Question 1

How does Jamm compare to Java VisualVM for memory measurement?

Accepted Answer

Jamm provides object-level memory measurement including JVM overhead and deep object graphs, whereas VisualVM focuses on heap dumps and high-level monitoring. Jamm is better for detailed optimization, but VisualVM is more user-friendly for general profiling.

Question 2

How to measure memory usage of ByteBuffer slabs in Jamm?

Accepted Answer

Use MemoryMeter.builder() and call measureDeep with ByteBufferMode set to SLAB_ALLOCATION_SLICE or SLAB_ALLOCATION_NO_SLICE, depending on how slabs are created (e.g., with slice() or duplicate()), to accurately omit shared buffer overhead.

Question 3

Does Jamm work with Java 17 records and new features?

Accepted Answer

Yes, Jamm 0.4.0 supports Java 17, but for records and hidden classes like lambdas, the Unsafe strategy may delegate to Specification, and using the Java agent is recommended for best accuracy due to reflection limitations.

Question 4

Can I skip measuring certain fields without modifying the source code in Jamm?

Accepted Answer

Yes, you can use custom filters through the MemoryMeter builder or apply the @Unmetered annotation at runtime if allowed, but annotation requires class access, so filters offer more flexibility for external classes.

Question 5

What's the performance overhead of using Jamm in production?

Accepted Answer

Jamm can have significant overhead, especially with deep object graph traversal using reflection or Unsafe, and it tracks visited objects, adding memory cost. It's designed for profiling, not continuous monitoring, so use it cautiously in production.

Question 6

Jamm or Apache Commons SizeOf for memory measurement?

Accepted Answer

Jamm is more accurate and up-to-date, with support for modern Java versions and deep graph analysis, while Commons SizeOf is simpler and lighter but may be less precise for complex object structures or JVM overhead.

Question 7

How to fix Jamm reflection errors in modular Java applications?

Accepted Answer

Add JVM arguments like --add-exports or --add-opens to allow reflective access. For example, --add-opens java.base/java.lang=ALL-UNNAMED might be needed for certain classes, but this can complicate deployment and security.

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