Java线程池实现原理

前言

很久以前看过JDK线程池实现源码,但最近的面试被问到线程实现原理的时候还说不清楚,甚至和连接池搞混。所以有必要重温一遍并记录下来以便日后翻阅。

线程池类图

线程池

类层次结构

Executor接口

Executor接口只有一个接受Runnable参数的execute方法,用来执行任务。

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package java.util.concurrent;
public interface Executor {
/**
* 接受执行一个任务
*/
void execute(Runnable command);
}

###ExecutorService接口
ExecutorService接口主要定义线程池内部管理的接口

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package java.util.concurrent;
public interface ExecutorService extends Executor {
/**
* 不会立即终止线程池,而是要等所有任务缓存队列中的任务都执行完后才终止,但再也不会接受新的任务
*/
void shutdown();

/**
* 立即终止线程池,并尝试打断正在执行的任务,并且清空任务缓存队列
* @return 待执行任务列表
*/
List<Runnable> shutdownNow();

boolean isShutdown();

boolean isTerminated();

boolean awaitTermination(long timeout, TimeUnit unit)
throws InterruptedException;

/**
* 提交任务
* @return 待完成任务future
*/
<T> Future<T> submit(Callable<T> task);

<T> Future<T> submit(Runnable task, T result);

Future<?> submit(Runnable task);

<T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
throws InterruptedException;

<T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit)
throws InterruptedException;

<T> T invokeAny(Collection<? extends Callable<T>> tasks)
throws InterruptedException, ExecutionException;

<T> T invokeAny(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}

AbstractExecutorService抽象类

AbstractExecutorService抽象类实现了大部分方法,但大部分方法都依赖Executor接口声明的execute方法,这里并没有实现该方法,而是把这个方法交由子类(java.util.concurrent.ThreadPoolExecutor)实现。

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package java.util.concurrent;
import java.util.*;
public abstract class AbstractExecutorService implements ExecutorService {

protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
return new FutureTask<T>(runnable, value);
}

protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
return new FutureTask<T>(callable);
}

public Future<?> submit(Runnable task) {
if (task == null) throw new NullPointerException(); // 任务为空,抛出空指针异常
RunnableFuture<Void> ftask = newTaskFor(task, null);
execute(ftask);
return ftask;
}

public <T> Future<T> submit(Runnable task, T result) {
if (task == null) throw new NullPointerException();// 任务为空,抛出空指针异常
RunnableFuture<T> ftask = newTaskFor(task, result);
execute(ftask);
return ftask;
}

public <T> Future<T> submit(Callable<T> task) {
if (task == null) throw new NullPointerException();// 任务为空,抛出空指针异常
RunnableFuture<T> ftask = newTaskFor(task);
execute(ftask);
return ftask;
}
// 其余方法略

}

#线程池原理

线程池的状态

  1. RUNNING:正在处理任务和接受队列中的任务。
  2. SHUTDOWN:不再接受新的任务,但是会继续处理完队列中的任务。
  3. STOP:不再接受新任务,也不继续处理队列中的任务,且会中止正在处理的任务。
  4. TIDYING:所有任务都处理结束,目前worker数为0,当线程池进入这个状态的时候,会调用terminated()方法。
  5. TERMINATED:线程池已经全部结束,并且terminated()方法执行完成。
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// 线程池状态计数器
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
private static final int COUNT_BITS = Integer.SIZE - 3;
private static final int CAPACITY = (1 << COUNT_BITS) - 1; // 线程池最大线程数不能超过(2^29)-1个线程,大约是500万个线程

