JUC（六）堵塞队列与线程池

堵塞队列

简介

def：在多线程中实现高效、安全的数据传输，主要是通过一个共享的队列，使得数据能够从一端输入，从另一端输出
当队列是空的，取数据的线程就会被堵塞，直到其他线程往空的队列中添加数据
当队列是满的，放数据的线程就会被堵塞，直到其他线程移除数据
使用堵塞队列能够实现简化线程之间的协作，使用生产者消费者模型而不必实现线程间的同步和协作

堵塞队列的分类

1 ArrayBlockingQueue：由数组结构组成的有界堵塞队列

2 LinkedBlockingQueue：由链表结构组成的有界堵塞队列

大小默认值为Integer.MAX_VALUE

3 DelayQueue：使用优先级队列实现的延迟无界的堵塞队列

4 PriorityBlockingQueue：支持优先级排序的无界堵塞队列

5 SynchronousQueue：不存储元素的堵塞队列，也即是单个元素的队列

6 LinkedTransferQueue：由链表结构组成的无界堵塞队列

7 LinkedTransferDeque：由链表结构组成的双向堵塞队列

堵塞队列的常用方法

Thread Pool线程池

简介

线程池是一种线程使用模式，线程池维护者多个线程，可以避免在执行短时间任务创建和销毁线程的代价。
线程池能够保证内核的充分利用，防止过分调度
线程池的主要工作是控制运行的线程数量，处理过程中将任务放入队列，然后在线程创建之后启动这些任务，当任务的数量超过线程数量时，就会将任务放入队列排队等候，等其他线程执行完毕，再从队列中取出任务执行

线程池的特点

降低资源消耗：重复利用自己创建的线程减少创建、销毁的资源消耗
提高响应速度：任务到达的时候，不需要等待线程创建就能直接执行
提高可管理型

线程池的使用

Executors.newFilexedThreadPool(int nThreads)，创建指定线程数量的线程池
```
ExecutorService threadPool1 = Executors.newFixedThreadPool(5);
try {
    for (int i = 0; i < 10; i++) {
        threadPool1.execute(() -> {
            System.out.println(Thread.currentThread().getName() + " is working...");
        });
    }
} catch (Exception e) {

} finally {
    threadPool1.shutdown();
}
```
pool-1-thread-3 is working...
pool-1-thread-3 is working...
pool-1-thread-3 is working...
pool-1-thread-3 is working...
pool-1-thread-3 is working...
pool-1-thread-3 is working...
pool-1-thread-5 is working...
pool-1-thread-4 is working...
pool-1-thread-1 is working...
pool-1-thread-2 is working...

Process finished with exit code 0

只会使用线程池中创建的五个线程去执行任务

Executors.newWorkStealingPool()，创建只含有一个线程的线程池

public static void main(String[] args) {
    ExecutorService threadPool1 = Executors.newSingleThreadExecutor();
    try {
        for (int i = 0; i < 10; i++) {
            threadPool1.execute(() -> {
                System.out.println(Thread.currentThread().getName() + " is working...");
            });
        }
    } catch (Exception e) {

    } finally {
        threadPool1.shutdown();
    }

}

pool-1-thread-1 is working...
pool-1-thread-1 is working...
pool-1-thread-1 is working...
pool-1-thread-1 is working...
pool-1-thread-1 is working...
pool-1-thread-1 is working...
pool-1-thread-1 is working...
pool-1-thread-1 is working...
pool-1-thread-1 is working...
pool-1-thread-1 is working...

Process finished with exit code 0

Executors.newCachedThreadPool()，创建可扩容的线程池

public static void main(String[] args) {
    ExecutorService threadPool1 = Executors.newCachedThreadPool();
    try {
        for (int i = 0; i < 10; i++) {
            threadPool1.execute(() -> {
                System.out.println(Thread.currentThread().getName() + " is working...");
            });
        }
    } catch (Exception e) {

    } finally {
        threadPool1.shutdown();
    }
}

pool-1-thread-3 is working...
pool-1-thread-9 is working...
pool-1-thread-4 is working...
pool-1-thread-1 is working...
pool-1-thread-6 is working...
pool-1-thread-7 is working...
pool-1-thread-5 is working...
pool-1-thread-8 is working...
pool-1-thread-2 is working...
pool-1-thread-10 is working...

三种线程池本质都是使用的：

new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                              60L, TimeUnit.SECONDS,
                              new SynchronousQueue<Runnable>())

线程池的七个参数

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

corePoolSize：核心常驻线程的数量
maximumPoolSize：线程池最大线程数量
keepAliveTime：非核心线程的存活时间
unit：存活时间的单位
workQueue：任务数量超过常驻线程的堵塞队列
threadFactory：线程工厂
handler：线程池拒接策略，即任务数量超过线程池最大线程数量的处理策略

线程池的工作流程和拒绝策略

工作流程

主线程首先通过线程池的execute()方法创建线程
线程数量达到常驻线程的数量，任务就会被放入堵塞队列中
如果堵塞队列也满了，主线程又通过execute()方法创建线程的时候，线程池就会为此任务创建线程并分配（而不是堵塞队列？这里很疑惑）
当堵塞队列满了，线程池也满了的时候，主线程再次执行execute()方法，线程池就会去执行拒绝策略

拒绝策略

AbortPolicy（默认）：直接抛出RejectExecutionException异常
CallerRunsPolicy：将任务回退给调用者，以降低新任务的流量
DisgardOldestPolicy：抛弃队列中等待最久的任务，而将当前任务添加到队列，尝试再次提交
DiscardPolicy：装死

自定义线程池

为什么要自定义线程池

FixedThreadPool和SingleThreadPool允许的请求队列的大小为Integer.MAX_VALUE，容易造成请求堆积，从而OOM
CachedThreadPool则是允许创建的最大线程数为Integer.MAX_VALUE，会创建大量的线程，从而OOM

public static void main(String[] args) {
    var threadPool = new ThreadPoolExecutor(2,
            5,
            5,
            TimeUnit.SECONDS,
            new ArrayBlockingQueue<>(3),
            Executors.defaultThreadFactory(),
            new ThreadPoolExecutor.AbortPolicy());
    for (int i = 0; i < 10; i++) {
        threadPool.execute(() -> {
            System.out.println(Thread.currentThread().getName() + " is working.");
        });
    }
}

Exception in thread "main" java.util.concurrent.RejectedExecutionException: Task com.hikaru.juc.threadPool.ThreadPoolDemo$$Lambda$14/0x00000008010031f0@7e6cbb7a rejected from java.util.concurrent.ThreadPoolExecutor@4769b07b[Running, pool size = 5, active threads = 5, queued tasks = 3, completed tasks = 0]
	at java.base/java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2081)
	at java.base/java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:841)
	at java.base/java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1376)
	at com.hikaru.juc.threadPool.ThreadPoolDemo.main(ThreadPoolDemo.java:15)
pool-1-thread-4 is working.
pool-1-thread-4 is working.
pool-1-thread-4 is working.
pool-1-thread-4 is working.
pool-1-thread-1 is working.
pool-1-thread-5 is working.
pool-1-thread-3 is working.
pool-1-thread-2 is working.