前言

在多線程開發(fā)中，應(yīng)該避免顯式創(chuàng)建線程，而是采用線程池里面的線程谭企。使用線程池可以減少手動(dòng)創(chuàng)建線程，減少線程創(chuàng)建和回收的損耗等评肆。那么使用線程池就需要了解它的原理债查。這里我們ThreadPoolExecutor.execute()方法內(nèi)部的具體實(shí)現(xiàn)邏輯

流程圖

image.png

源碼分析

    public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
       // 獲取狀態(tài)變量
        int c = ctl.get();
        if (workerCountOf(c) < corePoolSize) {
      // 如果worker個(gè)數(shù)小于核心線程數(shù)量
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        // 嘗試將任務(wù)丟入工作隊(duì)列中
        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);
        }
      // 工作隊(duì)列滿了的話再次嘗試增加worker
        else if (!addWorker(command, false))
            reject(command);
    }

  private boolean addWorker(Runnable firstTask, boolean core) {
        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 (;;) {
                 // 獲取worker數(shù)量
                int wc = workerCountOf(c);
                if (wc >= CAPACITY ||
                    wc >= (core ? corePoolSize : maximumPoolSize))
                    // 判斷是否大于核心線程數(shù)量或者超過線程池總大小
                    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
            }
        }

        boolean workerStarted = false;
        boolean workerAdded = false;
        Worker w = null;
        try {
            // 創(chuàng)建新的worker
            w = new Worker(firstTask);
            final Thread t = w.thread;
            if (t != null) {
                final ReentrantLock mainLock = this.mainLock;
                mainLock.lock();
                try {
                    // 單線程處理工作者入隊(duì)
                    int rs = runStateOf(ctl.get());

                    if (rs < SHUTDOWN ||
                        (rs == SHUTDOWN && firstTask == null)) {
                        if (t.isAlive()) // 這時(shí)候的worker不能已經(jīng)是在運(yùn)行中的
                            throw new IllegalThreadStateException();
                      // 將worker加入到工作者的集合中
                        workers.add(w);
                        int s = workers.size();
                      //   更新線程池的當(dāng)前最大size
                        if (s > largestPoolSize)
                            largestPoolSize = s;
                        workerAdded = true;
                    }
                } finally {
                    mainLock.unlock();
                }
                if (workerAdded) {
                    //   如果加入到工作者集合中成功，那么就啟動(dòng)工作者
                    t.start();
                    workerStarted = true;
                }
            }
        } finally {
            if (! workerStarted)
                addWorkerFailed(w);
        }
        return workerStarted;
    }

// 這個(gè)方法是工作者的實(shí)際工作內(nèi)容瓜挽，看下工作者是怎么處理任務(wù)的
  final void runWorker(Worker w) {
        Thread wt = Thread.currentThread();
        Runnable task = w.firstTask;
        w.firstTask = null;
        w.unlock(); // 允許中斷
        boolean completedAbruptly = true;
        try {
            // 如果工作者攜帶的任務(wù)或任務(wù)隊(duì)列不是空的盹廷，就會(huì)一直循環(huán)
            while (task != null || (task = getTask()) != null) {
                w.lock();
                // If pool is stopping, ensure thread is interrupted;
                // if not, ensure thread is not interrupted.  This
                // requires a recheck in second case to deal with
                // shutdownNow race while clearing interrupt
                if ((runStateAtLeast(ctl.get(), STOP) ||
                     (Thread.interrupted() &&
                      runStateAtLeast(ctl.get(), STOP))) &&
                    !wt.isInterrupted())
                    wt.interrupt();
                try {
                    beforeExecute(wt, task);
                    Throwable thrown = null;
                    try {
                        task.run();
                    } catch (RuntimeException x) {
                        thrown = x; throw x;
                    } catch (Error x) {
                        thrown = x; throw x;
                    } catch (Throwable x) {
                        thrown = x; throw new Error(x);
                    } finally {
                        // 處理任務(wù)執(zhí)行之后的邏輯
                        afterExecute(task, thrown);
                    }
                } finally {
                    task = null;
                    w.completedTasks++;
                    w.unlock();
                }
            }
            completedAbruptly = false;
        } finally {
            // 處理worker退出的邏輯
            processWorkerExit(w, completedAbruptly);
        }
    }