advancedJVM Internals & Memory

What is a DirectByteBuffer, and why can it cause OutOfMemoryError even when heap usage looks fine?

ByteBuffer.allocateDirect() allocates off-heap, native OS memory — invisible to normal heap monitoring tools. Exhausting it throws 'Direct buffer memory' OOM even with plenty of heap free; tune with -XX:MaxDirectMemorySize and check with jcmd VM.native_memory.

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