【Java并发】线程池的创建
注：《重构与模式》将构造重构成created method，在线程池的创建中隐约看到些影子，故希望能比较下
获得线程池实例
?
1.ThreadPoolExecutor构造
提供了四个构造如下：
ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue)ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, RejectedExecutionHandler handler)ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory)ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory, RejectedExecutionHandler handler)?
其中
int corePoolSize：保留在pool中的线程的数目，即使这些线程是idle（空闲）的，也不会terminate。换句话说就是pool会维持corePoolSize数目的线程。
Q:如果有idle thread，那么这些idle thread占用资源情况怎么样？
A:
int maximumPoolSize：pool所容纳线程的最大数目。
long keepAliveTime：当 number of thread > core thread时，this is the maximum time that excess idle threads will wait for new tasks before terminating.对于那些idle thread，会最长等待keepAliveTime时间来获取新任务，如果仍没有新任务的话则terminate。
BlockingQueue<Runnable> workQueue：任务队列，在任务被执行前，workqueue来hold这些任务。该queue只能用来hold那些由execute方法提交的Runnable型的任务。
Q:通过execute提交的任务都会放到任务队列中么？还是当线程池来不及处理execute提交的任务时放到队列里去？
A:
ThreadFactory threadFactory：线程池利用thread factory来创建线程。
RejectedExecutionHandler handler：当任务的执行被blocked（阻塞），会用到RejectedExecutionHandler
Q:何时任务的执行会blocked？
A:the thread bounds and queue capacities are reached。即当线程的数目已经达到maximumPoolSize，并且任务队列也满了。
源码如下：
public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue) { this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, Executors.defaultThreadFactory(), defaultHandler);}public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory) { this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, threadFactory, defaultHandler);}public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, RejectedExecutionHandler handler) { this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, Executors.defaultThreadFactory(), handler);}public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory, RejectedExecutionHandler handler) { if (corePoolSize < 0 || maximumPoolSize <= 0 || maximumPoolSize < corePoolSize || keepAliveTime < 0) throw new IllegalArgumentException(); if (workQueue == null || threadFactory == null || handler == null) throw new NullPointerException(); this.corePoolSize = corePoolSize; this.maximumPoolSize = maximumPoolSize; this.workQueue = workQueue; this.keepAliveTime = unit.toNanos(keepAliveTime); this.threadFactory = threadFactory; this.handler = handler;}?
?2. Executors静态方法
?
static ExecutorService?newFixedThreadPool(int nThreads)
static ExecutorService?newFixedThreadPool(int nThreads, ThreadFactory threadFactory)
static ExecutorService?newSingleThreadExecutor()
static ExecutorService?newSingleThreadExecutor(ThreadFactory threadFactory)
static ExecutorService?newCachedThreadPool()
static ExecutorService?newCachedThreadPool(ThreadFactory threadFactory)?
【示例代码】
代码1：
?
import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.ThreadFactory;import java.util.concurrent.atomic.AtomicInteger;ExecutorService executor = Executors.newFixedThreadPool(maxThread, new ThreadFactory() { private AtomicInteger seqNo = new AtomicInteger(1); public Thread newThread(Runnable r) { return new Thread(r,"SMS Log Search Engine Thread -- "+seqNo.getAndIncrement()); } });?源码如下
//nThreads ：fixed number of threads，在任何一个时刻，at most只有nThreads个活跃线程在执行任务public static ExecutorService newFixedThreadPool(int nThreads) { return new ThreadPoolExecutor(nThreads, nThreads, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>()); }public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) { return new ThreadPoolExecutor(nThreads, nThreads, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>(), threadFactory); }public static ExecutorService newSingleThreadExecutor() { return new FinalizableDelegatedExecutorService (new ThreadPoolExecutor(1, 1, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>())); }public static ExecutorService newSingleThreadExecutor(ThreadFactory threadFactory) { return new FinalizableDelegatedExecutorService (new ThreadPoolExecutor(1, 1, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>(), threadFactory)); }//当执行许多short-lived asynchronous tasks时，能显著提高性能，一个线程如果超过imin没有任务执行，就会terminated并从cache中移除//so 如果没有任务执行的话，线程池不会占用任何资源public static ExecutorService newCachedThreadPool() { return new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60L, TimeUnit.SECONDS, new SynchronousQueue<Runnable>()); }public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory) { return new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60L, TimeUnit.SECONDS, new SynchronousQueue<Runnable>(), threadFactory); }??