우리는 스레드 풀이 스레드를 반복적으로 관리하고 오버헤드를 증가시키기 위해 많은 수의 스레드를 생성하는 것을 방지하는 데 도움이 된다는 것을 알고 있습니다.
스레드 풀을 적절하게 사용하면 다음과 같은 3가지 분명한 이점을 얻을 수 있습니다.
1. 리소스 소비 감소: 이미 생성된 스레드를 재사용하여 스레드 생성 및 삭제 비용을 줄입니다.
2. 응답 속도 향상: 작업이 도착할 때 기다릴 필요가 없습니다. 즉시 실행됩니다.
3. 스레드 관리 효율성 향상: 스레드 풀을 통합된 방식으로 관리, 할당, 조정 및 모니터링할 수 있습니다.
Java 파생 스레드 풀은 오늘 토론의 초점이기도 한 ThreadPoolExecutor 클래스에서 구현됩니다.
Jdk는 ThreadPoolExecutor 클래스를 사용하여 스레드 풀을 생성하는 방법을 살펴보겠습니다.
/**
* Creates a new {@code ThreadPoolExecutor} with the given initial
* parameters.
*
* @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 threadFactory the factory to use when the executor
* creates a new thread
* @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 threadFactory} or {@code handler} is null
*/
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.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;
}int corePoolSize, //코어 스레드 수
int maximumPoolSize, //최대 스레드 수
long keepAliveTime, //코어 스레드 수 이외의 스레드가 생존하는 시간 코어 스레드가 유휴 상태입니다
TimeUnit 단위, //생존 시간 단위
BlockingQueue ThreadFactory threadFactory, //다음을 위한 팩토리 생성 사용할 새 스레드 RejectedExecutionHandler 핸들러 // 작업을 실행할 수 없을 때의 핸들러(스레드 거부 정책) ThreadPoolExecutor에는 ctl이라는 상태 제어 속성이 있습니다. AtomicInteger 유형의 변수입니다. int 값은 두 가지 개념의 정보를 저장할 수 있습니다. workerCount: WorkerCountOf 메소드를 통해 얻은 현재 풀의 유효 스레드 수를 나타내며, WorkerCount의 상한은 (2^ 29)-1. (최종적으로 ctl의 하위 29비트에 저장됨) runState: 현재 스레드 풀의 상태를 나타내며, WorkerCountOf 메소드를 통해 얻은 후 최종적으로 ctl의 상위 3비트에 저장됩니다. 다음 값을 갖는 전체 스레드 풀, 각각의 의미는 다음과 같습니다. RUNNING: 새 스레드를 추가할 수 있으며 대기열의 스레드를 동시에 처리할 수 있습니다. SHUTDOWN: 새 스레드를 추가하지 않지만 대기열에 있는 스레드를 처리합니다. STOP은 새 스레드를 추가하지 않으며 대기열의 스레드를 처리하지 않습니다. TIDYING 모든 스레드가 종료되고(큐에서) WorkerCount가 0이면 TIDYING TERMINATED 종결() 메소드가 종료되고 TERMINATED COUNT_BITS=32(정수 크기)가 됩니다. 3=29이므로 29비트 왼쪽으로 이동한 5개 상태는 다음과 같습니다. SHUTDOWN: 정지: 정리: 종료됨: 스레드 PoolExecutor는 runStateOf와 WorkerCountOf를 통해 두 개념의 값을 획득합니다. CAPACITY 변수 CAPACITY=(1 << COUNT_BITS) - 1을 바이너리로 변환하면 000 11111111111111111111111111111이며, 이는 스레드 풀이 이론적으로 허용할 수 있는 최대 스레드 수입니다. 입니다. HashSet Core 내부 클래스Worker 我们看worker类时，会发现最重要的几个部分在于它里面定义了一个Thread thread和Runnable firstTask。看到这里，我们可能会比较奇怪，我们只是要一个可以执行的线程，这里放一个Thread和一个Runnable的变量做什么呢？ 好了，基本上我们将线程池的几个主角，ctl，workQueue，workers，Worker简单介绍了一遍，现在，我们来看看线程池是怎么玩的。 