Java Unsafe－park操作

park/unpark操作

这两个操作通常配合在一起使用，park操作用于阻塞当前线程，unpark用于使阻塞在park操作代码处的线程退出阻塞。

 

park操作

该方法是一个native方法：

public native void park(boolean isAbsolute, long time);

该方法实现unsafe.cpp在\hotspot\src\share\vm\prims目录下。

UNSAFE_ENTRY(void, Unsafe_Park(JNIEnv *env, jobject unsafe, jboolean isAbsolute, jlong time))   UnsafeWrapper("Unsafe_Park"); #ifndef USDT2   HS_DTRACE_PROBE3(hotspot, thread__park__begin, thread->parker(), (int) isAbsolute, time); #else /* USDT2 */    HOTSPOT_THREAD_PARK_BEGIN(                              (uintptr_t) thread->parker(), (int) isAbsolute, time); #endif /* USDT2 */   JavaThreadParkedState jtps(thread, time != 0);   thread->parker()->park(isAbsolute != 0, time); #ifndef USDT2   HS_DTRACE_PROBE1(hotspot, thread__park__end, thread->parker()); #else /* USDT2 */   HOTSPOT_THREAD_PARK_END(                           (uintptr_t) thread->parker()); #endif /* USDT2 */ UNSAFE_END

以上代码需要展开来才能看得懂，展开后代码如下：

如果定义了USDT2宏

extern "C" {                                                           void JNICALL Unsafe_Park(JNIEnv *env, jobject unsafe, jboolean isAbsolute, jlong time) {                                           JavaThread* thread=JavaThread::thread_from_jni_environment(env);     ThreadInVMfromNative __tiv(thread);       /* do nothing */       HandleMarkCleaner __hm(thread);                                        Thread* THREAD = thread;       /* begin of body */       /*nothing, for the present*/;       ;     JavaThreadParkedState jtps(thread, time != 0);     thread->parker()->park(isAbsolute != 0, time);     ;   } }

 

如果没有定义USDT2宏

这里不展开USDT2宏没有定义的那段代码

 

其中主要的代码在：

thread->parker()->park(isAbsolute != 0, time);

 

这块代码实现在os_solaris.cpp，在\hotspot\src\os\solaris\vm目录下。

void Parker::park(bool isAbsolute, jlong time) {     // Optional fast-path check:   // Return immediately if a permit is available.   if (_counter > 0) {       _counter = 0 ;       OrderAccess::fence();       return ;   }     // Optional fast-exit: Check interrupt before trying to wait   Thread* thread = Thread::current();   assert(thread->is_Java_thread(), "Must be JavaThread");   JavaThread *jt = (JavaThread *)thread;   if (Thread::is_interrupted(thread, false)) {     return;   }     // First, demultiplex/decode time arguments   timespec absTime;   if (time < 0 || (isAbsolute && time == 0) ) { // don't wait at all     return;   }   if (time > 0) {     // Warning: this code might be exposed to the old Solaris time     // round-down bugs.  Grep "roundingFix" for details.     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.   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) ||       os::Solaris::mutex_trylock(_mutex) != 0) {     return;   }     int status ;     if (_counter > 0)  { // no wait needed     _counter = 0;     status = os::Solaris::mutex_unlock(_mutex);     assert (status == 0, "invariant") ;     OrderAccess::fence();     return;   }   #ifdef ASSERT   // Don't catch signals while blocked; let the running threads have the signals.   // (This allows a debugger to break into the running thread.)   sigset_t oldsigs;   sigset_t* allowdebug_blocked = os::Solaris::allowdebug_blocked_signals();   thr_sigsetmask(SIG_BLOCK, allowdebug_blocked, &oldsigs); #endif     OSThreadWaitState osts(thread->osthread(), false /* not Object.wait() */);   jt->set_suspend_equivalent();   // cleared by handle_special_suspend_equivalent_condition() or java_suspend_self()     // Do this the hard way by blocking ...   // See http://monaco.sfbay/detail.jsf?cr=5094058.   // TODO-FIXME: for Solaris SPARC set fprs.FEF=0 prior to parking.   // Only for SPARC >= V8PlusA #if defined(__sparc) && defined(COMPILER2)   if (ClearFPUAtPark) { _mark_fpu_nosave() ; } #endif     if (time == 0) {     status = os::Solaris::cond_wait (_cond, _mutex) ;   } else {     status = os::Solaris::cond_timedwait (_cond, _mutex, &absTime);   }   // Note that an untimed cond_wait() can sometimes return ETIME on older   // versions of the Solaris.   assert_status(status == 0 || status == EINTR ||                 status == ETIME || status == ETIMEDOUT,                 status, "cond_timedwait");   #ifdef ASSERT   thr_sigsetmask(SIG_SETMASK, &oldsigs, NULL); #endif   _counter = 0 ;   status = os::Solaris::mutex_unlock(_mutex);   assert_status(status == 0, status, "mutex_unlock") ;     // If externally suspended while waiting, re-suspend   if (jt->handle_special_suspend_equivalent_condition()) {     jt->java_suspend_self();   }   OrderAccess::fence(); }