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:
Java代码
- public class CacheThreadPool {
- public static void 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:
Java代码
- public interface ExecutorService extends 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:
Java代码
- 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:
Java代码
- 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:
Java代码
- public class ThreadPoolExecutor extends 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:
Java代码
- public class ThreadPoolExecutor extends 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:
Java代码
- public class ThreadPoolExecutor extends 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:
Java代码
- public class ThreadPoolExecutor extends 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:
Java代码
- public class ThreadPoolExecutor extends AbstractExecutorService {
- ..........
- private final class 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:
Java代码
- public class Executors {
- ..........
- static class DefaultThreadFactory implements 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。
转自:
http://paddy-w.iteye.com/blog/1039530
posted on 2012-10-30 21:56
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