开篇
这篇文章的主要目标是为了讲解清楚ThreadPoolExecutor的提交任务的过程，非常推荐静下心来仔细阅读。
java源码-ThreadPoolExecutor（1）
java源码-ThreadPoolExecutor（2）
java源码-ThreadPoolExecutor（3）
ThreadPoolExecutor状态介绍
ThreadPoolExecutor针对线程池一共维护了五种状态，实现上用用高3位表示ThreadPoolExecutor的执行状态，低29位维持线程池线程个数，分别是：
- RUNNING = -1 << COUNT_BITS = -1<<29 高三位为111
- SHUTDOWN = 0 << COUNT_BITS = 0<<29 高三位为000
- STOP = 1 << COUNT_BITS = 1<<29 高三位为001
- TIDYING = 2 << COUNT_BITS = 2<<29 高三位为010
- TERMINATED = 3 << COUNT_BITS = 3<<29 高三位为011
public class ThreadPoolExecutor extends AbstractExecutorService {
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
// Integer.SIZE=32,Integer.SIZE-3=29,COUNT_BITS=29
private static final int COUNT_BITS = Integer.SIZE - 3;
// 线程池最大线程数=536870911（2^29-1）,CAPACITY二进制中低29为为1，高3位为0
private static final int CAPACITY = (1 << COUNT_BITS) - 1;
// 用高3位表示ThreadPoolExecutor的执行状态
// RUNNING=111
private static final int RUNNING = -1 << COUNT_BITS;
// SHUTDOWN=000
private static final int SHUTDOWN = 0 << COUNT_BITS;
// STOP=001
private static final int STOP = 1 << COUNT_BITS;
// TIDYING=010
private static final int TIDYING = 2 << COUNT_BITS;
// TERMINATED=110
private static final int TERMINATED = 3 << COUNT_BITS;
// Packing and unpacking ctl
// runStateOf通过获取高3位来对比
private static int runStateOf(int c) { return c & ~CAPACITY; }
// workerCountOf通过比较低29位来获取线程数
private static int workerCountOf(int c) { return c & CAPACITY; }
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;
}
ThreadPoolExecutor任务提交过程
ThreadPoolExecutor提交任务代码是在AbstractExecutorService当中通过submit()方法实现的，按照两个步骤来实现：
- 通过newTaskFor()方法创建待提交任务，该方法内部的实现后面再分析。
- 通过execute()方法提交task，execute的在ThreadPoolExecutor类中实现重写。
- 进一步跟进ThreadPoolExecutor的execute方法。
public abstract class AbstractExecutorService implements ExecutorService {
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;
}
}
整个ThreadPoolExecutor的execute其实在源码自带的注释中已经写的很清楚了，怕自己翻译的不是特别所以这次直接把注释也贴在代码当中了，整个过程分为三个过程：
- 1、当前的线程数是否小于corePoolSize，新建core线程并运行第一个任务。
- 2、如果第一步不满足条件，那么就把任务提交到workQueue代表的队列当中。
- 3、如果第二步不满足条件，那么就就新建不属于corePoolSize计数的线程（也就是新建core以外的线程）来进行处理。
- 4、如果都失败那么就直接通过rejectHandler拒绝任务，步骤123当中任何检测到线程池关闭的情况直接执行任务拒绝。
public class ThreadPoolExecutor extends AbstractExecutorService {
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();
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
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);
}
}
ThreadPoolExecutor的addWorker方法有两个参数，Runnable firstTask代表待执行任务, boolean core代表是否启动核心线程，整个启动过程主要分为三个步骤：
- 前置检查：检查线程池是否处于关闭状态，在正常运行的情况下增加工作线程计数。
- 正常处理：创建Worker对象并在加锁的条件下将新建worker添加到workers集合当中，并通过调用t.start()方法启动线程。
- 后置处理：判断启动线程是否失败，如果失败那么就尝试中止线程池。
public class ThreadPoolExecutor extends AbstractExecutorService {
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
for (;;) {
int wc = workerCountOf(c);
// 判断是否超出线程限制，corePoolSize和core线程数，
// maximumPoolSize代表超出core部分的线程数
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
if (compareAndIncrementWorkerCount(c))
break retry;
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) {
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
private void addWorkerFailed(Worker w) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
if (w != null)
workers.remove(w);
decrementWorkerCount();
tryTerminate();
} finally {
mainLock.unlock();
}
}
}
ThreadPoolExecutor的worker介绍
ThreadPoolExecutor的worker实现Runnable接口，在worker的内部run()方法中通过执行runWorker()方法来启动task，启动方式会调用task.run()方法，所以从这个角度来看，task的执行线程其实ThreadPoolExecutor线程池中的worker。
- Worker类内部包含：Thread thread工作线程用于执行task、Runnable firstTask标识待执行任务。
- runWorker()方法内部负责执行来自提交的firstTask或者阻塞从任务队列通过getTask()方法取得待执行任务
- runWorker()方法内部通过执行task.run()负责真正执行任务。
public class ThreadPoolExecutor extends AbstractExecutorService {
private final class Worker
extends AbstractQueuedSynchronizer
implements Runnable
{
