ThreadPool使用

为什么要用线程池:

1.减少了创建和销毁线程的次数，每个工作线程都可以被重复利用，可执行多个任务。

2.可以根据系统的承受能力，调整线程池中工作线线程的数目，防止因为消耗过多的内存，线程开的越多，消耗的内存也就越大，最后死机)。

Java里面线程池的顶级接口是Executor，但是严格意义上讲Executor并不是一个线程池，而只是一个执行线程的工具。真正的线程池接口是ExecutorService。

java中自带的4种pool

1. newFixedThreadPool - 创建固定大小的线程.如果某个线程在执行过程中挂掉，那么会自动创建一个新的 

package com.zhuyang.concurrency.threadpool;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.ScheduledExecutorService;import java.util.concurrent.TimeUnit;public class ThreadPoolTest {public static void main(String[] args) {ExecutorService exe = Executors.newFixedThreadPool(2);// create thread pool,capacity is 2for (int i = 0; i < 4; i++) {// add 4 tasks to poolfinal int taskId = i;exe.execute(new Runnable() {@Overridepublic void run() {for (int j = 0; j < 5; j++) {// each task to print 0-4System.out.println("Thread name " + Thread.currentThread().getName() + " is printing " + j+ ", task id is " + taskId);}}});}}}

Thread name pool-1-thread-1 is printing 0, task id is 0Thread name pool-1-thread-2 is printing 0, task id is 1Thread name pool-1-thread-1 is printing 1, task id is 0Thread name pool-1-thread-1 is printing 2, task id is 0Thread name pool-1-thread-1 is printing 3, task id is 0Thread name pool-1-thread-2 is printing 1, task id is 1Thread name pool-1-thread-1 is printing 4, task id is 0Thread name pool-1-thread-2 is printing 2, task id is 1Thread name pool-1-thread-1 is printing 0, task id is 2Thread name pool-1-thread-1 is printing 1, task id is 2Thread name pool-1-thread-1 is printing 2, task id is 2Thread name pool-1-thread-1 is printing 3, task id is 2Thread name pool-1-thread-1 is printing 4, task id is 2Thread name pool-1-thread-1 is printing 0, task id is 3Thread name pool-1-thread-1 is printing 1, task id is 3Thread name pool-1-thread-1 is printing 2, task id is 3Thread name pool-1-thread-1 is printing 3, task id is 3Thread name pool-1-thread-1 is printing 4, task id is 3Thread name pool-1-thread-2 is printing 3, task id is 1Thread name pool-1-thread-2 is printing 4, task id is 1

2. newCachedThreadPool - 创建一个可缓存的线程池。如果线程池的大小超过了处理任务所需要的线程，那么就会回收部分空闲（60秒不执行任务）的线程，当任务数增加时，此线程池又可以智能的添加新线程来处理任务。此线程池不会对线程池大小做限制，线程池大小完全依赖于操作系统（或者说JVM）能够创建的最大线程大小。

package com.zhuyang.concurrency.threadpool;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.ScheduledExecutorService;import java.util.concurrent.TimeUnit;public class ThreadPoolTest {public static void main(String[] args) {ExecutorService exe =Executors.newCachedThreadPool();for (int i = 0; i < 4; i++) {// add 10 tasks to poolfinal int taskId = i;exe.execute(new Runnable() {@Overridepublic void run() {for (int j = 0; j < 5; j++) {// each task to print 0-4System.out.println("Thread name " + Thread.currentThread().getName() + " is printing " + j+ ", task id is " + taskId);}}});}exe.shutdown();//ScheduledExecutorService  schedulePool =Executors.newScheduledThreadPool(2);//schedulePool.scheduleAtFixedRate(new Runnable(){////@Override//public void run() {//System.out.println("time is up");//}//}, 10,5, TimeUnit.SECONDS);}}

3. newSingleThreadExecutor - 创建一个单线程的线程池。这个线程池只有一个线程在工作，也就是相当于单线程串行执行所有任务。如果这个唯一的线程因为异常结束，那么会有一个新的线程来替代它。此线程池保证所有任务的执行顺序按照任务的提交顺序执行

4. newScheduledThreadPool - 创建一个大小无限的线程池。此线程池支持定时以及周期性执行任务的需求。

ScheduledExecutorService  schedulePool =Executors.newScheduledThreadPool(2);schedulePool.scheduleAtFixedRate(new Runnable(){@Overridepublic void run() {System.out.println("time is up");}}, 10,5, TimeUnit.SECONDS);//创建一个每5秒执行一次的线程，第一次执行定时10秒