爲什麼要有線程池來,可以參考這篇Blog Java線程實現原理
首先來簡單看下Java中兩種啓動線程的方式。
//Extends Thread
public class MyThread extends Thread {
@Override
public void run(){
//TODO
}
public void main(String[] args){
new MyThread().start();
}
}
// Implements Runnable
public class MyRunnable implements Runnable{
@Override
public void run(){
//TODO
}
public void main(String[] args){
new Thread(new MyRunnable()).start();
}
}
第一種方式需要繼承一個Thread類,來擴展Thread的類的功能已達到運行自身邏輯的目的,從軟件設計上來看這種方式使用繼承來擴展功能,不是推薦的做法;從Java語言特性上來說,單繼承的規約限制了MyThread的對其他類的功能的複用。因此這種方式看似簡單,但是不是推薦啓動線程的方法。
第二種方式通過集成Runnable接口實現其run方法完成業務邏輯的編寫,並將MyRunnable的實例傳給一個Thread實例,並調用Thread實例的start方法進行線程的啓動。這種做法的好處是將邏輯功能與線程本事解耦出來,業務邏輯不用受到Thread的類的限制。因此這種方式算是比較合理的方式。
但是這兩種方式的一個共同特點是自己啓動一個Thread來運行,結合Java線程實現原理來看,Java的線程其實是映射爲Linux的一個內核線程來實現的,因而線程的個數也收到了限制,同事過多的啓動線程來完成任務也會找成CPU頻繁的進行上下文切換造成服務器不穩定,資源分配不合理。爲了解決這個問題Java線程池應允而生。
本文只介紹ThreadPoolExecutor因此本文涉及的類的繼承結構如下所示:
兩個比較重要的類
Executors: 快速創建ThreadPoolExecutor的工具類
ThreadPoolExecutor: 這正的線程池實現者
廢話不說,先看一個實例。
public class ThreadPoolTest{
private static ExecutorService executorService = Executors.newFixedThreadPool(5);
public static void main(String[] args){
executorService.execute(new MyRunnable());
}
}
怎麼樣使用起來簡單易懂。
在來看看ThreadPoolExecutor的構造方法
/**
* Creates a new {@code ThreadPoolExecutor} with the given initial
* parameters.
*
* @param corePoolSize the number of threads to keep in the pool, even
* if they are idle, unless {@code allowCoreThreadTimeOut} is set
* @param maximumPoolSize the maximum number of threads to allow in the
* pool
* @param keepAliveTime when the number of threads is greater than
* the core, this is the maximum time that excess idle threads
* will wait for new tasks before terminating.
* @param unit the time unit for the {@code keepAliveTime} argument
* @param workQueue the queue to use for holding tasks before they are
* executed. This queue will hold only the {@code Runnable}
* tasks submitted by the {@code execute} method.
* @param threadFactory the factory to use when the executor
* creates a new thread
* @param handler the handler to use when execution is blocked
* because the thread bounds and queue capacities are reached
* @throws IllegalArgumentException if one of the following holds:<br>
* {@code corePoolSize < 0}<br>
* {@code keepAliveTime < 0}<br>
* {@code maximumPoolSize <= 0}<br>
* {@code maximumPoolSize < corePoolSize}
* @throws NullPointerException if {@code workQueue}
* or {@code threadFactory} or {@code handler} is null
*/
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler)
註釋已經對這些參數解釋的足夠清晰,但是要真是使用起來,new出一個ThreadPoolExecutor實例也是比較麻煩的,好在Executors工具類提供了三個適合常見場景的工廠方法,使用起來非常方便快捷。
public static ExecutorService newFixedThreadPool(int nThreads);
public static ExecutorService newSingleThreadExecutor();
public static ExecutorService newCachedThreadPool();