JAVA-異步編程-線程和線程池實現異步編程,線程池源碼導讀

1.文章目錄

• 如何使用線程，線程池實現異步編程，以及其各自的優缺點；
• 線程池的原理，源碼導讀；

2.使用線程實現異步編程

    任務類

public class Task {

    // taskA
    public static void doSomethingA() {
        try {
            // 模擬耗時
            Thread.sleep(200);
            System.out.println("taskA done");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    // taskA
    public static void doSomethingB() {
        try {
            // 模擬耗時
            Thread.sleep(200);
            System.out.println("taskB done");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

同步編程：

測試:耗時約4s左右

public class SyncExample {

    public static void main(String[] argv) throws InterruptedException {
        long start = System.currentTimeMillis();
        Task.doSomethingA();
        Task.doSomethingB();
        System.out.println(System.currentTimeMillis() - start);
    }
}

顯式使用線程：

package AsynchronousProgramming;

import java.util.concurrent.CountDownLatch;

/**
 * @Author: SoftWareKang
 * @Name:JAVALEARN
 * @Date: 2020/5/30 15:16
 */
public class SyncExample {

    private static CountDownLatch countDownLatch = new CountDownLatch(2);

    public static void main(String[] argv) throws InterruptedException {
        long start = System.currentTimeMillis();
        // do A
        new Thread(() -> {
            try {
                Task.doSomethingA();
                countDownLatch.countDown();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        ).start();


        // do B
        new Thread(() -> {
            try {
                Task.doSomethingB();
                countDownLatch.countDown();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        ).start();

        countDownLatch.await();
        System.out.println(System.currentTimeMillis() - start);
    }
}

測試耗時：約2.5s左右

   如上代碼，我們使用lamba創建Java.lang.Runnable接口的實現類；上述代碼，耗時2.5左右，可以看出異步編程可以大大縮減任務，當任務數多的時候效果更明顯；

  JAVA中Deamon與No Deamon的區別，默認情況下我們創建的線程是No Deamon的，線程的類型與JVM退出條件有關，在JAVA中當JVM進程中不存在No Deamon線程就會退出。我們可以顯式的setDaemon(true)方法設置線程爲Deamon線程；

顯示用線程編程的缺點：

• 每次執行異步任務，會直接創建一個Thread來執行異步任務,生產環境是不可以的，因爲線程創建，銷燬是有開銷的，隨意的濫用會消耗完系統線程，從而出錯；推薦使用線程池來執行異步任務，線程池也有效的限制線程數量；
• 上述的線程執行異步任務沒有返回值，如果需要我們可以用JDK的Future;

3.線程池實現異步編程

public class ThreadPoolTest {
    // 定義線程池
    private final static int AVALIABLE_PROCESSORS = Runtime.getRuntime().availableProcessors();

    private final static ThreadPoolExecutor POOL_EXECUTOR = new ThreadPoolExecutor(
            AVALIABLE_PROCESSORS, AVALIABLE_PROCESSORS * 2, 1, TimeUnit.MINUTES,
            new LinkedBlockingQueue<>(5), new ThreadPoolExecutor.CallerRunsPolicy()
    );

    public static void main(String[] argv) throws InterruptedException {
        long start = System.currentTimeMillis();

        POOL_EXECUTOR.execute(() -> {
            Task.doSomethingA();
        });

        // Task.doSomethingB();
        POOL_EXECUTOR.execute(() -> {
            Task.doSomethingB();
        });
        System.out.println(System.currentTimeMillis() - start);

        Thread.currentThread().join();
    }

}

• 上述代碼創建了線程池：核心線程數爲CPU核數，最大線程數爲2*CPU核數；線程池阻塞隊列大小爲5；拒絕策略爲CallerRunsPolicy，當線程池任務飽和，不會丟棄新任務，而是使用調用線程執行；
• 上述代碼，我們減輕了main線程的負擔，把任務交給線程池處理，我們再去完成其他任務；

