阿里面试分析Synchronized原理，让面试官仰望
内容导航
从同步的字节码说起
什么是显示器
分析同步的源码
从同步的字节码说起
由于同步的实现是在JVM层面，所以我们如果要看它的源码，需要从字节码入手。这段代码演示了同步作为实例锁的两种用法，我们观察一下这段代码生成的字节码
public class App
{
public synchronized void test1(){
}
public void test2(){
synchronized (this){
}
}
public static void main( String[] args ){
System.out.println( "Hello World!" );
}
}
进入classpath目录下找到App.class文件，在cmd中输入javap -v App.class查看字节码
public synchronized void test1();
descriptor: ()V
flags: ACC_PUBLIC, ACC_SYNCHRONIZED
Code:
stack=0, locals=1, args_size=1
0: return
LineNumberTable:
line 10: 0
LocalVariableTable:
Start Length Slot Name Signature
0 1 0 this Lcom/gupaoedu/openclass/App;
public void test2();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=2, locals=3, args_size=1
0: aload_0
1: dup
2: astore_1
3: monitorenter //监视器进入，获取锁
4: aload_1
5: monitorexit //监视器退出，释放锁
6: goto 14
9: astore_2
10: aload_1
11: monitorexit
12: aload_2
13: athrow
14: return
通过字节码我们可以发现，修饰在方法层面的同步关键字，会多一个ACC_SYNCHRONIZED的旗帜;修饰在代码块层面的同步块会多一个monitorenter和monitorexit关键字。无论采用哪一种方式，本质上都是对一个对象的监视器（显示器）进行获取，而这个获取的过程是排他的，也就是同一个时刻只能有一个线程获得同步块对象的监视器。
在同步原理的分析这篇文章中，有提到对象监视器。
同步关键字经过编译之后，会在同步块的前后分别形成monitorenter和monitorexit这两个字节码指令。当我们的JVM把字节码加载到内存的时候，会对这两个指令进行解析。这两个字节码都需要一个对象类型的参数来指明要锁定和解锁的对象如果爪哇程序中的同步明确指定了对象参数，那么这个对象就是加锁和解锁的对象;如果没有明确指定，那就根据同步修饰的是实例方法还是类方法，获取对应的对象实例或类对象来作为锁对象
什么是显示器
在分析源代码之前需要了解oop，oopDesc，markOop等相关概念，在同步的原理分析这篇文章中，我们讲到了synchronized的同步锁实际上是存储在对象头中，这个对象头是一个Java对象在内存中的布局的一部分.Java中的每一个对象在JVM内部都会有一个本地的C ++对象oop / oopDesc与之对应。在hotspot源码oop.hpp中oopDesc的定义如下
class oopDesc {
friend class VMStructs;
private:
volatile markOop _mark;
union _metadata {
Klass* _klass;
narrowKlass _compressed_klass;
} _metadata;
其中markOop就是我们所说的Mark Word，用于存储锁的标识.hotspot
源码markOop.hpp文件代码片段
class markOopDesc: public oopDesc {
private:
// Conversion
uintptr_t value() const { return (uintptr_t) this; }
public:
// Constants
enum { age_bits = 4,
lock_bits = 2,
biased_lock_bits = 1,
max_hash_bits = BitsPerWord - age_bits - lock_bits - biased_lock_bits,
hash_bits = max_hash_bits > 31 ? 31 : max_hash_bits,
cms_bits = LP64_ONLY(1) NOT_LP64(0),
epoch_bits = 2
};
...
}
markOopDesc继承自oopDesc，并且扩展了自己的监控方法，这个方法返回一个ObjectMonitor指针对象，在热点虚拟机中，采用ObjectMonitor类来实现监控
bool has_monitor() const {
return ((value() & monitor_value) != 0);
}
ObjectMonitor* monitor() const {
assert(has_monitor(), "check");
// Use xor instead of &~ to provide one extra tag-bit check.
return (ObjectMonitor*) (value() ^ monitor_value);
}
在ObjectMonitor.hpp中，可以看到ObjectMonitor的定义
class ObjectMonitor {
...
