文章目录
- 1.线程池的概念
- 2.线程池的优点
- 3.JDK API
- 4.多线程使用引入样例
- 4.多线程执行流程
- 4.1.线程池使用入口:ThreadPoolExecutor.execute
- 4.2.线程池ThreadPoolExecutor构造方法参数
- 4.2.1.corePoolSize:核心线程池数量
- 4.2.2.maximumPoolSize:最大线程池数量
- 4.2.3.keepAliveTime:线程空闲时间长度
- 4.2.4.unit:线程空闲时间长度单位
- 4.2.5.workQueue:线程池中阻塞待处理的队列
- 4.2.6.handler:线程池无法处理新的请求之后的拒绝策略
- 4.3.线程整体执行流程图
- 4.3.1.线程池内部结构解析
- 4.3.2.线程池流程判断
- 4.3.3.线程池的状态
- 4.3.3.1.线程池的几种状态介绍
- 4.3.3.1.1.RUNNING:接收新任务，同时处理已经在队列中的任务
- 4.3.3.1.2.SHUTDOWN :不接受新任务，但是处理已经在队列中的任务
- 4.3.3.1.3.STOP状态:不接受新任务,不处理队列中任务,中断正在进行的任务
- 4.3.3.1.4.TIDYING状态:所有的任务终止,WorkerCount为0，进入TIDYING状态,开始调用terminated()方法
- 4.3.3.1.5.TERMINATED ：terminated() 调用结束的时候状态
- 4.3.3.2.线程池状态的变化关系
- 4.3.3.3.线程池状态的二进制标示介绍
- 4.3.4.从源码看流程
1.线程池的概念
1.我们这里说的线程池主要是JDK中提供的线程池；
2.线程池，主要是基于频繁的创建和销毁线程比较耗费资源，所以我们在项目启动的过程中，提前创建包含一定数量线程的
线程池(池子)；
3.后面请求过来的时候，直接从线程池中获取可用的线程即可；
2.线程池的优点
2.1.减少资源的消耗
1.这里减少资源的消耗，主要是由于提供了线程池，一定数量上，可以减少因为线程的创建和销毁而带来的CPU内存资源消耗；
2.2.提高请求访问速度(响应速度)
1.因为我们有了线程池，减少了线程的创建和销毁时间，这样响应速度和访问速度都能够提高；
2.3.便于对线程的管理
1.因为有了线程池，线程的数量是稳定的，保证了系统的稳定性
2.同时，我们可以便于对线程池中的线程进行统一的管理，调优等操作；
3.JDK API
3.1.线程池对象ThreadPoolExecutor(ExecutorService子类)
1.这里我们着重看一下构造一个线程池ThreadPoolExecutor所需要的因子参数;
2.下面是对参数的介绍
int corePoolSize, #核心线程数量
int maximumPoolSize, #最大线程数量
long keepAliveTime, #线程存活时间
TimeUnit unit, #线程存活时间单位，keepAliveTime的单位
BlockingQueue<Runnable> workQueue, #存储请求任务的线程队列
ThreadFactory threadFactory,
RejectedExecutionHandler handler
3.2.创建线程池的核心工具类对象-Executors
1.首先 Executors是一个创建线程池的工具类；
2.Executors中提供了一些列静态的创建线程池对象ExecutorService;的一系列的方法,返回线程池ExecutorService;
3.这里我们先简单的介绍一下ThreadPoolExecutor对象(ExecutorService子类)
3.3.Executors 创建线程池的核心方法
3.3.1.创建只有一个线程的线程池:newSingleThreadExecutor
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}
3.3.2.创建固定线程数量的线程池:newFixedThreadPool
public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>(),
threadFactory);
}
3.3.3.创建没有限制数量的线程池:newCachedThreadPool
public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory) {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>(),
threadFactory);
}
1.这里的线程数量所谓没有限制，是一个相对值，因为最大值为2^32-1，几乎是一个够用的一个数量
2.线程池会根据实际的需要去自动的调整线程池的大小;
3.60秒后线程自动被回收;
3.3.4.周期性任务的调度的线程池:ScheduledThreadPoolExecutor
public static ScheduledExecutorService newScheduledThreadPool(
int corePoolSize, ThreadFactory threadFactory) {
return new ScheduledThreadPoolExecutor(corePoolSize, threadFactory);
}
1.所谓周期性任务的调度，周期性的执行队列中的请求
4.多线程使用引入样例
package com.gaoxinfu.demo.jdk.rt.java.util.concurrent;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
/**
* @Description:
* @Author: gaoxinfu
* @Date: 2020-05-20 14:07
*/
public class ExecutorsDemo {
public static void main(String[] args) {
ExecutorService threadPoolExecutor= Executors.newCachedThreadPool();
for (int i = 0; i < 10; i++) {
final int index = i;
try {
Thread.sleep(index * 1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
threadPoolExecutor.execute(new Runnable() {
@Override
public void run() {
System.out.println("index = "+index);
}
});
}
}
}
4.多线程执行流程
4.1.线程池使用入口:ThreadPoolExecutor.execute
1.从上面的样例,我们可以知道，线程池使用的入口为:
ThreadPoolExecutor.execute(Runnable runnable)
4.2.线程池ThreadPoolExecutor构造方法参数
/**
* Creates a new {@code ThreadPoolExecutor} with the given initial
* parameters and default thread factory.
