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

2019年07月22日  | 移动技术网IT编程  | 我要评论

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

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