Java多线程系列--“JUC线程池”05之 线程池原理(四)

概要


本章介绍线程池的拒绝策略。内容包括:

  • 拒绝策略介绍
  • 拒绝策略对比和示例

拒绝策略介绍


线程池的拒绝策略,是指当任务添加到线程池中被拒绝,而采取的处理措施。
当任务添加到线程池中之所以被拒绝,可能是由于:第一,线程池异常关闭。第二,任务数量超过线程池的最大限制。

线程池共包括4种拒绝策略,它们分别是:AbortPolicy, CallerRunsPolicy, DiscardOldestPolicyDiscardPolicy

AbortPolicy         -- 当任务添加到线程池中被拒绝时,它将抛出 RejectedExecutionException 异常。
CallerRunsPolicy    -- 当任务添加到线程池中被拒绝时,会在线程池当前正在运行的Thread线程池中处理被拒绝的任务。
DiscardOldestPolicy -- 当任务添加到线程池中被拒绝时,线程池会放弃等待队列中最旧的未处理任务,然后将被拒绝的任务添加到等待队列中。
DiscardPolicy       -- 当任务添加到线程池中被拒绝时,线程池将丢弃被拒绝的任务。

线程池默认的处理策略是AbortPolicy

拒绝策略对比和示例


下面通过示例,分别演示线程池的4种拒绝策略。

  1. DiscardPolicy 示例;
  2. DiscardOldestPolicy 示例;
  3. AbortPolicy 示例;
  4. CallerRunsPolicy 示例;
1、DiscardPolicy 示例
import java.lang.reflect.Field;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.ThreadPoolExecutor.DiscardPolicy;

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();
    }
}

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();
        }
    }
}

运行结果:

task-0 is running.
task-1 is running.

结果说明:

  • 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
import java.lang.reflect.Field;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.ThreadPoolExecutor.DiscardOldestPolicy;

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();
    }
}

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(200);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

运行结果:

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

结果说明: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
import java.lang.reflect.Field;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.ThreadPoolExecutor.AbortPolicy;
import java.util.concurrent.RejectedExecutionException;

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();
        }
    }
}

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(200);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

(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.

结果说明: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
import java.lang.reflect.Field;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.ThreadPoolExecutor.CallerRunsPolicy;

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();
    }
}

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();
        }
    }
}

(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.

结果说明: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。






文章来源:http://www.cnblogs.com/skywang12345/p/3512947.html

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