一. java如何实现run()方法回调:
通常我们启动一个线程有两种方式,但是不管你通过start一个Thread还是通过实现Runnable的方式创建线程,都要实现它的run方法,run方法只是一个回调方法,那jvm什么时候去调用这个方法呢?
在java层面创建一个线程,本质上就是创建了一个native线程,本地线程对应到本地代码,java提供了一个线程统一函数, 由jvm调用java线程方法.启动一个线程实际上调用了start0()方法,而start0()实际调用了一个native方法.
public synchronized void start() {
/**
* This method is not invoked for the main method thread or "system"
* group threads created/set up by the VM. Any new functionality added
* to this method in the future may have to also be added to the VM.
*
* A zero status value corresponds to state "NEW".
*/
if (threadStatus != 0)
throw new IllegalThreadStateException();
/* Notify the group that this thread is about to be started
* so that it can be added to the group's list of threads
* and the group's unstarted count can be decremented. */
group.add(this);
boolean started = false;
try {
start0();
started = true;
} finally {
try {
if (!started) {
group.threadStartFailed(this);
}
} catch (Throwable ignore) {
/* do nothing. If start0 threw a Throwable then
it will be passed up the call stack */
}
}
}
private native void start0();
- start0()其实是由Thread这个类里面的静态的代码块registerNatives()注册进去的,几乎所有的native thread都是由它注册进去的.registerNatives在class load到jvm时候就被注册到本地方法.native修饰的是底层实现的方法,源码一般是由c或者c++实现.
public
class Thread implements Runnable {
/* Make sure registerNatives is the first thing <clinit> does. */
private static native void registerNatives();
static {
registerNatives();
}
}
- 本地方法registerNatives()实际是定义在Thread.c这个文件中
JNIEXPORT void JNICALL
Java_Java_lang_Thread_registerNatives (JNIEnv *env, jclass cls){
(*env)->RegisterNatives(env, cls, methods, ARRAY_LENGTH(methods));
}
static JNINativeMethod methods[] = {
{"start0", "()V",(void *)&JVM_StartThread},
{"stop0", "(" OBJ ")V", (void *)&JVM_StopThread},
{"isAlive","()Z",(void *)&JVM_IsThreadAlive},
{"suspend0","()V",(void *)&JVM_SuspendThread},
{"resume0","()V",(void *)&JVM_ResumeThread},
{"setPriority0","(I)V",(void *)&JVM_SetThreadPriority},
{"yield", "()V",(void *)&JVM_Yield},
{"sleep","(J)V",(void *)&JVM_Sleep},
{"currentThread","()" THD,(void *)&JVM_CurrentThread},
{"countStackFrames","()I",(void *)&JVM_CountStackFrames},
{"interrupt0","()V",(void *)&JVM_Interrupt},
{"isInterrupted","(Z)Z",(void *)&JVM_IsInterrupted},
{"holdsLock","(" OBJ ")Z",(void *)&JVM_HoldsLock},
{"getThreads","()[" THD,(void *)&JVM_GetAllThreads},
{"dumpThreads","([" THD ")[[" STE, (void *)&JVM_DumpThreads},
};
JNINativeMethod定义了一个静态数组,调用start0,需要调用JVM_StartThread方法,而JVM_StartThread方法是由jvm里面thread_entry调用,具体可以阅读jvm源码实现,这一部分还未深入研究.
二. 线程池复用原理:
2.1 AbstractExecutorService
- 本文从submit一个Runnable说起,我们可以看到提交一个任务实际上它内部还是调用了execute方法,只是帮我们用Future封装了一下线程执行结果.
/**
* @throws RejectedExecutionException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
public Future<?> submit(Runnable task) {
if (task == null) throw new NullPointerException();
RunnableFuture<Void> ftask = newTaskFor(task, null);
execute(ftask);
return ftask;
}
2.2 execute
- 如果运行的线程数小于核心线程池配置大小,开启一个线程
- 如果核心线程池都处于运行状态,任务丢到队列中去排队.实际上在这里,作者做了严谨的判断,采用双重检查方式,因为作者认为在上一次check后可能这个任务被kill掉或者被线程池shut down了,所以重新检查一下运行状态,必要时会滚排队,如果没有新线程stopped就返回新线程
- 如果队列已满拒绝
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
int c = ctl.get();
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
if (! isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
else if (!addWorker(command, false))
reject(command);
}
2.3 addWorker
只要new Worker(Runnable firstTask)，就会new一个线程，并且new一个thread的时候将这个内部类本身this传入进去当task,然后调用了start方法去开启这个线程
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()))
return false;
for (;;) {
int wc = workerCountOf(c);
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
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
}
}
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 {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
2.4 Worker
线程启动的时候我们将内部类worker对象传入进去了，内部类Worker是实现了runable接口的，重写了run()方法,线程获得cpu时间片以后, java虚拟机就会执行Worker中的run方法,run方法里面调用runWorker(this),这里的this就是指的就是当前的Worker对象
private final class Worker
extends AbstractQueuedSynchronizer
implements Runnable
{
/**
* This class will never be serialized, but we provide a
* serialVersionUID to suppress a javac warning.
*/
private static final long serialVersionUID = 6138294804551838833L;
/** Thread this worker is running in. Null if factory fails. */
final Thread thread;
/** Initial task to run. Possibly null. */
Runnable firstTask;
/** Per-thread task counter */
volatile long completedTasks;
/**
* Creates with given first task and thread from ThreadFactory.
* @param firstTask the first task (null if none)
*/
Worker(Runnable firstTask) {
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;
this.thread = getThreadFactory().newThread(this);
}
/** Delegates main run loop to outer runWorker */
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) {
}
}
}
}
2.5 runWorker
这里是一个while循环,当task不为空或者 getTask()不为空,核心逻辑在getTask()里面
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);
}
}
2.6 getTask
从队列中获取task,如果核心线程允许超时(默认false,不超时)或者工作线程数大于核心线程池,采取workQueue.poll方式从任务队列中poll数据,最大waitkeepAliveTime时间,没有返回null;否则采取take方式获取任务,take是阻塞式获取任务,这个就是核心线程池默认情况下,就是空闲也一直阻塞的原因,直到队列里有任务唤醒阻塞线程.
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;
}
}
}
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