java并发synchronized 锁的膨胀过程（锁的升级过程）深入剖析(2)

接下来我们分析两个批量偏向撤销的相关案例（禁止偏向锁延迟的情况下：-XX:+UseBiasedLocking -XX:BiasedLockingStartupDelay=0）：

案例一：

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public class TestDemo {

}

public class DemoExample6 {

public static void main(String[] args) throws InterruptedException {

test2();

}

/**

* 证明偏量偏向撤销

* @throws InterruptedException

*/

public static void test2() throws InterruptedException {

List<TestDemo> list = new ArrayList<TestDemo>();

for (int i = 0; i < 100; i++) {

list.add(new TestDemo());

}

Thread t1 = new Thread(()->{

System.out.println("加锁前 get(0) 应该是无锁可偏向 "+ClassLayout.parseInstance(list.get(0)).toPrintable());

for (TestDemo a:list ) {

synchronized (a){

System.out.print("加锁 >");

}

System.out.println();

System.out.println("加锁后 get(0) 应该是偏向锁"+ClassLayout.parseInstance(list.get(0)).toPrintable());

try {

TimeUnit.SECONDS.sleep(1000);//这里不让线程死，防止线程ID复用

} catch (InterruptedException e) {

e.printStackTrace();

}

});

t1.start();

TimeUnit.SECONDS.sleep(5);

Thread t2 = new Thread(()->{

for (int i = 0; i < 100; i++) {

TestDemo a = list.get(i);

synchronized (a){

System.out.println(Thread.currentThread().getId()+"加锁 >");

}

try {

TimeUnit.MILLISECONDS.sleep(100);

} catch (InterruptedException e) {

e.printStackTrace();

}

if (i==9){//这里刚好是第19个上锁的（同样是第19个偏向锁升级的）

System.out.println();

System.out.println("加锁后 get(9) 应该是无锁（轻量级锁释放） "+ClassLayout.parseInstance(list.get(i)).toPrintable());

}

if (i==10){//这里刚好是第21个上锁的

System.out.println();

System.out.println("加锁后 get(10) 应该是偏向锁偏向t2 "+ClassLayout.parseInstance(list.get(i)).toPrintable());

}

if (i==50){//50开始升级为轻量级锁（同样是第21个偏向锁升级的）

System.out.println();

System.out.println("加锁后 get(50) 无锁（轻量级锁释放） "+ClassLayout.parseInstance(list.get(i)).toPrintable());

}

if (i==59){//60（同样是第39个偏向锁升级的）

System.out.println();

System.out.println("加锁后 get(59) 无锁（轻量级锁释放） "+ClassLayout.parseInstance(list.get(i)).toPrintable());

}

if (i==69){//69（同样是第59个偏向锁升级的）

System.out.println();

System.out.println("加锁后 get(69) 无锁（轻量级锁释放） "+ClassLayout.parseInstance(list.get(i)).toPrintable());

TestDemo a1 = new TestDemo();

synchronized (a1){

System.out.println("偏向撤销发生后的该类新建的对象都不会再偏向任何线程 "+ClassLayout.parseInstance(a1).toPrintable());

}

});

Thread t3 = new Thread(()->{

for (int i = 99; i >= 0; i--) {

TestDemo a = list.get(i);

synchronized (a){

System.out.println(Thread.currentThread().getId()+"加锁 >");

}

try {

TimeUnit.MILLISECONDS.sleep(100);

} catch (InterruptedException e) {

e.printStackTrace();

}

/**

* 重点：重偏向撤销

*/

if (i==40){//40升级为轻量级锁（同样是第40个偏向锁升级的，这时候发生偏向撤销）

System.out.println();

System.out.println("加锁后 get("+i+") 应该是无锁（轻量级锁释放） "+ClassLayout.parseInstance(list.get(0)).toPrintable());

TestDemo a1 = new TestDemo();

synchronized (a1){

System.out.println("偏向撤销发生后的该类新建的对象都不会再偏向任何线程 "+ClassLayout.parseInstance(a1).toPrintable());

