偏向锁、轻量级锁测试工具类级相关命令

依赖包

依赖包推荐使用 0.9 版本的，其他版本可能有不一样的效果，珍重。

		<dependency>
			<groupId>org.openjdk.jol</groupId>
			<artifactId>jol-core</artifactId>
			<version>0.9</version>
		</dependency>

演示代码

package com.xxl.job.admin.mytest;


import org.openjdk.jol.info.ClassLayout;

import java.util.concurrent.TimeUnit;

public class ObjectMarkWordDemo {
    

    public void test() {
    
        Object obj = new Object();
        synchronized (obj) {
    
            System.out.println("111");
        }
        synchronized (obj) {
    
            System.out.println("111");
        }
        synchronized (obj) {
    
            System.out.println("111");
        }
    }

    public static void main(String[] args) throws InterruptedException {
    
        // 打印 JVM 相关的信息
        // System.out.println(VM.current().details());
        // 打印每个对象是否为 8 的整数倍大小
        // System.out.println(VM.current().objectAlignment());
        MyObject myObject = new MyObject();
        System.out.println(Integer.toHexString(myObject.hashCode()));
        new Thread(()->{
    
            // 在 myObject 对象头上进行加锁(默认直接干到轻量级锁,这里我非要把他干到偏向锁状态)
            // 默认是开启偏向锁的，所以这里我们只需要把开启偏向锁的延迟时间修改成 0 方便看效果 -XX:+BiasedLockingStartupDelay=0
            synchronized (myObject) {
    
                // 给这个线程加锁，并且还设置了偏向线程 ID
                System.out.println(ClassLayout.parseInstance(myObject).toPrintable());
            }
        }).start();

        TimeUnit.MICROSECONDS.sleep(500);

        // 锁被释放了，所以这里打印的肯定是无锁状态 001
        System.out.println(ClassLayout.parseInstance(myObject).toPrintable());

    }
}
class MyObject {
    

}

运行结果：

76fb509a
# 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
com.xxl.job.admin.mytest.MyObject object internals:
 OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           e8 29 c3 0a (11101000 00101001 11000011 00001010) (180562408)
      4     4        (object header)                           03 00 00 00 (00000011 00000000 00000000 00000000) (3)
      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

# 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
com.xxl.job.admin.mytest.MyObject object internals:
 OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0     4        (object header)                           e8 29 c3 0a (11101000 00101001 11000011 00001010) (180562408)
      4     4        (object header)                           03 00 00 00 (00000011 00000000 00000000 00000000) (3)
      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

常用命令

设置 JVM 堆大小

-Xms10m -Xmx10m

查询 JVM 启动运行了那些命令

java -XX:+PrintCommandLineFlags -version

关闭对象头中类指针的压缩配置

-XX:-UseCompressedClassPointers

查询偏向锁是否开启

java -XX:+PrintFlagsInitial | grep BiasedLock

运行结果：

intx BiasedLockingBulkRebiasThreshold          = 20                                  {
    product}
intx BiasedLockingBulkRevokeThreshold          = 40                                  {
    product}
intx BiasedLockingDecayTime                    = 25000                               {
    product}
// 然后偏向锁开启之后默认会有4s钟的延迟，测试的时候需要注意，可以将这个值设置成0，方便查看效果
intx BiasedLockingStartupDelay                 = 4000                                {
    product}
bool TraceBiasedLocking                        = false                               {
    product}
// JVM 默认开启了偏向锁的设置
bool UseBiasedLocking                          = true                                {
    product}

开启偏向锁设置

-XX:+UseBiasedLocking -XX:BiasedLockingStartupDelay=0