你的位置:首页 > Java教程

[Java教程]Java 线程池


系统启动一个线程的成本是比较高的,因为它涉及到与操作系统的交互,使用线程池的好处是提高性能,当系统中包含大量并发的线程时,会导致系统性能剧烈下降,甚至导致JVM崩溃,而线程池的最大线程数参数可以控制系统中并发线程数不超过次数。

一、Executors 工厂类用来产生线程池,该工厂类包含以下几个静态工厂方法来创建对应的线程池。创建的线程池是一个ExecutorService对象,使用该对象的submit方法或者是execute方法执行相应的Runnable或者是Callable任务。线程池本身在不再需要的时候调用shutdown()方法停止线程池,调用该方法后,该线程池将不再允许任务添加进来,但是会直到已添加的所有任务执行完成后才死亡。

1、newCachedThreadPool(),创建一个具有缓存功能的线程池,提交到该线程池的任务(Runnable或Callable对象)创建的线程,如果执行完成,会被缓存到CachedThreadPool中,供后面需要执行的任务使用。

import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;public class CacheThreadPool {  static class Task implements Runnable {    @Override    public void run() {      System.out.println(this + " " + Thread.currentThread().getName() + " AllStackTraces map size: "          + Thread.currentThread().getAllStackTraces().size());    }  }  public static void main(String[] args) {    ExecutorService cacheThreadPool = Executors.newCachedThreadPool();        //先添加三个任务到线程池    for(int i = 0 ; i < 3; i++) {      cacheThreadPool.execute(new Task());    }        //等三个线程执行完成后,再次添加三个任务到线程池    try {      Thread.sleep(3000);    } catch (InterruptedException e) {      e.printStackTrace();    }        for(int i = 0 ; i < 3; i++) {      cacheThreadPool.execute(new Task());    }  }}

执行结果如下:

CacheThreadPool$Task@2d312eb9 pool-1-thread-1 AllStackTraces map size: 7CacheThreadPool$Task@59522b86 pool-1-thread-3 AllStackTraces map size: 7CacheThreadPool$Task@73dbb89f pool-1-thread-2 AllStackTraces map size: 7CacheThreadPool$Task@5795cedc pool-1-thread-3 AllStackTraces map size: 7CacheThreadPool$Task@256d5600 pool-1-thread-1 AllStackTraces map size: 7CacheThreadPool$Task@7d1c5894 pool-1-thread-2 AllStackTraces map size: 7

线程池中的线程对象进行了缓存,当有新任务执行时进行了复用。但是如果有特别多的并发时,缓存线程池还是会创建很多个线程对象。

2、newFixedThreadPool(int nThreads) 创建一个指定线程个数,线程可复用的线程池。

import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;public class FixedThreadPool {  static class Task implements Runnable {    @Override    public void run() {      System.out.println(this + " " + Thread.currentThread().getName() + " AllStackTraces map size: "          + Thread.currentThread().getAllStackTraces().size());    }  }  public static void main(String[] args) {    ExecutorService fixedThreadPool = Executors.newFixedThreadPool(3);    // 先添加三个任务到线程池    for (int i = 0; i < 5; i++) {      fixedThreadPool.execute(new Task());    }    // 等三个线程执行完成后,再次添加三个任务到线程池    try {      Thread.sleep(3);    } catch (InterruptedException e) {      e.printStackTrace();    }    for (int i = 0; i < 3; i++) {      fixedThreadPool.execute(new Task());    }  }}

执行结果:

FixedThreadPool$Task@7045c12d pool-1-thread-2 AllStackTraces map size: 7
FixedThreadPool$Task@50fa0bef pool-1-thread-2 AllStackTraces map size: 7
FixedThreadPool$Task@ccb1870 pool-1-thread-2 AllStackTraces map size: 7
FixedThreadPool$Task@7392b4e3 pool-1-thread-1 AllStackTraces map size: 7
FixedThreadPool$Task@5bdeff18 pool-1-thread-2 AllStackTraces map size: 7
FixedThreadPool$Task@7d5554e1 pool-1-thread-1 AllStackTraces map size: 7
FixedThreadPool$Task@24468092 pool-1-thread-3 AllStackTraces map size: 7
FixedThreadPool$Task@fa7b978 pool-1-thread-2 AllStackTraces map size: 7


3、newSingleThreadExecutor(),创建一个只有单线程的线程池,相当于调用newFixedThreadPool(1)

4、newSheduledThreadPool(int corePoolSize),创建指定线程数的线程池,它可以在指定延迟后执行线程。也可以以某一周期重复执行某一线程,知道调用shutdown()关闭线程池。

示例如下:

import java.util.concurrent.Executors;import java.util.concurrent.ScheduledExecutorService;import java.util.concurrent.TimeUnit;public class ScheduledThreadPool {  static class Task implements Runnable {    @Override    public void run() {      System.out.println("time " + System.currentTimeMillis() + " " + Thread.currentThread().getName() + " AllStackTraces map size: "          + Thread.currentThread().getAllStackTraces().size());    }  }  public static void main(String[] args) {    ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(3);        scheduledExecutorService.schedule(new Task(), 3, TimeUnit.SECONDS);        scheduledExecutorService.scheduleAtFixedRate(new Task(), 3, 5, TimeUnit.SECONDS);      try {      Thread.sleep(30 * 1000);    } catch (InterruptedException e) {      e.printStackTrace();    }    scheduledExecutorService.shutdown();  }}

