Java5 min read2026-03-01

Java OutOfMemoryError: Java Heap Space

Diagnose java.lang.OutOfMemoryError: Java heap space, configure JVM -Xmx limits, and identify memory leaks.

Error Code / Stack Trace

java.lang.OutOfMemoryError: Java heap space

Problem Overview

The Garbage Collector cannot allocate memory for new objects because the JVM allocated maximum heap size (-Xmx) has been fully consumed.

Why Does This Happen?

Loading massive datasets (like querying 1,000,000 rows from a database) entirely into memory.
Unbounded static collections (e.g. Map caches) that retain object references forever.
Default JVM heap limit is too low for the container or workload.

Step-by-Step Solution

Step 1: Increase JVM Max Heap Size (-Xmx)

Specify appropriate minimum (-Xms) and maximum (-Xmx) heap size parameters.

bash

java -Xms2g -Xmx4g -jar app.jar

Step 2: Stream large database queries instead of List<T>

Use Spring Data JPA Stream or batch pagination to avoid loading all records at once.

java

@Transactional(readOnly = true)
public void exportUsers() {
    try (Stream<User> userStream = userRepository.streamAll()) {
        userStream.forEach(this::writeToCsv);
    }
}

Common Mistakes to Avoid

•Blindly increasing -Xmx when there is an actual memory leak, merely delaying the eventual crash.
•Ignoring container cgroup memory limits in Kubernetes, causing the Linux kernel OOMKiller to kill the pod.

Prevention & Best Practices

Configure -XX:+HeapDumpOnOutOfMemoryError to capture memory dumps for Eclipse Memory Analyzer (MAT).

Frequently Asked Questions

Does doubling -Xmx slow down garbage collection?

Yes, larger heaps can increase GC pause times unless using modern low-pause collectors like G1GC or ZGC (-XX:+UseZGC).