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Java同步锁Synchronized底层源码和原理剖析(推荐)

作者:赵广陆

这篇文章主要介绍了Java同步锁Synchronized底层源码和原理剖析,释放后,进入减减操作、直到为0然后唤醒队列,让他们去争夺锁,循环前面步骤,对Java同步锁Synchronized相关知识感兴趣的朋友一起看看吧

1 synchronized场景回顾

目标:
synchronized回顾(锁分类–>多线程)
概念
synchronized:是Java中的关键字,是一种同步锁。
Java中锁分为以下几种:
乐观锁、悲观锁(syn)
独享锁(syn)、共享锁
公平锁、非公平锁(syn)
互斥锁(syn)、读写锁
可重入锁(syn)
分段锁
synchronized JDK1.6锁升级(无锁 -> 偏向锁 (非锁)-> 轻量级锁 -> 重量级锁(1.6前都是)【面试常问】
tips:
为什么用到锁?大家肯定会想到多线程(并发)
接下来,我们一起简单回顾下多线程特性
多线程特性回顾(面试常问)
原子性:指一个操作或者多个操作,要么全部执行并且执行的过程不会被任何因素打断,要么就都不执

可见性:是指多个线程访问一个资源时,该资源的状态、值信息等对于其他线程都是可见的。
有序性:指程序中代码的执行顺序 (编译器会重排)

原子性实现回顾
保证了原子性?

com.syn.com.syn.th.SyncAtomicity

package com.syn.com.syn.th;
import java.util.concurrent.TimeUnit;
/*
  目标:测试原子性问题
  1、调用正常(不加锁)方法;两个线程都可以正常执行
  2、调用加锁方法,只能有一个线程正常执行,其他线程排队等候
*/
public class SyncAtomicity {
  public static void main(String[] args) throws InterruptedException {
    SyncAtomicity syncAtomicity = new SyncAtomicity();
    //synchronized修饰实例方法
    //new Thread(()->syncAtomicity.testSYNC()).start();
    //new Thread(()->syncAtomicity.testSYNC()).start();
    //synchronized修饰静态方法
    new Thread(() -> SyncAtomicity.testSYNCForStatic()).start();
    new Thread(() -> SyncAtomicity.testSYNCForStatic()).start();
    //正常方法
    //new Thread(() -> syncAtomicity.test()).start();
    //new Thread(() -> syncAtomicity.test()).start();
 }
  //加锁方法
  public synchronized void testSYNC() {
    System.out.println("进入testSYNC方法>>>>>>>>>>>>>>>>>>>>>");
    try {
      //模拟方法体尚未执行完毕
      TimeUnit.HOURS.sleep(1);
   } catch (InterruptedException e) {
      e.printStackTrace();
   }
 }
  //加锁方法
  public synchronized static void testSYNCForStatic() {
    System.out.println("进入testSYNC方法>>>>>>>>>>>>>>>>>>>>>");
    try {
      //模拟方法体尚未执行完毕
      TimeUnit.HOURS.sleep(1);
   } catch (InterruptedException e) {
      e.printStackTrace();
   }
 }
  //正常方法
  public void test() {
    System.out.println("进入test方法>>>>>>>>>>>>>>>>>>>>>");
    try {
      //模拟方法体尚未执行完毕
      TimeUnit.HOURS.sleep(1);
   } catch (InterruptedException e) {
      e.printStackTrace();
   }
 }
}

总结
我们发现在同一时刻确实只有一个线程进入,保证了原子性
这是什么原理呢?

2 反汇编寻找锁实现原理

目标 通过javap反汇编看一下synchronized到底是怎么加锁的

com.syn.BTest

public class BTest {
  private static Object object = new Object();
  public synchronized void testMethod() {
    System.out.println("Hello World -synchronized method ");
 }
  public static void main(String[] args) {
    synchronized (object) {
      System.out.println("Hello World -synchronized block ");
     
   }
 }
}

反汇编后,我们将看到什么?