}

if (i==30){//39升级为轻量级锁（同样是第42个偏向锁升级的）

System.out.println();

System.out.println("加锁后 get("+i+") 应该是无锁（轻量级锁释放） "+ClassLayout.parseInstance(list.get(0)).toPrintable());

TestDemo a1 = new TestDemo();

synchronized (a1){

System.out.println("偏向撤销发生后的该类新建的对象都不会再偏向任何线程 "+ClassLayout.parseInstance(a1).toPrintable());

}

});

t2.start();

TimeUnit.MILLISECONDS.sleep(50);

t3.start();

}

运行结果（截取部分）：

加锁前 get(0) 应该是无锁可偏向 com.boke.TestDemo 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) 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

加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >加锁 >

加锁后 get(0) 应该是偏向锁com.boke.TestDemo object internals:

OFFSET SIZE TYPE DESCRIPTION VALUE

0 4 (object header) 05 e0 84 08 (00000101 11100000 10000100 00001000) (142925829)

4 4 (object header) b1 7f 00 00 (10110001 01111111 00000000 00000000) (32689)

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

加锁后 get(9) 应该是无锁（轻量级锁释放） com.boke.TestDemo 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 (loss due to the next object alignment)

Instance size: 16 bytes

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

15加锁 >

加锁后 get(90) 应该是偏向锁偏向t3com.boke.TestDemo object internals:

OFFSET SIZE TYPE DESCRIPTION VALUE

0 4 (object header) 05 89 01 0c (00000101 10001001 00000001 00001100) (201427205)

4 4 (object header) b1 7f 00 00 (10110001 01111111 00000000 00000000) (32689)

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

加锁后 get(10) 应该是偏向锁偏向t2 com.boke.TestDemo object internals:

OFFSET SIZE TYPE DESCRIPTION VALUE

0 4 (object header) 05 b1 0a 08 (00000101 10110001 00001010 00001000) (134918405)

4 4 (object header) b1 7f 00 00 (10110001 01111111 00000000 00000000) (32689)

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

15加锁 >

加锁后 get(89) 应该是无锁（轻量级锁释放） com.boke.TestDemo 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 (loss due to the next object alignment)

Instance size: 16 bytes

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

加锁后 get(50) 无锁（轻量级锁释放） com.boke.TestDemo 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 (loss due to the next object alignment)

Instance size: 16 bytes

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

15加锁 >

加锁后 get(49) 应该是无锁（轻量级锁释放） com.boke.TestDemo 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 (loss due to the next object alignment)

Instance size: 16 bytes

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

加锁后 get(59) 无锁（轻量级锁释放） com.boke.TestDemo 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 (loss due to the next object alignment)

Instance size: 16 bytes

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

15加锁 >

加锁后 get(40) 应该是无锁（轻量级锁释放） com.boke.TestDemo 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 (loss due to the next object alignment)

Instance size: 16 bytes

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

偏向撤销发生后的该类新建的对象都不会再偏向任何线程 com.boke.TestDemo object internals:

OFFSET SIZE TYPE DESCRIPTION VALUE

0 4 (object header) 48 18 a6 09 (01001000 00011000 10100110 00001001) (161880136)

4 4 (object header) 00 70 00 00 (00000000 01110000 00000000 00000000) (28672)

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

加锁后 get(69) 无锁（轻量级锁释放） com.boke.TestDemo 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 (loss due to the next object alignment)

Instance size: 16 bytes

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

偏向撤销发生后的该类新建的对象都不会再偏向任何线程 com.boke.TestDemo object internals:

OFFSET SIZE TYPE DESCRIPTION VALUE

0 4 (object header) 50 e8 95 09 (01010000 11101000 10010101 00001001) (160819280)

4 4 (object header) 00 70 00 00 (00000000 01110000 00000000 00000000) (28672)

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

15加锁 >

加锁后 get(30) 应该是无锁（轻量级锁释放） com.boke.TestDemo 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 (loss due to the next object alignment)

Instance size: 16 bytes

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

偏向撤销发生后的该类新建的对象都不会再偏向任何线程 com.boke.TestDemo object internals:

