ThreadPoolExecutor线程池7个参数的含义、任务的提交、线程池的关闭和拒绝策略

柔情只为你懂 2022-12-14 12:27 97阅读 0赞

# ThreadPoolExecutor线程池的使用 #

## 线程池的创建 ##

我们可以通过ThreadPoolExecutor来创建一个线程池。

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

创建一个线程池时需要输入几个参数，如下：

1.  corePoolSize（最大核心线程的数量）：当提交一个任务到线程池时，线程池会创建一个线程来执行任务，即使此时其他核心线程空闲也会创建新的核心线程，直到核心线程的数量等于最大核心线程的数量corePoolSize。如果调用了线程池的prestartAllCoreThreads()方法，线程池会提前创建并启动所有核心线程。
2.  maximumPoolSize（最大线程的数量）：线程池允许创建的最大线程数。如果阻塞队列满了，并且已创建的线程数小于最大线程数，则线程池会再创建新的线程执行任务。值得注意的是，如果使用了无界队列这个参数就没有意义。
3.  keepAliveTime（非核心线程的空闲时间）：非核心线程等待keepAliveTime时间后还没有获取到任务就会自动销毁。
4.  unit：空闲时间的单位。
5.  workQueue（任务队列）：用于保存等待执行的任务的阻塞队列。
6.  ThreadFactory：用于创建线程的工厂，可以通过线程工厂给每个创建出来的线程设置更有意义的名字，后期方便定位问题。
7.  RejectedExecutionHandler（拒绝策略）：当线程池中的线程达到maximumPoolSize，说明线程池处于饱和状态，此时仍然有任务提交过来，那么必须采取一种策略处理提交的新任务。

## 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();
        }
    }

## 拒绝策略 ##

当线程池中的线程达到maximumPoolSize，说明线程池处于饱和状态，此时仍然有任务提交过来，那么必须采取一种策略处理提交的新任务。

### AbortPolicy ###

默认的拒绝策略为AbortPolicy，表示无法处理新任务时抛出异常。

package com.morris.concurrent.threadpool.threadpoolexecutor.construct;
    
    import lombok.extern.slf4j.Slf4j;
    
    import java.util.concurrent.ArrayBlockingQueue;
    import java.util.concurrent.ThreadPoolExecutor;
    import java.util.concurrent.TimeUnit;
    import java.util.stream.IntStream;
    
    /** * 拒绝策略之AbortPolicy的使用，此策略简单粗暴，直接抛出异常 */
    @Slf4j
    public class AbortPolicyDemo { 
    
        public static void main(String[] args) { 
            // 核心线程数为1，最大线程数为1，阻塞队列大小为1，此线程池最多同时容纳2个任务，提交第三个任务就会执行拒绝策略
            ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(1, 1, 0, TimeUnit.SECONDS, new ArrayBlockingQueue<>(1));
    
            IntStream.rangeClosed(1, 3).forEach(i->{ 
                threadPoolExecutor.submit(()->{ 
                    log.info("task begin ...");
                    try { 
                        TimeUnit.SECONDS.sleep(3);
                    } catch (InterruptedException e) { 
                        e.printStackTrace();
                    }
                    log.info("task end .");
                });
            });
        }
    }

运行结果如下：

Exception in thread "main" java.util.concurrent.RejectedExecutionException: Task java.util.concurrent.FutureTask@b1a58a3 rejected from java.util.concurrent.ThreadPoolExecutor@6438a396[Running, pool size = 1, active threads = 1, queued tasks = 1, completed tasks = 0]
    	at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2063)
    	at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:830)
    	at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1379)
    	at java.util.concurrent.AbstractExecutorService.submit(AbstractExecutorService.java:112)
    	at com.morris.concurrent.threadpool.threadpoolexecutor.construct.AbortPolicyDemo.lambda$main$1(AbortPolicyDemo.java:20)
    	at java.util.stream.Streams$RangeIntSpliterator.forEachRemaining(Streams.java:114)
    	at java.util.stream.IntPipeline$Head.forEach(IntPipeline.java:557)
    	at com.morris.concurrent.threadpool.threadpoolexecutor.construct.AbortPolicyDemo.main(AbortPolicyDemo.java:19)
    2020-10-13 17:17:50,504  INFO [pool-1-thread-1] (AbortPolicyDemo.java:21) - task begin ...
    2020-10-13 17:17:53,506  INFO [pool-1-thread-1] (AbortPolicyDemo.java:27) - task end .
    2020-10-13 17:17:53,506  INFO [pool-1-thread-1] (AbortPolicyDemo.java:21) - task begin ...
    2020-10-13 17:17:56,507  INFO [pool-1-thread-1] (AbortPolicyDemo.java:27) - task end .

