java线程池使用(二)------部分源码解析-阿里云开发者社区

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java线程池使用(二)------部分源码解析

简介: 简单介绍java线程池源码

前一篇博客介绍了构造参数分别是什么意思
现在介绍java线程池源码
1 init
线程池初始化,代码比较简单

public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue,
                              ThreadFactory threadFactory,
                              RejectedExecutionHandler handler) {
        if (corePoolSize < 0 || //核心线程数校验
            maximumPoolSize <= 0 || //非核心线程数校验
            maximumPoolSize < corePoolSize || 
            keepAliveTime < 0)
            throw new IllegalArgumentException();
        if (workQueue == null || threadFactory == null || handler == null) //必传参数校验
            throw new NullPointerException();
        this.acc = System.getSecurityManager() == null ?
                null :
                AccessController.getContext();
        this.corePoolSize = corePoolSize;
        this.maximumPoolSize = maximumPoolSize;
        this.workQueue = workQueue;
        this.keepAliveTime = unit.toNanos(keepAliveTime);
        this.threadFactory = threadFactory;
        this.handler = handler;
    }

2 线程池核心类Worker
线城池中,使用Worker来保证线程池线程的活跃,保证在没有任务执行时,也能保证线程资源不被释放掉
woker类实现了runable接口,本身就是一个线程

 private final class Worker
        extends AbstractQueuedSynchronizer
        implements Runnable
    {
        private static final long serialVersionUID = 6138294804551838833L;
        //worker运行的线程,是线程池中实际的线程,通过ThreadFactory生成
        final Thread thread;
       //线程池新增线程时的默认任务,可以为空
        Runnable firstTask;
        //已经执行过的线程任务数
        volatile long completedTasks;

        Worker(Runnable firstTask) {
            setState(-1); // inhibit interrupts until runWorker
            this.firstTask = firstTask;
            this.thread = getThreadFactory().newThread(this);
        }

        //将执行方法委托给ThreadPoolExecuter
        public void run() {
            runWorker(this);
        }

        // Lock methods
        //
        // The value 0 represents the unlocked state.
        // The value 1 represents the locked state.

        protected boolean isHeldExclusively() {
            return getState() != 0;
        }

        protected boolean tryAcquire(int unused) {
            if (compareAndSetState(0, 1)) {
                setExclusiveOwnerThread(Thread.currentThread());
                return true;
            }
            return false;
        }

        protected boolean tryRelease(int unused) {
            setExclusiveOwnerThread(null);
            setState(0);
            return true;
        }

        public void lock()        { acquire(1); }
        public boolean tryLock()  { return tryAcquire(1); }
        public void unlock()      { release(1); }
        public boolean isLocked() { return isHeldExclusively(); }

        void interruptIfStarted() {
            Thread t;
            if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
                try {
                    t.interrupt();
                } catch (SecurityException ignore) {
                }
            }
        }
    }

3 线程池新增任务
image

下面看源码
核心方法1 ThreadPoolExecuter.execute()


  public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
         
        int c = ctl.get();
        //判断当前线程数量和最大核心线程数
        if (workerCountOf(c) < corePoolSize) {//线程数<最大核心线程数
            if (addWorker(command, true))//尝试创建新的worker,如果创建成功则直接返回
                return;
            c = ctl.get();//获取最新的的数量
        }
        if (isRunning(c) && workQueue.offer(command)) {//1 检查线程池状态
                                                        //2当前线程数>=最大核心线程数。
                                                        //3尝试将新增的线程放入blockingqueue
            int recheck = ctl.get(); //对线程池状态进行二次校验,防止其他线程在offer期间对线程池状态进行了修改
            if (! isRunning(recheck) && remove(command)) //二次校验,失败后将新增线程从blockingqueue移除
                                                         //二次校验成功则不移除,利用java&&的特性
                reject(command);                        //校验失败进入过载策略
            else if (workerCountOf(recheck) == 0) //二次校验成功,判断当前线程池线程数量是否归零,
                                                   //如果归零,则创建新的空worker,由worker去进行处理queue
                                                    //中的任务
               addWorker(null, false);
        }
        else if (!addWorker(command, false))//如果queue.offer返回false,证明queue已经满了,
                                           // 尝试创建非核心线程
            reject(command);  //创建失败则证明已经到达最大线程数,进去过载策略
    }

2 核心方法二 ThreadPoolExecuter.addWorker()

private boolean addWorker(Runnable firstTask, boolean core) {
        retry:
        for (;;) {
            int c = ctl.get();
            int rs = runStateOf(c);//获取当前线程池状态

            // Check if queue empty only if necessary.
            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
            }
        }

        boolean workerStarted = false;
        boolean workerAdded = false;
        Worker w = null;
        try {
            w = new Worker(firstTask);
            final Thread t = w.thread;
            if (t != null) {
                final ReentrantLock mainLock = this.mainLock;
                mainLock.lock();
                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);
                        int s = workers.size();
                        if (s > largestPoolSize)
                            largestPoolSize = s;
                        workerAdded = true;
                    }
                } finally {
                    mainLock.unlock();
                }
                if (workerAdded) {
                    t.start();
                    workerStarted = true;
                }
            }
        } finally {
            if (! workerStarted)
                addWorkerFailed(w);
        }
        return workerStarted;
    }

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