java 线程池的使用
1.线程池的概念
线程池:创建线程的生命周期包括创建、就绪,阻塞和销毁阶段,当我们要执行的任务比较小且频繁时,会导致重复的创建线程,避免资源的消耗。线程池中存放我们需要数量的 线程,使用已经创建好的线程去执行我们的操作,避免频繁的创建。
Java通过Executors提供四种线程池,分别为:
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.为什要使用线程池?
1.单个的任务之多,可以使多个任务有序的执行
2.处理任务比较频繁,避免重复的创建和销毁线程,减少cpu和资源的使用
3.几种线程池具体的使用方法
newCachedThreadPool:可缓存的线程池的使用
ExecutorService cachedThreadPool = Executors.newCachedThreadPool();
for(int i = 1;i<=5;i++){
try {
Thread.sleep(i*1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
final int finalI = i;
cachedThreadPool.execute(new Runnable() {
@Override
public void run() {
System.out.println("当前执行的 idx=" + finalI);
}
});
int threadCount = ((ThreadPoolExecutor)cachedThreadPool).getActiveCount();//这边有事获取的是1有时是0
System.out.println("当前活动的线程数 for=" + threadCount);
int poolCount = ((ThreadPoolExecutor)cachedThreadPool).getPoolSize();
System.out.println("当前池线程数 for=" + poolCount); //这边使用了缓存的线程,获取到的线程数一直视1
}
int threadCount = ((ThreadPoolExecutor)cachedThreadPool).getActiveCount();
System.out.println("当前活动的线程数=" + threadCount);
int poolCount = ((ThreadPoolExecutor)cachedThreadPool).getPoolSize();
System.out.println("当前池线程数=" + poolCount);
getActiveCount:获取当前的线程池的活动的线程数 getPoolSize:获取当前的线程池的线程数
newFixedThreadPool:定长的线程池,超过消息队列会继续等待
ExecutorService fixedThreadPool = Executors.newFixedThreadPool(4);
for(int i = 1;i<=5;i++){
final int finalI = i;
if (!fixedThreadPool.isShutdown()) {
fixedThreadPool.execute(new Runnable() {
@Override
public void run() {
System.out.println("当前执行的 idx=" + finalI);
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});
}
//不要显示调用shutdown方法,可能会导致RejectedExecutionException,或者判断是否fixedThreadPool.isShutdown(),
// 如果已经shutdown就不要调用excute方法了
// fixedThreadPool.shutdown();
// List<Runnable> list = fixedThreadPool.shutdownNow();
// System.out.println("未完task list size=" + list.size());
}
这边需要注意的是shutdown方法的调用,在执行excute方法前,先判断是否已经shutdown了
newScheduledThreadPool:定长线程池,可以定时和周期的执行
ScheduledExecutorService scheduleThreadPool = Executors.newScheduledThreadPool(1);
scheduleThreadPool.schedule(new Runnable() {
@Override
public void run() {
System.out.println("让我们跑起来,我只执行一次" );
}
},1, TimeUnit.SECONDS);
final int[] idx = {0};
scheduleThreadPool.scheduleWithFixedDelay(new Runnable() {
@Override
public void run() {
idx[0] = idx[0] +1;
System.out.println("让我们跑起来,循环间隔=" + idx[0]);
}
},1,1,TimeUnit.SECONDS);
// scheduleThreadPool.shutdown();//这边调用shutdown没有问题
scheduleThreadPool:这边的schedule完全可以替换thread的定时操作了,^_^
newSingleThreadExecutor:单线程化的线程池,可以指定执行的顺序
ExecutorService singleThreadPool = Executors.newSingleThreadExecutor();
singleThreadPool.execute(new Runnable() {
@Override
public void run() {
System.out.println("我是一个单线程化线程池,ThreadName=" + Thread.currentThread().getName());
}
});
//newSingleThreadScheduledExecutor创建的就是ScheduledThreadPoolExecutor
/*
* public static ScheduledExecutorService newSingleThreadScheduledExecutor() {
return new DelegatedScheduledExecutorService
(new ScheduledThreadPoolExecutor(1));
}
*/
ScheduledExecutorService singleScheduleThreaPool= Executors.newSingleThreadScheduledExecutor();
4.遇到的问题总结
上面的常量操作只有数组可以进行自加的操作
ExecutorService singleThreadPool = Executors.newSingleThreadExecutor();
singleThreadPool.execute(new Runnable() {
@Override
public void run() {
System.out.println("我是一个单线程化线程池,ThreadName=" + Thread.currentThread().getName());
}
});
//newSingleThreadScheduledExecutor创建的就是ScheduledThreadPoolExecutor
/*
* public static ScheduledExecutorService newSingleThreadScheduledExecutor() {
return new DelegatedScheduledExecutorService
(new ScheduledThreadPoolExecutor(1));
}
*/
ScheduledExecutorService singleScheduleThreaPool= Executors.newSingleThreadScheduledExecutor();
Future a =singleScheduleThreaPool.submit(new Runnable() {
@Override
public void run() {
System.out.println("我执行了");
}
},1);
try {
System.out.println("给出结果了没=" + (Integer)a.get());
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
这边需要注意excute和submit方法都可执行当前的task,submit可以指定返回的结果,如果不需要返回值,直接用excute方法
初次使用线程池,如有不足,大家多多见谅,多多指教^-^