Java並行newFixedThreadPool方法

可以通過呼叫Executors類的static newFixedThreadPool()方法獲得一個固定執行緒池。

語法

ExecutorService fixedPool = Executors.newFixedThreadPool(2);

其中，

• 最多2個執行緒將處於活動狀態。
• 如果提交了兩個以上的執行緒，那麼它們將保持在佇列中，直到執行緒可用。
• 如果一個執行緒由於執行關閉期間的失敗而終止，則執行器尚未被呼叫，則建立一個新執行緒。
• 執行緒會一直存在，直到池關閉。

範例

以下TestThread程式顯示線上程環境中使用newFixedThreadPool方法。

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

public class TestThread {

   public static void main(final String[] arguments) throws InterruptedException {
      ExecutorService executor = Executors.newFixedThreadPool(2);

      // Cast the object to its class type
      ThreadPoolExecutor pool = (ThreadPoolExecutor) executor;

      //Stats before tasks execution
      System.out.println("Core threads: " + pool.getCorePoolSize());
      System.out.println("Largest executions: "
         + pool.getLargestPoolSize());
      System.out.println("Maximum allowed threads: "
         + pool.getMaximumPoolSize());
      System.out.println("Current threads in pool: "
         + pool.getPoolSize());
      System.out.println("Currently executing threads: "
         + pool.getActiveCount());
      System.out.println("Total number of threads(ever scheduled): "
         + pool.getTaskCount());

      executor.submit(new Task());
      executor.submit(new Task());

      //Stats after tasks execution
      System.out.println("Core threads: " + pool.getCorePoolSize());
      System.out.println("Largest executions: "
         + pool.getLargestPoolSize());
      System.out.println("Maximum allowed threads: "
         + pool.getMaximumPoolSize());
      System.out.println("Current threads in pool: "
         + pool.getPoolSize());
      System.out.println("Currently executing threads: "
         + pool.getActiveCount());
      System.out.println("Total number of threads(ever scheduled): "
         + pool.getTaskCount());

      executor.shutdown();
   }  

   static class Task implements Runnable {

      public void run() {
         try {
            Long duration = (long) (Math.random() * 5);
            System.out.println("Running Task! Thread Name: " + Thread.currentThread().getName());
               TimeUnit.SECONDS.sleep(duration);
            System.out.println("Task Completed! Thread Name: "+ Thread.currentThread().getName());
         } 
         catch (InterruptedException e) {
            e.printStackTrace();
         }
      }
   }
}

這將產生以下結果 -

Core threads: 2
Largest executions: 0
Maximum allowed threads: 2
Current threads in pool: 0
Currently executing threads: 0
Total number of threads(ever scheduled): 0
Core threads: 2
Largest executions: 2
Maximum allowed threads: 2
Current threads in pool: 2
Currently executing threads: 1
Total number of threads(ever scheduled): 1
Running Task! Thread Name: pool-1-thread-1
Running Task! Thread Name: pool-1-thread-2
Task Completed! Thread Name: pool-1-thread-1
Task Completed! Thread Name: pool-1-thread-2