Fixing Java OutOfMemoryError: Java heap space

Symptoms

The application crashes with the error: java.lang.OutOfMemoryError: Java heap space. This may occur during object creation, data processing, or after prolonged runtime. The JVM stops and the application becomes unresponsive.

Root Causes

• Insufficient heap size allocated via JVM options (e.g., -Xmx too low).
• Memory leaks: objects held by static collections, listeners, or caches not cleared.
• Large data sets loaded entirely into memory (e.g., huge files, database results).
• Infinite loops or recursion creating excessive objects.
• Fragmentation or inefficient garbage collection.

Step-by-step Fix

1. Increase heap size: Add JVM arguments: -Xms512m -Xmx2g (adjust values based on available RAM). For Tomcat, edit CATALINA_OPTS; for command line: java -Xmx2g -jar app.jar.
2. Enable GC logging: Add -verbose:gc -XX:+PrintGCDetails -XX:+PrintGCTimeStamps to monitor garbage collection.
3. Generate heap dump on OOM: Use -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=/path/dump.hprof.
4. Analyze heap dump: Use Eclipse MAT or VisualVM. Look for large objects, unexpected retention, or class instances that shouldn't persist.
5. Fix memory leaks: Clear static collections, close resources (streams, connections), remove listeners, use weak references where appropriate.
6. Optimize code: Stream large data instead of loading all at once. Use StringBuilder over string concatenation in loops. Reuse objects.

Alternative Fixes

• Switch to a different garbage collector: -XX:+UseG1GC or -XX:+UseParallelGC for better throughput.
• Reduce object size: Use primitives instead of wrappers, avoid unnecessary fields.
• Increase swap space or add more physical RAM to the server.
• If using containers, ensure container memory limits match JVM heap settings.

Prevention

• Monitor heap usage with tools like JConsole, VisualVM, or Prometheus + Grafana.
• Set appropriate -Xms and -Xmx based on load testing.
• Implement circuit breakers or backpressure to limit request load.
• Conduct regular code reviews for memory management.
• Use profiling during development to catch leaks early.

Example JVM Configuration

java -Xms1g -Xmx4g -XX:+UseG1GC -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=/var/log/heapdump.hprof -jar application.jar

Adjust memory settings based on your environment. For production, start with -Xmx equal to 70-80% of container/VM memory, leaving room for OS and other processes.