Java concurrency线程池之线程池原理(四)_动力节点Java学院整理

Java concurrency线程池之线程池原理(四)_动力节点Java学院整理

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, DiscardOldestPolicy和DiscardPolicy。

AbortPolicy         -- 当任务http://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

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(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 ArrayBlockingQuhttp://eue(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.chttp://oncurrent.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(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(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

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