并发中锁的原理

一篇对对象锁的原理，存储位置的笔记

无锁：启动VM参数不配置，4s 程序开始执行，无锁 01偏向锁： JVM启动，默认延迟4s启动偏向锁。轻量级锁：synchronized (userEntity) 一个线程调用， 轻量级锁 00重量级锁：synchronized (userEntity) 多个线程争抢， 重量级锁 10

演示锁的几种状态

pom.xml <dependency> <groupId>org.openjdk.jol</groupId> <artifactId>jol-core</artifactId> <version>0.8</version> </dependency> static UserEntity userEntity ; // 立即开启偏向锁： -XX:+UseBiasedLocking -XX:BiasedLockingStartupDelay=0 // JVM默认会延迟 4s 开启偏向锁， // 00000101 101 偏向锁 //轻量级锁 00 //重量级锁 10 //无锁 01 public static void main(String[] args) throws Exception { userEntity = new UserEntity(); out.println("before lock"); out.println(ClassLayout.parseInstance(userEntity).toPrintable()); // 此时 userEntity 无锁 Thread t1 = new Thread(()->{ synchronized (userEntity){ try { Thread.sleep(5000); } catch (InterruptedException e) { e.printStackTrace(); } } }); t1.start(); Thread.sleep(1000); out.println("t1 lock ing"); out.println(ClassLayout.parseInstance(userEntity).toPrintable()); // 此时 userEntity此时 轻量级锁 sync(); out.println("after lock"); out.println(ClassLayout.parseInstance(userEntity).toPrintable()); //重量级锁 System.gc(); out.println("after gc()"); out.println(ClassLayout.parseInstance(userEntity).toPrintable()); // 无锁 } private static void sync() { synchronized (userEntity){ try { Thread.sleep(4000); } catch (InterruptedException e) { e.printStackTrace(); } } } result: before lock com.lxu.threadlocal.UserEntity object internals: OFFSET SIZE TYPE DESCRIPTION VALUE 0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1) 4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0) 8 4 (object header) 43 c1 00 f8 (01000011 11000001 00000000 11111000) (-134168253) 12 4 java.lang.String UserEntity.name null 16 4 java.lang.Integer UserEntity.age null 20 4 java.lang.String UserEntity.sex null 24 4 java.lang.String UserEntity.desc null 28 4 java.lang.Long UserEntity.create_time null Instance size: 32 bytes Space losses: 0 bytes internal + 0 bytes external = 0 bytes total t1 lock ing com.lxu.threadlocal.UserEntity object internals: OFFSET SIZE TYPE DESCRIPTION VALUE 0 4 (object header) 20 f5 9e 20 (00100000 11110101 10011110 00100000) (547288352) 4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0) 8 4 (object header) 43 c1 00 f8 (01000011 11000001 00000000 11111000) (-134168253) 12 4 java.lang.String UserEntity.name null 16 4 java.lang.Integer UserEntity.age null 20 4 java.lang.String UserEntity.sex null 24 4 java.lang.String UserEntity.desc null 28 4 java.lang.Long UserEntity.create_time null Instance size: 32 bytes Space losses: 0 bytes internal + 0 bytes external = 0 bytes total after lock com.lxu.threadlocal.UserEntity object internals: OFFSET SIZE TYPE DESCRIPTION VALUE 0 4 (object header) ca 1c 41 1d (11001010 00011100 01000001 00011101) (490806474) 4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0) 8 4 (object header) 43 c1 00 f8 (01000011 11000001 00000000 11111000) (-134168253) 12 4 java.lang.String UserEntity.name null 16 4 java.lang.Integer UserEntity.age null 20 4 java.lang.String UserEntity.sex null 24 4 java.lang.String UserEntity.desc null 28 4 java.lang.Long UserEntity.create_time null Instance size: 32 bytes Space losses: 0 bytes internal + 0 bytes external = 0 bytes total after gc() com.lxu.threadlocal.UserEntity object internals: OFFSET SIZE TYPE DESCRIPTION VALUE 0 4 (object header) 09 00 00 00 (00001001 00000000 00000000 00000000) (9) // 00001001 第一位nouse,2-5 age ，6 是否是偏向锁， 7-8位 锁类别标识 4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0) 8 4 (object header) 43 c1 00 f8 (01000011 11000001 00000000 11111000) (-134168253) 12 4 java.lang.String UserEntity.name null 16 4 java.lang.Integer UserEntity.age null 20 4 java.lang.String UserEntity.sex null 24 4 java.lang.String UserEntity.desc null 28 4 java.lang.Long UserEntity.create_time null Instance size: 32 bytes Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