OFFSET SIZE TYPE DESCRIPTION VALUE

0 4 (object header) 48 18 a6 09 (01001000 00011000 10100110 00001001) (161880136)

4 4 (object header) 00 70 00 00 (00000000 01110000 00000000 00000000) (28672)

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

案例二：

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public class TestDemo {

}

public class DemoExample8 {

public static void main(String[] args) throws Exception {

List<TestDemo> list = new ArrayList<>();

List<TestDemo> list2 = new ArrayList<>();

List<TestDemo> list3 = new ArrayList<>();

for (int i = 0; i < 100; i++) {

list.add(new TestDemo());

list2.add(new TestDemo());

list3.add(new TestDemo());

}

//偏向锁

System.out.println("初始状态" + 10 + ClassLayout.parseClass(TestDemo.class).toPrintable());

Thread t1 = new Thread() {

String name = "1";

public void run() {

System.out.printf(name);

for (TestDemo a : list) {

synchronized (a) {

if (a == list.get(10)) {

//偏向锁

System.out.println("t1 预期是偏向锁" + 10 + ClassLayout.parseInstance(a).toPrintable());

}

try {

Thread.sleep(100000);

} catch (InterruptedException e) {

e.printStackTrace();

}

};

t1.start();

Thread.sleep(5000);

//偏向锁

System.out.println("main 预期是偏向锁" + 10 + ClassLayout.parseInstance(list.get(10)).toPrintable());

Thread t2 = new Thread() {

String name = "2";

public void run() {

System.out.printf(name);

for (int i = 0; i < 100; i++) {

TestDemo a = list.get(i);

synchronized (a) {

if (a == list.get(10)) {

System.out.println("t2 i=10 get(1)预期是无锁" + ClassLayout.parseInstance(list.get(1)).toPrintable());//偏向锁

System.out.println("t2 i=10 get(10) 预期轻量级锁 " + i + ClassLayout.parseInstance(a).toPrintable());//偏向锁

}

if (a == list.get(19)) {

System.out.println("t2 i=19 get(10)预期是无锁" + 10 + ClassLayout.parseInstance(list.get(10)).toPrintable());//偏向锁

System.out.println("t2 i=19 get(19) 满足重偏向条件20 预期偏向锁 " + i + ClassLayout.parseInstance(a).toPrintable());//偏向锁

System.out.println("类的对象累计撤销达到20");

}

try {

Thread.sleep(100000);

} catch (InterruptedException e) {

e.printStackTrace();

}

};

t2.start();

Thread.sleep(5000);

Thread t3 = new Thread() {

String name = "3";

public void run() {

System.out.printf(name);

for (TestDemo a : list2) {

synchronized (a) {

if (a == list2.get(10)) {

System.out.println("t3 预期是偏向锁" + 10 + ClassLayout.parseInstance(a).toPrintable());//偏向锁

}

try {

Thread.sleep(100000);

} catch (InterruptedException e) {

e.printStackTrace();

}

};

t3.start();

Thread.sleep(5000);

Thread t4 = new Thread() {

String name = "4";

public void run() {

System.out.printf(name);

for (int i = 0; i < 100; i++) {

TestDemo a = list2.get(i);

synchronized (a) {

if (a == list2.get(10)) {

System.out.println("t4 i=10 get(1)预期是无锁" + ClassLayout.parseInstance(list2.get(1)).toPrintable());//偏向锁

System.out.println("t4 i=10 get(10) 当前不满足重偏向条件 20 预期轻量级锁 " + i + ClassLayout.parseInstance(a).toPrintable());//偏向锁

}

if (a == list2.get(19)) {

System.out.println("t4 i=19 get(10)预期是无锁" + 10 + ClassLayout.parseInstance(list2.get(10)).toPrintable());//偏向锁

System.out.println("t4 i=19 get(19) 当前满足重偏向条件 20 但A类的对象累计撤销达到40 预期轻量级锁 " + i + ClassLayout.parseInstance(a).toPrintable());//偏向锁

System.out.println("类的对象累计撤销达到40");

