Jackson中那些你需要掌握的知识点

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.12.4</version>
</dependency>
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public static <T> String encode(T obj) {
    try {
        return new ObjectMapper().writeValueAsString(obj);
    } catch (Exception e) {
        throw new UnsupportedOperationException(e);
    }
}
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public static <T> T decode(String str, Class<T> clz) {
    try {
        return new ObjectMapper().readValue(str, clz);
    } catch (Exception e) {
        throw new UnsupportedOperationException(e);
    }
}
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public static <T> T decode(String str, Type type) {
    try {
        return objectMapper.readValue(str, objectMapper.getTypeFactory().constructType(type));
    } catch (Exception e) {
        throw new UnsupportedOperationException(e);
    }
}
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GenericBean<Map> gbean2 = JacksonUtil.decode(str, new com.fasterxml.jackson.core.type
      .TypeReference<GenericBean<Map>>() {}.getType());
System.out.println(gbean2);
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public static Map toMap(String str) {
    try {
        return objectMapper.readValue(str, Map.class);
    } catch (JsonProcessingException e) {
        throw new UnsupportedOperationException(e);
    }
}
public static List toList(String str) {
    try {
        return objectMapper.readValue(str, List.class);
    } catch (JsonProcessingException e) {
        throw new UnsupportedOperationException(e);
    }
}
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public static JsonNode toObj(String str) {
    try {
        return objectMapper.readTree(str);
    } catch (JsonProcessingException e) {
        throw new UnsupportedOperationException(e);
    }
}
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String str = "{\"userId\":12,\"userName\":\"yh\",\"userMoney\":12.3,\"userSkills\":[\"1\",\"2\",\"3\"],\"extra\":{\"a\":\"123\",\"b\":345,\"c\":[\"1\",\"2\",\"3\"],\"d\":35.1},\"hello\":\"你好\"}";
JsonNode bean = JacksonUtil.toObj(str);
int userId = bean.get("userId").asInt();
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/**
 * 驼峰转下换线
 *
 * @param obj
 * @return
 */
public static String toUnderStr(Object obj) {
    ObjectMapper objectMapper = new ObjectMapper();
    // 驼峰转下划线
    objectMapper.setPropertyNamingStrategy(PropertyNamingStrategies.SNAKE_CASE);
    try {
        return objectMapper.writerWithDefaultPrettyPrinter().writeValueAsString(obj);
    } catch (JsonProcessingException e) {
        throw new UnsupportedOperationException(e);
    }
}
/**
 * 下划线格式json串，转驼峰格式的Java bean
 */ 
public static <T> T fromUnderStr(String str, Class<T> clz) {
    ObjectMapper objectMapper = new ObjectMapper();
    // 驼峰转下划线
    objectMapper.setPropertyNamingStrategy(PropertyNamingStrategies.SNAKE_CASE);
    try {
        return objectMapper.readValue(str, clz);
    } catch (JsonProcessingException e) {
        throw new UnsupportedOperationException(e);
    }
}
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@JsonNaming(PropertyNamingStrategies.SnakeCaseStrategy.class)
public class SimpleBean implements Serializable {
    private static final long serialVersionUID = -9111747337710917591L;
    private Integer userId;
    private String userName;
    private double userMoney;
}
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@JsonProperty("user")
private String userName;
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@Data
public class SimpleBean implements Serializable {
    private static final long serialVersionUID = -9111747337710917591L;
    @JsonIgnore
    private SimpleBean self;
}
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private transient SimpleBean self;
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objectMapper = new ObjectMapper();
// 忽略 transient 关键字的配置
// case1
objectMapper.configure(MapperFeature.PROPAGATE_TRANSIENT_MARKER, true);
// case2
objectMapper.setVisibility(objectMapper.getSerializationConfig()
        .getDefaultVisibilityChecker()
        .withFieldVisibility(JsonAutoDetect.Visibility.ANY)
        .withGetterVisibility(JsonAutoDetect.Visibility.NONE)
        .withIsGetterVisibility(JsonAutoDetect.Visibility.NONE));
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public class SimpleBean {
  private String name;
  private Long userId;
  public String desc = "hello world";
  public SimpleBean() {
      name = "yhh";
      userId = 10L;
  }
}
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No serializer found for class com.git.hui.spring.json.bean.SimpleBean and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS)
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Cannot construct instance of `com.git.hui.spring.json.bean.SimpleBean` (although at least one Creator exists): cannot deserialize from Object value (no delegate- or property-based Creator)
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Unrecognized field "xxx" (class com.git.hui.spring.json.bean.GenericBean), not marked as ignorable
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new ObjectMapper().disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES)
    .readValue(str, Xxx.class)
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{
  "userId" : null,
  "userName" : null,
  "userMoney" : 0.0,
  "userSkills" : null,
  "extra" : null
}
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new ObjectMapper().setSerializationInclusion(JsonInclude.Include.NON_NULL)
    .writerWithDefaultPrettyPrinter().writeValueAsString(xxx)
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Map<String, String> map = new HashMap<>();
map.put(null, "123");
new ObjectMapper().writeValueAsString(map);
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Null key for a Map not allowed in JSON (use a converting NullKeySerializer?)
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ObjectMapper objectMapper = new ObjectMapper();
objectMapper.getSerializerProvider().setNullKeySerializer(new JsonSerializer<Object>() {
    @Override
    public void serialize(Object value, JsonGenerator gen, SerializerProvider serializers) throws IOException {
        gen.writeFieldName("null");
    }
});
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Map map = new HashMap();
map.put(new ArrayList<>(), 123);
new ObjectMapper().writeValueAsString(map);
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{"[]":123}
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