JDK自带的一个工具: javap ,对字节码进行反汇编:

//com.syn.BTest
javap -v -c BTest.class

反汇编后

解释
被synchronized修饰的代码块,多了两个指令
monitorenter、monitorexit
即JVM使用monitorenter和monitorexit两个指令实现同步

解释
被synchronized修饰的方法;增加 了ACC_SYNCHRONIZED 修饰。会隐式调用monitorenter和
monitorexit。
monitorenter原理(重要)
monitorenter首先我们来看一下JVM规范中对于monitorenter的描述

https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-6.html#jvms-6.5.monitorenter

翻译如下:
每一个对象都会和一个监视器monitor关联。
监视器被占用时会被锁住,其他线程无法来获取该monitor。
当JVM执行某个线程的某个方法内部的monitorenter时,它会尝试去获取当前对象对应的monitor的所有权。其过程如下:

monitor,不再拥有monitor的所有权,此时其他被这个monitor阻塞的线程可以尝试去获取这个

monitor的所有权

monitorexit释放锁。

monitorexit插入在方法结束处和异常处,JVM保证每个monitorenter必须有对应的monitorexit。

tips(重要)

上面文字太多,杜绝去念!!!!!!
用图说话!!!! !!!!!!!!

类:com.syn.BTest

public static void main(String[] args) {
    synchronized (object) {
      System.out.println("Hello World -synchronized block ");
   }
 }

总结:
通过上面的流程我们发现
1、synchronized是靠Monitor关联拿到锁的
2、如果竞争的时候拿不到锁,线程就去竞争队列
3、如果拿到锁了,第二次拿,它又拿到锁,其他线程进入阻塞队列
4、如果拿到锁的线程调用了wait方法,其他线程进入等待队列
5、释放锁,需要将计数器减减操作
6、出现异常,也释放锁。

3 synchronized虚拟机源码

synchronized是Java中的关键字,无法通过JDK源码查看它的实现,它是由JVM提供支持的,所以如果想要了解具体的实现需要查看JVM源码

目标:JVM虚拟机源码下载

http://hg.openjdk.java.net/jdk8/jdk8/hotspot/
或者
http://hg.openjdk.java.net/jdk8/jdk8/hotspot/archive/tip.zip

解压查看即可,无需环境搭建

3.1 HotSpot源码Monitor生成

目标: 通过JVM虚拟机源码分析synchronized监视器Monitor是怎么生成的
tips:
c++源码只看重点、弄懂原理
c++重要吗?不重要
但是面试时很重要,面试过去了就不重要!!!!!!!!!!!!
学别人不会的东西你才有价值!!!!你会、大家都会,没啥意思!!
在HotSpot虚拟机中,monitor监视器是由ObjectMonitor实现的。
构造器代码src/share/vm/runtime/objectMonitor.hpp
hpp可以include包含cpp的东西,两者都是c++的代码

//构造器
ObjectMonitor() {
_header = NULL;
_count = 0;
_waiters = 0,
_recursions = 0; // 递归:线程的重入次数,典型的System.out.println
_object = NULL; // 对应synchronized (object)对应里面的object
_owner = NULL; // 标识拥有该monitor的线程
_WaitSet = NULL; // 因为调用object.wait()方法而被阻塞的线程会被放在该队列中
_WaitSetLock = 0 ;
_Responsible = NULL;
_succ = NULL;
_cxq = NULL; // 竞争队列,所有请求锁的线程首先会被放在这个队列中
FreeNext = NULL;
_EntryList = NULL; // 阻塞;第二轮竞争锁仍然没有抢到的线程(偏向/可重入)
_SpinFreq = 0;
_SpinClock = 0;
OwnerIsThread = 0;
}

结论:正好印证了上面的流程图

3.2 HotSpot源码之Monitor竞争

目标: 通过JVM虚拟机源码分析synchronized多个线程抢夺锁,拿到锁之后要干什么?

monitorenter指令执行:
JVM源码:src/share/vm/interpreter/interpreterRuntime.cpp
JVM函数: InterpreterRuntime::monitorenter函数