// runState is stored in the high-order bits
// 运行状态
private static final int RUNNING = -1 << COUNT_BITS;
// 关闭状态
private static final int SHUTDOWN = 0 << COUNT_BITS;
// 停止状态
private static final int STOP = 1 << COUNT_BITS;
// 整理状态
private static final int TIDYING = 2 << COUNT_BITS;
// 终结状态
private static final int TERMINATED = 3 << COUNT_BITS;
// 计算线程池当前状态
private static int runStateOf(int c) { return c & ~CAPACITY; }
// 计算当前状态是否小于某个状态
private static boolean runStateLessThan(int c, int s) {
return c < s;
}
// 计算当前状态是否大于或等于某个状态
private static boolean runStateAtLeast(int c, int s) {
return c >= s;
}
// 计算当前状态是否RUNNING
private static boolean isRunning(int c) {
return c < SHUTDOWN;
}

##线程任务执行

线程任务执行涉及属性

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private final BlockingQueue<Runnable> workQueue;//任务队列
private final ReentrantLock mainLock = new ReentrantLock();//线程主锁
private final HashSet<Worker> workers = new HashSet<Worker>();
private final Condition termination = mainLock.newCondition();
private int largestPoolSize;
private long completedTaskCount;
private volatile ThreadFactory threadFactory;
private volatile RejectedExecutionHandler handler;//任务拒绝策略处理器
private volatile long keepAliveTime;//临时线程等待超时时间
private volatile boolean allowCoreThreadTimeOut;
private volatile int corePoolSize;//核心线程数
private volatile int maximumPoolSize; //最大线程数
  • workQueue
    任务队列。用来存放任务,在任务量比较大,核心线程在执行任务的时候,会把新提交的任务放到任务队列中,如果任务队列再满了的话,会尝试创建临时线程来处理任务,如果临时线程全部满了的话,就会启动拒绝策略。
  • mainLock
    线程池主锁。对线程池本身的一些状态操作的时候,必须使用这个锁。
  • handler
    拒绝策略处理器。这拒绝策略处理器可以由调用着传入。已实现的拒绝策略有java.util.concurrent.ThreadPoolExecutor.AbortPolicy、java.util.concurrent.ThreadPoolExecutor.DiscardPolicy、java.util.concurrent.ThreadPoolExecutor.CallerRunsPolicy和java.util.concurrent.ThreadPoolExecutor.DiscardOldestPolicy
  • keepAliveTime
    临时线程等待超时时间。这个属性表示创建的临时线程在空闲的时候最长的等待时间,如果超过这个时间,线程就会结束掉。
  • corePoolSize
    核心线程数。表示的是整个线程池中最少存活的线程数。实际上,在任务数比较多的情况下,线程池会创建一些新的临时线程来执行任务,等到任务数降下来之后,临时线程就会逐渐减少,总的线程数会回落到核心线程数。值得注意的是,当线程池任务队列中没有任务的时候,所有线程都会处于等待状态,但是临时线程等待会超时后就会结束,而核心线程是不允许等待超时的,因此核心线程不会减少,这也保证了线程池的线程数量不会少于核心线程数。

执行execute

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public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
int c = ctl.get(); // 当前线程总数
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true)) // 低于核心线程数,创建核心线程来执行任务
return;
c = ctl.get();
}
// 线程池运行中,尝试加入队列
if (isRunning(c) && workQueue.offer(command)) {
// 重新检查线程池状态
int recheck = ctl.get();
// 如果线程池已经不接收新的任务,则移除这个任务,并转入拒绝策略。
if (! isRunning(recheck) && remove(command))
reject(command);
// 状态没变,重新检查当前核心线程数,为0则调用addWorker(null, false)去队列中取任务并执行;不为0不做任何操作,等待线程执行完当前任务后自动去队列中获取新的任务并执行。
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
// running但队列已满,则进入拒绝策略
else if (!addWorker(command, false))
reject(command);
}

addWorker方法

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/**
* 尝试增加工作线程
* @param firstTask 新建线程后执行的第一个任务
* @param core 是否创建核心线程
*/
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
// 检查线程池状态
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 (;;) {
int wc = workerCountOf(c);
// 非核心线程数超过最大限制或核心线程数超过最大限制
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
// 线程计数器加1退出循环
if (compareAndIncrementWorkerCount(c))
break retry;
c = ctl.get(); // 重新读取线程池状态
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 {
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());

