Jdk线程池ThreadPoolExecutor源码解析
文章目录
- 一、基础模型
- 1.1 线程池任务的抽象 FutureTask
- 1.1.1 FutureTask类层次结构
- 1.1.2 FutureTask源码
- 1.2 线程池的抽象: 从Excutor、ExecutorService到AbstractExecutorService
- 1.2.1 线程池类层次结构
- 1.2.2 线程池接口定义
- 1.2.3 小结
- 二、线程池工作原理
- 2.1 线程池工作机制概述
- 2.2 任务提交、执行流程
- 三、ThreadPoolExecutor源码解析
- 3.1 线程池的数据模型
- 3.2 线程池的任务执行
- 四、总结
一、基础模型
1.1 线程池任务的抽象 FutureTask
1.1.1 FutureTask类层次结构
可以将RunnableFuture看做线程池对任务的统一抽象，这样线程池内部的执行逻辑就不用特意区分Runnable和Callable
1.1.2 FutureTask源码
FutureTask内部持有一个Callable对象，如果任务是Runable就通过RunnableAdapter适配成Callable；故无论提交的任务是Runable还是Callable，FutureTask都能Hold住。
public class FutureTask<V> implements RunnableFuture<V> {
/**
* The run state of this task, initially NEW. The run state
* transitions to a terminal state only in methods set,
* setException, and cancel. During completion, state may take on
* transient values of COMPLETING (while outcome is being set) or
* INTERRUPTING (only while interrupting the runner to satisfy a
* cancel(true)). Transitions from these intermediate to final
* states use cheaper ordered/lazy writes because values are unique
* and cannot be further modified.
*
* Possible state transitions:
* NEW -> COMPLETING -> NORMAL
* NEW -> COMPLETING -> EXCEPTIONAL
* NEW -> CANCELLED
* NEW -> INTERRUPTING -> INTERRUPTED
*/
private volatile int state;
private static final int NEW = 0;
private static final int COMPLETING = 1;
private static final int NORMAL = 2;
private static final int EXCEPTIONAL = 3;
private static final int CANCELLED = 4;
private static final int INTERRUPTING = 5;
private static final int INTERRUPTED = 6;
/** The underlying callable; nulled out after running */
private Callable<V> callable;
/** The result to return or exception to throw from get() */
private Object outcome; // non-volatile, protected by state reads/writes
/** The thread running the callable; CASed during run() */
private volatile Thread runner;
/** Treiber stack of waiting threads */
private volatile WaitNode waiters;
// 包装Callable
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW; // ensure visibility of callable
}
// 包装Runnable: 通过RunnableAdapter将Runnable适配成Callable
public FutureTask(Runnable runnable, V result) {
this.callable = Executors.callable(runnable, result);
this.state = NEW; // ensure visibility of callable
}
}
// 代码见 java.util.concurrent.Executors.RunnableAdapter
static final class RunnableAdapter<T> implements Callable<T> {
final Runnable task;
final T result;
RunnableAdapter(Runnable task, T result) {
this.task = task;
this.result = result;
}
public T call() {
task.run();
return result;
}
}
1.2 线程池的抽象: 从Excutor、ExecutorService到AbstractExecutorService
1.2.1 线程池类层次结构
1.2.2 线程池接口定义
- Executor
顶层接口，定义了最基本的execute(Runnable task)方法
public interface Executor {
void execute(Runnable command);
}
- ExecutorService
1、定义shutdown()、shutdownNow()等生命周期管理方法，让客户端能对线程池生命周期进行管控；
