Java线程池--拒绝策略RejectedExecutionHandler

2022-11-27 12:51:00

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*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。

码农公寓

相关文章