測試結果：我們發現JVM任務執行完，沒有退出，因爲線程池的線程不是Deamon線程，JVM檢測到存在NO Deamon線程所以不退出；

因此我們可以調用POOL_EXECUTOR.shutdown(); //POOL_EXECUTOR.shutdownNow();關閉線程池；

• 以上的沒有實現異步返回的任務，我們測試下；

4.線程池實現異步返回任務編程

創建任務：返回“taskC"

// TaskC
    public static String doSomethingC() {
        try {
            Thread.sleep(200);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(" taskC done");

        return "taskC";
    }

測試：

public class AsncThreadPoolTest {
    // 定義線程池
    private final static int AVALIABLE_PROCESSORS = Runtime.getRuntime().availableProcessors();

    private final static ThreadPoolExecutor POOL_EXECUTOR = new ThreadPoolExecutor(
            AVALIABLE_PROCESSORS, AVALIABLE_PROCESSORS * 2, 1, TimeUnit.MINUTES,
            new LinkedBlockingQueue<>(),new ThreadPoolExecutor.CallerRunsPolicy()
    );

    public static void main(String[] argv) throws ExecutionException, InterruptedException {
        Future<?> future = POOL_EXECUTOR.submit(() -> Task.doSomethingC());
        // 阻塞，等待異步任務結果
        System.out.println(future.get());
        POOL_EXECUTOR.shutdown();
    }
}

• 後續我們針對Future原理，源碼角度來看如果實現；

5.線程池源碼，原理解析

     基本屬性

    // 線程池狀態(高3位)  線程個數（低29)  
    private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
    // 線程掩碼位
    private static final int COUNT_BITS = Integer.SIZE - 3;
    // 容量
    private static final int CAPACITY   = (1 << COUNT_BITS) - 1;
    
    // runState is stored in the high-order bits
    // 線程池狀態
    // 111
    private static final int RUNNING    = -1 << COUNT_BITS;
    // 000
    private static final int SHUTDOWN   =  0 << COUNT_BITS;
    // 001
    private static final int STOP       =  1 << COUNT_BITS;
    // 010
    private static final int TIDYING    =  2 << COUNT_BITS;
    // 011
    private static final int TERMINATED =  3 << COUNT_BITS;

    // 獲取運行狀態
    private static int runStateOf(int c)     { return c & ~CAPACITY; }
    //線程個數
    private static int workerCountOf(int c)  { return c & CAPACITY; }
    // 計算ctl新值
    private static int ctlOf(int rs, int wc) { return rs | wc; }
    // 任務阻塞隊列
    private final BlockingQueue<Runnable> workQueue;
    // lock
    private final ReentrantLock mainLock = new ReentrantLock();
    // woker集合
    private final HashSet<Worker> workers = new HashSet<Worker>();
     // codition條件集合
    private final Condition termination = mainLock.newCondition();

    // 最大線程數
    private int largestPoolSize;
    // 任務完成個數
    private long completedTaskCount;
    // 線程工廠
    private volatile ThreadFactory threadFactory;
    // 拒絕策略
    private volatile RejectedExecutionHandler handler;

    // 線程空閒時間
    private volatile long keepAliveTime;

    // 是否允許timeount
    private volatile boolean allowCoreThreadTimeOut;
    // 核心線程數
    private volatile int corePoolSize;
    // 
    private volatile int maximumPoolSize;

    /**
     * The default rejected execution handler
     */
    private static final RejectedExecutionHandler defaultHandler =
        new AbortPolicy();

• corePoolSize:線程池核心線程個數.
• workQueue:用於保存等待執行任務的阻塞隊列；
• maximunPoolSize:線程池最大線程數量;
• threadFactory:線程工廠類；
• defaultHandler:飽和策略，當任務隊列滿後，線程個數達到了maximunPoolSize就執行飽和策略,默認AbortPolicy(拋出異常），Caller Runs Policy（使用調用者所在的線程來運行任務),DiscardOldestPolicy(丟棄一個任務，執行當前的任務），DiscardPolicy(直接丟棄);

初始化方法：

   

    // 使用默認threadFactory,拒絕策略
    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue) {
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
             Executors.defaultThreadFactory(), defaultHandler);
    }

    // 自定義線程工廠使用
    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue,
                              ThreadFactory threadFactory) {
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
             threadFactory, defaultHandler);
    }

