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
ExecutorService threadPool = Executors.newFixedThreadPool(N);
ExecutorService threadPool = Executors.newSingleThreadExecutor();
ExecutorService threadPool = Executors.newCachedThreadPool();
ExecutorService threadPool = Executors.newScheduledThreadPool();
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 = Executors.newFixedThreadPool(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= "+ num.getAndIncrement());
Thread.sleep(100);
}catch (Exception e){
e.printStackTrace();
}
}
}
}
}
结果:
只会使用唯一的工作线程执行任务的线程池:
public class SingleThreadExecutor {
private static AtomicInteger num = new AtomicInteger(0);
private static ExecutorService executorService = Executors.newSingleThreadExecutor();
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= "+ num.getAndIncrement());
Thread.sleep(100);
}catch (Exception e){
e.printStackTrace();
}
}
}
}
}
结果:
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
public class ScheduledThreadPool {
// corePoolSize = 2
private static ScheduledExecutorService service = Executors.newScheduledThreadPool(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:
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 = Executors.newScheduledThreadPool(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:
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 = Executors.newCachedThreadPool();
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= "+num.getAndIncrement());
}
}
}
}
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
若将Tread.sleep(1000)注释掉,你会发现有3个线程在跑
若感兴趣可以去了解一下它们的底层源码,对于CachedThreadPool而言,可新建线程最大数量为INTEGER.MAXIMUM
以newFixedThreadPool为例
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,Executors.defaultThreadFactory(), defaultHandler);
}
线程池七大参数
线程池四大流程
1)创建线程池后,开始等待请求
2)当调用execute()方法添加一个请求任务时,线程池会做以下判断:
3)当一个线程完成任务时,会从等待队列中取下一个任务来执行
4)当空闲线程超过keepAliveTime定义时间,会判断:
Note:阿里巴巴JAVA开发手册:线程池不允许使用Executors去创建线程池,而是通过使用ThreadPoolExecutor的方式自定义线程池,规避资源耗尽的风险
Executors返回的线程池对象的弊端:
1)FixedThreadPool和SingleThreadPool:
允许请求队列长度为Integer.MAX_VALUE,可能会堆积大量请求导致OOM
2)CachedThreadPool和ScheduledThreadPool:
允许创建线程数量为Integer.MAX_VALUE,可能会创建大量的线程导致OOM
拒绝策略
1)AbortPolicy
直接抛出RejectedExecutionException异常阻止系统正常运行
2)CallerRunsPolicy
"调用者运行"的调节机制,该策略既不会抛弃任务,也不会抛出异常,而是将某些任务回退到调用者,从而降低新任务的流量
3)DiscardPolicy
该策略抛弃无法处理的任务,不予任何处理也不抛出异常。如果允许任务丢失,这是最好的一种策略
4)DiscardOldestPolicy
抛弃队列中等待最久的任务,然后把当前任务加入队列中尝试再次提交当前任务
如何设置maximumPoolSize大小
Runtime.getRuntime().availableProcessors()方法获取核数
CPU密集型
maximumPoolSize设为核数+1
IO密集型
maximumPoolSize设为核数/阻塞系数
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原文链接 : https://blog.csdn.net/weixin_60707895/article/details/119805517
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