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

2024-07-13 10:09:01

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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<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

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