Unsafe和LockSupport
Java併發包下的類基本都是基於AQS (AbstractQueuedSynchronizer)框架實現的,關於AQS我在前面講解ReentrantLock源碼的文章中就已經有涉及了。
-->> 面試難點:深度解析ReentrantLock的實現原理
而AQS線程安全的實現,又是基於兩個很關鍵的類Unsafe和LockSupport,其中Unsafe主要直接提供CAS操作(關於cas,在文章 讀懂AtomicInteger源碼(多線程專題 )中講解過 ),LockSupport主要提供park/unpark操作,而park/unpark最終調用還是unsafe類,所以unsafe類纔是關鍵。
(如果不會下載JVM源碼可以後臺回覆 “ jdk ”,獲得下載壓縮包)
public static void park() {
UNSAFE.park(false, 0L);
}
public static void unpark(Thread thread) {
if (thread != null)
UNSAFE.unpark(thread);
}
Unsafe類實現:
//park
public native void park(boolean isAbsolute, long time);
//unpack
public native void unpark(Object var1);
由代碼可見,Unsafe類的park/unpark是native級別的實現。使用native關鍵字說明這個方法是原生函數,也就是這個方法是用C/C++語言實現的,並且被編譯成了DLL,由java去調用。
park函數是將當前調用線程阻塞,unpark函數則是將指定線程線程喚醒。
park和unpark作用:
park是等待一個許可,unpark是爲某線程提供一個許可。如果某線程A調用park,那麼除非另外一個線程調用unpark(A)給A一個許可,否則線程A將阻塞在park操作上。每次調用一次park,需要有一個unpark來解鎖。
並且unpark可以先於park調用,但是不管unpark先調用幾次,都只提供一個許可,不可疊加,只需要一次park來消費掉unpark帶來的許可,再次調用會阻塞。
Unsafe.park源碼
在Linux系統下,park和unpark是用的Posix線程庫pthread中的mutex(互斥量),condition(條件變量)來實現的。
簡單來說,mutex和condition保護了一個叫_counter的信號量。當park時,這個變量被設置爲0,當unpark時,這個變量被設置爲1。當_counter=0 時線程阻塞,當_counter>0直接設爲0並返回。
每個Java線程都有一個Parker實例,Parker類部分源碼如下:
class Parker : public os::PlatformParker {
private:
volatile int _counter ;
...
public:
void park(bool isAbsolute, jlong time);
void unpark();
...
}
class PlatformParker : public CHeapObj<mtInternal> {
protected:
pthread_mutex_t _mutex [1] ;
pthread_cond_t _cond [1] ;
...
}
由源碼可知Parker類繼承於PlatformParker,實際上時用Posix的mutex,condition來實現的。Parker類裏的_counter字段,就是用來記錄park和unpark是否需要阻塞的標識。
執行過程
具體的執行邏輯已經用註釋標記在代碼中,簡要來說,就是檢查_counter是不是大於0,如果是,則把_counter設置爲0,返回。如果等於零,繼續執行,阻塞等待。
void Parker::park(bool isAbsolute, jlong time) {
//判斷信號量counter是否大於0,如果大於設爲0返回
if (Atomic::xchg(0, &_counter) > 0) return;
//獲取當前線程
Thread* thread = Thread::current();
assert(thread->is_Java_thread(), "Must be JavaThread");
JavaThread *jt = (JavaThread *)thread;
//如果中途已經是interrupt了,那麼立刻返回,不阻塞
// Check interrupt before trying to wait
if (Thread::is_interrupted(thread, false)) {
return;
}
//記錄當前絕對時間戳
// Next, demultiplex/decode time arguments
timespec absTime;
//如果park的超時時間已到,則返回
if (time < 0 || (isAbsolute && time == 0) ) { // don't wait at all
return;
}
//更換時間戳
if (time > 0) {
unpackTime(&absTime, isAbsolute, time);
}
// Enter safepoint region
// Beware of deadlocks such as 6317397.
