Java源码阅读之Executors

Summary：

该类可以生成四种类型的线程池：

不限定数量的线程池
限定数量的线程池
只有一个线程的线程"池"
为预定执行而构建的线程池
前三者是用ThreadPoolExecutor来构建线程池的；返回的对象是一个ExecutorService；因为ThreadPoolExecutor继承了 AbstractExecutorService类，也就间接的实现了ExecutorService接口；
后者特殊单独讲

该类定义了内部适配器类，用于将Runnable转换为Callable类型，即变成带返回值的运行方法，当然这个返回值默认值是null；在FuctureTash构造器会用到
该类定义了一个线程工厂；从该工厂创建出来的线程都是非守护线程，优先级normal，的普通线程；在ThreadPoolExecutor构造器中被使用；

newCachedThreadPool：

//必要时创建新线程；空闲线程保留60秒
//这里的SynchronousQueue是一个同步队列，即size永远为0，存的数只有被取走才能再存一个；
//即如果当前没有空线程则创建一个新线程；空线程保留60s后自动销毁
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}

newFixedThreadPool：

//包含固定数量的线程，空线程一直保留
//创建nThreads个线程，请求如果没有空线程可以自行则存入LinkedBlockingQueue，该队列可以存入的线程是无界的；(可能会耗尽系统资源！！！任务疯涨的情况)
public static ExecutorService newFixedThreadPool(int nThreads){
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}

newSingleThreadExecutor：

//只有一个线程的线程池
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}

newScheduledThreadPool

//预定执行而构建的固定线程池
public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
return new ScheduledThreadPoolExecutor(corePoolSize);
}

内部类

DefaultThreadFactory
//ThreadPoolExecutor会用到
static class DefaultThreadFactory implements ThreadFactory {
private static final AtomicInteger poolNumber = new AtomicInteger(1);
private final ThreadGroup group;
private final AtomicInteger threadNumber = new AtomicInteger(1);
private final String namePrefix;

DefaultThreadFactory() {
SecurityManager s = System.getSecurityManager();
group = (s != null) ? s.getThreadGroup() :
Thread.currentThread().getThreadGroup();
namePrefix = "pool-" + poolNumber.getAndIncrement() +"-thread-";
}

public Thread newThread(Runnable r) {
Thread t = new Thread(group, r,
namePrefix + threadNumber.getAndIncrement(),
0);
if (t.isDaemon())
t.setDaemon(false); //非守护线程
if (t.getPriority() != Thread.NORM_PRIORITY)
t.setPriority(Thread.NORM_PRIORITY);
return t;
}
}

RunnableAdapter

//FuctureTask构造器会用到
//将Runnable转化为Callable<T>对象
public static <T> Callable<T> callable(Runnable task, T result) {
if (task == null)
throw new NullPointerException();
return new RunnableAdapter<T>(task, result);
}
//RunnableAdapter是内部类，适配器模式
static final class RunnableAdapter<T> implements Callable<T> {
final Runnable task;
final T result;
RunnableAdapter(Runnable task, T result) {
this.task = task;
this.result = result;
}
public T call() {
task.run();
return result;
}
}