Why Does FastJSON Call isChinaName() During Serialization? Deep Dive & Fix

Problem Overview

A simple log line added before release triggered a series of alarms. After rolling back the log statement the service started normally again. The root cause was a NullPointerException that occurred during FastJSON serialization because the method isChinaName() was executed while the field country was still null.

Reproducing the Issue

Define a DTO class and a test case:

public class CountryDTO {
    private String country;

    public void setCountry(String country) { this.country = country; }
    public String getCountry() { return this.country; }
    public Boolean isChinaName() { return this.country.equals("中国"); }
}

public class FastJonTest {
    @Test
    public void testSerialize() {
        CountryDTO countryDTO = new CountryDTO();
        String str = JSON.toJSONString(countryDTO);
        System.out.println(str);
    }
}

Running the test produces a NullPointerException because this.country is null when isChinaName() is called.

Why Does Serialization Call isChinaName() ?

FastJSON builds a dynamic serializer class (e.g., ASMSerializer_1_CountryDTO) using ASM bytecode generation. The generated write() method ultimately invokes JavaBeanSerializer.write(), which reflects over the bean to discover getters and boolean‑style methods.

Key Methods in the Serialization Path

JavaBeanSerializer.write()

SerializeConfig#createJavaBeanSerializer()

TypeUtils#computeGetters()

During computeGetters() FastJSON iterates over all public methods, skips those with a void return type or with parameters, and treats methods starting with get or is as property accessors. If a method is annotated with @JSONField(serialize = false) it is excluded.

Three Typical Scenarios That Influence Serialization

@JSONField(serialize = false, name = "xxx")

– explicit exclusion.

Getter methods returning void – ignored.

Boolean‑style methods ( isXxx()) whose return type is not boolean – ignored.

Visualization

The following images illustrate the call chain and flowchart of FastJSON serialization:

Best‑Practice Code Example

Use @JSONField(serialize = false) to clearly mark methods that should not participate in serialization:

@JSONType(ignores = "otherName")
public class CountryDTO {
    private String country;

    public void setCountry(String country) { this.country = country; }
    public String getCountry() { return this.country; }

    @JSONField(serialize = false)
    public Boolean isChinaName() { return "中国".equals(this.country); }

    @JSONField(serialize = false)
    public void queryCountryList() { System.out.println("queryCountryList() executed!!"); }

    public String getEnglishName() { System.out.println("getEnglishName() executed!!"); return "lucy"; }
}

Running the serializer now prints only the desired fields without triggering the NPE.

Conclusion

The investigation follows the pattern Problem → Principle Analysis → Solution → Coding Standards . Understanding FastJSON’s dynamic serializer generation helps developers avoid hidden method invocations, and using explicit @JSONField annotations provides a maintainable way to control serialization across multiple systems.