本章介紹線程池的拒絕策略。內容包括：
拒絕策略介紹
拒絕策略對比和示例
轉載請注明出處：http://www.cnblogs.com/skywang12345/p/3512947.html
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，可能是由於：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
2. DiscardOldestPolicy 示例
3. AbortPolicy 示例
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.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 // 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)。
15 ThreadPoolExecutor pool = new ThreadPoolExecutor(THREADS_SIZE, THREADS_SIZE, 0, TimeUnit.SECONDS,
16 new ArrayBlockingQueue<Runnable>(CAPACITY));
17 // 設置線程池的拒絕策略為"丟棄"
18 pool.setRejectedExecutionHandler(new ThreadPoolExecutor.DiscardPolicy());
19
20 // 新建10個任務，並將它們添加到線程池中。
21 for (int i = 0; i < 10; i++) {
22 Runnable myrun = new MyRunnable("task-"+i);
23 pool.execute(myrun);
24 }
25 // 關閉線程池
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 }
運行結果：
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)，這意味着"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。這也意味着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！
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 // 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)。
15 ThreadPoolExecutor pool = new ThreadPoolExecutor(THREADS_SIZE, THREADS_SIZE, 0, TimeUnit.SECONDS,
16 new ArrayBlockingQueue<Runnable>(CAPACITY));
17 // 設置線程池的拒絕策略為"DiscardOldestPolicy"
18 pool.setRejectedExecutionHandler(new ThreadPoolExecutor.DiscardOldestPolicy());
19
20 // 新建10個任務，並將它們添加到線程池中。
21 for (int i = 0; i < 10; i++) {
22 Runnable myrun = new MyRunnable("task-"+i);
23 pool.execute(myrun);
24 }
25 // 關閉線程池
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 }
運行結果：
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。
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 // 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)。
16 ThreadPoolExecutor pool = new ThreadPoolExecutor(THREADS_SIZE, THREADS_SIZE, 0, TimeUnit.SECONDS,
17 new ArrayBlockingQueue<Runnable>(CAPACITY));
18 // 設置線程池的拒絕策略為"拋出異常"
19 pool.setRejectedExecutionHandler(new ThreadPoolExecutor.AbortPolicy());
20
21 try {
22
23 // 新建10個任務，並將它們添加到線程池中。
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 // 關閉線程池
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)運行結果：
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.
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。
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 // 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)。
15 ThreadPoolExecutor pool = new ThreadPoolExecutor(THREADS_SIZE, THREADS_SIZE, 0, TimeUnit.SECONDS,
16 new ArrayBlockingQueue<Runnable>(CAPACITY));
17 // 設置線程池的拒絕策略為"CallerRunsPolicy"
18 pool.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());
19
20 // 新建10個任務，並將它們添加到線程池中。
21 for (int i = 0; i < 10; i++) {
22 Runnable myrun = new MyRunnable("task-"+i);
23 pool.execute(myrun);
24 }
25
26 // 關閉線程池
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)運行結果：
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.
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。