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;
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);
}
}
runWorker内部主要做两件事情，分别是：
- 获取任务：通过直接传进来firstTask或者通过getTask从任务队列中获取任务
- 执行任务：task.run()执行真正的task任务
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
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);
}
}
getTask()方法外层是一个for循环，然后内部从workQueue获取任务，区分设置超时或者阻塞等待。
- 阻塞等待直至线程获取到可消费任务。
- 超时等待使用的是keepAliveTime，用于超时后设置线程超时标记然后线程退出工作。
- 线程退出循环是通过返回task=null，外层循环直接结束实现。
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())) {
decrementWorkerCount();
return null;
}
int wc = workerCountOf(c);
// Are workers subject to culling?
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
// 标记线程退出工作部分的逻辑，通过返回task=null，从而在外层调用方实现退出while循环
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
try {
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}
}
ThreadPoolExecutor的task介绍
ThreadPoolExecutor的newTaskFor()方法负责创建task，创建的FutureTask的实例本身实现了Runnable、Future的接口。
- FutureTask内部可以创建入参为Runnable的对象的时候会创建一个代理器
- RunnableAdapter，创建入参为Callable的对象就比较直接了。
- FutureTask的运行函数run()负责执行Callable对象的call()方法并将返回值通过set()方法设置到outcome对象。
- FutureTask的get()方法负责获取返回值，就是我们submit()后返回的future的get()调用。
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 class Executors {
public static <T> Callable<T> callable(Runnable task, T result) {
if (task == null)
throw new NullPointerException();
return new RunnableAdapter<T>(task, result);
}
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;
}
}
}
public interface RunnableFuture<V> extends Runnable, Future<V> {
void run();
}
public class FutureTask<V> implements RunnableFuture<V> {
/**
* 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;
private Callable<V> callable;
private Object outcome; // non-volatile, protected by state reads/writes
private volatile Thread runner;
private volatile WaitNode waiters;
private V report(int s) throws ExecutionException {
Object x = outcome;
if (s == NORMAL)
return (V)x;
if (s >= CANCELLED)
throw new CancellationException();
throw new ExecutionException((Throwable)x);
}
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW; // ensure visibility of callable
}
public FutureTask(Runnable runnable, V result) {
this.callable = Executors.callable(runnable, result);
this.state = NEW; // ensure visibility of callable
}
public boolean isCancelled() {
return state >= CANCELLED;
}
public boolean isDone() {
return state != NEW;
}
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
s = awaitDone(false, 0L);
return report(s);
}
public V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {
if (unit == null)
throw new NullPointerException();
int s = state;
if (s <= COMPLETING &&
(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
throw new TimeoutException();
return report(s);
}
protected void set(V v) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
finishCompletion();
}
}
// 核心的逻辑，负责调用对象的call方法并赋值返回值
public void run() {
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))
return;
try {
Callable<V> c = callable;
if (c != null && state == NEW) {
V result;
boolean ran;
try {
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)
set(result);
}
} finally {
runner = null;
int s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE;
private static final long stateOffset;
private static final long runnerOffset;
private static final long waitersOffset;
static {
try {
UNSAFE = sun.misc.Unsafe.getUnsafe();
Class<?> k = FutureTask.class;
stateOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("state"));
runnerOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("runner"));
waitersOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("waiters"));
} catch (Exception e) {
throw new Error(e);
}
}
}
参考文章
ThreadPoolExecutor解析-主要源码研究
ThreadPoolExecutor(五)——线程池关闭相关操作
ThreadPoolExecutor(六)——线程池关闭之后
