Hello Jol!
Only works on HotSpot/OpenJDK VMs.
Quotes from VM specifictions
For example, the memory layout of run-time data areas, the garbage-collection algorithm used, and any internal optimization of the Java Virtual Machine instructions (for example, translating them into machine code) are left to the discretion of the implementor.
JOL (Java Object Layout) is the tiny toolbox to analyze object layout schemes in JVMs.
./gradlew clean check
./gradlew clean test
./gradlew clean run
guohai@kleokooe:~/work/jol_quickstart$ ./gradlew clean run
WARNING: JNI local refs: 33, exceeds capacity: 32
at com.sun.management.internal.DiagnosticCommandImpl.getDiagnosticCommandInfo(jdk.management@11.0.2/Native Method)
at com.sun.management.internal.DiagnosticCommandImpl.getMBeanInfo(jdk.management@11.0.2/DiagnosticCommandImpl.java:196)
at com.sun.jmx.interceptor.DefaultMBeanServerInterceptor.getNewMBeanClassName(java.management@11.0.2/DefaultMBeanServerInterceptor.java:329)
at com.sun.jmx.interceptor.DefaultMBeanServerInterceptor.registerMBean(java.management@11.0.2/DefaultMBeanServerInterceptor.java:315)
at com.sun.jmx.mbeanserver.JmxMBeanServer.registerMBean(java.management@11.0.2/JmxMBeanServer.java:522)
at java.lang.management.ManagementFactory.lambda$addMXBean$7(java.management@11.0.2/ManagementFactory.java:901)
at java.lang.management.ManagementFactory$$Lambda$47/0x0000000800127440.run(java.management@11.0.2/Unknown Source)
at java.security.AccessController.doPrivileged(java.base@11.0.2/Native Method)
at java.lang.management.ManagementFactory.addMXBean(java.management@11.0.2/ManagementFactory.java:891)
at java.lang.management.ManagementFactory.lambda$getPlatformMBeanServer$1(java.management@11.0.2/ManagementFactory.java:488)
at java.lang.management.ManagementFactory$$Lambda$46/0x0000000800127040.accept(java.management@11.0.2/Unknown Source)
at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(java.base@11.0.2/ForEachOps.java:183)
at java.util.Collections$2.tryAdvance(java.base@11.0.2/Collections.java:4745)
at java.util.Collections$2.forEachRemaining(java.base@11.0.2/Collections.java:4753)
at java.util.stream.ReferencePipeline$Head.forEach(java.base@11.0.2/ReferencePipeline.java:658)
at java.util.stream.ReferencePipeline$7$1.accept(java.base@11.0.2/ReferencePipeline.java:274)
at java.util.stream.ReferencePipeline$2$1.accept(java.base@11.0.2/ReferencePipeline.java:177)
at java.util.HashMap$ValueSpliterator.forEachRemaining(java.base@11.0.2/HashMap.java:1675)
at java.util.stream.AbstractPipeline.copyInto(java.base@11.0.2/AbstractPipeline.java:484)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(java.base@11.0.2/AbstractPipeline.java:474)
at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(java.base@11.0.2/ForEachOps.java:150)
at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(java.base@11.0.2/ForEachOps.java:173)
at java.util.stream.AbstractPipeline.evaluate(java.base@11.0.2/AbstractPipeline.java:234)
at java.util.stream.ReferencePipeline.forEach(java.base@11.0.2/ReferencePipeline.java:497)
at java.lang.management.ManagementFactory.getPlatformMBeanServer(java.management@11.0.2/ManagementFactory.java:488)
- locked <0x0000000751ac1ef8> (a java.lang.Class for java.lang.management.ManagementFactory)
at org.openjdk.jol.vm.VMOptions.getString(VMOptions.java:37)
at org.openjdk.jol.vm.VMOptions.pollCompressedOops(VMOptions.java:45)
at org.openjdk.jol.vm.HotspotUnsafe.<init>(HotspotUnsafe.java:133)
at org.openjdk.jol.vm.VM.current(VM.java:81)
at org.openjdk.jol.layouters.CurrentLayouter.layout(CurrentLayouter.java:52)
at org.openjdk.jol.info.ClassLayout.parseInstance(ClassLayout.java:102)
at org.openjdk.jol.info.ClassLayout.parseInstance(ClassLayout.java:85)
at jol.quickstart.App.main(App.java:32)
jol.quickstart.App$Car object internals:
OFF SZ TYPE DESCRIPTION VALUE
0 8 (object header: mark) 0x0000000000000005 (biasable; age: 0)
8 4 (object header: class) 0x00060250
12 4 int Car.id 3
16 8 double Car.price 133.9
24 2 char Car.level A
26 2 (alignment/padding gap)
28 4 java.lang.String Car.type (object)
Instance size: 32 bytes
Space losses: 2 bytes internal + 0 bytes external = 2 bytes total
[I object internals:
OFF SZ TYPE DESCRIPTION VALUE
0 8 (object header: mark) 0x0000000000000001 (non-biasable; age: 0)
8 4 (object header: class) 0x00000c10
12 4 (array length) 3
12 4 (alignment/padding gap)
16 12 int [I.<elements> N/A
28 4 (object alignment gap)
Instance size: 32 bytes
Space losses: 4 bytes internal + 4 bytes external = 8 bytes totalReference: