和线程数获取ctl\n private static int ctlOf(int rs, int wc) { return rs | wc; }\n \n // 任务等待队列\n private final BlockingQueue workQueue;\n // 可重入主锁（保证一些操作的线程安全）\n private final ReentrantLock mainLock = new ReentrantLock();\n // 线程的集合\n private final HashSet workers = new HashSet();\n \n // 在Condition中，用await()替换wait()，用signal()替换notify()，用signalAll()替换notifyAll()，\n // 传统线程的通信方式，Condition都可以实现，Condition和传统的线程通信没什么区别，Condition的强大之处在于它可以为多个线程间建立不同的Condition\n private final Condition termination = mainLock.newCondition();\n \n // 最大线程池大小\n private int largestPoolSize;\n // 完成的任务数量\n private long completedTaskCount;\n // 线程工厂\n private volatile ThreadFactory threadFactory;\n // 任务拒绝处理器\n private volatile RejectedExecutionHandler handler;\n // 非核心线程的存活时间\n private volatile long keepAliveTime;\n // 允许核心线程的超时时间\n private volatile boolean allowCoreThreadTimeOut;\n // 核心线程数\n private volatile int corePoolSize;\n // 工作线程最大容量\n private volatile int maximumPoolSize;\n // 默认的拒绝处理器（丢弃任务）\n private static final RejectedExecutionHandler defaultHandler =\n new AbortPolicy();\n // 运行时关闭许可\n private static final RuntimePermission shutdownPerm =\n new RuntimePermission(\"modifyThread\");\n // 上下文\n private final AccessControlContext acc;\n // 只有一个线程\n private static final boolean ONLY_ONE = true;\n}\n","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"线程池状态","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"从上面的代码可以看出，用一个32位的对象保存线程池的状态以及线程池的容量，高3位是线程池的状态，而剩下的29位，则是保存线程的数量：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":" // 状态量，存储在高位，32位中的前3位\n // 111（第一位是符号位，1表示负数），线程池运行中\n private static final int RUNNING = -1 << COUNT_BITS; \n // 000\n private static final int SHUTDOWN = 0 << COUNT_BITS;\n // 001\n private static final int STOP = 1 << COUNT_BITS;\n // 010\n private static final int TIDYING = 2 << COUNT_BITS;\n // 011\n private static final int TERMINATED = 3 << COUNT_BITS;\n","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"各种状态之间是不一样的，他们的状态之间变化如下：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/8d/8d9dae20a9436627fe9465b0a9a43108.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"RUNNING：运行状态，可以接受任务，也可以处理任务","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"SHUTDOWN：不可以接受任务，但是可以处理任务","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"STOP：不可以接受任务，也不可以处理任务，中断当前任务","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"TIDYING：所有线程停止","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"TERMINATED：线程池的最后状态","attrs":{}}]}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"Worker 实现","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"线程池，肯定得有池子，并且是放线程的地方，在 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"ThreadPoolExecutor","attrs":{}}],"attrs":{}},{"type":"text","text":" 中表现为 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"Worker","attrs":{}}],"attrs":{}},{"type":"text","text":"，这是内部类：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/01/011a2f829b87221becdb7b9dd50f7c1a.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"线程池其实就是 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"Worker","attrs":{}}],"attrs":{}},{"type":"text","text":" (打工人，不断的领取任务，完成任务)的集合，这里使用的是 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"HashSet","attrs":{}}],"attrs":{}},{"type":"text","text":":","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"private final HashSet workers = new HashSet();\n","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"Worker","attrs":{}}],"attrs":{}},{"type":"text","text":" 怎么实现的呢？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"Worker","attrs":{}}],"attrs":{}},{"type":"text","text":" 除了继承了 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"AbstractQueuedSynchronizer","attrs":{}}],"attrs":{}},{"type":"text","text":",也就是 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"AQS","attrs":{}}],"attrs":{}},{"type":"text","text":" ， ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"AQS","attrs":{}}],"attrs":{}},{"type":"text","text":" 本质上就是个队列锁，一个简单的互斥锁，一般是在中断或者修改 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"worker","attrs":{}}],"attrs":{}},{"type":"text","text":" 状态的时候使用。