Jackson JSON针对某些领域的自定义序列化

Question

有没有办法使用Jackson JSON处理器进行自定义字段级序列化？例如，我想要上课

public class Person {
    public String name;
    public int age;
    public int favoriteNumber;
}

序列化为以下JSON：

{ "name": "Joe", "age": 25, "favoriteNumber": "123" }

请注意，age = 25编码为数字，而favoriteNumber = 123编码为字符串。杰克逊开箱即用int编号。在这种情况下，我希望favoriteNumber被编码为字符串。

Answer 1

您可以按如下方式实现自定义序列化程序：

public class Person {
    public String name;
    public int age;
    @JsonSerialize(using = IntToStringSerializer.class, as=String.class)
    public int favoriteNumber:
}


public class IntToStringSerializer extends JsonSerializer<Integer> {

    @Override
    public void serialize(Integer tmpInt, 
                          JsonGenerator jsonGenerator, 
                          SerializerProvider serializerProvider) 
                          throws IOException, JsonProcessingException {
        jsonGenerator.writeObject(tmpInt.toString());
    }
}

Java应该为您处理从int到Integer的自动装箱。

Answer 2

Jackson-databind（至少2.1.3）提供特殊ToStringSerializer（com.fasterxml.jackson.databind.ser.std.ToStringSerializer）

示例：

public class Person {
    public String name;
    public int age;
    @JsonSerialize(using = ToStringSerializer.class)
    public int favoriteNumber:
}

Answer 3

为@JsonProperty字段添加String带注释的getter，返回favoriteNumber：

public class Person {
    public String name;
    public int age;
    private int favoriteNumber;

    public Person(String name, int age, int favoriteNumber) {
        this.name = name;
        this.age = age;
        this.favoriteNumber = favoriteNumber;
    }

    @JsonProperty
    public String getFavoriteNumber() {
        return String.valueOf(favoriteNumber);
    }

    public static void main(String... args) throws Exception {
        Person p = new Person("Joe", 25, 123);
        ObjectMapper mapper = new ObjectMapper();
        System.out.println(mapper.writeValueAsString(p)); 
        // {"name":"Joe","age":25,"favoriteNumber":"123"}
    }
}

Answer 4

jackson-annotations提供@JsonFormat，可以处理大量自定义而无需编写自定义序列化程序。

例如，为具有数字类型的字段请求STRING形状将输出数值作为字符串

public class Person {
    public String name;
    public int age;
    @JsonFormat(shape = JsonFormat.Shape.STRING)
    public int favoriteNumber;
}

将产生所需的输出

{"name":"Joe","age":25,"favoriteNumber":"123"}

Answer 5

在 @JsonView 的帮助下，我们可以决定要序列化的模型类的字段，它们满足最小标准（我们必须定义标准），就像我们可以拥有一个具有10个属性的核心类一样但只有5个属性可以序列化，仅供客户使用

通过简单地创建以下类来定义我们的视图：

public class Views
{
    static class Android{};
    static class IOS{};
    static class Web{};
}

带注释的带注释的模型类：

public class Demo 
{
    public Demo() 
    {
    }

@JsonView(Views.IOS.class)
private String iosField;

@JsonView(Views.Android.class)
private String androidField;

@JsonView(Views.Web.class)
private String webField;

 // getters/setters
...
..
}

现在我们必须通过简单地从spring扩展HttpMessageConverter类来编写自定义json转换器：

    public class CustomJacksonConverter implements HttpMessageConverter<Object> 
    {
    public CustomJacksonConverter() 
        {
            super();
        //this.delegate.getObjectMapper().setConfig(this.delegate.getObjectMapper().getSerializationConfig().withView(Views.ClientView.class));
        this.delegate.getObjectMapper().configure(MapperFeature.DEFAULT_VIEW_INCLUSION, true);
        this.delegate.getObjectMapper().setSerializationInclusion(Include.NON_NULL);

    }

    // a real message converter that will respond to methods and do the actual work
    private MappingJackson2HttpMessageConverter delegate = new MappingJackson2HttpMessageConverter();

    @Override
    public boolean canRead(Class<?> clazz, MediaType mediaType) {
        return delegate.canRead(clazz, mediaType);
    }

    @Override
    public boolean canWrite(Class<?> clazz, MediaType mediaType) {
        return delegate.canWrite(clazz, mediaType);
    }

