Biased lock, lightweight lock test tool class level related commands

Dependency package

Recommended for dependent packages 0.9 Version of , Other versions may have different effects , Treasure .

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

Demo code

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 {
    
        //  Print  JVM  Relevant information 
        // System.out.println(VM.current().details());
        //  Print whether each object is  8  Integer multiple size of 
        // System.out.println(VM.current().objectAlignment());
        MyObject myObject = new MyObject();
        System.out.println(Integer.toHexString(myObject.hashCode()));
        new Thread(()->{
    
            //  stay  myObject  Lock the object head ( By default, it works directly to the lightweight lock , Here I have to put him in a locked state )
            //  The default is to open the bias lock , So here we only need to modify the delay time of opening the bias lock to  0  Easy to see  -XX:+BiasedLockingStartupDelay=0
            synchronized (myObject) {
    
                //  Lock this thread , And also set the bias thread  ID
                System.out.println(ClassLayout.parseInstance(myObject).toPrintable());
            }
        }).start();

        TimeUnit.MICROSECONDS.sleep(500);

        //  The lock was released , So what is printed here must be unlocked  001
        System.out.println(ClassLayout.parseInstance(myObject).toPrintable());

    }
}
class MyObject {
    

}

Running results ：

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

Common commands

Set up JVM Heap size

-Xms10m -Xmx10m

Inquire about JVM Start and run those commands

java -XX:+PrintCommandLineFlags -version

Close the compression configuration of class pointer in the object header

-XX:-UseCompressedClassPointers

Check whether the deflection lock is turned on

java -XX:+PrintFlagsInitial | grep BiasedLock

Running results ：

intx BiasedLockingBulkRebiasThreshold          = 20                                  {
    product}
intx BiasedLockingBulkRevokeThreshold          = 40                                  {
    product}
intx BiasedLockingDecayTime                    = 25000                               {
    product}
//  After the deflection lock is opened, there will be 4s The delay of the clock , When testing, you need to pay attention to , You can set this value to 0, Easy to view 
intx BiasedLockingStartupDelay                 = 4000                                {
    product}
bool TraceBiasedLocking                        = false                               {
    product}
// JVM  The setting of deflection lock is enabled by default 
bool UseBiasedLocking                          = true                                {
    product}

Turn on the deflection lock setting

-XX:+UseBiasedLocking -XX:BiasedLockingStartupDelay=0