java 中ThreadPoolExecutor原理分析
线程池简介
Java线程池是开发中常用的工具，当我们有异步、并行的任务要处理时，经常会用到线程池，或者在实现一个服务器时，也需要使用线程池来接收连接处理请求。
线程池使用
JDK中提供的线程池实现位于java.util.concurrent.ThreadPoolExecutor。在使用时，通常使用ExecutorService接口，它提供了submit,invokeAll,shutdown等通用的方法。
在线程池配置方面，Executors类中提供了一些静态方法能够提供一些常用场景的线程池，如newFixedThreadPool,newCachedThreadPool,newSingleThreadExecutor等，这些方法最终都是调用到了ThreadPoolExecutor的构造函数。
ThreadPoolExecutor的包含所有参数的构造函数是
private static final long serialVersionUID = 6138294804551838833L;
final Thread thread;
Runnable firstTask;
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);
}
// Lock methods
//
// The value 0 represents the unlocked state.
// The value 1 represents the locked state.
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(Worker)是Worker的轮询执行逻辑，不断地从工作队列中获取任务并执行它们。Worker每次执行任务前需要进行lock，防止在执行任务时被interrupt。
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
while (task != null || (task = getTask()) != null) {
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();
try {
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 {
afterExecute(task, thrown);
}
} finally {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
processWorkerExit(w, completedAbruptly);
}
}
ThreadPoolExecutor的submit方法中将Callable包装成FutureTask后交给execute方法。
FutureTask
FutureTask继承于Runnable和Future，FutureTask定义的几个状态为
NEW, 尚未执行
COMPLETING, 正在执行
NORMAL, 正常执行完成得到结果
EXCEPTIONAL, 执行抛出异常
CANCELLED， 执行被取消
INTERRUPTING，执行正在被中断
INTERRUPTED, 已经中断。
其中关键的get方法
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
s = awaitDone(false, 0L);
return report(s);
}
先获取当前状态，如果还未执行完成并且正常，则进入等待结果流程。在awaitDone不断循环获取当前状态，如果没有结果，则将自己通过CAS的方式添加到等待链表的头部，如果设置了超时，则LockSupport.parkNanos到指定的时间。
static final class WaitNode {
volatile Thread thread;
volatile WaitNode next;
WaitNode() { thread = Thread.currentThread(); }
}
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()) {
removeWaiter(q);
throw new InterruptedException();
}
int s = state;
if (s > COMPLETING) {
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();
else if (!queued)
queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
q.next = waiters, q);
else if (timed) {
nanos = deadline - System.nanoTime();
if (nanos <= 0L) {
removeWaiter(q);
return state;
}
LockSupport.parkNanos(this, nanos);
}
else
LockSupport.park(this);
}
}
FutureTask的run方法是执行任务并设置结果的位置,首先判断当前状态是否为NEW并且将当前线程设置为执行线程，然后调用Callable的call获取结果后设置结果修改FutureTask状态。
public void run() {
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))
return;
try {
Callable c = callable;
if (c != null && state == NEW) {
V result;
boolean ran;
try {
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)
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);
}
}
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