对象头markword-锁升级

锁状态的“可视化”实践

通过工具查看对象的内存布局

MAVEN引入


<dependency>
  <groupId>org.openjdk.jolgroupId>
  <artifactId>jol-coreartifactId>
  <version>0.10version>   
dependency>


package cn.ly.test;
import org.openjdk.jol.info.ClassLayout;
public class JavaObjLayout {
    int[] aa = new int[3];
    public static void main(String[] args) {
        A a = new A();
        B b = new B();
        C c = new C();
        int[] aa = new int[3];
        //查看对象内部信息`
        System.out.println(ClassLayout.
                parseInstance(a).toPrintable());
        System.out.println(ClassLayout.
                parseInstance(b).toPrintable());
        System.out.println(ClassLayout.
                parseInstance(c).toPrintable());
        System.out.println(ClassLayout.
                parseInstance(aa).toPrintable());
    }
 
    static class A {
    }
 
    static class B {
        private long s;
    }
 
    static class C {
        private int a;
        private long d;
    }
}
 
 
################################################################################################################################################################################
# WARNING: Unable to attach Serviceability Agent. You can try again with escalated privileges. Two options: a) use -Djol.tryWithSudo=true to try with sudo; b) echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope
cn.ly.test.JavaObjLayout$A 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)                           44 c1 00 f8 (01000100 11000001 00000000 11111000) (-134168252)
     12     4        (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
 
 
cn.ly.test.JavaObjLayout$B 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)                           83 c1 00 f8 (10000011 11000001 00000000 11111000) (-134168189)
     12     4        (alignment/padding gap)                  
     16     8   long B.s                                       0
Instance size: 24 bytes
Space losses: 4 bytes internal + 0 bytes external = 4 bytes total
 
 
cn.ly.test.JavaObjLayout$C 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)                           c1 c1 00 f8 (11000001 11000001 00000000 11111000) (-134168127)
     12     4    int C.a                                       0
     16     8   long C.d                                       0
Instance size: 24 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
 
 
[I 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)                           6d 01 00 f8 (01101101 00000001 00000000 11111000) (-134217363)
     12     4        (object header)                           03 00 00 00 (00000011 00000000 00000000 00000000) (3)
     16    12    int [I.                             N/A
     28     4        (loss due to the next object alignment)
Instance size: 32 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
################################################################################################################################################################################

注意点：

jdk6默认开启偏向锁，但是是输入延时开启，也就是说，程序刚启动创建的对象是不会开启偏向锁的，几秒后后创建的对象才会开启偏向锁的

//关闭延迟开启偏向锁

-XX:BiasedLockingStartupDelay=0

//禁止偏向锁

-XX:-UseBiasedLocking

//启用偏向锁

-XX:+UseBiasedLocking

Jol layout 结果查看需要注意顺序

对象头看锁升级过程：

一、无锁状态和偏向锁状态

JVM的启动参数默认值。偏向锁相关的不做任何设置。这意味着默认会有几秒的锁偏向的延迟。
代码示例，及结果


public static void main(String[] args) throws InterruptedException {
    Object obj1 = new Object();
    // 由于偏向锁生效是有延迟的（JVM启动后相对时间），所以这里采用这种方式来演示延迟
    // 在偏向锁生效前先创建好一个对象 obj1 ，此时对象头默认是无锁状态：001
    TimeUnit.SECONDS.sleep(5);
    // sleep的目的是为了使"延迟的偏向锁"变得生效
    System.out.println(ClassLayout.
            parseInstance(obj1).toPrintable());
 
    
    // JVM偏向锁延迟生效以后，再创建对象Obj2这个时候会发现，默认对象的头标记就是偏向锁 101
    Object obj2 = new Object();
    System.out.println(ClassLayout.
            parseInstance(obj2).toPrintable());
}
 
################################################################################################################################################################################
# WARNING: Unable to attach Serviceability Agent. You can try again with escalated privileges. Two options: a) use -Djol.tryWithSudo=true to try with sudo; b) echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope
java.lang.Object 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)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
     12     4        (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
 
 
java.lang.Object 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)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
     12     4        (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
################################################################################################################################################################################

