java安全编码指南之:Thread API调用规则

原創

2020-10-19 14:33

{"type":"doc","content":[{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"简介"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"java中多线程的开发中少不了使用Thread，我们在使用Thread中提供的API过程中，应该注意些什么规则呢？"}]},{"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":"一起来看一看吧。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"start一个Thread"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Thread中有两个方法，一个是start方法，一个是run方法，两个都可以调用，那么两个有什么区别呢？"}]},{"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":"先看一下start方法："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"public synchronized void start() {\n\n if (threadStatus != 0)\n throw new IllegalThreadStateException();\n group.add(this);\n boolean started = false;\n try {\n start0();\n started = true;\n } finally {\n try {\n if (!started) {\n group.threadStartFailed(this);\n }\n } catch (Throwable ignore) {\n }\n }\n }\n\n private native void start0();\n"}]},{"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":"start()是一个synchronized的方法，通过它会去调用native的start0方法，而最终将会调用Thread的run()方法。"}]},{"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，创建一个Thread有两种方式，一种是传入一个Runnable，一个是继承Thread，并重写run()方法。"}]},{"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":"如果我们直接调用Thread的run()方法会发生什么事情呢？"}]},{"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":"先看一下run方法的定义："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":" public void run() {\n if (target != null) {\n target.run();\n }\n }\n"}]},{"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":"默认情况下，这个target就是一个Runnable对象，如果Thread是通过Runnable来构建的话，调用Thread.run()会在当前线程中运行run方法中的内容。"}]},{"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Thread是以其形式构建，并且没有重新run()方法，那么直接调用Thread.run()将什么都不会做。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":" public void wrongStart(){\n Runnable runnable= ()-> System.out.println(\"in thread running!\");\n Thread thread= new Thread(runnable);\n thread.run();\n }\n\n public void correctStart(){\n Runnable runnable= ()-> System.out.println(\"in thread running!\");\n Thread thread= new Thread(runnable);\n thread.start();\n }\n"}]},{"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":"所以，上面两种调用方式，只有第二种是正确的。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"不要使用ThreadGroup"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Thread中有个字段类型是java.lang.ThreadGroup，这个主要是用来给Thread进行分组，我们看下Thread的这个构造函数："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":" public Thread(ThreadGroup group, Runnable target) {\n this(group, target, \"Thread-\" + nextThreadNum(), 0);\n }\n"}]},{"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":"上面的构造函数可以在传入runnable的同时传递一个ThreadGroup对Thread进行分组。"}]},{"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":"如果没有指定ThreadGroup，那么将会为其分配一个默认的default group。"}]},{"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":"ThreadGroup是做什么的呢？ThreadGroup是java 1.0引入的方法，主要是一次性的对一组thread进行操作。我们可以调用ThreadGroup.interrupt()来一次性的对整个Group的Thread进行interrupts操作。"}]},{"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":"虽然ThreadGroup提供了很多有用的方法，但是其中很多方法都被废弃了，比如：allowThreadSuspension(), resume(), stop(), 和 suspend()，并且ThreadGroup中还有很多方法是非线程安全的："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"ThreadGroup.activeCount()"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这个方法主要是用来统计一个ThreadGroup中活动的线程个数，这个方法会统计还未启动的线程，同时也会受系统线程的影响，所以是不准确的。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"ThreadGroup.enumerate()"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这个方法是将ThreadGroup和子group的线程拷贝到一个数组中，但是如果数组太小了，多余的线程是会被自动忽略的。"}]},{"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":"ThreadGroup本身有一个 stop() 方法用来停止所有的线程，但是stop是不安全的，已经被废弃了。"}]},{"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":"那么我们该怎么去安全的停止很多个线程呢？"}]},{"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executor.shutdown()就可以了。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"不要使用stop()方法"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"刚刚讲了ThreadGroup中不要调用stop()方法，因为stop是不安全的。"}]},{"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":"调用stop方法会立马释放线程持有的所有的锁，并且会抛出ThreadDeath异常。"