这是线程池实现类外露供给外部实现提交线程任务command的核心方法，对于无需了解线程池内部的使用者来说，这个方法就是把某个任务交给线程池，正常情况下，这个任务会在未来某个时刻被执行，实现和注释如下： 我们可以简单归纳如下（注：图来源见水印，谢谢大神的归纳）： 在execute方法中，我们看到核心的逻辑是由addWorker方法来实现的，当我们将一个任务提交给线程池，线程池会如何处理，就是主要由这个方法加以规范： 该方法有两个参数： firstTask： worker线程的初始任务，可以为空 core： true：将corePoolSize作为上限，false：将maximumPoolSize作为上限 排列组合，addWorker方法有4种传参的方式： 在execute方法中就使用了前3种，结合这个核心方法进行以下分析 同样的，我们可以归纳一下： 在addWorker方法中，我们将一个新增进去的worker所组合的线程属性thread启动了，但我们知道，在worker的构造方法中，它将自己本身注入到了thread的target属性里，所以绕了一圈，线程启动后，调用的还是worker的run方法，而在这里面，runWorker定义了线程执行的逻辑： 我们归纳： runWorker方法中的getTask()方法是线程处理完一个任务后，从队列中获取新任务的实现，也是处理判断一个线程是否应该被销毁的逻辑所在： 归纳： 在runWorker方法中，我们看到当不满足while条件后，线程池会执行退出线程的操作，这个操作，就封装在processWorkerExit方法中。 总而言之：如果线程池还没有完全终止，就仍需要保持一定数量的线程。 线程池状态是running 或 shutdown的情况下： 前面我们讲过execute方法，其作用是将一个任务提交给线程池，以期在未来的某个时间点被执行。 源码很简单，submit方法，将任务task封装成FutureTask(newTaskFor方法中就是new了一个FutureTask)，然后调用execute。所以submit方法和execute的所有区别，都在这FutureTask所带来的差异化实现上。 总而言之，submit方法将一个任务task用future模式封装成FutureTask对象，提交给线程执行，并将这个FutureTask对象返回，以供主线程在该任务被线程池执行之后得到执行结果。 注意，获得执行结果的方法FutureTask.get()，会阻塞执行该方法的线程。 위 내용은 Java 기본 - 스레드 풀 소스 코드 분석의 상세 내용입니다. 자세한 내용은 PHP 중국어 웹사이트의 기타 관련 기사를 참조하세요!Core 클래스 변수
ctl 변수
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
private static final int COUNT_BITS = Integer.SIZE - 3;
private static final int CAPACITY = (1 << COUNT_BITS) - 1;
// 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;
// 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; }000000000000000000000000000
이런 식으로, WorkerCountOf 메서드에서 CAPACITY와 ctl이 & 연산을 수행하면 상위 3비트를 모두 얻을 수 있습니다. 0, ctl의 하위 29비트는 동일한 값을 갖습니다.
이 값은 WorkerCount
마찬가지로 runStateOf 메소드, CAPACITY의 반전 및 ctl의 & 연산을 사용하여 다음과 같은 값을 얻습니다. 상위 3비트는 ctl의 상위 3비트와 동일하고 하위 29비트는 모두 0입니다.
이 값은 runState;
workQueue/**
* The queue used for holding tasks and handing off to worker
* threads. We do not require that workQueue.poll() returning
* null necessarily means that workQueue.isEmpty(), so rely
* solely on isEmpty to see if the queue is empty (which we must
* do for example when deciding whether to transition from
* SHUTDOWN to TIDYING). This accommodates special-purpose
* queues such as DelayQueues for which poll() is allowed to
* return null even if it may later return non-null when delays
* expire.