注意结果：

二、无锁膨胀为轻量级锁

JVM参数不做任何调整。即存在偏向锁的延迟.
代码示例及结果


/**
* 轻量级锁
*/
public static void lightLock() throws InterruptedException {
    Object obj3 = new Object();
    System.out.println("无锁状态下的对象头markword标记：\n" + ClassLayout.
            parseInstance(obj3).toPrintable());
    System.out.println("------------------------------------------------------------------------------------------------------");
    Thread t1 = new Thread(()->{
        synchronized(obj3){
            System.out.println("线程t1获得锁obj3");
            System.out.println("偏向锁延迟生效时间内，无锁状态-膨胀为：轻量锁 ，其对象头markword标记：\n" + ClassLayout.
                    parseInstance(obj3).toPrintable());
        }
    });
    t1.start();
    t1.join();
}
 
################################################################################################################################################################################
无锁状态下的对象头markword标记：
java.lang.Object 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)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
     12     4        (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
 
 
------------------------------------------------------------------------------------------------------
线程t1获得锁obj3
偏向锁延迟生效时间内，无锁状态-膨胀为：轻量锁 ，其对象头markword标记：
java.lang.Object object internals:
OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           08 19 60 08 (00001000 00011001 01100000 00001000) (140515592)
      4     4        (object header)                           00 70 00 00 (00000000 01110000 00000000 00000000) (28672)
      8     4        (object header)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
     12     4        (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
################################################################################################################################################################################

三、偏向锁和轻量锁升级

JVM参数不做任何调整。即存在偏向锁的延迟.（代码中通过sleep来达到让延迟到期从而偏向生效来演示）
代码和结果如下：


/**
* 偏向锁-轻量级锁，在偏向生效以后
*/
public static void lightLockAfterBiasedLockingEffective() throws InterruptedException {
    // 由于偏向锁生效是有延迟的（JVM启动后相对时间），所以这里采用这种方式来演示延迟
    TimeUnit.SECONDS.sleep(5);
    Object obj4 = new Object();
    System.out.println("JVM偏向锁已经生效：偏向锁状态下的对象头markword标记：\n" + ClassLayout.
            parseInstance(obj4).toPrintable());
    System.out.println("------------------------------------------------------------------------------------------------------");
    Thread t2 = new Thread(()->{
        synchronized(obj4){
            System.out.println("线程t2获得锁obj4");
            System.out.println("偏向锁延迟生效后，线程t2 打印obj4 对象偏向锁状态，其对象头markword标记：\n" + ClassLayout.
                    parseInstance(obj4).toPrintable());
        }
    });
    t2.start();
    t2.join();
 
    TimeUnit.SECONDS.sleep(2);
    System.out.println("------------------------------------------------------------------------------------------------------");
    Thread t3 = new Thread(()->{
        synchronized(obj4){
            System.out.println("线程t3获得锁obj4");
            System.out.println("偏向锁延迟生效后，偏向锁状态-膨胀为：轻量锁 ，其对象头markword标记：\n" + ClassLayout.
                    parseInstance(obj4).toPrintable());
        }
    });
    t3.start();
    t3.join();
}
################################################################################################################################################################################
JVM偏向锁已经生效：偏向锁状态下的对象头markword标记：
java.lang.Object 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)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
     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 内存地址:31874192952,十六进制位：76bd99638,二进制位：11101101011110110011001011000111000
线程t2获得锁obj4
偏向锁延迟生效后，线程t2 打印obj4 对象偏向锁状态，其对象头markword标记：
java.lang.Object object internals:
OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           05 48 95 c8 (00000101 01001000 10010101 11001000) (-929740795)
      4     4        (object header)                           d3 7f 00 00 (11010011 01111111 00000000 00000000) (32723)
      8     4        (object header)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
     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 id:14,二进制位：1110
------------------------------------------------------------------------------------------------------
线程t3获得锁obj4
偏向锁延迟生效后，偏向锁状态-膨胀为：轻量锁 ，其对象头markword标记：
java.lang.Object object internals:
OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           08 b9 cc 0e (00001000 10111001 11001100 00001110) (248297736)
      4     4        (object header)                           00 70 00 00 (00000000 01110000 00000000 00000000) (28672)
      8     4        (object header)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
     12     4        (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
 