}

if (a == list2.get(20)) {

System.out.println("t4 i=20 get(20) 当前满足重偏向条件 20 预期轻量级锁 " + i + ClassLayout.parseInstance(a).toPrintable());//偏向锁

}

};

t4.start();

Thread.sleep(5000);

System.out.println("main 预期是偏向锁" + 10 + ClassLayout.parseInstance(list3.get(0)).toPrintable());//偏向锁

Thread t5 = new Thread() {

String name = "5";

public void run() {

System.out.printf(name);

for (TestDemo a : list3) {

synchronized (a) {

if (a == list3.get(10)) {

System.out.println("t5 预期是轻量级锁，类的对象累计撤销达到40 不可以用偏向锁了" + 10 + ClassLayout.parseInstance(a).toPrintable());//偏向锁

}

try {

Thread.sleep(100000);

} catch (InterruptedException e) {

e.printStackTrace();

}

};

t5.start();

Thread.sleep(5000);

System.out.println("main 预期是偏向锁" + 10 + ClassLayout.parseInstance(list.get(10)).toPrintable());//偏向锁

Thread t6 = new Thread() {

String name = "6";

public void run() {

System.out.printf(name);

for (int i = 0; i < 100; i++) {

TestDemo a = list3.get(i);

synchronized (a) {

if (a == list3.get(10)) {

System.out.println("t6 i=10 get(1)预期是无锁" + ClassLayout.parseInstance(list3.get(1)).toPrintable());//偏向锁

System.out.println("t6 i=10 get(10) 预期轻量级锁 " + i + ClassLayout.parseInstance(a).toPrintable());//偏向锁

}

if (a == list3.get(19)) {

System.out.println("t6 i=19 get(10)预期是无锁" + 10 + ClassLayout.parseInstance(list3.get(10)).toPrintable());//偏向锁

System.out.println("t6 i=19 get(19) 满足重偏向条件20 但类的对象累计撤销达到40 不可以用偏向锁了 " + i + ClassLayout.parseInstance(a).toPrintable());//偏向锁

}

try {

Thread.sleep(100000);

} catch (InterruptedException e) {

e.printStackTrace();

}

};

t6.start();

Thread.sleep(5000);

System.out.println("由于撤销锁次数达到默认的 BiasedLockingBulkRevokeThreshold=40 这里实例化的对象是无锁状态" + ClassLayout.parseInstance(new TestDemo()).toPrintable());//偏向锁

　　　　 System.out.println("撤销偏向后状态" + 10 + ClassLayout.parseInstance(new TestDemo()).toPrintable());//偏向锁

　　}

运行结果：

初始状态10 com.boke.TestDemo object internals:

OFFSET SIZE TYPE DESCRIPTION VALUE

0 12 (object header) N/A

12 4 (loss due to the next object alignment)

Instance size: 16 bytes

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

1t1 预期是偏向锁10 com.boke.TestDemo object internals:

OFFSET SIZE TYPE DESCRIPTION VALUE

0 4 (object header) 05 e0 86 8e (00000101 11100000 10000110 10001110) (-1903763451)

4 4 (object header) ec 7f 00 00 (11101100 01111111 00000000 00000000) (32748)

8 4 (object header) bf c3 00 f8 (10111111 11000011 00000000 11111000) (-134167617)

12 4 (loss due to the next object alignment)

Instance size: 16 bytes

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

main 预期是偏向锁10 com.boke.TestDemo object internals:

OFFSET SIZE TYPE DESCRIPTION VALUE

0 4 (object header) 05 e0 86 8e (00000101 11100000 10000110 10001110) (-1903763451)

4 4 (object header) ec 7f 00 00 (11101100 01111111 00000000 00000000) (32748)

8 4 (object header) bf c3 00 f8 (10111111 11000011 00000000 11111000) (-134167617)

12 4 (loss due to the next object alignment)

Instance size: 16 bytes

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

2t2 i=10 get(1)预期是无锁com.boke.TestDemo 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) bf c3 00 f8 (10111111 11000011 00000000 11111000) (-134167617)

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 i=10 get(10) 预期轻量级锁 10 com.boke.TestDemo object internals:

OFFSET SIZE TYPE DESCRIPTION VALUE

0 4 (object header) 08 99 7a 03 (00001000 10011001 01111010 00000011) (58366216)

4 4 (object header) 00 70 00 00 (00000000 01110000 00000000 00000000) (28672)

8 4 (object header) bf c3 00 f8 (10111111 11000011 00000000 11111000) (-134167617)

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 i=19 get(10)预期是无锁10 com.boke.TestDemo 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) bf c3 00 f8 (10111111 11000011 00000000 11111000) (-134167617)

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 i=19 get(19) 满足重偏向条件20 预期偏向锁 19com.boke.TestDemo object internals:

OFFSET SIZE TYPE DESCRIPTION VALUE

0 4 (object header) 05 09 90 91 (00000101 00001001 10010000 10010001) (-1852831483)

4 4 (object header) ec 7f 00 00 (11101100 01111111 00000000 00000000) (32748)

8 4 (object header) bf c3 00 f8 (10111111 11000011 00000000 11111000) (-134167617)

12 4 (loss due to the next object alignment)

Instance size: 16 bytes

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

类的对象累计撤销达到20

3t3 预期是偏向锁10com.boke.TestDemo object internals:

OFFSET SIZE TYPE DESCRIPTION VALUE

0 4 (object header) 05 09 89 90 (00000101 00001001 10001001 10010000) (-1870067451)

4 4 (object header) ec 7f 00 00 (11101100 01111111 00000000 00000000) (32748)

8 4 (object header) bf c3 00 f8 (10111111 11000011 00000000 11111000) (-134167617)

12 4 (loss due to the next object alignment)

Instance size: 16 bytes

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

4t4 i=10 get(1)预期是无锁com.boke.TestDemo 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) bf c3 00 f8 (10111111 11000011 00000000 11111000) (-134167617)

12 4 (loss due to the next object alignment)

Instance size: 16 bytes

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

t4 i=10 get(10) 当前不满足重偏向条件 20 预期轻量级锁 10com.boke.TestDemo object internals:

OFFSET SIZE TYPE DESCRIPTION VALUE

0 4 (object header) 08 f9 9a 03 (00001000 11111001 10011010 00000011) (60487944)

4 4 (object header) 00 70 00 00 (00000000 01110000 00000000 00000000) (28672)

8 4 (object header) bf c3 00 f8 (10111111 11000011 00000000 11111000) (-134167617)

12 4 (loss due to the next object alignment)

Instance size: 16 bytes

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

t4 i=19 get(10)预期是无锁10com.boke.TestDemo 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) bf c3 00 f8 (10111111 11000011 00000000 11111000) (-134167617)

12 4 (loss due to the next object alignment)

Instance size: 16 bytes

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

t4 i=19 get(19) 当前满足重偏向条件 20 但A类的对象累计撤销达到40 预期轻量级锁 19com.boke.TestDemo object internals:

OFFSET SIZE TYPE DESCRIPTION VALUE

0 4 (object header) 08 f9 9a 03 (00001000 11111001 10011010 00000011) (60487944)

4 4 (object header) 00 70 00 00 (00000000 01110000 00000000 00000000) (28672)

8 4 (object header) bf c3 00 f8 (10111111 11000011 00000000 11111000) (-134167617)

12 4 (loss due to the next object alignment)

Instance size: 16 bytes

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

类的对象累计撤销达到40

t4 i=20 get(20) 当前满足重偏向条件 20 预期轻量级锁 20com.boke.TestDemo object internals:

OFFSET SIZE TYPE DESCRIPTION VALUE

0 4 (object header) 08 f9 9a 03 (00001000 11111001 10011010 00000011) (60487944)

4 4 (object header) 00 70 00 00 (00000000 01110000 00000000 00000000) (28672)

8 4 (object header) bf c3 00 f8 (10111111 11000011 00000000 11111000) (-134167617)

12 4 (loss due to the next object alignment)

Instance size: 16 bytes

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

main 预期是偏向锁10com.boke.TestDemo 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) bf c3 00 f8 (10111111 11000011 00000000 11111000) (-134167617)

