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。

1. AbortPolicy         -- 当任务添加到线程池中被拒绝时，它将抛出 RejectedExecutionException 异常。
2. 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。
3. 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。
4. 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