*
* @param corePoolSize the number of threads to keep in the pool, even
* if they are idle, unless {@code allowCoreThreadTimeOut} is set
* @param maximumPoolSize the maximum number of threads to allow in the
* pool
* @param keepAliveTime when the number of threads is greater than
* the core, this is the maximum time that excess idle threads
* will wait for new tasks before terminating.
* @param unit the time unit for the {@code keepAliveTime} argument
* @param workQueue the queue to use for holding tasks before they are
* executed. This queue will hold only the {@code Runnable}
* tasks submitted by the {@code execute} method.
* @param handler the handler to use when execution is blocked
* because the thread bounds and queue capacities are reached
* @throws IllegalArgumentException if one of the following holds:<br>
* {@code corePoolSize < 0}<br>
* {@code keepAliveTime < 0}<br>
* {@code maximumPoolSize <= 0}<br>
* {@code maximumPoolSize < corePoolSize}
* @throws NullPointerException if {@code workQueue}
* or {@code handler} is null
*/
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
RejectedExecutionHandler handler) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), handler);
}
4.2.1.corePoolSize:核心线程池数量
1.corePoolSize：我们可以简单理解就是应用运行的过程中，需要保证的线程池中一个至少应该运行的线程的数量;
2.但是这里有一个特殊情况：就是我们
4.2.2.maximumPoolSize:最大线程池数量
4.2.3.keepAliveTime:线程空闲时间长度
4.2.4.unit:线程空闲时间长度单位
4.2.5.workQueue:线程池中阻塞待处理的队列
4.2.6.handler:线程池无法处理新的请求之后的拒绝策略
4.3.线程整体执行流程图
4.3.1.线程池内部结构解析
1.corePoolSize实际上我们可以认为是隶属于maxmumPoolSize的子集
2.WorkerQueue是一个队列，这个队列主要是基于在新的请求进来之后，线程池数量大于corePoolSize时，会进去这个
WorkerQueue队列
4.3.2.线程池流程判断
1.请求过来之后，进去线程池之后，首先查看核心线程池当前是否有空余的线程，如果有，则创建线程进行执行;
2.如果核心线程池corePoolSize没有多余的线程，那么判断WorkerQueue队列是否已经满;
如果已满，那么进行判断进行步骤3的处理;
如果未满，进入WorkerQueue队列，等待线程池中空余线程Thread的处理;
3.判断maxmumPoolSize是否已满
如果没有剩余线程，直接执行线程池指定的拒绝策略;
如果还有剩余线程，直接创建线程执行的任务，处理当前的请求;
4.3.3.线程池的状态
/**
* The main pool control state, ctl, is an atomic integer packing
* two conceptual fields
* workerCount, indicating the effective number of threads
* runState, indicating whether running, shutting down etc
*
* In order to pack them into one int, we limit workerCount to
* (2^29)-1 (about 500 million) threads rather than (2^31)-1 (2
* billion) otherwise representable. If this is ever an issue in
* the future, the variable can be changed to be an AtomicLong,
* and the shift/mask constants below adjusted. But until the need
* arises, this code is a bit faster and simpler using an int.
