【图灵学院10】高并发之java线程池源码分析

1. 提纲

1）线程池的模块结构

2）示例&原理解析

2. 问题

1）线程池包含哪些东西

2）线程池的运作原理

3）调度线程池的运作原理

4）线程池怎么实现FixRate，FixDelay，他们之间的区别

5）任务怎么取消

3. 源码解析

3.1 线程池框架

接口简介：

java.util.concurrent.Executor：执行器，执行方法

java.util.concurrent.ExecutorService：（执行服务）包含服务的生命周期

java.util.concurrent.ScheduledExecutorService：（调度相关的服务）

辅助类：

java.util.concurrent.Executors：

核心实现类：

java.util.concurrent.ThreadPoolExecutor：（普通的线程池实现类）

java.util.concurrent.ScheduledThreadPoolExecutor：（调度的核心实现类）

java.util.concurrent.ForkJoinPool

完成服务：

java.util.concurrent.CompletionService：

java.util.concurrent.ExecutorCompletionService：

3.2 ThreadPoolExecutor源码剖析

1）构造器

核心数量，任务队列容器，存活时间，线程工厂，处理器

ThreadPoolExecutor.execute()方法源码

    public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        /*
         * Proceed in 3 steps:
         *
         * 1. If fewer than corePoolSize threads are running, try to
         * start a new thread with the given command as its first
         * task.  The call to addWorker atomically checks runState and
         * workerCount, and so prevents false alarms that would add
         * threads when it shouldn't, by returning false.
         *
         * 2. If a task can be successfully queued, then we still need
         * to double-check whether we should have added a thread
         * (because existing ones died since last checking) or that
         * the pool shut down since entry into this method. So we
         * recheck state and if necessary roll back the enqueuing if
         * stopped, or start a new thread if there are none.
         *
         * 3. If we cannot queue task, then we try to add a new
         * thread.  If it fails, we know we are shut down or saturated
         * and so reject the task.
         */
        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);
    }

ThreadPoolExecutor.runWorker()方法源码

    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) {
                w.lock();
                // If pool is stopping, ensure thread is interrupted;
                // if not, ensure thread is not interrupted.  This
                // requires a recheck in second case to deal with
                // shutdownNow race while clearing interrupt
                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();
                    } 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++;
                    w.unlock();
                }
            }
            completedAbruptly = false;
        } finally {
            processWorkerExit(w, completedAbruptly);
        }
    }

3.3 ScheduledThreadPoolExecutor

1）

2）FixRate和FixDelay的区别

        /**
         * Sets the next time to run for a periodic task.
         */
        private void setNextRunTime() {
            long p = period;
            if (p > 0)
                time += p;
            else
                time = triggerTime(-p);
        }

FixRate严格按照设定的时间

FixDelay按照上一个线程执行完成的时间间隔

FixRate： 21:40，21:41，21:42

FixDelay：21:40，21:43

    void reExecutePeriodic(RunnableScheduledFuture<?> task) {
        if (canRunInCurrentRunState(true)) {
　　　　　　　//此处没有捕获异常，如果单个任务发生异常，会导致后续任务无法继续执行
            super.getQueue().add(task);
            if (!canRunInCurrentRunState(true) && remove(task))
                task.cancel(false);
            else
                ensurePrestart();
        }
    }