新的线程加入后，如果正在运行的线程达到了上限，则会阻塞，直到有了空闲的线程来运行。

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

/**
 * 线程池newFixedThreadPool的使用。
 *
 * @author 赵学庆，Java世纪网(java2000.net)
 *
 */
public class ExecutorTest {
  public static void main(String args[]) {
    Random random = new Random();
    // 建立一个容量为3的固定尺寸的线程池
    ExecutorService executor = Executors.newFixedThreadPool(3);
    // 判断可是线程池可以结束
    int waitTime = 500;
    for (int i = 0; i < 10; i++) {
      String name = "线程 " + i;
      int time = random.nextInt(1000);
      waitTime += time;
      Runnable runner = new ExecutorThread(name, time);
      System.out.println("增加: " + name + " / " + time);
      executor.execute(runner);
    }
    try {
      Thread.sleep(waitTime);
      executor.shutdown();
      executor.awaitTermination(waitTime, TimeUnit.MILLISECONDS);
    } catch (InterruptedException ignored) {
    }
  }
}

class ExecutorThread implements Runnable {
  private final String name;
  private final int delay;

  public ExecutorThread(String name, int delay) {
    this.name = name;
    this.delay = delay;
  }

  public void run() {

    System.out.println("启动: " + name);
    try {
      Thread.sleep(delay);
    } catch (InterruptedException ignored) {
    }
    System.out.println("完成: " + name);
  }
}