java.util.concurrent.Executor

简介:

An object that executes submitted {@link Runnable} tasks.

这个接口提供了一种将任务提交与每个任务的运行机制分离的方法。通常使用{@code Executor}而不是显式创建线程。

例如:

不是为每一个任务调度

new Thread(new(RunnableTask())).start()

而是

Executor executor =  ... ;
executor.execute(new RunnableTask1());
executor.execute(new RunnableTask2());

注意:Executor接口并不严格要求执行是异步的。

在最简单的情况下，执行者可以立即在调用者的线程中运行提交的任务：

class DirectExecutor implements Executor {
       public void execute(Runnable r) {
              r.run();
       }
}    

更典型地，任务在调用者线程之外的某个线程中执行。

例如:下面的执行程序为每个任务生成一个新线程：

public class ThreadPerTaskExecutor implements Executor {
    public void execute(Runnable r) {
        new Thread(r).start();
    }
}

许多Executor实现对计划任务的方式和时间施加了某种限制。

下面的执行程序将任务提交序列化到第二个执行程序，演示了一个复合执行器(a composite executor)。

import java.util.ArrayDeque;
import java.util.Queue;
import java.util.concurrent.Executor;




public class SerialExecutor implements Executor {
    final Queue<Runnable> tasks = new ArrayDeque<Runnable>();
    final Executor executor;
    Runnable active;

    SerialExecutor(Executor executor) {
        this.executor = executor;
    }

    public synchronized void execute(final Runnable r) {
        tasks.offer(new Runnable() {
            public void run() {
                try {
                    r.run();
                } finally {
                    scheduleNext();
                }
            }
        });

        if (active == null) {
            System.out.println("active == null!");
            scheduleNext();
        }
    }

    protected synchronized void scheduleNext() {
        if ((active = tasks.poll()) != null) {
            executor.execute(active);
        }
    }
}

综合演示:

import java.util.ArrayDeque;
import java.util.Queue;
import java.util.concurrent.Executor;

class DirectExecutor implements Executor {
    public void execute(Runnable r) {
        r.run();
    }
}


class ThreadPerTaskExecutor implements Executor {
    public void execute(Runnable r) {
        new Thread(r).start();
    }
}


public class SerialExecutor implements Executor {
    final Queue<Runnable> tasks = new ArrayDeque<Runnable>();
    final Executor executor;
    Runnable active;

    SerialExecutor(Executor executor) {
        this.executor = executor;
    }

    public synchronized void execute(final Runnable r) {
        tasks.offer(new Runnable() {
            public void run() {
                try {
                    r.run();
                } finally {
                    scheduleNext();
                }
            }
        });

        if (active == null) {
            System.out.println("active == null!");
            scheduleNext();
        }
    }

    protected synchronized void scheduleNext() {
        if ((active = tasks.poll()) != null) {
            executor.execute(active);
        }
    }
}


class SerialExecutorMain {
    public static void main(String[] args) {
        SerialExecutor serialExecutor = new SerialExecutor(new ThreadPerTaskExecutor());
        /**or**/
        //SerialExecutor serialExecutor = new SerialExecutor(new DirectExecutor());
        for (int i = 0; i < 100; i++) {
            serialExecutor.execute(new Runnable() {
                @Override
                public void run() {
                    System.out.println("Hello!");
                }
            });
        }
    }
}