//锁竞争InterpreterRuntime::monitorenter
IRT_ENTRY_NO_ASYNC(void, InterpreterRuntime::monitorenter(JavaThread* thread,
BasicObjectLock* elem))
#ifdef ASSERT
 thread->last_frame().interpreter_frame_verify_monitor(elem);
#endif
 if (PrintBiasedLockingStatistics) {
  Atomic::inc(BiasedLocking::slow_path_entry_count_addr());
}
 Handle h_obj(thread, elem->obj());
 assert(Universe::heap()->is_in_reserved_or_null(h_obj()),
    "must be NULL or an object");
 //偏向锁(非锁:jdk14废弃)
 if (UseBiasedLocking) {
  // Retry fast entry if bias is revoked to avoid unnecessary inflation
  ObjectSynchronizer::fast_enter(h_obj, elem->lock(), true, CHECK);
} else {
 // 重量级锁,最终调用了objectMonitor.cpp中的ObjectMonitor::enter
  ObjectSynchronizer::slow_enter(h_obj, elem->lock(), CHECK);
 ...略

最终调用objectMonitor.cpp文件中的 ObjectMonitor::enter

src/share/vm/runtime/objectMonitor.cpp

//重量级锁入口
void ATTR ObjectMonitor::enter(TRAPS) {
 Thread * const Self = THREAD ;
 void * cur ;
// 1、通过CAS(原子操作)操作尝试把monitor的_owner字段设置为当前线程(开始竞争)
 cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
 if (cur == NULL) {
  // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
  assert (_recursions == 0  , "invariant") ;
  assert (_owner    == Self, "invariant") ;
  // CONSIDER: set or assert OwnerIsThread == 1
  return ;
}
// 2、拿到锁;计数+1,recursions++
 if (cur == Self) {
  _recursions ++ ;//第一次进入(计数+1)
  return ;
}
 if (Self->is_lock_owned ((address)cur)) {
  assert (_recursions == 0, "internal state error");
  _recursions = 1 ;
  _owner = Self ;
  OwnerIsThread = 1 ;
  return ;
}
 assert (Self->_Stalled == 0, "invariant") ;
 Self->_Stalled = intptr_t(this) ;
 if (Knob_SpinEarly && TrySpin (Self) > 0) {
  assert (_owner == Self   , "invariant") ;
  assert (_recursions == 0  , "invariant") ;
  assert (((oop)(object()))->mark() == markOopDesc::encode(this),
"invariant") ;
  Self->_Stalled = 0 ;
  return ;
}
 assert (_owner != Self     , "invariant") ;
 assert (_succ  != Self     , "invariant") ;
 assert (Self->is_Java_thread() , "invariant") ;
 JavaThread * jt = (JavaThread *) Self ;
 assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
 assert (jt->thread_state() != _thread_blocked  , "invariant") ;
 assert (this->object() != NULL , "invariant") ;
 assert (_count >= 0, "invariant") ;
 Atomic::inc_ptr(&_count);
 EventJavaMonitorEnter event;
{
  JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
  DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
  if (JvmtiExport::should_post_monitor_contended_enter()) {
   JvmtiExport::post_monitor_contended_enter(jt, this);
 }
  OSThreadContendState osts(Self->osthread());
  ThreadBlockInVM tbivm(jt);
  Self->set_current_pending_monitor(this);
  for (;;) {
   jt->set_suspend_equivalent();
   // cleared by handle_special_suspend_equivalent_condition()
   // or java_suspend_self()
// 3、获取锁失败的线程,则等待!!!!!!!!!!!!!!!!!!!!!!!!
   EnterI (THREAD) ;
   if (!ExitSuspendEquivalent(jt)) break ;
     _recursions = 0 ;
   _succ = NULL ;
   exit (false, Self) ;
   jt->java_suspend_self();
 }
  Self->set_current_pending_monitor(NULL);
}

总结

3.3 HotSpot源码之Monitor等待

目标: 通过JVM虚拟机源码分析synchronized拿不到锁的线程他们都去干什么了?

