How to deal with the 'java.lang.OutOfMemoryError: Java heap space' error?

Abdul D.

—November 15, 2024

The Problem

I don’t know how to deal with the java.lang.OutOfMemoryError: Java heap space error in Java.

The Solution

The java.lang.OutOfMemoryError: Java heap space error occurs when a Java virtual machine (JVM) runs out of memory to allocate new objects.

There are multiple ways to ensure the memory allocated to your JVM is sufficient for the requirements of an application, including the following common solutions:

• Using -Xms and -Xmx to increase the heap size
• Using a profiling tool to analyze memory usage
• Using -XX:+HeapDumpOnOutOfMemoryError to generate a heap dump
• Tuning the garbage collector

Increase Heap Size

You can increase the heap size allocated to the JVM by using the -Xms and -Xmx options:

java -Xms512m -Xmx1024m YourApplication

• The -Xms parameter sets the initial heap size (for example, 512m for 512 MB).
• The -Xmx parameter sets the maximum heap size (for example, 1024m for 1 GB).

Analyze Memory Usage

You can use a profiling tool like VisualVM or YourKit to monitor memory usage and identify memory leaks or excessive memory consumption. This helps you identify which part of your code uses the most memory.

Use -XX:+HeapDumpOnOutOfMemoryError

You can use the -XX:+HeapDumpOnOutOfMemoryError flag to generate a heap dump when the error occurs. You can then analyze the heap dump to find the root cause of the problem:

java -Xmx1024m -XX:+HeapDumpOnOutOfMemoryError YourApplication

Garbage Collection Tuning

Tuning the garbage collector sometimes helps resolve memory issues. Experiment with different garbage collection algorithms (such as G1GC) and JVM options (like -XX:+UseG1GC) to see whether you can improve the JVM memory management.

Example

Suppose you have an application that reads a large file into memory all at once:

import java.nio.file.Files;
import java.nio.file.Paths;
import java.io.IOException;

public class Main {
    public static void main(String[] args) {
        try {
            byte[] data = Files.readAllBytes(Paths.get("largefile.txt"));
            System.out.println("File size: " + data.length);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

The code above will throw an OutOfMemoryError error if the file being read in is too large

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
    at java.base/java.util.Arrays.copyOf(Arrays.java:3745)
    at java.base/java.io.ByteArrayOutputStream.grow(ByteArrayOutputStream.java:118)
    at java.base/java.io.ByteArrayOutputStream.ensureCapacity(ByteArrayOutputStream.java:93)
    at java.base/java.io.ByteArrayOutputStream.write(ByteArrayOutputStream.java:153)
    at java.base/java.io.ByteArrayOutputStream.write(ByteArrayOutputStream.java:160)
    at java.base/java.nio.file.Files.read(Files.java:3273)
    at java.base/java.nio.file.Files.readAllBytes(Files.java:3326)
    at Main.main(Main.java:8)

This approach can easily lead to an OutOfMemoryError if the file is too large. To solve this, you could read the file in smaller chunks:

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;

public class Main {
    public static void main(String[] args) {
        try (BufferedReader reader = new BufferedReader(new FileReader("largefile.txt"))) {
            String line;
            while ((line = reader.readLine()) != null) {
                System.out.println(line);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}