*
* The workerCount is the number of workers that have been
* permitted to start and not permitted to stop. The value may be
* transiently different from the actual number of live threads,
* for example when a ThreadFactory fails to create a thread when
* asked, and when exiting threads are still performing
* bookkeeping before terminating. The user-visible pool size is
* reported as the current size of the workers set.
*
* The runState provides the main lifecycle control, taking on values:
*
* RUNNING: Accept new tasks and process queued tasks
* SHUTDOWN: Don't accept new tasks, but process queued tasks
* STOP: Don't accept new tasks, don't process queued tasks,
* and interrupt in-progress tasks
* TIDYING: All tasks have terminated, workerCount is zero,
* the thread transitioning to state TIDYING
* will run the terminated() hook method
* TERMINATED: terminated() has completed
*
* The numerical order among these values matters, to allow
* ordered comparisons. The runState monotonically increases over
* time, but need not hit each state. The transitions are:
*
* RUNNING -> SHUTDOWN
* On invocation of shutdown(), perhaps implicitly in finalize()
* (RUNNING or SHUTDOWN) -> STOP
* On invocation of shutdownNow()
* SHUTDOWN -> TIDYING
* When both queue and pool are empty
* STOP -> TIDYING
* When pool is empty
* TIDYING -> TERMINATED
* When the terminated() hook method has completed
*
* Threads waiting in awaitTermination() will return when the
* state reaches TERMINATED.
*
* Detecting the transition from SHUTDOWN to TIDYING is less
* straightforward than you'd like because the queue may become
* empty after non-empty and vice versa during SHUTDOWN state, but
* we can only terminate if, after seeing that it is empty, we see
* that workerCount is 0 (which sometimes entails a recheck -- see
* below).
*/
// runState is stored in the high-order bits
private static final int RUNNING = -1 << COUNT_BITS;
private static final int SHUTDOWN = 0 << COUNT_BITS;
private static final int STOP = 1 << COUNT_BITS;
private static final int TIDYING = 2 << COUNT_BITS;
private static final int TERMINATED = 3 << COUNT_BITS;
4.3.3.1.线程池的几种状态介绍
4.3.3.1.1.RUNNING:接收新任务，同时处理已经在队列中的任务
RUNNING: Accept new tasks and process queued tasks
4.3.3.1.2.SHUTDOWN :不接受新任务，但是处理已经在队列中的任务
SHUTDOWN:Don't accept new tasks, but process queued tasks
4.3.3.1.3.STOP状态:不接受新任务,不处理队列中任务,中断正在进行的任务
Don't accept new tasks, don't process queued tasks,and interrupt in-progress tasks
4.3.3.1.4.TIDYING状态:所有的任务终止,WorkerCount为0，进入TIDYING状态,开始调用terminated()方法
TIDYING:All tasks have terminated, workerCount is zero,
the thread transitioning to state TIDYING
will run the terminated() hook method
4.3.3.1.5.TERMINATED ：terminated() 调用结束的时候状态
TERMINATED:terminated() has completed
4.3.3.2.线程池状态的变化关系
* RUNNING -> SHUTDOWN
* On invocation of shutdown(), perhaps implicitly in finalize()
* (RUNNING or SHUTDOWN) -> STOP
* On invocation of shutdownNow()
* SHUTDOWN -> TIDYING
* When both queue and pool are empty
* STOP -> TIDYING
* When pool is empty
* TIDYING -> TERMINATED
* When the terminated() hook method has completed
1.应用启动的时候，线程池就进入了RUNNING状态;