2、在Executor基础上扩充了线程池任务提交方式，如submit(Callable task)、submit(Runable task)；不再局限于只能提交Runnable任务。
public interface ExecutorService extends Executor {
void shutdown();
List<Runnable> shutdownNow();
boolean isShutdown();
boolean isTerminated();
boolean awaitTermination(long timeout, TimeUnit unit)
throws InterruptedException;
<T> Future<T> submit(Callable<T> task);
<T> Future<T> submit(Runnable task, T result);
Future<?> submit(Runnable task);
<T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
throws InterruptedException;
<T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,
long timeout, TimeUnit unit)
throws InterruptedException;
<T> T invokeAny(Collection<? extends Callable<T>> tasks)
throws InterruptedException, ExecutionException;
<T> T invokeAny(Collection<? extends Callable<T>> tasks,
long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}
- AbstractExecutorService
1、封装并实现对Runnable和Callable两种类型任务的适配逻辑: 对提交的Runnable任务和Callable任务，统一包装成RunnableFuture
2、定义了关心返回值类型的任务提交模板方法：先适配成RunnableFuture类型的task，在交给子类去execute(task)
public abstract class AbstractExecutorService implements ExecutorService {
protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
// 将Runnable包装成FutureTask, 对Runnable的包装比对Callable的包装稍复杂一点，用到的RunnableAdapter将Runnable适配为返回值为Void的Callable
// FutureTask本身实现了Runable接口
return new FutureTask<T>(runnable, value);
}
protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
// 将Callable包装成FutureTask
return new FutureTask<T>(callable);
}
public Future<?> submit(Runnable task) {
if (task == null) throw new NullPointerException();
RunnableFuture<Void> ftask = newTaskFor(task, null);
// execute()来源于Executor接口定义的方法void execute(Runnable command); 本类并没有实现，而是交给子类去实现。妥妥滴模板方法设计模式！
execute(ftask);
return ftask;
}
public <T> Future<T> submit(Runnable task, T result) {
if (task == null) throw new NullPointerException();
RunnableFuture<T> ftask = newTaskFor(task, result);
execute(ftask);
return ftask;
}
public <T> Future<T> submit(Callable<T> task) {
if (task == null) throw new NullPointerException();
// 将Callable包装成RunnableFuture
RunnableFuture<T> ftask = newTaskFor(task);
execute(ftask);
return ftask;
}
}
1.2.3 小结
从 AbstractExecutorService 类的任务提交方式可以看出，线程池对Callable和Runnable的任务提交，二者传入的参数实际类型都是FutureTask，最终都是交给execute(Runnable runnable)来执行；
在Jdk8中通过CompletableFuture.supplyAsync()提交任务到线程池，稍微有点不一样，没有封装成FutureTask。传入参数实际类型为AsyncSupply，它也是Runnable的子类。不过最终执行的也是 execute(Runnable runnable)；故后面看线程池任务执行流程，重点关注ThreadPoolExecutor的execute方法即可。
public static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier) {
return asyncSupplyStage(asyncPool, supplier);
}
static <U> CompletableFuture<U> asyncSupplyStage(Executor e,
Supplier<U> f) {
if (f == null) throw new NullPointerException();
CompletableFuture<U> d = new CompletableFuture<U>();
e.execute(new AsyncSupply<U>(d, f));
return d;
}
二、线程池工作原理
2.1 线程池工作机制概述
线程池的作用是将任务提交与任务执行进行解耦，怎么理解？我们先从整体上把握下线程池执行流程