*/
private final BlockingQueue<Runnable> workQueue;workers
/**
* Set containing all worker threads in pool. Accessed only when
* holding mainLock.
*/
private final HashSet<Worker> workers = new HashSet<Worker>();
으로 직관적으로 이해하는 것이기도 합니다.
Worker 클래스는 스레드 풀의 스레드를 구현하는 객체이며 스레드 풀의 핵심입니다. 소스 코드를 살펴보겠습니다.
/**
* Class Worker mainly maintains interrupt control state for
* threads running tasks, along with other minor bookkeeping.
* This class opportunistically extends AbstractQueuedSynchronizer
* to simplify acquiring and releasing a lock surrounding each
* task execution. This protects against interrupts that are
* intended to wake up a worker thread waiting for a task from
* instead interrupting a task being run. We implement a simple
* non-reentrant mutual exclusion lock rather than use
* ReentrantLock because we do not want worker tasks to be able to
* reacquire the lock when they invoke pool control methods like
* setCorePoolSize. Additionally, to suppress interrupts until
* the thread actually starts running tasks, we initialize lock
* state to a negative value, and clear it upon start (in
* runWorker).
*/
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;
/** Thread this worker is running in. Null if factory fails. */
final Thread thread;
/** Initial task to run. Possibly null. */
Runnable firstTask;
/** 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) {
//设置AQS的同步状态private volatile int state，是一个计数器，大于0代表锁已经被获取
// 在调用runWorker()前，禁止interrupt中断，在interruptIfStarted()方法中会判断 getState()>=0
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;
this.thread = getThreadFactory().newThread(this);//根据当前worker创建一个线程对象
//当前worker本身就是一个runnable任务，也就是不会用参数的firstTask创建线程，而是调用当前worker.run()时调用firstTask.run()
//后面在addworker中，我们会启动worker对象中组合的Thread，而我们的执行逻辑runWorker方法是在worker的run方法中被调用。
//为什么执行thread的run方法会调用worker的run方法呢，原因就是在这里进行了注入，将worker本身this注入到了thread中
}
/** Delegates main run loop to outer runWorker */
public void run() {
runWorker(this);
}//runWorker()是ThreadPoolExecutor的方法
// Lock methods
//
// The value 0 represents the unlocked state. 0代表“没被锁定”状态
// The value 1 represents the locked state. 1代表“锁定”状态
protected boolean isHeldExclusively() {
return getState() != 0;
}
/**
* 尝试获取锁
* 重写AQS的tryAcquire()，AQS本来就是让子类来实现的
*/
protected boolean tryAcquire(int unused) {
//尝试一次将state从0设置为1，即“锁定”状态，但由于每次都是state 0->1，而不是+1，那么说明不可重入
//且state==-1时也不会获取到锁
if (compareAndSetState(0, 1)) {
setExclusiveOwnerThread(Thread.currentThread());
return true;
}
return false;
}
/**
* 尝试释放锁
* 不是state-1，而是置为0
*/
protected boolean tryRelease(int unused) {
setExclusiveOwnerThread(null);
setState(0);
return true;
}
public void lock() { acquire(1); }
public boolean tryLock() { return tryAcquire(1); }
public void unlock() { release(1); }
public boolean isLocked() { return isHeldExclusively(); }
/**
* 中断（如果运行）
* shutdownNow时会循环对worker线程执行
* 且不需要获取worker锁，即使在worker运行时也可以中断
*/
void interruptIfStarted() {
Thread t;
//如果state>=0、t!=null、且t没有被中断
//new Worker()时state==-1，说明不能中断
if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
try {
t.interrupt();
} catch (SecurityException ignore) {
}
}
}
}
其实之所以Worker自己实现Runnable，并创建Thread，在firstTask外包一层，是因为要通过Worker负责控制中断，而firstTask这个工作任务只是负责执行业务，worker的run方法调用了runWorker方法，在这里面，worker里的firstTask的run方法被执行。稍后我们会聚焦这个执行任务的runWorker方法。核心方法
execute方法
/**
* Executes the given task sometime in the future. The task
* may execute in a new thread or in an existing pooled thread.