ObjectMonitor() {
_header = NULL; //markOop对象头
_count = 0;
_waiters = 0, //等待线程数
_recursions = 0; //重入次数
_object = NULL;
_owner = NULL; //获得ObjectMonitor对象的线程
_WaitSet = NULL; //处于wait状态的线程，会被加入到waitSet
_WaitSetLock = 0 ;
_Responsible = NULL ;
_succ = NULL ;
_cxq = NULL ;
FreeNext = NULL ;
_EntryList = NULL ; //处于等待锁BLOCKED状态的线程
_SpinFreq = 0 ;
_SpinClock = 0 ;
OwnerIsThread = 0 ;
_previous_owner_tid = 0; //监视器前一个拥有线程的ID
}
...
简单总结一下，同步块的实现使用monitorenter和monitorexit指令，而同步方法是依靠方法修饰符上的旗帜ACC_SYNCHRONIZED来完成。其本质是对一个对象监视器（monitor）进行获取，这个获取过程是排他的，也就是同一个时刻只能有一个线程获得由同步所保护对象的监视器。所谓的监视器，实际上可以理解为一个同步工具，它是由Java的对象进行描述的。在热点中，是通过ObjectMonitor来实现，每个对象中都会内置一个ObjectMonitor对象
简单分析同步的源码
从monitorenter和monitorexit这两个指令来开始阅读源码，JVM将字节码加载到内存以后，会对这两个指令进行解释执行，monitorenter，monitorexit的指令解析是通过InterpreterRuntime.cpp中的两个方法实现
InterpreterRuntime::monitorenter(JavaThread* thread, BasicObjectLock* elem)
InterpreterRuntime::monitorexit(JavaThread* thread, BasicObjectLock* elem)
//JavaThread 当前获取锁的线程
//BasicObjectLock 基础对象锁
我们基于monitorenter为入口，沿着偏向锁 - >轻量级锁 - >重量级锁的路径来分析同步的实现过程
IRT_ENTRY_NO_ASYNC(void, InterpreterRuntime::monitorenter(JavaThread* thread, BasicObjectLock* elem))
#ifdef ASSERT
thread->last_frame().interpreter_frame_verify_monitor(elem);
#endif
...
if (UseBiasedLocking) {
// Retry fast entry if bias is revoked to avoid unnecessary inflation
ObjectSynchronizer::fast_enter(h_obj, elem->lock(), true, CHECK);
} else {
ObjectSynchronizer::slow_enter(h_obj, elem->lock(), CHECK);
}
...
#ifdef ASSERT
thread->last_frame().interpreter_frame_verify_monitor(elem);
#endif
IRT_END
UseBiasedLocking是在JVM启动的时候，是否启动偏向锁的标识
如果支持偏向锁，则执行ObjectSynchronizer :: fast_enter的逻辑
如果不支持偏向锁，则执行ObjectSynchronizer :: slow_enter逻辑，绕过偏向锁，直接进入轻量级锁
ObjectSynchronizer :: fast_enter的实现在synchronizer.cpp文件中，代码如下
void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock, bool attempt_rebias, TRAPS) {
if (UseBiasedLocking) { //判断是否开启了偏向锁
if (!SafepointSynchronize::is_at_safepoint()) { //如果不处于全局安全点
//通过`revoke_and_rebias`这个函数尝试获取偏向锁
BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD);
if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) {//如果是撤销与重偏向直接返回
return;
}
} else {//如果在安全点，撤销偏向锁
assert(!attempt_rebias, "can not rebias toward VM thread");
BiasedLocking::revoke_at_safepoint(obj);
}
assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
}
slow_enter (obj, lock, THREAD) ;
}
fast_enter方法的主要流程做一个简单的解释
再次检查偏向锁是否开启
当处于不安全点时，通过revoke_and_rebias尝试获取偏向锁，如果成功则直接返回，如果失败则进入轻量级锁获取过程
revoke_and_rebias这个偏向锁的获取逻辑在biasedLocking.cpp中
如果偏向锁未开启，则进入slow_enter获取轻量级锁的流程
偏向锁的获取逻辑
BiasedLocking :: revoke_and_rebias是用来获取当前偏向锁的状态（可能是偏向锁撤销后重新偏向）。这个方法的逻辑在biasedLocking.cpp中
BiasedLocking::Condition BiasedLocking::revoke_and_rebias(Handle obj, bool attempt_rebias, TRAPS) {
assert(!SafepointSynchronize::is_at_safepoint(), "must not be called while at safepoint");
markOop mark = obj->mark(); //获取锁对象的对象头
//判断mark是否为可偏向状态，即mark的偏向锁标志位为1，锁标志位为 01，线程id为null