2.RUNNING状态下，一旦调用shutdown()方法，进入SHUTDOWN状态
3.TIDYING 翻译为整理，为调用terminated()之前的一个状态
4.3.3.3.线程池状态的二进制标示介绍
private static final int RUNNING = -1 << COUNT_BITS;
private static final int SHUTDOWN = 0 << COUNT_BITS;
private static final int STOP = 1 << COUNT_BITS;
private static final int TIDYING = 2 << COUNT_BITS;
private static final int TERMINATED = 3 << COUNT_BITS;
转换为二进制 CPU内存中的存储形式
@Test
public void ctl(){
System.out.println(Integer.toBinaryString(-1));
System.out.println("RUNNING = " +Integer.toBinaryString((-1 << (Integer.SIZE - 3))));
System.out.println("SHUTDOWN = " +Integer.toBinaryString((0 << (Integer.SIZE - 3))));
System.out.println("STOP = " +Integer.toBinaryString((1 << (Integer.SIZE - 3))));
System.out.println("TIDYING = " +Integer.toBinaryString((2 << (Integer.SIZE - 3))));
System.out.println("TERMINATED = " +Integer.toBinaryString((3 << (Integer.SIZE - 3))));
}
11111111111111111111111111111111
RUNNING = 11100000000000000000000000000000 #相当于 011100000 00000000 00000000 00000000
SHUTDOWN = 0 #相当于 000000000 00000000 00000000 00000000
STOP = 100000000000000000000000000000 #相当于 000100000 00000000 00000000 00000000
TIDYING = 1000000000000000000000000000000 #相当于 001000000 00000000 00000000 00000000
TERMINATED = 1100000000000000000000000000000 #相当于 001100000 00000000 00000000 00000000
第一位是符号位，也就是这5个状态是通过高三位的保存了状态
关于位移有符号位移运算的介绍可以参考下面的地址
https://blog.csdn.net/u014636209/article/details/106405242
4.3.4.从源码看流程
/**
* Executes the given task sometime in the future. The task
* may execute in a new thread or in an existing pooled thread.
*
* If the task cannot be submitted for execution, either because this
* executor has been shutdown or because its capacity has been reached,
* the task is handled by the current {@code RejectedExecutionHandler}.
*
* @param command the task to execute
* @throws RejectedExecutionException at discretion of
* {@code RejectedExecutionHandler}, if the task
* cannot be accepted for execution
* @throws NullPointerException if {@code command} is null
*/
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
/*
* Proceed in 3 steps:
*
* 1. If fewer than corePoolSize threads are running, try to
* start a new thread with the given command as its first
* task. The call to addWorker atomically checks runState and
* workerCount, and so prevents false alarms that would add
* threads when it shouldn't, by returning false.
*
* 2. If a task can be successfully queued, then we still need
* to double-check whether we should have added a thread
* (because existing ones died since last checking) or that
* the pool shut down since entry into this method. So we
* recheck state and if necessary roll back the enqueuing if
* stopped, or start a new thread if there are none.
*
* 3. If we cannot queue task, then we try to add a new
* thread. If it fails, we know we are shut down or saturated
* and so reject the task.
*/
//获取当前线程池的状态
int c = ctl.get();
//1.判断Worker(正在运行的线程)数量是否小于核心线程池的数量
if (workerCountOf(c) < corePoolSize) {
//核心线程池还有空闲的线程，将当前请求加入到Worker之中，启动新的线程
//具体addWorker的方法 可以查看4.3.3.1
if (addWorker(command, true))
return;
c = ctl.get();
}
//2.
