谈谈java的线程池（创建、机制）

今天药忘吃喽~ 2023-03-02 10:52 5阅读 0赞

**目录**

Executors创建线程池默认方法

自定义线程池

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

# Executors创建线程池默认方法 #

newFixedThreadPool()方法，该方法返回一个固定数量的线程池，该方法的线程数始终不变，当有一个任务提交时，若线程池中空闲，则立即执行，若没有，则会被暂缓在一个任务队列中等待有空闲的线程去执行。

newSingleThreadExecutor()方法，创建一个线程的线程池，若空闲则执行，若没有空闲线程则暂缓在任务队列。

newCachedThreadPool()方法，返回一个可根据实际情况调整线程个数的线程池，不限制最大线程数量，若有空闲的线程则直接执行任务，若无空闲则创建线程，若无任务则不创建线程。并且每一个空闲线程会在60秒后自动回收。

newScheduledThreadPool()该方法返回一个ScheduledExecutorService对象，但该线程池可以指定线程的数量，可以定时。

前三种线程池添加线程：
    ExecutorService pool = Executors.newSingleThreadExecutor();
    pool.execute(new Thread());
    //submit和execute的区别： 第一点是submit可以传入实现Callable接口的实例对象， 第二点是submit方法有返回值
    Future f1 = pool.submit(new Thread());
    f1.get()//如果为空，则线程执行完毕了，否则会一直阻塞在这里。

//ScheduledThreadPool相当于定时任务
    class Temp extends Thread {
        public void run() {
            System.out.println("run");
        }
    }
    public class ScheduledJob {
         
        public static void main(String args[]) throws Exception {
        
          Temp command = new Temp();
            ScheduledExecutorService scheduler =  Executors.newScheduledThreadPool(1);
            
            ScheduledFuture<?> scheduleTask =  scheduler.scheduleWithFixedDelay(command, 5, 1,  TimeUnit.SECONDS);//5秒初始化之后执行一次，以后每1秒执行一次
        
        }
    }

其实底层都是new了ThreadPoolExecutor。

若Executors工厂类无法满足我们的需求，可以自己去创建自定义的线程池，其实Executors工厂类里面的创建线程方法其内部实现均是用了ThreadPoolExecutor这个类，这个类可以自定义线程。构造方法如下：

public ThreadPoolExecutor(int corePoolSize,
                　　　　　　　　　　　int maximumPoolSize,
                                  long keepAliveTime,
                                  TimeUnit unit,
                                  BlockingQueue<Runnable> workQueue,
    　　　　　　　　　　　　　　　　　　ThreadFactory threadFactory,
                                  RejectedExecutionHandler handler) {}
    * corePoolSize： 线程池核心线程数(线程池初始化存在的线程)
    * maximumPoolSize：线程池最大数
    * keepAliveTime： 空闲线程存活时间
    * unit： 时间单位
    * workQueue： 线程池所使用的缓冲队列
    * threadFactory：线程池创建线程使用的工厂
    * handler： 线程池对拒绝任务的处理策略

# 自定义线程池 #

这个构造方法对于队列是什么类型的比较关键。

**使用有界队列时**：若有新的任务需要执行，如果线程池实际线程小于corePoolSize，则优先创建线程，若大于corePoolSize，则会将任务加入队列，若队列已满，则在总线程数不大于maximumPoolSize的前提下，创建新的线程去执行新任务，若线程数大于maximumPoolSize，则执行拒绝策略。或其他自定义方式。

**使用无界队列时**：LinkedBlockingQueue。与有界队列相比，除非系统资源耗尽，否则无界的任务队列不存在任务入队失败的情况。当有新任务到来，系统的线程数小于corePoolSize时，则新建线程执行任务。当达到corePoolSize后，就不会继续增加。若后续仍有新的任务加入，而又没有空闲的线程资源，则任务直接进入队列等待。若任务创建和处理的速度差异很大，无界队列会保持快速增长，直到耗尽系统内存。