- 1、客户端直接提交任务给线程池
- 2、线程池获取到任务并执行任务
- 3、如果客户端关心返回值，线程池将任务执行结果返回给Client
1和3比较直观，没什么逻辑。复杂的地方在第二步执行任务：任务可以被Worker直接执行，也可以放到任务队列workQueue存起来缓冲起来，甚至可以由指定的饱和策略RejectedExecutionHandler来决定怎么执行。那什么条件对应这三条执行分支呢？带着这个疑问，看下线程池从任务提交到执行具体干了什么。
2.2 任务提交、执行流程
从上图可以看出，当客户端提交一个新任务到线程池时，线程池按如下处理流程执行：
- 1.首先，判断核心线程数是否达到上限？否，则新建工作线程直接执行任务；是，则进入下个流程。
- 2.其次，判断线程池判断任务队列是否已满？否，则将新提交的任务缓冲到队列；是，则进入下个流程。
- 3.最后，判断线程数是否已达线程池最大限制？否，则新建工作线程直接执行任务；是，则交给饱和策略来处理这个任务。
Q1: 假如coreSize=3，那么线程池创建完成的时候就已经new了3个线程吗？
A1: No，是0个。线程池比较懒，一开始是有任务提交进来才开始新建线程的。
Q2: 客户端提交的任务，执行顺序是不是先提交就先执行？
A2: NO，当队列满了之后，如果为达到线程上限，那之后再提交的任务，不入队而是直接由新起的线程处理，这样就出现客户端后提交的任务反而先被执行。
三、ThreadPoolExecutor源码解析
3.1 线程池的数据模型
- a.线程池状态相关属性
// 线程池控制状态变量，这个ctl变量，包含两层含义：高3位表示线程池运行状态、低29位表示有效的线程个数
// RUNNING状态是 111 00000 00000000 00000000 00000000 , 所以初始化的ctl是 111 00000 00000000 00000000 00000000
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
// 29
private static final int COUNT_BITS = Integer.SIZE - 3;
// 线程池理论上的最大容量(536870911=2^29 -1)
// 左移29位，再减一，得到 000 11111 11111111 11111111 11111111 (536870911=2^29 -1)
private static final int CAPACITY = (1 << COUNT_BITS) - 1;
// 高3位用来表示运行状态
// runState is stored in the high-order bits
// -1的原码为 100 00000 00000000 00000000 00000001
// -1的反码为 111 11111 11111111 11111111 11111110 (负数的反码为: 对原码除符号位之外，其余位取反)
// -1的补码为 111 11111 11111111 11111111 11111111 (负数的补码为: 对反码+1)
// 左移29位，故RUNNING的值为: 111 00000 00000000 00000000 00000000
private static final int RUNNING = -1 << COUNT_BITS;
// 000 00000 00000000 00000000 00000000
private static final int SHUTDOWN = 0 << COUNT_BITS;
// 001 00000 00000000 00000000 00000000
private static final int STOP = 1 << COUNT_BITS;
// 010 00000 00000000 00000000 00000000
private static final int TIDYING = 2 << COUNT_BITS;
// 011 00000 00000000 00000000 00000000
private static final int TERMINATED = 3 << COUNT_BITS;
// 实时获取线程池运行状态runState：CAPACITY 的高三位都是0，先取反得到111 00000 00000000 00000000 00000000 ；故方法的实质就是取ctl变量的高3位
// ctl的初始值为RUNNING状态，值为 111 00000 00000000 00000000 00000000
// Packing and unpacking ctl
private static int runStateOf(int c) { return c & ~CAPACITY; }
// 获取线程池当前有效线程数: CAPACITY 的高三位都是0，再与ctl做且运算；故方法的实质就是取ctl变量的低29位
private static int workerCountOf(int c) { return c & CAPACITY; }
// 原子整型变量ctl的初始化方法
private static int ctlOf(int rs, int wc) { return rs | wc; }
源代码里面出现线程池运行状态判断非常多，一幅图掌握下线程池运行状态的状态机(来源于源码作者写的状态机注释)
- b.其他属性
这里比较重要的是工作队列workQueue和工作线程的集合两个属性workers；工作队列是BlockingQueue，本身是线程安全的；而workers仅仅是HashSet，非线程安全，故添加删除worker时，都需要加锁。源码里面是使用ReentrantLock来做同步操作的。
// 工作队列
private final BlockingQueue<Runnable> workQueue;
/**
* Lock held on access to workers set and related bookkeeping.
* While we could use a concurrent set of some sort, it turns out
* to be generally preferable to use a lock. Among the reasons is
* that this serializes interruptIdleWorkers, which avoids
* unnecessary interrupt storms, especially during shutdown.