还是 /objectMonitor.cpp
还是EnterI函数
路径:src/share/vm/runtime/objectMonitor.cpp的

//拿不到锁的线程他们都去干什么了??
void ATTR ObjectMonitor::EnterI (TRAPS) {
  Thread * Self = THREAD ;
  assert (Self->is_Java_thread(), "invariant") ;
  assert (((JavaThread *) Self)->thread_state() == _thread_blocked  ,
"invariant") ;
 // 没拿到锁,还是要尝试TryLock一次
  if (TryLock (Self) > 0) {
   //拿到锁执行,在返回
    assert (_succ != Self       , "invariant") ;
    assert (_owner == Self       , "invariant") ;
    assert (_Responsible != Self    , "invariant") ;
    return ;//成功获取
 }
  DeferredInitialize () ;
 //没拿到锁,开始TrySpin自旋(CAS,while循环)
  if (TrySpin (Self) > 0) {
    assert (_owner == Self    , "invariant") ;
    assert (_succ != Self     , "invariant") ;
    assert (_Responsible != Self , "invariant") ;
    return ;
 }
  assert (_succ  != Self      , "invariant") ;
  assert (_owner != Self      , "invariant") ;
  assert (_Responsible != Self   , "invariant") ;
// 实在拿不到锁;当前线程被封装成ObjectWaiter对象node,状态设置成ObjectWaiter::TS_CXQ
//即将放入竞争队列
  ObjectWaiter node(Self) ;
  Self->_ParkEvent->reset() ;
  node._prev  = (ObjectWaiter *) 0xBAD ;
  node.TState  = ObjectWaiter::TS_CXQ ;
  ObjectWaiter * nxt ;
  for (;;) {
   node._next = nxt = _cxq ;
    //使用内核函数cmpxchg_ptr 将没有拿到锁线程(node)放到竞争队列
    if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ;
    if (TryLock (Self) > 0) {
      assert (_succ != Self     , "invariant") ;
      assert (_owner == Self    , "invariant") ;
      assert (_Responsible != Self , "invariant") ;
      return ;
   }
 }
  if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
    Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
 }
  TEVENT (Inflated enter - Contention) ;
  int nWakeups = 0 ;
  int RecheckInterval = 1 ;
//将竞争队列线程挂起
  for (;;) {
// 线程在被挂起前做一下挣扎,看能不能获取到锁
    if (TryLock (Self) > 0) break ;
    assert (_owner != Self, "invariant") ;
    if ((SyncFlags & 2) && _Responsible == NULL) {
     Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
   }
    // park self
    if (_Responsible == Self || (SyncFlags & 1)) {
      TEVENT (Inflated enter - park TIMED) ;
      Self->_ParkEvent->park ((jlong) RecheckInterval) ;
      // Increase the RecheckInterval, but clamp the value.
      RecheckInterval *= 8 ;
      if (RecheckInterval > 1000) RecheckInterval = 1000 ;
   } else {
      TEVENT (Inflated enter - park UNTIMED) ;
     // 挂起!!!!!!::通过park将当前线程挂起(不被执行了),等待被唤
醒!!!!!!!!!!!
      Self->_ParkEvent->park() ;
   }
  //当该线程被唤醒时,执行TryLock----->ObjectMonitor::TryLoc 
!!!!!!!!!!!!!!!!!!!!!
    if (TryLock(Self) > 0) break ;

当该线程被唤醒时,会从挂起的点继续执行,通过 ObjectMonitor::TryLock 尝试获取锁
总结
4. 竞争失败的线程被封装成ObjectWaiter对象node,状态设置成ObjectWaiter::TS_CXQ(竞争队
列)
5. 在for循环中,通过CAS把node节点push到_cxq列表中,(竞争队列)
6. node节点push到_cxq列表之后,通过自旋尝试获取锁,如果还是没有获取到锁,则通过park将当
前线程挂起,等待被唤醒。
7. 当该线程被唤醒时,会从挂起的点继续执行,通过 ObjectMonitor::TryLock 尝试获取锁。

一句话总结:没拿到,尝试拿一次、在自旋去拿、实在拿不到就去竞争队列、等待唤醒

3.4 HotSpot源码之Monitor释放

目标: 通过JVM虚拟机源码分析synchronized拿到锁的线程最后是怎么释放锁的?

