线程池--拒绝策略RejectedExecutionHandler

当线程池的任务缓存队列已满并且线程池中的线程数目达到maximumPoolSize,如果还有任务到来就会采取任务拒绝策略,通常有以下四种策略:

ThreadPoolExecutor.AbortPolicy:丢弃任务并抛出RejectedExecutionException异常。
ThreadPoolExecutor.DiscardPolicy:也是丢弃任务,但是不抛出异常。
ThreadPoolExecutor.DiscardOldestPolicy:丢弃队列最前面的任务,然后重新尝试执行任务(重复此过程)
ThreadPoolExecutor.CallerRunsPolicy:由调用线程处理该任务

当Executor已经关闭(即执行了executorService.shutdown()方法后),并且Executor将有限边界用于最大线程和工作队列容量,且已经饱和时,在方法execute()中提交的新任务将被拒绝.
在以上述情况下,execute 方法将调用其 RejectedExecutionHandler 的RejectedExecutionHandler.rejectedExecution(java.lang.Runnable, java.util.concurrent.ThreadPoolExecutor) 方法。
线程池默认会采用的是defaultHandler策略。首先看defaultHandler的定义:

private static final RejectedExecutionHandler defaultHandler = new AbortPolicy(); // 使用默认的拒绝策略 
//丢弃任务并抛出RejectedExecutionException异常。
public static class AbortPolicy implements RejectedExecutionHandler {  
    public AbortPolicy() { }  
    // 抛出异常  
    public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {  
        throw new RejectedExecutionException("Task " + r.toString() +  " rejected from " +  e.toString());  
    }  
}  

如下是一个测试任务的例子,下面编写4个测试用例来测试。

class MyRunnable implements Runnable {  
    private String name;  
    public MyRunnable(String name) {  
        this.name = name;  
    }  
    @Override  
    public void run() {  
        try {  
            System.out.println(this.name + " is running.");  
            Thread.sleep(100);  
        } catch (Exception e) {  
            e.printStackTrace();  
        }  
    }  
}  

看一下其他拒绝策略的具体实现。
1.DiscardPolicy 示例(也是丢弃任务,但是不抛出异常。)

public class DiscardPolicyDemo {  

    private static final int THREADS_SIZE = 1;  
    private static final int CAPACITY = 1;  

    public static void main(String[] args) throws Exception {  

        // 创建线程池。线程池的"最大池大小"和"核心池大小"都为1(THREADS_SIZE),"线程池"的阻塞队列容量为1(CAPACITY)。  
        ThreadPoolExecutor pool = new ThreadPoolExecutor(THREADS_SIZE, THREADS_SIZE, 0, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(CAPACITY));  
        // 设置线程池的拒绝策略为"丢弃"  
        pool.setRejectedExecutionHandler(new ThreadPoolExecutor.DiscardPolicy());  

        // 新建10个任务,并将它们添加到线程池中。  
        for (int i = 0; i < 10; i++) {  
            Runnable myrun = new MyRunnable("task-"+i);  
            pool.execute(myrun);  
        }  
        // 关闭线程池  
        pool.shutdown();  
    }  
}  

线程池pool的”最大池大小”和”核心池大小”都为1(THREADS_SIZE),这意味着”线程池能同时运行的任务数量最大只能是1”。
线程池pool的阻塞队列是ArrayBlockingQueue,ArrayBlockingQueue是一个有界的阻塞队列,ArrayBlockingQueue的容量为1。这也意味着线程池的阻塞队列只能有一个线程池阻塞等待。   
根据”“中分析的execute()代码可知:线程池中共运行了2个任务。第1个任务直接放到Worker中,通过线程去执行;第2个任务放到阻塞队列中等待。其他的任务都被丢弃了!
2.DiscardOldestPolicy 示例(丢弃队列最前面的任务,然后重新尝试执行任务(重复此过程))

public class DiscardOldestPolicyDemo {  

    private static final int THREADS_SIZE = 1;  
    private static final int CAPACITY = 1;  

    public static void main(String[] args) throws Exception {  

        // 创建线程池。线程池的"最大池大小"和"核心池大小"都为1(THREADS_SIZE),"线程池"的阻塞队列容量为1(CAPACITY)。  
        ThreadPoolExecutor pool = new ThreadPoolExecutor(THREADS_SIZE, THREADS_SIZE, 0, TimeUnit.SECONDS,  
                new ArrayBlockingQueue<Runnable>(CAPACITY));  
        // 设置线程池的拒绝策略为"DiscardOldestPolicy"  
        pool.setRejectedExecutionHandler(new ThreadPoolExecutor.DiscardOldestPolicy());  

