PermGen vs Metaspace in Java: Key Differences and Usage

In Java, PermGen (Permanent Generation) is a fixed-size memory area used in older JVMs to store class metadata, while Metaspace replaces it in Java 8 and later, dynamically resizing and storing class metadata outside the heap. Metaspace improves memory management by reducing out-of-memory errors related to class metadata.

Quick Comparison

This table summarizes the main differences between PermGen and Metaspace in Java.

Factor	PermGen	Metaspace
Memory Location	Part of JVM heap	Native memory (outside JVM heap)
Size	Fixed size, set by JVM options	Dynamically resizable by default
Java Version	Used in Java 7 and earlier	Introduced in Java 8 and later
Class Metadata Storage	Stores class metadata and interned strings	Stores class metadata only; interned strings moved to heap
OutOfMemoryError Cause	PermGen space exhaustion	Metaspace space exhaustion
Tuning	Requires manual size tuning	Less tuning needed, but max size can be set

Key Differences

PermGen is a fixed-size memory area inside the JVM heap that stores class metadata, method objects, and interned strings. Because its size is fixed, if the JVM loads many classes or large metadata, it can run out of space, causing java.lang.OutOfMemoryError: PermGen space. Developers had to tune its size manually using JVM flags like -XX:PermSize and -XX:MaxPermSize.

Starting with Java 8, Metaspace replaced PermGen. It stores class metadata in native memory outside the JVM heap, allowing it to grow dynamically as needed. This reduces the risk of out-of-memory errors related to class metadata. Interned strings were moved to the regular heap, improving memory management. Developers can still limit Metaspace size with -XX:MaxMetaspaceSize, but it usually requires less tuning.

Overall, Metaspace offers better flexibility and reduces memory management headaches compared to PermGen, making Java applications more stable and easier to maintain.

Code Comparison

Here is an example showing how to set JVM options for PermGen in Java 7 or earlier to avoid OutOfMemoryError:

bash

java -XX:PermSize=64m -XX:MaxPermSize=256m -jar YourApp.jar

Metaspace Equivalent

In Java 8 and later, you control Metaspace size with these JVM options. By default, it grows dynamically, but you can limit it if needed:

bash

java -XX:MetaspaceSize=64m -XX:MaxMetaspaceSize=256m -jar YourApp.jar

When to Use Which

Choose PermGen tuning only if you are running Java 7 or older, as it is the fixed memory area for class metadata in those versions. You must carefully set its size to avoid memory errors.

For Java 8 and newer, use Metaspace, which automatically manages class metadata memory more efficiently. You generally do not need to tune it unless your application loads many classes dynamically or you want to limit native memory usage.

In summary, prefer Metaspace for modern Java applications and only worry about PermGen if maintaining legacy Java versions.

Key Takeaways

PermGen is fixed-size and inside JVM heap; Metaspace is dynamic and outside heap.

Metaspace replaced PermGen starting Java 8 for better memory management.

Tuning PermGen size is critical in Java 7 and earlier to avoid errors.

Metaspace usually requires less tuning but can be limited with JVM flags.

Use Metaspace for modern Java apps and PermGen only for legacy support.