* Otherwise exiting threads would concurrently interrupt those
* that have not yet interrupted. It also simplifies some of the
* associated statistics bookkeeping of largestPoolSize etc. We
* also hold mainLock on shutdown and shutdownNow, for the sake of
* ensuring workers set is stable while separately checking
* permission to interrupt and actually interrupting.
*/
// 操作workers时必须持有这个锁；workers、largestPoolSize、completedTaskCount 这些变量的修改都需要持有这把锁
private final ReentrantLock mainLock = new ReentrantLock();
// 工作线程的集合
/**
* Set containing all worker threads in pool. Accessed only when
* holding mainLock.
*/
private final HashSet<Worker> workers = new HashSet<Worker>();
/**
* Wait condition to support awaitTermination
*/
private final Condition termination = mainLock.newCondition();
/**
* Tracks largest attained pool size. Accessed only under
* mainLock.
*/
private int largestPoolSize;
/**
* Counter for completed tasks. Updated only on termination of
* worker threads. Accessed only under mainLock.
*/
private long completedTaskCount;
// 线程工厂
private volatile ThreadFactory threadFactory;
/**
* Handler called when saturated or shutdown in execute.
*/
private volatile RejectedExecutionHandler handler;
/**
* Timeout in nanoseconds for idle threads waiting for work.
* Threads use this timeout when there are more than corePoolSize
* present or if allowCoreThreadTimeOut. Otherwise they wait
* forever for new work.
*/
private volatile long keepAliveTime;
/**
* If false (default), core threads stay alive even when idle.
* If true, core threads use keepAliveTime to time out waiting
* for work.
*/
// 允许核心线程超时，默认为false
private volatile boolean allowCoreThreadTimeOut;
/**
* Core pool size is the minimum number of workers to keep alive
* (and not allow to time out etc) unless allowCoreThreadTimeOut
* is set, in which case the minimum is zero.
*/
private volatile int corePoolSize;
/**
* Maximum pool size. Note that the actual maximum is internally
* bounded by CAPACITY.
*/
private volatile int maximumPoolSize;
/**
* The default rejected execution handler
*/
private static final RejectedExecutionHandler defaultHandler =
new AbortPolicy();
- c.构造方法
ThreadPoolExecutor提供了多个构造函数来创建一个线程，但最终都会走到下面这个带7个参数的构造函数，看参数名就能知道每个参数含义。
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {
if (corePoolSize < 0 ||
maximumPoolSize <= 0 ||
maximumPoolSize < corePoolSize ||
keepAliveTime < 0)
throw new IllegalArgumentException();
if (workQueue == null || threadFactory == null || handler == null)
throw new NullPointerException();
this.acc = System.getSecurityManager() == null ?
null :
AccessController.getContext();
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;
}
3.2 线程池的任务执行
- execute
线程池最核心的方法，就是execute方法，整个执行流程包含了任务调度、线程池新建/回收线程等逻辑。
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.
