Core Java

Inspect the contents of the Java Metaspace region

JVM Memory has following regions:

a. Young Generation

b. Old Generation

c. Metaspace

d. Others region

To see what objects are stored in what region, you may refer to this video clip. Sometimes your application might run into ‘java.lang.OutOfMemoryError: Metaspace’ as discussed in this post. In such circumstances you might want to see what are the Contents loaded in the Metaspace region of the JVM. In nutshell, the Metaspace region in the JVM memory contains the class metadata definitions that are required to execute your application. If you want to understand what class metadata definitions means, you can refer to this documentation. It has intense details, you may not have to understand all the details of it. Basically if you can understand what are the classes that are loaded into memory, it will give a good idea what are the Contents that are present in the Metaspace region of the JVM memory. In this post let’s explore the options that are available to see the classes that are loaded into the Metaspace.

Below are the options to see the classes that are loaded in the Metaspace:

1. -verbose:class

2. -Xlog:class+load

3. jcmd GC.class_histogram

4. Programmatic approach

5. Heap Dump analysis

Let’s discuss each option in detail in this post.

1. -verbose:class

If you are running on Java version 8 or below then you can use this option. When you pass the ‘-verbose:class’ option to your application during startup, it will print all the classes that are loaded into memory. Loaded classes will be printed in the standard error stream (i.e. console, if you aren’t routing your error stream to a log file).

java {app_name} -verbose:class

Following is the sample output of the open source BuggyApp program when ‘-verbose:class’ argument is passed:

[Opened C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded java.lang.Object from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded java.lang.Comparable from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded java.lang.CharSequence from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded java.lang.String from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded java.lang.reflect.AnnotatedElement from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded java.lang.reflect.GenericDeclaration from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded java.lang.reflect.Type from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded java.lang.Class from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded java.lang.Cloneable from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded java.lang.ClassLoader from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded java.lang.System from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded java.lang.Throwable from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded java.lang.Error from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded java.lang.ThreadDeath from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded java.lang.Exception from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded java.lang.RuntimeException from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded java.lang.SecurityManager from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded java.lang.ReflectiveOperationException from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded java.lang.ClassNotFoundException from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded java.lang.LinkageError from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded java.lang.NoClassDefFoundError from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded java.lang.ClassCastException from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded java.lang.ArrayStoreException from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded java.lang.VirtualMachineError from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded java.lang.OutOfMemoryError from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]
[Loaded java.lang.StackOverflowError from C:\Program Files\Java\jre1.8.0_171\lib\rt.jar]

2. -Xlog:class+load

If you are running on Java version 9 or above then you can use this option. When you pass the ‘-Xlog:class+load’ option to your application during startup, it will print all the classes that are loaded into memory. Loaded classes will be printed in the file path you have configured.

java {app_name} -Xlog:class+load=info:/opt/log/loadedClasses.txt

Following is the sample output of a java program when ‘-Xlog:class+load’ argument is passed:

[0.004s][info][class,load] opened: /home/ec2-user/jdk-9.0.4/lib/modules
[0.006s][info][class,load] java.lang.Object source: jrt:/java.base
[0.007s][info][class,load] source: jrt:/java.base
[0.007s][info][class,load] java.lang.Comparable source: jrt:/java.base
[0.007s][info][class,load] java.lang.CharSequence source: jrt:/java.base
[0.007s][info][class,load] java.lang.String source: jrt:/java.base
[0.007s][info][class,load] java.lang.reflect.AnnotatedElement source: jrt:/java.base
[0.007s][info][class,load] java.lang.reflect.GenericDeclaration source: jrt:/java.base
[0.007s][info][class,load] java.lang.reflect.Type source: jrt:/java.base
[0.008s][info][class,load] java.lang.Class source: jrt:/java.base
[0.008s][info][class,load] java.lang.Cloneable source: jrt:/java.base
[0.008s][info][class,load] java.lang.ClassLoader source: jrt:/java.base
[0.008s][info][class,load] java.lang.System source: jrt:/java.base
[0.008s][info][class,load] java.lang.Throwable source: jrt:/java.base
[0.008s][info][class,load] java.lang.Error source: jrt:/java.base
[0.008s][info][class,load] java.lang.ThreadDeath source: jrt:/java.base
[0.008s][info][class,load] java.lang.Exception source: jrt:/java.base
[0.008s][info][class,load] java.lang.RuntimeException source: jrt:/java.base
[0.008s][info][class,load] java.lang.SecurityManager source: jrt:/java.base
[0.008s][info][class,load] source: jrt:/java.base
[0.009s][info][class,load] source: jrt:/java.base
[0.009s][info][class,load] source: jrt:/java.base
[0.009s][info][class,load] java.lang.ReflectiveOperationException source: jrt:/java.base
[0.009s][info][class,load] java.lang.ClassNotFoundException source: jrt:/java.base
[0.009s][info][class,load] java.lang.LinkageError source: jrt:/java.base
[0.009s][info][class,load] java.lang.NoClassDefFoundError source: jrt:/java.base
[0.009s][info][class,load] java.lang.ClassCastException source: jrt:/java.base
[0.009s][info][class,load] java.lang.ArrayStoreException source: jrt:/java.base
[0.009s][info][class,load] java.lang.VirtualMachineError source: jrt:/java.base
[0.009s][info][class,load] java.lang.OutOfMemoryError source: jrt:/java.base
[0.009s][info][class,load] java.lang.StackOverflowError source: jrt:/java.base
[0.009s][info][class,load] java.lang.IllegalMonitorStateException source: jrt:/java.base
[0.009s][info][class,load] java.lang.ref.Reference source: jrt:/java.base
[0.009s][info][class,load] java.lang.ref.SoftReference source: jrt:/java.base
[0.009s][info][class,load] java.lang.ref.WeakReference source: jrt:/java.base
[0.009s][info][class,load] java.lang.ref.FinalReference source: jrt:/java.base
[0.009s][info][class,load] java.lang.ref.PhantomReference source: jrt:/java.base
[0.009s][info][class,load] java.lang.ref.Finalizer source: jrt:/java.base
[0.009s][info][class,load] java.lang.Runnable source: jrt:/java.base
[0.009s][info][class,load] java.lang.Thread source: jrt:/java.base
[0.009s][info][class,load] java.lang.Thread$UncaughtExceptionHandler source: jrt:/java.base
[0.009s][info][class,load] java.lang.ThreadGroup source: jrt:/java.base
[0.010s][info][class,load] java.util.Map source: jrt:/java.base
[0.010s][info][class,load] java.util.Dictionary source: jrt:/java.base
[0.010s][info][class,load] java.util.Hashtable source: jrt:/java.base
[0.010s][info][class,load] java.util.Properties source: jrt:/java.base
[0.010s][info][class,load] java.lang.Module source: jrt:/java.base
[0.010s][info][class,load] java.lang.reflect.AccessibleObject source: jrt:/java.base

3. jcmd GC.class_histogram

JDK contains a tool called ‘jcmd’. You can invoke this tool when JVM is running to inspect the Contents of the Metaspace region. When you invoke this tool with ‘GC.class_histogram’ argument, it will print the list of classes that are loaded into the memory.  You can invoke this tool in two modes:

a. Print loaded classes on the console

jcmd {pid} GC.class_histogram

When you invoke the ‘jcmd’ as shown above it will print all the loaded classes in the console. Here {pid} is the process id of your java application. 

b. Print loaded classes on a File

jcmd {pid} GC.class_histogram filename={file-path}

When you invoke the ‘jcmd’ as shown above, it will print all the loaded classes in the file path specified in the ‘filename’ argument. Here {pid} is the process id of your java application.

Here is a blog post which helps you to identify the process id quickly.

