线程池源码解读
参考文档： https://www.jianshu.com/p/a977ab6704d7
源码位置： java.util.concurrent.ThreadPoolExecutor#execute
jdk 版本： 1.8
3 * 4 * RUNNING: Accept new tasks and process queued tasks 5 * SHUTDOWN: Don't accept new tasks, but process queued tasks 6 * STOP: Don't accept new tasks, don't process queued tasks, 7 * and interrupt in-progress tasks 8 * TIDYING: All tasks have terminated, workerCount is zero, 9 * the thread transitioning to state TIDYING 10 * will run the terminated() hook method 11 * TERMINATED: terminated() has completed 12 * 13 * The numerical order among these values matters, to allow 14 * ordered comparisons. The runState monotonically increases over 15 * time, but need not hit each state. The transitions are: 16 * 17 * RUNNING -> SHUTDOWN 18 * On invocation of shutdown(), perhaps implicitly in finalize() 19 * (RUNNING or SHUTDOWN) -> STOP 20 * On invocation of shutdownNow() 21 * SHUTDOWN -> TIDYING 22 * When both queue and pool are empty 23 * STOP -> TIDYING 24 * When pool is empty 25 * TIDYING -> TERMINATED 26 * When the terminated() hook method has completed terminated() 27 */ 28 29 // 前三位表示运行状态，后面存储当前运行 workerCount 30 private static final int COUNT_BITS = Integer.SIZE - 3; // 32 - 3 31 32 // 最大容量 33 private static final int CAPACITY = (1 << COUNT_BITS) - 1; // 00011111111111111111111111111111 34 35 /** 36 * Maximum pool size. Note that the actual maximum is internally 37 * bounded by CAPACITY. 实际线程池大小还是由 CAPACITY 决定 38 */ 39 private volatile int maximumPoolSize; 40 41 // 线程池的几个状态 官方注释在最上方 42 // 接受新的任务 43 private static final int RUNNING = -1 << COUNT_BITS; // 11100000000000000000000000000000 44 45 // 不接受新的任务，但是已在队列中的任务，还会继续处理 46 private static final int SHUTDOWN = 0 << COUNT_BITS; // 00000000000000000000000000000000 47 48 // 不接受，不处理新的任务，且中断正在进行中的任务 49 private static final int STOP = 1 << COUNT_BITS; // 00100000000000000000000000000000 50 51 // 所有任务已停止，workerCount 清零，注意 workerCount 是由 workerCountOf(int c) 计算得出的 52 private static final int TIDYING = 2 << COUNT_BITS; // 01000000000000000000000000000000 53 54 // 所有任务已完成 55 private static final int TERMINATED = 3 << COUNT_BITS; // 01100000000000000000000000000000 56 57 // 线程池运行状态和已工作的 workerCount 初始化为 RUNNING 和 0 58 private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0)); 59 60 // 计算当前 state 61 // ~CAPACITY 为 11100000000000000000000000000000 & c（假如前三位为 000）说明线程池已经 SHUTDOWN 62 private static int runStateOf(int c) { return c & ~CAPACITY; } 63 64 // 同时拿到 state workerCount 65 private static int ctlOf(int rs, int wc) { return rs | wc; } 66 67 // & 可以计算出当前工作的 workerCount 68 private static int workerCountOf(int c) { return c & CAPACITY; } 69 70 // 线程入列 71 public void execute(Runnable command) { 72 if (command == null) 73 throw new NullPointerException(); 74 75 // 获取线程池 state 和 workerCount 76 // 判断是否满足加入核心线程 77 int c = ctl.get(); 78 if (workerCountOf(c) < corePoolSize) { 79 // 以核心线程的方式加入队列 80 if (addWorker(command, true)) 