JOL
JOL 是OpenJdk发布的用查看对象在JVM中信息，全称是Java Object Layout。可以通过命令行使用。
下面例子中仅使用到了ClassLayout，有兴趣可以看看官网介绍。
pom依赖，jol 0.16版本没有使用字节码展示对象头而是直接展示了锁信息，与jstack日志一致，如果想查看对象头变化请使用 0.10版
<dependency>
<groupId>org.openjdk.jol</groupId>
<artifactId>jol-core</artifactId>
<version>0.16</version>
</dependency>synchronization
synchronization是同步关键字，下面的示例使用synchronization+jol查看锁信息。有兴趣可以直接运行看看效果，也可以直接拖到最后看看结论或者看看oracle官方解释。
JAVA代码
import lombok.extern.slf4j.Slf4j;
import org.openjdk.jol.info.ClassLayout;
@Slf4j
public class SynchronizedDemo {
public static void main(String[] args) {
SynchronizedDemo demo = new SynchronizedDemo();
SynchronizedDemo demo1 = new SynchronizedDemo();
SynchronizedDemo demo2 = new SynchronizedDemo();
new Thread(() -> {
demo.lockMethod();
}).start();
new Thread(() -> {
demo.lockInstance(demo1);
}).start();
new Thread(() -> {
SynchronizedDemo.lockStaticMethod();
}).start();
log.info("打印demo头信息");
log.info(ClassLayout.parseInstance(demo).toPrintable());
log.info("打印demo1头信息");
log.info(ClassLayout.parseInstance(demo1).toPrintable());
log.info("打印demo2头信息");
log.info(ClassLayout.parseInstance(demo2).toPrintable());
log.info("打印Class头信息");
log.info(ClassLayout.parseInstance(SynchronizedDemo.class).toPrintable());
}
public synchronized static void lockStaticMethod() {
try {
Thread.sleep(1000000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
public synchronized void lockMethod() {
try {
Thread.sleep(1000000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
public void lockInstance(SynchronizedDemo demo) {
synchronized (demo) {
try {
Thread.sleep(1000000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}打印日志
2022-06-15 15:28:48 INFO --- [main ] c.m.basis.concurrent.SynchronizedDemo : 打印demo头信息
2022-06-15 15:28:49 INFO --- [main ] c.m.basis.concurrent.SynchronizedDemo : com.meijm.basis.concurrent.SynchronizedDemo object internals:
OFF SZ TYPE DESCRIPTION VALUE
0 8 (object header: mark) 0x000000002097f050 (thin lock: 0x000000002097f050)
8 4 (object header: class) 0xf800c105
12 4 (object alignment gap)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
2022-06-15 15:28:49 INFO --- [main ] c.m.basis.concurrent.SynchronizedDemo : 打印demo1头信息
2022-06-15 15:28:49 INFO --- [main ] c.m.basis.concurrent.SynchronizedDemo : com.meijm.basis.concurrent.SynchronizedDemo object internals:
OFF SZ TYPE DESCRIPTION VALUE
0 8 (object header: mark) 0x0000000020a7ee10 (thin lock: 0x0000000020a7ee10)
8 4 (object header: class) 0xf800c105
12 4 (object alignment gap)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
2022-06-15 15:28:49 INFO --- [main ] c.m.basis.concurrent.SynchronizedDemo : 打印demo2头信息
2022-06-15 15:28:49 INFO --- [main ] c.m.basis.concurrent.SynchronizedDemo : com.meijm.basis.concurrent.SynchronizedDemo object internals:
OFF SZ TYPE DESCRIPTION VALUE
0 8 (object header: mark) 0x0000000000000001 (non-biasable; age: 0)
8 4 (object header: class) 0xf800c105
12 4 (object alignment gap)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
2022-06-15 15:28:49 INFO --- [main ] c.m.basis.concurrent.SynchronizedDemo : 打印Class头信息
2022-06-15 15:28:49 INFO --- [main ] c.m.basis.concurrent.SynchronizedDemo : java.lang.Class object internals:
OFF SZ TYPE DESCRIPTION VALUE
0 8 (object header: mark) 0x000000001ba5570a (fat lock: 0x000000001ba5570a)
8 4 (object header: class) 0xf80003df
12 4 java.lang.reflect.Constructor Class.cachedConstructor null
16 4 java.lang.Class Class.newInstanceCallerCache null
20 4 java.lang.String Class.name (object)
24 4 (alignment/padding gap)
28 4 java.lang.ref.SoftReference Class.reflectionData (object)
32 4 sun.reflect.generics.repository.ClassRepository Class.genericInfo null
36 4 java.lang.Object[] Class.enumConstants null
40 4 java.util.Map Class.enumConstantDirectory null
44 4 java.lang.Class.AnnotationData Class.annotationData (object)
48 4 sun.reflect.annotation.AnnotationType Class.annotationType null
52 4 java.lang.ClassValue.ClassValueMap Class.classValueMap null
56 32 (alignment/padding gap)
88 4 int Class.classRedefinedCount 0
92 4 (object alignment gap)
Instance size: 512 bytes
Space losses: 36 bytes internal + 4 bytes external = 40 bytes total在代码运行时可使用jstack -l 进程id，查看具体锁信息，
结论
synchronization锁定的是JVM中运行的对象，静态方法锁定的是Class对象，代码块锁定的是括号中的对象。
参考资料
https://github.com/openjdk/jol
https://docs.oracle.com/javase/tutorial/essential/concurrency/locksync.html
