Executor框架是一种将线程的创建和执行分离的机制。它基于Executor和ExecutorService接口，及这两个接口的实现类ThreadPoolExecutor展开，Executor有一个内部线程池，并提供了将任务传递到池中线程以获得执行的方法，可传递的任务有如下两种：通过Runnable接口实现的任务和通过Callable接口实现的任务。在这两种情况下，只需要传递任务到执行器，执行器即可使用线程池中的线程或新创建的线程来执行任务。执行器也决定了任务执行时间。
java提供了四种线程池的实现：
（1）newCachedThreadPool创建一个可缓存线程池，如果线程池长度超过处理需要，可灵活回收空闲线程，若无可回收，则新建线程。
（2）newFixedThreadPool 创建一个定长线程池，可控制线程最大并发数，超出的线程会在队列中等待。
（3）newScheduledThreadPool 创建一个定长线程池，支持定时及周期性任务执行。
（4）newSingleThreadExecutor 创建一个单线程化的线程池，它只会用唯一的工作线程来执行任务，保证所有任务按照指定顺序(FIFO, LIFO, 优先级)执行。
下面就对这些线程池的使用方式进行简要的代码介绍：
首先是可缓存线程池：
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class ThreadPoolDemo {
public static void main (String[] args) {
ExecutorService cachedThreadPool = Executors.newCachedThreadPool();
for(int i = 0; i < 10; i++) {
final int index = i;
try {
Thread.sleep(index*1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
cachedThreadPool.execute(new Runnable () {
public void run () {
System.out.println(index);
}
});
}
}
}
然后是定长线程池：
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class ThreadPoolDemo {
public static void main (String[] args) {
ExecutorService fixedThreadPool = Executors.newFixedThreadPool(3);
for(int i = 0; i < 10; i++) {
final int index = i;
fixedThreadPool.execute(new Runnable () {
public void run () {
System.out.println(index);
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
});
}
}
}
然后是定长线程池支持定时和周期性任务:
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
public class ThreadPoolDemo {
public static void main (String[] args) {
ScheduledExecutorService scheduleThreadPool = Executors.newScheduledThreadPool(5);
scheduleThreadPool.schedule(new Runnable() {
public void run() {
System.out.println("delay 3 seconds");
}
}, 3, TimeUnit.SECONDS);
}
}
最后是：单线程化线程池
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
public class ThreadPoolDemo {
public static void main (String[] args) {
ExecutorService singleThreadExecutor = Executors.newSingleThreadExecutor();
for(int i = 0; i < 10; i++) {
final int index = i;
singleThreadExecutor.execute(new Runnable() {
public void run() {
try {
System.out.println(index);
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});
}
}
}
好了四种线程池的使用上面已经介绍完了，现在来看看线程池的原理吧，其中最重要的就是ThreadPoolExecutor类的构造函数，你会有疑惑，上面程序压根没有出现这个ThreadPoolExecutor类
/**
*Executors.newSingleThreadExecutor()
*中查看你会发现如下代码：
*/
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}
这四类线程池类底层都是ThreadPoolExecutor类进行初始化的，四大线程池是通过使用ThreadPoolExecutor构造函数实现的:
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
return new ScheduledThreadPoolExecutor(corePoolSize);//这边居然不是直接调ThreadPoolExecutor构造函数，但是我们追一下代码看看下面这个函数你就会明白
}
public ScheduledThreadPoolExecutor(int corePoolSize) {
super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
new DelayedWorkQueue());//里面使用了父类的构造函数，下面就是本类和父类的继承关系，看看他的父类是什么，你就明白了
}
public class ScheduledThreadPoolExecutor
extends ThreadPoolExecutor
implements ScheduledExecutorService
OK。到这里，你就应该知道我们今天的主角是谁了---------ThreadPoolExecutor
好了我们可以来看看这个类的构造函数源码了：
/**
* 看见有好多参数啊，现在主要对其中的构造参数进行解释：
* @corePoolSize:核心池大小，意思是当超过这个范围的时候，就需要将新的线程放到等待队列中了即workQueue；
* @maximumPoolSize:线程池最大线程数量，表明线程池能创建的最大线程数
* @keepAlivertime:当活跃线程数大于核心线程数，空闲的多余线程最大存活时间。
* @unit:存活时间的单位
* @workQueue:存放任务的队列---阻塞队列
* @handler:超出线程范围（maximumPoolSize）和队列容量的任务的处理程序
*/
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), defaultHandler);
}
我们执行线程时都会调用到ThreadPoolExecutor的execute()方法，现在我们来看看这个方法的源码（就是下面这段代码了，这里面有一些注释解析），我直接来解释一下吧：在这段代码中我们至少要看懂一个逻辑：当当前线程数小于核心池线程数时，只需要添加一个线程并且启动它，如果线程数数目大于核心线程池数目，我们将任务放到workQueue中，如果连workQueue满了，那么就要拒绝任务了。详细的函数我就不介绍了
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);
}
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