文章目录

预备知识源码分析submit()源码分析shutdownNow()源码分析 代码输出设计目的与优点

预备知识

Java常用的二进制操作（左移、右移、无符号右移、加法、减法）Java中的位掩码BitMask

源码分析

我们把ThreadPoolExecutor中的状态和状态相关的方法复制出来，然后创建一个线程池，在运行中的时候分析线程池的状态和线程数，于是有了下面例子：

@Slf4j public class ThreadPoolExecutorCtlAnalysis { private static final int COUNT_BITS = Integer.SIZE - 3; private static final int CAPACITY = (1 << COUNT_BITS) - 1;// 000,11111111111111111111111111111 // runState is stored in the high-order bits private static final int RUNNING = -1 << COUNT_BITS; // 111,00000000000000000000000000000 private static final int SHUTDOWN = 0 << COUNT_BITS; // 000,00000000000000000000000000000 private static final int STOP = 1 << COUNT_BITS; // 001,00000000000000000000000000000 private static final int TIDYING = 2 << COUNT_BITS; // 010,00000000000000000000000000000 private static final int TERMINATED = 3 << COUNT_BITS;// 011,00000000000000000000000000000 // Packing and unpacking ctl // RUNNING(3'thread) 111,00000000000000000000000000011 // ~CAPACITY 111,00000000000000000000000000000 // RESULT 111,00000000000000000000000000000 // 与操作取高位获取的就是ctl中保存的的线程池的状态 private static int runStateOf(int c) { return c & ~CAPACITY; } // RUNNING(3'thread) 111,00000000000000000000000000011 // CAPACITY 000,11111111111111111111111111111 // RESULT 000,00000000000000000000000000011 // 与操作取低位获取的就是ctl中保存的worker数量 private static int workerCountOf(int c) { return c & CAPACITY; } private static Runnable buildRunnableTask() { return () -> { try { Thread.sleep(3000); } catch (InterruptedException e) { e.printStackTrace(); } log.info("Task finished."); }; } private static int getCtlValue(ThreadPoolExecutor executor, Field field) { //noinspection ConstantConditions return ((AtomicInteger) ReflectionUtils.getField(field, executor)).get(); } private static String formatBinaryString(int state) { StringBuilder binaryString = new StringBuilder(Integer.toBinaryString(state)); if (binaryString.length() < Integer.SIZE) { for (int i = binaryString.length(); i < Integer.SIZE; i++) { binaryString.insert(0, "0"); } } return binaryString.substring(0, 3) + "," + binaryString.substring(3, Integer.SIZE); } private static void peekThreadPoolExecuteState(ThreadPoolExecutor executor, Field ctlField) { log.info("------------------- ThreadPoolExecuteState -------------------"); int ctlValue = getCtlValue(executor, ctlField); log.info("getCtlValue : {}", formatBinaryString(ctlValue)); log.info("workerCountOf: {}", workerCountOf(ctlValue)); log.info("Is RUNNING: {}", runStateOf(ctlValue) == RUNNING); log.info("Is SHUTDOWN: {}", runStateOf(ctlValue) == SHUTDOWN); log.info("Is STOP: {}", runStateOf(ctlValue) == STOP); log.info("Is TIDYING: {}", runStateOf(ctlValue) == TIDYING); log.info("Is TERMINATED: {}", runStateOf(ctlValue) == TERMINATED); } public static void main(String[] args) throws NoSuchFieldException, InterruptedException { // 打印出来看看几种状态的二进制表示 log.info("{} --> CAPACITY", formatBinaryString(CAPACITY)); log.info("{} --> RUNNING", formatBinaryString(RUNNING)); log.info("{} --> STOP", formatBinaryString(STOP)); log.info("{} --> TERMINATED", formatBinaryString(TERMINATED)); // 创建一个线程池，运行3个任务 ThreadPoolExecutor executor = new ThreadPoolExecutor( 1, 2, 0L, TimeUnit.MILLISECONDS, new ArrayBlockingQueue<>(1)); executor.submit(buildRunnableTask()); executor.submit(buildRunnableTask()); executor.submit(buildRunnableTask()); // 休眠一秒钟，可以拿到中间状态的ctl Thread.sleep(1000); log.info("getActiveCount(): {}", executor.getActiveCount()); // 通过反射能拿到ThreadPoolExecutor的ctl的值 Field ctlField = ThreadPoolExecutor.class.getDeclaredField("ctl"); ctlField.setAccessible(true); // 线程池运行中的状态可通过ctl拿到 peekThreadPoolExecuteState(executor, ctlField); // 终止线程池，再来看看线程池中ctl的状态 executor.shutdownNow(); peekThreadPoolExecuteState(executor, ctlField); // 休眠2秒钟，看看线程池最终的状态 Thread.sleep(2000); peekThreadPoolExecuteState(executor, ctlField); } }

