ThreadPoolExecutor 可擴展性

 

到這裏對於這個線程池還只是介紹了一小部分。ThreadPoolExecutor 具有很強的可擴展性，不過擴展它的前提是要熟悉它的工作方式。後面的文章將會介紹如何擴展 ThreadPoolExecutor 類。 

　　java.util.concurrent.ThreadPoolExecutor 類提供了豐富的可擴展性。你可以通過創建它的子類來自定義它的行爲。例如，我希望當每個任務結束之後打印一條消息，但我又無法修改任務對象，那麼我可以這樣寫： 

　　除了 afterExecute 方法之外，ThreadPoolExecutor 類還有 beforeExecute() 和 terminated() 方法可以重寫，分別是在任務執行之前和整個線程池停止之後執行。 

package concurrent; import java.util.concurrent.BlockingQueue; import java.util.concurrent.ThreadPoolExecutor; import java.util.concurrent.TimeUnit; public class ThreadPoolExecutorExtention extends ThreadPoolExecutor { public ThreadPoolExecutorExtention(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue) { super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue); } @Override public void beforeExecute(Thread t, Runnable r){ System.out.println("===beforeExecute==="); super.beforeExecute(t, r); } @Override public void afterExecute(Runnable r, Throwable t){ System.out.println("===afterExecute==="); super.afterExecute(r, t); } } package concurrent; import java.util.LinkedList; import java.util.Queue; import java.util.concurrent.ExecutorService; import java.util.concurrent.LinkedBlockingQueue; import java.util.concurrent.TimeUnit; public class ThreadPoolExtentionManager implements Runnable { private Queue<Runnable> tasks = new LinkedList<Runnable>(); private final ExecutorService pool; private static Object lock = new Object(); public ThreadPoolExtentionManager(){ pool = new ThreadPoolExecutorExtention(10, 50,0L , TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>()); } /* (non-Javadoc) * @see java.lang.Runnable#run() */ public void run() { while(true){ if(tasks.size() > 0){ pool.execute(tasks.poll()); } else{ try { synchronized (lock) { lock.wait(); } } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } } } } public void executeTasks(){ int size = tasks.size(); for (int i = 0; i < size; i++) { pool.execute(tasks.poll()); System.out.println("===============" + i); } } public void shutDown(){ pool.shutdown(); System.out.println("======shut down========"); } /** * @return the tasks */ public Queue<Runnable> getTasks() { return tasks; } /** * @param tasks the tasks to set */ public void addTask(Runnable task) { this.tasks.add(task); synchronized (lock) { lock.notify(); } } } /** * */ package concurrent; /** * @author Administrator * */ public class SimpleTask implements Runnable { /* (non-Javadoc) * @see java.lang.Runnable#run() */ public void run() { System.out.println("hello=" + Thread.currentThread().getName()); try { Thread.sleep(1000); } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } } } package concurrent; public class Main { /** * @param args */ public static void main(String[] args) { // TODO Auto-generated method stub Main main = new Main(); main.testPoolExtention(); System.out.println("the end"); } private void testPoolExtention(){ ThreadPoolExtentionManager pool = new ThreadPoolExtentionManager(); new Thread(pool).start(); SimpleTask task = null; for (int i = 0; i < 50; i++) { task = new SimpleTask(); pool.addTask(task); } } }  

　 　除了可以添加任務執行前後的動作之外， ThreadPoolExecutor 還允許你自定義當添加任務失敗後的執行策略。你可以調用線程池的 setRejectedExecutionHandler() 方法，用自定義的 RejectedExecutionHandler 對象替換現有的策略。 ThreadPoolExecutor 提供 4 個現有的策略，分別是： 

　　ThreadPoolExecutor.AbortPolicy：表示拒絕任務並拋出異常 

　　ThreadPoolExecutor.DiscardPolicy：表示拒絕任務但不做任何動作 

　　ThreadPoolExecutor.CallerRunsPolicy：表示拒絕任務，並在調用者的線程中直接執行該任務 

　　ThreadPoolExecutor.DiscardOldestPolicy：表示先丟棄任務隊列中的第一個任務，然後把這個任務加進隊列。 

　　這裏是一個例子： 

ThreadPoolExecutor executor = new ThreadPoolExecutor(size, maxSize, 1, TimeUnit.DAYS, queue); 

executor.setRejectedExecutionHandler(new ThreadPoolExecutor.DiscardPolicy()); 

　　除此之外，你也可以通過實現 RejectedExecutionHandler 接口來編寫自己的策略。下面是一個例子： 

ThreadPoolExecutor executor = new ThreadPoolExecutor(3, 6, 1, TimeUnit.SECONDS, queue, 

new RejectedExecutionHandler() { 

public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) { 

System.out.println(String.format("Task %d rejected.", r.hashCode())); 

} 

}