JAVA-异步编程-线程和线程池实现异步编程,线程池源码导读

1.文章目录

• 如何使用线程，线程池实现异步编程，以及其各自的优缺点；
• 线程池的原理，源码导读；

2.使用线程实现异步编程

    任务类

public class Task {

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

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

同步编程：

测试:耗时约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);
    }
}

显式使用线程：

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);
    }
}

测试耗时：约2.5s左右

   如上代码，我们使用lamba创建Java.lang.Runnable接口的实现类；上述代码，耗时2.5左右，可以看出异步编程可以大大缩减任务，当任务数多的时候效果更明显；

  JAVA中Deamon与No Deamon的区别，默认情况下我们创建的线程是No Deamon的，线程的类型与JVM退出条件有关，在JAVA中当JVM进程中不存在No Deamon线程就会退出。我们可以显式的setDaemon(true)方法设置线程为Deamon线程；

显示用线程编程的缺点：

• 每次执行异步任务，会直接创建一个Thread来执行异步任务,生产环境是不可以的，因为线程创建，销毁是有开销的，随意的滥用会消耗完系统线程，从而出错；推荐使用线程池来执行异步任务，线程池也有效的限制线程数量；
• 上述的线程执行异步任务没有返回值，如果需要我们可以用JDK的Future;

3.线程池实现异步编程

public class ThreadPoolTest {
    // 定义线程池
    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();
    }

}

• 上述代码创建了线程池：核心线程数为CPU核数，最大线程数为2*CPU核数；线程池阻塞队列大小为5；拒绝策略为CallerRunsPolicy，当线程池任务饱和，不会丢弃新任务，而是使用调用线程执行；
• 上述代码，我们减轻了main线程的负担，把任务交给线程池处理，我们再去完成其他任务；

测试结果：我们发现JVM任务执行完，没有退出，因为线程池的线程不是Deamon线程，JVM检测到存在NO Deamon线程所以不退出；

因此我们可以调用POOL_EXECUTOR.shutdown(); //POOL_EXECUTOR.shutdownNow();关闭线程池；

• 以上的没有实现异步返回的任务，我们测试下；

4.线程池实现异步返回任务编程

创建任务：返回“taskC"

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

        return "taskC";
    }

测试：

public class AsncThreadPoolTest {
    // 定义线程池
    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，等待异步任务结果
        System.out.println(future.get());
        POOL_EXECUTOR.shutdown();
    }
}

• 后续我们针对Future原理，源码角度来看如果实现；

5.线程池源码，原理解析

     基本属性

    // 线程池状态(高3位)  线程个数（低29)  
    private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
    // 线程掩码位
    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
    // 线程池状态
    // 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;

    // 获取运行状态
    private static int runStateOf(int c)     { return c & ~CAPACITY; }
    //线程个数
    private static int workerCountOf(int c)  { return c & CAPACITY; }
    // 计算ctl新值
    private static int ctlOf(int rs, int wc) { return rs | wc; }
    // 任务阻塞队列
    private final BlockingQueue<Runnable> workQueue;
    // lock
    private final ReentrantLock mainLock = new ReentrantLock();
    // woker集合
    private final HashSet<Worker> workers = new HashSet<Worker>();
     // codition条件集合
    private final Condition termination = mainLock.newCondition();

    // 最大线程数
    private int largestPoolSize;
    // 任务完成个数
    private long completedTaskCount;
    // 线程工厂
    private volatile ThreadFactory threadFactory;
    // 拒绝策略
    private volatile RejectedExecutionHandler handler;

    // 线程空闲时间
    private volatile long keepAliveTime;

    // 是否允许timeount
    private volatile boolean allowCoreThreadTimeOut;
    // 核心线程数
    private volatile int corePoolSize;
    // 
    private volatile int maximumPoolSize;

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

• corePoolSize:线程池核心线程个数.
• workQueue:用于保存等待执行任务的阻塞队列；
• maximunPoolSize:线程池最大线程数量;
• threadFactory:线程工厂类；
• defaultHandler:饱和策略，当任务队列满后，线程个数达到了maximunPoolSize就执行饱和策略,默认AbortPolicy(抛出异常），Caller Runs Policy（使用调用者所在的线程来运行任务),DiscardOldestPolicy(丢弃一个任务，执行当前的任务），DiscardPolicy(直接丢弃);

初始化方法：

   

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

    // 自定义线程工厂使用
    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue,
                              ThreadFactory threadFactory) {
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
             threadFactory, defaultHandler);
    }

   // 自定义线程工厂，拒绝策略
    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) {
         // 参数校验赋值
        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;
    }

默认线程工厂:defaultThreadFactory

 static class DefaultThreadFactory implements ThreadFactory {
        // 原子类
        private static final AtomicInteger poolNumber = new AtomicInteger(1);
        // 线程组
        private final ThreadGroup group;
        // 技术
        private final AtomicInteger threadNumber = new AtomicInteger(1);
        // threadname前缀
        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) {
            // 创建一个线程
            Thread t = new Thread(group, r,
                                  namePrefix + threadNumber.getAndIncrement(),
                                  0);
            // 如果线程位Daemon，置为非Daemon
            if (t.isDaemon())
                t.setDaemon(false);
            // 重置线程优先级为正常5
            if (t.getPriority() != Thread.NORM_PRIORITY)
                t.setPriority(Thread.NORM_PRIORITY);
            return t;
        }
    }

execut:线程池执行任务的方法：

 public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        // 获取ctl(运行状态&线程数)
        int c = ctl.get();
        // 如果线程数小于核心线程数，直接创建一个线程
        if (workerCountOf(c) < corePoolSize) {
            // true表示，当前创建线程数量应<=核心线程数
            if (addWorker(command, true))
                return;
            // 如果没有添加成功，获取新的ctl
            c = ctl.get();
        }
           
