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Gowtham Kalyan
Gowtham Kalyan

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What Happens When Memory is Full in JVM?

#jvm #memory

When the JVM runs out of memory, it tries to recover using Garbage Collection (GC).

πŸ‘‰ If GC cannot free enough memory, the JVM throws an:

java.lang.OutOfMemoryError
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This error indicates that the application cannot allocate more memory.

πŸ” Types of OutOfMemoryError

1️⃣ Heap Space Error 

java.lang.OutOfMemoryError: [Java ](https://ashokitech.com/core-java-online-training/)heap space
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πŸ“Œ Happens when:

  • Too many objects are created
  • Memory is not released properly
  • Large data structures are used

2️⃣ Metaspace Error 

java.lang.OutOfMemoryError: Metaspace
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πŸ“Œ Happens when:

  • Too many classes are loaded
  • Class metadata exceeds limit

3️⃣ Stack Overflow Error 

java.lang.StackOverflowError
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πŸ“Œ Happens when:

  • Deep or infinite recursion
  • Large stack memory usage

4️⃣ GC Overhead Limit Exceeded 

java.lang.OutOfMemoryError: GC overhead limit exceeded
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πŸ“Œ Happens when:

  • JVM spends too much time in GC
  • Very little memory is actually freed

5️⃣ Direct Buffer Memory Error 

java.lang.OutOfMemoryError: Direct buffer memory
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πŸ“Œ Happens when:

  • Excessive use of NIO buffers
  • Native memory exhaustion

πŸ”„ JVM Behavior When Memory is Full

  1. JVM tries minor GC (young generation cleanup)
  2. Then tries major/full GC
  3. If still no memory β†’ throws OutOfMemoryError
  4. Application may:
  • ❌ Crash
  • ❌ Slow down drastically
  • ❌ Become unresponsive

🧠 Real-Time Example

In a Spring Boot application:

  • Memory leaks (like unclosed DB connections or static collections)
  • Large file processing
  • High traffic

πŸ‘‰ Can easily lead to JVM memory exhaustion

βš™οΈ How to Prevent This?

βœ… Best Practices

  • Use proper object lifecycle management
  • Avoid memory leaks
  • Close resources (DB, streams)
  • Use connection pooling
  • Optimize collections

βš™οΈ JVM Tuning Options 

-Xms512m
-Xmx1024m
-XX:MaxMetaspaceSize=256m
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πŸ‘‰ Helps control memory allocation

🎯 Interview Tip

πŸ‘‰ Common question:
What happens before OutOfMemoryError occurs?

βœ”οΈ Answer:
JVM attempts multiple Garbage Collection cycles. If memory is still insufficient, it throws OutOfMemoryError.

πŸ“š Learn More Java Internals

Master JVM, memory management, and real-time debugging:

πŸ‘‰ https://ashokitech.com/core-java-online-training/

πŸ”₯ Final Thoughts

When JVM memory is full, it’s not just an error β€” it’s a sign of:

  • Poor memory management
  • High load
  • Inefficient code

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