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Jackson is a suite of data-processing tools for Java (and the JVM platform)

Jackson最常用的Json序列化功能，引入如下的包即可：

<properties>...<!-- Use the latest version whenever possible. --><jackson.version>2.17.1</jackson.version>...
</properties><dependencies>...<dependency><groupId>com.fasterxml.jackson.core</groupId><artifactId>jackson-databind</artifactId><version>${jackson.version}</version></dependency>...
</dependencies>

Json序列化的坑

首先来看一个Json序列化异常。
JsonUtil工具类如下：

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.*;public class JsonUtil {private static final ObjectMapper OBJECT_MAPPER = buildLowerCamelCase();private static ObjectMapper buildLowerCamelCase() {ObjectMapper objectMapper = new ObjectMapper();objectMapper.disable(SerializationFeature.FAIL_ON_EMPTY_BEANS);objectMapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);objectMapper.setPropertyNamingStrategy(PropertyNamingStrategies.LOWER_CAMEL_CASE);return objectMapper;}public static String json(Object obj) {try {return OBJECT_MAPPER.writeValueAsString(obj);} catch (JsonProcessingException e) {throw new RuntimeException(e);}}}

另外有一个实体类CodeMonkey:

import lombok.Data;
import java.util.Objects;@Data
public class CodeMonkey {private Long id;private String name;public boolean isJavaer () {// mock throw exceptionthrow new IllegalArgumentException("should be javaer");}
}

我们对改实体对象进行Json序列化：

public class JacksonTestApplication {public static void main(String[] args) {CodeMonkey codeMonkey = new CodeMonkey();codeMonkey.setId(1L);codeMonkey.setName("not bty");log.info("hello , {}", JsonUtil.json(codeMonkey));}}

运行发现报错：

Exception in thread "main" java.lang.RuntimeException: com.fasterxml.jackson.databind.JsonMappingException: should be javaer (through reference chain: io.github.bty834.jacksontest.CodeMonkey["javaer"])at io.github.bty834.jacksontest.util.JsonUtil.json(JsonUtil.java:22)at io.github.bty834.jacksontest.JacksonTestApplication.main(JacksonTestApplication.java:16)
Caused by: com.fasterxml.jackson.databind.JsonMappingException: should be javaer (through reference chain: io.github.bty834.jacksontest.CodeMonkey["javaer"])at com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:392)at com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:351)...

可以看到Jackson将isJavaer方法当作字段来序列化，但isJavaer其实是不需要序列化的，
可以在方法上添加 com.fasterxml.jackson.annotation.JsonIgnore 注解忽略该方法。

经过调试，Jackson使用com.fasterxml.jackson.databind.introspect.POJOPropertiesCollector#collectAll方法获取某个类型需要序列化的属性信息:

public class POJOPropertiesCollector {...protected void collectAll() {LinkedHashMap<String, POJOPropertyBuilder> props = new LinkedHashMap<String, POJOPropertyBuilder>();// 最终调用 java.lang.Class#getDeclaredFields 获取字段// final字段和transient字段可配置，默认都序列化_addFields(props); // 最终调用 java.lang.Class#getDeclaredMethods // 获取get/set/is开头的方法（且方法入参分别为0或1，any getter这里不讨论）_addMethods(props);...// 去掉不需要的属性， 如isJavaer方法产生的javaer prop_removeUnwantedProperties(props);...}...
}

Json反序列化的坑

回到CodeMonkey类：

@Data
public class CodeMonkey {private Long id;private String name;private Language language;@JsonIgnorepublic boolean isJavaer () {if (Objects.equals(this.name, "bty")) {return true;}throw new IllegalArgumentException("should be javaer");}
}@Getter
@AllArgsConstructor
public enum Language {JAVA(1, "Java"),C_PLUS_PLUS(2, "C++"),GO(10, "Go");private final int code;private final String name;
}

