ExecutorService | Executor框架
Excutor框架结构图:
上一篇讲了Executor接口,本篇来讲一下ExecutorService接口,从图中可以看出ExecutorService扩展了Executor接口,一般扩展后,都会新增一些功能方法,它有哪些新方法呢?
一:关于ExecutorService的源码
package java.util.concurrent; import java.util.List; import java.util.Collection; import java.security.PrivilegedAction; import java.security.PrivilegedExceptionAction; publicinterface ExecutorService extends Executor { /** * 启动一次顺序关闭,执行以前提交的任务,但不接受新任务。如果已经关闭,则调用没有其他作用。 */ void shutdown(); /** * 试图停止所有正在执行的活动任务,暂停处理正在等待的任务,并返回等待执行的任务列表。 * 无法保证能够停止正在处理的活动执行任务,但是会尽力尝试。例如,通过 Thread.interrupt() * 来取消典型的实现,所以任何任务无法响应中断都可能永远无法终止。 */ List<Runnable> shutdownNow(); /** * 如果此执行程序已关闭,则返回 true。 */ boolean isShutdown(); /** * 如果关闭后所有任务都已完成,则返回 true。注意,除非首先调用 shutdown 或 shutdownNow * 否则 isTerminated 永不为 true。 */ boolean isTerminated(); /** * 请求关闭、发生超时或者当前线程中断,无论哪一个首先发生之后,都将导致阻塞,直到所有任务完成执行。 */ boolean awaitTermination(long timeout, TimeUnit unit)throws InterruptedException; /** * Submits a value-returning task for execution and returns a * Future representing the pending results of the task. The * Future's <tt>get</tt> method will return the task's result upon * successful completion. */ <T> Future<T> submit(Callable<T> task); /** * Submits a Runnable task for execution and returns a Future * representing that task. The Future's <tt>get</tt> method will * return the given result upon successful completion. */ <T> Future<T> submit(Runnable task, T result); /** * Submits a Runnable task for execution and returns a Future * representing that task. The Future's <tt>get</tt> method will * return <tt>null</tt> upon <em>successful</em> completion. */ Future<?> submit(Runnable task); /** * Executes the given tasks, returning a list of Futures holding * their status and results when all complete. * {@link Future#isDone} is <tt>true</tt> for each * element of the returned list. * Note that a <em>completed</em> task could have * terminated either normally or by throwing an exception. * The results of this method are undefined if the given * collection is modified while this operation is in progress. */ <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)throws InterruptedException; /** * Executes the given tasks, returning a list of Futures holding * their status and results * when all complete or the timeout expires, whichever happens first. * {@link Future#isDone} is <tt>true</tt> for each * element of the returned list. * Upon return, tasks that have not completed are cancelled. * Note that a <em>completed</em> task could have * terminated either normally or by throwing an exception. * The results of this method are undefined if the given * collection is modified while this operation is in progress. */ <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,long timeout, TimeUnit unit) throws InterruptedException; /** * Executes the given tasks, returning the result * of one that has completed successfully (i.e., without throwing * an exception), if any do. Upon normal or exceptional return, * tasks that have not completed are cancelled. * The results of this method are undefined if the given * collection is modified while this operation is in progress. */ <T> T invokeAny(Collection<? extends Callable<T>> tasks)throws InterruptedException, ExecutionException; /** * Executes the given tasks, returning the result * of one that has completed successfully (i.e., without throwing * an exception), if any do before the given timeout elapses. * Upon normal or exceptional return, tasks that have not * completed are cancelled. * The results of this method are undefined if the given * collection is modified while this operation is in progress. * */ <T> T invokeAny(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException; }
Executor提供了管理终止的方法,以及可为跟踪一个或多个异步任务执行状况而生成 Future 的方法。
可以关闭ExecutorService,这将导致其拒绝新任务。提供两个方法来关闭 ExecutorService。shutdown()方法在终止前允许执行以前提交的任务,而 shutdownNow() 方法阻止等待任务启动并试图停止当前正在执行的任务。在终止时,执行程序没有任务在执行,也没有任务在等待执行,并且无法提交新任务。应该关闭未使用的 ExecutorService 以允许回收其资源。
Executor框架新手,看了ExecutorService的源码及源码中的说明,必定仍是一头雾水!没关系,看不懂没关系,但至少应该明白的是:
- Executor 接口定义了最基本的 execute 方法,用于接收用户提交的任务
- ExecutorService接口扩展自Executor接口,并添加了一些生命周期管理的方法。例如:ExecutorService 定义了线程池终止和创建及提交 futureTask 任务支持的方法。
一个Executor的生命周期有三种状态:运行、关闭、终止
二:关于ExecutorService的实现
ExecutorService的实现类有:
- AbstractExecutorService 抽象类主要实现了 ExecutorService和 futureTask 相关的一些任务创建和提交的方法
- ScheduledThreadPoolExecutor
- ThreadPoolExecutor Executor框架核心类,线程池的功能都在这里实现了
抽象类AbstractExecutorService:
package java.util.concurrent; import java.util.*; publicabstractclass AbstractExecutorService implements ExecutorService { /** * 为给定可运行任务和默认值返回一个 RunnableFuture. * @since 1.6 */ protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) { returnnew FutureTask<T>(runnable, value); } /** * 为给定可调用任务返回一个 RunnableFuture. * @since 1.6 */ protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) { returnnew FutureTask<T>(callable); } /** * 提交一个 Runnable 任务用于执行,并返回一个表示该任务的 Future. */ public Future<?> submit(Runnable task) { if (task == null) thrownew NullPointerException(); RunnableFuture<Void> ftask = newTaskFor(task, null); execute(ftask); return ftask; } /** * 提交一个 Runnable 任务用于执行,并返回一个表示该任务的 Future. */ public <T> Future<T> submit(Runnable task, T result) { if (task == null) thrownew NullPointerException(); RunnableFuture<T> ftask = newTaskFor(task, result); execute(ftask); return ftask; } /** * 提交一个返回值的任务用于执行,返回一个表示任务的未决结果的 Future. */ public <T> Future<T> submit(Callable<T> task) { if (task == null) thrownew NullPointerException(); RunnableFuture<T> ftask = newTaskFor(task); execute(ftask); return ftask; } }
说明:AbstractExecutorService类有关键字 abstart修饰,则它是一个抽象类,不允许被实例化。它提供了一些执行任务(Runnable类型)的方法。
参考资料:
JDK API 1.6.0
http://www.iteye.com/topic/366591
http://singleant.iteye.com/blog/1423931