Metaspace

Learn the class metadata region with Metaspace.

The Metaspace region

Metaspace holds class metadata (method tables, field info, the constant pool). Before Java 8 this was the job of PermGen, whose fixed size caused frequent OutOfMemoryError. Metaspace uses native memory and grows as needed.

When a class is loaded its metadata is written to Metaspace. If many classes are generated dynamically, Metaspace grows; the limit can be set with -XX:MaxMetaspaceSize. Objects themselves still live on the heap; Metaspace holds only class-level information.

The Class object and Metaspace

Every loaded class has a Class object that refers to its metadata in Metaspace. You reach this object with obj.getClass() or Topic.class.

Example: Class<?> c = new Topic().getClass(). Here c.getSimpleName() returns the class's short name; this data comes from Metaspace. Runtime type inspection (reflection) happens through this object. Metaspace grows as the number of classes increases.

Exercise Read class metadata

Define a Topic class. In main, get the Class<?> object with new Topic().getClass() and print the getSimpleName() result with System.out.println.

Show example solution

class Topic { }

public class MetaspaceDemo {
    public static void main(String[] args) {
        Class<?> c = new Topic().getClass();
        System.out.println(c.getSimpleName());
    }
}

Test yourself

Match the Metaspace concepts

Metaspace

Class metadata
PermGen

Old region
getClass()

Refers to Metaspace
Stack

Local variables
What does Metaspace store?
- Objects
- Local variables
- Running threads
- Class metadata
Show answer

Class metadata — Metaspace stores class metadata.

Get the class name

String name = c.____();

Show answer

String name = c.getSimpleName();

Next lesson: Summary - Memory model

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