################################################################################################################################################################################

三、升级到重量级锁

JVM参数不做任何调整。即存在偏向锁的延迟.（代码中通过sleep来达到让延迟到期从而偏向生效来演示）
代码和结果如下：


public static void weightAfterBiasedLockingEffective() throws InterruptedException {
 
 
    // 由于偏向锁生效是有延迟的（JVM启动后相对时间），所以这里采用这种方式来演示延迟
    TimeUnit.SECONDS.sleep(5);
    Object obj5 = new Object();
    System.out.println("JVM偏向锁已经生效：偏向锁状态下的对象头markword标记：\n" + ClassLayout.
            parseInstance(obj5).toPrintable());
    System.out.println("------------------------------------------------------------------------------------------------------");
 
 
    Thread t2 = new Thread(()->{
        synchronized(obj5){
            System.out.println("线程t2获得锁obj4");
            System.out.println("偏向锁延迟生效后，线程t2 打印obj5 对象偏向锁状态，其对象头markword标记：\n" + ClassLayout.
                    parseInstance(obj5).toPrintable());
            long tid = Thread.currentThread().getId();
            System.out.println("t2 id:"+ tid + ",二进制位：" + Long.toBinaryString(tid));
        }
    });
    t2.start();
    long t2Addr =  VM.current().addressOf(t2);
    System.out.println("t2 内存地址:"+ t2Addr + ",十六进制位：" + Long.toHexString(t2Addr)+ ",二进制位：" + Long.toBinaryString(t2Addr));
    t2.join();
    System.out.println("------------------------------------------------------------------------------------------------------");
 
 
 
 
    TimeUnit.MILLISECONDS.sleep(500);
    Thread t3 = new Thread(()->{
        synchronized(obj5){
            System.out.println("[线程t3] 获得锁obj5");
            System.out.println("偏向锁延迟生效后，偏向锁状态-膨胀为：??? ，其对象头markword标记：\n" + ClassLayout.
                    parseInstance(obj5).toPrintable());
        }
    });
    t3.start();
 
 
    Thread t4 = new Thread(()->{
        synchronized(obj5){
            System.out.println("[线程t4] 获得锁obj5");
            System.out.println("偏向锁延迟生效后，偏向锁状态-膨胀为：??? ，其对象头markword标记：\n" + ClassLayout.
                    parseInstance(obj5).toPrintable());
        }
    });
    t4.start();
 
}

备注：升级重量级锁的过程稍微复杂一些，会根据线程的调度情况和JVM运行时的情况不一样产生三种不一样的结果。

以下是多次跑代码后得到的3种不一样的结果


##########################################################################情况一######################################################################################################
JVM偏向锁已经生效：偏向锁状态下的对象头markword标记：
java.lang.Object 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)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
     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 内存地址:31874198304,十六进制位：76bd9ab20,二进制位：11101101011110110011010101100100000
线程t2获得锁obj4
偏向锁延迟生效后，线程t2 打印obj5 对象偏向锁状态，其对象头markword标记：
java.lang.Object object internals:
OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           05 c8 aa dd (00000101 11001000 10101010 11011101) (-576010235)
      4     4        (object header)                           c4 7f 00 00 (11000100 01111111 00000000 00000000) (32708)
      8     4        (object header)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
     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 id:14,二进制位：1110
------------------------------------------------------------------------------------------------------
[线程t3] 获得锁obj5
偏向锁延迟生效后，偏向锁状态-膨胀为：重量级锁 ，其对象头markword标记：
java.lang.Object object internals:
OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           7a 0c 81 dc (01111010 00001100 10000001 11011100) (-595522438)
      4     4        (object header)                           c4 7f 00 00 (11000100 01111111 00000000 00000000) (32708)
      8     4        (object header)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
     12     4        (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
 
