java线程池源码解析

主要介绍线程池相关知识，关于线程池，首先我们思考下为什么要用线程池。如果单纯的使用线程，线程的创建和销毁都是自己来完成，如果并发请求过多，可能造成资源耗尽。线程池可以对线程进行统一分配，调优和监控。本篇文章为《图灵学院》课程笔记

• 降低资源消耗（线程无限制地创建，然后使用完毕后销毁）
• 提高响应速度（无须创建线程）
• 提高线程的可管理性

java是如何实现和管理线程池的，jdk5开始把工作单元和任务执行分离，工作单元包括callable、runnable，而执行机制由Executor提供，Executor的实现还提供了对线程生命周期的管理

相关接口

image

接口介绍

• java.util.concurrent.Executor （执行器，执行方法）

• java.util.concurrent.ExecutorService （执行服务） 包含服务的生命周期

• java.util.concurrent.ScheduledExecutorService （调度相关的服务）

核心接口实现

• java.util.concurrent.ThreadPoolExecutor （普通的的线程池实现类）
• java.util.concurrent.ScheduledThreadPoolExecutor （调度的核心实现类）

名称	方法	说明	类型
java.util.concurrent. Executor	execute	执行接口	接口
java.util.concurrent. ExecutorService	submit(java.util.concurrent.Callable)	提交接口	接口
java.util.concurrent. AbstractExecutorService	submit(Callable task)	把执行和提交接口 进行合并区别:有 返回值和无返回值	抽象类
java.util.concurrent. ThreadPoolExecutor	execute(Runnable command)	调 用 runwork 方 法 getTask（从队列 拿数据）	实现类
java.util.concurrent. ScheduledExecutorService	scheduleAtFixedRate、scheduleWithFixedDelay	定义方法	接口
java.util.concurrent. ScheduledThreadPoolExecutor	delayedExecute	具体实现 add>task>addWo rk	实现类

image

内部类分为两种

• policy 策略
• worker 工作

内部工作原理（构造方法赋值）

• corePool：核心线程池大小
• maximumPool：最大线程池大小
• BlockingQueue：任务工作队列
• keepAliveTime：线程活跃时间，如果线程数量大于核心线程数量，多余线程空闲时间超时候被销毁
• RejectedExecutionHandler：当ThreadPoolExecutor关闭或最大线程池已经满了，executor将调用的handler
• ThreadFactory：使用ThreadFactory创建线程，默认使用defaultThreadFactory

线程池的运行思路

• 如果当前线程池中的线程数目小于corePoolSize，则每来一个任务，就会创建一个线程去执行这个任务；
• 如果当前线程池中的线程数目>=corePoolSize，则每来一个任务，会尝试将其添加到任务缓存队列当中，若添加成功，则该任务会等待空闲线程将其取出去执行；若添加失败（一般来说是任务缓存队列已满），则会尝试创建新的线程去执行这个任务；
• 如果当前线程池中的线程数目达到maximumPoolSize，则会采取任务拒绝策略进行处理；
• 如果线程池中的线程数量大于 corePoolSize时，如果某线程空闲时间超过keepAliveTime，线程将被终止，直至线程池中的线程数目不大于corePoolSize；如果允许为核心池中的线程设置存活时间，那么核心池中的线程空闲时间超过keepAliveTime，线程也会被终止

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拒接策略

 ThreadPoolExecutor.AbortPolicy:丢弃任务并抛出RejectedExecutionException异常(默认)
 ThreadPoolExecutor.DiscardPolicy：也是丢弃任务，但是不抛出异常
 ThreadPoolExecutor.DiscardOldestPolicy：丢弃队列最前面的任务，然后重新尝试执行任务（重复此过程）
 ThreadPoolExecutor.CallerRunsPolicy：由调用线程处理该任务

源码解析

线程池的执行原理

初始化构造器

public ThreadPoolExecutor(int corePoolSize,
                          int maximumPoolSize,
                          long keepAliveTime,
                          TimeUnit unit,
                          BlockingQueue workQueue,
                          ThreadFactory threadFactory,
                          RejectedExecutionHandler handler) {
    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;
}

ThreadPoolExecutor#execute

public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        int c = ctl.get();
  //判断是否小于核心数量，是直接新增work成功后直接退出
        if (workerCountOf(c) < corePoolSize) {
            if (addWorker(command, true))
                return;
          // 增加失败后继续获取标记 
            c = ctl.get();
        }
  //判断是运行状态并且扔到workQueue里成功后 
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();
          //再次check判断运行状态如果是非运行状态就移除出去&reject掉
            if (! isRunning(recheck) && remove(command))
                reject(command);
          //否则发现可能运行线程数是0那么增加一个null的worker
            else if (workerCountOf(recheck) == 0)
                addWorker(null, false);
         }
      //直接增加worker如果不成功直接reject
        else if (!addWorker(command, false))
            reject(command);
    }