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();
}
if (workerAdded) {
t.start();
workerStarted = true;
}
}
} finally {
// 线程没运行则回滚
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}

Worker类

每一个Worker类对象代表一个线程

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private final class Worker  extends AbstractQueuedSynchronizer implements Runnable   {
private static final long serialVersionUID = 6138294804551838833L;
final Thread thread;
/** 线程创建后执行的第一个任务。允许为空 */
Runnable firstTask;
/** 完成任务计数器 */
volatile long completedTasks;
Worker(Runnable firstTask) {
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;
this.thread = getThreadFactory().newThread(this);
}
public void run() {
runWorker(this);
}

protected boolean isHeldExclusively() {
return getState() != 0;
}

protected boolean tryAcquire(int unused) {
if (compareAndSetState(0, 1)) {
setExclusiveOwnerThread(Thread.currentThread());
return true;
}
return false;
}

protected boolean tryRelease(int unused) {
setExclusiveOwnerThread(null);
setState(0);
return true;
}

public void lock() { acquire(1); }
public boolean tryLock() { return tryAcquire(1); }
public void unlock() { release(1); }
public boolean isLocked() { return isHeldExclusively(); }

void interruptIfStarted() {
Thread t;
if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
try {
t.interrupt();
} catch (SecurityException ignore) {
}
}
}
}

###runWorker方法

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final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
// 获取待执行的任务
while (task != null || (task = getTask()) != null) {
// 获取当前线程锁
w.lock();
// 检查当前线程状态是否允许运行
if ((runStateAtLeast(ctl.get(), STOP) ||
(Thread.interrupted() &&
runStateAtLeast(ctl.get(), STOP))) &&
!wt.isInterrupted())
wt.interrupt();
try {
// 执行前处理。默认不处理,留给扩展实现
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 {
// 执行后处理。默认不处理,留给扩展实现
afterExecute(task, thrown);
}
} finally {
task = null;
// 计数器++
w.completedTasks++;
// 释放锁
w.unlock();
}
}
completedAbruptly = false;
} finally {
processWorkerExit(w, completedAbruptly);
}
}

getTask方法

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/**
* 取新的任务执行,如果没有新的任务,线程就会阻塞在getTask()方法内
*/
private Runnable getTask() {
boolean timedOut = false; // Did the last poll() time out?

for (;;) {
int c = ctl.get();
int rs = runStateOf(c);

// Check if queue empty only if necessary.
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
decrementWorkerCount();
return null;
}
int wc = workerCountOf(c);
// Are workers subject to culling?
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}

try {
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}

processWorkerExit方法

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private void processWorkerExit(Worker w, boolean completedAbruptly) {
if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
decrementWorkerCount();//减少工作线程数

final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
completedTaskCount += w.completedTasks;
workers.remove(w);
} finally {
mainLock.unlock();
}

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);
}
}

总结

  1. 线程池(Thread Pool)是限制程序同一时刻运行线程数量的一种实现。线程池最多不会超过maximumPoolSize个线程在运行
  2. 提交给线程池的任务也是一个线程,有线程池工作线程负责调度执行。当线程池的活跃线程未超过corePoolSize个时,线程池创建核心线程调度执行任务;当线程的活跃线程数超过corePoolSize,且小于maximumPoolSize,线程池创建临时线程调度执行;当线程池活跃线程等于maximumPoolSize时,提交任务到任务队列;任务队列已满,则转入拒绝策略
  3. 线程池中每个活跃线程在调度执行完一个任务后,会从任务队列阻塞获取新的任务调度执行。非核心工作线程阻塞到超时,核心工作线程默认阻塞到队列有任务
  4. 临时工作线程(非核心)从任务队列获取新任务超时后进入回收逻辑
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