jdk源代码concurrent说明及底层执行简单流程

摘要：实现初始化线程池数量策略并执行线程。二简述下从定义线程池，到其底层执行的简单流程简单定义了如上两句代码，自动创建个固定大小的线程的线程池，实现了接口的对象以后，自动启动线程并执行对象的方法。关键的调用类方法，管理线程池并执行任务。

一.类图说明

Executor接口类，执行Runnable接口execute。

ExecutorService接口类继承Executor接口，包含提交执行Runnable和Callable接口submit以及shutdown，invokeAll接口。

ScheduledExecutorService接口类继承ExecutorService接口，主要包含计划执行接口schedule，scheduleAtFixedRate以及scheduleWithFixedDelay。

虚类AbstractExecutorService，实现ExecutorService接口。实现ExecutorService相关接口。

ThreadPoolExecutor类，继承虚类AbstractExecutorService。实现初始化线程池数量、策略并执行线程。

ScheduledThreadPoolExecutor类，继承ThreadPoolExecutor类并实现ScheduledExecutorService接口。实现计划执行相关接口。

执行类，定义ThreadPoolExecutor和ScheduledThreadPoolExecutor类，并使用相关concurrent类方法。

二.简述下从定义线程池，到其底层执行的简单流程

ExecutorService executorService = Executors.newFixedThreadPool(3);
executorService.submit(new NewTask());

简单定义了如上两句代码，JDK自动创建3个固定大小的线程的线程池，submit实现了Runnable接口的NewTask对象以后，JDK自动启动线程并执行NewTask对象的run方法。流程是如何的呢？

1.Executors的newFixedThreadPool方法new了一个ThreadPoolExecutor对象，且new了一个LinkedBlockingQueue对象。

public static ExecutorService newFixedThreadPool(int nThreads) {
        return new ThreadPoolExecutor(nThreads, nThreads,
                                      0L, TimeUnit.MILLISECONDS,
                                      new LinkedBlockingQueue());
    }
    

2.调用ThreadPoolExecutor的构造函数，其中ThreadFactory默认使用defaultThreadFactory，defaultHandler为AbortPolicy。

public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue workQueue) {
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
             Executors.defaultThreadFactory(), defaultHandler);
    }

public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue 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;
    }
    

3.初始化以后，调用AbstractExecutorService类的submit方法，执行Runnable对象。

public Future submit(Runnable task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture ftask = newTaskFor(task, null);
        execute(ftask);
        return ftask;
    }
    

4.RunnableAdapter实现了Callable接口，FutureTask类包含callable类成员对象。
FutureTask实现RunnableFuture接口，RunnableFuture继承Runnable接口。所以newTaskFor(task, null)方法返回一个FutureTask对象。

public class FutureTask implements RunnableFuture

public interface RunnableFuture extends Runnable, Future

RunnableFuture ftask = newTaskFor(task, null);

protected  RunnableFuture newTaskFor(Runnable runnable, T value) {
    return new FutureTask(runnable, value);
}

public FutureTask(Runnable runnable, V result) {
    this.callable = Executors.callable(runnable, result);
    this.state = NEW;       // ensure visibility of callable
}

public static  Callable callable(Runnable task, T result) {
    if (task == null)
        throw new NullPointerException();
    return new RunnableAdapter(task, result);
}

static final class RunnableAdapter implements Callable {
    final Runnable task;
    final T result;
    RunnableAdapter(Runnable task, T result) {
        this.task = task;
        this.result = result;
    }
    public T call() {
        task.run();
        return result;
    }
}

5.关键的调用ThreadPoolExecutor类execute方法，管理线程池并执行Runnable任务。主要逻辑如下：
1）如果线程数量小于corePoolSize，新来一个任务，创建一个新线程。
2）如果线程数量等于corePoolSize，则将任务缓存到workQueue中。
3）如果线程数量等于corePoolSize并且workQueue队列已满，则继续创建线程直到等于maximumPoolSize。
4）如果线程数量等于maximumPoolSize，且workQueue队列已满，则根据defaultHandler策略执行相应措施。默认是AbortPolicy，抛出一个运行时异常RejectedExecutionException。另外还有3种策略：CallerRunsPolicy->如果线程池没有shutdown，则直接调用Runnable的run方法执行。如果线程池shutdown，则直接丢弃；DiscardPolicy->直接丢弃，没有任何异常；DiscardOldestPolicy->丢弃最老的任务，并调用线程池的execute方法执行，如果线程池没有shutdown。

public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        int c = ctl.get();
        if (workerCountOf(c) < corePoolSize) {
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();
            if (! isRunning(recheck) && remove(command))
                reject(command);
            else if (workerCountOf(recheck) == 0)
                addWorker(null, false);
        }
        else if (!addWorker(command, false))
            reject(command);
    }