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"内部引入","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"AQS","attrs":{}}],"attrs":{}},{"type":"text","text":"，是为了线程安全，线程执行任务的时候，调用的是","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"runWorker(Worker w)","attrs":{}}],"attrs":{}},{"type":"text","text":"，这个方法不是worker的方法，而是 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"ThreadPoolExecutor","attrs":{}}],"attrs":{}},{"type":"text","text":"的方法。从下面的代码可以看出，每次修改","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"Worke","attrs":{}}],"attrs":{}},{"type":"text","text":"r的状态的时候，都是线程安全的。","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"Worker","attrs":{}}],"attrs":{}},{"type":"text","text":"里面，持有了一个线程","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"Thread","attrs":{}}],"attrs":{}},{"type":"text","text":",可以理解为是对线程的封装。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"至于","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"runWorker(Worker w)","attrs":{}}],"attrs":{}},{"type":"text","text":"是怎么运行的？先保持这个疑问，后面详细讲解。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":" // 实现 Runnable，封装了线程\n private final class Worker\n extends AbstractQueuedSynchronizer\n implements Runnable\n {\n // 序列化id\n private static final long serialVersionUID = 6138294804551838833L;\n\n // worker运行的线程\n final Thread thread;\n \n // 初始化任务，有可能是空的，如果任务不为空的时候，其他进来的任务，可以直接运行，不在添加到任务队列\n Runnable firstTask;\n // 线程任务计数器\n volatile long completedTasks;\n\n // 指定一个任务让工人忙碌起来，这个任务可能是空的\n Worker(Runnable firstTask) {\n // 初始化AQS队列锁的状态\n setState(-1); // 禁止中断直到 runWorker\n this.firstTask = firstTask;\n // 从线程工厂，取出一个线程初始化\n this.thread = getThreadFactory().newThread(this);\n }\n\n // 实际上运行调用的是runWorker\n public void run() {\n // 不断循环获取任务进行执行\n runWorker(this);\n }\n\n // 0表示没有被锁\n // 1表示被锁的状态\n protected boolean isHeldExclusively() {\n return getState() != 0;\n }\n // 独占，尝试获取锁，如果成功返回true，失败返回false\n protected boolean tryAcquire(int unused) {\n // CAS 乐观锁\n if (compareAndSetState(0, 1)) {\n // 成功，当前线程独占锁\n setExclusiveOwnerThread(Thread.currentThread());\n return true;\n }\n return false;\n }\n // 独占方式，尝试释放锁\n protected boolean tryRelease(int unused) {\n setExclusiveOwnerThread(null);\n setState(0);\n return true;\n }\n // 上锁，调用的是AQS的方法\n public void lock() { acquire(1); }\n // 尝试上锁\n public boolean tryLock() { return tryAcquire(1); }\n // 解锁\n public void unlock() { release(1); }\n // 是否锁住\n public boolean isLocked() { return isHeldExclusively(); }\n\n // 如果开始可就中断\n void interruptIfStarted() {\n Thread t;\n if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {\n try {\n t.interrupt();\n } catch (SecurityException ignore) {\n }\n }\n }\n }\n","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"任务队列","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"除了放线程池的地方，要是任务很多，没有那么多线程，肯定需要一个地方放任务，充当缓冲作用，也就是任务队列，在代码中表现为：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"private final BlockingQueue workQueue;\n","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"拒绝策略和处理器","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"计算机的内存总是有限的，我们不可能一直往队列里面增加内容，所以线程池为我们提供了选择，可以选择多种队列。同时当任务实在太多，占满了线程，并且把任务队列也占满的时候，我们需要做出一定的反应，那就是拒绝还是抛出错误，丢掉任务？丢掉哪些任务，这些都是可能需要定制的内容。