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();
}
}
}
}