Lombok's hashCode and the "java.lang.StackOverflowError: null" Mystery
Introduction
Lombok is a popular Java library that simplifies code by automatically generating boilerplate code like getters, setters, constructors, and toString methods. While incredibly convenient, Lombok can sometimes lead to unexpected issues, especially when working with its generated hashCode method. This article will explore the "java.lang.StackOverflowError: null" error specifically related to Lombok's hashCode implementation and provide solutions to mitigate this problem.
The Problem
Imagine you have a simple Java class with a few fields:
import lombok.Data;
@Data
public class MyObject {
private String name;
private Integer age;
private MyObject parent;
}
Now, let's say you have a scenario where the parent field of MyObject can recursively reference itself, creating a circular dependency. This seemingly innocuous setup can lead to a StackOverflowError when calling hashCode on an instance of MyObject.
Understanding the Issue
Lombok's generated hashCode method calculates a hash code based on the object's fields. In the case of MyObject, it includes the parent field. When hashCode is called on an instance, it recursively calls hashCode on the parent field, potentially leading to an endless loop if the parent field keeps referencing itself.
The StackOverflowError occurs because the Java Virtual Machine (JVM) has a finite stack space. When the recursive calls to hashCode exceed this limit, the stack overflows, resulting in the error.
Solution: Breaking the Cycle
To prevent the StackOverflowError, we need to break the cycle of recursive calls. Here are two approaches:
- Use a
WeakReference: Replace theparentfield with aWeakReference. This ensures that theparentobject is not strongly referenced, preventing the infinite loop.
import lombok.Data;
import java.lang.ref.WeakReference;
@Data
public class MyObject {
private String name;
private Integer age;
private WeakReference<MyObject> parent;
}
- Override
hashCode: Manually override thehashCodemethod in your class and handle the recursive calls yourself. This allows you to break the cycle by checking for circular references and skipping thehashCodecall for theparentfield.
import lombok.Data;
import java.util.Objects;
@Data
public class MyObject {
private String name;
private Integer age;
private MyObject parent;
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((name == null) ? 0 : name.hashCode());
result = prime * result + ((age == null) ? 0 : age.hashCode());
// Check for circular reference and skip parent field
if (this == parent || parent == null) {
return result;
} else {
return prime * result + parent.hashCode();
}
}
}
Conclusion
The StackOverflowError caused by Lombok's hashCode in the presence of circular references can be a frustrating issue. By understanding the root cause and applying the solutions outlined above, you can avoid this error and ensure your application runs smoothly. Remember to always be mindful of potential recursive dependencies when designing your object relationships and choose the solution that best suits your needs and coding style.