* * 在未来的某个时刻执行给定的任务。这个任务用一个新线程执行，或者用一个线程池中已经存在的线程执行
*
* 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}.
* 如果任务无法被提交执行，要么是因为这个Executor已经被shutdown关闭，要么是已经达到其容量上限，任务会被当前的RejectedExecutionHandler处理
*
* @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.
* 如果运行的线程少于corePoolSize，尝试开启一个新线程去运行command，command作为这个线程的第一个任务
*
* 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.
* 如果任务成功放入队列，我们仍需要一个双重校验去确认是否应该新建一个线程（因为可能存在有些线程在我们上次检查后死了）
* 或者 从我们进入这个方法后，pool被关闭了
* 所以我们需要再次检查state，如果线程池停止了需要回滚入队列，如果池中没有线程了，新开启 一个线程
*
* 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.
* 如果无法将任务入队列（可能队列满了），需要新开区一个线程（自己：往maxPoolSize发展）
* 如果失败了，说明线程池shutdown 或者 饱和了，所以我们拒绝任务
*/
int c = ctl.get();
// 1、如果当前线程数少于corePoolSize（可能是由于addWorker()操作已经包含对线程池状态的判断，如此处没加，而入workQueue前加了）
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
/**
* 没有成功addWorker()，再次获取c（凡是需要再次用ctl做判断时，都会再次调用ctl.get()）
* 失败的原因可能是：
* 1、线程池已经shutdown，shutdown的线程池不再接收新任务
* 2、workerCountOf(c) < corePoolSize 判断后，由于并发，别的线程先创建了worker线程，导致workerCount>=corePoolSize
*/
c = ctl.get();
}
/**
* 2、如果线程池RUNNING状态，且入队列成功
*/
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
/**
* 再次校验放入workerQueue中的任务是否能被执行
* 1、如果线程池不是运行状态了，应该拒绝添加新任务，从workQueue中删除任务
* 2、如果线程池是运行状态，或者从workQueue中删除任务失败（刚好有一个线程执行完毕，并消耗了这个任务），
* 确保还有线程执行任务（只要有一个就够了）
*/
//如果再次校验过程中，线程池不是RUNNING状态，并且remove(command)--workQueue.remove()成功，拒绝当前command
if (! isRunning(recheck) && remove(command))
reject(command);
//如果当前worker数量为0，通过addWorker(null, false)创建一个线程，其任务为null
//为什么只检查运行的worker数量是不是0呢？？ 为什么不和corePoolSize比较呢？？
//只保证有一个worker线程可以从queue中获取任务执行就行了？？
//因为只要还有活动的worker线程，就可以消费workerQueue中的任务
else if (workerCountOf(recheck) == 0)
addWorker(null, false);//第一个参数为null，说明只为新建一个worker线程，没有指定firstTask
////第二个参数为true代表占用corePoolSize，false占用maxPoolSize
}
/**
* 3、如果线程池不是running状态 或者 无法入队列
* 尝试开启新线程，扩容至maxPoolSize，如果addWork(command, false)失败了，拒绝当前command
*/
else if (!addWorker(command, false))
reject(command);
}
addWorker
1、addWorker(command, true)
2、addWorker(command, false)
3、addWorker(null, false)
4、addWorker(null, true)
第一个：线程数小于corePoolSize时，放一个需要处理的task进Workers Set。如果Workers Set长度超过corePoolSize，就返回false
第二个：当队列被放满时，就尝试将这个新来的task直接放入Workers Set，而此时Workers Set的长度限制是maximumPoolSize。如果线程池也满了的话就返回false
第三个：放入一个空的task进workers Set，长度限制是maximumPoolSize。这样一个task为空的worker在线程执行的时候会去任务队列里拿任务，这样就相当于创建了一个新的线程，只是没有马上分配任务
第四个：这个方法就是放一个null的task进Workers Set，而且是在小于corePoolSize时，如果此时Set中的数量已经达到corePoolSize那就返回false，什么也不干。实际使用中是在prestartAllCoreThreads()方法，这个方法用来为线程池预先启动corePoolSize个worker等待从workQueue中获取任务执行
/**
* 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.