12 4 (loss due to the next object alignment)

Instance size: 16 bytes

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

5t5 预期是轻量级锁，A类的对象累计撤销达到40 不可以用偏向锁了10com.boke.TestDemo object internals:

OFFSET SIZE TYPE DESCRIPTION VALUE

0 4 (object header) 08 f9 9a 03 (00001000 11111001 10011010 00000011) (60487944)

4 4 (object header) 00 70 00 00 (00000000 01110000 00000000 00000000) (28672)

8 4 (object header) bf c3 00 f8 (10111111 11000011 00000000 11111000) (-134167617)

12 4 (loss due to the next object alignment)

Instance size: 16 bytes

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

main 预期是偏向锁10com.boke.TestDemo 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) bf c3 00 f8 (10111111 11000011 00000000 11111000) (-134167617)

12 4 (loss due to the next object alignment)

Instance size: 16 bytes

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

6t6 i=10 get(1)预期是无锁com.boke.TestDemo 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) bf c3 00 f8 (10111111 11000011 00000000 11111000) (-134167617)

12 4 (loss due to the next object alignment)

Instance size: 16 bytes

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

t6 i=10 get(10) 预期轻量级锁 10com.boke.TestDemo object internals:

OFFSET SIZE TYPE DESCRIPTION VALUE

0 4 (object header) 08 29 ab 03 (00001000 00101001 10101011 00000011) (61548808)

4 4 (object header) 00 70 00 00 (00000000 01110000 00000000 00000000) (28672)

8 4 (object header) bf c3 00 f8 (10111111 11000011 00000000 11111000) (-134167617)

12 4 (loss due to the next object alignment)

Instance size: 16 bytes

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

t6 i=19 get(10)预期是无锁10com.boke.TestDemo 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) bf c3 00 f8 (10111111 11000011 00000000 11111000) (-134167617)

12 4 (loss due to the next object alignment)

Instance size: 16 bytes

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

t6 i=19 get(19) 满足重偏向条件20 但A类的对象累计撤销达到40 不可以用偏向锁了 19com.boke.TestDemo object internals:

OFFSET SIZE TYPE DESCRIPTION VALUE

0 4 (object header) 08 29 ab 03 (00001000 00101001 10101011 00000011) (61548808)

4 4 (object header) 00 70 00 00 (00000000 01110000 00000000 00000000) (28672)

8 4 (object header) bf c3 00 f8 (10111111 11000011 00000000 11111000) (-134167617)

12 4 (loss due to the next object alignment)

Instance size: 16 bytes

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

由于类撤销锁次数达到默认的 BiasedLockingBulkRevokeThreshold=40 这里实例化的对象是无锁状态com.boke.TestDemo 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) bf c3 00 f8 (10111111 11000011 00000000 11111000) (-134167617)

12 4 (loss due to the next object alignment)

Instance size: 16 bytes

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

撤销偏向后状态10com.boke.TestDemo 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) 　　　　　　　　　　　　　　 bf c3 00 f8 (10111111 11000011 00000000 11111000) (-134167617)
　　12　　 4 　　　　　　 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total

