Java线程池ThreadPoolExecutor使用与解析
概述
JDK提供了一个工具类Executors来很是方便的建立线程池，下面主要经过一个示例来分析Java线程池的实现原理。java
使用
Runnable runnable = new Runnable() {
@Override
public void run() {
// do something
}
};
ExecutorService executorService = Executors.newFixedThreadPool(2);
executorService.submit(runnable);
executorService.shutdown();
例子里面使用了Executors.newFixedThreadPool(2)建立了一个固定只有2个线程的线程池，返回了一个ExecutorService对象，而后调用executorService.submit()方法来启动一个线程，最后调用executorService.shutdown()来关闭线程池。编程
使用起来很是的方便，接下来经过深刻源代码看一下背后的原理。并发
源码分析
ExecutorService
看一下ExecutorService的定义异步
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);
...
}
ExecutorService继承自Executoride
public interface Executor {
void execute(Runnable command);
}
列出了一部分的接口，主要是提供了几个启动线程执行线程任务的方法，接收不一样的参数，以及关闭线程池的方法。submit方法接收Runnable或者Callable方法，返回一个Future对象用于异步获取执行结果。execute方法只接收一个Runnable参数，而且没有返回值。工具
Executors
再看一下Executors工具类的定义oop
public class Executors {
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
public static ExecutorService newWorkStealingPool() {
return new ForkJoinPool
(Runtime.getRuntime().availableProcessors(),
ForkJoinPool.defaultForkJoinWorkerThreadFactory,
null, true);
}
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
...
}
大体是这个样子的，这里列出了一部分，提供了建立固定线程数的线程池（newFixedThreadPool），工做窃取的线程池（newWorkStealingPool），单个线程的线程池（newSingleThreadExecutor），不知道怎么称呼的线程池（newCachedThreadPool）。源码分析
ThreadPoolExecutor
声明
以FixedThreadPool为例一探究竟，看一下FixedThreadPool返回的 ThreadPoolExecutor到底是什么东西学习
public class ThreadPoolExecutor extends AbstractExecutorService {...}
public abstract class AbstractExecutorService implements ExecutorService {
...
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(Callable<T> task) {
if (task == null) throw new NullPointerException();
RunnableFuture<T> ftask = newTaskFor(task);
execute(ftask);
return ftask;
}
...
}
ThreadPoolExecutor继承自AbstractExecutorService，AbstractExecutorService是一个实现了ExecutorService的抽象类。ui
抽象类中提供了submit方法的具体实现，将传入的Runnable或者Callable方法经过newTaskFor方法转换成一个FutureTask对象（它是RunnableFuture）的实现类，而后调用父类的execute方法执行任务，最终返回runnableFuture对象。从这能够看出来ExecutorService.submit()方法内部仍是经过调用Executor.execute()方法来执行的，只是将参数转换成一个Future对象，经过Future对象来获取执行结果。
内部结构
/**
* 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
*/
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; }
定义了5种线程池的状态
- RUNNING 表示线程池能够接受新的任务
- SHUTDOWN 表示不接受新的任务，可是继续处理队列中的任务
- STOP 表示不接受新的任务，中断当前处理的任务和队列种的任务
- TIDYING 表示全部任务都已经停止了，全部线程都中止了，将要执行 terminated()方法以前的状态
- TERMINATED 表示 terminated()方法已经执行完了
有5种状态变化的流程
- RUNNING -> SHUTDOWN 调用了 shutdown()
- (RUNNING or SHUTDOWN) -> STOP 调用了 shutdownNow()
- SHUTDOWN -> TIDYING 等待处理的队列和线程池都为空的时候
- STOP -> TIDYING 线程池为空尚未执行terminated()以前的状态
- TIDYING -> TERMINTED 已经执行完terminated()方法
AtomicInteger类型的ctl变量存着当前worker（Worker是一个内部类，下面会详细解释）的数量。
ThreadPoolExecutor用一个32位整型的高3位表示运行的状态，剩下的29位表示能够支持的线程数。
COUNT_BITS 为32-3 = 29， 好比 RUNNING 是 -1 << COUNT_BITS，即-1带符号位左移29位，就是101000...0，STOP为001000...0，TIDYING为010000...0。
CAPACITY 为 (1 << COUNT_BITS) - 1，1左移29位以后-1，最后的结果位 0001111...1，最高3位是0 剩下的29位都是1。
workerCountOf(int c) 用来计算当前线程数，用的方法是 c & CAPACITY 即 c & 0001111...1，取除了高3位的剩下29位来判断。
runStateOf(int c) 用来查看当前的线程状态， c & ~CAPACITY 即 c & 1110000...0，取高3位来判断。
private final BlockingQueue<Runnable> workQueue;
private final ReentrantLock mainLock = new ReentrantLock();