* 检查根据当前线程池的状态和给定的边界(core or maximum)是否可以创建一个新的worker
* 如果是这样的话，worker的数量做相应的调整，如果可能的话，创建一个新的worker并启动，参数中的firstTask作为worker的第一个任务
* 如果方法返回false，可能因为pool已经关闭或者调用过了shutdown
* 如果线程工厂创建线程失败，也会失败，返回false
* 如果线程创建失败，要么是因为线程工厂返回null，要么是发生了OutOfMemoryError
*
* @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) {
//外层循环，负责判断线程池状态
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
/**
* 线程池的state越小越是运行状态，runnbale=-1，shutdown=0,stop=1,tidying=2，terminated=3
* 1、如果线程池state已经至少是shutdown状态了
* 2、并且以下3个条件任意一个是false
* rs == SHUTDOWN （隐含：rs>=SHUTDOWN）false情况： 线程池状态已经超过shutdown，
* 可能是stop、tidying、terminated其中一个，即线程池已经终止
* firstTask == null （隐含：rs==SHUTDOWN）false情况： firstTask不为空，rs==SHUTDOWN 且 firstTask不为空，
* return false，场景是在线程池已经shutdown后，还要添加新的任务，拒绝
* ! workQueue.isEmpty() （隐含：rs==SHUTDOWN，firstTask==null）false情况： workQueue为空，
* 当firstTask为空时是为了创建一个没有任务的线程，再从workQueue中获取任务，
* 如果workQueue已经为空，那么就没有添加新worker线程的必要了
* return false，即无法addWorker()
*/
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
//内层循环，负责worker数量+1
for (;;) {
int wc = workerCountOf(c);
//入参core在这里起作用，表示加入的worker是加入corePool还是非corepool，换句话说，受到哪个size的约束
//如果worker数量>线程池最大上限CAPACITY（即使用int低29位可以容纳的最大值）
//或者( worker数量>corePoolSize 或 worker数量>maximumPoolSize )，即已经超过了给定的边界,不添加worker
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
//CAS尝试增加线程数，,如果成功加了wc，那么break跳出检查
//如果失败，证明有竞争，那么重新到retry。
if (compareAndIncrementWorkerCount(c))
break retry;
//如果不成功，重新获取状态继续检查
c = ctl.get(); // Re-read ctl
//如果状态不等于之前获取的state，跳出内层循环，继续去外层循环判断
if (runStateOf(c) != rs)
continue retry;
// else CAS failed due to workerCount change; retry inner loop
// else CAS失败时因为workerCount改变了，继续内层循环尝试CAS对worker数量+1
}
}
//worker数量+1成功的后续操作
// 添加到workers Set集合，并启动worker线程
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
//新建worker//构造方法做了三件事//1、设置worker这个AQS锁的同步状态state=-1
w = new Worker(firstTask); //2、将firstTask设置给worker的成员变量firstTask
//3、使用worker自身这个runnable，调用ThreadFactory创建一个线程，并设置给worker的成员变量thread
final Thread t = w.thread;
if (t != null) {
//获取重入锁，并且锁上
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
int rs = runStateOf(ctl.get());
// rs!=SHUTDOWN ||firstTask!=null
// 如果线程池在运行running<shutdown 或者
// 线程池已经shutdown，且firstTask==null（可能是workQueue中仍有未执行完成的任务，创建没有初始任务的worker线程执行）
// worker数量-1的操作在addWorkerFailed()
if (rs < SHUTDOWN ||
(rs == SHUTDOWN && firstTask == null)) {
if (t.isAlive()) // // precheck that t is startable 线程已经启动，抛非法线程状态异常
throw new IllegalThreadStateException();
workers.add(w);
//设置最大的池大小largestPoolSize，workerAdded设置为true
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
if (workerAdded) {//如果往HashSet中添加worker成功，启动线程
//通过t.start()方法正式执行线程。在这里一个线程才算是真正的执行起来了。
t.start();
workerStarted = true;
}
}
} finally {
//如果启动线程失败
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
runWorker方法
/**
* 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.