以上案例证实了偏向锁的批量重偏向和批量撤销，接下来我们讲解轻量级锁；

轻量级锁：

当锁是偏向锁的时候，被另外的线程所访问，偏向锁就会升级为轻量级锁，其他线程会通过自旋的形式尝试获取锁，不会阻塞，从而提高性能。
在代码进入同步块的时候，如果同步对象锁状态为无锁状态（锁标志位为“01”状态，是否为偏向锁为“0”），虚拟机首先将在当前线程的栈帧中建立一个名为锁记录（Lock Record）的空间，用于存储锁对象目前的Mark Word的拷贝，然后拷贝对象头中的Mark Word复制到锁记录中。
拷贝成功后，虚拟机将使用CAS操作尝试将对象的Mark Word更新为指向Lock Record的指针，并将Lock Record里的owner指针指向对象的Mark Word。
如果这个更新动作成功了，那么这个线程就拥有了该对象的锁，并且对象Mark Word的锁标志位设置为“00”，表示此对象处于轻量级锁定状态。
如果轻量级锁的更新操作失败了，虚拟机首先会检查对象的Mark Word是否指向当前线程的栈帧，如果是就说明当前线程已经拥有了这个对象的锁，那就可以直接进入同步块继续执行，否则说明多个线程竞争锁。
若当前只有一个等待线程，则该线程通过自旋进行等待。但是当自旋超过一定的次数，或者一个线程在持有锁，一个在自旋，又有第三个来访时，轻量级锁升级为重量级锁。
多个线程在不同的时间段请求同一把锁，也就是说没有锁竞争。针对这种情形，Java 虚拟机采用了轻量级锁，来避免重量级锁的阻塞以及唤醒
在没有锁竞争的前提下，减少传统锁使用OS互斥量产生的性能损耗
在竞争激烈时，轻量级锁会多做很多额外操作，导致性能下降
可以认为两个线程交替执行的情况下请求同一把锁

分析一个由偏向锁膨胀成轻量级锁的案例：

public class TestDemo {

}

public class DemoExample9 {

public static void main(String[] args) throws Exception {

TestDemo testDemo = new TestDemo();

//子线程

Thread t1 = new Thread(){

@Override

public void run() {

synchronized (testDemo){

System.out.println("t1 lock ing");

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

}

};

t1.join();

//主线程

synchronized (testDemo){

System.out.println("main lock ing");

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

}

运行结果（两个线程交替执行的情况下）：

main lock ing

com.boke.TestDemo object internals:

OFFSET SIZE TYPE DESCRIPTION VALUE

0 4 (object header) e8 48 95 09 (11101000 01001000 10010101 00001001) (160778472)

4 4 (object header) 00 70 00 00 (00000000 01110000 00000000 00000000) (28672)

8 4 (object header) a0 c1 00 f8 (10100000 11000001 00000000 11111000) (-134168160)

12 4 (loss due to the next object alignment)

Instance size: 16 bytes

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

重量级锁：

多个线程竞争同一个锁的时候，虚拟机会阻塞加锁失败的线程，并且在目标锁被释放的时候，唤醒这些线程；

Java 线程的阻塞以及唤醒，都是依靠操作系统来完成的：os pthread_mutex_lock() ；

升级为重量级锁时，锁标志的状态值变为“10”，此时Mark Word中存储的是指向重量级锁的指针，此时等待锁的线程都会进入阻塞状态

分析一个由轻量级锁膨胀成重量级锁的案例：

public class TestDemo {

}

public class DemoExample9 {

public static void main(String[] args) throws Exception {

TestDemo testDemo = new TestDemo();

Thread t1 = new Thread(){

@Override

public void run() {

synchronized (testDemo){

System.out.println("t1 lock ing");

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

}

};

t1.start();

synchronized (testDemo){

System.out.println("main lock ing");

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

}

运行结果：

main lock ing

com.boke.TestDemo object internals:

OFFSET SIZE TYPE DESCRIPTION VALUE

0 4 (object header) 5a ad 00 b0 (01011010 10101101 00000000 10110000) (-1342132902)

4 4 (object header) cf 7f 00 00 (11001111 01111111 00000000 00000000) (32719)

8 4 (object header) a0 c1 00 f8 (10100000 11000001 00000000 11111000) (-134168160)

12 4 (loss due to the next object alignment)

Instance size: 16 bytes

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

t1 lock ing

com.boke.TestDemo object internals:

OFFSET SIZE TYPE DESCRIPTION VALUE

0 4 (object header) 5a ad 00 b0 (01011010 10101101 00000000 10110000) (-1342132902)

4 4 (object header) cf 7f 00 00 (11001111 01111111 00000000 00000000) (32719)

8 4 (object header) a0 c1 00 f8 (10100000 11000001 00000000 11111000) (-134168160)

12 4 (loss due to the next object alignment)

