线程池ThreadPoolExecutor实现原理

1.ThreadPoolExecutor源码解析

ctl 代表线程池当前状态和线程数量. 32位,前三位代表状态,后29位为线程最大的线程数量

    private static int runStateOf(int c)     { return c & ~CAPACITY; } //获取当前线程池的状态
    private static int workerCountOf(int c)  { return c & CAPACITY; } //获取当前线程池中线程数量
    private static int ctlOf(int rs, int wc) { return rs | wc; } //构造ctl值
    private final HashSet workers = new HashSet(); //线程池线程
    private final ReentrantLock mainLock = new ReentrantLock(); //在add to workes的时候用的全局锁

构造方法: 默认handler是AbortPolicy,ThreadFactory默认是DefaultThreadFactory

public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue workQueue,
                              ThreadFactory threadFactory,
                              RejectedExecutionHandler handler) 

将一个Runnable任务扔进线程池的关键代码

public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        int c = ctl.get();
        if (workerCountOf(c) < corePoolSize) {  //小于corePoolSize,则新增线程,处理任务.
            if (addWorker(command, true))
                return;
            c = ctl.get(); //重新获取状态,以免其它线程修改
        }
        if (isRunning(c) && workQueue.offer(command)) { //否则入队列
            int recheck = ctl.get();  //重新获取状态,以免其它线程修改
            if (! isRunning(recheck) && remove(command))  // 如果当前线程已经不在运行状态,将任务移除队列
                reject(command);  // 实施拒绝策略
            else if (workerCountOf(recheck) == 0)  //corePoolSize=0
                addWorker(null, false);
        }
        else if (!addWorker(command, false)) //如果队列满了,创建新线程也失败
            reject(command); // 实施拒绝策略
    }

addWorker, 新增worker到线程池内部的workers属性中,并且让当前新增的worker自旋起来,firtstTask执行起来.

private boolean addWorker(Runnable firstTask, boolean core) {
        retry:
        for (;;) {
            int c = ctl.get();
            int rs = runStateOf(c);

            // Check if queue empty only if necessary. 检查线程池状态和队列情况
            if (rs >= SHUTDOWN &&
                ! (rs == SHUTDOWN &&
                   firstTask == null &&
                   ! workQueue.isEmpty()))
                return false;

            for (;;) {
                int wc = workerCountOf(c);
                if (wc >= CAPACITY ||
                    wc >= (core ? corePoolSize : maximumPoolSize))
                    return false;
                if (compareAndIncrementWorkerCount(c))
                    break retry;
                c = ctl.get();  // Re-read ctl
                if (runStateOf(c) != rs)
                    continue retry;
                // else CAS failed due to workerCount change; retry inner loop
            }
        }
		//如果符合条件可以添加,则执行下面代码
        boolean workerStarted = false;
        boolean workerAdded = false;
        Worker w = null;
        try {
        	//这两句很重要,new Worker时会生成一个线程,线程内部的任务就是当前创建的Worker,即t就是包含了当前worker的线程,
        	//当后面执行t.start的时候就会执行Worker的run方法,最终执行当前worker的runWorker方法.
        	//让线程执行了任务之后,不断的自旋,不懂可以继续看后面的runWorker方法
            w = new Worker(firstTask);
            final Thread t = w.thread; 
            if (t != null) {
                final ReentrantLock mainLock = this.mainLock;
                mainLock.lock(); // workers.add非线程安全,所以需要锁
                try {
                    // Recheck while holding lock.
                    // Back out on ThreadFactory failure or if
                    // shut down before lock acquired.
                    int rs = runStateOf(ctl.get());

                    if (rs < SHUTDOWN ||
                        (rs == SHUTDOWN && firstTask == null)) {
                        if (t.isAlive()) // precheck that t is startable
                            throw new IllegalThreadStateException();
                        workers.add(w);  //添加到workers属性中,即当前线程池所有存活的workers
                        int s = workers.size();
                        if (s > largestPoolSize)
                            largestPoolSize = s;
                        workerAdded = true;
                    }
                } finally {
                    mainLock.unlock(); // 完了后解锁
                }
                if (workerAdded) {
                    t.start(); //最终执行了runWorker方法
                    workerStarted = true;
                }
            }
        } finally {
            if (! workerStarted)
                addWorkerFailed(w);
        }
        return workerStarted;
    }

2.Worker

类定义:
 private final class Worker
        extends AbstractQueuedSynchronizer
        implements Runnable  --实现了Runnable,继承AQS
 
属性:
final Thread thread;  在worker上运行的线程
Runnable firstTask;  生成worker时的第一个runnable task.

run方法:
public void run() {
       runWorker(this);
}

runWorker方法就是执行firstTask任务,然后自旋,循环从队列中获取待执行的任务执行的过程.
之前一直思考runWorker是何时执行的,可以回到addWorker那段代码的注释中就知道,在add方法创建worker同时就已经开始让worker的run方法执行了.产生了自旋.

