java线程池的原理学习（三）
接上文：java线程池的原理学习（二）
ThreadPoolExecutor深入剖析
线程池的五种状态
ThreadPoolExecutor 类中将线程状态（ runState）分为了以下五种：
RUNNING：可以接受新任务并且处理进入队列中的任务SHUTDOWN：不接受新任务，但是仍然执行队列中的任务STOP：不接受新任务也不执行队列中的任务TIDYING：所有任务中止，队列为空，进入该状态下的任务会执行terminated()方法TERMINATED：terminated()方法执行完成后进入该状态
状态之间的转换
RUNNING->SHUTDOWN
调用了 shutdown()方法，可能是在 finalize()方法中被隐式调用
(RUNNING or SHUTDOWN)->STOP
调用 shutdownNow()
SHUTDOWN->TIDYING
当队列和线程池都为空时
STOP->TIDYING
线程池为空时
TIDYING->TERMINATED
terminated()方法执行完成
线程池状态实现
如果查看 ThreadPoolExecutor的源码，会发现开头定义了这几个变量来代表线程状态和活动线程的数量：
//原子变量 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;这个类中将二进制数分为了两部分，高位代表线程池状态（ runState），低位代表活动线程数（ workerCount）， CAPACITY代表最大的活动线程数，为2^29-1,下面为了更直观的看到这些数我做了些打印：
public class Test1 { public static void main(String[] args) {final int COUNT_BITS = Integer.SIZE - 3;final int CAPACITY = (1 << COUNT_BITS) - 1;final int RUNNING = -1 << COUNT_BITS;final int SHUTDOWN = 0 << COUNT_BITS;final int STOP = 1 << COUNT_BITS;final int TIDYING = 2 << COUNT_BITS;final int TERMINATED = 3 << COUNT_BITS;System.out.println(Integer.toBinaryString(CAPACITY));System.out.println(Integer.toBinaryString(RUNNING));System.out.println(Integer.toBinaryString(SHUTDOWN));System.out.println(Integer.toBinaryString(STOP));System.out.println(Integer.toBinaryString(TIDYING));System.out.println(Integer.toBinaryString(TERMINATED)); }}输出：
1111111111111111111111111111111100000000000000000000000000000010000000000000000000000000000010000000000000000000000000000001100000000000000000000000000000打印的时候会将高位0省略
可以看到，第一行代表线程容量，后面5行提取高3位得到：
111 - RUNNING000 - SHUTDOWN001 - STOP010 - TIDYING011 - TERMINATED分别对应5种状态，可以看到这样定义之后，只需要通过简单的移位操作就可以进行状态的转换。
重要方法
execute方法：
public void execute(Runnable command) { if (command == null)throw new NullPointerException(); int c = ctl.get(); /**分三步执行 * 如果workerCount<corePoolSize,则创建一个新线程执行该任务 */ if (workerCountOf(c) < corePoolSize) {if (addWorker(command, true)) //创建成功则return return;c = ctl.get(); //创建失败重新读取状态，随时保持状态的最新 } /** * workerCount>=corePoolSize,判断线程池是否处于运行状态，再将任务加入队列 * */ if (isRunning(c) && workQueue.offer(command)) {int recheck = ctl.get(); //用于double check//如果线程池处于非运行态，则将任务从缓存队列中删除if (! isRunning(recheck) && remove(command)) reject(command); //拒绝任务else if (workerCountOf(recheck) == 0) //如果活动线程数为0，则创建新线程 addWorker(null, false); } //如果线程池不处于RUNNING状态，或者workQueue满了，则执行以下代码 else if (!addWorker(command, false))reject(command); }可以看到，在类中使用了 Work类来代表任务，下面是 Work类的简单摘要：
private final class Worker extends AbstractQueuedSynchronizer implements Runnable { /** Thread this worker is running in. Null if factory fails. */ final Thread thread; /** Initial task to run. Possibly null. */ Runnable firstTask; /** Per-thread task counter */ volatile long completedTasks; /** * Creates with given first task and thread from ThreadFactory. * @param firstTask the first task (null if none) */ Worker(Runnable firstTask) {this.firstTask = firstTask;this.thread = getThreadFactory().newThread(this); } /** Delegates main run loop to outer runWorker */ public void run() {runWorker(this); } ...Work类实现了 Runnable接口，使用了线程工厂创建线程，使用 runWork方法来运行任务
创建新线程时用到了 addWorker()方法:
/** * 检查在当前线程池状态和限制下能否创建一个新线程，如果可以，会相应改变workerCount， * 每个worker都会运行他们的firstTask * @param firstTask 第一个任务 * @param core true使用corePoolSize作为边界，false使用maximumPoolSize * @return false 线程池关闭或者已经具备关闭的条件或者线程工厂没有创建新线程 */private boolean addWorker(Runnable firstTask, boolean core) { retry: for (;;) {int c = ctl.get();int rs = runStateOf(c);// 只有当rs < SHUTDOWN才有可能接受新任务if (rs >= SHUTDOWN && ! (rs == SHUTDOWN && firstTask == null && ! workQueue.isEmpty())) return false;for (;;) { int wc = workerCountOf(c); //工作线程数量 if (wc >= CAPACITY || wc >= (core ? corePoolSize : maximumPoolSize)) //不合法则返回 return false; if (compareAndIncrementWorkerCount(c)) //将工作线程数量+1 break retry; c = ctl.get(); // Re-read ctl if (runStateOf(c) != rs) //判断线程池状态有没有改变，改变了则进行外循环，否则只进行内循环 continue retry; // else CAS failed due to workerCount change; retry inner loop} } //创建新线程 Worker w = new Worker(firstTask); Thread t = w.thread; final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try {//再次检查状态，防止ThreadFactory创建线程失败或者状态改变了int c = ctl.get();int rs = runStateOf(c);if (t == null || (rs >= SHUTDOWN && ! (rs == SHUTDOWN && firstTask == null))) { decrementWorkerCount(); //减少线程数量 tryTerminate();//尝试中止线程 return false;}workers.add(w);//添加到工作线程Set集合中int s = workers.size();if (s > largestPoolSize) largestPoolSize = s; } finally {mainLock.unlock(); } t.start();//执行任务 //状态变成了STOP（调用了shutdownNow方法） if (runStateOf(ctl.get()) == STOP && ! t.isInterrupted())t.interrupt(); return true; }再看 Work中的 runWork方法：
final void runWorker(Worker w) { Runnable task = w.firstTask; w.firstTask = null; boolean completedAbruptly = true;//线程是否异常中止 try {//先取firstTask，再从队列中取任务直到为nullwhile (task != null || (task = getTask()) != null) { w.lock(); clearInterruptsForTaskRun(); try { beforeExecute(w.thread, task);//实现钩子方法 Throwable thrown = null; try {task.run();//运行任务 } 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);//实现钩子方法 } } finally { task = null; w.completedTasks++; w.unlock(); }}completedAbruptly = false;//成功运行，说明没有异常中止 } finally {processWorkerExit(w, completedAbruptly); } }