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
if (! isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
else if (!addWorker(command, false))
reject(command);
}
4.3.4.1.我们先来了解一个AtomicInteger ctl变量
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
// Packing and unpacking ctl
private static int runStateOf(int c) {
return c & ~CAPACITY;
}
private static int workerCountOf(int c) {
return c & CAPACITY;
}
private static int ctlOf(int rs, int wc) {
return rs | wc;
}
4.3.4.1.1.ctlOf的两个属性:线程池的运行状态和线程池的Worker数量
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
private static int ctlOf(int rs, int wc) {
return rs | wc;
}
1.首先，使用AtomicInteger类型的变量存储了线程池的两个属性：线程池的运行状态和线程池的Worker数量
我们前面看知道线程池的一个状态的二进制存储的问题,如下
RUNNING = 11100000000000000000000000000000 #相当于 011100000 00000000 00000000 00000000
SHUTDOWN = 0 #相当于 000000000 00000000 00000000 00000000
STOP = 100000000000000000000000000000 #相当于 000100000 00000000 00000000 00000000
TIDYING = 1000000000000000000000000000000 #相当于 001000000 00000000 00000000 00000000
TERMINATED = 1100000000000000000000000000000 #相当于 001100000 00000000 00000000 00000000
从上面，可以看到，第一位为符号位(0标示正) 然后第二位到第四位保存类线程池的状态
我们的Worker线程池的数量估计认为3*8+5=29，也就是29^2-1 个
那么可以看出，前面四个是线程池的状态，后面29个二进制可以认为是Worker数量
这样我们就可以对他俩的二进制进行运算，可以算的线程池的状态或者Worker的数量，具体见下面的分析
4.3.4.1.2.获取线程池的状态:runStateOf
private static int runStateOf(int c) {
return c & ~CAPACITY;
}
1.因为我们知道CAPACITY可以认为是Worker的最大数量
00011111 11111111 11111111 11111111
按位取反之后
11100000 00000000 00000000 00000000
然后跟前面存储的线程池的状态和Worker数量的字段ctl去进行按位与运算
因为我们上面按位取反之后的二进制后面29位都是0，
所以进行按位与运算的时候，肯定只剩下高位四位(加上符号位)，实际上就是我们线程池的状态
4.3.4.1.3.获取Worker线程的数量
private static int workerCountOf(int c) {
return c & CAPACITY;
}
1.因为前面我们说过CAPACITY，二进制内存中 00011111 11111111 11111111 11111111
1.这个理解上就比较简单了，因为我们Worker的数量的高三位都是0，然后CAPACITY，低位都是1
进行按位与运算之后，相同为1，那么结果为Worker的数量了
4.3.4.2.addWorker(Runnable firstTask, boolean core)解析
/**
* 大家注意下，这里的Worker 其实就是一个线程，这个线程执行的请求的任务
*
* Checks if a new worker can be added with respect to current
* pool state and the given bound (either core or maximum). If so,
* the worker count is adjusted accordingly, and, if possible, a
* new worker is created and started, running firstTask as its
* first task. This method returns false if the pool is stopped or
* eligible to shut down. It also returns false if the thread
* factory fails to create a thread when asked. If the thread
* creation fails, either due to the thread factory returning
* null, or due to an exception (typically OutOfMemoryError in
* Thread.start()), we roll back cleanly.
* 翻译:
* 根据当前线程池的状态和给定的线程池的范围(核心线程池数量和最大的线程池数量)检查是否可以创建新Worker;
* 如果可以,那么WorkerCount会相应的调整,同时,新创建的Worker会执行我们传递进来的firstTask任务作为
* 他的第一个任务(这句话，我们可以认为,新创建的Worker实际上就是线程池中一个线程,一旦firstTask任务
* 结束，实际上这个线程池中Worker线程并不会消亡,他会继续处理新的任务请求);
* 如果线程池处理停止状态或者SHUTDOWN状态，该方法将返回false，表示创建任务失败;
* 如果线程创建失败，不管是由于线程工厂返回null(线程创建失败)还是OOM内存溢出导致的异常等，
* 我们都将彻底回滚当前操作;
*
* @param firstTask the task the new thread should run first (or
* null if none). Workers are created with an initial first task
* (in method execute()) to bypass queuing when there are fewer
* than corePoolSize threads (in which case we always start one),
* or when the queue is full (in which case we must bypass queue).
* Initially idle threads are usually created via
* prestartCoreThread or to replace other dying workers.
*
* @param core if true use corePoolSize as bound, else
* maximumPoolSize. (A boolean indicator is used here rather than a
* value to ensure reads of fresh values after checking other pool
* state).