        // 如果线程池处于Running状态，添加任务到阻塞队列
        if (isRunning(c) && workQueue.offer(command)) {
            // 二次校验
            int recheck = ctl.get();
            // 如果不是running则从队列删除任务，并执行拒绝策略
            if (! isRunning(recheck) && remove(command))
                reject(command);
            // 如果线程数为0
            else if (workerCountOf(recheck) == 0)
                // 添加一个线程
                addWorker(null, false);
        }
        // 如果线程数达到了核心线程数，且添加阻塞队列失败，则创建一个线程
        else if (!addWorker(command, false))
            // 失败则，拒绝
            reject(command);
    }

addWorker（Runnable, bool)方法：创建一个线程，运行runnable任务，bool决定最大线程为核心线程数，还是最大线程数来执行拒绝策略

private boolean addWorker(Runnable firstTask, boolean core) {
        retry:
        for (;;) {
            // 获取ctl
            int c = ctl.get();
            // 获取运行状态
            int rs = runStateOf(c);

            // 检测运行状态以及队列状态
            if (rs >= SHUTDOWN &&
                ! (rs == SHUTDOWN &&
                   firstTask == null &&
                   ! workQueue.isEmpty()))
                return false;

            for (;;) {
                // 获取线程数
                int wc = workerCountOf(c);
                // 如果线程数大于容量/（核心线程数/最大线程数)core决定
                if (wc >= CAPACITY ||
                    wc >= (core ? corePoolSize : maximumPoolSize))
                    return false;
                // CAS算法，增加线程数，成功跳出循环
                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
            }
        }

        // 任务start,worker添加成功标志
        boolean workerStarted = false;
        boolean workerAdded = false;
        Worker w = null;
        try {
            // worker包装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());
                    // 校验线程池状态
                    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，运行
                if (workerAdded) {
                    t.start();
                    workerStarted = true;
                }
            }
        } finally {
            if (! workerStarted)
                addWorkerFailed(w);
        }
        return workerStarted;
    }

几种拒绝策略：

    // 使用调用者的线程运行
    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();
            }
        }
    }

    // 抛出异常
    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());
        }
    }

   // 默认不管
    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) {
        }
    }

   // 抛出队列中的一个，然后运行他
    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，后置方法;子类可以进行扩展

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

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

线程池提供的三种异步Future方式：都是包装为RunnableFuture执行

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:我们线程池调度这东西，看下他源码；

// 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为null 任务队列获取的也是null
            while (task != null || (task = getTask()) != null) {
                w.lock();
                // shutDownNow可以打断当前线程，可以自己测试下
                if ((runStateAtLeast(ctl.get(), STOP) ||
                     (Thread.interrupted() &&
                      runStateAtLeast(ctl.get(), STOP))) &&
                    !wt.isInterrupted())
                    wt.interrupt();
                try {
                     // 执行前
                    beforeExecute(wt, task);
                    Throwable thrown = null;
                    try {
                        // 核心，也是我们写的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 {
                        // 执行后处理
                        afterExecute(task, thrown);
                    }
                } finally {
                    task = null;
                    w.completedTasks++;
                    w.unlock();
                }
            }
            completedAbruptly = false;
        } finally {
           // 执行清理
            processWorkerExit(w, completedAbruptly);
        }
    }

• 可以看出来，我们的worker就是一个thread,不断的执行任务；

任务运行完：执行清理

  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 {
            // 完成任务数
            completedTaskCount += w.completedTasks;
            // woker集合删除此woker
            workers.remove(w);
        } finally {
            mainLock.unlock();
        }
        
        // 尝试设置线程池状态为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);
        }
    }

关闭线程池：

ShutDown:关闭线程池，线程池不会接受新任务，工作队列的任务执行完；

public void shutdown() {
        final ReentrantLock mainLock = this.mainLock;
        // 保证线程同步
        mainLock.lock();
        try {
            checkShutdownAccess();
           // 设置线程状态
            advanceRunState(SHUTDOWN);
            // 设置中断标志
            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;
                // 如果线程没有被中断，并且没有运行
                if (!t.isInterrupted() && w.tryLock()) {
                    try {
                        // 中断
                        t.interrupt();
                    } catch (SecurityException ignore) {
                    } finally {
                        w.unlock();
                    }
                }
                if (onlyOne)
                    break;
            }
        } finally {
            mainLock.unlock();
        }
    }

shutDownNow:中断所有线程，包括正在运行的

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

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

6.总结

• 本文描述了异步执行的好处，以及其具体实现；
• 显式线程的优缺点，线程池优点以及其实现原理，重要源码导读；
• 线程池线程增加策略：如果当前线程数<=核心线程数，新建一个线程；然后给阻塞队列添加，队列满了，当线程数<=最大线程数则新建线程；
• 上述的异步并不是比较好的实现，Future必须调用Get阻塞当前线程才可以拿到任务返回值，后续文章，会对Future源码，以及JDK新增的CompletableAbleFuture实现异步编程；