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"如何创建线程池","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"关于如何创建线程池，其实 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"ThreadPoolExecutor","attrs":{}}],"attrs":{}},{"type":"text","text":"提供了构造方法，主要参数如下，不传的话会使用默认的：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"核心线程数：核心线程数，一般是指常驻的线程，没有任务的时候通常也不会销毁","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"最大线程数：线程池允许创建的最大的线程数量","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"非核心线程的存活时间：指的是没有任务的时候，非核心线程能够存活多久","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"时间的单位：存活时间的单位","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"存放任务的队列：用来存放任务","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"线程工厂","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"拒绝处理器:如果添加任务失败，将由该处理器处理","attrs":{}}]}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":" // 指定核心线程数，最大线程数，非核心线程没有任务的存活时间，时间单位，任务队列 \n public ThreadPoolExecutor(int corePoolSize,\n int maximumPoolSize,\n long keepAliveTime,\n TimeUnit unit,\n BlockingQueue workQueue) {\n this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,\n Executors.defaultThreadFactory(), defaultHandler);\n }\n // 指定核心线程数，最大线程数，非核心线程没有任务的存活时间，时间单位，任务队列，线程池工厂 \n public ThreadPoolExecutor(int corePoolSize,\n int maximumPoolSize,\n long keepAliveTime,\n TimeUnit unit,\n BlockingQueue workQueue,\n ThreadFactory threadFactory) {\n this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,\n threadFactory, defaultHandler);\n }\n // 指定核心线程数，最大线程数，非核心线程没有任务的存活时间，时间单位，任务队列，拒绝任务处理器\n public ThreadPoolExecutor(int corePoolSize,\n int maximumPoolSize,\n long keepAliveTime,\n TimeUnit unit,\n BlockingQueue workQueue,\n RejectedExecutionHandler handler) {\n this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,\n Executors.defaultThreadFactory(), handler);\n }\n // 最后其实都是调用了这个方法\n public ThreadPoolExecutor(int corePoolSize,\n int maximumPoolSize,\n long keepAliveTime,\n TimeUnit unit,\n BlockingQueue workQueue,\n ThreadFactory threadFactory,\n RejectedExecutionHandler handler) {\n ...\n }\n","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"其实，除了显示的指定上面的参数之外，JDK也封装了一些直接创建线程池的方法给我们，那就是","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"Executors","attrs":{}}],"attrs":{}},{"type":"text","text":":","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":" // 固定线程数量的线程池，无界的队列\n public static ExecutorService newFixedThreadPool(int nThreads) {\n return new ThreadPoolExecutor(nThreads, nThreads,\n 0L, TimeUnit.MILLISECONDS,\n new LinkedBlockingQueue());\n }\n // 单个线程的线程池，无界的队列，按照任务提交的顺序，串行执行 \n public static ExecutorService newSingleThreadExecutor(ThreadFactory threadFactory) {\n return new FinalizableDelegatedExecutorService\n (new ThreadPoolExecutor(1, 1,\n 0L, TimeUnit.MILLISECONDS,\n new LinkedBlockingQueue(),\n threadFactory));\n }\n // 动态调节，没有核心线程，全部都是普通线程，每个线程存活60s，使用容量为1的阻塞队列\n public static ExecutorService newCachedThreadPool() {\n return new ThreadPoolExecutor(0, Integer.MAX_VALUE,\n 60L, TimeUnit.SECONDS,\n new SynchronousQueue());\n }\n // 定时任务线程池\n public static ScheduledExecutorService newSingleThreadScheduledExecutor() {\n return new DelegatedScheduledExecutorService\n (new ScheduledThreadPoolExecutor(1));\n }\n","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"但是一般是不推荐使用上面别人封装的线程池的哈！！！","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"线程池的底层参数以及核心方法","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"看完上面的创建参数大家可能会有点懵，但是没关系，一一为大家道来：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/eb/ebf5c72919a6fba8ad2e732040b7ef29.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"可以看出，当有任务进来的时候，先判断核心线程池是不是已经满了，如果还没有，将会继续创建线程。注意，如果一个任务进来，创建线程执行，执行完成，线程空闲下来，这时候再来一个任务，是会继续使用之前的线程，还是重新创建一个线程来执行呢？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"答案是重新创建线程，这样线程池可以快速达到核心线程数的规模大小，以便快速响应后面的任务。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果线程数量已经到达核心线程数，来了任务，线程池的线程又都不是空闲状态，那么就会判断队列是不是满的，倘若队列还有空间，那么就会把任务放进去队列中，等待线程领取执行。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果任务队列已经满了，放不下任务，那么就会判断线程数是不是已经到最大线程数了，要是还没有到达，就会继续创建线程并执行任务，这个时候创建的是非核心部分线程。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果已经到达最大线程数，那么就不能继续创建线程了，只能执行拒绝策略，默认的拒绝策略是丢弃任务，我们可以自定义拒绝策略。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"值得注意的是，倘若之前任务比较多，创建出了一些非核心线程，那么任务少了之后，领取不到任务，过了一定时间，非核心线程就会销毁，只剩下核心线程池的数量的线程。这个时间就是前面说的","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"keepAliveTime","attrs":{}}],"attrs":{}},{"type":"text","text":"。