if (mark->is_biased_anonymously() && !attempt_rebias) {
//这个分支是进行对象的hashCode计算时会进入，在一个非全局安全点进行偏向锁撤销
markOop biased_value = mark;
//创建一个非偏向的markword
markOop unbiased_prototype = markOopDesc::prototype()->set_age(mark->age());
//Atomic:cmpxchg_ptr是CAS操作，通过cas重新设置偏向锁状态
markOop res_mark = (markOop) Atomic::cmpxchg_ptr(unbiased_prototype, obj->mark_addr(), mark);
if (res_mark == biased_value) {//如果CAS成功，返回偏向锁撤销状态
return BIAS_REVOKED;
}
} else if (mark->has_bias_pattern()) {//如果锁对象为可偏向状态（biased_lock:1, lock:01，不管线程id是否为空）,尝试重新偏向
Klass* k = obj->klass();
markOop prototype_header = k->prototype_header();
//如果已经有线程对锁对象进行了全局锁定，则取消偏向锁操作
if (!prototype_header->has_bias_pattern()) {
markOop biased_value = mark;
//CAS 更新对象头markword为非偏向锁
markOop res_mark = (markOop) Atomic::cmpxchg_ptr(prototype_header, obj->mark_addr(), mark);
assert(!(*(obj->mark_addr()))->has_bias_pattern(), "even if we raced, should still be revoked");
return BIAS_REVOKED; //返回偏向锁撤销状态
} else if (prototype_header->bias_epoch() != mark->bias_epoch()) {
//如果偏向锁过期，则进入当前分支
if (attempt_rebias) {//如果允许尝试获取偏向锁
assert(THREAD->is_Java_thread(), "");
markOop biased_value = mark;
markOop rebiased_prototype = markOopDesc::encode((JavaThread*) THREAD, mark->age(), prototype_header->bias_epoch());
//通过CAS 操作， 将本线程的 ThreadID 、时间错、分代年龄尝试写入对象头中
markOop res_mark = (markOop) Atomic::cmpxchg_ptr(rebiased_prototype, obj->mark_addr(), mark);
if (res_mark == biased_value) { //CAS成功，则返回撤销和重新偏向状态
return BIAS_REVOKED_AND_REBIASED;
}
} else {//不尝试获取偏向锁，则取消偏向锁
//通过CAS操作更新分代年龄
markOop biased_value = mark;
markOop unbiased_prototype = markOopDesc::prototype()->set_age(mark->age());
markOop res_mark = (markOop) Atomic::cmpxchg_ptr(unbiased_prototype, obj->mark_addr(), mark);
if (res_mark == biased_value) { //如果CAS操作成功，返回偏向锁撤销状态
return BIAS_REVOKED;
}
}
}
}
...//省略
}
偏向锁的撤销
当到达一个全局安全点时，这时会根据偏向锁的状态来判断是否需要撤销偏向锁，调用revoke_at_safepoint方法，这个方法也是在biasedLocking.cpp中定义的
void BiasedLocking::revoke_at_safepoint(Handle h_obj) {
assert(SafepointSynchronize::is_at_safepoint(), "must only be called while at safepoint");
oop obj = h_obj();
//更新撤销偏向锁计数，并返回偏向锁撤销次数和偏向次数
HeuristicsResult heuristics = update_heuristics(obj, false);
if (heuristics == HR_SINGLE_REVOKE) {//可偏向且未达到批量处理的阈值(下面会单独解释)
revoke_bias(obj, false, false, NULL); //撤销偏向锁
} else if ((heuristics == HR_BULK_REBIAS) ||
(heuristics == HR_BULK_REVOKE)) {//如果是多次撤销或者多次偏向
//批量撤销
bulk_revoke_or_rebias_at_safepoint(obj, (heuristics == HR_BULK_REBIAS), false, NULL);
}
clean_up_cached_monitor_info();
}
偏向锁的释放，需要等待全局安全点（在这个时间点上没有正在执行的字节码），首先暂停拥有偏向锁的线程，然后检查持有偏向锁的线程是否还活着，如果线程不处于活动状态，则将对象头设置成无锁状态。如果线程仍然活着，则会升级为轻量级锁，遍历偏向对象的所记录。栈帧中的锁记录和对象头的Mark Word要么重新偏向其他线程，要么恢复到无锁，或者标记对象不适合作为偏向锁。最后唤醒暂停的线程。
JVM内部为每个类维护了一个偏向锁撤销计数器，对偏向锁撤销进行计数，当这个值达到指定阈值时，JVM会认为这个类的偏向锁有问题，需要重新偏向（rebias），对所有属于这个类的对象进行重偏向的操作成为批量重偏向（bulk rebias）。在批量rebias时，会对这个类的epoch的值做递增，这个epoch会存储在对象头中的epoch字段。在判断这个对象是否获得偏向锁的条件是：markword的biased_lock：1，lock：01，threadid和当前线程id相等，epoch字段和所属类的epoch值相同，如果epoch的值不一样，要么就是撤销偏向锁，要么就是rebias;如果这个类的revoke计数器的值继续增加到一个阈值，那么jvm会认为这个类不适合偏向锁，就需要进行bulk revoke操作