执行monitorexit指令
还是 /objectMonitor.cpp
里面的exit函数
Osrc/share/vm/runtime/objectMonitor.cpp

//线程释放调用exit方法
void ATTR ObjectMonitor::exit(bool not_suspended, TRAPS) {
 Thread * Self = THREAD ;
 if (THREAD != _owner) {
  if (THREAD->is_lock_owned((address) _owner)) {
   assert (_recursions == 0, "invariant") ;
   _owner = THREAD ;
   _recursions = 0 ;
   OwnerIsThread = 1 ;
  } else {
   TEVENT (Exit - Throw IMSX) ;
   assert(false, "Non-balanced monitor enter/exit!");
   if (false) {
     THROW(vmSymbols::java_lang_IllegalMonitorStateException());
   }
   return;
  }
 }
//_recursions计数不等于0;说明还没出代码块;进入减减操作,
 if (_recursions != 0) {
  _recursions--;     // this is simple recursive enter
  TEVENT (Inflated exit - recursive) ;
  return ;
 }
 if ((SyncFlags & 4) == 0) {
    _Responsible = NULL ;
 }
#if INCLUDE_TRACE
 if (not_suspended && Tracing::is_event_enabled(TraceJavaMonitorEnterEvent)) {
  _previous_owner_tid = SharedRuntime::get_java_tid(Self);
 }
#endif
 for (;;) {
   assert (THREAD == _owner, "invariant") ;
   if (Knob_ExitPolicy == 0) {
    OrderAccess::release_store_ptr (&_owner, NULL) ;  // drop the lock
    OrderAccess::storeload() ;             // See if we need to
wake a successor
    if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
      TEVENT (Inflated exit - simple egress) ;
      return ;
    }
    TEVENT (Inflated exit - complex egress) ;
    if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
      return ;
    }
    TEVENT (Exit - Reacquired) ;
  } else {
    if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
      OrderAccess::release_store_ptr (&_owner, NULL) ;  // drop the lock
      OrderAccess::storeload() ;
      // Ratify the previously observed values.
      if (_cxq == NULL || _succ != NULL) {
        TEVENT (Inflated exit - simple egress) ;
        return ;
     }
      if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
       TEVENT (Inflated exit - reacquired succeeded) ;
       return ;
     }
      TEVENT (Inflated exit - reacquired failed) ;
    } else {
      TEVENT (Inflated exit - complex egress) ;
    }
  }
   guarantee (_owner == THREAD, "invariant") ;
// 计数为0;开始唤醒cq竞争队列、enteryList阻塞队列
   ObjectWaiter * w = NULL ;//w就是被唤醒的线程
   int QMode = Knob_QMode ;
// qmode = 2:直接绕过EntryList阻塞队列,从cxq(竞争)队列中获取线程用于竞争锁
   if (QMode == 2 && _cxq != NULL) {
     w = _cxq ;
     assert (w != NULL, "invariant") ;
     assert (w->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
     ExitEpilog (Self, w) ;
     return ;
  }
// qmode =3:cxq(竞争)队列插入EntryList(阻塞)尾部;
   if (QMode == 3 && _cxq != NULL) {
     w = _cxq ;
     for (;;) {
      assert (w != NULL, "Invariant") ;
      ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL,
&_cxq, w) ;
      if (u == w) break ;
      w = u ;
    }
     assert (w != NULL       , "invariant") ;
     ObjectWaiter * q = NULL ;
     ObjectWaiter * p ;
     for (p = w ; p != NULL ; p = p->_next) {
       guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
       p->TState = ObjectWaiter::TS_ENTER ;
       p->_prev = q ;
       q = p ;
    }
     // Append the RATs to the EntryList
     // TODO: organize EntryList as a CDLL so we can locate the tail in
constant-time.
     ObjectWaiter * Tail ;
     for (Tail = _EntryList ; Tail != NULL && Tail->_next != NULL ; Tail =
Tail->_next) ;
     if (Tail == NULL) {
       _EntryList = w ;
    } else {
       Tail->_next = w ;
       w->_prev = Tail ;
    }
  }
// qmode =4:cxq队列插入到_EntryList头部
   if (QMode == 4 && _cxq != NULL) {
     // Aggressively drain cxq into EntryList at the first opportunity.
     // This policy ensure that recently-run threads live at the head of
EntryList.
     // Drain _cxq into EntryList - bulk transfer.
     // First, detach _cxq.
     // The following loop is tantamount to: w = swap (&cxq, NULL)
     w = _cxq ;
     for (;;) {
      assert (w != NULL, "Invariant") ;
      ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL,
&_cxq, w) ;
      if (u == w) break ;
      w = u ;
    }
     assert (w != NULL       , "invariant") ;
     ObjectWaiter * q = NULL ;
     ObjectWaiter * p ;
     for (p = w ; p != NULL ; p = p->_next) {
       guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
       p->TState = ObjectWaiter::TS_ENTER ;
       p->_prev = q ;
       q = p ;
    }
     // Prepend the RATs to the EntryList
     if (_EntryList != NULL) {
       q->_next = _EntryList ;
       _EntryList->_prev = q ;
    }
     _EntryList = w ;
     // Fall thru into code that tries to wake a successor from EntryList
  }
   w = _EntryList ;
   if (w != NULL) {
     assert (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
     ExitEpilog (Self, w) ;//唤醒w!!!!!!!!!!!!!!!!!!!!!! ------->当前
类的ExitEpilog
     return ;
  }

