Understanding FastJSON Serialization: Why isChinaName() Is Invoked and How to Control Serialized Methods

Incident Review

Recently a simple logging line was added to a new feature, but after deployment a cascade of alerts forced a rollback. The root cause was a NullPointerException triggered during JSON serialization.

Reproducing the Problem

Define a CountryDTO class and a test class FastJonTest that serializes an instance using FastJSON:

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 throws a NullPointerException because isChinaName() is invoked while country is still null.

Why isChinaName() Is Executed?

FastJSON uses ASM to generate a serializer class (e.g., ASMSerializer_1_CountryDTO ) that replaces reflection. The generated write() method calls JavaBeanSerializer.write() , which obtains an ObjectWriter that inspects getter methods.

During the inspection FastJSON’s computeGetters method treats any public method with no parameters and a non‑void return type that starts with get or is as a property accessor, unless it is excluded by annotations.

public static List
computeGetters(Class
clazz, JSONType jsonType, ... ) {
    Method[] methods = clazz.getMethods();
    for (Method method : methods) {
        if (method.getReturnType().equals(Void.TYPE)) continue;
        if (method.getParameterTypes().length != 0) continue;
        JSONField annotation = TypeUtils.getAnnotation(method, JSONField.class);
        if (annotation != null && !annotation.serialize()) continue;
        if (methodName.startsWith("get")) { /* handle getter */ }
        if (methodName.startsWith("is")) { /* handle boolean getter */ }
    }
}

Thus isChinaName() is considered a boolean getter and is called during serialization, leading to the NPE.

What Methods Are Invoked During Serialization?

@JSONField(serialize = false, name = "xxx") – excluded.

Methods starting with get – treated as getters.

Methods starting with is – treated as boolean getters.

Recommended Coding Practice

To avoid accidental invocation, explicitly mark methods that should not be serialized with @JSONField(serialize = false) . The following example shows a clean version of CountryDTO where only intended properties are serialized:

public class CountryDTO {
    private String country;
    public void setCountry(String country) { this.country = country; }
    public String getCountry() { return this.country; }
    @JSONField(serialize = false)
    public static void queryCountryList() { System.out.println("queryCountryList() executed!!"); }
    public Boolean isChinaName() { System.out.println("isChinaName() executed!!"); return true; }
    public String getEnglishName() { System.out.println("getEnglishName() executed!!"); return "lucy"; }
    @JSONField(serialize = false)
    public String getOtherName() { System.out.println("getOtherName() executed!!"); return "lucy"; }
    @JSONField(serialize = false)
    public String getEnglishName2() { System.out.println("getEnglishName2() executed!!"); return "lucy"; }
    @JSONField(serialize = false)
    public void getEnglishName3() { System.out.println("getEnglishName3() executed!!"); }
    @JSONField(serialize = false)
    public String isChinaName2() { System.out.println("isChinaName2() executed!!"); return "isChinaName2"; }
}

Running the test now produces the expected JSON without NPE:

isChinaName() executed!!
getEnglishName() executed!!
{"chinaName":true,"englishName":"lucy"}

Conclusion

The debugging process followed the pattern: discover the issue → analyze the underlying FastJSON serialization mechanism → apply a solution (explicit exclusion) → formalize a coding guideline. Understanding which methods FastJSON treats as properties helps prevent similar bugs in backend Java projects.