《学习多线程02》线程池的概念和使用

痛定思痛。 2023-06-28 11:19 22阅读 0赞

# 核心线程类ThreadPoolExecutor #

public ThreadPoolExecutor(int corePoolSize,
                                  int maximumPoolSize,
                                  long keepAliveTime,
                                  TimeUnit unit,
                                  BlockingQueue<Runnable> workQueue,
                                  ThreadFactory threadFactory,
                                  RejectedExecutionHandler handler)

corePoolSize：线程池的大小（核心线程）

maximumPoolSize：线程池能创建的最大线程数（非核心线程）

keepAliveTime：非核心线程能够空闲的最长时间，超过时间线程终止

unit：时间单位，配合 ↑ 一起用

workQueue：缓存队列，存放等待被执行的任务

threadFactory：线程工厂，用来创建线程，一般有三种  
（1）ArrayBlockQueue  
（2）LinkedBlockingQueue  
（3）SynchronousQueue

handler：拒绝处理策略（4种），线程数量大于最大线程数就会采用

**拒绝处理策略**  
1.ThreadPoolExecutor.AbortPolicy:丢弃任务并抛出RejectedExecutionException异常。

2.ThreadPoolExecutor.DiscardPolicy：也是丢弃任务，但是不抛出异常。

3.ThreadPoolExecutor.DiscardOldestPolicy：丢弃队列最前面的任务，然后重新尝试执行任务（重复此过程）

4.ThreadPoolExecutor.CallerRunsPolicy：由调用线程处理该任务

关闭线程池的两个方法shutdown和submit，区别是后者会有一个结果返回，一般使用第一种。

# Executor #

线程池的最上层接口Executor，这个接口定义了一个方法execute(Runnable command)，然后会被线程池实现，execute负责传递任务。

**关于该方法的四个总结**：  
1.如果当前线程池中的线程数目小于corePoolSize，则每来一个任务，就会创建一个线程去执行这个任务。

2.如果当前线程池中的线程数目>=corePoolSize，则每来一个任务，会尝试将其添加到任务缓存队列当中，若添加成功，则该任务会等待空闲线程将其取出去执行；若添加失败（一般来说是任务缓存队列已满），则会尝试创建新的线程去执行这个任务；

3.如果当前线程池中的线程数目达到maximumPoolSize，则会采取任务拒绝策略进行处理；

4.如果线程池中的线程数量大于 corePoolSize时，如果某线程空闲时间超过keepAliveTime，线程将被终止，直至线程池中的线程数目不大于corePoolSize；如果允许为核心池中的线程设置存活时间，那么核心池中的线程空闲时间超过keepAliveTime，线程也会被终止。

# 线程池状态 #

1.RUNNING：线程池创建后的状态

2.SHUTDOWN：调用shutdown处于SHUTDOWN状态，不能接收新的任务，但是需要等待队列中的任务完成

3.STOP：调用shutdownNow处于STOP状态，不能接收新的任务，尝试终止正在执行的任务

4.TERMINATED(结束)：线程池处于SHUTDOWN或STOP状态，所有工作线程摧毁，队列清空或执行结束，会进入这个状态

# Executors #

一般通过Executors来选择线程池

*      1.Executors.newCachedThreadPool() 缓冲池为Integer.MAX_VALUE （相当于无线）
 *      2.Executors.newSingleThreadExecutor() 缓冲池为1
 *      3.Executors.newFixedThreadPool(int n) 缓冲池为n
 *      4.Executors.newScheduledThreadPool(int n) 缓冲池为n，且支持时间调度

# 线程池使用 #

首先先定义三个任务  
**TaskOne**

public class TaskOne implements Runnable{
        private volatile String taskOneName;
        TaskOne(String name){
            taskOneName = name;
            System.out.println("TaskOne名字为："+taskOneName);
        }
        @Override
        public void run() {
            System.out.println(taskOneName+"，开始执行");
            try {
                Thread.sleep(3000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(taskOneName+"睡了3s");
        }
    }

**TaskTwo**

public class TaskTwo implements Runnable {
        private volatile String taskTwoName;
        TaskTwo(String name){
            taskTwoName = name;
            System.out.println("TaskTwo名字为："+taskTwoName);
        }
        @Override
        public void run() {
            System.out.println(taskTwoName+"，开始执行");
            try {
                Thread.sleep(5000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(taskTwoName+"睡了5s");
        }
    }

**TaskThree**

public class TaskThree implements Runnable{
        private volatile String taskThreeName;
        TaskThree(String name){
            taskThreeName = name;
            System.out.println("TaskThree名字为："+taskThreeName);
        }
        @Override
        public void run() {
            System.out.println(taskThreeName+"，开始执行");
            try {
                Thread.sleep(7000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(taskThreeName+"睡了7s");
        }
    }

## CachedThreadPool ##

public class CachedThreadPoolTest {
        public static void main(String[] args) {
            ExecutorService executorService = Executors.newCachedThreadPool();
            for (int i=0;i<2;i++){
                executorService.execute(new TaskOne("One"));
                executorService.execute(new TaskTwo("Two"));
                executorService.execute(new TaskThree("Three"));
            }
            executorService.shutdown();
            System.out.println("任务全部执行完毕(假)");
            while (!executorService.isTerminated()){
            }
            System.out.println("执行结束(真!)");
        }
    }

执行结果  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2JheHhkd3lw_size_16_color_FFFFFF_t_70]

