JavaSE-线程池（1）- 线程池概念

前提

使用多线程可以并发处理任务，提高程序执行效率。但同时创建和销毁线程会消耗操作系统资源，虽然java 使用线程的方式有多种，但是在实际使用过程中并不建议使用 new Thread 的方式手动创建线程。

线程池概念

线程池可以理解成一个存放线程的容器，当需要使用线程处理任务时从线程池中取，而并非直接创建一个线程

使用线程池的优势

1. 降低资源消耗。通过重复利用已创建的线程降低线程创建和销毁造成的消耗。
2. 提高响应速度。当任务到达时，任务可以不需要的等到线程创建就能立即执行。
3. 提高线程的可管理性。线程是稀缺资源，如果无限制的创建，不仅会消耗系统资源，还会降低系统的稳定性，使用线程池可以进行统一的分配，调优和监控。

来自 《Java 并发编程的艺术》

线程池相关接口以及类

Runnable

可以理解成一个不需要获取返回结果的任务

@FunctionalInterface
public interface Runnable {public abstract void run();
}

Callable

类似于 Runnable ，是一个有返回结果的任务

@FunctionalInterface
public interface Callable<V> {/*** Computes a result, or throws an exception if unable to do so.** @return computed result* @throws Exception if unable to compute a result*/V call() throws Exception;
}

Future

异步任务提交后使用 Future 接收，从 Future get 方法可以获取异步任务的返回值

public interface Future<V> {boolean cancel(boolean mayInterruptIfRunning);boolean isCancelled();boolean isDone();V get() throws InterruptedException, ExecutionException;V get(long timeout, TimeUnit unit)throws InterruptedException, ExecutionException, TimeoutException;
}

RunnableFuture

Runnable 和 Future 的结合体

public interface RunnableFuture<V> extends Runnable, Future<V> {/*** Sets this Future to the result of its computation* unless it has been cancelled.*/void run();
}

FutureTask

RunnableFuture 接口的实现类

public class FutureTask<V> implements RunnableFuture<V> {
}

使用demo：

import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;public class FutureTaskTest {public static void main(String[] args) throws ExecutionException, InterruptedException {FutureTask futureTask = new FutureTask(() -> {Thread.sleep(1000);return 100;});new Thread(futureTask).start();System.out.println(futureTask.get());}
}

Executor

执行器，用来执行 Runnable 任务，通过实现 Executor 接口可以自定义任务的执行方式，比方使用线程池来执行任务，避免使用 new Thread 的方式来执行

public interface Executor {/*** 执行方法，执行一个具体的 Runnable 任务*/void execute(Runnable command);
}

