Java线程池：简明易懂的源码分析
Java线程池的一些基础知识，可以参考博客
本文将从源码角度分析线程池原理，加深对线程池原理的理解，简单背几个原理知识，其实很难得到面试官的青睐，了解源码知识，可以由内而外得征服面试官。
Java线程池状态转换
使用Integer类型(32bit) ctl中高3位记录Java线程池的状态,低29位记录线程数量
RUNNING:111、SHUTDOWN:000、STOP:001
TIDYING:010、TERMINATED:011.后面均又29位零
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
private static final int COUNT_BITS = Integer.SIZE - 3; // 29
//线程池最大容量2^29-1
private static final int CAPACITY = (1 << COUNT_BITS) - 1;
private static final int RUNNING = -1 << COUNT_BITS;
private static final int SHUTDOWN = 0 << COUNT_BITS;
private static final int STOP = 1 << COUNT_BITS;
private static final int TIDYING = 2 << COUNT_BITS;
private static final int TERMINATED = 3 << COUNT_BITS;
//取得线程池运行状态
private static int runStateOf(int c) { return c & ~CAPACITY; }
//取得线程池线程数量
private static int workerCountOf(int c) { return c & CAPACITY; }
private static int ctlOf(int rs, int wc) { return rs | wc; }
下面从线程池的submit方法作为出发点，从源码角度分析，线程池的实现原理
submit方法
submit方法将提交的任务task,包装成RunnableFuture对象,既实现了Runnable接口，又实现了Future接口。之后会调用execute方法
public Future<?> submit(Runnable task) {
if (task == null) throw new NullPointerException();
RunnableFuture<Void> ftask = newTaskFor(task, null);
execute(ftask);
return ftask;
}
execute方法
当提交一个新任务，线程池的处理流程如下：
-
判断线程池中核心线程数是否达到
corePoolSize,若否，则创建一个新核心线程执行任务 -
若核心线程数已达阈值,判断
workQueue是否已满，若未满，则将新任务添加进阻塞队列 -
若满，再判断，线程池中线程数是否达到阈值
maximumPoolSize,若否，则新建一个非核心线程执行任务。若达到阈值，则执行线程池饱和策略。
execute方法，各种参数
| 参数类型 | 用途 |
|---|---|
addWorker(firstTask, true) |
创建核心线程，执行提交的任务 |
addWorker(firstTask, false) |
创建非核心线程，执行提交的任务 |
addWorker(null, false) |
创建非核心线程，执行工作队列中任务 |
addWorker(null, true) |
创建核心线程，执行工作队列中任务 |
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
int c = ctl.get();
//线程数小于corePoolSize
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
//线程数大于等于corePoolSize,尝试添加进workQueue
if (isRunning(c) && workQueue.offer(command)) {
//再次检查
int recheck = ctl.get();
//如果状态不为Running，则从队列中移除任务
if (!isRunning(recheck) && remove(command))
reject(command);
//如果线程池数量为0，则添加一个非核心线程执行任务
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
//队列满，尝试新建非核心线程，执行任务
else if (!addWorker(command, false))
//否则执行拒绝策略
reject(command);
}
addWorker方法
首先通过自旋操作，将线程总数加1.之后用独占锁锁住，构建一个Worker对象，将Worker对象添加进workers(HashSet<Worker>),释放锁，启动线程。
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
//以下情况直接返回
//线程池处于STOP、TYDING、TERMINATED状态
//线程池处于SHUTDOWN状态，firstTask!=null
//线程池处于SHUTDOWN状态，firstTask==null, workQueue为空
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
//循环CAS操作，增加元素个数
for (;;) {
int wc = workerCountOf(c);
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
if (compareAndIncrementWorkerCount(c))
break retry;
//CAS失败，查看线程池状态是否改变，变化则跳到最外层循环
c = ctl.get();
if (runStateOf(c) != rs)
continue retry;
}
}
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;
//独占锁进行同步
mainLock.lock();
try {
int rs = runStateOf(ctl.get());
//重新check一下线程池状态
if (rs < SHUTDOWN ||
(rs == SHUTDOWN && firstTask == null)) {
if (t.isAlive())
throw new IllegalThreadStateException();
//添加任务
workers.add(w);
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
//添加成功，执行任务
if (workerAdded) {
t.start();
workerStarted = true;
}
}
} finally {
if (!workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
Worker类( 实现了 AQS)
Worker对象，包括thread和firstTask成员变量。thread成员变量的构造函数的参数是自身的Worker对象。
所以调用thread成员变量的run方法，其实就是调用本身Worker对象的run方法。
本身Worker对象的run方法，会调用runWorker方法，参数为本身Worker对象
private final class Worker extends AbstractQueuedSynchronizer
implements Runnable
{
final Thread thread;
Runnable firstTask;
volatile long completedTasks;
Worker(Runnable firstTask) {
this.firstTask = firstTask;
this.thread = getThreadFactory().newThread(this);
}
public void run() {
runWorker(this);
}
}
runWorker方法
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); //允许中断
boolean completedAbruptly = true;
try {
//循环过程，执行完第一个任务后，一直从队列中拿取任务
while (task != null || (task = getTask()) != null) {
w.lock();
//处于SHUTDOWN状态，并不会中断正在运行的任务
if ((runStateAtLeast(ctl.get(), STOP) ||
(Thread.interrupted() &&
runStateAtLeast(ctl.get(), STOP))) &&
!wt.isInterrupted())
wt.interrupt();
try {
//执行任务前干一些事
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 {
//执行任务后干一些事
afterExecute(task, thrown);
}
} finally {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
processWorkerExit(w, completedAbruptly);
}
}
getTask方法
从工作队列中，拿取任务，如果满足某几个条件(线程超时与否、线程池状态、工作队列是否为空)，直接返回null.
private Runnable getTask() {
boolean timedOut = false;
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
//线程池状态为SHUTDOWN，并且workQueue为空
//线程池处于STOP、TYDING、TERMINATED状态
//以上两种情况,返回null
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
//线程总数减一
decrementWorkerCount();
return null;
}
int wc = workerCountOf(c);
//allowCoreThreadTimeOut为true,运行核心线程受超时机制影响
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
//空闲线程超时，直接返回null
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
try {
//从队列中取任务
//如果空闲线程超时，workQueue.poll方法返回null
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}
参考文章
