如同数据库连接一样，线程的创建、切换和销毁同样会耗费大量的系统资源。为了复用创建好的线程，减少频繁创建线程的次数，提高线程利用率可以引用线程池技术。使用线程池的优势有如下几点：
1、保持一定数量的线程，减少了线程频繁创建和销毁资源消耗。
2、使用线程的时候直接由线程池中取出线程，省去了创建线程的时间，侧面提高了系统的响应时间。
3、需要使用线程的时候直接从线程池中取出，避免了人为的不合理创建线程，减少了潜在的风险。
Doug Lea在实现JUC中提供了原生的线程池，并提供了各种线程管理策略来适应于不同的使用场景。使用的时候可通过Executors获取各种线程池实例。
这里提供了六对12个方法来创建ExecutorService，其中每种类型的ExecutorService可以适用于不同的应用场景，对线程的管理策略也各不相同。下面就看一下各个方法的注释：
/**
* Creates a thread pool that creates new threads as needed, but will reuse previously constructed threads when they are available.
* 创建一个线程池，需要的时候会创建新的线程，如果有可用的线程则会复用以前已经创建好的线程。
* These pools will typically improve the performance of programs that execute many short-lived asynchronous tasks.
* 这些线程池通常情况下可以提升哪些短期异步任务的性能
* Calls to {@code execute} will reuse previously constructed threads if available.
* 如果以创建的线程状态可用的话，调用execute可以复用他们
* If no existing thread is available, a new thread will be created and added to the pool.
* 如果不存在可用状态的线程，那么将会创建一个新线程同时会把该线程添加到线程池中
* Threads that have not been used for sixty seconds are terminated and removed from the cache.
*那些超过60s没用的线程将会被销毁同时从缓存中移除
* Thus, a pool that remains idle for long enough will not consume any resources.
*因此长时间空闲的线程池不会消耗任何资源
* Note that pools with similar properties but different details (for example, timeout parameters) may be created using {@link ThreadPoolExecutor} constructors.
*可以使用ThreadPoolExecutor创建性质相似但实现细节不同的线程池
* @return the newly created thread pool
*/
public static ExecutorService newCachedThreadPool();
//可以使用自定义的ThreadFactory 类创建线程，其它和无参方法一致
public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory);
/**
* Creates a thread pool that reuses a fixed number of threads operating off a shared unbounded queue.
*创建一个可重用、固定数量线程的线程池
* At any point, at most {@code nThreads} threads will be active processing tasks.
*任何时间最多只有 nThreads 个线程被激活来执行任务
* If additional tasks are submitted when all threads are active, they will wait in the queue until a thread is available.
* 当无可用空闲线程的时候，如果有新任务被提交，这些新任务将会一直等待直至有可用线程来执行。
* If any thread terminates due to a failure during execution prior to shutdown, a new one will take its place if needed to execute subsequent tasks.
*如果任何线程正常关闭之前在执行过程中因失败而提前终止，那么如果有未被执行的后续任务，则会创建新的线程来继续执行。
* The threads in the pool will exist until it is explicitly {@link ExecutorService#shutdown shutdown}.
* 线程池中的所有线程在明确掉用shutdown之后将会退出
*
* @param nThreads the number of threads in the pool
* @return the newly created thread pool
* @throws IllegalArgumentException if {@code nThreads <= 0}
*/
public static ExecutorService newFixedThreadPool(int nThreads);
//可自定义ThreadFactory
public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory);
/**
* Creates a thread pool that can schedule commands to run after a given delay, or to execute periodically.
* 创建一个线程池，该线程在延迟指定时间之后可以周期性的执行线程体
* @param corePoolSize the number of threads to keep in the pool,
* even if they are idle
* @return a newly created scheduled thread pool 注意返回值类型是ScheduledExecutorService，不要使用ExecutorService来接收，否则找不到schedule执行方法
* @throws IllegalArgumentException if {@code corePoolSize < 0}
*/
public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize);
//可自定义ThreadFactory
public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize, ThreadFactory threadFactory);
/**
* Creates an Executor that uses a single worker thread operating off an unbounded queue.
