synchronized锁升级的过程
synchronized锁一共有四种状态，级别从低到高依次是：无锁状态，偏向锁状态，轻量级锁状态和重量级锁状态，这几个状态随着竞争情况逐渐升级，为了提高获得锁和释放锁的效率，锁可以升级但不能降级，意味着偏向锁升级为轻量级锁后不能降级为偏向锁，这种升级过程叫做锁膨胀
无锁状态：未拥有线程抢占锁
偏向锁：当线程抢占偏向锁，也就是把markword的线程ID改为当前抢占锁的线程ID的过程。
轻量级锁：当多个线程竞争一个锁时，升级轻量级锁，线程在自己的线程栈帧中会创建一个LockRecord，用CAS操作把markword设置为指向自己这个线程的LR的指针，设置成功后表示抢占到锁。
重量级锁：当竞争的线程次数超过10次，升级为重量级锁，用户状态调用内核态等待，并且把阻塞线程添加添加到等待队列当中。效率慢
synchronized锁升级解析图
对象在JVM当中分配内存图
一个Java对象在内存中包括3个部分：对象头、实例数据和对齐填充
对象头：
mark word存储的是一些标记位【哈希值，分代年龄，锁状态】 8个字节
class Pointer 记录类信息【指向对象对应的类元数据的内存地址】 8字节
length 数组长度 4字节
实例数据：根据不同的类型决定不同的字节
对齐补充：为保证对象的大小为8字节的整数倍【为了是让cup解析和寻址方便补充】
在64位虚拟机下，Mark Word是64bit大小的
代码实现 [ 请对质Mark Word是64bit大小的图当中的 偏向锁和锁标志位 状态码观看 ]
无锁状态 - > 轻量级锁状态
public class Test { public static void main(String[] args) { Test test = new Test(); System.out.println("加锁前=========="); System.out.println(ClassLayout.parseInstance(test).toPrintable()); synchronized (test) { System.out.println("加锁后=========="); System.out.println(ClassLayout.parseInstance(test).toPrintable()); } } } 加锁前========== com.gupaoedu.vip.pattern.observer.Test object internals: OFFSET SIZE TYPE DESCRIPTION VALUE 0 4 (object header) 09 00 00 00 (00001001 00000000 00000000 00000000) (9) 4 4 (object header) b0 22 e3 14 (10110000 00100010 11100011 00010100) (350429872) Instance size: 8 bytes Space losses: 0 bytes internal + 0 bytes external = 0 bytes total 加锁后========== com.gupaoedu.vip.pattern.observer.Test object internals: OFFSET SIZE TYPE DESCRIPTION VALUE 0 4 (object header) c4 f8 c1 00 (11000100 11111000 11000001 00000000) (12712132) 4 4 (object header) b0 22 e3 14 (10110000 00100010 11100011 00010100) (350429872) Instance size: 8 bytes Space losses: 0 bytes internal + 0 bytes external = 0 bytes total org.openjdk.jol jol-core 0.9
偏向锁 - > 偏向锁
配置偏向锁延迟加载配置 -XX:BiasedLockingStartupDelay=0
public class Test {
public static void main(String[] args) {
Test test = new Test();
System.out.println("加锁前==========");
System.out.println(ClassLayout.parseInstance(test).toPrintable());
synchronized (test) {
System.out.println("加锁后==========");
System.out.println(ClassLayout.parseInstance(test).toPrintable());
}
// 这里的第一个对象和第二个对象的锁状态都是101，是因为偏向锁打开状态下，默认会有配置匿名的对象获得偏向锁。
}
}
加锁前==========
com.gupaoedu.vip.pattern.observer.Test object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 0d 00 00 00 (00001101 00000000 00000000 00000000) (13)
4 4 (object header) b0 22 a3 15 (10110000 00100010 10100011 00010101) (363012784)
Instance size: 8 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
加锁后==========
com.gupaoedu.vip.pattern.observer.Test object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 0d dc 4e 03 (00001101 11011100 01001110 00000011) (55499789)
4 4 (object header) b0 22 a3 15 (10110000 00100010 10100011 00010101) (363012784)
Instance size: 8 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
无锁状态 - > 轻量级 - > 重量级 【未开启偏向锁默认0秒启动】
public class Test {
public static void main(String[] args) {
Test test = new Test();
System.out.println("加锁前==========");
System.out.println(ClassLayout.parseInstance(test).toPrintable());
new Thread(() -> {
synchronized (test) {
System.out.println("多线程抢锁状态==========");
System.out.println(ClassLayout.parseInstance(test).toPrintable());
}
}).start();
synchronized (test) {
System.out.println("mian线程加锁后==========");
System.out.println(ClassLayout.parseInstance(test).toPrintable());
}
}
}
加锁前==========
com.gupaoedu.vip.pattern.observer.Test object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 09 00 00 00 (00001001 00000000 00000000 00000000) (9)
4 4 (object header) a8 24 63 15 (10101000 00100100 01100011 00010101) (358818984)
Instance size: 8 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
mian线程加锁后==========
com.gupaoedu.vip.pattern.observer.Test object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 24 f9 3d 01 (00100100 11111001 00111101 00000001) (20838692)
4 4 (object header) a8 24 63 15 (10101000 00100100 01100011 00010101) (358818984)
Instance size: 8 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
多线程抢锁状态==========
com.gupaoedu.vip.pattern.observer.Test object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 1e 93 87 15 (00011110 10010011 10000111 00010101) (361206558)
4 4 (object header) a8 24 63 15 (10101000 00100100 01100011 00010101) (358818984)
Instance size: 8 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
无锁状态 - > 偏向锁 - > 重量级 【开启偏向锁默认0秒启动】
public class Test {
public static void main(String[] args) {
Test test = new Test();
System.out.println("加锁前==========");
System.out.println(ClassLayout.parseInstance(test).toPrintable());
new Thread(() -> {
synchronized (test) {
System.out.println("多线程抢锁状态==========");
System.out.println(ClassLayout.parseInstance(test).toPrintable());
}
}).start();
synchronized (test) {
System.out.println("mian线程加锁后==========");
System.out.println(ClassLayout.parseInstance(test).toPrintable());
}
}
}
加锁前==========
com.gupaoedu.vip.pattern.observer.Test object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 0d 00 00 00 (00001101 00000000 00000000 00000000) (13)
4 4 (object header) a8 24 ec 14 (10101000 00100100 11101100 00010100) (351020200)
Instance size: 8 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
mian线程加锁后==========
com.gupaoedu.vip.pattern.observer.Test object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 0d dc a0 02 (00001101 11011100 10100000 00000010) (44096525)
4 4 (object header) a8 24 ec 14 (10101000 00100100 11101100 00010100) (351020200)
Instance size: 8 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
多线程抢锁状态==========
com.gupaoedu.vip.pattern.observer.Test object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 5e 91 19 15 (01011110 10010001 00011001 00010101) (353997150)
4 4 (object header) a8 24 ec 14 (10101000 00100100 11101100 00010100) (351020200)
Instance size: 8 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