private final HashSet<Worker> workers = new HashSet<Worker>();
private final Condition termination = mainLock.newCondition();
private int largestPoolSize;
private long completedTaskCount;
private volatile ThreadFactory threadFactory;
private volatile RejectedExecutionHandler handler;
private volatile long keepAliveTime;
private volatile boolean allowCoreThreadTimeOut;
private volatile int corePoolSize;
private volatile int maximumPoolSize;
private static final RejectedExecutionHandler defaultHandler = new AbortPolicy();
private static final RuntimePermission shutdownPerm = new RuntimePermission("modifyThread");
在来看一些其余的全局属性。workerQueue 一个BlockingQueue存放Runnable对象，workers 一个HashSet存放Worker对象，还有一些corePoolSize maximumPoolSize等就是平时配置链接池的参数。
构造方法
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), defaultHandler);
}
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;
}
Executors提供的newFIxedThreadPool方法其实建立的是一个ThreadPoolExecutor对象，以 newFixedPoolSize(2) 为例，经过将corePoolSize maximumPoolSize都是设置为2来实现固定数量的线程池。keepAliveTime设置为0微秒。workerQueue传入了一个LinkedBlockingQueue对象。
execute()方法
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);
}
通读几遍代码加上上面的注释，基本能够理解整个方法的意思。主要的思想是
- 当前线程数小于corePoolSize的时候调用addWorker来建立Worker类
- 当前线程数量大于corePoolSize的时候执行 workQueue.offer() 将任务加到等待队列里面
- 若是加不进等待队列而且建立Worker失败，就使用reject策略来拒绝当前任务
注释中的第二点作了不少检查，将任务加到等待队列以后还要作一次检查看看是否须要建立Worker，防止以前建立的Worker已经出现异常中止了。不理解不要紧，不影响对线程池原理的学习。
addWorker
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);
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;
}
第一个for循环检查线程数有没有超过corePoolSize或者maximunPoolSize。过了这个for循环以后就是建立Worker的地方了
private final class Worker extends AbstractQueuedSynchronizer implements Runnable {
/** 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) {
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;
this.thread = getThreadFactory().newThread(this);
}
public void run() {
runWorker(this);
}
...
}
Worker 类继承自 AbstractQueuedSynchronizer 实现了 Runnable接口， AbstractQueuedSynchronizer 这个玩意特别厉害，是并发编程的核心类，因为内容很是多本文不做解析。Worker类中维护了一个Thread对象，存了当前运行的线程，还维护了一个Runnable对象（firstTask），存了当前线程须要执行的对象。再回顾addWorker方法，其实就是用传入的firstTask参数建立一个Worker对象，并使worker对象启动一个线程去执行firstTask。重点在Worker对象的run方法，调用了一个runWorker(this)方法。
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);
}
}
runWorker方法接受一个Worker参数，将参数里面的firstTask拿出来，而后调用 task.run() 方法直接运行这个task，运行完将task变量设置为null。而后这里有一个while循环 while (task != null || (task = getTask()) != null)，当task等于null的时候调用getTask()获取任务。
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;
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;
}
}
}
getTask()方法里面有一个死循环，boolean timed = allowCoreThreadTimeOut || wc > corePoolSize; timed变量判断wc变量是否大于corePoolSize (allowCoreThreadTimeOut 默认为 false)。而后下面有一行代码判断timed时候为ture，若是为true，调用 workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS)，不然调用workQueue.take()，从等待队列中获取等待被处理的线程，而后返回出去。poll和take的区别是 当队列里面没有数据的时候poll立刻返回false，而take会堵塞当前线程直到队列里面有数据。这里解释了为何线程池可以维持线程不释放。
总结
当设置了corePoolSize的时候，这个参数表明了可以运行的线程数，当用户执行submit方法的时候首先会去判断当前线程数有没有达到corePoolSize，若是没有达到，就建立Worker对象并启动线程执行任务，一个对象内维护一个线程，当线程数超过corePoolSize的时候，用户执行submit方法的时候只是将任务放到等待队列里面，核心线程不断从等待队列里面取出任务执行，没有任务的时候一直被堵塞住，当有任务来的时候直接取出执行，避免了不断建立线程带来的开销，以及增长了系统资源的利用率。
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