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
转载请注明出处：http://www.cnblogs.com/skywang12345/p/3512947.html
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
2. DiscardOldestPolicy 示例
3. AbortPolicy 示例
4. CallerRunsPolicy 示例
1. DiscardPolicy 示例
1 import java.lang.reflect.Field;
2 import java.util.concurrent.ArrayBlockingQueue; 3 import java.util.concurrent.ThreadPoolExecutor; 4 import java.util.concurrent.TimeUnit; 5 import java.util.concurrent.ThreadPoolExecutor.DiscardPolicy; 6 7 public class DiscardPolicyDemo { 8 9 private static final int THREADS_SIZE = 1; 10 private static final int CAPACITY = 1; 11 12 public static void main(String[] args) throws Exception { 13 14 // 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)。 15 ThreadPoolExecutor pool = new ThreadPoolExecutor(THREADS_SIZE, THREADS_SIZE, 0, TimeUnit.SECONDS, 16 new ArrayBlockingQueue(CAPACITY)); 17 // 设置线程池的拒绝策略为"丢弃" 18 pool.setRejectedExecutionHandler(new ThreadPoolExecutor.DiscardPolicy()); 19 20 // 新建10个任务，并将它们添加到线程池中。 21 for (int i = 0; i < 10; i++) { 22 Runnable myrun = new MyRunnable("task-"+i); 23 pool.execute(myrun); 24 } 25 // 关闭线程池 26 pool.shutdown(); 27 } 28 } 29 30 class MyRunnable implements Runnable { 31 private String name; 32 public MyRunnable(String name) { 33 this.name = name; 34 } 35 @Override 36 public void run() { 37 try { 38 System.out.println(this.name + " is running."); 39 Thread.sleep(100); 40 } catch (Exception e) { 41 e.printStackTrace(); 42 } 43 } 44 }
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
1 import java.lang.reflect.Field;
2 import java.util.concurrent.ArrayBlockingQueue; 3 import java.util.concurrent.ThreadPoolExecutor; 4 import java.util.concurrent.TimeUnit; 5 import java.util.concurrent.ThreadPoolExecutor.DiscardOldestPolicy; 6 7 public class DiscardOldestPolicyDemo { 8 9 private static final int THREADS_SIZE = 1; 10 private static final int CAPACITY = 1; 11 12 public static void main(String[] args) throws Exception { 13 14 // 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)。 15 ThreadPoolExecutor pool = new ThreadPoolExecutor(THREADS_SIZE, THREADS_SIZE, 0, TimeUnit.SECONDS, 16 new ArrayBlockingQueue(CAPACITY)); 17 // 设置线程池的拒绝策略为"DiscardOldestPolicy" 18 pool.setRejectedExecutionHandler(new ThreadPoolExecutor.DiscardOldestPolicy()); 19 20 // 新建10个任务，并将它们添加到线程池中。 21 for (int i = 0; i < 10; i++) { 22 Runnable myrun = new MyRunnable("task-"+i); 23 pool.execute(myrun); 24 } 25 // 关闭线程池 26 pool.shutdown(); 27 } 28 } 29 30 class MyRunnable implements Runnable { 31 private String name; 32 public MyRunnable(String name) { 33 this.name = name; 34 } 35 @Override 36 public void run() { 37 try { 38 System.out.println(this.name + " is running."); 39 Thread.sleep(200); 40 } catch (Exception e) { 41 e.printStackTrace(); 42 } 43 } 44 }
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
1 import java.lang.reflect.Field;
2 import java.util.concurrent.ArrayBlockingQueue; 3 import java.util.concurrent.ThreadPoolExecutor; 4 import java.util.concurrent.TimeUnit; 5 import java.util.concurrent.ThreadPoolExecutor.AbortPolicy; 6 import java.util.concurrent.RejectedExecutionException; 7 8 public class AbortPolicyDemo { 9 10 private static final int THREADS_SIZE = 1; 11 private static final int CAPACITY = 1; 12 13 public static void main(String[] args) throws Exception { 14 15 // 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)。 16 ThreadPoolExecutor pool = new ThreadPoolExecutor(THREADS_SIZE, THREADS_SIZE, 0, TimeUnit.SECONDS, 17 new ArrayBlockingQueue(CAPACITY)); 18 // 设置线程池的拒绝策略为"抛出异常" 19 pool.setRejectedExecutionHandler(new ThreadPoolExecutor.AbortPolicy()); 20 21 try { 22 23 // 新建10个任务，并将它们添加到线程池中。 24 for (int i = 0; i < 10; i++) { 25 Runnable myrun = new MyRunnable("task-"+i); 26 pool.execute(myrun); 27 } 28 } catch (RejectedExecutionException e) { 29 e.printStackTrace(); 30 // 关闭线程池 31 pool.shutdown(); 32 } 33 } 34 } 35 36 class MyRunnable implements Runnable { 37 private String name; 38 public MyRunnable(String name) { 39 this.name = name; 40 } 41 @Override 42 public void run() { 43 try { 44 System.out.println(this.name + " is running."); 45 Thread.sleep(200); 46 } catch (Exception e) { 47 e.printStackTrace(); 48 } 49 } 50 }
(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
1 import java.lang.reflect.Field;
2 import java.util.concurrent.ArrayBlockingQueue; 3 import java.util.concurrent.ThreadPoolExecutor; 4 import java.util.concurrent.TimeUnit; 5 import java.util.concurrent.ThreadPoolExecutor.CallerRunsPolicy; 6 7 public class CallerRunsPolicyDemo { 8 9 private static final int THREADS_SIZE = 1; 10 private static final int CAPACITY = 1; 11 12 public static void main(String[] args) throws Exception { 13 14 // 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)。 15 ThreadPoolExecutor pool = new ThreadPoolExecutor(THREADS_SIZE, THREADS_SIZE, 0, TimeUnit.SECONDS, 16 new ArrayBlockingQueue(CAPACITY)); 17 // 设置线程池的拒绝策略为"CallerRunsPolicy" 18 pool.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy()); 19 20 // 新建10个任务，并将它们添加到线程池中。 21 for (int i = 0; i < 10; i++) { 22 Runnable myrun = new MyRunnable("task-"+i); 23 pool.execute(myrun); 24 } 25 26 // 关闭线程池 27 pool.shutdown(); 28 } 29 } 30 31 class MyRunnable implements Runnable { 32 private String name; 33 public MyRunnable(String name) { 34 this.name = name; 35 } 36 @Override 37 public void run() { 38 try { 39 System.out.println(this.name + " is running."); 40 Thread.sleep(100); 41 } catch (Exception e) { 42 e.printStackTrace(); 43 } 44 } 45 }
(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。