* 我们可能使用一个初始化任务开始，即firstTask为null
* 然后只要线程池在运行，我们就从getTask()获取任务
* 如果getTask()返回null，则worker由于改变了线程池状态或参数配置而退出
* 其它退出因为外部代码抛异常了，这会使得completedAbruptly为true，这会导致在processWorkerExit()方法中替换当前线程
*
* 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.
* 在任何任务执行之前，都需要对worker加锁去防止在任务运行时，其它的线程池中断操作
* clearInterruptsForTaskRun保证除非线程池正在stoping，线程不会被设置中断标示
*
* 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.
* 每个任务执行前会调用beforeExecute()，其中可能抛出一个异常，这种情况下会导致线程die（跳出循环，且completedAbruptly==true），没有执行任务
* 因为beforeExecute()的异常没有cache住，会上抛，跳出循环
*
* 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
//标识线程是不是异常终止的
boolean completedAbruptly = true;
try {
//task不为null情况是初始化worker时，如果task为null，则去队列中取线程--->getTask()
//可以看到，只要getTask方法被调用且返回null，那么worker必定被销毁，而确定一个线程是否应该被销毁的逻辑，在getTask方法中
while (task != null || (task = getTask()) != null) {
w.lock();
if ((runStateAtLeast(ctl.get(), STOP) ||
(Thread.interrupted() &&
runStateAtLeast(ctl.get(), STOP))) &&
!wt.isInterrupted())
wt.interrupt();
try {
//线程开始执行之前执行此方法，可以实现Worker未执行退出，本类中未实现
beforeExecute(wt, task);
Throwable thrown = null;
try {
task.run();//runWorker方法最本质的存在意义，就是调用task的run方法
} catch (RuntimeException x) {
thrown = x; throw x;
} catch (Error x) {
thrown = x; throw x;
} catch (Throwable x) {
thrown = x; throw new Error(x);
} finally {
//线程执行后执行，可以实现标识Worker异常中断的功能，本类中未实现
afterExecute(task, thrown);
}
} finally {
task = null;//运行过的task标null
w.completedTasks++;
w.unlock();
}
}
//标识线程不是异常终止的,是因为不满足while条件，被迫销毁的
completedAbruptly = false;
} finally {
//处理worker退出的逻辑
processWorkerExit(w, completedAbruptly);
}
}
getTask方法
/**
* Performs blocking or timed wait for a task, depending on
* current configuration settings, or returns null if this worker
* must exit because of any of: 以下情况会返回null
* 1. There are more than maximumPoolSize workers (due to
* a call to setMaximumPoolSize).
* 超过了maximumPoolSize设置的线程数量（因为调用了setMaximumPoolSize()）
* 2. The pool is stopped.
* 线程池被stop
* 3. The pool is shutdown and the queue is empty.
* 线程池被shutdown，并且workQueue空了
* 4. This worker timed out waiting for a task, and timed-out
* workers are subject to termination (that is,
* {@code allowCoreThreadTimeOut || workerCount > corePoolSize})
* both before and after the timed wait.
* 线程等待任务超时
*
* @return task, or null if the worker must exit, in which case
* workerCount is decremented
* 返回null表示这个worker要结束了，这种情况下workerCount-1
*/
private Runnable getTask() {
// timedOut 主要是判断后面的poll是否要超时
boolean timedOut = false; // Did the last poll() time out?