**JDK拒绝策略**：

AbortPolicy：直接抛出异常组织系统正常工作。

CallerRunsPolicy：只要线程池未关闭，该策略直接在调用者线程中，运行当前被丢弃的任务。

DiscardOldestPolicy：丢弃最老的一个请求，尝试再次提交当前任务。

DiscardPolicy：丢弃无法处理的任务，不给予任何处理。

//如果需要自定义拒绝策略可以实现RejectedExecutionHandler接口。
    public class MyTask implements Runnable {
         private int taskId;
         private String taskName;
         
         public MyTask(int taskId, String taskName){
               this.taskId = taskId;
               this.taskName = taskName;
         }
         
         public int getTaskId() {
               return taskId;
         }
         public void setTaskId(int taskId) {
               this.taskId = taskId;
         }
         public String getTaskName() {
               return taskName;
         }
         public void setTaskName(String taskName) {
               this.taskName = taskName;
         }
         @Override
         public void run() {
               try {
                    System.out.println("run taskId =" +  this.taskId);
                    Thread.sleep(5*1000);
                    //System.out.println("end taskId =" +  this.taskId);
               } catch (InterruptedException e) {
                    e.printStackTrace();
               }          
         }
         
         public String toString(){
               return Integer.toString(this.taskId);
         }
    }
    
    
    public class UseThreadPoolExecutor1 {
         public static void main(String[] args) {
               /**
                * 在使用有界队列时，若有新的任务需要执行，如果线程池实际线程数小于corePoolSize，则优先创建线程，
                * 若大于corePoolSize，则会将任务加入队列，
                * 若队列已满，则在总线程数不大于maximumPoolSize的前提下，创建新的线程，
                * 若线程数大于maximumPoolSize，则执行拒绝策略。或其他自定义方式。
                *
                */  
               ThreadPoolExecutor pool = new ThreadPoolExecutor(
                         1,                   //coreSize
                         2,                   //MaxSize
                         60,             //60
                         TimeUnit.SECONDS,
                         new ArrayBlockingQueue<Runnable>(3)             //指定一种队列 （有界队列）
                         //new LinkedBlockingQueue<Runnable>()
                         , new MyRejected()
                         //, new DiscardOldestPolicy()
                         );
               
               MyTask mt1 = new MyTask(1, "任务1");
               MyTask mt2 = new MyTask(2, "任务2");
               MyTask mt3 = new MyTask(3, "任务3");
               MyTask mt4 = new MyTask(4, "任务4");
               MyTask mt5 = new MyTask(5, "任务5");
               MyTask mt6 = new MyTask(6, "任务6");
               
               pool.execute(mt1);
               pool.execute(mt2);
               pool.execute(mt3);
               pool.execute(mt4);
               pool.execute(mt5);
               pool.execute(mt6);
               
               pool.shutdown();
               
         }
    }
    
    public class MyRejected implements RejectedExecutionHandler{
         
         public MyRejected(){
         }
         
         @Override
         public void rejectedExecution(Runnable r,  ThreadPoolExecutor executor) {
               System.out.println("自定义处理..");
               System.out.println("当前被拒绝任务为：" +  r.toString());
               
         }
    }

public class UseThreadPoolExecutor2 implements Runnable{
         private static AtomicInteger count = new AtomicInteger(0);
         
         @Override
         public void run() {
               try {
                    int temp = count.incrementAndGet();
                    System.out.println("任务" + temp);
                    Thread.sleep(2000);
               } catch (InterruptedException e) {
                    e.printStackTrace();
               }
         }
         
         public static void main(String[] args) throws Exception{
               //System.out.println(Runtime.getRuntime().availableProcessors());
               BlockingQueue<Runnable> queue =
                         //new LinkedBlockingQueue<Runnable>();
                         new ArrayBlockingQueue<Runnable>(10);
               ExecutorService executor  = new ThreadPoolExecutor(
                               5,         //core
                               10,  //max无界队列的这个参数其实没啥作用了
                               120L,      //2fenzhong
                               TimeUnit.SECONDS,
                               queue);
               
               for(int i = 0 ; i < 20; i++){
                    executor.execute(new UseThreadPoolExecutor2());
               }
               Thread.sleep(1000);
               System.out.println("queue size:" + queue.size());         //10
               Thread.sleep(2000);
         }
    }

# 线程池的方法 #

## 向线程池提交任务 ##

### execute() ###

execute()方法用于提交不需要返回值的任务Runnable，所以无法判断任务是否被线程池执行成功。

运行结果如下：

2020-10-13 14:58:09,500  INFO [pool-1-thread-1] (ExecuteDemo.java:12) - Task 1 working...
    2020-10-13 14:58:09,502  INFO [pool-1-thread-1] (ExecuteDemo.java:12) - Task 2 working...
    2020-10-13 14:58:09,502  INFO [pool-1-thread-1] (ExecuteDemo.java:12) - Task 3 working...

submit()  
submit()方法用于提交需要返回值的任务。线程池会返回一个future类型的对象，通过这个future对象可以判断任务是否执行成功，并且可以通过future的get()方法来获取返回值，get()方法会阻塞当前线程直到任务完成，get()方法也支持带超时时间的阻塞。

package com.morris.concurrent.threadpool.threadpoolexecutor.api;
    
    import lombok.extern.slf4j.Slf4j;
    
    import java.util.concurrent.*;
    import java.util.stream.IntStream;
    