        // 新建10个任务,并将它们添加到线程池中。  
        for (int i = 0; i < 10; i++) {  
            Runnable myrun = new MyRunnable("task-"+i);  
            pool.execute(myrun);  
        }  
        // 关闭线程池  
        pool.shutdown();  
    }  
}  

运行结果:

task-0 is running.
task-9 is running.

将”线程池的拒绝策略”由DiscardPolicy修改为DiscardOldestPolicy之后,当有任务添加到线程池被拒绝时,线程池会丢弃阻塞队列中末尾的任务,然后将被拒绝的任务添加到末尾。

3.AbortPolicy 示例(丢弃任务并抛出RejectedExecutionException异常。)

public class AbortPolicyDemo {  

    private static final int THREADS_SIZE = 1;  
    private static final int CAPACITY = 1;  

    public static void main(String[] args) throws Exception {  

        // 创建线程池。线程池的"最大池大小"和"核心池大小"都为1(THREADS_SIZE),"线程池"的阻塞队列容量为1(CAPACITY)。  
        ThreadPoolExecutor pool = new ThreadPoolExecutor(THREADS_SIZE, THREADS_SIZE, 0, TimeUnit.SECONDS,  
                new ArrayBlockingQueue<Runnable>(CAPACITY));  
        // 设置线程池的拒绝策略为"抛出异常"  
        pool.setRejectedExecutionHandler(new ThreadPoolExecutor.AbortPolicy());  

        try {  

            // 新建10个任务,并将它们添加到线程池中。  
            for (int i = 0; i < 10; i++) {  
                Runnable myrun = new MyRunnable("task-"+i);  
                pool.execute(myrun);  
            }  
        } catch (RejectedExecutionException e) {  
            e.printStackTrace();  
            // 关闭线程池  
            pool.shutdown();  
        }  
    }  
}  

(某一次)运行结果:

java.util.concurrent.RejectedExecutionException
    at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:1774)
    at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:768)
    at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:656)
    at AbortPolicyDemo.main(AbortPolicyDemo.java:27)
task-0 is running.
task-1 is running.

将”线程池的拒绝策略”由DiscardPolicy修改为AbortPolicy之后,当有任务添加到线程池被拒绝时,会抛出RejectedExecutionException。

4.CallerRunsPolicy 示例(由调用线程处理该任务)

public class CallerRunsPolicyDemo {  

    private static final int THREADS_SIZE = 1;  
    private static final int CAPACITY = 1;  

    public static void main(String[] args) throws Exception {  

        // 创建线程池。线程池的"最大池大小"和"核心池大小"都为1(THREADS_SIZE),"线程池"的阻塞队列容量为1(CAPACITY)。  
        ThreadPoolExecutor pool = new ThreadPoolExecutor(THREADS_SIZE, THREADS_SIZE, 0, TimeUnit.SECONDS,  
                new ArrayBlockingQueue<Runnable>(CAPACITY));  
        // 设置线程池的拒绝策略为"CallerRunsPolicy"  
        pool.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());  

        // 新建10个任务,并将它们添加到线程池中。  
        for (int i = 0; i < 10; i++) {  
            Runnable myrun = new MyRunnable("task-"+i);  
            pool.execute(myrun);  
        }  

        // 关闭线程池  
        pool.shutdown();  
    }  
}  

(某一次)运行结果:

task-2 is running.
task-3 is running.
task-4 is running.
task-5 is running.
task-6 is running.
task-7 is running.
task-8 is running.
task-9 is running.
task-0 is running.
task-1 is running.

将”线程池的拒绝策略”由DiscardPolicy修改为CallerRunsPolicy之后,当有任务添加到线程池被拒绝时,线程池会将被拒绝的任务添加到”线程池正在运行的线程”中取运行。

参考来自http://blog.csdn.net/zhoufenqin/article/details/51012666

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