ExecutorService与Executors例子的简单剖析

对于多线程有了一点了解之后，那么来看看java.lang.concurrent包下面的一些东西。在此之前，我们运行一个线程都是显式调用了Thread的start()方法。我们用concurrent下面的类来实现一下线程的运行，而且这将成为以后常用的方法或者实现思路。

看一个简单的例子：

public class CacheThreadPool {
public staticvoid main(String[] args) {
ExecutorService exec=Executors.newCachedThreadPool();
for(int i=0;i<5;i++)
exec.execute(new LiftOff());
exec.shutdown();//并不是终止线程的运行，而是禁止在这个Executor中添加新的任务
}
}

这个例子其实很容易看懂，ExecutorService中有一个execute方法，这个方法的参数是Runnable类型。也就是说，将一个实现了Runnable类型的类的实例作为参数传入execute方法并执行，那么线程就相应的执行了。

一、ExecutorService
先看看ExecutorService，这是一个接口，简单的列一下这个接口：

public interface ExecutorServiceextends Executor {

void shutdown();

List<Runnable> shutdownNow();

boolean isShutdown();

boolean isTerminated();

boolean awaitTermination(long timeout, TimeUnit unit)

<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)

<T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,long timeout, TimeUnit unit)

<T> T invokeAny(Collection<? extends Callable<T>> tasks)

<T> T invokeAny(Collection<? extends Callable<T>> tasks,
long timeout, TimeUnit unit)
}

ExecuteService继承了Executor，Executor也是一个接口，里面只有一个方法：

void execute(Runnable command)

二、Executors
Executors是一个类，直接援引JDK文档的说明来说一下这个类的作用：

Factory and utility methods for Executor, ExecutorService, ScheduledExecutorService, ThreadFactory, and Callable classes defined in this package. This class supports the following kinds of methods:

• Methods that create and return an ExecutorService set up with commonly useful configuration settings.
• Methods that create and return a ScheduledExecutorService set up with commonly useful configuration settings.
• Methods that create and return a "wrapped" ExecutorService, that disables reconfiguration by making implementation-specific methods inaccessible.
• Methods that create and return a ThreadFactory that sets newly created threads to a known state.
• Methods that create and return a Callable out of other closure-like forms, so they can be used in execution methods requiring Callable.

在上面的例子中，我们用到了newCachedThreadPool()方法。看一下这个方法：

public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}

在源码中我们可以知道两点，1、这个方法返回类型是ExecutorService；2、此方法返回值实际是另一个类的实例。看一下这个类的信息：

public class ThreadPoolExecutorextends AbstractExecutorService {
..........
private final BlockingQueue<Runnable> workQueue;//这个变量在下面会提到
..........
}

ThreadPoolExecutor继承了AbstractExecutorService,而AbstractExecutorService又实现了ExecutorService接口。所以，根据多态，ThreadPoolExecutor可以看作是ExecutorService类型。

线程执行的最关键的一步是执行了executor方法，根据java的动态绑定，实际执行的是ThreadPoolExecutor所实现的executor方法。看看源码：

public class ThreadPoolExecutorextends AbstractExecutorService {
..........
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
if (poolSize >= corePoolSize || !addIfUnderCorePoolSize(command)) {
if (runState == RUNNING && workQueue.offer(command)) {
if (runState != RUNNING || poolSize == 
0)
ensureQueuedTaskHandled(command);
}
else if (!addIfUnderMaximumPoolSize(command))
reject(command); // is shutdown or saturated
}
}
..........
}