    @Slf4j
    public class SubmitDemo {
        public static void main(String[] args) throws ExecutionException, InterruptedException {
            ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(1, 1, 0, TimeUnit.SECONDS, new LinkedBlockingQueue<>());
    
            Future<String> future = threadPoolExecutor.submit(() -> {
                log.info("Task begin...");
                try {
                    TimeUnit.SECONDS.sleep(3);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                log.info("Task end.");
                return "task result";
            });
    
            threadPoolExecutor.shutdown(); // 关闭线程池
    
            String result = future.get(); // 阻塞等待获取结果
            log.info("get result:{}", result);
        }
    }

运行结果如下：

2020-10-13 15:03:22,306  INFO [pool-1-thread-1] (SubmitDemo.java:14) - Task begin...
    2020-10-13 15:03:25,315  INFO [pool-1-thread-1] (SubmitDemo.java:20) - Task end.
    2020-10-13 15:03:25,316  INFO [main] (SubmitDemo.java:27) - get result:task result

### invokeAll() ###

package com.morris.concurrent.threadpool.threadpoolexecutor.api;
    
    import lombok.extern.slf4j.Slf4j;
    
    import java.util.List;
    import java.util.concurrent.*;
    import java.util.stream.Collectors;
    import java.util.stream.IntStream;
    
    @Slf4j
    public class InvokeAllDemo {
    
        public static void main(String[] args) throws InterruptedException, ExecutionException {
            ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(1, 1, 0, TimeUnit.SECONDS, new LinkedBlockingQueue<>());
    
            List<Callable<String>> tasks = IntStream.rangeClosed(1, 3).boxed().map(i -> (Callable<String>) () -> "task " + i).collect(Collectors.toList());
    
            List<Future<String>> futures = threadPoolExecutor.invokeAll(tasks);// 批量提交任务
    
            for (Future<String> future : futures) {
                log.info("result:{}", future.get()); // 阻塞获取结果
            }
    
            threadPoolExecutor.shutdown();
        }
    }

运行结果如下：

2020-10-13 16:09:00,518  INFO [main] (InvokeAllDemo.java:25) - result:task 1
    2020-10-13 16:09:00,520  INFO [main] (InvokeAllDemo.java:25) - result:task 2
    2020-10-13 16:09:00,520  INFO [main] (InvokeAllDemo.java:25) - result:task 3

### invokeAny() ###

invokeAny批量提交任务，哪个任务先执行完毕，立刻返回此任务执行结果，其它任务取消。

package com.morris.concurrent.threadpool.threadpoolexecutor.api;
    
    import lombok.extern.slf4j.Slf4j;
    
    import java.util.List;
    import java.util.Random;
    import java.util.concurrent.*;
    import java.util.stream.Collectors;
    import java.util.stream.IntStream;
    
    @Slf4j
    public class InvokeAnyDemo {
    
        public static void main(String[] args) throws ExecutionException, InterruptedException {
            ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(3, 3, 0, TimeUnit.SECONDS, new LinkedBlockingQueue<>());
    
            List<Callable<String>> tasks = IntStream.rangeClosed(1, 3).boxed().map(i -> (Callable<String>) () -> {
                log.info("task{} begin ...", i);
                TimeUnit.SECONDS.sleep(new Random().nextInt(10));
                log.info("task{} end .", i);
                return "task" + i;
            }).collect(Collectors.toList());
    
            String result = threadPoolExecutor.invokeAny(tasks); // 哪个任务执行完就立马返回，其他任务直接中断
    
            log.info("result:{}", result); // 阻塞获取结果
    
            threadPoolExecutor.shutdown();
    
        }
    }

运行结果如下：

2020-10-13 16:09:35,121  INFO [pool-1-thread-1] (InvokeAnyDemo.java:21) - task1 begin ...
    2020-10-13 16:09:35,121  INFO [pool-1-thread-2] (InvokeAnyDemo.java:21) - task2 begin ...
    2020-10-13 16:09:35,121  INFO [pool-1-thread-3] (InvokeAnyDemo.java:21) - task3 begin ...
    2020-10-13 16:09:38,125  INFO [pool-1-thread-3] (InvokeAnyDemo.java:23) - task3 end .
    2020-10-13 16:09:38,125  INFO [main] (InvokeAnyDemo.java:29) - result:task3