81 return; 82 // 添加失败 获取最新的线程池 state 和 workerCount 83 c = ctl.get(); 84 } 85 // 在运行且成功加入队列 86 if (isRunning(c) && workQueue.offer(command)) { 87 int recheck = ctl.get(); 88 //防止线程池状态发生了变化， 再检查一次，不是运行状态就拒绝任务 89 if (!isRunning(recheck) && remove(command)) 90 reject(command); 91 else if (workerCountOf(recheck) == 0) 92 // 加入一个 null 93 addWorker(null, false); 94 } 95 // 加入失败就拒绝任务 96 else if (!addWorker(command, false)) 97 reject(command); 98 } 99
100 // 实际的操作 101 private boolean addWorker(Runnable firstTask, boolean core) { 102 retry: 103 for (;;) { 104 // 获得当前 state 和 workerCount 105 int c = ctl.get(); 106 int rs = runStateOf(c); 107 108 // 大于等于 SHUTDOWN 即 SHUTDOWN, STOP TIDYING TERMINATED （SHUTDOWN需要排除 队列中不为空的情况， 这个状态不接受新任务， 需要把当前任务处理完） 109 // Check if queue empty only if necessary. 110 if (rs >= SHUTDOWN && 111 ! (rs == SHUTDOWN && 112 firstTask == null && 113 ! workQueue.isEmpty())) 114 return false; 115 116 for (;;) { 117 // 计算 workerCount 118 int wc = workerCountOf(c); 119 if (wc >= CAPACITY || 120 wc >= (core ? corePoolSize : maximumPoolSize)) 121 return false; 122 // 成功了就退出 123 if (compareAndIncrementWorkerCount(c)) 124 break retry; 125 c = ctl.get(); // Re-read ctl 126 if (runStateOf(c) != rs) 127 // 状态发了变化， 进行重试 128 continue retry; 129 // else CAS failed due to workerCount change; retry inner loop 130 } 131 }
// 上面的逻辑， 主要是对线程（worker）数量进行加一操作（原子性），下面 132 133 boolean workerStarted = false; 134 boolean workerAdded = false; 135 Worker w = null; 136 try { 137 // 统一线程的名字 138 // 设置 daemon 和 priority 139 w = new Worker(firstTask); 140 final Thread t = w.thread; 141 if (t != null) {
// 加入可重入锁 142 final ReentrantLock mainLock = this.mainLock; 143 mainLock.lock(); 144 try { 145 // Recheck while holding lock. 146 // Back out on ThreadFactory failure or if 147 // shut down before lock acquired. 148 int rs = runStateOf(ctl.get()); 149 150 // 异常检查 （rs== SHUTDOWN， firstTask必然是null, 原因是这个状态拒绝接受新任务， 这么写更严谨一些 ） 151 if (rs < SHUTDOWN || 152 (rs == SHUTDOWN && firstTask == null)) {
// 检查线程状态， 165行才启动线程， 按道理线程线程状态不可能是alive 状态 153 if (t.isAlive()) // precheck that t is startable 154 throw new IllegalThreadStateException(); 155 workers.add(w); 156 int s = workers.size(); 157 if (s > largestPoolSize) 158 largestPoolSize = s; 159 workerAdded = true; 160 } 161 } finally { 162 mainLock.unlock(); 163 } 164 // 添加成功 启动线程 165 if (workerAdded) { 166 t.start(); 167 workerStarted = true; 168 } 169 } 170 } finally { 171 // 加入失败 172 if (! workerStarted) 173 addWorkerFailed(w); 174 } 175 return workerStarted; 176 } 177
178 // 加入失败 做一些扫尾清理（回滚操作）
// 1. 工作线程数量减1 2. 工作线程队列移除当前worker 179 private void addWorkerFailed(Worker w) { 180 final ReentrantLock mainLock = this.mainLock; 181 mainLock.lock(); 182 try { 183 if (w != null) 184 workers.remove(w); 185 // workerCount-1 186 decrementWorkerCount(); 187 // 尝试更新状态 何为尝试，即需要满足一定条件，而不是冒然去做某事 188 tryTerminate(); 189 } finally { 190 mainLock.unlock(); 191 } 192 }
