Fix Java Heap Space Error

What's Actually Happening

Java applications run out of heap memory. OutOfMemoryError is thrown, applications crash, or garbage collection causes performance issues.

The Error You'll See

OutOfMemoryError Java heap space:

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
    at java.util.Arrays.copyOf(Arrays.java:3332)
    at java.util.Arrays.copyOf(Arrays.java:3332)

OutOfMemoryError GC overhead:

java.lang.OutOfMemoryError: GC overhead limit exceeded

Memory warning:

```bash tail -f /var/log/app.log

WARNING: java.lang.OutOfMemoryError: Java heap space ```

Why This Happens

1. 1.Heap too small - Initial heap size insufficient
2. 2.Memory leak - Objects not being garbage collected
3. 3.Large data processing - Processing data larger than heap
4. 4.Too many objects - Creating too many objects
5. 5.GC tuning needed - Garbage collection not optimized
6. 6.Off-heap memory - Not enough native memory
7. 7.Memory fragmentation - Heap fragmentation

Step 1: Check Current Heap Settings

```bash ps aux | grep java

# Look for -Xmx and -Xms flags

# Check JVM arguments: jcmd <pid> VM.flags

# Get heap usage: jcmd <pid> GC.heap_info

# Check memory: jmap -heap <pid> ```

Step 2: Increase Heap Size

```bash # Set initial and max heap: java -Xms512m -Xmx2g MyApp

# Or in environment: export JAVA_OPTS="-Xms512m -Xmx2g"

# Common settings: # Small: -Xms256m -Xmx512m # Medium: -Xms512m -Xmx2g # Large: -Xms2g -Xmx4g

# As fraction of system memory: # Leave ~25% for OS and other processes ```

Step 3: Check for Memory Leaks

```bash # Generate heap dump: jcmd <pid> GC.heap_dump /tmp/heap.hprof

# Or use jmap: jmap -dump:format=b,file=/tmp/heap.hprof <pid>

# Analyze with tools: # - Eclipse MAT # - VisualVM # - YourKit # - JProfiler

# Check live objects: jcmd <pid> GC.class_histogram | head -20 ```

Step 4: Monitor Memory Usage

```bash # JConsole: jconsole <pid>

# VisualVM: jvisualvm

# Command line: jstat -gc <pid> 1000

# Memory pools: jcmd <pid> VM.info | grep -A 20 "Memory" ```

Step 5: Analyze GC Logs

```bash # Enable GC logging: -XX:+PrintGCDetails -XX:+PrintGCDateStamps -Xloggc:/var/log/gc.log

# Modern Java (11+): -Xlog:gc*:file=/var/log/gc.log

# Check GC logs: tail -f /var/log/gc.log

# Analyze with: # - GCViewer # - GCEasy.io ```

Step 6: Tune Garbage Collection

```bash # Use G1GC (Java 11+ default): -XX:+UseG1GC

# Set max GC pause: -XX:MaxGCPauseMillis=200

# For throughput: -XX:+UseParallelGC

# For low latency: -XX:+UseZGC

# Common settings: -XX:+UseG1GC -XX:MaxGCPauseMillis=200 -XX:InitiatingHeapOccupancyPercent=45 ```

Step 7: Check Metaspace

```bash # Metaspace is for class metadata # If OutOfMemoryError: Metaspace

# Increase metaspace: -XX:MetaspaceSize=256m -XX:MaxMetaspaceSize=512m

# Check metaspace usage: jcmd <pid> GC.heap_info | grep Metaspace

# Enable class unloading: -XX:+CMSClassUnloadingEnabled ```

Step 8: Handle Large Objects

```bash # For large data: # Process in chunks # Use streaming APIs # Avoid loading all data into memory

# Use off-heap memory: -XX:MaxDirectMemorySize=1g

# For large files: # Use streaming I/O # Use memory-mapped files ```

Step 9: Container Memory Limits

```bash # In Docker/container: # Set JVM heap based on container limit

# Java 10+ automatically detects container limits # For older versions: -XX:+UnlockExperimentalVMOptions -XX:+UseCGroupMemoryLimitForHeap

# Or set explicitly: java -Xmx$(echo $(cat /sys/fs/cgroup/memory/memory.limit_in_bytes) * 0.75 | bc) MyApp

# In Kubernetes: # Set JVM heap based on container limit env: - name: JAVA_OPTS value: "-Xms512m -Xmx768m" ```

Step 10: Monitor and Alert

```bash # JMX monitoring: -Dcom.sun.management.jmxremote -Dcom.sun.management.jmxremote.port=9010 -Dcom.sun.management.jmxremote.authenticate=false -Dcom.sun.management.jmxremote.ssl=false

# Prometheus JMX exporter: java -javaagent:jmx_prometheus_javaagent.jar=8080:config.yaml -jar app.jar

# Key metrics: # - jvm_memory_used_bytes # - jvm_gc_collection_seconds # - jvm_threads_current ```

Java Heap Space Error Checklist

Verify the Fix

```bash jcmd <pid> VM.flags | grep -E "Xmx|Xms"

jcmd <pid> GC.heap_info

jstat -gc <pid> 1000 5

tail -f /var/log/gc.log

jcmd <pid> GC.class_histogram | head -10

ps aux | grep java ```

Related Issues

• [Fix Java OutOfMemoryError](/articles/fix-java-outofmemoryerror)
• [Fix Java Application Crash](/articles/fix-java-application-crash)
• [Fix Docker Out of Memory](/articles/fix-docker-out-of-memory)