    @Override
    public List<MediaType> getSupportedMediaTypes() {
        return delegate.getSupportedMediaTypes();
    }

    @Override
    public Object read(Class<? extends Object> clazz,
            HttpInputMessage inputMessage) throws IOException,
            HttpMessageNotReadableException {
        return delegate.read(clazz, inputMessage);
    }

    @Override
    public void write(Object obj, MediaType contentType, HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException 
    {
        synchronized(this) 
        {
            String userAgent = ((ServletRequestAttributes) RequestContextHolder.getRequestAttributes()).getRequest().getHeader("userAgent");
            if ( userAgent != null ) 
            {
                switch (userAgent) 
                {
                case "IOS" :
                    this.delegate.getObjectMapper().setConfig(this.delegate.getObjectMapper().getSerializationConfig().withView(Views.IOS.class));
                    break;
                case "Android" :
                    this.delegate.getObjectMapper().setConfig(this.delegate.getObjectMapper().getSerializationConfig().withView(Views.Android.class));
                    break;
                case "Web" :
                    this.delegate.getObjectMapper().setConfig(this.delegate.getObjectMapper().getSerializationConfig().withView( Views.Web.class));
                    break;
                default:
                    this.delegate.getObjectMapper().setConfig(this.delegate.getObjectMapper().getSerializationConfig().withView( null ));
                    break;
                }
            }
            else
            {
                // reset to default view
                this.delegate.getObjectMapper().setConfig(this.delegate.getObjectMapper().getSerializationConfig().withView( null ));
            }
            delegate.write(obj, contentType, outputMessage);
        }
    }

}

现在需要告诉spring使用这个自定义json转换，只需将它放在dispatcher-servlet.xml中

<mvc:annotation-driven>
        <mvc:message-converters register-defaults="true">
            <bean id="jsonConverter" class="com.mactores.org.CustomJacksonConverter" >
            </bean>
        </mvc:message-converters>
    </mvc:annotation-driven>

您将如何决定序列化哪些字段。

Answer 6

如果您不想使用批注污染模型并希望执行一些自定义操作，则可以使用mixins。

ObjectMapper mapper = new ObjectMapper();
SimpleModule simpleModule = new SimpleModule();
simpleModule.setMixInAnnotation(Person.class, PersonMixin.class);
mapper.registerModule(simpleModule);

替代年龄：

public abstract class PersonMixin {
    @JsonSerialize(using = PersonAgeSerializer.class)
    public String age;
}

随年龄而变：

public class PersonAgeSerializer extends JsonSerializer<Integer> {
    @Override
    public void serialize(Integer integer, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException {
        jsonGenerator.writeString(String.valueOf(integer * 52) + " months");
    }
}

Answer 7

您可以在mixin中内联创建自定义序列化程序。然后用它注释一个字段。请参见下面的示例，在lang字段后附加“ -其他”。这是 hack 的一种-如果您的序列化程序需要诸如存储库之类的东西或在spring之前注入的东西，那么这将是一个问题。最好使用自定义解串器/序列化器代替mixin。

package com.test;

import com.fasterxml.jackson.annotation.JsonAutoDetect;
import com.fasterxml.jackson.annotation.JsonAutoDetect.Visibility;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.annotation.JsonPropertyOrder;
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.SerializerProvider;
import com.fasterxml.jackson.databind.annotation.JsonSerialize;
import com.test.Argument;
import java.io.IOException;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
//Serialize only fields explicitly mentioned by this mixin.
@JsonAutoDetect(
    fieldVisibility = Visibility.NONE,
    setterVisibility = Visibility.NONE,
    getterVisibility = Visibility.NONE,
    isGetterVisibility = Visibility.NONE,
    creatorVisibility = Visibility.NONE
)
@JsonPropertyOrder({"lang", "name", "value"})
public abstract class V2ArgumentMixin {

  @JsonProperty("name")
  private String name;

  @JsonSerialize(using = LangCustomSerializer.class, as=String.class)
  @JsonProperty("lang")
  private String lang;

  @JsonProperty("value")
  private Object value;


  
  public static class LangCustomSerializer extends JsonSerializer<String> {

    @Override
    public void serialize(String value,
                          JsonGenerator jsonGenerator,
                          SerializerProvider serializerProvider)
        throws IOException, JsonProcessingException {
      jsonGenerator.writeObject(value.toString() + "  - something else");
    }
  }
}