轻量级锁的获取逻辑
轻量级锁的获取，是调用:: slow_enter方法，该方法同样位于synchronizer.cpp文件中
void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) {
markOop mark = obj->mark();
assert(!mark->has_bias_pattern(), "should not see bias pattern here");
if (mark->is_neutral()) { //如果当前是无锁状态, markword的biase_lock:0，lock:01
//直接把mark保存到BasicLock对象的_displaced_header字段
lock->set_displaced_header(mark);
//通过CAS将mark word更新为指向BasicLock对象的指针，更新成功表示获得了轻量级锁
if (mark == (markOop) Atomic::cmpxchg_ptr(lock, obj()->mark_addr(), mark)) {
TEVENT (slow_enter: release stacklock) ;
return ;
}
// Fall through to inflate() ...
}
//如果markword处于加锁状态、且markword中的ptr指针指向当前线程的栈帧，表示为重入操作，不需要争抢锁
else if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
assert(lock != mark->locker(), "must not re-lock the same lock");
assert(lock != (BasicLock*)obj->mark(), "don't relock with same BasicLock");
lock->set_displaced_header(NULL);
return;
}
#if 0
// The following optimization isn't particularly useful.
if (mark->has_monitor() && mark->monitor()->is_entered(THREAD)) {
lock->set_displaced_header (NULL) ;
return ;
}
#endif
//代码执行到这里，说明有多个线程竞争轻量级锁，轻量级锁通过`inflate`进行膨胀升级为重量级锁
lock->set_displaced_header(markOopDesc::unused_mark());
ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);
}
轻量级锁的获取逻辑简单再整理一下
mark-> is_neutral（）方法，is_neutral这个方法是在markOop.hpp中定义，如果biased_lock：0且锁定：01表示无锁状态
如果标记处于无锁状态，则进入步骤（3），否则执行步骤（5）
把标记保存到BasicLock对象的displacedheader字段
通过CAS尝试将markword更新为指向BasicLock对象的指针，如果更新成功，表示竞争到锁，则执行同步代码，否则执行步骤（5）
如果当前标记处于加锁状态，且标记中的PTR指针指向当前线程的栈帧，则执行同步代码，否则说明有多个线程竞争轻量级锁，轻量级锁需要膨胀升级为重量级锁
轻量级锁的释放逻辑
轻量级锁的释放是通过monitorexit调用
IRT_ENTRY_NO_ASYNC(void, InterpreterRuntime::monitorexit(JavaThread* thread, BasicObjectLock* elem))
#ifdef ASSERT
thread->last_frame().interpreter_frame_verify_monitor(elem);
#endif
Handle h_obj(thread, elem->obj());
assert(Universe::heap()->is_in_reserved_or_null(h_obj()),
"must be NULL or an object");
if (elem == NULL || h_obj()->is_unlocked()) {
THROW(vmSymbols::java_lang_IllegalMonitorStateException());
}
ObjectSynchronizer::slow_exit(h_obj(), elem->lock(), thread);
// Free entry. This must be done here, since a pending exception might be installed on
// exit. If it is not cleared, the exception handling code will try to unlock the monitor again.
elem->set_obj(NULL);
#ifdef ASSERT
thread->last_frame().interpreter_frame_verify_monitor(elem);
#endif
IRT_END
这段代码中主要是通过ObjectSynchronizer :: slow_exit来执行
void ObjectSynchronizer::slow_exit(oop object, BasicLock* lock, TRAPS) {
fast_exit (object, lock, THREAD) ;
}
ObjectSynchronizer :: fast_exit的代码如下
void ObjectSynchronizer::fast_exit(oop object, BasicLock* lock, TRAPS) {
assert(!object->mark()->has_bias_pattern(), "should not see bias pattern here");
// if displaced header is null, the previous enter is recursive enter, no-op
markOop dhw = lock->displaced_header(); //获取锁对象中的对象头
markOop mark ;
if (dhw == NULL) {
// Recursive stack-lock.