## 线程池的关闭 ##

### shutdown() ###

shutdown()方法会先将线程池的状态改为SHUTDOWN，然后中断空闲的线程，不会再接收新任务，但已提交任务会执行完。

package com.morris.concurrent.threadpool.threadpoolexecutor.api;
    
    import lombok.extern.slf4j.Slf4j;
    
    import java.util.Random;
    import java.util.concurrent.*;
    import java.util.stream.IntStream;
    
    /**
     * 线程池的关闭之shutdown()
     */
    @Slf4j
    public class ShutdownDemo {
    
        public static void main(String[] args) {
            ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(2, 2, 0, TimeUnit.SECONDS, new LinkedBlockingQueue<>());
    
            IntStream.rangeClosed(1, 10).forEach(i -> threadPoolExecutor.submit(() -> {
                log.info("task{} begin ...", i);
                try {
                    TimeUnit.SECONDS.sleep(new Random().nextInt(3));
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                log.info("task{} end .", i);
            }));
    
            threadPoolExecutor.shutdown();
    
            threadPoolExecutor.submit(() -> log.info("submit task after shutdown"));
        }
    }

运行结果如下：

Exception in thread "main" java.util.concurrent.RejectedExecutionException: Task java.util.concurrent.FutureTask@18769467 rejected from java.util.concurrent.ThreadPoolExecutor@46ee7fe8[Shutting down, pool size = 2, active threads = 2, queued tasks = 8, completed tasks = 0]
    	at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2063)
    	at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:830)
    	at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1379)
    	at java.util.concurrent.AbstractExecutorService.submit(AbstractExecutorService.java:112)
    	at com.morris.concurrent.threadpool.threadpoolexecutor.api.ShutdownDemo.main(ShutdownDemo.java:30)
    2020-10-13 16:54:42,637  INFO [pool-1-thread-1] (ShutdownDemo.java:19) - task1 begin ...
    2020-10-13 16:54:42,637  INFO [pool-1-thread-2] (ShutdownDemo.java:19) - task2 begin ...
    2020-10-13 16:54:43,639  INFO [pool-1-thread-2] (ShutdownDemo.java:25) - task2 end .
    2020-10-13 16:54:43,639  INFO [pool-1-thread-1] (ShutdownDemo.java:25) - task1 end .
    2020-10-13 16:54:43,640  INFO [pool-1-thread-2] (ShutdownDemo.java:19) - task3 begin ...
    2020-10-13 16:54:43,640  INFO [pool-1-thread-1] (ShutdownDemo.java:19) - task4 begin ...
    2020-10-13 16:54:43,641  INFO [pool-1-thread-1] (ShutdownDemo.java:25) - task4 end .
    2020-10-13 16:54:43,641  INFO [pool-1-thread-1] (ShutdownDemo.java:19) - task5 begin ...
    2020-10-13 16:54:44,642  INFO [pool-1-thread-1] (ShutdownDemo.java:25) - task5 end .
    2020-10-13 16:54:44,642  INFO [pool-1-thread-1] (ShutdownDemo.java:19) - task6 begin ...
    2020-10-13 16:54:45,642  INFO [pool-1-thread-2] (ShutdownDemo.java:25) - task3 end .
    2020-10-13 16:54:45,642  INFO [pool-1-thread-2] (ShutdownDemo.java:19) - task7 begin ...
    2020-10-13 16:54:46,642  INFO [pool-1-thread-2] (ShutdownDemo.java:25) - task7 end .
    2020-10-13 16:54:46,642  INFO [pool-1-thread-1] (ShutdownDemo.java:25) - task6 end .
    2020-10-13 16:54:46,642  INFO [pool-1-thread-2] (ShutdownDemo.java:19) - task8 begin ...
    2020-10-13 16:54:46,642  INFO [pool-1-thread-1] (ShutdownDemo.java:19) - task9 begin ...
    2020-10-13 16:54:47,642  INFO [pool-1-thread-2] (ShutdownDemo.java:25) - task8 end .
    2020-10-13 16:54:47,642  INFO [pool-1-thread-2] (ShutdownDemo.java:19) - task10 begin ...
    2020-10-13 16:54:47,642  INFO [pool-1-thread-2] (ShutdownDemo.java:25) - task10 end .
    2020-10-13 16:54:48,643  INFO [pool-1-thread-1] (ShutdownDemo.java:25) - task9 end .