我们再来说一下Java 虚拟机是怎么区分轻量级锁和重量级锁的：

当进行加锁操作时，Java 虚拟机会判断是否已经是重量级锁。如果不是，它会在当前线程的当前栈桢中划出一块空间，作为该锁的锁记录，并且将锁对象的标记字段复制到该锁记录中。
然后，Java 虚拟机会尝试用 CAS（compare-and-swap）操作替换锁对象的标记字段。这里解释一下，CAS 是一个原子操作，它会比较目标地址的值是否和期望值相等，如果相等，则替换为一个新的值。
假设当前锁对象的标记字段为 X…XYZ，Java 虚拟机会比较该字段是否为 X…X01。如果是，则替换为刚才分配的锁记录的地址。由于内存对齐的缘故，它的最后两位为 00。此时，该线程已成功获得这把锁，可以继续执行了。
如果不是 X…X01，那么有两种可能。第一，该线程重复获取同一把锁。此时，Java 虚拟机会将锁记录清零，以代表该锁被重复获取。第二，其他线程持有该锁。此时，Java 虚拟机会将这把锁膨胀为重量级锁，并且阻塞当前线程。
当进行解锁操作时，如果当前锁记录（你可以将一个线程的所有锁记录想象成一个栈结构，每次加锁压入一条锁记录，解锁弹出一条锁记录，当前锁记录指的便是栈顶的锁记录）的值为 0，则代表重复进入同一把锁，直接返回即可。
否则，Java 虚拟机会尝试用 CAS 操作，比较锁对象的标记字段的值是否为当前锁记录的地址。如果是，则替换为锁记录中的值，也就是锁对象原本的标记字段。此时，该线程已经成
功释放这把锁。
如果不是，则意味着这把锁已经被膨胀为重量级锁。此时，Java 虚拟机会进入重量级锁的释放过程，唤醒因竞争该锁而被阻塞了的线程

到此为止本篇就讲完了锁的膨胀过程：

总结一下：

偏向锁只会在第一次请求时采用 CAS 操作，在锁对象的标记字段中记录下当前线程的地址。在之后的运行过程中，持有该偏向锁的线程的加锁操作将直接返回。它针对的是锁仅会被同一线程持有的情况。
轻量级锁采用 CAS 操作，将锁对象的标记字段替换为一个指针，指向当前线程栈上的一块空间，存储着锁对象原本的标记字段。它针对的是多个线程在不同时间段申请同一把锁的情况。
重量级锁会阻塞、唤醒请求加锁的线程。它针对的是多个线程同时竞争同一把锁的情况。Java 虚拟机采取了自适应自旋，来避免线程在面对非常小的 synchronized 代码块时，仍会被阻塞、唤醒的情况。

说完了锁的膨胀过程，那么会不会有锁的降级呢？

我在hotspot源码中找到了这样的注释：

// We create a list of in-use monitors for each thread.//// deflate_thread_local_monitors() scans a single thread's in-use list, while// deflate_idle_monitors() scans only a global list of in-use monitors which// is populated only as a thread dies (see omFlush()).//// These operations are called at all safepoints, immediately after mutators// are stopped, but before any objects have moved. Collectively they traverse// the population of in-use monitors, deflating where possible. The scavenged// monitors are returned to the monitor free list.//// Beware that we scavenge at *every* stop-the-world point. Having a large// number of monitors in-use could negatively impact performance. We also want// to minimize the total # of monitors in circulation, as they incur a small// footprint penalty.//// Perversely, the heap size -- and thus the STW safepoint rate --// typically drives the scavenge rate.  Large heaps can mean infrequent GC,// which in turn can mean large(r) numbers of objectmonitors in circulation.// This is an unfortunate aspect of this design.//大概意思是：锁降级确实是会发生的，当 JVM 进入安全点（SafePoint）的时候，会检查是否有闲置的 Monitor，然后试图进行降级有兴趣的大佬可以在https://hg.openjdk.java.net/jdk/jdk/file/896e80158d35/src/hotspot/share/runtime/synchronizer.cpp链接中：研究一下deflate_idle_monitors是分析锁降级逻辑的入口，这部分行为还在进行持续改进，因为其逻辑是在安全点内运行，处理不当可能拖长 JVM 停顿（STW，stop-the-world）的时间。