// Diagnostics -- Could be: stack-locked, inflating, inflated.
mark = object->mark() ;
assert (!mark->is_neutral(), "invariant") ;
if (mark->has_locker() && mark != markOopDesc::INFLATING()) {
assert(THREAD->is_lock_owned((address)mark->locker()), "invariant") ;
}
if (mark->has_monitor()) {
ObjectMonitor * m = mark->monitor() ;
assert(((oop)(m->object()))->mark() == mark, "invariant") ;
assert(m->is_entered(THREAD), "invariant") ;
}
return ;
}
mark = object->mark() ; //获取线程栈帧中锁记录(LockRecord)中的markword
// If the object is stack-locked by the current thread, try to
// swing the displaced header from the box back to the mark.
if (mark == (markOop) lock) {
assert (dhw->is_neutral(), "invariant") ;
//通过CAS尝试将Displaced Mark Word替换回对象头，如果成功，表示锁释放成功。
if ((markOop) Atomic::cmpxchg_ptr (dhw, object->mark_addr(), mark) == mark) {
TEVENT (fast_exit: release stacklock) ;
return;
}
}
//锁膨胀，调用重量级锁的释放锁方法
ObjectSynchronizer::inflate(THREAD, object)->exit (true, THREAD) ;
}
轻量级锁的释放也比较简单，就是将当前线程栈帧中锁记录空间中的Mark Word替换到锁对象的对象头中，如果成功表示锁释放成功。否则，锁膨胀成重量级锁，实现重量级锁的释放锁逻辑
锁膨胀的过程分析
重量级锁是通过对象内部的监视器（显示器）来实现，而监视的本质是依赖操作系统底层的MutexLock实现的。我们先来看锁的膨胀过程，从前面的分析中已经知道了所膨胀的过程是通过ObjectSynchronizer :: inflate方法实现的，代码如下
ObjectMonitor * ATTR ObjectSynchronizer::inflate (Thread * Self, oop object) {
// Inflate mutates the heap ...
// Relaxing assertion for bug 6320749.
assert (Universe::verify_in_progress() ||
!SafepointSynchronize::is_at_safepoint(), "invariant") ;
for (;;) { //通过无意义的循环实现自旋操作
const markOop mark = object->mark() ;
assert (!mark->has_bias_pattern(), "invariant") ;
if (mark->has_monitor()) {//has_monitor是markOop.hpp中的方法，如果为true表示当前锁已经是重量级锁了
ObjectMonitor * inf = mark->monitor() ;//获得重量级锁的对象监视器直接返回
assert (inf->header()->is_neutral(), "invariant");
assert (inf->object() == object, "invariant") ;
assert (ObjectSynchronizer::verify_objmon_isinpool(inf), "monitor is invalid");
return inf ;
}
if (mark == markOopDesc::INFLATING()) {//膨胀等待，表示存在线程正在膨胀，通过continue进行下一轮的膨胀
TEVENT (Inflate: spin while INFLATING) ;
ReadStableMark(object) ;
continue ;
}
if (mark->has_locker()) {//表示当前锁为轻量级锁，以下是轻量级锁的膨胀逻辑
ObjectMonitor * m = omAlloc (Self) ;//获取一个可用的ObjectMonitor
// Optimistically prepare the objectmonitor - anticipate successful CAS
// We do this before the CAS in order to minimize the length of time
// in which INFLATING appears in the mark.
m->Recycle();
m->_Responsible = NULL ;
m->OwnerIsThread = 0 ;
m->_recursions = 0 ;
m->_SpinDuration = ObjectMonitor::Knob_SpinLimit ; // Consider: maintain by type/class
/**将object->mark_addr()和mark比较，如果这两个值相等，则将object->mark_addr()
改成markOopDesc::INFLATING()，相等返回是mark，不相等返回的是object->mark_addr()**/
markOop cmp = (markOop) Atomic::cmpxchg_ptr (markOopDesc::INFLATING(), object->mark_addr(), mark) ;
if (cmp != mark) {//CAS失败
omRelease (Self, m, true) ;//释放监视器
continue ; // 重试
}
markOop dmw = mark->displaced_mark_helper() ;
assert (dmw->is_neutral(), "invariant") ;
//CAS成功以后，设置ObjectMonitor相关属性
m->set_header(dmw) ;
m->set_owner(mark->locker());
m->set_object(object);
// TODO-FIXME: assert BasicLock->dhw != 0.