### shutdownNow() ###

shutdownNow()会先将会先将线程池的状态改为STOP，不会再接收新任务，然后中断所有的线程，如果执行中的任务没有对中断进行处理，那么这个任务将会继续执行直至完成，如果执行中的任务对中断进行的处理，那么将按中断进行执行，最后方法会返回等待队列中未执行的任务。

package com.morris.concurrent.threadpool.threadpoolexecutor.api;
    
    import lombok.extern.slf4j.Slf4j;
    
    import java.util.List;
    import java.util.Random;
    import java.util.concurrent.*;
    import java.util.stream.IntStream;
    
    /**
     * 线程池的关闭之shutdownNow()
     */
    @Slf4j
    public class ShutdownNowDemo {
    
        public static void main(String[] args) {
            ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(2, 2, 0, TimeUnit.SECONDS, new LinkedBlockingQueue<>());
    
            IntStream.rangeClosed(1, 10).forEach(i -> threadPoolExecutor.submit(() -> {
                log.info("task{} begin ...", i);
                try {
                    TimeUnit.SECONDS.sleep(new Random().nextInt(3));
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                log.info("task{} end .", i);
            }));
    
            List<Runnable> runnables = threadPoolExecutor.shutdownNow();
            log.info("tasks size:{}", runnables.size());
        }
    }

运行结果如下：

2020-10-13 16:55:08,985  INFO [main] (ShutdownNowDemo.java:30) - tasks size:8
    2020-10-13 16:55:08,987  INFO [pool-1-thread-2] (ShutdownNowDemo.java:20) - task2 begin ...
    2020-10-13 16:55:08,987  INFO [pool-1-thread-1] (ShutdownNowDemo.java:20) - task1 begin ...
    2020-10-13 16:55:08,988  INFO [pool-1-thread-2] (ShutdownNowDemo.java:26) - task2 end .
    2020-10-13 16:55:08,989  INFO [pool-1-thread-1] (ShutdownNowDemo.java:26) - task1 end .

### awaitTermination() ###

调用shutdown()后，调用线程并不会等待所有任务运行结束,可以利用awaitTermination()方法设置一个超时时间进行阻塞等待，此方法返回了有两种情况，要么所有的任务执行完成，要么超时时间到了。

package com.morris.concurrent.threadpool.threadpoolexecutor.api;
    
    import lombok.extern.slf4j.Slf4j;
    
    import java.util.concurrent.LinkedBlockingQueue;
    import java.util.concurrent.ThreadPoolExecutor;
    import java.util.concurrent.TimeUnit;
    /**
     * threadPoolExecutor.awaitTermination()的使用
     */
    @Slf4j
    public class AwaitTerminationDemo {
        public static void main(String[] args) throws InterruptedException {
            ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(1, 1, 0, TimeUnit.SECONDS, new LinkedBlockingQueue<>());
            threadPoolExecutor.submit(() -> {
                log.info("task begin ...");
                try {
                    TimeUnit.SECONDS.sleep(3);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                log.info("task end .");
            });
            threadPoolExecutor.shutdown(); // 关闭线程池
    
            threadPoolExecutor.awaitTermination(10, TimeUnit.SECONDS);
            log.info("main thread exit");
        }
    }

运行结果如下：

2020-10-13 17:01:45,397  INFO [pool-1-thread-1] (AwaitTerminationDemo.java:16) - task begin ...
    2020-10-13 17:01:48,398  INFO [pool-1-thread-1] (AwaitTerminationDemo.java:22) - task end .
    2020-10-13 17:01:48,398  INFO [main] (AwaitTerminationDemo.java:27) - main thread exit

## 创建线程的工厂ThreadFactory ##

可以自定义ThreadFactory为线程池中的线程取一个有意义的名字，方便后面快速定位问题。

package com.morris.concurrent.threadpool.threadpoolexecutor.construct;
    
    import lombok.extern.slf4j.Slf4j;
    
    import java.util.concurrent.LinkedBlockingQueue;
    import java.util.concurrent.ThreadFactory;
    import java.util.concurrent.ThreadPoolExecutor;
    import java.util.concurrent.TimeUnit;
    import java.util.concurrent.atomic.AtomicInteger;
    
    /**
     * 使用ThreadFactory自定义线程名
     */
    @Slf4j
    public class ThreadFactoryDemo {
        public static void main(String[] args) {
            ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(1, 1, 0, TimeUnit.SECONDS,
                    new LinkedBlockingQueue<>(), new ThreadFactory() {
    
                private AtomicInteger idx = new AtomicInteger(1);
    
                private static final String THREAD_NAME_PREFIX = "mythread-pool-";
    
                @Override
                public Thread newThread(Runnable r) {
                    return new Thread(r, THREAD_NAME_PREFIX + idx.getAndIncrement());
                }
            });
    
            threadPoolExecutor.submit(()->{
                log.info("task begin ...");
                try {
                    TimeUnit.SECONDS.sleep(3);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                log.info("task end .");
            });
    
            threadPoolExecutor.shutdown();
        }
    }