(十七)java多线程之ThreadPoolExecutor

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引言

在之前的例子,我们要创建多个线程处理一批任务的时候.我是通过创建线程数组,或者使用线程集合来管理的.但是这样做不太好,因为这些线程没有被重复利用.所以这里要引入线程池,今天我们就讲线程池执行器ThreadPoolExecutor.

理论

首先我们来看一下它的构造器:
ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory, RejectedExecutionHandler handler) 这个是ThreadPoolExecutor完整的构造器,其他的构造器其实也是在内部调用这个.

• corePoolSize 核心线程数,线程池保留线程的数量,即使这些线程是空闲.除非设置了allowCoreThreadTimeOut
• maximumPoolSize 线程池最大允许的线程数.
• keepAliveTime 当当前的线程数大于核心线程数,那么这些多余的空闲的线程在被终止之前能等待新任务的时间.
• unit keepAliveTime时间的单位
• workQueue 这个是用来保留将要执行的工作队列.
• threadFactory 用于创建新线程的工厂
• handler 如果工作队列(workQueue)满了,那么这个handler是将会被执行.

ThreadPoolExecutor还有几个可不带threadFactory或handler惨的构造器,说明java提供了一些默认的配置,让我们看一下.

如果构造不带threadFactory,那么默认使用java.util.concurrent.Executors.DefaultThreadFactory创建出一个新的工厂对象.通过阅读源代码,主要是在创建新的线程的时候修改了线程名为pool-全局线程池递增数编号-thread-当前线程池线程递增编号,让线程改为非守护线程,并设置线程的优先级为NORM_PRIORITY.

ok,再看一下handler有什么默认值.

• java.util.concurrent.ThreadPoolExecutor.AbortPolicy 这个是默认使用的拒绝策略,如果有要执行的任务队列已满,且还有任务提交,则直接抛出异常信息
• java.util.concurrent.ThreadPoolExecutor.DiscardPolicy 这个是忽略策略,如果有要执行的任务队列已满,且还有任务提交,则直接忽略掉这个任务,即不抛出异常也不做任何处理.
• java.util.concurrent.ThreadPoolExecutor.DiscardOldestPolicy 忽略最早提交的任务.如果有要执行的任务队列已满,此时若还有任务提交且线程池还没有停止,则把队列中最早提交的任务抛弃掉,然后把当前任务加入队列中.
• java.util.concurrent.ThreadPoolExecutor.CallerRunsPolicy 这个是来着不拒策略.如果有要执行的任务队列已满,此时若还有任务提交且线程池还没有停止,则直接运行任务的run方法.

例子 使用默认的拒绝策略AbortPolicy

public class Demo1 {
    public static void main(String[] args) {
        BlockingQueue<Runnable> queue = new ArrayBlockingQueue<Runnable>(10);
        RejectedExecutionHandler handler = new ThreadPoolExecutor.AbortPolicy();
        ThreadPoolExecutor pool = new ThreadPoolExecutor(3, 3, 0, TimeUnit.SECONDS, queue, handler);
        for (int i = 0; i < 20; i ++){
            final int temp = i;
            pool.execute(() -> {
                System.out.println("客户" + temp + "来了.......");
                try {
                    Thread.sleep(10);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            });
        }
        pool.shutdown();
    }
}

Exception in thread "main" java.util.concurrent.RejectedExecutionException: Task com.kco.test17.demo1.Demo1$$Lambda$1/15497079@ca494b rejected from java.util.concurrent.ThreadPoolExecutor@1a4f24f[Running, pool size = 5, active threads = 5, queued tasks = 10, completed tasks = 0]
at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2047)
at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:823)
at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1369)
at com.kco.test17.demo1.Demo1.main(Demo1.java:16)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)
pool-1-thread-1客户0来了.......
pool-1-thread-2客户1来了.......
pool-1-thread-3客户2来了.......
pool-1-thread-5客户14来了.......
pool-1-thread-4客户13来了.......
pool-1-thread-2客户3来了.......
pool-1-thread-1客户4来了.......
pool-1-thread-5客户5来了.......
pool-1-thread-3客户6来了.......
pool-1-thread-4客户7来了.......
pool-1-thread-2客户9来了.......
pool-1-thread-1客户8来了.......
pool-1-thread-3客户10来了.......
pool-1-thread-5客户11来了.......
pool-1-thread-4客户12来了.......

从结果看出来,可以看出线程是重复被使用的,而且当执行的任务超过工作队列的容量时,线程确实抛出了异常.