*/
int c = ctl.get();
// 若工作线程数 < 核心线程数
if (workerCountOf(c) < corePoolSize) {
// 【核心点1-核心线程未满，则 新起线程】添加任务到worker集合，addWorker返回值为线程是否启动成功
if (addWorker(command, true))
// 成功则返回
return;
// 失败了，再次获取控线程池制状态；失败的原因是什么？？线程池状态变更了，处于SHUTDOWN中，主动抛出了异常
c = ctl.get();
}
// 【核心点2-任务入队】如果线程池是运行状态，且任务能放入阻塞队列(表明队列未满)
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
// 再次检查线程池状态，如果线程池未运行，则从队列移除任务，交给饱和策略去处理
if (! isRunning(recheck) && remove(command))
reject(command);
// workerCount = 0；线程池里没有线程，则创建新的线程去执行获取阻塞队列的任务执行
else if (workerCountOf(recheck) == 0)
// 注意，这里人为设置任务为null，所以这个线程不是立即去执行任务，而是要从队列取
addWorker(null, false);
}
// 【核心点3-队列满了，但未达到线程池最大线程数，则新开线程直接处理】任务放入阻塞队列失败(表明队列满了)，则尝试以线程池最大线程数新开线程去执行该任务
// 新开的线程直接处理当前任务，而不是把任务丢队列
else if (!addWorker(command, false))
// 新起worker失败(失败原因是线程数量达到线程池最大线程数)，则交给饱和策略去处理
reject(command);
}
- addWorker
见名知意，用来判断是否需要新增线程去处理任务；如果需要，则新建线程并且后续会由该线程直接处理当前任务，任务无需入队。注意入参core=true或false，代表是按核心线程数上限（true)还是按线程池最大数量(false)上限来约束线程的创建。
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// 如果线程池runState至少已经是SHUTDOWN（注:初始状态是RUNNING，是小于SHUTDOWN的）
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
/** 线程池处于RUNNING 状态 */
// 内层循环
for (;;) {
// 获取Worker数量
int wc = workerCountOf(c);
// worker数量超出 核心线程数 或 线程池数量上限，返回false；表明此时不允许新建线程了
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
// CAS操作，对workerCount数量+1，成功则跳出循环回到retry标记
if (compareAndIncrementWorkerCount(c))
break retry;
// CAS操作失败，再次获取线程池的控制状态
c = ctl.get(); // Re-read ctl
// 如果当前runState不等于刚开始获取的runState，则跳出内层循环，继续外层循环
if (runStateOf(c) != rs)
continue retry;
// else CAS failed due to workerCount change; retry inner loop
}
}
// 通过以上循环，对workerCount成功+1了； 但仅仅只做了对ctl的值更改，还没真正新增worker，新增worker并启动就是下面代码的逻辑
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
// new一个worker，worker实现了Runnable接口，每个Worker都持有一个线程
w = new Worker(firstTask);
final Thread t = w.thread;
if (t != null) {
final ReentrantLock mainLock = this.mainLock;
// 加锁
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
// 线程是活的，但是，线程不是刚new出来还没调start吗，这种情况肯定不对所以要报错！
throw new IllegalThreadStateException();
// 【核心】将worker加入到集合中，该集合是一个HashSet，为什么不是用List，是为了方便对线程做删除操作！！
workers.add(w);
int s = workers.size();
// largestPoolSize 用来记录线程池历史上达到过的最大值
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
// 解锁
mainLock.unlock();
}
if (workerAdded) {
// 如果Worker添加成功，则启动线程
t.start();
// 启动没异常，则标记启动成功
workerStarted = true;
}
}
} finally {
// 如果worker没有启动成功（或线程为null？）
if (! workerStarted)
// worker计数-1（对最开始的woker计数+1的回滚操作)
addWorkerFailed(w);
}
// 返回worker是否启动的标记
return workerStarted;
}
- Woker对象
Woker对象继承了AQS，为何要用到AQS？Worker的本职工作就是用来执行Runnable类型的的task，但task本身不一定是线程安全的，故需要通过在执行task前后加锁、解锁，来保障同一个时刻只一个线程在执行task，做到了task的执行是线程安全的。
另外，观察tryAcquire和tryRelease两个方法可知，传入的参数都没用到，Jdk大师是故意将Worker对象设计成不可重入的。作用是不希望在调用setCorePoolSize等线程池控制方法时能让worker锁重入。（如果重入会怎样？？这里还没理解透，mark下）
private final class Worker
extends AbstractQueuedSynchronizer
implements Runnable
{
/**
* This class will never be serialized, but we provide a
* serialVersionUID to suppress a javac warning.
*/
private static final long serialVersionUID = 6138294804551838833L;
// 工作线程，线程工厂创建失败的时候为null
/** Thread this worker is running in. Null if factory fails. */
final Thread thread;
// 初始化任务，有可能为null（人为设置的null)
/** Initial task to run. Possibly null. */
Runnable firstTask;
// 已完成的任务计数（无论成功失败都算已完成？）
/** Per-thread task counter */
volatile long completedTasks;
/**
* Creates with given first task and thread from ThreadFactory.