   // 自定義線程工廠，拒絕策略
    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue,
                              RejectedExecutionHandler handler) {
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
             Executors.defaultThreadFactory(), handler);
    }

    // 核心方法
    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue,
                              ThreadFactory threadFactory,
                              RejectedExecutionHandler handler) {
         // 參數校驗賦值
        if (corePoolSize < 0 ||
            maximumPoolSize <= 0 ||
            maximumPoolSize < corePoolSize ||
            keepAliveTime < 0)
            throw new IllegalArgumentException();
        if (workQueue == null || threadFactory == null || handler == null)
            throw new NullPointerException();
        this.acc = System.getSecurityManager() == null ?
                null :
                AccessController.getContext();
        this.corePoolSize = corePoolSize;
        this.maximumPoolSize = maximumPoolSize;
        this.workQueue = workQueue;
        this.keepAliveTime = unit.toNanos(keepAliveTime);
        this.threadFactory = threadFactory;
        this.handler = handler;
    }

默認線程工廠:defaultThreadFactory

 static class DefaultThreadFactory implements ThreadFactory {
        // 原子類
        private static final AtomicInteger poolNumber = new AtomicInteger(1);
        // 線程組
        private final ThreadGroup group;
        // 技術
        private final AtomicInteger threadNumber = new AtomicInteger(1);
        // threadname前綴
        private final String namePrefix;

        DefaultThreadFactory() {
            SecurityManager s = System.getSecurityManager();
            group = (s != null) ? s.getThreadGroup() :
                                  Thread.currentThread().getThreadGroup();
            namePrefix = "pool-" +
                          poolNumber.getAndIncrement() +
                         "-thread-";
        }

        public Thread newThread(Runnable r) {
            // 創建一個線程
            Thread t = new Thread(group, r,
                                  namePrefix + threadNumber.getAndIncrement(),
                                  0);
            // 如果線程位Daemon，置爲非Daemon
            if (t.isDaemon())
                t.setDaemon(false);
            // 重置線程優先級爲正常5
            if (t.getPriority() != Thread.NORM_PRIORITY)
                t.setPriority(Thread.NORM_PRIORITY);
            return t;
        }
    }

execut:線程池執行任務的方法：

 public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        // 獲取ctl(運行狀態&線程數)
        int c = ctl.get();
        // 如果線程數小於核心線程數，直接創建一個線程
        if (workerCountOf(c) < corePoolSize) {
            // true表示，當前創建線程數量應<=核心線程數
            if (addWorker(command, true))
                return;
            // 如果沒有添加成功，獲取新的ctl
            c = ctl.get();
        }
           
        // 如果線程池處於Running狀態，添加任務到阻塞隊列
        if (isRunning(c) && workQueue.offer(command)) {
            // 二次校驗
            int recheck = ctl.get();
            // 如果不是running則從隊列刪除任務，並執行拒絕策略
            if (! isRunning(recheck) && remove(command))
                reject(command);
            // 如果線程數爲0
            else if (workerCountOf(recheck) == 0)
                // 添加一個線程
                addWorker(null, false);
        }
        // 如果線程數達到了核心線程數，且添加阻塞隊列失敗，則創建一個線程
        else if (!addWorker(command, false))
            // 失敗則，拒絕
            reject(command);
    }

addWorker（Runnable, bool)方法：創建一個線程，運行runnable任務，bool決定最大線程爲核心線程數，還是最大線程數來執行拒絕策略

private boolean addWorker(Runnable firstTask, boolean core) {
        retry:
        for (;;) {
            // 獲取ctl
            int c = ctl.get();
            // 獲取運行狀態
            int rs = runStateOf(c);

            // 檢測運行狀態以及隊列狀態
            if (rs >= SHUTDOWN &&
                ! (rs == SHUTDOWN &&
                   firstTask == null &&
                   ! workQueue.isEmpty()))
                return false;

            for (;;) {
                // 獲取線程數
                int wc = workerCountOf(c);
                // 如果線程數大於容量/（核心線程數/最大線程數)core決定
                if (wc >= CAPACITY ||
                    wc >= (core ? corePoolSize : maximumPoolSize))
                    return false;
                // CAS算法，增加線程數，成功跳出循環
                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
            }
        }