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"提交任务","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"提交任务，我们看","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"execute()","attrs":{}}],"attrs":{}},{"type":"text","text":"，会先获取线程池的状态和个数，要是线程个数还没达到核心线程数，会直接添加线程，否则会放到任务队列，如果任务队列放不下，会继续增加线程，但是不是增加核心线程。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":" public void execute(Runnable command) {\n if (command == null)\n throw new NullPointerException();\n // 获取状态和个数\n int c = ctl.get();\n // 如果个数小于核心线程数\n if (workerCountOf(c) < corePoolSize) {\n // 直接添加\n if (addWorker(command, true))\n return;\n // 添加失败则继续获取\n c = ctl.get();\n }\n // 判断线程池状态是不是运行中，任务放到队列中\n if (isRunning(c) && workQueue.offer(command)) {\n // 再次检查\n int recheck = ctl.get();\n // 判断线程池是不是还在运行\n if (! isRunning(recheck) && remove(command))\n // 如果不是，那么就拒绝并移除任务\n reject(command);\n else if (workerCountOf(recheck) == 0)\n // 如果线程数为0，并且还在运行，那么就直接添加\n addWorker(null, false);\n }else if (!addWorker(command, false))\n // 添加任务队列失败，拒绝\n reject(command);\n }\n","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"上面的源码中，调用了一个重要的方法：","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"addWorker(Runnable firstTask, boolean core)","attrs":{}}],"attrs":{}},{"type":"text","text":",该方法主要是为了增加工作的线程，我们来看看它是如何执行的：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":" private boolean addWorker(Runnable firstTask, boolean core) {\n // 回到当前位置重试\n retry:\n for (;;) {\n // 获取状态\n int c = ctl.get();\n int rs = runStateOf(c);\n\n // 大于SHUTDOWN说明线程池已经停止\n // ! (rs == SHUTDOWN && firstTask == null && ! workQueue.isEmpty()) 表示三个条件至少有一个不满足\n // 不等于SHUTDOWN说明是大于shutdown\n // firstTask ！= null 任务不是空的\n // workQueue.isEmpty() 队列是空的\n if (rs >= SHUTDOWN &&\n ! (rs == SHUTDOWN &&\n firstTask == null &&\n ! workQueue.isEmpty()))\n return false;\n\n for (;;) {\n // 工作线程数\n int wc = workerCountOf(c);\n // 是否符合容量\n if (wc >= CAPACITY ||\n wc >= (core ? corePoolSize : maximumPoolSize))\n return false;\n // 添加成功，跳出循环\n if (compareAndIncrementWorkerCount(c))\n break retry;\n c = ctl.get(); // Re-read ctl\n // cas失败，重新尝试\n if (runStateOf(c) != rs)\n continue retry;\n // else CAS failed due to workerCount change; retry inner loop\n }\n }\n\n // 前面线程计数增加成功\n boolean workerStarted = false;\n boolean workerAdded = false;\n Worker w = null;\n try {\n // 创建了一个worker，包装了任务\n w = new Worker(firstTask);\n final Thread t = w.thread;\n // 线程创建成功\n if (t != null) {\n // 获取锁\n final ReentrantLock mainLock = this.mainLock;\n mainLock.lock();\n try {\n // 再次确认状态\n int rs = runStateOf(ctl.get());\n if (rs < SHUTDOWN ||\n (rs == SHUTDOWN && firstTask == null)) {\n // 如果线程已经启动，失败\n if (t.isAlive()) // precheck that t is startable\n throw new IllegalThreadStateException();\n // 新增线程到集合\n workers.add(w);\n // 获取大小\n int s = workers.size();\n // 判断最大线程池数量\n if (s > largestPoolSize)\n largestPoolSize = s;\n // 已经添加工作线程\n workerAdded = true;\n }\n } finally {\n // 解锁\n mainLock.unlock();\n }\n // 如果已经添加\n if (workerAdded) {\n // 启动线程\n t.start();\n workerStarted = true;\n }\n }\n } finally {\n // 如果没有启动\n if (! workerStarted)\n // 失败处理\n addWorkerFailed(w);\n }\n return workerStarted;\n }\n","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"处理任务","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"前面在介绍","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"Worker","attrs":{}}],"attrs":{}},{"type":"text","text":"这个类的时候，我们讲解到其实它的","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"run()","attrs":{}}],"attrs":{}},{"type":"text","text":"方法调用的是外部的","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"runWorker()","attrs":{}}],"attrs":{}},{"type":"text","text":"方法，那么我们来看看","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"runWorkder()","attrs":{}}],"attrs":{}},{"type":"text","text":"方法：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"首先，它会直接处理自己的firstTask,这个任务并没有在任务队列里面，而是它自己持有的：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"final void runWorker(Worker w) {\n // 当前线程\n Thread wt = Thread.currentThread();\n // 第一个任务\n Runnable task = w.firstTask;\n // 重置为null\n w.firstTask = null;\n // 允许打断\n w.unlock();\n boolean completedAbruptly = true;\n try {\n // 任务不为空，或者获取的任务不为空\n while (task != null || (task = getTask()) != null) {\n // 加锁\n w.lock();\n //如果线程池停止，确保线程被中断;\n //如果不是，确保线程没有被中断。