实现如下

void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) {
assert (_owner == Self, "invariant") ;
_succ = Knob_SuccEnabled ? Wakee->_thread : NULL ;
ParkEvent * Trigger = Wakee->_event ;
Wakee = NULL ;
// Drop the lock
OrderAccess::release_store_ptr (&_owner, NULL) ;
OrderAccess::fence() ; // ST _owner vs LD in
unpark()
if (SafepointSynchronize::do_call_back()) {
TEVENT (unpark before SAFEPOINT) ;
}
DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
  // 唤醒之前被park()挂起的线程.
 Trigger->unpark() ;// invoke ObjectMonitor::EnterI 方法,继续竞争
   if (ObjectMonitor::_sync_Parks != NULL) {
   ObjectMonitor::_sync_Parks->inc() ;
 }
}

被唤醒的线程,回到 ObjectMonitor::EnterI (TRAPS) 的第600行,继续执行monitor 的竞争。

// park self
if (_Responsible == Self || (SyncFlags & 1)) {
TEVENT (Inflated enter - park TIMED) ;
Self->_ParkEvent->park ((jlong) RecheckInterval) ;
// Increase the RecheckInterval, but clamp the value.
RecheckInterval *= 8 ;
if (RecheckInterval > 1000) RecheckInterval = 1000 ;
} else {
TEVENT (Inflated enter - park UNTIMED) ;
Self->_ParkEvent->park() ;
}
//唤醒之后就开始抢夺锁
if (TryLock(Self) > 0) break ;

TryLock方 法实现如下:

//线程尝试获取锁(or 线程被唤醒后获取)
int ObjectMonitor::TryLock (Thread * Self) {
 for (;;) {
   void * own = _owner ;
   if (own != NULL) return 0 ;
   //获取
   if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
    // Either guarantee _recursions == 0 or set _recursions = 0.
    assert (_recursions == 0, "invariant") ;
    assert (_owner == Self, "invariant") ;
   // 尝试拿到锁返回1
    return 1 ;
  }
//拿不到锁返回-1
   if (true) return -1 ;
 }
}

总结
1、先进入减减操作,直到为0
2、为0后,唤醒竞争队列的线程
3、唤醒线程后,继续争夺锁,循环前面的步骤(锁竞争-----等待----释放)
一句话总结:释放后,进入减减操作、直到为0然后唤醒队列,让他们去争夺锁,循环前面步骤

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