我们选取两个json字符串模拟反序列化：

@Slf4j
public class JacksonTestApplication {public static void main(String[] args) {// language取1String jsonStr1 = "{\"id\":1,\"name\":\"bty\",\"language\": 1}";CodeMonkey codeMonkey1 = JsonUtil.fromJson(jsonStr1, CodeMonkey.class);System.out.println(codeMonkey1); // CodeMonkey(id=1, name=bty, language=C_PLUS_PLUS)// language取JAVAString jsonStr2 = "{\"id\":1,\"name\":\"bty\",\"language\": \"JAVA\"}";CodeMonkey codeMonkey2 = JsonUtil.fromJson(jsonStr2, CodeMonkey.class);System.out.println(codeMonkey2); // CodeMonkey(id=1, name=bty, language=JAVA)}
}

为啥我们language传1时，反序列化为C_PLUS_PLUS枚举了呢？C_PLUS_PLUS的code字段也不是1啊

在不添加jackson额外注解时，枚举序列化后是枚举值即大写英文字母，而反序列化会经过多个流程。

具体来说，枚举的反序列化默认通过com.fasterxml.jackson.databind.deser.std.EnumDeserializer#deserialize完成：

public class EnumDeserializerextends StdScalarDeserializer<Object>implements ContextualDeserializer
{...@Overridepublic Object deserialize(JsonParser p, DeserializationContext ctxt) throws IOException{if (p.hasToken(JsonToken.VALUE_STRING)) {// 1. 如果传的是字符串，则解析字符串return _fromString(p, ctxt, p.getText());}// 如果是int能表示的数字（包含能转为int的long类型）if (p.hasToken(JsonToken.VALUE_NUMBER_INT)) { if (_isFromIntValue) {  // 判断根据@JsonValue的值来解析// 将数字转换为字符串根据@JsonValue注解的字段或方法解析return _fromString(p, ctxt, p.getText());}// 根据反序列化配置的CoercionAction取值或报错// CoercionAction 分为 Fail , TryConvert , AsNull , AsEmpty// 枚举这里为TryConvert，具体逻辑参考：com.fasterxml.jackson.databind.cfg.CoercionConfigs#findCoercion// TryConvert会使用枚举值在枚举中的index来匹配这里的int，很坑！return _fromInteger(p, ctxt, p.getIntValue());}// 解析嵌套json对象if (p.isExpectedStartObjectToken()) {return _fromString(p, ctxt,ctxt.extractScalarFromObject(p, this, _valueClass));}// 解析嵌套json数组return _deserializeOther(p, ctxt);}...
}

如果规避以上的坑呢？

@JsonValue

code字段添加@JsonValue，则反序列化只能根据code值(int或int的字符串都行)进行，除了会影响反序列化，序列化时会将该枚举写成其code值

@Getter
@AllArgsConstructor
public enum Language {JAVA(1L, "Java"),C_PLUS_PLUS(2L, "C++"),GO(10L, "Go");@JsonValueprivate final long code;private final String name;
}

@JsonCreator

2. 如果我既想要根据code字段也想要根据Enum.name()来反序列化呢？自定义反序列化方法并注解@JsonCreator

@Getter
@AllArgsConstructor
public enum Language {JAVA(1L, "Java"),C_PLUS_PLUS(2L, "C++"),GO(10L, "Go");@JsonValueprivate final long code;private final String name;@JsonCreatorpublic static Language parse(String val) {for (Language value : Language.values()) {// 先根据Enum.name()判断if (value.name().equals(val)) {return value;}// 再根据code字段判断if (Objects.equals(String.valueOf(value.code), val)) {return value;}}return null;}
}

DeserializationFeature.FAIL_ON_NUMBERS_FOR_ENUMS

可以打开反序列化配置objectMapper.enable(DeserializationFeature.FAIL_ON_NUMBERS_FOR_ENUMS); 来避免int匹配不到时根据index来匹配，会直接报错。但并不影响@JsonValue和@JsonCreator的使用。