### CallerRunsPolicy ###

CallerRunsPolicy策略会将任务交由调用者线程来执行。

package com.morris.concurrent.threadpool.threadpoolexecutor.construct;
    
    import lombok.extern.slf4j.Slf4j;
    
    import java.util.concurrent.ArrayBlockingQueue;
    import java.util.concurrent.ThreadPoolExecutor;
    import java.util.concurrent.TimeUnit;
    import java.util.stream.IntStream;
    
    /** * 拒绝策略之CallerRunsPolicy，将任务交由调用者线程来执行 */
    @Slf4j
    public class CallerRunsPolicyDemo { 
        public static void main(String[] args) { 
            // 核心线程数为1，最大线程数为1，阻塞队列大小为1，此线程池最多同时容纳2个任务，提交第三个任务就会执行拒绝策略
            // 构造时指定拒绝策略为CallerRunsPolicy
            ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(1, 1, 0, TimeUnit.SECONDS,
                    new ArrayBlockingQueue<>(1), new ThreadPoolExecutor.CallerRunsPolicy());
    
            IntStream.rangeClosed(1, 3).forEach(i->{ 
                threadPoolExecutor.submit(()->{ 
                    log.info("task begin ...");
                    try { 
                        TimeUnit.SECONDS.sleep(3);
                    } catch (InterruptedException e) { 
                        e.printStackTrace();
                    }
                    log.info("task end .");
                });
            });
            threadPoolExecutor.shutdown();
        }
    }

运行结果如下：

2020-10-13 17:23:01,542  INFO [main] (CallerRunsPolicyDemo.java:23) - task begin ...
    2020-10-13 17:23:01,542  INFO [pool-1-thread-1] (CallerRunsPolicyDemo.java:23) - task begin ...
    2020-10-13 17:23:04,544  INFO [main] (CallerRunsPolicyDemo.java:29) - task end .
    2020-10-13 17:23:04,545  INFO [pool-1-thread-1] (CallerRunsPolicyDemo.java:29) - task end .
    2020-10-13 17:23:04,545  INFO [pool-1-thread-1] (CallerRunsPolicyDemo.java:23) - task begin ...
    2020-10-13 17:23:07,545  INFO [pool-1-thread-1] (CallerRunsPolicyDemo.java:29) - task end .

### DiscardOldestPolicy ###

DiscardOldestPolicy策略会丢弃放入任务队列中最早的任务，然后将当前的任务入队，等待线程池执行。

package com.morris.concurrent.threadpool.threadpoolexecutor.construct;
    
    import lombok.extern.slf4j.Slf4j;
    
    import java.util.concurrent.ArrayBlockingQueue;
    import java.util.concurrent.ThreadPoolExecutor;
    import java.util.concurrent.TimeUnit;
    import java.util.stream.IntStream;
    
    /** * 拒绝策略之DiscardOldestPolicy的使用，丢弃最早放入任务队列中的任务 */
    @Slf4j
    public class DiscardOldestPolicyDemo { 
        public static void main(String[] args) { 
            // 核心线程数为1，最大线程数为1，阻塞队列大小为5，此线程池最多同时容纳6个任务，提交第7个任务就会执行拒绝策略
            // 最早放入任务队列中的任务为task2，所以task会被丢弃，不会被执行
            ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(1, 1, 0, TimeUnit.SECONDS,
                    new ArrayBlockingQueue<>(5), new ThreadPoolExecutor.DiscardOldestPolicy());
            
            IntStream.rangeClosed(1, 7).forEach(i -> { 
                threadPoolExecutor.submit(new Task("Task" + i));
            });
            threadPoolExecutor.shutdown();
        }
    
        private static class Task implements Runnable { 
    
            private String name;
    
            public Task(String name) { 
                this.name = name;
            }
    
            @Override
            public void run() { 
                log.info("{} is running...", name);
                try { 
                    TimeUnit.SECONDS.sleep(1);
                } catch (InterruptedException e) { 
                    e.printStackTrace();
                }
            }
        }
    }

运行结果如下：

2020-10-13 17:35:32,536  INFO [pool-1-thread-1] (DiscardOldestPolicyDemo.java:36) - Task1 is running...
    2020-10-13 17:35:33,538  INFO [pool-1-thread-1] (DiscardOldestPolicyDemo.java:36) - Task3 is running...
    2020-10-13 17:35:34,538  INFO [pool-1-thread-1] (DiscardOldestPolicyDemo.java:36) - Task4 is running...
    2020-10-13 17:35:35,539  INFO [pool-1-thread-1] (DiscardOldestPolicyDemo.java:36) - Task5 is running...
    2020-10-13 17:35:36,539  INFO [pool-1-thread-1] (DiscardOldestPolicyDemo.java:36) - Task6 is running...
    2020-10-13 17:35:37,539  INFO [pool-1-thread-1] (DiscardOldestPolicyDemo.java:36) - Task7 is running...

