Java執行緒池

Java執行緒池表示一組正在等待作業並重複使用多次的工作執行緒。

線上程池的情況下，建立一組固定大小的執行緒。 來自執行緒池中的執行緒被拉出並由服務提供者分配作業。 完成作業後，執行緒再次包含線上程池中。

Java執行緒池的優點

提供了更好的效能，因為不需要重新建立新執行緒，所以節省了時間。

實時使用

在Servlet和JSP中使用，容器建立一個執行緒池來處理請求。

Java執行緒池的範例
下面來看看使用ExecutorService和Executors的java執行緒池的簡單範例。

檔案: WorkerThread.java

package com.yiibai;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

class WorkerThread implements Runnable {
    private String message;

    public WorkerThread(String s) {
        this.message = s;
    }

    public void run() {
        System.out.println(Thread.currentThread().getName() + " (Start) message = " + message);
        processmessage();// call processmessage method that sleeps the thread for 2 seconds
        System.out.println(Thread.currentThread().getName() + " (End)");// prints thread name
    }

    private void processmessage() {
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

檔案: WorkerThread.java

package com.yiibai;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class TestThreadPool {  
    public static void main(String[] args) {  
       ExecutorService executor = Executors.newFixedThreadPool(5);//creating a pool of 5 threads  
       for (int i = 0; i < 10; i++) {  
           Runnable worker = new WorkerThread("" + i);  
           executor.execute(worker);//calling execute method of ExecutorService  
         }  
       executor.shutdown();  
       while (!executor.isTerminated()) {   }  

       System.out.println("Finished all threads");  
   }  
}

執行上面範例程式碼，得到以下結果：

pool-1-thread-2 (Start) message = 1
pool-1-thread-4 (Start) message = 3
pool-1-thread-5 (Start) message = 4
pool-1-thread-3 (Start) message = 2
pool-1-thread-1 (Start) message = 0
pool-1-thread-5 (End)
pool-1-thread-2 (End)
pool-1-thread-4 (End)
pool-1-thread-1 (End)
pool-1-thread-3 (End)
pool-1-thread-1 (Start) message = 6
pool-1-thread-3 (Start) message = 7
pool-1-thread-2 (Start) message = 5
pool-1-thread-4 (Start) message = 8
pool-1-thread-5 (Start) message = 9
pool-1-thread-3 (End)
pool-1-thread-5 (End)
pool-1-thread-4 (End)
pool-1-thread-1 (End)
pool-1-thread-2 (End)
Finished all threads