java并发编程学习之线程池ThreadPoolExecutor三

jiezi

5 年前

ThreadPoolExecutor 是所有线程池实现的父类，我们先看看构造函数

corePoolSize：线程核心数
maximumPoolSize：最大线程数
keepAliveTime：线程空闲后，存活的时间，只有线程数大于 corePoolSize 的时候生效
unit：存活时间的单位
workQueue：任务的阻塞队列
threadFactory：创建线程的工程，给线程起名字
handler：当线程池满了，选择新加入的任务应该使用什么策略，比如抛异常、丢弃当前任务、丢弃阻塞队列的最老任务等，也可以自定义。

判断是否超过线程核心数 corePoolSize，没超过创建线程
超过线程核心数，则判断队列是否已满，没有满，放入队列
队列也满了，判断是否超过 maximumPoolSize，没有就创建线程
超过了，根据策略执行

//32 为，前 3 位作为线程池的状态，后三位是线程数
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
private static final int COUNT_BITS = Integer.SIZE - 3;//28
private static final int CAPACITY   = (1 << COUNT_BITS) - 1;00011111 11111111 11111111 11111110
//- 1 的二进制是 11111111 11111111 11111111 11111111
private static final int RUNNING    = -1 << COUNT_BITS;//- 1 如上，左移 28 位后，就是 111000000 00000000 00000000 00000000
private static final int SHUTDOWN   =  0 << COUNT_BITS;// 0 左移 28 位，还是 0，00000000 00000000 00000000 00000000
private static final int STOP       =  1 << COUNT_BITS;//00100000 00000000 00000000 00000000
private static final int TIDYING    =  2 << COUNT_BITS;//01000000 00000000 00000000 00000000
private static final int TERMINATED =  3 << COUNT_BITS;//01100000 00000000 00000000 00000000
private static int runStateOf(int c)     {return c & ~CAPACITY;}//~CAPACITY 为 11100000000000000000000000000000，与完就是线程的状态
private static int workerCountOf(int c)  {return c & CAPACITY;}// 与完，是线程的数量
private static int ctlOf(int rs, int wc) {return rs | wc;}
private static boolean isRunning(int c) {return c < SHUTDOWN;// 小于 0，说明是 RUNNING，RUNNING=-1}

public void execute(Runnable command) {if (command == null)
        throw new NullPointerException();
    int c = ctl.get();
    if (workerCountOf(c) < corePoolSize) {// 如果线程数少于线程核心数
        if (addWorker(command, true))// 增加任务成功，返回 true，没成功，继续往下
            return;
        c = ctl.get();}
    // 判断队列
    if (isRunning(c) && workQueue.offer(command)) {// 如果线程池还在跑，并且可以插入队列
        int recheck = ctl.get();
        if (! isRunning(recheck) && remove(command))// 线程池不是运行状态，就移除刚刚插入的任务
            reject(command);// 执行策略
        else if (workerCountOf(recheck) == 0)//
            addWorker(null, false);
    }
    // 队列也满了，判断最大线程数
    else if (!addWorker(command, false))
        reject(command);// 执行策略
}

private boolean addWorker(Runnable firstTask, boolean core) {//core 为 true，使用 corePoolSize 判断，否则使用 maximumPoolSize
    retry:
    for (;;) {int c = ctl.get();
        int rs = runStateOf(c);// 获取当前线程状态

        // Check if queue empty only if necessary.
        if (rs >= SHUTDOWN && // 就是 STOP、TIDYING、TERMINATED，此时不让任务进来
            ! (rs == SHUTDOWN &&
               firstTask == null &&
               ! workQueue.isEmpty()))//
            return false;

        for (;;) {int wc = workerCountOf(c);
            if (wc >= CAPACITY ||
                wc >= (core ? corePoolSize : maximumPoolSize))
                return false;// 超过了线程核心数或最大线程数，不让新增
            if (compareAndIncrementWorkerCount(c))// 返回 true，说明成功了，跳出 retry 循环
                break retry;
            // 失败了，说明被其他符号条件的线程占了，就再判断线程状态是否跟之前一样，不一样重新获取，跳到 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();// 获取锁
            try {int rs = runStateOf(ctl.get());// 获取线程池的状态

                if (rs < SHUTDOWN ||
                    (rs == SHUTDOWN && firstTask == null)) {if (t.isAlive()) // 没通过 start 来启动 run 的
                        throw new IllegalThreadStateException();
                    workers.add(w);// 加点 hashset
                    int s = workers.size();
                    if (s > largestPoolSize)
                        largestPoolSize = s;// 更新当前最大值
                    workerAdded = true;// 增加成功
                }
            } finally {mainLock.unlock();
            }
            if (workerAdded) {t.start();// 启动线程
                workerStarted = true;// 启动成功
            }
        }
    } finally {if (! workerStarted)
            addWorkerFailed(w);// 失败，线程数 -1，从 hashset 移除，并尝试 Terminate
    }
    return workerStarted;
}

上面执行 t.start(); 的时候，就会通过 run 方法调用下面的方法

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) {// 任务不为空或者获取的任务也不为空
            w.lock();           
            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();// 调用 run 方法，这里没有通过 start，也就是说没有启动新线程
                } 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++;// 完成任务数加 1
                w.unlock();// 释放}
        }
        completedAbruptly = false;
    } finally {processWorkerExit(w, completedAbruptly);// 移除 w，在 task 为空的时候，比如线程池状态停止或者启动的线程太多
    }
}

getTask 方法
当 Worker 第一次启动的时候，调用 run 方法，后面就一直从队列里获取任务

private Runnable getTask() {boolean timedOut = false; // Did the last poll() time out?

    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 >= STOP || workQueue.isEmpty())) {//
            decrementWorkerCount();// 线程数量 -1
            return null;
        }

        int wc = workerCountOf(c);// 线程数
        //allowCoreThreadTimeOut 为 true，说明线程数要根据是否超过核心线程数判断 keepAliveTime
        boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;// 是否超过核心线程数

        if ((wc > maximumPoolSize || (timed && timedOut))// 超过了最大线程数
            && (wc > 1 || workQueue.isEmpty())) {if (compareAndDecrementWorkerCount(c))// 线程数 -1
                return null;// 返回空
            continue;
        }

        try {
            Runnable r = timed ?
                workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                workQueue.take();// 获取任务
            if (r != null)
                return r;
            timedOut = true;
        } catch (InterruptedException retry) {timedOut = false;}
    }
}

java并发编程学习之线程池ThreadPoolExecutor三

ThreadPoolExecutor

构造参数

流程

源码解析

execute 方法

addWorker 方法

runWorker 方法