/**
* 用于判断线程池状态
*/
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
/**
* 对线程池状态的判断，两种情况会workerCount-1，并且返回null
* 线程池状态为shutdown，且workQueue为空（反映了shutdown状态的线程池还是要执行workQueue中剩余的任务的）
* 线程池状态为>=stop()（只有TIDYING和TERMINATED会大于stop）（shutdownNow()会导致变成STOP）（此时不用考虑workQueue的情况）
*/
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
decrementWorkerCount();//循环的CAS减少worker数量，直到成功
return null;
}
int wc = workerCountOf(c);
// Are workers subject to culling?
//allowCoreThreadTimeOut字段，表示是否允许核心线程超过闲置时间后被摧毁，默认为false
//我们前面说过，如果getTask方法返回null，那么这个worker只有被销毁一途
//于是这个timed有3种情况
//(1)当没有超过核心线程，且默认allowCoreThreadTimeOut为false时
// timed值为false，除非目前线程数大于最大值,否则下面的if始终进不去，该方法不可能返回null，worker也就不可能被销毁
//(2)当超过核心线程数，且默认allowCoreThreadTimeOut为false时//timed值为true，
//(3)如果allowCoreThreadTimeOut为true，则timed始终为true
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
//wc > maximumPoolSize则必销毁，因为wc>1也肯定满足
//wc <= maximumPoolSize,如果(timed && timedOut) = true 一般情况下也意味着worker要被销毁，因为超时一般是由阻塞队列为空造成的
//一般情况是这样，那不一般的情况呢？阻塞队列没有为空，但是因为一些原因，还是超时了，这时候取决于wc > 1，它为真就销毁，为假就不销毁。
// 也就是说，如果阻塞队列还有任务，但是wc=1，线程池里只剩下自己这个线程了，那么就不能销毁,这个if不满足，我们的代码继续往下走
//当核心线程数<线程数<最大线程数，或者允许核心线程超时时，
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
try {
//如果timed为true那么使用poll取线程。否则使用take()
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
//workQueue.poll()：如果在keepAliveTime时间内，阻塞队列还是没有任务，返回null
workQueue.take();
//workQueue.take()：如果阻塞队列为空，当前线程会被挂起等待；当队列中有任务加入时，线程被唤醒，take方法返回任务
//如果正常返回，那么返回取到的task。
if (r != null)
return r;
//否则，设为超时，重新执行循环,
timedOut = true;
} catch (InterruptedException retry) {
//在阻塞从workQueue中获取任务时，可以被interrupt()中断，代码中捕获了InterruptedException，重置timedOut为初始值false，再次执行第1步中的判断，满足就继续获取任务，不满足return null，会进入worker退出的流程
timedOut = false;
}
}
processWorkerExit方法
/**
* Performs cleanup and bookkeeping for a dying worker. Called
* only from worker threads. Unless completedAbruptly is set,
* assumes that workerCount has already been adjusted to account
* for exit. This method removes thread from worker set, and
* possibly terminates the pool or replaces the worker if either
* it exited due to user task exception or if fewer than
* corePoolSize workers are running or queue is non-empty but
* there are no workers.
