文章简介
前面我有文章介绍了synchronized的基本原理，这篇文章我会从jvm源码分析synchronized的实现逻辑，希望让大家有一个更加深度的认识
内容导航
- 从synchronized的字节码说起
- 什么是monitor
- 分析synchronized的源码
从synchronized的字节码说起
由于synchronized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synchronized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flag；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(monitor)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 synchronized的原理分析这篇文章中，有提到对象监视器。
synchronized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Object类型的参数来指明要锁定和解锁的对象。如果Java程序中的synchronized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synchronized修饰的是实例方法还是类方法，获取对应的对象实例或Class对象来作为锁对象
什么是monitor
在分析源代码之前需要了解oop, oopDesc, markOop等相关概念，在Synchronized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Object在JVM内部都会有一个native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monitor方法，这个方法返回一个ObjectMonitor指针对象，在hotspot虚拟机中，采用ObjectMonitor类来实现monitor
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flag 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synchronized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对象进行描述的。在Hotspot中，是通过ObjectMonitor来实现，每个对象中都会内置一个ObjectMonitor对象
synchroinzed简图
简单分析synchronized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synchronized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revoke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)。在做bulk 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lock:01表示无锁状态
- 如果mark处于无锁状态，则进入步骤(3)，否则执行步骤(5)
- 把mark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mark处于加锁状态，且mark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(monitor)来实现，而monitor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::enter方法里面
重量级锁的竞争逻辑
重量级锁的竞争，在 ObjectMonitor::enter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monitor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node，状态设置成TS_CXQ
- 通过自旋操作将node节点push到_cxq队列
- 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
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monitor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
- 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::exit()，直到重入锁次数为0为止
- 释放当前锁，并根据QMode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QMode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unpark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() ;
}
}
分析源码，需要很大的耐心，希望大家能有耐心看下去，有疑问欢迎微信留言
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