// The per-thread Parker:: mutex is a classic leaf-lock.
// In particular a thread must never block on the Threads_lock while
// holding the Parker:: mutex. If safepoints are pending both the
// the ThreadBlockInVM() CTOR and DTOR may grab Threads_lock.
//進入安全點,利用該thread構造一個ThreadBlockInVM
ThreadBlockInVM tbivm(jt);
// Don't wait if cannot get lock since interference arises from
// unblocking. Also. check interrupt before trying wait
if (Thread::is_interrupted(thread, false) || pthread_mutex_trylock(_mutex) != 0) {
return;
}
//記錄等待狀態
int status ;
//中途再次檢查許可,有則直接返回不等帶。
if (_counter > 0) { // no wait needed
_counter = 0;
status = pthread_mutex_unlock(_mutex);
assert (status == 0, "invariant") ;
// Paranoia to ensure our locked and lock-free paths interact
// correctly with each other and Java-level accesses.
OrderAccess::fence();
return;
}
OSThreadWaitState osts(thread->osthread(), false /* not Object.wait() */);
jt->set_suspend_equivalent();
// cleared by handle_special_suspend_equivalent_condition() or java_suspend_self()
assert(_cur_index == -1, "invariant");
if (time == 0) {
_cur_index = REL_INDEX; // arbitrary choice when not timed
//線程條件等待 線程等待信號觸發,如果沒有信號觸發,無限期等待下去。
status = pthread_cond_wait (&_cond[_cur_index], _mutex) ;
} else {
_cur_index = isAbsolute ? ABS_INDEX : REL_INDEX;
//線程等待一定的時間,如果超時或有信號觸發,線程喚醒
status = os::Linux::safe_cond_timedwait (&_cond[_cur_index], _mutex, &absTime) ;
if (status != 0 && WorkAroundNPTLTimedWaitHang) {
pthread_cond_destroy (&_cond[_cur_index]) ;
pthread_cond_init (&_cond[_cur_index], isAbsolute ? NULL : os::Linux::condAttr());
}
}
_cur_index = -1;
assert_status(status == 0 || status == EINTR ||
status == ETIME || status == ETIMEDOUT,
status, "cond_timedwait");
_counter = 0 ;
status = pthread_mutex_unlock(_mutex) ;
assert_status(status == 0, status, "invariant") ;
// Paranoia to ensure our locked and lock-free paths interact
// correctly with each other and Java-level accesses.
OrderAccess::fence();
// If externally suspended while waiting, re-suspend
if (jt->handle_special_suspend_equivalent_condition()) {
jt->java_suspend_self();
}
}
Unsafe.unpark源碼
unpark直接設置_counter爲1,再unlock mutex返回。如果_counter之前的值是0,則還要調用pthread_cond_signal喚醒在park中等待的線程。
源碼如下:
void Parker::unpark() {
//定義兩個變量,staus用於判斷是否獲取鎖
int s, status ;
//獲取鎖
status = pthread_mutex_lock(_mutex);
//判斷是否成功
assert (status == 0, "invariant") ;
//存儲原先變量_counter
s = _counter;
//把_counter設爲1
_counter = 1;
if (s < 1) {
// thread might be parked
if (_cur_index != -1) {
// thread is definitely parked
if (WorkAroundNPTLTimedWaitHang) {
status = pthread_cond_signal (&_cond[_cur_index]);
assert (status == 0, "invariant");
status = pthread_mutex_unlock(_mutex);
assert (status == 0, "invariant");
} else {
status = pthread_mutex_unlock(_mutex);
assert (status == 0, "invariant");
status = pthread_cond_signal (&_cond[_cur_index]);
assert (status == 0, "invariant");
}
} else {
//釋放鎖
pthread_mutex_unlock(_mutex);
assert (status == 0, "invariant") ;
}
} else {
//釋放鎖
pthread_mutex_unlock(_mutex);
assert (status == 0, "invariant") ;
}
}