about_newSingleThreadExecutor

关于 Executors.newSingleThreadExecutor()

本以为只是一个简单的单线程线程池，与 Executors.newFixedThreadExecutor(1) 类似，但它的 DOC 注释中又显式提到：

Unlike the otherwise equivalent {@code newFixedThreadPool(1)} the returned executor is guaranteed not to be reconfigurable to use additional threads.

与类似功能的 newFixedThreadPool(1)不一样，该方法返回的线程池执行器不可以被重新配置以使用额外的线程。

关键在于不可以重新配置。来看看它的构造器：

public static ExecutorService newSingleThreadExecutor() {
    // FinalizableDelegatedExecutorService 装饰器
    return new FinalizableDelegatedExecutorService
        (new ThreadPoolExecutor(1, 1, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>()));
}

/**
 * FinalizableDelegatedExecutorService 装饰器，普普通通。但多了一个 finalize() 方法。
 */
static class FinalizableDelegatedExecutorService extends DelegatedExecutorService {
    FinalizableDelegatedExecutorService(ExecutorService executor) {
        super(executor);
    }
    // 关于 finalize()，在『深入理解Java虚拟机』中有提到：可达性分析后，没有与 GC Roots 相连接的对象
    // 如果 Override 了 finalize()，且该方法还没有被执行过，则该对象会被添加到 F-Queue 的队列中，并
    // 稍后在一个由虚拟机发起的自动建立的、低优先级的 Finalizer 线程中去执行 finalize()，但并不承诺会
    // 等到该方法执行完成。
    // FinalizableDelegatedExecutorService 会尝试在被 GC 时关闭线程池。
    protected void finalize() {
        super.shutdown();
    }
}

/**
  * A wrapper class that exposes only the ExecutorService methods
  * of an ExecutorService implementation.
  * 仅暴露 ExecutorService 接口的方法
  */
static class DelegatedExecutorService extends AbstractExecutorService {
    private final ExecutorService e;
    // 构造方法
    DelegatedExecutorService(ExecutorService executor) { e = executor; }
    public void execute(Runnable command) { e.execute(command); }
    public void shutdown() { e.shutdown(); }
    ...
}

再看看 ThreadPoolExecutor 的实现：👇

/**
 * ThreadPoolExecutor 和 DelegatedExecutorService 一样都是 AbstractExecutorService 抽象类的子类
 */
public class ThreadPoolExecutor extends AbstractExecutorService

那我们试试，reconfigurable newSingleThreadExecutor 会怎样：👇

@Test
public void singleThreadPoolExecutorTest(){
    ExecutorService fixed = Executors.newFixedThreadPool(1);
    ThreadPoolExecutor t1 = (ThreadPoolExecutor)fixed;
    // 正常执行
    t1.setCorePoolSize(2);
  
    ExecutorService single = Executors.newSingleThreadExecutor();
    // 强转报错:java.lang.ClassCastException: java.util.concurrent.Executors$FinalizableDelegatedExecutorService cannot be cast to java.util.concurrent.ThreadPoolExecutor
    // 都是 AbstractExecutorService 的子类，没法儿强转
    ThreadPoolExecutor t2 = (ThreadPoolExecutor)single;
    t2.setCorePoolSize(2);
} 

/**
 * newFixedThreadPool 返回的是 ThreadPoolExecutor，ThreadPoolExecutor 有 setCorePoolSize()，
 * 自然没问题。
 */
public static ExecutorService newFixedThreadPool(int nThreads) {
    return new ThreadPoolExecutor(nThreads, nThreads, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>());
}