根据程序正常执行的路线来看，这个方法中比较重要的两个地方分别是：
1、workQueue.offer(command)
workQueue在上面提到过，是BlockingQueue<Runnable>类型的变量，这条语句就是将Runnable类型的实例加入到队列中。
2、ensureQueuedTaskHandled(command)
这个是线程执行的关键语句。看看它的源码：

public class ThreadPoolExecutorextends AbstractExecutorService {
..........
private void ensureQueuedTaskHandled(Runnable command) {
final ReentrantLock mainLock = 
this.mainLock;
mainLock.lock();
boolean reject = false;
Thread t = null;
try {
int state = runState;
if (state != RUNNING && workQueue.remove(command))
reject = true;
else if (state < STOP &&
poolSize < Math.max(corePoolSize, 1) &&
!workQueue.isEmpty())
t = addThread(null);
} finally {
mainLock.unlock();
}
if (reject)
reject(command);
else if (t !=null)
t.start();
}
..........
}

在这里我们就可以看到最终执行了t.start()方法来运行线程。在这之前的重点是t=addThread(null)方法，看看addThread方法的源码：

public class ThreadPoolExecutorextends AbstractExecutorService {
..........
private Thread addThread(Runnable firstTask) {
Worker w = new Worker(firstTask);
Thread t = threadFactory.newThread(w);
if (t != null) {
w.thread = t;
workers.add(w);
int nt = ++poolSize;
if (nt > largestPoolSize)
largestPoolSize = nt;
}
return t;
}
..........
}

这里两个重点，很明显：
1、Worker w = new Worker(firstTask)
2、Thread t = threadFactory.newThread(w)
先看Worker是个什么结构：

public class ThreadPoolExecutorextends AbstractExecutorService {
..........
private finalclass Worker
implements Runnable {
..........
Worker(Runnable firstTask) {
this.firstTask = firstTask;
}

private Runnable firstTask;
..........

public void run() {
try {
Runnable task = firstTask;
firstTask = null;
while (task != null || (task = getTask()) !=null) {
runTask(task);
task = null;
}
} finally {
workerDone(this);
}
}
}

Runnable getTask() {
for (;;) {
try {
int state = runState;
if (state > SHUTDOWN)
return null;
Runnable r;
if (state == SHUTDOWN) 
// Help drain queue
r = workQueue.poll();
else if (poolSize > corePoolSize || allowCoreThreadTimeOut)
r = workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS);
else
r = workQueue.take();
if (r != null)
return r;
if (workerCanExit()) {
if (runState >= SHUTDOWN) 
// Wake up others
interruptIdleWorkers();
return null;
}
// Else retry
} catch (InterruptedException ie) {
// On interruption, re-check runState
}
}
}
}
..........
}

Worker是一个内部类。根据之前可以知道，传入addThread的参数是null，也就是说Work中firstTask为null。
在看看newThread是一个什么方法：

public class Executors {
..........
static class DefaultThreadFactoryimplements ThreadFactory {
..........
public Thread newThread(Runnable r) {
Thread t = new Thread(group, r,
namePrefix + threadNumber.getAndIncrement(),
0);
if (t.isDaemon())
t.setDaemon(false);
if (t.getPriority() != Thread.NORM_PRIORITY)
t.setPriority(Thread.NORM_PRIORITY);
return t;
}
..........
}
..........
}

通过源码可以得知threadFactory的实际类型是DefaultThreadFactory，而DefaultThreadFactory是Executors的一个嵌套内部类。

之前我们提到了t.start()这个方法执行了线程。那么现在从头顺一下，看看到底是执行了谁的run方法。首先知道，t=addThread(null)，而addThread内部执行了下面三步，Worker w = new Worker(null);Thread t = threadFactory.newThread(w);return t;这里两个t是一致的。
从这里可以看出，t.start()实际上执行的是Worker内部的run方法。run()内部会在if条件里面使用“短路”：判断firstTask是否为null，若不是null则直接执行firstTask的run方法；如果是null，则调用getTask()方法来获取Runnable类型实例。从哪里获取呢？workQueue！在execute方法中，执行ensureQueuedTaskHandled(command)之前就已经把Runnable类型实例放入到workQueue中了，所以这里可以从workQueue中获取到。