这\n //在第二种情况下需要复查处理\n // shutdown - now竞赛同时清除中断\n if ((runStateAtLeast(ctl.get(), STOP) ||\n (Thread.interrupted() &&\n runStateAtLeast(ctl.get(), STOP))) &&\n !wt.isInterrupted())\n wt.interrupt();\n try {\n // 执行之前回调方法（可以由我们自己实现）\n beforeExecute(wt, task);\n Throwable thrown = null;\n try {\n // 执行任务\n task.run();\n } catch (RuntimeException x) {\n thrown = x; throw x;\n } catch (Error x) {\n thrown = x; throw x;\n } catch (Throwable x) {\n thrown = x; throw new Error(x);\n } finally {\n // 执行之后回调方法\n afterExecute(task, thrown);\n }\n } finally {\n // 置为null\n task = null;\n // 更新完成任务\n w.completedTasks++;\n w.unlock();\n }\n }\n // 完成\n completedAbruptly = false;\n } finally {\n // 处理线程退出相关工作\n processWorkerExit(w, completedAbruptly);\n }\n }\n","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"上面可以看到如果当前的任务是null，会去获取一个task，我们看看","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"getTask()","attrs":{}}],"attrs":{}},{"type":"text","text":"，里面涉及到了两个参数，一个是是不是允许核心线程销毁，另外一个是线程数是不是大于核心线程数，如果满足条件，就从队列中取出任务，如果超时取不到，那就返回空，表示没有取到任务，没有取到任务，就不会执行前面的循环，就会触发线程销毁","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"processWorkerExit()","attrs":{}}],"attrs":{}},{"type":"text","text":"等工作。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"private Runnable getTask() {\n // 是否超时\n boolean timedOut = false; // Did the last poll() time out?\n\n for (;;) {\n int c = ctl.get();\n int rs = runStateOf(c);\n\n // SHUTDOWN状态继续处理队列中的任务，但是不接收新的任务\n if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {\n decrementWorkerCount();\n return null;\n }\n // 线程数\n int wc = workerCountOf(c);\n\n // 是否允许核心线程超时或者线程数大于核心线程数\n boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;\n\n if ((wc > maximumPoolSize || (timed && timedOut))\n && (wc > 1 || workQueue.isEmpty())) {\n // 减少线程成功，就返回null，后面由processWorkerExit()处理\n if (compareAndDecrementWorkerCount(c))\n return null;\n continue;\n }\n\n try {\n // 如果允许核心线程关闭，或者超过了核心线程，就可以在超时的时间内获取任务，或者直接取出任务\n Runnable r = timed ?\n workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :\n workQueue.take();\n // 如果能取到任务，那就肯定可以执行\n if (r != null)\n return r;\n // 否则就获取不到任务，超时了\n timedOut = true;\n } catch (InterruptedException retry) {\n timedOut = false;\n }\n }\n}\n","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"销毁线程","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"前面提到，如果线程当前任务为空，又允许核心线程销毁，或者线程超过了核心线程数，等待了一定时间，超时了却没有从任务队列获取到任务的话，就会跳出循环执行到后面的线程销毁（结束）程序。那销毁线程的时候怎么做呢？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":" private void processWorkerExit(Worker w, boolean completedAbruptly) {\n // 如果是突然结束的线程，那么之前的线程数是没有调整的，这里需要调整\n if (completedAbruptly)\n decrementWorkerCount();\n // 获取锁\n final ReentrantLock mainLock = this.mainLock;\n mainLock.lock();\n \n try {\n // 完成的任务数\n completedTaskCount += w.completedTasks;\n // 移除线程\n workers.remove(w);\n } finally {\n // 解锁\n mainLock.unlock();\n }\n // 试图停止\n tryTerminate();\n // 获取状态\n int c = ctl.get();\n // 比stop小，至少是shutdown\n if (runStateLessThan(c, STOP)) {\n // 如果不是突然完成\n if (!completedAbruptly) {\n // 最小值要么是0，要么是核心线程数，要是允许核心线程超时销毁，那么就是0\n int min = allowCoreThreadTimeOut ? 0 : corePoolSize;\n // 如果最小的是0或者队列不是空的，那么保留一个线程\n if (min == 0 && ! workQueue.isEmpty())\n min = 1;\n // 只要大于等于最小的线程数，就结束当前线程\n if (workerCountOf(c) >= min)\n return; // replacement not needed\n }\n // 否则的话，可能还需要新增工作线程\n addWorker(null, false);\n }\n }\n","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"如何停止线程池","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"停止线程池可以使用","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"shutdown()","attrs":{}}],"attrs":{}},{"type":"text","text":"或者","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"shutdownNow()","attrs":{}}],"attrs":{}},{"type":"text","text":"，","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"shutdown()","attrs":{}}],"attrs":{}},{"type":"text","text":"可以继续处理队列中的任务，而","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"shutdownNow()","attrs":{}}],"attrs":{}},{"type":"text","text":"会立即清理任务，并返回未执行的任务。