## FixedThreadPool ##

public class FixedThreadPoolTest {
        public static void main(String[] args) {
            ExecutorService executorService = Executors.newFixedThreadPool(2);
            for (int i=0;i<2;i++){
                executorService.execute(new TaskOne("One"));
                executorService.execute(new TaskTwo("Two"));
                executorService.execute(new TaskThree("Three"));
            }
            executorService.shutdown();
            System.out.println("执行结束(假结束)");
            while (!executorService.isTerminated()){
            }
            System.out.println("执行结束(真!)");
        }
    }

执行结果  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2JheHhkd3lw_size_16_color_FFFFFF_t_70 1]

## SingleThreePool ##

public class SingleThreePoolTest {
        public static void main(String[] args) {
            ExecutorService executorService = Executors.newSingleThreadExecutor();
            executorService.execute(new TaskOne("one"));
            executorService.execute(new TaskTwo("two"));
            executorService.execute(new TaskThree("three"));
            executorService.shutdown();
            while (!executorService.isTerminated()){
            }
            System.out.println("执行结束（真）");
        }
    }

执行结果  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2JheHhkd3lw_size_16_color_FFFFFF_t_70 2]

## ScheduledThread ##

public class ScheduledThreadPoolTest {
        public static void main(String[] args) {
            //周期时间
            scheduledThreadPoolTestOne();
        }
    
        public static void scheduledThreadPoolTestOne(){
            ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(2);
            scheduledExecutorService.schedule(new TaskOne("One"),1000,TimeUnit.MILLISECONDS);
            scheduledExecutorService.schedule(new TaskTwo("Two"),2000,TimeUnit.MILLISECONDS);
            scheduledExecutorService.schedule(new TaskThree("Three"),3000,TimeUnit.MILLISECONDS);
            scheduledExecutorService.shutdown();
            while (!scheduledExecutorService.isTerminated()){
            }
            System.out.println("执行结束(真!)");
        }
    }

执行结果  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2JheHhkd3lw_size_16_color_FFFFFF_t_70 3]

## ThreadPoolExecutor ##

public class ThreadPoolExecutorTest {
    
        public static void main(String[] args) {
            int corePoolSize = 1;
            int maximumPoolSize = 3;
            long keepAliveTime = 1L;
            threadPoolExecutorOneTest(corePoolSize,maximumPoolSize,keepAliveTime);
    //        threadPoolExecutorTwoTest(corePoolSize,maximumPoolSize,keepAliveTime);
        }
        //无拒绝处理策略
        public static void threadPoolExecutorOneTest(int corePoolSize,int maximumPoolSize,long keepAliveTime) {
            ThreadPoolExecutor threadPoolExecutorOne = new ThreadPoolExecutor(corePoolSize, maximumPoolSize, keepAliveTime, TimeUnit.SECONDS, new ArrayBlockingQueue<>(3));
            threadPoolExecutorOne.execute(new TaskOne("One1"));
            threadPoolExecutorOne.execute(new TaskOne("One2"));
            threadPoolExecutorOne.execute(new TaskOne("One3"));
            threadPoolExecutorOne.execute(new TaskTwo("Two1"));
            threadPoolExecutorOne.execute(new TaskThree("Three"));
            threadPoolExecutorOne.execute(new TaskTwo("Two2"));
            threadPoolExecutorOne.execute(new TaskTwo("Two3"));
    
            threadPoolExecutorOne.shutdown();
            while (!threadPoolExecutorOne.isTerminated()) {
            }
            System.out.println("threadPoolExecutor执行完毕");
        }
    
        //添加拒绝处理策略（线程数超过最大线程+队列容量）
        public static void threadPoolExecutorTwoTest(int corePoolSize,int maximumPoolSize,long keepAliveTime) {
            ThreadPoolExecutor threadPoolExecutorTwo = new ThreadPoolExecutor(corePoolSize, maximumPoolSize, keepAliveTime, TimeUnit.SECONDS, new ArrayBlockingQueue<>(2), Executors.defaultThreadFactory(), new ThreadPoolExecutor.DiscardOldestPolicy());
            threadPoolExecutorTwo.execute(new TaskOne("One"));
            threadPoolExecutorTwo.execute(new TaskOne("1"));
            threadPoolExecutorTwo.execute(new TaskOne("2"));
            threadPoolExecutorTwo.execute(new TaskTwo("Two 1"));
            threadPoolExecutorTwo.execute(new TaskThree("Three"));
            threadPoolExecutorTwo.execute(new TaskTwo("Two2"));
            threadPoolExecutorTwo.execute(new TaskTwo("Two3"));
            threadPoolExecutorTwo.shutdown();
            while (!threadPoolExecutorTwo.isTerminated()) {
            }
            System.out.println("threadPoolExecutor执行完毕");
        }
    }

**注意**：线程数不能超过最大线程数+队列，若超过，执行拒绝策略，核心线程为1个，最大为5个，队列容量3个，核心线程先执行，后面任务进队列，若队列满，使用子线程（最大线程减核心）执行。

继续跟进↓  
[《学习多线程03》关键字volatile][03_volatile]

--------------------

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![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2JheHhkd3lw_size_16_color_FFFFFF_t_70 4]

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2JheHhkd3lw_size_16_color_FFFFFF_t_70]: https://img-blog.csdnimg.cn/202001090923058.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2JheHhkd3lw,size_16,color_FFFFFF,t_70
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[03_volatile]: https://blog.csdn.net/baxxdwyp/article/details/103959753
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