Following is the sample output of the open source BuggyApp program when ‘jcmd GC.class_histogram’ argument is passed:

jcmd 19684 GC.class_histogram

 num     #instances         #bytes  class name
   1:        143036       75523008  [Ljavassist.bytecode.ConstInfo;
   2:        718060       70032224  [C
   3:       1573553       50353696  java.util.HashMap$Node
   4:        430124       24732832  [Ljava.lang.Object;
   5:       1001290       24030960  javassist.bytecode.Utf8Info
   6:        858268       20598432  java.util.ArrayList
   7:        718037       17232888  java.lang.String
   8:        144011       14987488  java.lang.Class
   9:        143081       11447152  [Ljava.util.HashMap$Node;
  10:        143036        9154304  javassist.bytecode.ClassFile
  11:        143035        9154240  javassist.CtNewClass
  12:        286124        6892400  [B
  13:        143085        6868080  java.util.HashMap
  14:        286078        6865872  javassist.bytecode.ClassInfo
  15:        143036        6865728  [[Ljavassist.bytecode.ConstInfo;
  16:        143049        5721960  javassist.bytecode.MethodInfo
  17:        143042        5721680  javassist.bytecode.CodeAttribute
  18:        143323        4586336  java.util.Hashtable$Entry
  19:        143038        4577216  java.lang.ref.WeakReference
  20:        143036        4577152  javassist.bytecode.ConstPool
  21:        143045        3433080  javassist.bytecode.MethodrefInfo
  22:        143045        3433080  javassist.bytecode.NameAndTypeInfo
  23:        143042        3433008  javassist.bytecode.ExceptionTable
  24:        143036        3432864  javassist.bytecode.LongVector
  25:        143036        3432864  javassist.bytecode.SourceFileAttribute
  26:        143622        2323336  [I
  27:            10         788688  [Ljava.util.Hashtable$Entry;
  28:           642          20544  java.util.concurrent.ConcurrentHashMap$Node
  29:           244          13664  java.lang.invoke.MemberName
  30:           341          10912  sun.misc.FDBigInteger
  31:           212           8480  java.lang.ref.SoftReference
  32:           140           8400  [Ljava.lang.ref.SoftReference;
  33:           234           7488  java.lang.invoke.LambdaForm$Name
  34:           176           7040  java.lang.invoke.MethodType
  35:           256           6144  java.lang.Long
  36:            16           6016  java.lang.Thread
  37:           173           5880  [Ljava.lang.Class;
  38:           366           5856  java.lang.Object
  39:           177           5664  java.lang.invoke.MethodType$ConcurrentWeakInternSet$WeakEntry
  40:            10           5280  [Ljava.util.concurrent.ConcurrentHashMap$Node;
  41:           256           4096  java.lang.Byte
  42:           256           4096  java.lang.Integer
  43:           256           4096  java.lang.Short
  44:            73           4088  java.lang.invoke.MethodTypeForm
  45:            82           3808  [Ljava.lang.invoke.LambdaForm$Name;
  46:            77           3696  java.lang.invoke.LambdaForm

4. Programmatic approach

You can also use a programmatic approach to print the classes that are loaded into the memory. Open source Guava library provides APIs to print the loaded classes. Below is the code sample that leverage Guava library to print the loaded classes in the memory:

ClassPath classPath = ClassPath.from(BuggyAppLoader.class.getClassLoader());
Set<ClassInfo> classes = classPath.getAllClasses();
for(ClassInfo classInfo : classes) {;

5. Heap Dump analysis

Another option to see the classes that are loaded into memory is to inspect Heap Dump. Heap dump reports all the data, objects, classes that are loaded into memory. You can use one of the approaches given here to capture the heap dump. Once a heap dump is captured, you can use the heap dump analysis tools such as Eclipse MAT, HeapHero,… to analyze the heap dump.

Below is the excerpt from the report generated by the HeapHero tool that shows the classes that are loaded into the memory.

Note: All the approaches mentioned above will not add noticeable overhead to your application, however the heap dump approach is an intrusive option and it will add considerable overhead to your application. When heap dump is captured your application will be paused until capturing is complete.


Published on Java Code Geeks with permission by Ram Lakshmanan, partner at our JCG program. See the original article here: Inspect the Contents of the Java Metaspace region

Opinions expressed by Java Code Geeks contributors are their own.

Ram Lakshmanan

Ram Lakshmanan developed world's finest DevOps tools:,, Every single day, millions & millions of people in North America—bank, travel, and commerce—use the applications that Ram Lakshmanan has architected. Ram is an acclaimed speaker in major conferences on scalability, availability, and performance topics. Recently, he has founded a startup, which specializes in troubleshooting performance problems.
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