Why FastJSON Invokes isChinaName() During Serialization and How to Control It

Online Incident Review

Recently a simple feature was added with an extra log line; after deployment a flood of alerts appeared, prompting an immediate rollback and removal of the log statement.

Scenario Reproduction

Define a CountryDTO class

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");
    }
}

Define a test class FastJonTest

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 the serialization process invokes isChinaName() while this.country is null.

The error reveals that FastJSON calls isChinaName() during serialization, leading to the following questions:

Why does serialization execute isChinaName()?

What other methods are invoked during serialization?

Source Code Analysis

Debugging the call stack shows that FastJSON uses ASM to dynamically generate a class ASMSerializer_1_CountryDTO whose write method performs the serialization.

The ASMSerializer_1_CountryDTO.write method is generated by FastJSON using ASM, which replaces reflection with dynamically generated bytecode to improve performance.

One use case of ASM is to generate classes at runtime to avoid the overhead of Java reflection.

JavaBeanSerializer Serialization Principle

The serialization process mainly calls the write() method of JavaBeanSerializer.

JavaBeanSerializer

obtains an ObjectWriter via getObjectWriter(). Debugging this flow leads to SerializeConfig#createJavaBeanSerializer and ultimately to TypeUtils#computeGetters:

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;
            }
            if (annotation.name().length() != 0) {
                // ...
            }
        }
        if (methodName.startsWith("get")) {
            // ...
        }
        if (methodName.startsWith("is")) {
            // ...
        }
    }
}

The method selection follows three rules:

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

Methods starting with get Methods starting with

is

Serialization Flowchart

Example Code

/**
 * case1: @JSONField(serialize = false)
 * case2: getXxx() returns void
 * case3: isXxx() returns non‑boolean
 * case4: @JSONType(ignores = "xxx")
 */
@JSONType(ignores = "otherName")
public class CountryDTO {
    private String country;

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

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

    public static void queryCountryList() {
        System.out.println("queryCountryList()执行!!");
    }

    public Boolean isChinaName() {
        System.out.println("isChinaName()执行!!");
        return true;
    }

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

    public String getOtherName() {
        System.out.println("getOtherName()执行!!");
        return "lucy";
    }

    @JSONField(serialize = false)
    public String getEnglishName2() {
        System.out.println("getEnglishName2()执行!!");
        return "lucy";
    }

    public void getEnglishName3() {
        System.out.println("getEnglishName3()执行!!");
    }

    public String isChinaName2() {
        System.out.println("isChinaName2()执行!!");
        return "isChinaName2";
    }
}

Running the test produces:

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

Code Guidelines

Serialization rules are numerous; developers must pay attention to return types, parameter counts, and annotations such as @JSONType and @JSONField. To make the codebase clearer, it is recommended to explicitly mark non‑serializable methods with @JSONField(serialize = false). The following code shows the recommended style:

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()执行!!");
    }

    public Boolean isChinaName() {
        System.out.println("isChinaName()执行!!");
        return true;
    }

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

    @JSONField(serialize = false)
    public String getOtherName() {
        System.out.println("getOtherName()执行!!");
        return "lucy";
    }

    @JSONField(serialize = false)
    public String getEnglishName2() {
        System.out.println("getEnglishName2()执行!!");
        return "lucy";
    }

    @JSONField(serialize = false)
    public void getEnglishName3() {
        System.out.println("getEnglishName3()执行!!");
    }

    @JSONField(serialize = false)
    public String isChinaName2() {
        System.out.println("isChinaName2()执行!!");
        return "isChinaName2";
    }
}

Three Common Serialization Cases

The overall process follows: discover the problem → analyze the principle → solve the problem → elevate to coding standards.