        // 任務start,worker添加成功標誌
        boolean workerStarted = false;
        boolean workerAdded = false;
        Worker w = null;
        try {
            // worker包裝firsrTask
            w = new Worker(firstTask);
            final Thread t = w.thread;
            if (t != null) {
                final ReentrantLock mainLock = this.mainLock;
                mainLock.lock();
                try {
                    // Recheck while holding lock.
                    // Back out on ThreadFactory failure or if
                    // shut down before lock acquired.
                    int rs = runStateOf(ctl.get());
                    // 校驗線程池狀態
                    if (rs < SHUTDOWN ||
                        (rs == SHUTDOWN && firstTask == null)) {
                        if (t.isAlive()) // precheck that t is startable
                            throw new IllegalThreadStateException();
                        workers.add(w);
                        int s = workers.size();
                        if (s > largestPoolSize)
                            largestPoolSize = s;
                        workerAdded = true;
                    }
                } finally {
                    mainLock.unlock();
                }
                // 如果添加成功，運行
                if (workerAdded) {
                    t.start();
                    workerStarted = true;
                }
            }
        } finally {
            if (! workerStarted)
                addWorkerFailed(w);
        }
        return workerStarted;
    }

幾種拒絕策略：

    // 使用調用者的線程運行
    public static class CallerRunsPolicy implements RejectedExecutionHandler {
        /**
         * Creates a {@code CallerRunsPolicy}.
         */
        public CallerRunsPolicy() { }

        /**
         * Executes task r in the caller's thread, unless the executor
         * has been shut down, in which case the task is discarded.
         *
         * @param r the runnable task requested to be executed
         * @param e the executor attempting to execute this task
         */
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
            if (!e.isShutdown()) {
                r.run();
            }
        }
    }

    // 拋出異常
    public static class AbortPolicy implements RejectedExecutionHandler {
        /**
         * Creates an {@code AbortPolicy}.
         */
        public AbortPolicy() { }

        /**
         * Always throws RejectedExecutionException.
         *
         * @param r the runnable task requested to be executed
         * @param e the executor attempting to execute this task
         * @throws RejectedExecutionException always
         */
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
            throw new RejectedExecutionException("Task " + r.toString() +
                                                 " rejected from " +
                                                 e.toString());
        }
    }

   // 默認不管
    public static class DiscardPolicy implements RejectedExecutionHandler {
        /**
         * Creates a {@code DiscardPolicy}.
         */
        public DiscardPolicy() { }

        /**
         * Does nothing, which has the effect of discarding task r.
         *
         * @param r the runnable task requested to be executed
         * @param e the executor attempting to execute this task
         */
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
        }
    }

   // 拋出隊列中的一個，然後運行他
    public static class DiscardOldestPolicy implements RejectedExecutionHandler {
        /**
         * Creates a {@code DiscardOldestPolicy} for the given executor.
         */
        public DiscardOldestPolicy() { }

        /**
         * Obtains and ignores the next task that the executor
         * would otherwise execute, if one is immediately available,
         * and then retries execution of task r, unless the executor
         * is shut down, in which case task r is instead discarded.
         *
         * @param r the runnable task requested to be executed
         * @param e the executor attempting to execute this task
         */
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
            if (!e.isShutdown()) {
                e.getQueue().poll();
                e.execute(r);
            }
        }
    }

前置，後置方法;子類可以進行擴展

 
    protected void beforeExecute(Thread t, Runnable r) { }

    protected void afterExecute(Runnable r, Throwable t) { }

線程池提供的三種異步Future方式：都是包裝爲RunnableFuture執行

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

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     */
    public <T> Future<T> submit(Runnable task, T result) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<T> ftask = newTaskFor(task, result);
        execute(ftask);
        return ftask;
    }

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     */
    public <T> Future<T> submit(Callable<T> task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<T> ftask = newTaskFor(task);
        execute(ftask);
        return ftask;
    }

