类继承结构
ThreadPoolExecutor源码解析，需要一定的线程池基础知识，否则这篇文章看起来会比较困难。 我们直接开门见山看类继承结构图：
可以看到，ThreadPoolExecutor实现了Executor、ExecutorService两个接口，实现了AbstractExecutorService抽象类，我们看下这些接口和抽象类中的核心方法。
Executor
java
public interface Executor {
//提交Runnable任务到线程池
void execute(Runnable command);
}可以看到，ThreadPoolExecutor实现了顶级接口Executor，在该接口中仅定一个了一个方法void execute(Runnable command)。
ExecutorService
java
public interface ExecutorService extends Executor {
//关闭线程池
void shutdown();
//也是关闭线程池，和上面的关闭相比是会立刻关闭，及时当前的任务还未执行完
List<Runnable> shutdownNow();
//是否已关闭
boolean isShutdown();
......
//提交一个Callable类型的任务，返回一个Future
<T> Future<T> submit(Callable<T> task);
//提交一个Runnable类型的任务及结果参数，返回一个Future
<T> Future<T> submit(Runnable task, T result);
//提交Runnable任务到线程池
Future<?> submit(Runnable task);
......
}AbstractExecutorService
java
public abstract class AbstractExecutorService implements ExecutorService {
protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
return new FutureTask<T>(runnable, value);
}
protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
return new FutureTask<T>(callable);
}
public Future<?> submit(Runnable task) {
if (task == null) throw new NullPointerException();
RunnableFuture<Void> ftask = newTaskFor(task, null);
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();
RunnableFuture<T> ftask = newTaskFor(task);
execute(ftask);
return ftask;
}
......
}这里可以看到这些submit方法最终都是将参数封装成RunnableFuture类型的FutureTask，再交给execute方法执行。
ThreadPoolExecutor
终于到ThreadPoolExecutor类了，我们先来看下里面的核心变量。
java
public class ThreadPoolExecutor extends AbstractExecutorService {
//原子变量，这里设计的比较巧妙，32位的Integer类型，高三位保存线程池的运行状态，后29位保存线程数
//量，一个变量保存两个值，无锁化的保证了数据一致，并且使用了位运算，极致的性能
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
//线程池的五种状态RUNNING < SHUTDOWN < STOP < TIDYING < TERMINATED
//正常运行状态
private static final int RUNNING = -1 << COUNT_BITS;
//关闭状态，不再接受任务，队列中未处理完的任务会继续处理
private static final int SHUTDOWN = 0 << COUNT_BITS;
//不接受任务，未处理完的任务会中断
private static final int STOP = 1 << COUNT_BITS;
//任务都终止了 线程数为0
private static final int TIDYING = 2 << COUNT_BITS;
//执行完terminated方法后
private static final int TERMINATED = 3 << COUNT_BITS;
// Packing and unpacking ctl
//当前线程池运行状态
private static int runStateOf(int c) { return c & ~CAPACITY; }
//当前线程池线程数
private static int workerCountOf(int c) { return c & CAPACITY; }
//传入的状态及线程数生成ctl
private static int ctlOf(int rs, int wc) { return rs | wc; }
private static boolean runStateLessThan(int c, int s) {
return c < s;
}
private static boolean runStateAtLeast(int c, int s) {
return c >= s;
}
private static boolean isRunning(int c) {
return c < SHUTDOWN;
}运行机制
execute
我们看到execute方法的具体实现
java
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
//获取线程池运行状态
int c = ctl.get();
if (workerCountOf(c) < corePoolSize) {
//如果运行线程数小于核心线程数，添加一个worker来执行任务，第二个参数true表示使用核心线程
if (addWorker(command, true))
//addWorker如果返回true，直接返回，如果返回false，说明添加失败，走后续处理逻辑
return;
c = ctl.get();
}
//如果线程池是Running状态，则将任务添加至队列中
if (isRunning(c) && workQueue.offer(command)) {
//到这里说明任务已成功进入队列，
int recheck = ctl.get();
//如果线程池不处于Running状态，将任务从刚刚添加的队列中移除
if (! isRunning(recheck) && remove(command))
//执行拒绝策略
reject(command);
//线程池处于Running状态&线程数量为0，开启一个线程进行处理
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
//线程池不处于Running状态或者队列满了，添加一个worker来执行任务
//第二个参数false表示使用非核心线程
else if (!addWorker(command, false))
//失败的话执行拒绝策略
reject(command);
}worker
上面这段代码，出现了好几处addWorker方法，我们点进去看下，究竟怎么个事。
java
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)
*/
//构造函数，传入的Runnable会赋值为Worker的firstTask，并且会new一个线程出来