在看运行结果之前，我们先看下ThreadPoolExecutor中的几处涉及到状态变更的方法实现。

submit()源码分析

public Future<?> submit(Runnable task) { if (task == null) throw new NullPointerException(); RunnableFuture<Void> ftask = newTaskFor(task, null); execute(ftask); return ftask; }

最终调用的是内部的execute方法：

public void execute(Runnable command) { if (command == null) throw new NullPointerException(); 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); }

这个方法不是特别复杂，我们本文的重点是要看看它的addWorker()方法，这个不复制太多逻辑，关键在两行：

private boolean addWorker(Runnable firstTask, boolean core) { int c = ctl.get(); ... compareAndIncrementWorkerCount(c) ... } private boolean compareAndIncrementWorkerCount(int expect) { return ctl.compareAndSet(expect, expect + 1); }

这里控制的是ctl中工作线程数(wc:WorkerCount)的变更，即整形低29位的自增不会影响到高3位的状态：

RUNNING(0'wc) 111,00000000000000000000000000000 RUNNING(1'wc) 111,00000000000000000000000000001

所以可预见的输出结果就是：

workerCountOf(): 1 Is Running: true Is Stop: false

注意的是这些值都从ctl属性中得来。

shutdownNow()源码分析

在我们的例子中，我们调用了shutdownNow()方法来改变线程池的状态。

public List<Runnable> shutdownNow() { List<Runnable> tasks; final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { checkShutdownAccess(); advanceRunState(STOP); interruptWorkers(); tasks = drainQueue(); } finally { mainLock.unlock(); } tryTerminate(); return tasks; }

这里我们关注的是advanceRunState(STOP)方法：

/** * Transitions runState to given target, or leaves it alone if * already at least the given target. * * @param targetState the desired state, either SHUTDOWN or STOP * (but not TIDYING or TERMINATED -- use tryTerminate for that) */ private void advanceRunState(int targetState) { for (;;) { int c = ctl.get(); if (runStateAtLeast(c, targetState) || ctl.compareAndSet(c, ctlOf(targetState, workerCountOf(c)))) break; } }

该方法最终是要把当前状态变为STOP状态。

注意点一：

ThreadPoolExecutor中状态定义的值大小是有序的，即：

TERMINATED > TIDYING > STOP > SHUTDOWN > RUNNING（最高位1是负数）

注意点二：

advance的含意是推进、前进的意思，Java并发包里的很多方法都使用了该命名，所以当前方法表示的意思是要推进运行状态（advanceRunState），因此方法中才有了runStateAtLeast()判断。 即要推进状态，那如果当前状态已经大于目标状态了，本次方法直接跳过。否则才去做cas操作。这也正是原方法注释想表达的意思。

注意点三：

最后在做cas操作的时候合并当前wc和rs的值，使用的是ctlOf方法：

private static int ctlOf(int rs, int wc) { return rs | wc; }

通过与运算把RunState和WorkerCount的值合并到一处，即最终的ctl的值：

STOP 001,00000000000000000000000000000 WorkCount 000,00000000000000000000000000001 ctl value 001,00000000000000000000000000001