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":" public void shutdown() {\n // 获取锁\n final ReentrantLock mainLock = this.mainLock;\n mainLock.lock();\n try {\n // 检查停止权限\n checkShutdownAccess();\n // 更新状态\n advanceRunState(SHUTDOWN);\n // 中断所有线程\n interruptIdleWorkers();\n // 回调钩子\n onShutdown(); // hook for ScheduledThreadPoolExecutor\n } finally {\n mainLock.unlock();\n }\n tryTerminate();\n }\n // 立刻停止\n public List shutdownNow() {\n List tasks;\n // 获取锁\n final ReentrantLock mainLock = this.mainLock;\n mainLock.lock();\n try {\n // 检查停止权限\n checkShutdownAccess();\n // 更新状态到stop\n advanceRunState(STOP);\n // 中断所有线程\n interruptWorkers();\n // 清理队列\n tasks = drainQueue();\n } finally {\n mainLock.unlock();\n }\n tryTerminate();\n // 返回任务列表（未完成）\n return tasks;\n }\n","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"execute()和submit()方法","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"execute() ","attrs":{}}],"attrs":{}},{"type":"text","text":"方法可以提交不需要返回值的任务，无法判断任务是否被线程池执行是否成功","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"submit()","attrs":{}}],"attrs":{}},{"type":"text","text":"方法用于提交需要返回值的任务。线程池会返回一个future类型的对象，通过这个对象，我们调用","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"get()","attrs":{}}],"attrs":{}},{"type":"text","text":"方法就可以","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"阻塞","attrs":{}},{"type":"text","text":"，直到获取到线程执行完成的结果，同时我们也可以使用有超时时间的等待方法","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"get（long timeout，TimeUnit unit）","attrs":{}}],"attrs":{}},{"type":"text","text":",这样不管线程有没有执行完成，如果到时间，也不会阻塞，直接返回null。返回的是","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"RunnableFuture","attrs":{}}],"attrs":{}},{"type":"text","text":"对象，继承了","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"Runnable, Future","attrs":{}}],"attrs":{}},{"type":"text","text":"两个接口：","attrs":{}}]}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"public interface RunnableFuture extends Runnable, Future {\n /**\n * Sets this Future to the result of its computation\n * unless it has been cancelled.\n */\n void run();\n}\n","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"线程池为什么使用阻塞队列？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"阻塞队列，首先是一个队列，肯定具有先进先出的属性。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"而阻塞，则是这个模型的演化，一般队列，可以用在生产消费者模型，也就是数据共享，有人往里面放任务，有人不断的往里面取出任务，这是一个理想的状态。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"但是倘若不理想，产生任务和消费任务的速度不一样，要是任务放在队列里面比较多，消费比较慢，还可以慢慢消费，或者生产者得暂停一下产生任务（阻塞生产者线程）。可以使用 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"offer(E o, long timeout, TimeUnit unit)","attrs":{}}],"attrs":{}},{"type":"text","text":"设定等待的时间，如果在指定的时间内，还不能往队列中加入","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"BlockingQueue","attrs":{}}],"attrs":{}},{"type":"text","text":"，则返回失败,也可以使用","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"put(Object)","attrs":{}}],"attrs":{}},{"type":"text","text":",将对象放到阻塞队列里面，如果没有空间，那么这个方法会阻塞到有空间才会放进去。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果消费速度快，生产者来不及生产，获取任务的时候，可以使用","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"poll(time)","attrs":{}}],"attrs":{}},{"type":"text","text":",有数据则直接取出来，没数据则可以等待","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"time","attrs":{}}],"attrs":{}},{"type":"text","text":"时间后，返回","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"null","attrs":{}}],"attrs":{}},{"type":"text","text":"。也可以使用","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"take()","attrs":{}}],"attrs":{}},{"type":"text","text":"取出第一个任务，没有任务就会一直阻塞到队列有任务为止。