ThreadPoolExecutor#addWorker

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()))
      // 两种情况
      //1.如果非运行状态 
      //2.不是这种情况（停止状态并且是null对象并且workQueue不等于null）
        return false;

    for (;;) {
        int wc = workerCountOf(c);
        if (wc >= CAPACITY ||
            wc >= (core ? corePoolSize : maximumPoolSize))
            return false;// 判断是否饱和容量了
        if (compareAndIncrementWorkerCount(c)) //增加一个work数量 然后跳出去
            break retry;
        c = ctl.get();  // Re-read ctl  增加work失败后继续递归
        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 {
  //增加一个worker
    w = new Worker(firstTask);
    final Thread t = w.thread;
  //判断是否 为null
    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.  锁定后并重新检查下 是否存在线程工厂的失败或者锁定前的关闭
            int rs = runStateOf(ctl.get());

            if (rs < SHUTDOWN ||
                (rs == SHUTDOWN && firstTask == null)) {
                if (t.isAlive()) // precheck that t is startable
                    throw new IllegalThreadStateException();  
                workers.add(w);   //增加work
                int s = workers.size();
                if (s > largestPoolSize)
                    largestPoolSize = s;
                workerAdded = true;
            }
        } finally {
            mainLock.unlock();
        }
        if (workerAdded) { //本次要是新增加work成功就调用start运行
            t.start();
            workerStarted = true;
        }
    }
} finally {
    if (! workerStarted)
        addWorkerFailed(w);
}
return workerStarted;
    }

ThreadPoolExecutor#runWorker

final void runWorker(Worker w) {
Thread wt = Thread.currentThread();//1.取到当前线程
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
    while (task != null || (task = getTask()) != null) { //获取任务 看看是否能拿到
        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();// 确保线程是能中断的
        try {
            beforeExecute(wt, task); //开始任务前的钩子
            Throwable thrown = null;
            try {
                task.run();//执行任务
            } catch (RuntimeException x) {
                thrown = x; throw x;
            } catch (Error x) {
                thrown = x; throw x;
            } catch (Throwable x) {
                thrown = x; throw new Error(x);
            } finally {
                afterExecute(task, thrown); //任务后的钩子
            }
        } finally {
            task = null;
            w.completedTasks++;
            w.unlock();
        }
    }
    completedAbruptly = false;
} finally {
    processWorkerExit(w, completedAbruptly);
}
}

ThreadPoolExecutor#processWorkerExit

 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 {
    completedTaskCount += w.completedTasks;
    workers.remove(w);  //移除work
} finally {
    mainLock.unlock();
}

tryTerminate();

int c = ctl.get();
if (runStateLessThan(c, STOP)) { //判断是否还有任务
    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);
}
}

image

线程池调度原理

调度核心构造器

public ScheduledThreadPoolExecutor(int corePoolSize) {
    super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
          new DelayedWorkQueue());
}

ScheduledThreadPoolExecutor#delayedExecute

private void delayedExecute(RunnableScheduledFuture task) {
if (isShutdown())
    reject(task);
else {
    super.getQueue().add(task);//增加任务
    if (isShutdown() &&
        !canRunInCurrentRunState(task.isPeriodic()) &&
        remove(task))
        task.cancel(false);
    else
        ensurePrestart();
}
}

通过DelayedWorkQueue 延迟队列实现 offer获取对象的延迟

ScheduledThreadPoolExecutor.DelayedWorkQueue#offer

public boolean offer(Runnable x) {
if (x == null)
    throw new NullPointerException();
RunnableScheduledFuture e = (RunnableScheduledFuture)x; //当前对象
final ReentrantLock lock = this.lock;
lock.lock();
try {
    int i = size;
    if (i >= queue.length) //扩容
        grow();
    size = i + 1;
    if (i == 0) {
        queue[0] = e;
        setIndex(e, 0); //第一个直接设置索引和下标0
    } else {
        siftUp(i, e); //筛选到上边
    }
    if (queue[0] == e) {
        leader = null;
        available.signal(); //唤醒所有的被挤压的wait线程
    }
} finally {
    lock.unlock();
}
return true;
}

ScheduledThreadPoolExecutor.DelayedWorkQueue#siftUp

 private void siftUp(int k, RunnableScheduledFuture key) {
while (k > 0) {
    int parent = (k - 1) >>> 1; 
    RunnableScheduledFuture e = queue[parent];
    if (key.compareTo(e) >= 0)
        break; 
    queue[k] = e;
    setIndex(e, k);
    k = parent;
}
queue[k] = key;
setIndex(key, k);
 }

ScheduledThreadPoolExecutor.ScheduledFutureTask#compareTo

 public int compareTo(Delayed other) {
if (other == this) // compare zero if same object
    return 0;
if (other instanceof ScheduledFutureTask) {
    ScheduledFutureTask x = (ScheduledFutureTask)other;
    long diff = time - x.time; //判断time
    if (diff < 0)
        return -1;
    else if (diff > 0)
        return 1;
    else if (sequenceNumber < x.sequenceNumber)
        return -1;
    else
        return 1;
}
long diff = getDelay(NANOSECONDS) - other.getDelay(NANOSECONDS);
return (diff < 0) ? -1 : (diff > 0) ? 1 : 0;
   }

