Why Does FastJSON Invoke isChinaName() During Serialization? Explore the Mechanics

Online Incident Review

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Scenario Reconstruction

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 FastJSON calls isChinaName() during serialization while country is null.

Root Cause Questions

Why does serialization invoke isChinaName()?

What methods are executed during FastJSON serialization?

Source Code Analysis

Debugging reveals the call chain reaches ASMSerializer_1_CountryDTO.write, a class dynamically generated by FastJSON using the ASM bytecode framework.

The core of FastJSON serialization is the JavaBeanSerializer class, whose write() method is invoked. It obtains an object writer via SerializeConfig#createJavaBeanSerializer, which eventually calls TypeUtils#computeGetters to decide which getters and isXxx methods participate.

public static List<FieldInfo> computeGetters(Class<?> clazz, JSONType jsonType, Map<String, String> aliasMap, Map<String, Field> fieldCacheMap, boolean sorted, PropertyNamingStrategy propertyNamingStrategy) {
    // ... omitted code ...
    Method[] methods = clazz.getMethods();
    for (Method method : methods) {
        // skip void return or methods with parameters
        // process @JSONField annotation
        // handle methods starting with "get" or "is"
    }
}

The method selects three kinds of members:

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

Methods starting with get Methods starting with

is

Serialization Flowchart

Example Code

/** case1: @JSONField(serialize = false) */
@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"; }

Running the test produces:

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

Code Guidelines

To avoid accidental method invocation during serialization, explicitly mark methods with @JSONField(serialize = false). The revised CountryDTO looks like:

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"; }
}

Three Frequently Encountered Serialization Cases

The overall workflow follows: discover the problem → analyze the principle → solve the issue → formalize coding standards.

Source: 老鹰汤, juejin.cn/post/7156439842958606349