SpringBoot中处理JSON的三大库应用全指南

// 对象转 JSON
User user = new User("Eve", 29, null);
String json = JSON.toJSONString(user);

// JSON 转对象
User userFromJson = JSON.parseObject(json, User.class);

ObjectMapper objectMapper = new ObjectMapper();
User user = new User("Frank", 31, null);

// 对象转 JSON
String json = objectMapper.writeValueAsString(user);

// JSON 转对象
User userFromJson = objectMapper.readValue(json, User.class);

Gson gson = new Gson();
User user = new User("Grace", 27, null);

// 对象转 JSON
String json = gson.toJson(user);

// JSON 转对象
User userFromJson = gson.fromJson(json, User.class);

// 局部保留：仅当前转换生效
String jsonWithNull = JSON.toJSONString(user, SerializerFeature.WriteMapNullValue);
// 输出：{"address":null,"age":29,"name":"Eve"}

// 全局保留：所有转换默认保留 null
JSON.DEFAULT_GENERATE_FEATURE |= SerializerFeature.WriteMapNullValue.getMask();
String globalJson = JSON.toJSONString(user); // 同样保留 null 字段

// 局部忽略：覆盖全局配置，仅当前转换忽略 null
String jsonWithoutNull = JSON.toJSONString(user, SerializerFeature.IgnoreNullValue);
// 输出：{"age":29,"name":"Eve"}

// 全局保留：所有类默认保留 null
ObjectMapper objectMapper = new ObjectMapper();
objectMapper.setSerializationInclusion(JsonInclude.Include.ALWAYS);

// 局部保留：仅当前类保留 null（注解优先级更高）
@JsonInclude(JsonInclude.Include.ALWAYS)
public class User {
    private String name;
    private Integer age;
    private String address; // null 时仍会序列化
}

// 输出结果：{"name":"Frank","age":31,"address":null}

// 全局忽略：所有类默认忽略 null
objectMapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);

// 局部忽略：仅当前类忽略 null
@JsonInclude(JsonInclude.Include.NON_NULL)
public class User { ... }

// 输出结果：{"name":"Frank","age":31}（address 为 null 时被忽略）

Gson gson = new Gson();
String json = gson.toJson(user); // user.address 为 null 时，输出 {"address":null,...}

Gson gson = new GsonBuilder()
    .setExclusionStrategies(new ExclusionStrategy() {
        @Override
        public boolean shouldSkipField(FieldAttributes f) {
            // 忽略所有 null 字段
            try {
                return f.get(null) == null;
            } catch (Exception e) {
                return false;
            }
        }
        @Override
        public boolean shouldSkipClass(Class<?> clazz) {
            return false;
        }
    })
    .create();

// 输出结果：{"name":"Grace","age":27}（address 为 null 时被忽略）