阿⾥⼆⾯被问JavaThreadPool线程池，看完这篇，⼀招怒怼
⾯试官
线程池的优势
线程池做的⼯作主要是控制运⾏的线程数量，处理过程中将任务放⼊队列，然后在线程创建后启动这些任务，如果线程数量超过了最⼤数量，超出的线程排队等候，等待其他线程执⾏完毕，再从队列中取出任务来执⾏
线程池的特点
线程复⽤、控制最⼤并发数、管理线程
降低资源消耗。重复利⽤已创建的线程，降低创建和销毁线程的开销
提⾼响应速度。当任务到达时，任务可以不需要等待线程创建就能⽴刻执⾏
提⾼线程的可管理性。使⽤线程池可以对线程进⾏统⼀的分配、调优和监控
1 线程池的⽅法
java线程池创建的四种执⾏长期任务性能好，创建⼀个线程池，⼀池有N个固定的线程，可以控制线程最⼤并发数，有固定线程数的线程池ExecutorService threadPool = wFixedThreadPool(N);
单个任务执⾏，它只会使⽤单个⼯作线程，⼀池⼀线程
ExecutorService threadPool = wSingleThreadExecutor();
执⾏短期异步任务，可缓存线程池，线程池根据需要创建新线程，但在先前构造的线程可以复⽤，也可灵活回收空闲的线程，可扩容的池
ExecutorService threadPool = wCachedThreadPool();
周期性线程池；⽀持定时及周期性任务执⾏
ExecutorService threadPool = wScheduledThreadPool();
(1) newFixedThreadPool
可以控制线程最⼤并发数的线程池：
public class FixedThreadPool {
private static AtomicInteger num = new AtomicInteger(0);
private static ExecutorService executorService = wFixedThreadPool(2);
public static void main(String[] args) {
countSum c= new countSum();
//将coutSum作为Task，submit⾄线程池
for (int i = 0; i < 2; i++) {
executorService.submit(c);
}
//Task执⾏完成后关闭
executorService.shutdown();
}
static class countSum implements Runnable{
@Override
public void run() {
for (int i = 0; i < 500; i++) {
try{
System.out.println("Thread - "+Thread.currentThread().getName()+" count= "+ AndIncrement()); Thread.sleep(100);
}catch (Exception e){
e.printStackTrace();
}
}
}
}
}
结果：
阿⾥⼆⾯被问Java ThreadPool线程池，看完这篇，⼀招怒怼⾯试官
(2) newSingleThreadExecutor
只会使⽤唯⼀的⼯作线程执⾏任务的线程池：
private static AtomicInteger num = new AtomicInteger(0);
private static ExecutorService executorService = wSingleThreadExecutor();
public static void main(String[] args) {
//将coutSum作为Task，submit⾄线程池
for (int i = 0; i < 2; i++) {
executorService.submit(new countSum());
}
//Task执⾏完成后关闭
executorService.shutdown();
}
static class countSum implements Runnable{
@Override
public void run() {
for (int i = 0; i < 500; i++) {
try{
System.out.println("Thread - "+Thread.currentThread().getName()+" count= "+ AndIncrement()); Thread.sleep(100);
}catch (Exception e){
e.printStackTrace();
}
}
}
}
}
结果：
阿⾥⼆⾯被问Java ThreadPool线程池，看完这篇，⼀招怒怼⾯试官
(3) newScheduledThreadPool
传参值为corePoolSize⼤⼩，⽀持定时及周期性任务执⾏
延期执⾏⽰例：调⽤schedule⽅法，三个参数：Task，Delay，TimeUnit
// corePoolSize = 2
private static ScheduledExecutorService service = wScheduledThreadPool(2);
public static void main(String[] args) {
System.out.println("Thread - "+Thread.currentThread().getName()+" BEGIN "+ new Date());
service.schedule(new print(),5, TimeUnit.SECONDS);
service.shutdown();
}
static class print implements Runnable{
@Override
public void run() {
for (int i = 0; i < 10; i++) {
try{
System.out.println("Thread - "+Thread.currentThread().getName()+" Delay 5 second and sleep 2 second "+ new Date()); Thread.sleep(2000);
}catch (Exception e){
e.printStackTrace();
}
}
}
}
}
结果：
阿⾥⼆⾯被问Java ThreadPool线程池，看完这篇，⼀招怒怼⾯试官
定时执⾏⽰例：调⽤scheduleAtFixedRate⽅法，四个参数：Task，initialDelay，Period，TimeUnit
public class ScheduledThreadPool {
// corePoolSize = 1
private static ScheduledExecutorService service = wScheduledThreadPool(1);
public static void main(String[] args) {
System.out.println("Thread - "+Thread.currentThread().getName()+" BEGIN "+ new Date());
service.scheduleAtFixedRate(new print(),5,3,TimeUnit.SECONDS);
}
static class print implements Runnable{
@Override
public void run() {
System.out.println("Thread - "+Thread.currentThread().getName()+" Delay 5 second and period 3 second "+ new Date());
}
}
}
结果：
阿⾥⼆⾯被问Java ThreadPool线程池，看完这篇，⼀招怒怼⾯试官
(4) newCachedThreadPool
可缓存线程池，如果线程池长度超过处理需要，回收空闲线程，若⽆可回收，则新建线程。即若前⼀个任务已完成，则会接着复⽤该线程：public class CachedThreadPool {
private static AtomicInteger num = new AtomicInteger(0);
private static ExecutorService service = wCachedThreadPool();
public static void main(String[] args) {
countSum c = new countSum();
for (int i = 0; i < 3; i++) {
try {
service.submit(c);
Thread.sleep(1000);
}catch (Exception e){
e.printStackTrace();
}
}
service.shutdown();
}
static class countSum implements Runnable{
@Override
public void run() {
for (int i = 0; i < 1000; i++) {
System.out.println("Thread - "+Thread.currentThread().getName()+" countSum= "+AndIncrement());
}
}
}
}
结果：Thread.sleep(1000)即sleep⼀秒，上个任务完成可继续复⽤该线程，不需要创建新的线程
阿⾥⼆⾯被问Java ThreadPool线程池，看完这篇，⼀招怒怼⾯试官
若将Tread.sleep(1000)注释掉，你会发现有3个线程在跑
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