* @param firstTask the first task (null if none)
*/
Worker(Runnable firstTask) {
// 初始化AQS的同步状态为-1；在runWorker()方法一开始会做一次unlock将同步状态重置为0，这些都是为了在线程真正开始运行任务之前禁止线程中断！！？？
setState(-1); // inhibit interrupts until runWorker
// 初始化第一个任务，这个任务有可能是null
this.firstTask = firstTask;
// 创建线程，因为自身就是Runnalbe，故传入 this
this.thread = getThreadFactory().newThread(this);
}
/** Delegates main run loop to outer runWorker */
public void run() {
// 线程start后调用run方法，run方法内部调用ThreadPoolExecutor的runWorker方法
runWorker(this);
}
// Lock methods
//
// The value 0 represents the unlocked state.
// The value 1 represents the locked state.
// 代表是否独占锁
protected boolean isHeldExclusively() {
return getState() != 0;
}
// 重写AQS的tryAcquire方法尝试获取锁
protected boolean tryAcquire(int unused) {
if (compareAndSetState(0, 1)) {
// CAS操作成功，则将当前当前线程记录为独占模式线程
setExclusiveOwnerThread(Thread.currentThread());
return true;
}
return false;
}
// 重写AQS的tryRelease尝试释放锁
protected boolean tryRelease(int unused) {
// 设置为null，表示清空当前独占模式线程记录
setExclusiveOwnerThread(null);
// 不管传入的入参unused是多少，AQS同步状态统一置为0
setState(0);
return true;
}
public void lock() { acquire(1); }
public boolean tryLock() { return tryAcquire(1); }
public void unlock() { release(1); }
// 是否被独占
public boolean isLocked() { return isHeldExclusively(); }
void interruptIfStarted() {
Thread t;
if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
try {
t.interrupt();
} catch (SecurityException ignore) {
}
}
}
}
- runWorker
真正执行Runnable任务，调用其run方法；任务的来源有2个途径：一个是直接提交给worker的，一个是worker监听任务队列，从队列拉取得到的。任务的执行需要Worker对象加锁、解锁来保障线程安全。注意，这里面有个while循环调用getTask()方法从任务队列获取任务，榨干线程让其不断执行任务干到天荒地老。除非任务执行过程中出现了异常或线程数超过核心线程数被清退，否则线程会一直常驻。
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
// 获取worker里面的任务
Runnable task = w.firstTask;
// 将worker的任务清掉；（worker的任务已经记录到task了，清掉只是断开了w.firstTask对任务的引用）
w.firstTask = null;
// unlock方法会调用AQS的release方法, 将AQS状态置为0；(cgx: 上来先解锁，而不是加锁，目的是对Worker的同步状态做一次初始化？？)
w.unlock(); // allow interrupts
// 是否突然完成。（cgx: 发生了中断、异常？)
boolean completedAbruptly = true;
try {
// worker实例的task不为null；如果为null，则通过getTask获取task；这里是个whie循环，所以线程会一直死死盯着工作队列看有没有任务
while (task != null || (task = getTask()) != null) {
// 获取锁；(cgx: 这个锁是为了保护Task同一个时刻只能有一个线程在调用其run方法，保证了Task的线程安全)
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())
// 设置线程中断标识；如果此时线程刚好处于TIMED_WAITING或WAITING状态，则会抛出InterruptedException
wt.interrupt();
// 若Runnalbe任务执行过程中出现异常会进入到最外层finally代码块的processWorkerExit方法来退出任务
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设置为null；已完成任务数+1；解锁
task = null;
w.completedTasks++;
// 解锁
w.unlock();
}
}
// 能走到这里，说明task和getTask()都拿不到任务了，队列已经空了，线程变得多余。退出了while循环，走到finally代码块去做线程回收
completedAbruptly = false;
} finally {
// 退出当前Worker（走到这里，表明while循环退出了，要么是任务光了，要么是执行任务的线程异常了）
processWorkerExit(w, completedAbruptly);
}
}
- getTask
工作线程从任务队列阻塞获取任务。这里面还有个非常重要的逻辑：线程空闲超过一定时间就会被清理。具体而言，当线程数>核心线程数时，采用超时时间为keepAliveTime的poll方法从队列获取任务。只有一种情况会取不到任务，那就是队列已经空了。那么问题来了，队列都空了，线程数又大于核心线程数，那肯定要对超出的工作线程进行清退了，清退逻辑见processWorkerExit方法。
private Runnable getTask() {
boolean timedOut = false; // Did the last poll() time out?
