OutOfMemoryError: Java Heap Space

Why This Happens

The JVM allocates objects on the heap. When the garbage collector cannot free enough memory to satisfy a new allocation, this error is thrown. Common causes include loading large datasets into memory, memory leaks from unclosed resources, or growing collections without bounds.

The Problem

List<byte[]> list = new ArrayList<>();
while (true) {
    list.add(new byte[1_000_000]); // Unbounded allocation
}

The Fix

// Process data in chunks instead of loading everything
try (BufferedReader reader = new BufferedReader(new FileReader("large.txt"))) {
    String line;
    while ((line = reader.readLine()) != null) {
        processLine(line); // Process one line at a time
    }
}

Step-by-Step Fix

1. 1

   Identify the allocation source

   Use a heap dump (jmap -dump) or profiler to find which objects consume the most memory.

2. 2

   Check for memory leaks

   Look for collections that grow without bound, unclosed streams, or static fields holding large objects.

3. 3

   Fix the leak or increase heap

   Fix memory leaks by closing resources, limiting collection sizes, or processing data in streams. As a temporary fix, increase heap with -Xmx.