拒絕策略介紹
線程池的拒絕策略，是指當任務添加到線程池中被拒絕，而采取的處理措施。
當任務添加到線程池中之所以被拒絕，可能是由于：第一，線程池異常關閉。第二，任務數量超過線程池的最大限制。
線程池共包括4種拒絕策略，它們分別是：AbortPolicy, CallerRunsPolicy, DiscardOldestPolicy和DiscardPolicy。
- AbortPolicy -- 當任務添加到線程池中被拒絕時，它將拋出 RejectedExecutionException 異常。
- CallerRunsPolicy -- 當任務添加到線程池中被拒絕時，會在線程池當前正在運行的Thread線程池中處理被拒絕的任務。
- DiscardOldestPolicy -- 當任務添加到線程池中被拒絕時，線程池會放棄等待隊列中最舊的未處理任務，然后將被拒絕的任務添加到等待隊列中。
- DiscardPolicy -- 當任務添加到線程池中被拒絕時，線程池將丟棄被拒絕的任務。
線程池默認的處理策略是AbortPolicy！
拒絕策略對比和示例
下面通過示例，分別演示線程池的4種拒絕策略。
1. DiscardPolicy 示例
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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 { // 創建線程池。線程池的"最大池大小"和"核心池大小"都為1(THREADS_SIZE)，"線程池"的阻塞隊列容量為1(CAPACITY)。 ThreadPoolExecutor pool = new ThreadPoolExecutor(THREADS_SIZE, THREADS_SIZE, 0, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(CAPACITY)); // 設置線程池的拒絕策略為"丟棄" pool.setRejectedExecutionHandler(new ThreadPoolExecutor.DiscardPolicy()); // 新建10個任務，并將它們添加到線程池中。 for (int i = 0; i < 10; i++) { Runnable myrun = new MyRunnable("task-"+i); pool.execute(myrun); } // 關閉線程池 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(); } }} |
運行結果：
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task-0 is running.task-1 is running. |
結果說明：線程池pool的"最大池大小"和"核心池大小"都為1(THREADS_SIZE)，這意味著"線程池能同時運行的任務數量最大只能是1"。
線程池pool的阻塞隊列是ArrayBlockingQueue，ArrayBlockingQueue是一個有界的阻塞隊列，ArrayBlockingQueue的容量為1。這也意味著線程池的阻塞隊列只能有一個線程池阻塞等待。
根據""中分析的execute()代碼可知：線程池中共運行了2個任務。第1個任務直接放到Worker中，通過線程去執行；第2個任務放到阻塞隊列中等待。其他的任務都被丟棄了！
2. DiscardOldestPolicy 示例
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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 { // 創建線程池。線程池的"最大池大小"和"核心池大小"都為1(THREADS_SIZE)，"線程池"的阻塞隊列容量為1(CAPACITY)。 ThreadPoolExecutor pool = new ThreadPoolExecutor(THREADS_SIZE, THREADS_SIZE, 0, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(CAPACITY)); // 設置線程池的拒絕策略為"DiscardOldestPolicy" pool.setRejectedExecutionHandler(new ThreadPoolExecutor.DiscardOldestPolicy()); // 新建10個任務，并將它們添加到線程池中。 for (int i = 0; i < 10; i++) { Runnable myrun = new MyRunnable("task-"+i); pool.execute(myrun); } // 關閉線程池 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(); } }} |
運行結果：
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task-0 is running.task-9 is running. |
結果說明：將"線程池的拒絕策略"由DiscardPolicy修改為DiscardOldestPolicy之后，當有任務添加到線程池被拒絕時，線程池會丟棄阻塞隊列中末尾的任務，然后將被拒絕的任務添加到末尾。
3. AbortPolicy 示例
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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 { // 創建線程池。線程池的"最大池大小"和"核心池大小"都為1(THREADS_SIZE)，"線程池"的阻塞隊列容量為1(CAPACITY)。 ThreadPoolExecutor pool = new ThreadPoolExecutor(THREADS_SIZE, THREADS_SIZE, 0, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(CAPACITY)); // 設置線程池的拒絕策略為"拋出異常" pool.setRejectedExecutionHandler(new ThreadPoolExecutor.AbortPolicy()); try { // 新建10個任務，并將它們添加到線程池中。 for (int i = 0; i < 10; i++) { Runnable myrun = new MyRunnable("task-"+i); pool.execute(myrun); } } catch (RejectedExecutionException e) { e.printStackTrace(); // 關閉線程池 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)運行結果：
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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. |
結果說明：將"線程池的拒絕策略"由DiscardPolicy修改為AbortPolicy之后，當有任務添加到線程池被拒絕時，會拋出RejectedExecutionException。
4. CallerRunsPolicy 示例
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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 { // 創建線程池。線程池的"最大池大小"和"核心池大小"都為1(THREADS_SIZE)，"線程池"的阻塞隊列容量為1(CAPACITY)。 ThreadPoolExecutor pool = new ThreadPoolExecutor(THREADS_SIZE, THREADS_SIZE, 0, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(CAPACITY)); // 設置線程池的拒絕策略為"CallerRunsPolicy" pool.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy()); // 新建10個任務，并將它們添加到線程池中。 for (int i = 0; i < 10; i++) { Runnable myrun = new MyRunnable("task-"+i); pool.execute(myrun); } // 關閉線程池 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)運行結果：
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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. |
結果說明：將"線程池的拒絕策略"由DiscardPolicy修改為CallerRunsPolicy之后，當有任務添加到線程池被拒絕時，線程池會將被拒絕的任務添加到"線程池正在運行的線程"中取運行