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"上面说了阻塞队列的属性，那么为啥要用呢？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果产生任务，来了就往队列里面放，资源很容易被耗尽。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"创建线程需要获取锁，这个一个线程池的全局锁，如果各个线程不断的获取锁，解锁，线程上下文切换之类的开销也比较大，不如在队列为空的时候，然一个线程阻塞等待。","attrs":{}}]}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"常见的阻塞队列","attrs":{}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/0b/0b3e290a52869e3e8c1298cf7befe739.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"ArrayBlockingQueue","attrs":{}},{"type":"text","text":"：基于数组实现，内部有一个定长的数组，同时保存着队列头和尾部的位置。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"LinkedBlockingQueue","attrs":{}},{"type":"text","text":"：基于链表的阻塞对垒，生产者和消费者使用独立的锁，并行能力强，如果不指定容量，默认是无效容量，容易系统内存耗尽。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"DelayQueue","attrs":{}},{"type":"text","text":"：延迟队列，没有大小限制，生产数据不会被阻塞，消费数据会，只有指定的延迟时间到了，才能从队列中获取到该元素。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"PriorityBlockingQueue","attrs":{}},{"type":"text","text":"：基于优先级的阻塞队列，按照优先级进行消费，内部控制同步的是公平锁。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"SynchronousQueue","attrs":{}},{"type":"text","text":"：没有缓冲，生产者直接把任务交给消费者，少了中间的缓存区。","attrs":{}}]}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"线程池如何复用线程的？执行完成的线程怎么处理","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"前面的源码分析，其实已经讲解过这个问题了，线程池的线程调用的","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"run()","attrs":{}}],"attrs":{}},{"type":"text","text":"方法，其实调用的是","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"runWorker()","attrs":{}}],"attrs":{}},{"type":"text","text":"，里面是死循环，除非获取不到任务，如果没有了任务firstTask并且从任务队列中获取不到任务，超时的时候，会再判断是不是可以销毁核心线程，或者超过了核心线程数，满足条件的时候，才会让当前的线程结束。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"否则，一直都在一个循环中，不会结束。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我们知道","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"start()","attrs":{}}],"attrs":{}},{"type":"text","text":"方法只能调用一次,因此调用到","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"run()","attrs":{}}],"attrs":{}},{"type":"text","text":"方法的时候，调用外面的","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"runWorker()","attrs":{}}],"attrs":{}},{"type":"text","text":",让其在","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"runWorker()","attrs":{}}],"attrs":{}},{"type":"text","text":"的时候，不断的循环，获取任务。获取到任务，调用任务的","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"run()","attrs":{}}],"attrs":{}},{"type":"text","text":"方法。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"执行完成的线程会调用","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"processWorkerExit（）","attrs":{}}],"attrs":{}},{"type":"text","text":",前面有分析，里面会获取锁，把线程数减少，从工作线程从集合中移除，移除掉之后，会判断线程是不是太少了，如果是，会再加回来，个人以为是一种补救。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"如何配置线程池参数？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"一般而言，有个公式，如果是计算（CPU）密集型的任务，那么核心线程数设置为","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"处理器核数-1","attrs":{}}],"attrs":{}},{"type":"text","text":"，如果是io密集型（很多网络请求），那么就可以设置为","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"2*处理器核数","attrs":{}}],"attrs":{}},{"type":"text","text":"。但是这并不是一个银弹，一切要从实际出发，最好就是在测试环境进行压测，实践出真知，并且很多时候一台机器不止一个线程池或者还会有其他的线程，因此参数不可设置得太过饱满。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"一般 8 核的机器，设置 10-12 个核心线程就差不多了，这一切必须按照业务具体值进行计算。设置过多的线程数，上下文切换，竞争激烈，设置过少，没有办法充分利用计算机的资源。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"计算（CPU）密集型消耗的主要是 CPU 资源，可以将线程数设置为 N（CPU 核心数）+1，比 CPU 核心数多出来的一个线程是为了防止线程偶发的缺页中断，或者其它原因导致的任务暂停而带来的影响。一旦任务暂停，CPU 就会处于空闲状态，而在这种情况下多出来的一个线程就可以充分利用 CPU 的空闲时间。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"io密集型系统会用大部分的时间来处理 I/O 交互，而线程在处理 I/O 的时间段内不会占用 CPU 来处理，这时就可以将 CPU 交出给其它线程使用。因此在 I/O 密集型任务的应用中，我们可以多配置一些线程，具体的计算方法是 2N。","attrs":{}}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"为什么不推荐默认的线程池创建方式？