Fastjson Boolean Property Naming Issue and How to Resolve It

Background

A colleague noticed that after serializing a Java object with Fastjson, the resulting JSON string unexpectedly contained an extra property and omitted the original boolean field prefixed with is . The issue appears when boolean getters follow the isXxx naming convention.

Reproduction

The following class reproduces the problem:

public class MyClass {
    // boolean property
    private boolean isActive;
    private boolean valid;
    // int property
    private int id;

    public MyClass() {}

    public MyClass(boolean isActive, boolean valid, int id) {
        this.isActive = isActive;
        this.valid = valid;
        this.id = id;
    }

    public boolean isActive() { return isActive; }
    public void setActive(boolean isActive) { this.isActive = isActive; }
    public boolean getValid() { return valid; }
    public void setValid(boolean valid) { this.valid = valid; }
    public int getId() { return id; }
    public void setId(int id) { this.id = id; }
}

Test code:

import com.alibaba.fastjson.JSON;

public class MyClassMain {
    public static void main(String[] args) {
        MyClass myClass = new MyClass(true, false, 123);
        String jsonString = JSON.toJSONString(myClass);
        System.out.println(jsonString);
    }
}

The output is {"active":true,"id":123,"valid":false} , showing the isActive field was renamed to active and the original isActive name disappeared.

Analysis

Fastjson builds bean information via com.alibaba.fastjson.serializer.SerializeConfig#createJavaBeanSerializer , which ultimately calls com.alibaba.fastjson.util.TypeUtils#computeGetters . When a method name starts with is , Fastjson treats it as a boolean getter and derives the property name by stripping the is prefix and decapitalizing the remainder. This conversion turns isActive into active , causing the mismatch.

Solutions

1. Follow Alibaba Java Development Manual : Avoid using the is prefix for boolean fields; name the field active instead of isActive . This prevents Fastjson from mis‑interpreting the getter.

2. Use @JSONField Alias : Annotate the field with @JSONField(name="isActive") to explicitly set the JSON property name, though this introduces Fastjson‑specific coupling.

public class MyClass {
    @JSONField(name="isActive")
    private boolean isActive;
    private boolean valid;
    // other members omitted
}

Conclusion

For Java developers, reading the Alibaba Java Development Manual can prevent many subtle bugs like this, and when necessary, the @JSONField annotation offers a quick fix, albeit with higher library coupling.