00001001 第一位nouse,2-5 age ，6 是否是偏向锁， 7-8位 锁类别标识

调用hashcode 方法后，对象头 前56位 就不能存放偏向锁信息java 对象头组成： mark_word 8byte 、klass_word 4bytemark_word 64bit 由 hash、gc标识、是否偏向锁、lock 组成， 但格式不固定 锁类型对象头信息描述偏向锁54bit threadinfo 2bit epoch、 gc 、1bit biasedlock轻量锁62-pro_to_record 、lock、重量锁62-ptr_to_monitorObject lock

① 无锁时 ② 偏向锁时 ③

锁的膨胀过程

如果调用wait()方法，会立即变成重量级锁两个线程逐个执行，依次为同一类的对象加锁多次，不满20次，锁升级 偏向锁-> 轻量锁 ，达20次，则jvm会把这个对象原来偏向的线程，全都指定为另一个线程，且这些对象都会不升级轻量锁，还是保持偏向锁， 偏向锁-> 轻量锁 复杂且费时 当jvm发现同一个线程，同一个类 在多次膨胀，就会发现这个类有问题，停止膨胀，批量偏量到指定线程 验证 // VM启动配置 立即开启偏向锁： -XX:+UseBiasedLocking -XX:BiasedLockingStartupDelay=0 public static void main(String[] args) throws InterruptedException { List<A> list = new ArrayList<>(); System.out.println("t1 start..."); Thread t1 = new Thread(() -> { for (int i = 0; i < 25; i++) { System.out.println("111"); A a = new A(); synchronized (a){ //此时a 为偏向锁，偏向 t1线程 list.add(a); } } },"t1"); t1.start(); t1.join(); System.out.println("t2 before"); System.out.println(ClassLayout.parseInstance(list.get(1)).toPrintable()); Thread thread1 = new Thread(() -> { int k = 0; for (A a: list) { synchronized (a){ if (k == 21){ //达到20次后，list中所以a对象， 偏向t2线程，保持偏向锁状态 System.out.println("t2 ing"); System.out.println(ClassLayout.parseInstance(a).toPrintable()); } k++; } } },"t2"); thread1.start(); thread1.join(); System.out.println("t2 after"); System.out.println(ClassLayout.parseInstance(list.get(1)).toPrintable()); } result: t1 start... t2 before com.lxu.th.A object internals: OFFSET SIZE TYPE DESCRIPTION VALUE 0 4 (object header) 05 d8 aa 1f (00000101 11011000 10101010 00011111) (531290117) // 00000101 101 标识偏向锁 531290117 标识偏向的线程 4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0) 8 4 (object header) 43 c1 00 f8 (01000011 11000001 00000000 11111000) (-134168253) 12 4 (loss due to the next object alignment) Instance size: 16 bytes Space losses: 0 bytes internal + 4 bytes external = 4 bytes total t2 ing com.lxu.th.A object internals: OFFSET SIZE TYPE DESCRIPTION VALUE 0 4 (object header) 05 d9 bc 1f (00000101 11011001 10111100 00011111) (532470021) //达到20次以后，都为偏向锁，且偏向的线程ID 532470021 变了 4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0) 8 4 (object header) 43 c1 00 f8 (01000011 11000001 00000000 11111000) (-134168253) 12 4 (loss due to the next object alignment) Instance size: 16 bytes Space losses: 0 bytes internal + 4 bytes external = 4 bytes total t2 after //释放锁后 com.lxu.th.A object internals: OFFSET SIZE TYPE DESCRIPTION VALUE 0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1) // 01 无锁 4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0) 8 4 (object header) 43 c1 00 f8 (01000011 11000001 00000000 11111000) (-134168253) 12 4 (loss due to the next object alignment) Instance size: 16 bytes Space losses: 0 bytes internal + 4 bytes external = 4 bytes total