### DiscardPolicy ###

DiscardPolicy策略会将当前任务直接忽略丢弃，不做任何处理。

package com.morris.concurrent.threadpool.threadpoolexecutor.construct;
    
    import lombok.extern.slf4j.Slf4j;
    
    import java.util.concurrent.ArrayBlockingQueue;
    import java.util.concurrent.ThreadPoolExecutor;
    import java.util.concurrent.TimeUnit;
    import java.util.stream.IntStream;
    
    /** * 拒绝策略之DiscardPolicy的使用，直接丢弃 */
    @Slf4j
    public class DiscardPolicyDemo { 
    
        public static void main(String[] args) { 
            // 核心线程数为1，最大线程数为1，阻塞队列大小为1，此线程池最多同时容纳2个任务，提交第三个任务就会执行拒绝策略
            // 构造时指定拒绝策略为CallerRunsPolicy
            ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(1, 1, 0, TimeUnit.SECONDS,
                    new ArrayBlockingQueue<>(1), new ThreadPoolExecutor.DiscardPolicy());
    
            IntStream.rangeClosed(1, 3).forEach(i -> { 
                threadPoolExecutor.submit(() -> { 
                    log.info("task begin ...");
                    try { 
                        TimeUnit.SECONDS.sleep(3);
                    } catch (InterruptedException e) { 
                        e.printStackTrace();
                    }
                    log.info("task end .");
                });
            });
            threadPoolExecutor.shutdown();
        }
    }

运行结果如下：

2020-10-13 17:39:37,842  INFO [pool-1-thread-1] (DiscardPolicyDemo.java:24) - task begin ...
    2020-10-13 17:39:40,843  INFO [pool-1-thread-1] (DiscardPolicyDemo.java:30) - task end .
    2020-10-13 17:39:40,843  INFO [pool-1-thread-1] (DiscardPolicyDemo.java:24) - task begin ...
    2020-10-13 17:39:43,844  INFO [pool-1-thread-1] (DiscardPolicyDemo.java:30) - task end .

### 自定义拒绝策略 ###

当然，也可以根据应用场景需要来实现RejectedExecutionHandler接口自定义策略。如记录日志或持久化存储不能处理的任务。

package com.morris.concurrent.threadpool.threadpoolexecutor.construct;
    
    import lombok.extern.slf4j.Slf4j;
    
    import java.util.concurrent.ArrayBlockingQueue;
    import java.util.concurrent.ThreadPoolExecutor;
    import java.util.concurrent.TimeUnit;
    import java.util.stream.IntStream;
    
    /** * 拒绝策略之自定义拒绝策略 */
    @Slf4j
    public class CustomPolicyDemo { 
    
        public static void main(String[] args) { 
            // 核心线程数为1，最大线程数为1，阻塞队列大小为1，此线程池最多同时容纳2个任务，提交第三个任务就会执行拒绝策略
            // 构造时指定拒绝策略为CallerRunsPolicy
            ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(1, 1, 0, TimeUnit.SECONDS,
                    new ArrayBlockingQueue<>(1), (r, executor) -> { 
                log.info("send alert email notify admin");
            });
    
            IntStream.rangeClosed(1, 3).forEach(i -> { 
                threadPoolExecutor.submit(() -> { 
                    log.info("task begin ...");
                    try { 
                        TimeUnit.SECONDS.sleep(3);
                    } catch (InterruptedException e) { 
                        e.printStackTrace();
                    }
                    log.info("task end .");
                });
            });
            threadPoolExecutor.shutdown();
        }
    }

运行结果如下：

2020-10-13 17:56:34,906  INFO [main] (CustomPolicyDemo.java:21) - send alert email notify admin
    2020-10-13 17:56:34,906  INFO [pool-1-thread-1] (CustomPolicyDemo.java:26) - task begin ...
    2020-10-13 17:56:37,909  INFO [pool-1-thread-1] (CustomPolicyDemo.java:32) - task end .
    2020-10-13 17:56:37,909  INFO [pool-1-thread-1] (CustomPolicyDemo.java:26) - task begin ...
    2020-10-13 17:56:40,909  INFO [pool-1-thread-1] (CustomPolicyDemo.java:32) - task end .