java 线程及线程池

说明：
线程共包括以下5种状态。
1. 新建状态(New)         : 线程对象被创建后，就进入了新建状态。例如，Thread thread = new Thread()。
2. 就绪状态(Runnable): 也被称为“可执行状态”。线程对象被创建后，其它线程调用了该对象的start()方法，从而来启动该线程。例如，thread.start()。处于就绪状态的线程，随时可能被CPU调度执行。
3. 运行状态(Running) : 线程获取CPU权限进行执行。需要注意的是，线程只能从就绪状态进入到运行状态。
4. 阻塞状态(Blocked)  : 阻塞状态是线程因为某种原因放弃CPU使用权，暂时停止运行。直到线程进入就绪状态，才有机会转到运行状态。阻塞的情况分三种：
    (01) 等待阻塞 -- 通过调用线程的wait()方法，让线程等待某工作的完成。
    (02) 同步阻塞 -- 线程在获取synchronized同步锁失败(因为锁被其它线程所占用)，它会进入同步阻塞状态。
    (03) 其他阻塞 -- 通过调用线程的sleep()或join()或发出了I/O请求时，线程会进入到阻塞状态。当sleep()状态超时、join()等待线程终止或者超时、或者I/O处理完毕时，线程重新转入就绪状态。
5. 死亡状态(Dead)    : 线程执行完了或者因异常退出了run()方法，该线程结束生命周期。

线程池对象

1. 一个int类型的前三位实现线程池的状态变化

private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
private static final int COUNT_BITS = Integer.SIZE - 3;
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. 添加任务流程

/*
 * 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);

3. addWorker 自旋设计使用retry

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

    // 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
    }
}