static inline bool is_MP() {

// During bootstrap if _processor_count is not yet initialized

// we claim to be MP as that is safest. If any platform has a

// stub generator that might be triggered in this phase and for

// which being declared MP when in fact not, is a problem - then

// the bootstrap routine for the stub generator needs to check

// the processor count directly and leave the bootstrap routine

// in place until called after initialization has ocurred.

return (_processor_count != 1) || AssumeMP;

}

jdk8u: atomic_linux_x86.inline.hpp

#define LOCK_IF_MP(mp) "cmp $0, " #mp "; je 1f; lock; 1: "

最终实现：

cmpxchg = cas修改变量值

lock cmpxchg 指令

硬件：

lock指令在执行后面指令的时候锁定一个北桥信号

（不采用锁总线的方式）

markword

========

工具：JOL = Java Object Layout

===========================

org.openjdk.jol

jol-core

0.9

jdk8u: markOop.hpp

// Bit-format of an object header (most significant first, big endian layout below):

// 32 bits:

// --------

// hash:25 ------------>| age:4 biased_lock:1 lock:2 (normal object)

// JavaThread*:23 epoch:2 age:4 biased_lock:1 lock:2 (biased object)

// size:32 ------------------------------------------>| (CMS free block)

// PromotedObject*:29 ---------->| promo_bits:3 ----->| (CMS promoted object)

// 64 bits:

// --------

// unused:25 hash:31 -->| unused:1 age:4 biased_lock:1 lock:2 (normal object)

// JavaThread*:54 epoch:2 unused:1 age:4 biased_lock:1 lock:2 (biased object)

// PromotedObject*:61 --------------------->| promo_bits:3 ----->| (CMS promoted object)

// size:64 ----------------------------------------------------->| (CMS free block)

// unused:25 hash:31 -->| cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && normal object)

// JavaThread*:54 epoch:2 cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && biased object)

// narrowOop:32 unused:24 cms_free:1 unused:4 promo_bits:3 ----->| (COOPs && CMS promoted object)

// unused:21 size:35 -->| cms_free:1 unused:7 ------------------>| (COOPs && CMS free block)

synchronized的横切面详解

==================

synchronized原理
升级过程
汇编实现
vs reentrantLock的区别

java源码层级

synchronized(o)

字节码层级

monitorenter moniterexit

JVM层级（Hotspot）

package com.mashibing.insidesync;

import org.openjdk.jol.info.ClassLayout;

public class T01_Sync1 {

public static void main(String[] args) {

Object o = new Object();

System.out.println(ClassLayout.parseInstance(o).toPrintable());

}

com.mashibing.insidesync.T01_Sync1$Lock object internals:

OFFSET SIZE TYPE DESCRIPTION VALUE

0 4 (object header) 05 00 00 00 (00000101 00000000 00000000 00000000) (5)

4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)

8 4 (object header) 49 ce 00 20 (01001001 11001110 00000000 00100000) (536923721)

12 4 (loss due to the next object alignment)

Instance size: 16 bytes

Space losses: 0 bytes internal + 4 bytes external = 4 bytes total

com.mashibing.insidesync.T02_Sync2$Lock object internals:

OFFSET SIZE TYPE DESCRIPTION VALUE

0 4 (object header) 05 90 2e 1e (00000101 10010000 00101110 00011110) (506368005)

4 4 (object header) 1b 02 00 00 (00011011 00000010 00000000 00000000) (539)

8 4 (object header) 49 ce 00 20 (01001001 11001110 00000000 00100000) (536923721)

12 4 (loss due to the next object alignment)

Instance size: 16 bytes

Space losses: 0 bytes internal + 4 bytes external = 4 bytes tota

InterpreterRuntime:: monitorenter方法

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”);

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);

}

assert(Universe::heap()->is_in_reserved_or_null(elem->obj()),

“must be NULL or an object”);

#ifdef ASSERT

thread->last_frame().interpreter_frame_verify_monitor(elem) 《一线大厂Java面试题解析+后端开发学习笔记+最新架构讲解视频+实战项目源码讲义》开源 ;

#endif

IRT_END

synchronizer.cpp

revoke_and_rebias

void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock, bool attempt_rebias, TRAPS) {

if (UseBiasedLocking) {

if (!SafepointSynchronize::is_at_safepoint()) {

BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD);

if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) {

return;

}

} else {

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) ;

}

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()) {

// Anticipate successful CAS – the ST of the displaced mark must

// be visible <= the ST performed by the CAS.

lock->set_displaced_header(mark);

if (mark == (markOop) Atomic::cmpxchg_ptr(lock, obj()->mark_addr(), mark)) {

TEVENT (slow_enter: release stacklock) ;

return ;