guarantee (object->mark() == markOopDesc::INFLATING(), "invariant") ;
object->release_set_mark(markOopDesc::encode(m));
if (ObjectMonitor::_sync_Inflations != NULL) ObjectMonitor::_sync_Inflations->inc() ;
TEVENT(Inflate: overwrite stacklock) ;
if (TraceMonitorInflation) {
if (object->is_instance()) {
ResourceMark rm;
tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
(void *) object, (intptr_t) object->mark(),
object->klass()->external_name());
}
}
return m ; //返回ObjectMonitor
}
//如果是无锁状态
assert (mark->is_neutral(), "invariant");
ObjectMonitor * m = omAlloc (Self) ; ////获取一个可用的ObjectMonitor
//设置ObjectMonitor相关属性
m->Recycle();
m->set_header(mark);
m->set_owner(NULL);
m->set_object(object);
m->OwnerIsThread = 1 ;
m->_recursions = 0 ;
m->_Responsible = NULL ;
m->_SpinDuration = ObjectMonitor::Knob_SpinLimit ; // consider: keep metastats by type/class
/**将object->mark_addr()和mark比较，如果这两个值相等，则将object->mark_addr()
改成markOopDesc::encode(m)，相等返回是mark，不相等返回的是object->mark_addr()**/
if (Atomic::cmpxchg_ptr (markOopDesc::encode(m), object->mark_addr(), mark) != mark) {
//CAS失败，说明出现了锁竞争，则释放监视器重行竞争锁
m->set_object (NULL) ;
m->set_owner (NULL) ;
m->OwnerIsThread = 0 ;
m->Recycle() ;
omRelease (Self, m, true) ;
m = NULL ;
continue ;
// interference - the markword changed - just retry.
// The state-transitions are one-way, so there's no chance of
// live-lock -- "Inflated" is an absorbing state.
}
if (ObjectMonitor::_sync_Inflations != NULL) ObjectMonitor::_sync_Inflations->inc() ;
TEVENT(Inflate: overwrite neutral) ;
if (TraceMonitorInflation) {
if (object->is_instance()) {
ResourceMark rm;
tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
(void *) object, (intptr_t) object->mark(),
object->klass()->external_name());
}
}
return m ; //返回ObjectMonitor对象
}
}
锁膨胀的过程稍微有点复杂，整个锁膨胀的过程是通过自旋来完成的，具体的实现逻辑简答总结以下几点
mark-> has_monitor（）判断如果当前锁对象为重量级锁，也就是lock：10，则执行（2），否则执行（3）
通过mark-> monitor获得重量级锁的对象监视器ObjectMonitor并返回，锁膨胀过程结束
如果当前锁处于INFLATING，说明有其他线程在执行锁膨胀，那么当前线程通过自旋等待其他线程锁膨胀完成
如果当前是轻量级锁状态mark-> has_locker（），则进行锁膨胀。首先，通过omAlloc方法获得一个可用的ObjectMonitor，并设置初始数据;然后通过CAS将对象头设置为`markOopDesc：INFLATING，表示当前锁正在膨胀，如果CAS失败，继续自旋
如果是无锁状态，逻辑类似第4步骤
锁膨胀的过程实际上是获得一个ObjectMonitor对象监视器，而真正抢占锁的逻辑，在ObjectMonitor ::进入方法里面
重量级锁的竞争逻辑
重量级锁的竞争，在ObjectMonitor ::输入方法中，代码文件在objectMonitor.cpp重量级锁的代码就不一一分析了，简单说一下下面这段代码主要做的几件事
通过CAS将监视器的_owner字段设置为当前线程，如果设置成功，则直接返回
如果之前的_owner指向的是当前的线程，说明是重入，执行_recursions ++增加重入次数
如果当前线程获取监视器锁成功，将_recursions设置为1，_owner设置为当前线程
如果获取锁失败，则等待锁释放
void ATTR ObjectMonitor::enter(TRAPS) {
// The following code is ordered to check the most common cases first
// and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
Thread * const Self = THREAD ;
void * cur ;
cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
if (cur == NULL) {//CAS成功
// Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
assert (_recursions == 0 , "invariant") ;
assert (_owner == Self, "invariant") ;
// CONSIDER: set or assert OwnerIsThread == 1
return ;
}
if (cur == Self) {
// TODO-FIXME: check for integer overflow! BUGID 6557169.
