java 线程池详解及四种线程池用法介绍

java 线程池详解

     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 blocke.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类啊，其实，如果你追代码到

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，你不信你可以一个一个点进去看一下，而且我告诉你，四大线程池是通过使用ThreadPoolExecutor构造函数实现的;你看看下面实现就知道了

    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：
    public ThreadPoolExecutor(int corePoolSize,                              int maximumPoolSize,                              long keepAliveTime,                              TimeUnit unit,                              BlockingQueue<Runnable> workQueue) {        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,             Executors.defaultThreadFactory(), defaultHandler);    }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);    }