serialization - jackson 注释 : Difference Between JsonIgnoreProperties(ignoreUnknown=true) and JsonInclude(Include. NON_EMPTY)

Question

我很好奇 Jackson 注释 @JsonIgnoreProperties(ignoreUnknown=true) 和 @JsonInclude(Include.NON_EMPTY) 在类级别上有区别吗？一个只是另一个的更新版本吗？谢谢!

jackson docs声明:

ignoreUnknown Property that defines whether it is ok to just ignore any unrecognized properties during deserialization.

这和空属性一样吗？

Answer 1

简短的回答:

@JsonIgnoreProperties(ignoreUnknown=true)仅适用于将 JSON 反序列化为 Java 对象 ( POJO )。如果您的 POJO 不包含 JSON 包含的某些属性，它们将被忽略并且不会引发错误。

另一方面@JsonInclude(Include.NON_EMPTY)用于将 POJO 序列化为 JSON，它说，跳过以下 POJO 属性:

null or what is considered empty are not to be included. Definition of emptiness is data type-specific.

长答案:

@JsonInclude

它仅在序列化时使用。它表示，如果所讨论的属性(或所有属性)的值等于某个值( null 、 empty - 无论这意味着什么，或默认值)，则该属性不会序列化。

如果没有这个注解，属性值总是被序列化。注释有助于减少传输的属性数量(当接收端不存在属性默认值时，必须指定它)。

例子:

public class Person {
    public String firstName = "Mark";
    public String middleName;
    public String lastName = "Watney";
}

ObjectMapper mapper = new ObjectMapper();
Person p = new Person();
System.out.println(mapper.writeValueAsString(p));

产生以下输出:

{"firstName":"Mark", "middleName":null, "lastName":"Watney"}

但如果 Person用 @JsonInclude(Include.NON_EMPTY) 注释, middleName从输出中省略，因为它的值为“空”(在本例中为 null):

@JsonInclude(Include.NON_EMPTY)
public static class Person {
    [....]
}

控制台输出为: {"firstName":"Mark", "lastName":"Watney"}
@JsonIgnoreProperties

用于忽略序列化和反序列化中的某些属性，而不管其值如何:

to prevent specified fields from being serialized or deserialized (i.e. not include in JSON output; or being set even if they were included): @JsonIgnoreProperties({ "internalId", "secretKey" })

To ignore any unknown properties in JSON input without exception: @JsonIgnoreProperties(ignoreUnknown=true)

如果 JSON 输入是:

{
    "firstName": "Homer",
    "middleName": "Jay",
    "lastName": "Simpson"
}

类(class)是:

public class Person {
    public String firstName;
    public String lastName;
}

反序列化 mapper.readValue(json, Person.class)将产生 UnrecognizedPropertyException异常(exception):

Exception in thread "main" com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: Unrecognized field "middleName" .....

因为属性(property) middleName不属于 Person类(class)。

但是如果类 Person用 @JsonIgnoreProperties(ignoreUnknown=true) 注释，在反序列化为 POJO 时将忽略未知属性(如 middleName)。

@JsonIgnoreProperties(ignoreUnknown=true)
public class person {
    [...]
}

另一个常见用例是抑制敏感属性的序列化，例如密码:

@JsonIgnoreProperties("password")
public static class User {
    public String login = "simpsonh";
    public String password = "D00nut";
    public String firstName = "Homer";
    public String middleName = "Jay";
    public String lastName = "Simpson";
}

现在，如果您序列化 User class ，则输出中将省略密码:

User u = new User();
System.out.println(mapper.writeValueAsString(u));

控制台输出:

{
    "login":"simpsonh",
    "firstName":"Homer",
    "middleName":"Jay",
    "lastName":"Simpson"
}