_recursions ++ ;
return ;
}
if (Self->is_lock_owned ((address)cur)) {
assert (_recursions == 0, "internal state error");
_recursions = 1 ;
// Commute owner from a thread-specific on-stack BasicLockObject address to
// a full-fledged "Thread *".
_owner = Self ;
OwnerIsThread = 1 ;
return ;
}
// We've encountered genuine contention.
assert (Self->_Stalled == 0, "invariant") ;
Self->_Stalled = intptr_t(this) ;
// Try one round of spinning *before* enqueueing Self
// and before going through the awkward and expensive state
// transitions. The following spin is strictly optional ...
// Note that if we acquire the monitor from an initial spin
// we forgo posting JVMTI events and firing DTRACE probes.
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") ;
// Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
// Ensure the object-monitor relationship remains stable while there's contention.
Atomic::inc_ptr(&_count);
EventJavaMonitorEnter event;
{ // Change java thread status to indicate blocked on monitor enter.
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);
// TODO-FIXME: change the following for(;;) loop to straight-line code.
for (;;) {
jt->set_suspend_equivalent();
// cleared by handle_special_suspend_equivalent_condition()
// or java_suspend_self()
EnterI (THREAD) ;
if (!ExitSuspendEquivalent(jt)) break ;
//
// We have acquired the contended monitor, but while we were
// waiting another thread suspended us. We don't want to enter
// the monitor while suspended because that would surprise the
// thread that suspended us.
//
_recursions = 0 ;
_succ = NULL ;
exit (false, Self) ;
jt->java_suspend_self();
}
Self->set_current_pending_monitor(NULL);
}
...//此处省略无数行代码
如果获取锁失败，则需要通过自旋的方式等待锁释放，自旋执行的方法是ObjectMonitor :: EnterI，部分代码如下
将当前线程封装成ObjectWaiter对象节点，状态设置成TS_CXQ
通过自旋操作将节点节点推到_cxq队列
节点节点添加到_cxq队列之后，继续通过自旋尝试获取锁，如果在指定的阈值范围内没有获得锁，则通过公园将当前线程挂起，等待被唤醒
void ATTR ObjectMonitor::EnterI (TRAPS) {
Thread * Self = THREAD ;
...//省略很多代码
ObjectWaiter node(Self) ;
Self->_ParkEvent->reset() ;
node._prev = (ObjectWaiter *) 0xBAD ;
node.TState = ObjectWaiter::TS_CXQ ;
// Push "Self" onto the front of the _cxq.
// Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
// Note that spinning tends to reduce the rate at which threads
// enqueue and dequeue on EntryList|cxq.
ObjectWaiter * nxt ;
for (;;) { //自旋，讲node添加到_cxq队列
node._next = nxt = _cxq ;
if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ;
// Interference - the CAS failed because _cxq changed. Just retry.
// As an optional optimization we retry the lock.
if (TryLock (Self) > 0) {
assert (_succ != Self , "invariant") ;
assert (_owner == Self , "invariant") ;
assert (_Responsible != Self , "invariant") ;
return ;
}
}
...//省略很多代码
//node节点添加到_cxq队列之后，继续通过自旋尝试获取锁，如果在指定的阈值范围内没有获得锁，则通过park将当前线程挂起，等待被唤醒
for (;;) {
if (TryLock (Self) > 0) break ;
assert (_owner != Self, "invariant") ;
if ((SyncFlags & 2) && _Responsible == NULL) {
Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
}
// park self //通过park挂起当前线程
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() ;//当前线程挂起
}
if (TryLock(Self) > 0) break ; //当线程被唤醒时，会从这里继续执行
TEVENT (Inflated enter - Futile wakeup) ;
if (ObjectMonitor::_sync_FutileWakeups != NULL) {
ObjectMonitor::_sync_FutileWakeups->inc() ;
}
++ nWakeups ;
if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ;
if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
Self->_ParkEvent->reset() ;
OrderAccess::fence() ;
}
if (_succ == Self) _succ = NULL ;
// Invariant: after clearing _succ a thread *must* retry _owner before parking.
