java.lang.OutOfMemoryError: Java heap space

The JVM ran out of heap memory.

Why this happens

The JVM allocates a fixed maximum amount of heap memory at startup (default varies by platform). When your application tries to allocate an object and the garbage collector cannot free enough space, this error is thrown. It usually indicates either a memory leak or that your application genuinely needs more memory than allocated.

Fix 1: Increase heap size

java -Xmx2g -Xms512m MyApp

• -Xmx2g — max heap 2GB
• -Xms512m — initial heap 512MB

Fix 2: Find memory leaks

Common causes:

• Collections that grow without bounds
• Static references holding large objects
• Unclosed resources (streams, connections)

// ❌ Memory leak — list grows forever
static List<byte[]> cache = new ArrayList<>();
void process() {
    cache.add(new byte[1024 * 1024]);
}

Fix 3: Use a profiler

# Generate heap dump on OOM
java -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=/tmp MyApp

Then analyze with VisualVM or Eclipse MAT.

Alternative solutions

Use weak references for caches so the GC can reclaim memory when needed:

Map<String, SoftReference<byte[]>> cache = new HashMap<>();

Stream large datasets instead of loading everything into memory — use BufferedReader, database cursors, or pagination.

Prevention

• Set -Xmx explicitly in production rather than relying on JVM defaults.
• Monitor heap usage with JMX metrics and alert before OOM occurs.

Related: Java: NullPointerException fix · Kubernetes: OOMKilled fix