ThreadPoolExecutor 可扩展性

 

到这里对于这个线程池还只是介绍了一小部分。ThreadPoolExecutor 具有很强的可扩展性，不过扩展它的前提是要熟悉它的工作方式。后面的文章将会介绍如何扩展 ThreadPoolExecutor 类。 

　　java.util.concurrent.ThreadPoolExecutor 类提供了丰富的可扩展性。你可以通过创建它的子类来自定义它的行为。例如，我希望当每个任务结束之后打印一条消息，但我又无法修改任务对象，那么我可以这样写： 

　　除了 afterExecute 方法之外，ThreadPoolExecutor 类还有 beforeExecute() 和 terminated() 方法可以重写，分别是在任务执行之前和整个线程池停止之后执行。 

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); } }} 

　 　除了可以添加任务执行前后的动作之外， ThreadPoolExecutor 还允许你自定义当添加任务失败后的执行策略。你可以调用线程池的 setRejectedExecutionHandler() 方法，用自定义的 RejectedExecutionHandler 对象替换现有的策略。 ThreadPoolExecutor 提供 4 个现有的策略，分别是： 

　　ThreadPoolExecutor.AbortPolicy：表示拒绝任务并抛出异常 

　　ThreadPoolExecutor.DiscardPolicy：表示拒绝任务但不做任何动作 

　　ThreadPoolExecutor.CallerRunsPolicy：表示拒绝任务，并在调用者的线程中直接执行该任务 

　　ThreadPoolExecutor.DiscardOldestPolicy：表示先丢弃任务队列中的第一个任务，然后把这个任务加进队列。 

　　这里是一个例子： 

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

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

　　除此之外，你也可以通过实现 RejectedExecutionHandler 接口来编写自己的策略。下面是一个例子： 

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())); 

} 

}