* @return true if successful
*/
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
/**
*1.rs >= SHUTDOWN 说明当前线程不是RUNNING状态
*2.! (rs == SHUTDOWN &&firstTask == null &&! workQueue.isEmpty())
*
* 简单这两种条件:
* 1.如果当前线程池的状态是RUNNINN状态，可以进行线程的处理
* 2.如果是SHUTDOWN状态,并且当前任务是空，并且队列中非空，也是可以进行线程的处理
* 这里线程的处理，就是Worker任务的线程处理
* 所以下面的if判断是基于上面两种都不是的情况下，直接返回false，不进行线程任务的处理
*/
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&firstTask == null &&! workQueue.isEmpty()))
return false;
for (;;) {
int wc = workerCountOf(c);//获取当前Worker线程的数量
//如果当前Worker线程任务已经最大值或者大于临街corePoolSize|maximumPoolSize
if (wc >= CAPACITY ||wc >= (core ? corePoolSize : maximumPoolSize))
return false;
//如果Worker线程任务比临界corePoolSize|maximumPoolSize小,那么WorkerCount+1
if (compareAndIncrementWorkerCount(c))
break retry;
//下面的操作主要是基于我们上面compareAndIncrementWorkerCount
c = ctl.get(); // Re-read ctl
if (runStateOf(c) != rs)
continue retry;
// else CAS failed due to workerCount change; retry inner loop
}
}
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
w = new Worker(firstTask);
final Thread t = w.thread;
if (t != null) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
// Recheck while holding lock.
// Back out on ThreadFactory failure or if
// shut down before lock acquired.
int rs = runStateOf(ctl.get());
if (rs < SHUTDOWN ||(rs == SHUTDOWN && firstTask == null)) {
if (t.isAlive()) // precheck that t is startable
throw new IllegalThreadStateException();
workers.add(w);
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
if (workerAdded) {
//这里启动的时候会去调用Worker的Run方法，本质上是runWorker方法
//runWorker见下面分析
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
4.3.4.3.runWorker方法解析
/**
* Main worker run loop. Repeatedly gets tasks from queue and
* executes them, while coping with a number of issues:
*
* 1. We may start out with an initial task, in which case we
* don't need to get the first one. Otherwise, as long as pool is
* running, we get tasks from getTask. If it returns null then the
* worker exits due to changed pool state or configuration
* parameters. Other exits result from exception throws in
* external code, in which case completedAbruptly holds, which
* usually leads processWorkerExit to replace this thread.
*
* 2. Before running any task, the lock is acquired to prevent
* other pool interrupts while the task is executing, and then we
* ensure that unless pool is stopping, this thread does not have
* its interrupt set.
*
* 3. Each task run is preceded by a call to beforeExecute, which
* might throw an exception, in which case we cause thread to die
* (breaking loop with completedAbruptly true) without processing
* the task.
*
* 4. Assuming beforeExecute completes normally, we run the task,
* gathering any of its thrown exceptions to send to afterExecute.
* We separately handle RuntimeException, Error (both of which the
* specs guarantee that we trap) and arbitrary Throwables.
* Because we cannot rethrow Throwables within Runnable.run, we
* wrap them within Errors on the way out (to the thread's
* UncaughtExceptionHandler). Any thrown exception also
* conservatively causes thread to die.
*
* 5. After task.run completes, we call afterExecute, which may
* also throw an exception, which will also cause thread to
* die. According to JLS Sec 14.20, this exception is the one that
* will be in effect even if task.run throws.
*
* The net effect of the exception mechanics is that afterExecute
* and the thread's UncaughtExceptionHandler have as accurate
* information as we can provide about any problems encountered by
* user code.
*
* @param w the worker
*/
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
//这里的worker会一直运行，首先判断当前的firstTask是否为空
//如果不为空，执行该任务，如果为空，getTask那么去WorkerQueue里拿一个
while (task != null || (task = getTask()) != null) {
w.lock();
// If pool is stopping, ensure thread is interrupted;
// if not, ensure thread is not interrupted. This
// requires a recheck in second case to deal with
// shutdownNow race while clearing interrupt
if ((runStateAtLeast(ctl.get(), STOP) ||
(Thread.interrupted() &&
runStateAtLeast(ctl.get(), STOP))) &&
!wt.isInterrupted())
wt.interrupt();
try {
beforeExecute(wt, task);
Throwable thrown = null;
try {
task.run();
} catch (RuntimeException x) {
thrown = x; throw x;
} catch (Error x) {
thrown = x; throw x;
} catch (Throwable x) {
thrown = x; throw new Error(x);
} finally {
afterExecute(task, thrown);
}
} finally {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
processWorkerExit(w, completedAbruptly);
}
}