// 注意，这里又是自旋操作！！
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// 若runState大于等于SHUTDOWN状态 且 runState大于等于STOP或者阻塞队列为空，则执行workerCount-1并返回null
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
decrementWorkerCount();
return null;
}
int wc = workerCountOf(c);
// 是否允许core Thread超时，默认false
// Are workers subject to culling?
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
// worker数大于线程池最大个数 或 超时(此时是timeOut=true, corePoolSize < wc <=maximumPoolSize)或 队列为空
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
// 执行CAS操作进行 worker数-1
if (compareAndDecrementWorkerCount(c))
return null;
// 操作失败，则继续循环
continue;
}
try {
// 若 wc大于核心线程数，则执行poll()，最多阻塞等待keepAliveTime；获取不到就返回null，然后timeOut=true, 下一轮循环用到timeOut, 就会对work计数减1
// 若 wc <= 核心线程数，则执行take(), 死等任务到来！
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
// 过了keepAliveTime这么长时间都没没获得到任务，就认为超时了！
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}
- processWorkerExit
线程执行任务，会有全部任务已执行完或者出现执行异常的时候，这时就会触发对线程的清理
private void processWorkerExit(Worker w, boolean completedAbruptly) {
// 如果是突然完成的(如发生了线程中断)，需执行CAS对worker计数减1；否则，不需要，因为已经在getTask()做了减1操作
if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
decrementWorkerCount();
final ReentrantLock mainLock = this.mainLock;
// 操作workers集合都要加锁
mainLock.lock();
try {
// 整个线程池完成任务计数加上worker完成的任务数
completedTaskCount += w.completedTasks;
// 将worker从HashSet删除
workers.remove(w);
} finally {
mainLock.unlock();
}
// 上面操作是删除work，所以顺带尝试下是否可以终止线程池
tryTerminate();
int c = ctl.get();
// 判断runState是否小于STOP，即是否处于RUNNING或SHUTDOWN；如果是，代表没有成功终止线程池
if (runStateLessThan(c, STOP)) {
// 是否突然完成，如果不是，代表已经没有任务可获取完成，因为getTask当中是用while循环死等任务
if (!completedAbruptly) {
int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
// 如果队列不为空，则至少保留一个线程断后
if (min == 0 && ! workQueue.isEmpty())
min = 1;
// 如果workerCount>=min，则表示满足所需，可以直接返回；
if (workerCountOf(c) >= min)
return; // replacement not needed
}
// 如果是突然完成，添加一个空任务的worker线程 ？？？
addWorker(null, false);
}
}
四、总结
因为线程池不是孤立存在的，肯定要配合其他数据结构发挥作用。故本文在讲解ThreadPoolExecutor之前，先从线程池周边比较重要的数据结构FutureTask讲起，接着论述线程池相关接口定义，最后通过流程图 + 源码注释的方式，详细剖析线程池内部数据结构及任务执行流程。但是，还有核心知识点(比如线程池的关闭，线程池大小动态调整、饱和处理策略等)，楼主遗漏了未曾讲到，等有时间在补上笔记！
线程池源码涉及的知识点还是很多的，以下是楼主认为比较重要的，本文没有细讲。但如果读者能提前掌握下，相信看ThreadPoolExecutor源码时会轻松很多。
- Future、Runnable、Callable层次关系
- AQS框架机制
- Unsafe类的CAS操作
- 线程中断
全文完~