Worker(Runnable firstTask) {
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;
this.thread = getThreadFactory().newThread(this);
}
/** Delegates main run loop to outer runWorker */
public void run() {
runWorker(this);
}addWorker
可以看到，Worker继承了大名鼎鼎的AQS，并且实现了Runnable接口。我们点开addWorker方法。
java
//第二个参数表示使用核心线程还是最大线程
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
//一个死循环
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
//线程池状态已关闭并且不为（线程池处于SHUTDOWN且firstTask为null且队列非空）则返回false
//逻辑很绕
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
//又一个死循环
for (;;) {
//线程池当前线程数
int wc = workerCountOf(c);
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
//不能再创建线程了
return false;
//cas操作，增加一个线程数量，如果失败，再次循环
if (compareAndIncrementWorkerCount(c))
break retry;
c = ctl.get(); // Re-read ctl
//这里考虑了并发情况下，线程池状态发生了变更，跳到外层
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 {
//构造worker
w = new Worker(firstTask);
final Thread t = w.thread;
if (t != null) {
final ReentrantLock mainLock = this.mainLock;
//加锁，保证只有一个线程能进行下面的操作
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)
//记录最大的worker数
largestPoolSize = s;
//成功添加标识
workerAdded = true;
}
} finally {
mainLock.unlock();
}
if (workerAdded) {
//已成功添加，启动这个线程
t.start();
//启动成功表示
workerStarted = true;
}
}
} finally {
//如果没有启动成功
if (! workerStarted)
//worker数量减1
addWorkerFailed(w);
}
return workerStarted;
}runWorker
前面我们看到worker其实也是Runnable，t.start()启动后执行的run方法会执行runWorker方法，点开它。
java
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
//死循环，当firstTask不为空或者getTask()不为空
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())
//线程中断
wt.interrupt();
try {
beforeExecute(wt, task);
Throwable thrown = null;
try {
//任务执行
task.run();
} catch (RuntimeException x) {
//注意细节，这里三个catch抓到异常后再次抛出了异常
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 {
//这个任务已处理了，置为空
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
//需要关闭线程，
//1.已经没有任务了,completedAbruptly == false
//2.或者任务执行出现了未被捕获的异常（很重要）completedAbruptly == true
processWorkerExit(w, completedAbruptly);
}
}getTask
我们先看getTask()是如何获取队列中的任务
java
private Runnable getTask() {
boolean timedOut = false; // Did the last poll() time out?
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
//线程池处于关闭或者队列为空，线程数减1，方法返回null
decrementWorkerCount();
return null;
}
int wc = workerCountOf(c);
// Are workers subject to culling?
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
//线程数减1
if (compareAndDecrementWorkerCount(c))
//cas成功直接返回
return null;
continue;
}
try {
//获取任务
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}processWorkerExit
最后我们看下processWorkerExit方法，关闭线程。
java
private void processWorkerExit(Worker w, boolean completedAbruptly) {
//true:说明任务执行出现异常，将工作线程减1
//false:没有任务，这里就不用减1了，因为前面的getTask已经减了1
if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
decrementWorkerCount();
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
completedTaskCount += w.completedTasks;
//从set中移除
workers.remove(w);
} finally {
mainLock.unlock();
}
tryTerminate();
int c = ctl.get();
if (runStateLessThan(c, STOP)) {
//当前线程池状态小于STOP
if (!completedAbruptly) {
//任务执行出现异常
int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
if (min == 0 && ! workQueue.isEmpty())
min = 1;
if (workerCountOf(c) >= min)
//如果当前线程数量不小于最小的，方法返回
return; // replacement not needed
}
//添加一个非核心线程
addWorker(null, false);
}
}这里我们要明白一个道理，当线程池中的线程在执行任务时出现了没有被捕获的异常，会导致这个当前这个线程回收，剩余线程不够用的时候，会创建一个新的线程。如果异常经常出现的话，会导致线程池频繁创建线程，增加资源开销，所以我们将任务交给线程池处理时最好的加一层try-catch捕获异常。
总结
这篇文章花了较大篇幅从源码上分析了ThreadPoolExecutor的核心实现原理，希望能够帮助大家对java线程池的学习。