}

// Fall through to inflate() …

} 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

// The object header will never be displaced to this lock,

// so it does not matter what the value is, except that it

// must be non-zero to avoid looking like a re-entrant lock,

// and must not look locked either.

lock->set_displaced_header(markOopDesc::unused_mark());

ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);

}

inflate方法：膨胀为重量级锁

锁升级过程

=====

JDK8 markword实现表：

无锁 - 偏向锁 - 轻量级锁（自旋锁，自适应自旋）- 重量级锁

synchronized优化的过程和markword息息相关

用markword中最低的三位代表锁状态其中1位是偏向锁位两位是普通锁位

Object o = new Object() 锁 = 0 01 无锁态
o.hashCode() 001 + hashcode00000001 10101101 00110100 00110110 01011001 00000000 00000000 00000000 little endian big endian00000000 00000000 00000000 01011001 00110110 00110100 10101101 00000000
默认sy Java开源项目【ali1024.coding.net/public/P7/Java/git】 nchronized(o) 00 -> 轻量级锁默认情况偏向锁有个时延，默认是4秒 why? 因为JVM虚拟机自己有一些默认启动的线程，里面有好多sync代码，这些sync代码启动时就知道肯定会有竞争，如果使用偏向锁，就会造成偏向锁不断的进行锁撤销和锁升级的操作，效率较低。-XX:BiasedLockingStartupDelay=0
如果设定上述参数 new Object () - > 101 偏向锁 ->线程ID为0 -> Anonymous BiasedLock 打开偏向锁，new出来的对象，默认就是一个可偏向匿名对象101
如果有线程上锁上偏向锁，指的就是，把markword的线程ID改为自己线程ID的过程偏向锁不可重偏向批量偏向批量撤销
如果有线程竞争撤销偏向锁，升级轻量级锁线程在自己的线程栈生成LockRecord ，用CAS操作将markword设置为指向自己这个线程的LR的指针，设置成功者得到锁
如果竞争加剧竞争加剧：有线程超过10次自旋， -XX:PreBlockSpin，或者自旋线程数超过CPU核数的一半， 1.6之后，加入自适应自旋 Adapative Self Spinning ， JVM自己控制升级重量级锁：-> 向操作系统申请资源，linux mutex , CPU从3级-0级系统调用，线程挂起，进入等待队列，等待操作系统的调度，然后再映射回用户空间

(以上实验环境是JDK11，打开就是偏向锁，而JDK8默认对象头是无锁)

偏向锁默认是打开的，但是有一个时延，如果要观察到偏向锁，应该设定参数

没错，我就是厕所所长

加锁，指的是锁定对象

锁升级的过程

JDK较早的版本 OS的资源互斥量用户态 -> 内核态的转换重量级效率比较低

现代版本进行了优化

无锁 - 偏向锁 -轻量级锁（自旋锁）-重量级锁

偏向锁 - markword 上记录当前线程指针，下次同一个线程加锁的时候，不需要争用，只需要判断线程指针是否同一个，所以，偏向锁，偏向加锁的第一个线程。hashCode备份在线程栈上线程销毁，锁降级为无锁

有争用 - 锁升级为轻量级锁 - 每个线程有自己的LockRecord在自己的线程栈上，用CAS去争用markword的LR的指针，指针指向哪个线程的LR，哪个线程就拥有锁

自旋超过10次，升级为重量级锁 - 如果太多线程自旋 CPU消耗过大，不如升级为重量级锁，进入等待队列（不消耗CPU）-XX:PreBlockSpin

自旋锁在 JDK1.4.2 中引入，使用 -XX:+UseSpinning 来开启。JDK 6 中变为默认开启，并且引入了自适应的自旋锁（适应性自旋锁）。

自适应自旋锁意味着自旋的时间（次数）不再固定，而是由前一次在同一个锁上的自旋时间及锁的拥有者的状态来决定。如果在同一个锁对象上，自旋等待刚刚成功获得过锁，并且持有锁的线程正在运行中，那么虚拟机就会认为这次自旋也是很有可能再次成功，进而它将允许自旋等待持续相对更长的时间。如果对于某个锁，自旋很少成功获得过，那在以后尝试获取这个锁时将可能省略掉自旋过程，直接阻塞线程，避免浪费处理器资源。

偏向锁由于有锁撤销的过程revoke，会消耗系统资源，所以，在锁争用特别激烈的时候，用偏向锁未必效率高。还不如直接使用轻量级锁。

synchronized最底层实现

public class T {

static volatile int i = 0;

public static void n() { i++; }

public static synchronized void m() {}

publics static void main(String[] args) {

for(int j=0; j<1000_000; j++) {

m();

n();

}

java -XX:+UnlockDiagonositicVMOptions -XX:+PrintAssembly T