程池中狀態與執行緒數的設計分析(ThreadPoolExecutor中ctl的設計分析)
目錄
預備知識
可以先看下我的另一篇文章對於Java中的位掩碼BitMask的解釋。
- 1、一個整數在jvm中佔用了4個位元組,共32bits
- 2、最高位的bit代表符號位,0為正數、1為負,剩餘的31bits則代表數字部分
- 3、反碼加1即為補碼
- 4、對於負數而言,是以補碼的形式儲存在記憶體中的。以-7(int)為例
- 1)、將-7的絕對值轉化為二進位制:
0000 0000 0000 0000 0000 0000 0000 0111 - 2):將上面的二進位制以反碼錶示:
1111 1111 1111 1111 1111 1111 1111 1000 - 3):轉化為補碼:
1111 1111 1111 1111 1111 1111 1111 1001
- 1)、將-7的絕對值轉化為二進位制:
原始碼分析
我們把ThreadPoolExecutor
中的狀態和狀態相關的方法複製出來,然後建立一個執行緒池,在執行中的時候分析執行緒池的狀態和執行緒數,於是有了下面例子:
@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 // 與操作取高位獲取的就是ctl中儲存的的執行緒池的狀態 private static int runStateOf(int c) { return c & ~CAPACITY; } // RUNNING(3'thread) 111,00000000000000000000000000011 // CAPACITY 000,11111111111111111111111111111 // RESULT 000,00000000000000000000000000011 // 與操作取低位獲取的就是ctl中儲存的worker數量 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 { // 打印出來看看幾種狀態的二進位制表示 log.info("{} --> CAPACITY", formatBinaryString(CAPACITY)); log.info("{} --> RUNNING", formatBinaryString(RUNNING)); log.info("{} --> STOP", formatBinaryString(STOP)); log.info("{} --> TERMINATED", formatBinaryString(TERMINATED)); // 建立一個執行緒池,執行兩個任務 ThreadPoolExecutor executor = new ThreadPoolExecutor( 1, 2, 0L, TimeUnit.MILLISECONDS, new ArrayBlockingQueue<>(1)); executor.submit(buildRunnableTask()); executor.submit(buildRunnableTask()); executor.submit(buildRunnableTask()); // 休眠一秒鐘,可以拿到中間狀態的ctl Thread.sleep(1000); log.info("getActiveCount(): {}", executor.getActiveCount()); // 通過反射能拿到ThreadPoolExecutor的ctl的值 Field ctlField = ThreadPoolExecutor.class.getDeclaredField("ctl"); ctlField.setAccessible(true); // 執行緒池執行中的狀態可通過ctl拿到 peekThreadPoolExecuteState(executor, ctlField); // 終止執行緒池,再來看看執行緒池中ctl的狀態 executor.shutdownNow(); peekThreadPoolExecuteState(executor, ctlField); // 休眠2秒鐘,看看執行緒池最終的狀態 Thread.sleep(2000); peekThreadPoolExecuteState(executor, ctlField); } }
在看執行結果之前,我們先看下ThreadPoolExecutor中的幾處涉及到狀態變更的方法實現。
submit()原始碼分析
public Future<?> submit(Runnable task) {
if (task == null) throw new NullPointerException();
RunnableFuture<Void> ftask = newTaskFor(task, null);
execute(ftask);
return ftask;
}
最終呼叫的是內部的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); }
這個方法不是特別複雜,我們本文的重點是要看看它的addWorker()方法,這個不復制太多邏輯,關鍵在兩行:
private boolean addWorker(Runnable firstTask, boolean core) {
int c = ctl.get();
...
compareAndIncrementWorkerCount(c)
...
}
private boolean compareAndIncrementWorkerCount(int expect) {
return ctl.compareAndSet(expect, expect + 1);
}
這裡控制的是ctl中工作執行緒數(wc:WorkerCount)的變更,即整形低29位的自增不會影響到高3位的狀態:
RUNNING(0'wc) 111,00000000000000000000000000000
RUNNING(1'wc) 111,00000000000000000000000000001
所以可預見的輸出結果就是:
workerCountOf(): 1
Is Running: true
Is Stop: false
注意的是這些值都從ctl屬性中得來。
shutdownNow()原始碼分析
在我們的例子中,我們呼叫了shutdownNow()方法來改變執行緒池的狀態。
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;
}
這裡我們關注的是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;
}
}
該方法最終是要把當前狀態變為STOP
狀態。
注意點一:
ThreadPoolExecutor中狀態定義的值大小是有序的,即:
TERMINATED > TIDYING > STOP > SHUTDOWN > RUNNING(最高位1是負數)
注意點二:
advance的含意是推進、前進的意思,Java併發包裡的很多方法都使用了該命名,所以當前方法表示的意思是要推進執行狀態
(advanceRunState),因此方法中才有了runStateAtLeast()
判斷。
即要推進狀態,那如果當前狀態已經大於目標狀態了,本次方法直接跳過。否則才去做cas操作。這也正是原方法註釋想表達的意思。
注意點三:
最後在做cas操作的時候合併當前wc和rs的值,使用的是ctlOf
方法:
private static int ctlOf(int rs, int wc) {
return rs | wc;
}
通過與運算把RunState和WorkerCount的值合併到一處,即最終的ctl的值:
STOP 001,00000000000000000000000000000
WorkCount 000,00000000000000000000000000001
ctl value 001,00000000000000000000000000001
所以可預見的輸出結果就是:
workerCountOf(): 1
Is Running: false
Is Stop: true
程式碼輸出
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
一個欄位可以獲取到兩個值,並且這兩個值不會有併發不一致的情況,每次都是一次cas更新值。
設計目的與優點
執行緒池自身的狀態和執行緒數量都維護在一個原子變數ctl
中,目的不是為了減少儲存空間,而是將執行緒池狀態與執行緒個數合二為一,這樣就可以用一次cas
原子操作進行賦值,更容易保證在多執行緒環境下保證執行狀態和執行緒數量的統一。這真是大師的設計智慧啊!