*创建一个Executor，使用一个线程来工作，该线程存储在LinkedBlockingQueue中
* (Note however that if this single thread terminates due to a failure during execution prior to shutdown, a new one will take its place if needed to execute subsequent tasks.)
*注，如果任何线程正常关闭之前在执行过程中因失败而提前终止，那么如果有未被执行的后续任务，则会创建新的线程来继续执行。
* Tasks are guaranteed to execute sequentially, and no more than one task will be active at any given time.
* 任务是按顺序执行的，任何时间都只有一个线程来执行任务
* Unlike the otherwise equivalent {@code newFixedThreadPool(1)} the returned executor is guaranteed not to be reconfigurable to use additional threads.
*
* @return the newly created single-threaded Executor
*/
public static ExecutorService newSingleThreadExecutor();
//可自定义ThreadFactory
public static ExecutorService newSingleThreadExecutor(ThreadFactory threadFactory);
/**
* Creates a single-threaded executor that can schedule commands
* to run after a given delay, or to execute periodically.
* (Note however that if this single
* thread terminates due to a failure during execution prior to
* shutdown, a new one will take its place if needed to execute
* subsequent tasks.) Tasks are guaranteed to execute
* sequentially, and no more than one task will be active at any
* given time. Unlike the otherwise equivalent
* {@code newScheduledThreadPool(1)} the returned executor is
* guaranteed not to be reconfigurable to use additional threads.
* @return the newly created scheduled executor
*/
public static ScheduledExecutorService newSingleThreadScheduledExecutor();
public static ScheduledExecutorService newSingleThreadScheduledExecutor(ThreadFactory threadFactory)
使用示例：
package thread.blogs.threadpool;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
/**
* Created by PerkinsZhu on 2017/8/31 13:59.
*/
public class PoolTest {
public static void main(String[] args) {
// testCachedThreadPool();
// testSingleThreadExecutor();
// testFixedThreadPool();
testScheduledThreadPool();
}
private static ThreadFactory myFactory = new MyThreadFactory();
private static void testSingleThreadExecutor() {
//一个一个的依次执行
doHandle(Executors.newSingleThreadExecutor(myFactory));
}
private static void testFixedThreadPool() {
//两个两个的一起执行
doHandle(Executors.newFixedThreadPool(2,myFactory));
}
private static void testCachedThreadPool() {
//10个一起一次性执行完
doHandle(Executors.newCachedThreadPool(myFactory));
}
private static void testScheduledThreadPool() {
//定时周期执行
Executors.newScheduledThreadPool(1,myFactory).scheduleAtFixedRate(runnable, 500, 2000, TimeUnit.MILLISECONDS);
}
private static Runnable runnable = () -> {
sleep(1000);
System.out.println(Thread.currentThread().getName() + " work!!!");
};
private static void doHandle(ExecutorService executorService) {
for (int i = 0; i < 10; i++) {
executorService.execute(runnable);
}
executorService.shutdown();
}
private static void sleep(int time) {
try {
Thread.sleep(time);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
class MyThreadFactory implements ThreadFactory {//自定义ThreadFactory
private static final AtomicInteger poolNumber = new AtomicInteger(1);
private final ThreadGroup group;
private final AtomicInteger threadNumber = new AtomicInteger(1);
private final String namePrefix;
MyThreadFactory() {
SecurityManager s = System.getSecurityManager();
group = (s != null) ? s.getThreadGroup() : Thread.currentThread().getThreadGroup();
namePrefix = "☆☆☆--" + poolNumber.getAndIncrement() + "-****-";
}
public Thread newThread(Runnable r) {
Thread t = new Thread(group, r, namePrefix + threadNumber.getAndIncrement(), 0);
if (t.isDaemon()) {
t.setDaemon(false);
}
if (t.getPriority() != Thread.NORM_PRIORITY) {
t.setPriority(Thread.NORM_PRIORITY);
}
return t;
}
}
在使用的时候必须明白各种线程池是否适用于自己的情景，选取合适的线程池进行使用。
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原文链接：多线程(七)JDK原生线程池转载请注明出处！
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