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"阿里的编程规范里面，不建议使用默认的方式来创建线程，是因为这样创建出来的线程很多时候参数都是默认的，可能创建者不太了解，很容易出问题，最好通过","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"new ThreadPoolExecutor()","attrs":{}}],"attrs":{}},{"type":"text","text":"来创建，方便控制参数。默认的方式创建的问题如下：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Executors.newFixedThreadPool()：无界队列，内存可能被打爆","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Executors.newSingleThreadExecutor()：单个线程，效率低，串行。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Executors.newCachedThreadPool()：没有核心线程，最大线程数可能为无限大，内存可能还会爆掉。","attrs":{}}]}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"使用具体的参数创建线程池，开发者必须了解每个参数的作用，不会胡乱设置参数，减少内存溢出等问题。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"一般体现在几个问题：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"任务队列怎么设置？","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"核心线程多少个？","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"最大线程数多少？","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"怎么拒绝任务？","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"创建线程的时候没有名称，追溯问题不好找。","attrs":{}}]}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"线程池的拒绝策略","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"线程池一般有以下四种拒绝策略，其实我们可以从它的内部类看出来：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/28/28f00a98a7f56844f2ff93a1640736b6.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"AbortPolicy: 不执行新的任务，直接抛出异常，提示线程池已满","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"DisCardPolicy：不执行新的任务，但是也不会抛出异常，默默的","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"DisCardOldSetPolicy：丢弃消息队列中最老的任务，变成新进来的任务","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"CallerRunsPolicy：直接调用当前的execute来执行任务","attrs":{}}]}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"一般而言，上面的拒绝策略都不会特别理想，一般要是任务满了，首先需要做的就是看任务是不是必要的，如果非必要，非核心，可以考虑拒绝掉，并报错提醒，如果是必须的，必须把它保存起来，不管是使用mq消息，还是其他手段，不能丢任务。在这些过程中，日志是非常必要的。既要保护线程池，也要对业务负责。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"线程池监控与动态调整","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"线程池提供了一些API，可以动态获取线程池的状态，并且还可以设置线程池的参数，以及状态：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"查看线程池的状态：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/af/af7dedc627c677e378981cb613543582.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"修改线程池的状态：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/74/743a317802323ca86f8ab2105491ad66.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"关于这一点，美团的线程池文章讲得很清楚，甚至做了一个实时调整线程池参数的平台，可以进行跟踪监控，线程池活跃度、任务的执行Transaction（频率、耗时）、Reject异常、线程池内部统计信息等等。这里我就不展开了，原文：https://tech.meituan.com/2020/04/02/java-pooling-pratice-in-meituan.html ，这是我们可以参考的思路。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"线程池隔离","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"线程隔离，很多同学可能知道，就是不同的任务放在不同的线程里面运行，而线程池隔离，一般是按照业务类型来隔离，比如订单的处理线程放在一个线程池，会员相关的处理放在一个线程池。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"也可以通过核心和非核心来隔离，核心处理流程放在一起，非核心放在一起，两个使用不一样的参数，不一样的拒绝策略，尽量保证多个线程池之间不影响，并且最大可能保住核心线程的运行，非核心线程可以忍受失败。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"Hystrix","attrs":{}}],"attrs":{}},{"type":"text","text":"里面运用到这个技术，","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"Hystrix","attrs":{}}],"attrs":{}},{"type":"text","text":"的线程隔离技术，来防止不同的网络请求之间的雪崩，即使依赖的一个服务的线程池满了，也不会影响到应用程序的其他部分。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"关于作者","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"秦怀，公众号【","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"秦怀杂货店","attrs":{}},{"type":"text","text":"】作者，技术之路不在一时，山高水长，纵使缓慢，驰而不息。个人写作方向：Java源码解析，JDBC，Mybatis，Spring，redis，分布式，剑指Offer，LeetCode等，认真写好每一篇文章，不喜欢标题党，不喜欢花里胡哨，大多写系列文章，不能保证我写的都完全正确，但是我保证所写的均经过实践或者查找资料。遗漏或者错误之处，还望指正。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"http://aphysia.cn/archives/2020","title":"","type":null},"content":[{"type":"text","text":"2020年我写了什么？","attrs":{}}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://damaer.github.io/Coding/#/","title":"","type":null},"content":[{"type":"text","text":"开源编程笔记","attrs":{}}]}]}]}