final void runWorker(Worker w) {
        Thread wt = Thread.currentThread();
        Runnable task = w.firstTask;
        w.firstTask = null;
        w.unlock(); // allow interrupts
        boolean completedAbruptly = true;
        try {
            while (task != null || (task = getTask()) != null) { //如果有执行的任务就执行,没有循环就从队列中获取,
            //没有的话就结束线程.当设置了allowCoreThreadTimeOut,keepAliveTime时,超时的情况就会
            //getTask=null,就会结束线程执行
                w.lock(); //执行任务时,锁住当前worker,一个线程只能同时执行一个任务,你懂得
                if ((runStateAtLeast(ctl.get(), STOP) ||
                     (Thread.interrupted() &&
                      runStateAtLeast(ctl.get(), STOP))) &&
                    !wt.isInterrupted())
                    wt.interrupt();
                try {
                    beforeExecute(wt, task);
                    Throwable thrown = null;
                    try {
                        task.run();   //执行任务
                    } catch (RuntimeException x) {
                        thrown = x; throw x;
                    } catch (Error x) {
                        thrown = x; throw x;
                    } catch (Throwable x) {
                        thrown = x; throw new Error(x);
                    } finally {
                        afterExecute(task, thrown);
                    }
                } finally {
                    task = null;
                    w.completedTasks++; //当前worker执行的线程数+1
                    w.unlock(); //worker解锁
                }
            }
            completedAbruptly = false;
        } finally {
            processWorkerExit(w, completedAbruptly);
        }
    }




getTask比较重要的一段:,就是判断线程是否需要回收:
//当设置了allowCoreThreadTimeOut = true || 当前线程数 > corePoolSize 就要考虑是否回收线程
 boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
//当超时后就需要回收, poll方法就是在keepAliveTime时间后还没获取到任务就会返回null, 
//不需要判断超时的话就采用take方法,take会阻塞一直一定拿到任务.
Runnable r = timed ?
                    workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                    workQueue.take();

所以总结一下.当一个Runnable扔进线程后:

• 当前线程池数量 run起来并自旋,不断从队列中获取可执行的任务执行.
• 当前线程池数量>=corePoolSize,将任务添加到阻塞队列中去
• 当队列已经满了,继续创建创建新线程(新Worker),直到最大的线程池数量maxPoolSize
• 执行拒绝策略.

上面是Runnable在线程池中的执行,如果是Callable方法.使用submit时.会不一样.继续看代码,先看使用方法方式:

ExecutorService runnableService = Executors.newFixedThreadPool(3);
Future r1 =  runnableService.submit(new TestCallable(1));
r1.get() 就能获取到result

那它是怎么实现的呢. submit提交callable时会包装返回一个FutureTask实例.
FutureTask实现了RunnableFuture、Runnable、Future.简而言之,它就是一个可以获取结果的Runnable而已

public  Future submit(Callable task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture ftask = newTaskFor(task);
        execute(ftask);
        return ftask;
}
    
protected  RunnableFuture newTaskFor(Callable callable) {
        return new FutureTask(callable);
}

submit一个Callable任务之后,执行了execute方法,和之前执行Runnable步骤上没有任何差别,但是在执行当前任务时,FutureTask执行的是自己的run方法.下面看代码

3.FutureTask

重要属性

/** The underlying callable; nulled out after running */ callable方法
    private Callable callable;
    /** The result to return or exception to throw from get() */  //callable方法返回值
    private Object outcome; // non-volatile, protected by state reads/writes
    /** The thread running the callable; CASed during run() */
    private volatile Thread runner; // 执行的线程
    /** Treiber stack of waiting threads */
    private volatile WaitNode waiters; //get方法阻塞队列

run方法源码如下.跟runnable实例方法不同的在于会保存任务执行后的结果.

public void run() {
        if (state != NEW ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return;
        try {
            Callable c = callable;  //拿到callble实例
            if (c != null && state == NEW) {
                V result;
                boolean ran;
                try {
                    result = c.call();  //执行callable的call方法,并返回result
                    ran = true;
                } catch (Throwable ex) {
                    result = null;
                    ran = false;
                    setException(ex);
                }
                if (ran)
                    set(result); //将当前task设置成已完成状态,并且将result设置到outcome属性,并唤起线程.
                                       FutureTask的get方法会返回这个result
            }
        } finally {
            // runner must be non-null until state is settled to
            // prevent concurrent calls to run()
            runner = null;
            // state must be re-read after nulling runner to prevent
            // leaked interrupts
            int s = state;
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
    }

get方法源码如下:
如果当前task状态是未完成,也就是result还没计算出来,就有人来get了.它会将线程阻塞起来,直到状态已完成会被唤醒,然后获取到result返回.

public V get() throws InterruptedException, ExecutionException {
        int s = state;
        if (s <= COMPLETING)
            s = awaitDone(false, 0L);
        return report(s);
}

get方法里面的awaitDone阻塞线程,将需要唤起的线程串成队列
run方法里面的set唤起线程,依次唤起队列中的等待线程.并将节点=null,以便gc.
这两个方法的代码可以自行观看.