java四种线程池的使用

小鱼儿 2022-07-14 15:53 234阅读 0赞

ava通过Executors提供四种线程池，分别为：   
**newCachedThreadPool**创建一个可缓存线程池，如果线程池长度超过处理需要，可灵活回收空闲线程，若无可回收，则新建线程。   
**newFixedThreadPool**  创建一个定长线程池，可控制线程最大并发数，超出的线程会在队列中等待。   
**newScheduledThreadPool** 创建一个定长线程池，支持定时及周期性任务执行。

**newSingleThreadExecutor** 创建一个单线程化的线程池，它只会用唯一的工作线程来执行任务，保证所有任务按照指定顺序(FIFO, LIFO, 优先级)执行。

**(1) newCachedThreadPool**   
创建一个可缓存线程池，如果线程池长度超过处理需要，可灵活回收空闲线程，若无可回收，则新建线程。示例代码如下：

package test;
    import java.util.concurrent.ExecutorService;
    import java.util.concurrent.Executors;
    public class ThreadPoolExecutorTest {
     public static void main(String[] args) {
      ExecutorService cachedThreadPool = Executors.newCachedThreadPool();
      for (int i = 0; i < 10; i++) {
       final int index = i;
       try {
        Thread.sleep(index * 1000);
       } catch (InterruptedException e) {
        e.printStackTrace();
       }
       cachedThreadPool.execute(new Runnable() {
        public void run() {
         System.out.println(index);
        }
       });
      }
     }
    }

线程池为无限大，当执行第二个任务时第一个任务已经完成，会复用执行第一个任务的线程，而不用每次新建线程。   
    
(2) newFixedThreadPool   
创建一个定长线程池，可控制线程最大并发数，超出的线程会在队列中等待。示例代码如下：

package test;
    import java.util.concurrent.ExecutorService;
    import java.util.concurrent.Executors;
    public class ThreadPoolExecutorTest {
     public static void main(String[] args) {
      ExecutorService fixedThreadPool = Executors.newFixedThreadPool(3);
      for (int i = 0; i < 10; i++) {
       final int index = i;
       fixedThreadPool.execute(new Runnable() {
        public void run() {
         try {
          System.out.println(index);
          Thread.sleep(2000);
         } catch (InterruptedException e) {
          e.printStackTrace();
         }
        }
       });
      }
     }
    }

因为线程池大小为3，每个任务输出index后sleep 2秒，所以每两秒打印3个数字。   
定长线程池的大小最好根据系统资源进行设置。如Runtime.getRuntime().availableProcessors()

(3)  newScheduledThreadPool   
创建一个定长线程池，支持定时及周期性任务执行。延迟执行示例代码如下：

package test;
    import java.util.concurrent.Executors;
    import java.util.concurrent.ScheduledExecutorService;
    import java.util.concurrent.TimeUnit;
    public class ThreadPoolExecutorTest {
     public static void main(String[] args) {
      ScheduledExecutorService scheduledThreadPool = Executors.newScheduledThreadPool(5);
      scheduledThreadPool.schedule(new Runnable() {
       public void run() {
        System.out.println("delay 3 seconds");
       }
      }, 3, TimeUnit.SECONDS);
     }
    }

表示延迟3秒执行。

定期执行示例代码如下：

package test;
    import java.util.concurrent.Executors;
    import java.util.concurrent.ScheduledExecutorService;
    import java.util.concurrent.TimeUnit;
    public class ThreadPoolExecutorTest {
     public static void main(String[] args) {
      ScheduledExecutorService scheduledThreadPool = Executors.newScheduledThreadPool(5);
      scheduledThreadPool.scheduleAtFixedRate(new Runnable() {
       public void run() {
        System.out.println("delay 1 seconds, and excute every 3 seconds");
       }
      }, 1, 3, TimeUnit.SECONDS);
     }
    }

表示延迟1秒后每3秒执行一次。

(4) newSingleThreadExecutor   
创建一个单线程化的线程池，它只会用唯一的工作线程来执行任务，保证所有任务按照指定顺序(FIFO, LIFO, 优先级)执行。示例代码如下：

package test;
    import java.util.concurrent.ExecutorService;
    import java.util.concurrent.Executors;
    public class ThreadPoolExecutorTest {
     public static void main(String[] args) {
      ExecutorService singleThreadExecutor = Executors.newSingleThreadExecutor();
      for (int i = 0; i < 10; i++) {
       final int index = i;
       singleThreadExecutor.execute(new Runnable() {
        public void run() {
         try {
          System.out.println(index);
          Thread.sleep(2000);
         } catch (InterruptedException e) {
          e.printStackTrace();
         }
        }
       });
      }
     }
    }

结果依次输出，相当于顺序执行各个任务。

你可以使用JDK自带的监控工具来监控我们创建的线程数量，运行一个不终止的线程，创建指定量的线程，来观察：   
工具目录： **C:\\Program Files\\Java\\jdk1.6.0\_06\\bin\\jconsole.exe**   
运行程序做稍微修改：

package test;
    import java.util.concurrent.ExecutorService;
    import java.util.concurrent.Executors;
    public class ThreadPoolExecutorTest {
     public static void main(String[] args) {
      ExecutorService singleThreadExecutor = Executors.newCachedThreadPool();
      for (int i = 0; i < 100; i++) {
       final int index = i;
       singleThreadExecutor.execute(new Runnable() {
        public void run() {
         try {
          while(true) {
           System.out.println(index);
           Thread.sleep(10 * 1000);
          }
         } catch (InterruptedException e) {
          e.printStackTrace();
         }
        }
       });
       try {
        Thread.sleep(500);
       } catch (InterruptedException e) {
        e.printStackTrace();
       }
      }
     }
    }

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