java基础----threadpoolexecutor

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线程池创建以及执行任务过程分析：

1. 创建线程池

Creates a new ThreadPoolExecutor with the given initial parameters.
Params:
corePoolSize – the number of threads to keep in the pool, even if they are idle, unless allowCoreThreadTimeOut is set
maximumPoolSize – the maximum number of threads to allow in the pool
keepAliveTime – when the number of threads is greater than the core, this is the maximum time that excess idle threads will wait for new tasks before terminating.
unit – the time unit for the keepAliveTime argument
workQueue – the queue to use for holding tasks before they are executed. This queue will hold only the Runnable tasks submitted by the execute method.
threadFactory – the factory to use when the executor creates a new thread
handler – the handler to use when execution is blocked because the thread bounds and queue capacities are reached
Throws:
IllegalArgumentException – if one of the following holds: corePoolSize < 0 keepAliveTime < 0 maximumPoolSize <= 0 maximumPoolSize < corePoolSize
NullPointerException – if workQueue or threadFactory or handler is null

1. public ThreadPoolExecutor(int corePoolSize, //线程池核心线程数，不管线程是否空闲
                          int maximumPoolSize, //线程池最大线程数目，当workqueue满了以后，在线程池中还可以创建线程的最大数目
                          long keepAliveTime,//超过核心线程数的线程在线程池中能呆的最久时间
                          TimeUnit unit,//上述时间的单位
                          BlockingQueue<Runnable> workQueue,//核心线程数满了以后，新来的任务需要被放到任务队列中
                          ThreadFactory threadFactory,
                          RejectedExecutionHandler handler)

2. 参考https://www.cnblogs.com/moonfair/p/13477974.html

private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));//初始状态是Running 状态，0个工作线程，用来记录线程池状态和工作线程数量

private static final int COUNT_BITS = Integer.SIZE - 3; //将原子类的AtomicInteger的二进制位数（32位），拆分高3位（表示线程的运行状态）和低29位（表示工作线程数量）

private static final int CAPACITY   = (1 << COUNT_BITS) - 1;

// runState is stored in the high-order bits
private static final int RUNNING    = -1 << COUNT_BITS;
private static final int SHUTDOWN   =  0 << COUNT_BITS;
private static final int STOP       =  1 << COUNT_BITS;
private static final int TIDYING    =  2 << COUNT_BITS;
private static final int TERMINATED =  3 << COUNT_BITS;

// Packing and unpacking ctl
private static int runStateOf(int c)     { return c & ~CAPACITY; }
private static int workerCountOf(int c)  { return c & CAPACITY; }
private static int ctlOf(int rs, int wc) { return rs | wc; }

 2.执行任务过程分析：

• 当有任务进来时候，先通过ctl判断当前工作线程数目是否小于核心线程数

• 如果当前线程数<corePoolSize, 排除异常情况，通过compareAndIncrementWorkerCount增加工作线程数量

• 创建worker添加到 workers,添加过程需要添加ReentrantLock的lock锁，保证加入workers的hashset期间的线程安全。添加成功以后运行worker(本质是thread)的thread

• 如果当前线程数>=corePoolSize, 在当前线程池是运行状态的前提下，将任务添加到workQueue

• 如果workQueue的队列已经满了， 在当前工作线程小于最大线程数maximumPoolSize的前提下，和上面方式一样添加worker到workers的hashset。

• 如果当前工作线程大于最大线程数maximumPoolSize，则通过RejectedExecutionHandler的处理策略来处理。

3. 当有空闲线程时候如何从workQueue里取任务？

工作线程worker的run方法中，会一直循环运行，当当前的任务运行完以后，会从workQueue当中取任务来执行。

4. 最大线程超过核心线程的线程在线程池中的存在时间如何计算？

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

    for (;;) {
        int c = ctl.get();
        int rs = runStateOf(c);

        // Check if queue empty only if necessary.
        if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
            decrementWorkerCount();
            return null;
        }

        int wc = workerCountOf(c);

        // Are workers subject to culling?
        boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;

        if ((wc > maximumPoolSize || (timed && timedOut))
            && (wc > 1 || workQueue.isEmpty())) {
            if (compareAndDecrementWorkerCount(c))
                return null;
            continue;
        }

        try {
            Runnable r = timed ?workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) ://如果timed为true说明worker有可能要被关闭，这里调用的代码含义：如果超过keepAliveTime纳秒还没取到任务，就返回null，后面会调用processWorkerExit把worker关闭
   
             workQueue.take();//否则任务队列为空就阻塞在这里，直到任务队列再有任务

            if (r != null)
                return r;
            timedOut = true;
        } catch (InterruptedException retry) {
            timedOut = false;
        }
    }
}

5. 核心线程一直存在吗？

通过参数allowCoreThreadTimeOut来设置，默认为false,表示即便核心工作线程已经空闲，也还存活。如果设置为true，通过keepAliveTime来计算超时。

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