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：
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1.public class CacheThreadPool {
2. public static void main(String[] args) {
3. ExecutorService exec=Executors.newCachedThreadPool();
4. for(int i=0;i<5;i++)
5. exec.execute(new LiftOff());
6. 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
7. }
8.}
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：
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1.public interface ExecutorService extends Executor {
2.
3. void shutdown();
4.
5. List<Runnable> shutdownNow();
6.
7. boolean isShutdown();
8.
9. boolean isTerminated();
10.
11. boolean awaitTermination(long timeout, TimeUnit unit)
12.
13. <T> Future<T> submit(Callable<T> task);
14.
15. <T> Future<T> submit(Runnable task, T result);
16.
17. Future<?> submit(Runnable task);
18.
19. <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
20.
21. <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit)
22.
23. <T> T invokeAny(Collection<? extends Callable<T>> tasks)
24.
25. <T> T invokeAny(Collection<? extends Callable<T>> tasks,
26. long timeout, TimeUnit unit)
27.}
ExecuteService继承了Executor，Executor也是一个接口，里面只有一个方法：
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1.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：
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1.public static ExecutorService newCachedThreadPool() {
2. return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
3. 60L, TimeUnit.SECONDS,
4. new SynchronousQueue<Runnable>());
5. }
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：
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1.public class ThreadPoolExecutor extends AbstractExecutorService {
2. ..........
3. private final BlockingQueue<Runnable> workQueue;//这个变量在下面会提到
4. ..........
5.}
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：
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1.public class ThreadPoolExecutor extends AbstractExecutorService {
2. ..........
3. public void execute(Runnable command) {
4. if (command == null)
5. throw new NullPointerException();
6. if (poolSize >= corePoolSize || !addIfUnderCorePoolSize(command)) {
7. if (runState == RUNNING && workQueue.offer(command)) {
8. if (runState != RUNNING || poolSize == 0)
9. ensureQueuedTaskHandled(command);
10. }
11. else if (!addIfUnderMaximumPoolSize(command))
12. reject(command); // is shutdown or saturated
13. }
14. }
15. ..........
16.}
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：
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1.public class ThreadPoolExecutor extends AbstractExecutorService {
2. ..........
3. private void ensureQueuedTaskHandled(Runnable command) {
4. final ReentrantLock mainLock = this.mainLock;
5. mainLock.lock();
6. boolean reject = false;
7. Thread t = null;
8. try {
9. int state = runState;
10. if (state != RUNNING && workQueue.remove(command))
11. reject = true;
12. else if (state < STOP &&
13. poolSize < Math.max(corePoolSize, 1) &&
14. !workQueue.isEmpty())
15. t = addThread(null);
16. } finally {
17. mainLock.unlock();
18. }
19. if (reject)
20. reject(command);
21. else if (t != null)
22. t.start();
23. }
24. ..........
25.}
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：
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1.public class ThreadPoolExecutor extends AbstractExecutorService {
2. ..........
3. private Thread addThread(Runnable firstTask) {
4. Worker w = new Worker(firstTask);
5. Thread t = threadFactory.newThread(w);
6. if (t != null) {
7. w.thread = t;
8. workers.add(w);
9. int nt = ++poolSize;
10. if (nt > largestPoolSize)
11. largestPoolSize = nt;
12. }
13. return t;
14. }
15. ..........
16.}
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：
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1.public class ThreadPoolExecutor extends AbstractExecutorService {
2. ..........
3. private final class Worker implements Runnable {
4. ..........
5. Worker(Runnable firstTask) {
6. this.firstTask = firstTask;
7. }
8.
9. private Runnable firstTask;
10. ..........
11.
12. public void run() {
13. try {
14. Runnable task = firstTask;
15. firstTask = null;
16. while (task != null || (task = getTask()) != null) {
17. runTask(task);
18. task = null;
19. }
20. } finally {
21. workerDone(this);
22. }
23. }
24. }
25.
26. Runnable getTask() {
27. for (;;) {
28. try {
29. int state = runState;
30. if (state > SHUTDOWN)
31. return null;
32. Runnable r;
33. if (state == SHUTDOWN) // Help drain queue
34. r = workQueue.poll();
35. else if (poolSize > corePoolSize || allowCoreThreadTimeOut)
36. r = workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS);
37. else
38. r = workQueue.take();
39. if (r != null)
40. return r;
41. if (workerCanExit()) {
42. if (runState >= SHUTDOWN) // Wake up others
43. interruptIdleWorkers();
44. return null;
45. }
46. // Else retry
47. } catch (InterruptedException ie) {
48. // On interruption, re-check runState
49. }
50. }
51. }
52. }
53. ..........
54.}
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：
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1.public class Executors {
2. ..........
3. static class DefaultThreadFactory implements ThreadFactory {
4. ..........
5. public Thread newThread(Runnable r) {
6. Thread t = new Thread(group, r,
7. namePrefix + threadNumber.getAndIncrement(),
8. 0);
9. if (t.isDaemon())
10. t.setDaemon(false);
11. if (t.getPriority() != Thread.NORM_PRIORITY)
12. t.setPriority(Thread.NORM_PRIORITY);
13. return t;
14. }
15. ..........
16. }
17. ..........
18.}
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。
ExecutorService与Executors例子的简单剖析
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转载自2560240061.iteye.com/blog/2310022
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