 
[线程t4] 获得锁obj5
偏向锁延迟生效后，偏向锁状态-膨胀为：重量级锁 ，其对象头markword标记：
java.lang.Object object internals:
OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           7a 0c 81 dc (01111010 00001100 10000001 11011100) (-595522438)
      4     4        (object header)                           c4 7f 00 00 (11000100 01111111 00000000 00000000) (32708)
      8     4        (object header)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
     12     4        (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
################################################################################################################################################################################
 
##########################################################################情况二############################################################################################
JVM偏向锁已经生效：偏向锁状态下的对象头markword标记：
java.lang.Object 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)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
     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 内存地址:31874198752,十六进制位：76bd9ace0,二进制位：11101101011110110011010110011100000
线程t2获得锁obj4
偏向锁延迟生效后，线程t2 打印obj5 对象偏向锁状态，其对象头markword标记：
java.lang.Object object internals:
OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           05 00 91 86 (00000101 00000000 10010001 10000110) (-2037317627)
      4     4        (object header)                           ed 7f 00 00 (11101101 01111111 00000000 00000000) (32749)
      8     4        (object header)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
     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 id:14,二进制位：1110
------------------------------------------------------------------------------------------------------
[线程t3] 获得锁obj5
偏向锁延迟生效后，偏向锁状态-保持：偏向锁 ，其对象头markword标记：
java.lang.Object object internals:
OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           05 00 91 86 (00000101 00000000 10010001 10000110) (-2037317627)
      4     4        (object header)                           ed 7f 00 00 (11101101 01111111 00000000 00000000) (32749)
      8     4        (object header)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
     12     4        (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
 
 
[线程t4] 获得锁obj5
偏向锁延迟生效后，偏向锁状态-膨胀为：重量级锁 ，其对象头markword标记：
java.lang.Object object internals:
OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           0a 9c 80 88 (00001010 10011100 10000000 10001000) (-2004837366)
      4     4        (object header)                           ed 7f 00 00 (11101101 01111111 00000000 00000000) (32749)
      8     4        (object header)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
     12     4        (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
 
################################################################################################################################################################################
 
##########################################################################情况三############################################################################################
JVM偏向锁已经生效：偏向锁状态下的对象头markword标记：
java.lang.Object 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)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
     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 内存地址:31874198880,十六进制位：76bd9ad60,二进制位：11101101011110110011010110101100000
线程t2获得锁obj4
偏向锁延迟生效后，线程t2 打印obj5 对象偏向锁状态，其对象头markword标记：
java.lang.Object object internals:
OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           05 f8 8e cd (00000101 11111000 10001110 11001101) (-846268411)
      4     4        (object header)                           c9 7f 00 00 (11001001 01111111 00000000 00000000) (32713)
      8     4        (object header)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
     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 id:14,二进制位：1110
------------------------------------------------------------------------------------------------------
[线程t3] 获得锁obj5
偏向锁延迟生效后，偏向锁状态-膨胀为：轻量级锁 ，其对象头markword标记：
java.lang.Object object internals:
OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           08 09 8d 0e (00001000 00001001 10001101 00001110) (244123912)
      4     4        (object header)                           c9 7f 00 00 (11001001 01111111 00000000 00000000) (32713)
      8     4        (object header)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
     12     4        (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
 
 
[线程t4] 获得锁obj5
偏向锁延迟生效后，偏向（轻量）锁状态-膨胀为：重量级锁 ，其对象头markword标记：
java.lang.Object object internals:
OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           0a 2c 01 cc (00001010 00101100 00000001 11001100) (-872338422)
      4     4        (object header)                           c9 7f 00 00 (11001001 01111111 00000000 00000000) (32713)
      8     4        (object header)                           e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
     12     4        (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
 
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原文地址：https://blog.csdn.net/PYXLY1314/article/details/126021584