}]},{"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":"因为会释放所有的锁，所以可能会造成受这些锁保护的对象的状态发生不一致的情况。"}]},{"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":"替代的方法有两种，一种是使用volatile flag变量，来控制线程的循环执行："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":" private volatile boolean done = false;\n\n public void shutDown(){\n this.done= true;\n }\n\n public void stopWithFlag(){\n\n Runnable runnable= ()->{\n while(!done){\n System.out.println(\"in Runnable\");\n }\n };\n\n Thread thread= new Thread(runnable);\n thread.start();\n shutDown();\n }\n"}]},{"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":"另外一种方法就是调用interrupt(), 这里我们要注意interrupt()的使用要点："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"如果当前线程实例在调用Object类的wait（），wait（long）或wait（long，int）方法或join（），join（long），join（long，int）方法，或者在该实例中调用了Thread.sleep（long）或Thread.sleep（long，int）方法，并且正在阻塞状态中时，则其中断状态将被清除，并将收到InterruptedException。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"如果此线程在InterruptibleChannel上的I/O操作中处于被阻塞状态，则该channel将被关闭，该线程的中断状态将被设置为true，并且该线程将收到java.nio.channels.ClosedByInterruptException异常。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"如果此线程在java.nio.channels.Selector中处于被被阻塞状态，则将设置该线程的中断状态为true，并且它将立即从select操作中返回。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":4,"align":null,"origin":null},"content":[{"type":"text","text":"如果上面的情况都不成立，则设置中断状态为true。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"先看下面的例子："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":" public static void main(String[] args) {\n Runnable runnable= ()->{\n while (!Thread.interrupted()) {\n System.out.println(\"in thread\");\n }\n };\n Thread thread= new Thread(runnable);\n thread.start();\n Thread.sleep(5000);\n thread.interrupt();\n }\n"}]},{"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":"我们在while循环中调用了Thread.interrupted()方法用来判断线程是否被设置了中断位，然后在main方法中调用了thread.interrupt()来设置中断，最终可以正确的停止Thread。"}]},{"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","marks":[{"type":"italic"}],"text":"注意，这里运行的Thread并没有被阻塞，所以并不满足我们上面提到的第一个条件。"}]}]},{"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":"下面我们再看一个例子："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":" public static void main(String[] args) {\n Runnable runnable= ()->{\n while (!Thread.interrupted()) {\n System.out.println(\"in thread\");\n try {\n Thread.sleep(5000);\n } catch (InterruptedException e) {\n e.printStackTrace();\n }\n }\n };\n\n Thread thread= new Thread(runnable);\n thread.start();\n thread.interrupt();\n }\n"}]},{"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":"这个例子和上面的例子不同之处就是在于，Thread中调用了sleep方法，导致Thread被阻塞了，最终满足了第一个条件，从而不会设置终端位，只会抛出InterruptedException，所以这个例子中线程是不会被停止的，大家一定要注意。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"wait 和 await 需要放在循环中调用"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"为什么要放在循环中呢？因为我们希望wait不是被错误的被唤醒，所以我们需要在wait被唤醒之后，重新检测一遍条件。"}]},{"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":"错误的调用是放在if语句中："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"synchronized (object) {\n if () {\n object.wait();\n }\n // Proceed when condition holds\n}\n"}]},{"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":"正确的方法是放在while循环中："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"synchronized (object) {\n while () {\n object.wait();\n }\n // Proceed when condition holds\n}\n"}]},{"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":"本文的代码："}]},{"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://github.com/ddean2009/learn-java-base-9-to-20/tree/master/security","title":null},"content":[{"type":"text","text":"learn-java-base-9-to-20/tree/master/security"}]}]},{"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","marks":[{"type":"italic"}],"text":"本文已收录于 "},{"type":"link","attrs":{"href":"http://www.flydean.com/java-security-code-line-thread/","title":null},"content":[{"type":"text","text":"http://www.flydean.com/java-security-code-line-thread/"}],"marks":[{"type":"italic"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"italic"}],"text":"最通俗的解读，最深刻的干货，最简洁的教程，众多你不知道的小技巧等你来发现！"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"italic"}],"text":"欢迎关注我的公众号:「程序那些事」,懂技术，更懂你！"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}}]}