Worker:我們線程池調度這東西，看下他源碼；

// run方法
 public void run() {
            runWorker(this);
 }
// 核心方法
final void runWorker(Worker w) {
        Thread wt = Thread.currentThread();
        Runnable task = w.firstTask;
        w.firstTask = null;
        w.unlock(); // allow interrupts
        boolean completedAbruptly = true;
        try {
            //如果task爲null 任務隊列獲取的也是null
            while (task != null || (task = getTask()) != null) {
                w.lock();
                // shutDownNow可以打斷當前線程，可以自己測試下
                if ((runStateAtLeast(ctl.get(), STOP) ||
                     (Thread.interrupted() &&
                      runStateAtLeast(ctl.get(), STOP))) &&
                    !wt.isInterrupted())
                    wt.interrupt();
                try {
                     // 執行前
                    beforeExecute(wt, task);
                    Throwable thrown = null;
                    try {
                        // 核心，也是我們寫的task
                        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 {
                        // 執行後處理
                        afterExecute(task, thrown);
                    }
                } finally {
                    task = null;
                    w.completedTasks++;
                    w.unlock();
                }
            }
            completedAbruptly = false;
        } finally {
           // 執行清理
            processWorkerExit(w, completedAbruptly);
        }
    }

• 可以看出來，我們的worker就是一個thread,不斷的執行任務；

任務運行完：執行清理

  private void processWorkerExit(Worker w, boolean completedAbruptly) {
        if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
            decrementWorkerCount();

        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            // 完成任務數
            completedTaskCount += w.completedTasks;
            // woker集合刪除此woker
            workers.remove(w);
        } finally {
            mainLock.unlock();
        }
        
        // 嘗試設置線程池狀態爲terminated
        tryTerminate();
        
        int c = ctl.get();
        if (runStateLessThan(c, STOP)) {
            if (!completedAbruptly) {
                int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
                if (min == 0 && ! workQueue.isEmpty())
                    min = 1;
                if (workerCountOf(c) >= min)
                    return; // replacement not needed
            }
            addWorker(null, false);
        }
    }

關閉線程池：

ShutDown:關閉線程池，線程池不會接受新任務，工作隊列的任務執行完；

public void shutdown() {
        final ReentrantLock mainLock = this.mainLock;
        // 保證線程同步
        mainLock.lock();
        try {
            checkShutdownAccess();
           // 設置線程狀態
            advanceRunState(SHUTDOWN);
            // 設置中斷標誌
            interruptIdleWorkers();
            onShutdown(); // hook for ScheduledThreadPoolExecutor
        } finally {
            mainLock.unlock();
        }
        tryTerminate();
    }

  private void advanceRunState(int targetState) {
        for (;;) {
            int c = ctl.get();
            if (runStateAtLeast(c, targetState) ||
                ctl.compareAndSet(c, ctlOf(targetState, workerCountOf(c))))
                break;
        }
    }

  private void interruptIdleWorkers(boolean onlyOne) {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            for (Worker w : workers) {
                Thread t = w.thread;
                // 如果線程沒有被中斷，並且沒有運行
                if (!t.isInterrupted() && w.tryLock()) {
                    try {
                        // 中斷
                        t.interrupt();
                    } catch (SecurityException ignore) {
                    } finally {
                        w.unlock();
                    }
                }
                if (onlyOne)
                    break;
            }
        } finally {
            mainLock.unlock();
        }
    }

shutDownNow:中斷所有線程，包括正在運行的

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

 private void interruptWorkers() {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
             // 全部打斷
            for (Worker w : workers)
                w.interruptIfStarted();
        } finally {
            mainLock.unlock();
        }
    }

6.總結

• 本文描述了異步執行的好處，以及其具體實現；
• 顯式線程的優缺點，線程池優點以及其實現原理，重要源碼導讀；
• 線程池線程增加策略：如果當前線程數<=核心線程數，新建一個線程；然後給阻塞隊列添加，隊列滿了，當線程數<=最大線程數則新建線程；
• 上述的異步並不是比較好的實現，Future必須調用Get阻塞當前線程纔可以拿到任務返回值，後續文章，會對Future源碼，以及JDK新增的CompletableAbleFuture實現異步編程；