OrderAccess::fence() ;
}
...//省略很多代码
}
TryLock（self）的代码是在ObjectMonitor :: TryLock定义的，代码的实现如下
代码的实现原理很简单，通过自旋，CAS设置监视器的_owner字段为当前线程，如果成功，表示获取到了锁，如果失败，则继续被挂起
int ObjectMonitor::TryLock (Thread * Self) {
for (;;) {
void * own = _owner ;
if (own != NULL) return 0 ;
if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
// Either guarantee _recursions == 0 or set _recursions = 0.
assert (_recursions == 0, "invariant") ;
assert (_owner == Self, "invariant") ;
// CONSIDER: set or assert that OwnerIsThread == 1
return 1 ;
}
// The lock had been free momentarily, but we lost the race to the lock.
// Interference -- the CAS failed.
// We can either return -1 or retry.
// Retry doesn't make as much sense because the lock was just acquired.
if (true) return -1 ;
}
}
重量级锁的释放
重量级锁的释放是通过ObjectMonitor :: exit来实现的，释放以后会通知被阻塞的线程去竞争锁
判断当前锁对象中的所有者没有指向当前线程，如果车主指向的BasicLock在当前线程栈上，那么将_owner指向当前线程
如果当前锁对象中的_owner指向当前线程，则判断当前线程重入锁的次数，如果不为0，继续执行ObjectMonitor ::出口（），直到重入锁次数为0为止
释放当前锁，并根据Q模式的模式判断，是否将_cxq中挂起的线程唤醒。还是其他操作
void ATTR ObjectMonitor::exit(bool not_suspended, TRAPS) {
Thread * Self = THREAD ;
if (THREAD != _owner) {//如果当前锁对象中的_owner没有指向当前线程
//如果_owner指向的BasicLock在当前线程栈上,那么将_owner指向当前线程
if (THREAD->is_lock_owned((address) _owner)) {
// Transmute _owner from a BasicLock pointer to a Thread address.
// We don't need to hold _mutex for this transition.
// Non-null to Non-null is safe as long as all readers can
// tolerate either flavor.
assert (_recursions == 0, "invariant") ;
_owner = THREAD ;
_recursions = 0 ;
OwnerIsThread = 1 ;
} else {
// NOTE: we need to handle unbalanced monitor enter/exit
// in native code by throwing an exception.
// TODO: Throw an IllegalMonitorStateException ?
TEVENT (Exit - Throw IMSX) ;
assert(false, "Non-balanced monitor enter/exit!");
if (false) {
THROW(vmSymbols::java_lang_IllegalMonitorStateException());
}
return;
}
}
//如果当前，线程重入锁的次数，不为0，那么就重新走ObjectMonitor::exit，直到重入锁次数为0为止
if (_recursions != 0) {
_recursions--; // this is simple recursive enter
TEVENT (Inflated exit - recursive) ;
return ;
}
...//此处省略很多代码
for (;;) {
if (Knob_ExitPolicy == 0) {
OrderAccess::release_store(&_owner, (void*)NULL); //释放锁
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);
//省略部分代码...
}
//省略部分代码...
ObjectWaiter * w = NULL;
int QMode = Knob_QMode;
//根据QMode的模式判断，
//如果QMode == 2则直接从_cxq挂起的线程中唤醒
if (QMode == 2 && _cxq != NULL) {
w = _cxq;
ExitEpilog(Self, w);
return;
}
//省略部分代码... 省略的代码为根据QMode的不同，不同的唤醒机制
}
}
根据不同的策略（由Q模式指定），从CXQ或EntryList中获取头节点，通过ObjectMonitor :: ExitEpilog方法唤醒该节点封装的线程，唤醒操作最终由取消驻留完成
void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) {
{
assert (_owner == Self, "invariant") ;
// Exit protocol:
// 1. ST _succ = wakee
// 2. membar #loadstore|#storestore;
// 2. ST _owner = NULL
// 3. unpark(wakee)
_succ = Knob_SuccEnabled ? Wakee->_thread : NULL ;
ParkEvent * Trigger = Wakee->_event ;
// Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
// The thread associated with Wakee may have grabbed the lock and "Wakee" may be
// out-of-scope (non-extant).
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);
Trigger->unpark() ; //unpark唤醒线程
// Maintain stats and report events to JVMTI
if (ObjectMonitor::_sync_Parks != NULL) {
ObjectMonitor::_sync_Parks->inc() ;
}
}
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