例子2 使用忽略策略 DiscardPolicy

将RejectedExecutionHandler handler = new ThreadPoolExecutor.AbortPolicy();改为 RejectedExecutionHandler handler = new ThreadPoolExecutor.DiscardPolicy();
运行结果如下:

pool-1-thread-1客户0来了.......
pool-1-thread-3客户2来了.......
pool-1-thread-4客户13来了.......
pool-1-thread-5客户14来了.......
pool-1-thread-3客户3来了.......
pool-1-thread-4客户4来了.......
pool-1-thread-1客户5来了.......
pool-1-thread-5客户6来了.......
pool-1-thread-2客户1来了.......
pool-1-thread-3客户7来了.......
pool-1-thread-4客户8来了.......
pool-1-thread-5客户9来了.......
pool-1-thread-1客户10来了.......
pool-1-thread-2客户11来了.......
pool-1-thread-4客户12来了.......

现在线程池正确退出了,而且也不抛出异常了,但是超过工作队列容量的任务全部被忽略了.

例子3 使用忽略最早任务策略 DiscardOldestPolicy

RejectedExecutionHandler改为RejectedExecutionHandler handler = new ThreadPoolExecutor.DiscardOldestPolicy();

pool-1-thread-1客户0来了.......
pool-1-thread-2客户1来了.......
pool-1-thread-3客户2来了.......
pool-1-thread-5客户14来了.......
pool-1-thread-4客户13来了.......
pool-1-thread-4客户8来了.......
pool-1-thread-1客户11来了.......
pool-1-thread-5客户10来了.......
pool-1-thread-3客户9来了.......
pool-1-thread-2客户12来了.......
pool-1-thread-1客户15来了.......
pool-1-thread-4客户16来了.......
pool-1-thread-5客户17来了.......
pool-1-thread-2客户19来了.......
pool-1-thread-3客户18来了.......

从以上结果,我们可以看出除了客户0到客户2刚好是3个核心线程被执行后,客户3到客户7直接被忽略掉了.

例子4 使用来着不拒策略 CallerRunsPolicy

同样讲拒绝策略改为RejectedExecutionHandler handler = new ThreadPoolExecutor.CallerRunsPolicy();
运行程序,结果如下:

pool-1-thread-1客户0来了.......
pool-1-thread-2客户1来了.......
pool-1-thread-3客户2来了.......
pool-1-thread-4客户13来了.......
main客户15来了.......
pool-1-thread-5客户14来了.......
pool-1-thread-2客户3来了.......
pool-1-thread-1客户4来了.......
main客户18来了.......
pool-1-thread-3客户5来了.......
pool-1-thread-4客户7来了.......
pool-1-thread-5客户6来了.......
pool-1-thread-5客户8来了.......
pool-1-thread-1客户9来了.......
pool-1-thread-4客户10来了.......
pool-1-thread-3客户12来了.......
pool-1-thread-2客户11来了.......
pool-1-thread-1客户16来了.......
pool-1-thread-5客户19来了.......
pool-1-thread-3客户17来了.......

结果,我们可以发现所有的任务都被执行,而且竟然还有两个是在主线程执行的.现在明白我之前说的则直接运行任务的run方法的意思了吧,没错是直接调用run方法,而不是开启线程去执行任务.

例子5 使用自定义的拒绝策略

现在我们自己写一个拒绝策略,要求所有的任务都必须被线程池执行,而且都要在线程池中执行.

public class Demo5 {
    public static void main(String[] args) {
        BlockingQueue<Runnable> queue = new ArrayBlockingQueue<Runnable>(10);
        RejectedExecutionHandler handler = new RejectedExecutionHandler() {
            @Override
            public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
                if (!executor.isShutdown()){
                    try {
                        executor.getQueue().put(r);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        };
        ThreadPoolExecutor pool = new ThreadPoolExecutor(3, 5, 0, TimeUnit.SECONDS, queue, handler);
        for (int i = 0; i < 20; i ++){
            final int temp = i;
            pool.execute(() -> {
                String name = Thread.currentThread().getName();
                System.out.println(name + "客户" + temp + "来了.......");
                try {
                    Thread.sleep(2000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            });
        }
        pool.shutdown();
    }
}

运行结果:

pool-1-thread-1客户0来了.......
pool-1-thread-3客户2来了.......
pool-1-thread-5客户14来了.......
pool-1-thread-4客户13来了.......
pool-1-thread-2客户1来了.......
pool-1-thread-1客户3来了.......
pool-1-thread-3客户4来了.......
pool-1-thread-5客户5来了.......
pool-1-thread-2客户6来了.......
pool-1-thread-4客户7来了.......
pool-1-thread-1客户8来了.......
pool-1-thread-3客户9来了.......
pool-1-thread-5客户10来了.......
pool-1-thread-4客户11来了.......
pool-1-thread-2客户12来了.......
pool-1-thread-1客户15来了.......
pool-1-thread-3客户16来了.......
pool-1-thread-5客户17来了.......
pool-1-thread-4客户19来了.......
pool-1-thread-2客户18来了.......

ok.所有任务都被线程池执行了.而且我们自定义的拒绝策略也很简单,就是让工作队列调用put让其一直等待,直到有可用的容量存放任务.

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