FastJSON Serialization Mechanics: Why isChinaName() Executes and How to Control Serialized Output

Online incident review : A simple feature was added with an extra log statement before deployment; after going live, a flood of alerts forced an immediate rollback.

Scenario reconstruction : The CountryDTO class and a test class FastJonTest were defined. During serialization, a NullPointerException occurred because the method isChinaName() was invoked while the country field was still null.

Source code analysis : FastJSON uses ASM to dynamically generate a class named ASMSerializer_1_CountryDTO . The actual serialization is performed by JavaBeanSerializer.write() . The method com.alibaba.fastjson.util.TypeUtils.computeGetters determines which getters are included based on return type, parameters, and annotations.

public static List<FieldInfo> computeGetters(Class<?> clazz, JSONType jsonType, Map<String,String> aliasMap, Map<String,Field> fieldCacheMap, boolean sorted, PropertyNamingStrategy propertyNamingStrategy) {
    // ...
    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) {
            if (!annotation.serialize()) continue;
            // handle name alias
        }
        String methodName = method.getName();
        if (methodName.startsWith("get")) {
            // treat as getter
        }
        if (methodName.startsWith("is")) {
            // treat as boolean getter
        }
    }
}

Three exclusion cases :

@JSONField(serialize = false, name = "xxx") annotation explicitly disables serialization.

Getter methods with a void return type are ignored.

Methods starting with is that do not return a boolean are ignored.

Serialization flow : The process follows the pattern – problem discovery → principle analysis → solution implementation → coding standards improvement.

Example code demonstrating the cases:

@JSONType(ignores = "otherName")
public class CountryDTO {
    private String country;
    public void setCountry(String country) { this.country = country; }
    public String getCountry() { return this.country; }
    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 getEnglishName2() { System.out.println("getEnglishName2() executed!!"); return "lucy"; }
    // other methods omitted for brevity
}

Running result : The console prints isChinaName() executed!! and getEnglishName() executed!! , and the final JSON output is {"chinaName":true,"englishName":"lucy"} .

Code conventions : To avoid accidental serialization, it is recommended to annotate non‑serializable methods with @JSONField(serialize = false) , making the intent explicit and improving code readability.