JAVA线程池

从JDK1.5之后的版本，加入了java.util.concurentt包，其中有java.util.concurrent.Executors和java.util.concurrent.ExecutorService实现线程池操作。线程池可以更加有效地管理线程，避免对资源的浪费和线程过多造成的拥挤。

线程池的顶级接口是Executors。严格得说Executors并不是实际的接口，而是为了实现线程池的工具，真正的接口是ExecutorService

要实现线程池是很不容易的，因此JAVA提供了静态工厂，生成一些常用的线程池。

1、newCachedThreadPool ：创建一个可缓存线程池，如果线程池长度超过处理需要，可灵活回收空闲线程，若无可回收，则新建线程。

2、newFixedThreadPool  ：创建一个定长线程池，可控制线程最大并发数，超出的线程会在队列中等待。

3、newSheduledThreadPool  ：创建一个定长线程池，支持定时及周期性任务执行。

4、newSingledThreadExecutor  ：创建唯一的线程，它只会用唯一的工作线程来执行任务，保证所有任务按照指定顺序(FIFO, LIFO, 优先级)执行

(1)newCachedThreadPool 

<span style="font-size:18px;">import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;public class Problem5test {public static void main(String[] args) {ExecutorService cachedThreadPool = Executors.newCachedThreadPool();for (int i = 0; i < 10; i++) {final int index = i;try {Thread.sleep(index * 1000);}catch (InterruptedException e) {e.printStackTrace();}cachedThreadPool.execute(new Runnable() {public void run() {System.out.println(index);}                 }   );      }   }}</span>

线程池为无限大，当执行第二个任务时第一个任务已经完成，会复用执行第一个任务的线程，而不用每次新建线程。
(2)newFixedThreadPool

import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;public class Problem5test {public static void main(String[] args) {ExecutorService fixedThreadPool = Executors.newFixedThreadPool(3);//定下是3个线程for (int i = 0; i < 10; i++) {final int index = i;fixedThreadPool.execute( new Runnable() {public void run() {try {System.out.println(index);Thread.sleep(2000);} catch (InterruptedException e) {e.printStackTrace();}}});}}}

因为线程池大小为3，每个任务输出index后sleep 2秒，所以每两秒打印3个数字。
定长线程池的大小最好根据系统资源进行设置。如Runtime.getRuntime().availableProcessors()
(3)newScheduledThreaPool

import java.util.concurrent.Executors;import java.util.concurrent.ScheduledExecutorService;import java.util.concurrent.TimeUnit;public class Problem5test {public static void main(String[] args) {ScheduledExecutorService scheduledThreadPool = Executors.newScheduledThreadPool(5);scheduledThreadPool.schedule(new Runnable() {public void run() {System.out.println("delay 3 seconds");}}, 3, TimeUnit.SECONDS);}}

延迟3秒执行run();

(4)newSingledTheadExecutor

import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;public class Problem5test {public static void main(String[] args) {ExecutorService singleThreadExecutor = Executors.newSingleThreadExecutor();for (int i = 0; i < 10; i++) {final int index = i;singleThreadExecutor.execute(new Runnable() {public void run() {try {System.out.println(index);Thread.sleep(2000);} catch (InterruptedException e) {e.printStackTrace();}}});}}}

每2秒执行一次。

import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;public class Problem5test {public static void main(String[] args) {ExecutorService singleThreadExecutor = Executors.newCachedThreadPool();for (int i = 0; i < 10; i++) {final int index = i;singleThreadExecutor.execute(new Runnable() {public void run() {try {while(true) {System.out.println(index);Thread.sleep(10 * 1000);}} catch (InterruptedException e) {e.printStackTrace();}}});try {Thread.sleep(5000);} catch (InterruptedException e) {e.printStackTrace();}}}}