*
* @param w the worker
* @param completedAbruptly if the worker died due to user exception
*/
private void processWorkerExit(Worker w, boolean completedAbruptly) {
//参数：
//worker： 要结束的worker
//completedAbruptly： 是否突然完成（是否因为异常退出）
/**
* 1、worker数量-1
* 如果是突然终止，说明是task执行时异常情况导致，即run()方法执行时发生了异常，那么正在工作的worker线程数量需要-1
* 如果不是突然终止，说明是worker线程没有task可执行了，不用-1，因为已经在getTask()方法中-1了
*/
if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted 代码和注释正好相反啊
decrementWorkerCount();
/**
* 2、从Workers Set中移除worker
*/
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
completedTaskCount += w.completedTasks; //把worker的完成任务数加到线程池的完成任务数
workers.remove(w); //从HashSet<Worker>中移除
} finally {
mainLock.unlock();
}
/**
* 3、在对线程池有负效益的操作时，都需要“尝试终止”线程池
* 主要是判断线程池是否满足终止的状态
* 如果状态满足，但线程池还有线程，尝试对其发出中断响应，使其能进入退出流程
* 没有线程了，更新状态为tidying->terminated
*/
tryTerminate();
/**
* 4、是否需要增加worker线程
* 线程池状态是running 或 shutdown
* 如果当前线程是突然终止的，addWorker()
* 如果当前线程不是突然终止的，但当前线程数量 < 要维护的线程数量，addWorker()
* 故如果调用线程池shutdown()，直到workQueue为空前，线程池都会维持corePoolSize个线程，然后再逐渐销毁这corePoolSize个线程
*/
int c = ctl.get();
//如果状态是running、shutdown，即tryTerminate()没有成功终止线程池，尝试再添加一个worker
if (runStateLessThan(c, STOP)) {
//不是突然完成的，即没有task任务可以获取而完成的，计算min，并根据当前worker数量判断是否需要addWorker()
if (!completedAbruptly) {
int min = allowCoreThreadTimeOut ? 0 : corePoolSize; //allowCoreThreadTimeOut默认为false，即min默认为corePoolSize
//如果min为0，即不需要维持核心线程数量，且workQueue不为空，至少保持一个线程
if (min == 0 && ! workQueue.isEmpty())
min = 1;
//如果线程数量大于最少数量，直接返回，否则下面至少要addWorker一个
if (workerCountOf(c) >= min)
return; // replacement not needed
}
//添加一个没有firstTask的worker
//只要worker是completedAbruptly突然终止的，或者线程数量小于要维护的数量，就新添一个worker线程，即使是shutdown状态
addWorker(null, false);
}
}A、如果当前线程是突然终止的，addWorker()
B、如果当前线程不是突然终止的，但当前线程数量 < 要维护的线程数量，addWorker()
故如果调用线程池shutdown()，直到workQueue为空前，线程池都会维持corePoolSize个线程，然后再逐渐销毁这corePoolSize个线程
submit方法
submit方法在作用上，和execute方法是一样的，将某个任务提交给线程池，让线程池调度线程去执行它。
那么它和execute方法有什么区别呢？我们来看看submit方法的源码：
submit方法的实现在ThreadPoolExecutor的父类AbstractExecutorService类中，有三种重载方法： /**
* 提交一个 Runnable 任务用于执行，并返回一个表示该任务的 Future。该Future的get方法在成功完成时将会返回null。
* submit 参数： task - 要提交的任务 返回：表示任务等待完成的 Future
* @throws RejectedExecutionException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
public Future<?> submit(Runnable task) {
if (task == null) throw new NullPointerException();
RunnableFuture<Void> ftask = newTaskFor(task, null);
execute(ftask);
return ftask;
}
/**
* 提交一个Runnable 任务用于执行，并返回一个表示该任务的 Future。该 Future 的 get 方法在成功完成时将会返回给定的结果。
* submit 参数： task - 要提交的任务 result - 完成任务时要求返回的结果
* 返回： 表示任务等待完成的 Future
* @throws RejectedExecutionException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
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;
}
/**
* 提交一个Callable的任务用于执行，返回一个表示任务的未决结果的 Future。该 Future 的 get
方法在成功完成时将会返回该任务的结果。
* 如果想立即阻塞任务的等待，则可以使用 result =
exec.submit(aCallable).get(); 形式的构造。
* 参数： task - 要提交的任务 返回： 表示任务等待完成的Future
* @throws RejectedExecutionException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
public <T> Future<T> submit(Callable<T> task) {
if (task == null) throw new NullPointerException();
RunnableFuture<T> ftask = newTaskFor(task);
execute(ftask);
return ftask;
}未完待续
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