ExecutorService

继承自 Executor ，提供更多的方法，实现线程池的类一般继承这个接口

public interface ExecutorService extends Executor {/*** 关闭执行器，但是会等待已经提交的任务执行完成，不再接收新的任务*/void shutdown();/*** 尝试停止所有正在执行的任务，不再接收新的任务*/List<Runnable> shutdownNow();/***判断执行器是否关闭，如果此执行器已关闭，则返回true。*/boolean isShutdown();/*** 如果关闭后（调用 shutdown 或 shutdownNow 方法）所有任务都已完成，则返回true。* 请注意，除非首先调用shutdown或shutdownNow，否则isTerminated       永远不会为true。*/boolean isTerminated();/*** Blocks until all tasks have completed execution after a shutdown* request, or the timeout occurs, or the current thread is* interrupted, whichever happens first.*/boolean awaitTermination(long timeout, TimeUnit unit)throws InterruptedException;/*** 提交一个有返回值的任务并使用 Future 接收*/<T> Future<T> submit(Callable<T> task);/*** Submits a Runnable task for execution and returns a Future* representing that task. The Future's {@code get} method will* return the given result upon successful completion.*/<T> Future<T> submit(Runnable task, T result);/*** 提交任务并使用 Future 接收*/Future<?> submit(Runnable task);/*** Executes the given tasks, returning a list of Futures holding* their status and results when all complete.* {@link Future#isDone} is {@code true} for each* element of the returned list.* Note that a <em>completed</em> task could have* terminated either normally or by throwing an exception.* The results of this method are undefined if the given* collection is modified while this operation is in progress.*/<T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)throws InterruptedException;/*** Executes the given tasks, returning a list of Futures holding* their status and results* when all complete or the timeout expires, whichever happens first.* {@link Future#isDone} is {@code true} for each* element of the returned list.* Upon return, tasks that have not completed are cancelled.* Note that a <em>completed</em> task could have* terminated either normally or by throwing an exception.* The results of this method are undefined if the given* collection is modified while this operation is in progress.*/<T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,long timeout, TimeUnit unit)throws InterruptedException;/*** Executes the given tasks, returning the result* of one that has completed successfully (i.e., without throwing* an exception), if any do. Upon normal or exceptional return,* tasks that have not completed are cancelled.* The results of this method are undefined if the given* collection is modified while this operation is in progress.*/<T> T invokeAny(Collection<? extends Callable<T>> tasks)throws InterruptedException, ExecutionException;/*** Executes the given tasks, returning the result* of one that has completed successfully (i.e., without throwing* an exception), if any do before the given timeout elapses.* Upon normal or exceptional return, tasks that have not* completed are cancelled.* The results of this method are undefined if the given* collection is modified while this operation is in progress.*<T> T invokeAny(Collection<? extends Callable<T>> tasks,long timeout, TimeUnit unit)throws InterruptedException, ExecutionException, TimeoutException;
}

AbstractExecutorService

public abstract class AbstractExecutorService implements ExecutorService {
}

实现 ExecutorService 接口，提供ExecutorService执行方法的默认实现

ThreadPoolExecutor

public class ThreadPoolExecutor extends AbstractExecutorService {
}

线程池的具体实现类，继承自 AbstractExecutorService

类结构图：

ThreadPoolExecutor 使用方法

可以使用工具类 Executors 提供的方法创建线程池，比如：

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;public class ExecutorServiceTest1 {static class MyTask implements Runnable {private int i;public MyTask(int i) {this.i = i;}@Overridepublic void run() {try {Thread.sleep(2000);} catch (InterruptedException e) {e.printStackTrace();}System.out.println(Thread.currentThread() + " 任务" + i);}}public static void main(String[] args) {ExecutorService executorService = Executors.newFixedThreadPool(2);for (int i = 1; i <= 10; i++) {executorService.execute(new MyTask(i));}executorService.shutdown();}
}

以上 Executors.newFixedThreadPool 方法创建了一个拥有固定线程数的线城池

public static ExecutorService newFixedThreadPool(int nThreads) {return new ThreadPoolExecutor(nThreads, nThreads,0L, TimeUnit.MILLISECONDS,new LinkedBlockingQueue<Runnable>());
}

执行结果：

Thread[pool-1-thread-1,5,main] 任务1
Thread[pool-1-thread-2,5,main] 任务2
Thread[pool-1-thread-2,5,main] 任务4
Thread[pool-1-thread-1,5,main] 任务3
Thread[pool-1-thread-1,5,main] 任务6
Thread[pool-1-thread-2,5,main] 任务5
Thread[pool-1-thread-2,5,main] 任务8
Thread[pool-1-thread-1,5,main] 任务7
Thread[pool-1-thread-2,5,main] 任务9
Thread[pool-1-thread-1,5,main] 任务10

通过结果可以看出，10个任务都是由两个线程执行的，由于这两个线程一次只能处理两个任务，其他任务只有在线程空闲时才能被处理，实际上线程池不仅维护了一组线程的引用，还维护了这组任务，而任务则是放在队列中，即上文的 LinkedBlockingQueue 参数

参考：
https://blog.csdn.net/qq_36881887/article/details/125707550
https://www.mianshigee.com/note/detail/20134hnk/