ThreadPoolExecutor#ensurePrestart

确保有work执行

 void ensurePrestart() {
        int wc = workerCountOf(ctl.get());
        if (wc < corePoolSize)
            addWorker(null, true);
        else if (wc == 0)
            addWorker(null, false);
   }

ScheduledThreadPoolExecutor.DelayedWorkQueue#take

work运行的时候调用queue的take方法

public RunnableScheduledFuture take() throws InterruptedException {
            final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
    for (;;) {
        RunnableScheduledFuture first = queue[0];//获取第一个对象
        if (first == null)
            available.await();
        else {
            long delay = first.getDelay(NANOSECONDS);//延迟时间
            if (delay <= 0)//到时间了
                return finishPoll(first);
            first = null; // don't retain ref while waiting
            if (leader != null)
                available.await();//因为没有执行线程初始化，所以等等什么时候有了自己被他人唤醒
            else {
                Thread thisThread = Thread.currentThread();
                leader = thisThread;
                try {
                    available.awaitNanos(delay); //各种condition的awaitNanos
                } finally {
                    if (leader == thisThread)
                        leader = null;
                }
            }
        }
    }
} finally {
    if (leader == null && queue[0] != null)
        available.signal();
    lock.unlock();
}

        }

ScheduledThreadPoolExecutor.DelayedWorkQueue#finishPoll

 private RunnableScheduledFuture finishPoll(RunnableScheduledFuture f) {
int s = --size;
RunnableScheduledFuture x = queue[s];  //重排序队列
queue[s] = null;
if (s != 0)
    siftDown(0, x);
setIndex(f, -1);
return f
}

ScheduledThreadPoolExecutor.ScheduledFutureTask#run

public void run() {
boolean periodic = isPeriodic();
if (!canRunInCurrentRunState(periodic))
    cancel(false);
else if (!periodic)
    ScheduledFutureTask.super.run();
else if (ScheduledFutureTask.super.runAndReset()) {//有period的要执行成功设置下次执行时间和增加额外任务
    setNextRunTime();
    reExecutePeriodic(outerTask);
}
}

异步结果源码分析

怎么拿到的异步任务结果？

FutureTask#awaitDone

  private int awaitDone(boolean timed, long nanos)
        throws InterruptedException {
final long deadline = timed ? System.nanoTime() + nanos : 0L;
WaitNode q = null;
boolean queued = false;
for (;;) {
    if (Thread.interrupted()) { //check线程中断
        removeWaiter(q);
        throw new InterruptedException();
    }

    int s = state;
    if (s > COMPLETING) {  //判断是否完成
        if (q != null)
            q.thread = null;
        return s;
    }
    else if (s == COMPLETING) // cannot time out yet
        Thread.yield();
    else if (q == null)
        q = new WaitNode();  //生成一个waint对象
    else if (!queued)
        queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
                                             q.next = waiters, q);//链表的对象下一个置成当前的waitNode
    else if (timed) {
        nanos = deadline - System.nanoTime();
        if (nanos <= 0L) {
            removeWaiter(q);
            return state;
        }
        LockSupport.parkNanos(this, nanos); //等待时间阻塞
    }
    else
        LockSupport.park(this); //一直阻塞
}
  }

什么时候回填的结果

FutureTask#run

 public void run() {
   if (state != NEW ||
    !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                 null, Thread.currentThread())) //如果状态不是new 或者 runner状态置不成功直接退出
    return;
try {
    Callable c = callable;
    if (c != null && state == NEW) {
        V result;
        boolean ran;
        try {
            result = c.call();//运行ok 的时候返回result
            ran = true;
        } catch (Throwable ex) {
            result = null;
            ran = false;
            setException(ex);
        }
        if (ran)//正常成功set result对象
            set(result);
    }
} finally {
    // runner must be non-null until state is settled to
    // prevent concurrent calls to run()
    runner = null;
    // state must be re-read after nulling runner to prevent
    // leaked interrupts
    int s = state;
    if (s >= INTERRUPTING)
        handlePossibleCancellationInterrupt(s);
}

 }

FutureTask#cancel

 public boolean cancel(boolean mayInterruptIfRunning) {
   if (!(state == NEW &&
      UNSAFE.compareAndSwapInt(this, stateOffset, NEW,
          mayInterruptIfRunning ? INTERRUPTING : CANCELLED))) //CAS 置成stateOffset 的中断或者取消
    return false;
try {    // in case call to interrupt throws exception
    if (mayInterruptIfRunning) {  //如果线程运行中，可能中断
        try {
            Thread t = runner;
            if (t != null)
                t.interrupt();
        } finally { // final state
            UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);
        }
    }
} finally {
    finishCompletion();
}
return true;

 }

原文地址

http://cbaj.gitee.io/blog/2020/08/22/java%E7%BA%BF%E7%A8%8B%E6%B1%A0%E6%BA%90%E7%A0%81%E8%A7%A3%E6%9E%90/#more