运行结果如下:

time 1458921795240 pool-1-thread-1 AllStackTraces map size: 6time 1458921795241 pool-1-thread-2 AllStackTraces map size: 6time 1458921800240 pool-1-thread-1 AllStackTraces map size: 7time 1458921805240 pool-1-thread-1 AllStackTraces map size: 7time 1458921810240 pool-1-thread-1 AllStackTraces map size: 7time 1458921815240 pool-1-thread-1 AllStackTraces map size: 7time 1458921820240 pool-1-thread-1 AllStackTraces map size: 7

由运行时间可看出,任务是按照5秒的周期执行的。

5、newSingleThreadScheduledExecutor() 创建一个只有一个线程的线程池,同调用newScheduledThreadPool(1)。

二、ForkJoinPool和ForkJoinTask

ForkJoinPool是ExecutorService的实现类,支持将一个任务划分为多个小任务并行计算,在把多个小任务的计算结果合并成总的计算结果。它有两个构造函数

ForkJoinPool(int parallelism)创建一个包含parallelism个并行线程的ForkJoinPool。

ForkJoinPool(),以Runtime.availableProcessors()方法返回值作为parallelism参数来创建ForkJoinPool。

ForkJoinTask 代表一个可以并行,合并的任务。它是实现了Future<T>接口的抽象类,它有两个抽象子类,代表无返回值任务的RecuriveAction和有返回值的RecursiveTask。可根据具体需求继承这两个抽象类实现自己的对象,然后调用ForkJoinPool的submit 方法执行。

RecuriveAction 示例如下,实现并行输出0-300的数字。

import java.util.concurrent.ForkJoinPool;import java.util.concurrent.RecursiveAction;import java.util.concurrent.TimeUnit;public class ActionForkJoinTask {  static class PrintTask extends RecursiveAction {    private static final int THRESHOLD = 50;    private int start;    private int end;    public PrintTask(int start, int end) {      this.start = start;      this.end = end;    }    @Override    protected void compute() {      if (end - start < THRESHOLD) {        for(int i = start; i < end; i++) {          System.out.println(Thread.currentThread().getName() + " " + i);        }      } else {        int middle = (start + end) / 2;        PrintTask left = new PrintTask(start, middle);        PrintTask right = new PrintTask(middle, end);        left.fork();        right.fork();      }    }  }  public static void main(String[] args) {    ForkJoinPool pool = new ForkJoinPool();        pool.submit(new PrintTask(0, 300));    try {      pool.awaitTermination(2, TimeUnit.SECONDS);    } catch (InterruptedException e) {      e.printStackTrace();    }        pool.shutdown();  }}

在拆分小任务后,调用任务的fork()方法,加入到ForkJoinPool中并行执行。

RecursiveTask示例,实现并行计算100个整数求和。拆分为每20个数求和后获取结果,在最后合并为最后的结果。

import java.util.Random;import java.util.concurrent.ExecutionException;import java.util.concurrent.ForkJoinPool;import java.util.concurrent.Future;import java.util.concurrent.RecursiveTask;public class TaskForkJoinTask {  static class CalTask extends RecursiveTask<Integer> {    private static final int THRESHOLD = 20;    private int arr[];    private int start;    private int end;    public CalTask(int[] arr, int start, int end) {      this.arr = arr;      this.start = start;      this.end = end;    }    @Override    protected Integer compute() {      int sum = 0;      if (end - start < THRESHOLD) {        for (int i = start; i < end; i++) {          sum += arr[i];        }        System.out.println(Thread.currentThread().getName() + " sum:" + sum);        return sum;      } else {        int middle = (start + end) / 2;        CalTask left = new CalTask(arr, start, middle);        CalTask right = new CalTask(arr, middle, end);        left.fork();        right.fork();        return left.join() + right.join();      }    }  }  public static void main(String[] args) {    int arr[] = new int[100];    Random random = new Random();    int total = 0;    for (int i = 0; i < arr.length; i++) {      int tmp = random.nextInt(20);      total += (arr[i] = tmp);    }    System.out.println("total " + total);    ForkJoinPool pool = new ForkJoinPool(4);    Future<Integer> future = pool.submit(new CalTask(arr, 0, arr.length));    try {      System.out.println("cal result: " + future.get());    } catch (InterruptedException e) {      e.printStackTrace();    } catch (ExecutionException e) {      e.printStackTrace();    }    pool.shutdown();  }}

执行结果如下:

total 912ForkJoinPool-1-worker-2 sum:82ForkJoinPool-1-worker-2 sum:123ForkJoinPool-1-worker-2 sum:144ForkJoinPool-1-worker-3 sum:119ForkJoinPool-1-worker-2 sum:106ForkJoinPool-1-worker-2 sum:128ForkJoinPool-1-worker-2 sum:121ForkJoinPool-1-worker-3 sum:89cal result: 912

子任务执行完后,调用任务的join()方法获取子任务执行结果,再相加获得最后的结果。