所以可预见的输出结果就是：

workerCountOf(): 1 Is Running: false Is Stop: true

代码输出

12:56:19.473 [main] ThreadPoolExecutorCtlAnalysis - 000,11111111111111111111111111111 --> CAPACITY 12:56:19.476 [main] ThreadPoolExecutorCtlAnalysis - 111,00000000000000000000000000000 --> RUNNING 12:56:19.476 [main] ThreadPoolExecutorCtlAnalysis - 001,00000000000000000000000000000 --> STOP 12:56:19.476 [main] ThreadPoolExecutorCtlAnalysis - 011,00000000000000000000000000000 --> TERMINATED 12:56:20.520 [main] ThreadPoolExecutorCtlAnalysis - getActiveCount(): 2 12:56:20.520 [main] ThreadPoolExecutorCtlAnalysis - ------------------- ThreadPoolExecuteState ------------------- 12:56:20.533 [main] ThreadPoolExecutorCtlAnalysis - getCtlValue : 111,00000000000000000000000000010 12:56:20.533 [main] ThreadPoolExecutorCtlAnalysis - workerCountOf: 2 12:56:20.533 [main] ThreadPoolExecutorCtlAnalysis - Is RUNNING: true 12:56:20.533 [main] ThreadPoolExecutorCtlAnalysis - Is SHUTDOWN: false 12:56:20.533 [main] ThreadPoolExecutorCtlAnalysis - Is STOP: false 12:56:20.533 [main] ThreadPoolExecutorCtlAnalysis - Is TIDYING: false 12:56:20.533 [main] ThreadPoolExecutorCtlAnalysis - Is TERMINATED: false 12:56:20.533 [main] ThreadPoolExecutorCtlAnalysis - ------------------- ThreadPoolExecuteState ------------------- 12:56:20.533 [main] ThreadPoolExecutorCtlAnalysis - getCtlValue : 001,00000000000000000000000000010 12:56:20.533 [main] ThreadPoolExecutorCtlAnalysis - workerCountOf: 2 12:56:20.533 [main] ThreadPoolExecutorCtlAnalysis - Is RUNNING: false 12:56:20.533 [main] ThreadPoolExecutorCtlAnalysis - Is SHUTDOWN: false 12:56:20.533 [main] ThreadPoolExecutorCtlAnalysis - Is STOP: true 12:56:20.533 [main] ThreadPoolExecutorCtlAnalysis - Is TIDYING: false 12:56:20.533 [main] ThreadPoolExecutorCtlAnalysis - Is TERMINATED: false 12:56:20.534 [pool-1-thread-1] ThreadPoolExecutorCtlAnalysis - Task finished. 12:56:20.534 [pool-1-thread-2] ThreadPoolExecutorCtlAnalysis - Task finished. 12:56:22.538 [main] ThreadPoolExecutorCtlAnalysis - ------------------- ThreadPoolExecuteState ------------------- 12:56:22.538 [main] ThreadPoolExecutorCtlAnalysis - getCtlValue : 011,00000000000000000000000000000 12:56:22.538 [main] ThreadPoolExecutorCtlAnalysis - workerCountOf: 0 12:56:22.539 [main] ThreadPoolExecutorCtlAnalysis - Is RUNNING: false 12:56:22.539 [main] ThreadPoolExecutorCtlAnalysis - Is SHUTDOWN: false 12:56:22.539 [main] ThreadPoolExecutorCtlAnalysis - Is STOP: false 12:56:22.539 [main] ThreadPoolExecutorCtlAnalysis - Is TIDYING: false 12:56:22.539 [main] ThreadPoolExecutorCtlAnalysis - Is TERMINATED: true

可以看到使用ctl一个字段可以获取到两个值，并且这两个值不会有并发不一致的情况，每次都是一次cas更新值。

设计目的与优点

线程池自身的状态和线程数量都维护在一个原子变量ctl中，目的不是为了减少存储空间，而是将线程池状态与线程个数合二为一，这样就可以用一次cas原子操作进行赋值，更容易保证在多线程环境下保证运行状态和线程数量的统一。这真是大师的设计智慧啊！