如何在Jackson中使用自定义Serializer？

Question

我有两个Java类,我想使用Jackson序列化为JSON:

public class User {
    public final int id;
    public final String name;

    public User(int id, String name) {
        this.id = id;
        this.name = name;
    }
}

public class Item {
    public final int id;
    public final String itemNr;
    public final User createdBy;

    public Item(int id, String itemNr, User createdBy) {
        this.id = id;
        this.itemNr = itemNr;
        this.createdBy = createdBy;
    }
}

我想将Item序列化为此JSON:

{"id":7, "itemNr":"TEST", "createdBy":3}

用户序列化只包括id.我还可以将所有用户对象serilize为JSON,如:

{"id":3, "name": "Jonas", "email": "jonas@example.com"}

所以我想我需要编写一个自定义序列化器Item并尝试使用它:

public class ItemSerializer extends JsonSerializer<Item> {

@Override
public void serialize(Item value, JsonGenerator jgen,
        SerializerProvider provider) throws IOException,
        JsonProcessingException {
    jgen.writeStartObject();
    jgen.writeNumberField("id", value.id);
    jgen.writeNumberField("itemNr", value.itemNr);
    jgen.writeNumberField("createdBy", value.user.id);
    jgen.writeEndObject();
}

}

我使用Jackson How-to:Custom Serializers中的代码序列化JSON :

ObjectMapper mapper = new ObjectMapper();
SimpleModule simpleModule = new SimpleModule("SimpleModule", 
                                              new Version(1,0,0,null));
simpleModule.addSerializer(new ItemSerializer());
mapper.registerModule(simpleModule);
StringWriter writer = new StringWriter();
try {
    mapper.writeValue(writer, myItem);
} catch (JsonGenerationException e) {
    e.printStackTrace();
} catch (JsonMappingException e) {
    e.printStackTrace();
} catch (IOException e) {
    e.printStackTrace();
}

但我得到这个错误:

Exception in thread "main" java.lang.IllegalArgumentException: JsonSerializer of type com.example.ItemSerializer does not define valid handledType() (use alternative registration method?)
    at org.codehaus.jackson.map.module.SimpleSerializers.addSerializer(SimpleSerializers.java:62)
    at org.codehaus.jackson.map.module.SimpleModule.addSerializer(SimpleModule.java:54)
    at com.example.JsonTest.main(JsonTest.java:54)

如何在Jackson中使用自定义Serializer？

---

我就是这样用Gson做的:

public class UserAdapter implements JsonSerializer<User> {

    @Override 
    public JsonElement serialize(User src, java.lang.reflect.Type typeOfSrc,
            JsonSerializationContext context) {
        return new JsonPrimitive(src.id);
    }
}

    GsonBuilder builder = new GsonBuilder();
    builder.registerTypeAdapter(User.class, new UserAdapter());
    Gson gson = builder.create();
    String json = gson.toJson(myItem);
    System.out.println("JSON: "+json);

但我现在需要和杰克逊一起做,因为Gson不支持接口.

Answer 1

您可以放置@JsonSerialize(using = CustomDateSerializer.class)要序列化的对象的任何日期字段.

public class CustomDateSerializer extends SerializerBase<Date> {

    public CustomDateSerializer() {
        super(Date.class, true);
    }

    @Override
    public void serialize(Date value, JsonGenerator jgen, SerializerProvider provider)
        throws IOException, JsonProcessingException {
        SimpleDateFormat formatter = new SimpleDateFormat("EEE MMM dd yyyy HH:mm:ss 'GMT'ZZZ (z)");
        String format = formatter.format(value);
        jgen.writeString(format);
    }

}

Answer 2

如上所述,@ JsonValue是一个好方法.但是如果你不介意自定义序列化程序,则不需要为Item编写一个,而是为User写一个 - 如果是这样,它就像下面这样简单:

public void serialize(Item value, JsonGenerator jgen,
    SerializerProvider provider) throws IOException,
    JsonProcessingException {
  jgen.writeNumber(id);
}

另一种可能性是实施JsonSerializable,在这种情况下不需要注册.

至于错误; 这很奇怪 - 你可能想要升级到更高版本.但扩展也更安全,org.codehaus.jackson.map.ser.SerializerBase因为它将具有非必要方法的标准实现(即除了实际的序列化调用之外的所有内容).

Answer 3

我也尝试过这样做,并且在Jackson网页上的示例代码中有一个错误,它无法在调用addSerializer方法中包含类型(.class),该方法应如下所示:

simpleModule.addSerializer(Item.class, new ItemSerializer());

换句话说,这些是实例化simpleModule并添加序列化程序的行(前面的错误行被注释掉):

ObjectMapper mapper = new ObjectMapper();
SimpleModule simpleModule = new SimpleModule("SimpleModule", 
                                          new Version(1,0,0,null));
// simpleModule.addSerializer(new ItemSerializer());
simpleModule.addSerializer(Item.class, new ItemSerializer());
mapper.registerModule(simpleModule);

仅供参考:以下是正确示例代码的参考:http://wiki.fasterxml.com/JacksonFeatureModules

希望这可以帮助!

Answer 4

使用@JsonValue:

public class User {
    int id;
    String name;

    @JsonValue
    public int getId() {
        return id;
    }
}

@JsonValue仅适用于方法,因此您必须添加getId方法.您应该可以完全跳过自定义序列化程序.

Answer 5

这些是我在试图理解杰克逊序列化时注意到的行为模式.

1)假设有一个对象Classroom和一个Student类.为了方便起见,我把一切都公之于众.

public class Classroom {
    public final double double1 = 1234.5678;
    public final Double Double1 = 91011.1213;
    public final Student student1 = new Student();
}

public class Student {
    public final double double2 = 1920.2122;
    public final Double Double2 = 2324.2526;
}

2)假设这些是我们用于将对象序列化为JSON的序列化器.writeObjectField如果向对象映射器注册,则使用对象自己的序列化器; 如果没有,那么它将它序列化为POJO.writeNumberField专门只接受基元作为参数.

public class ClassroomSerializer extends StdSerializer<Classroom> {
    public ClassroomSerializer(Class<Classroom> t) {
        super(t);
    }

    @Override
    public void serialize(Classroom value, JsonGenerator jgen, SerializerProvider provider) throws IOException, JsonGenerationException {
        jgen.writeStartObject();
        jgen.writeObjectField("double1-Object", value.double1);
        jgen.writeNumberField("double1-Number", value.double1);
        jgen.writeObjectField("Double1-Object", value.Double1);
        jgen.writeNumberField("Double1-Number", value.Double1);
        jgen.writeObjectField("student1", value.student1);
        jgen.writeEndObject();
    }
}

public class StudentSerializer extends StdSerializer<Student> {
    public StudentSerializer(Class<Student> t) {
        super(t);
    }

    @Override
    public void serialize(Student value, JsonGenerator jgen, SerializerProvider provider) throws IOException, JsonGenerationException {
        jgen.writeStartObject();
        jgen.writeObjectField("double2-Object", value.double2);
        jgen.writeNumberField("double2-Number", value.double2);
        jgen.writeObjectField("Double2-Object", value.Double2);
        jgen.writeNumberField("Double2-Number", value.Double2);
        jgen.writeEndObject();
    }
}

3)###,##0.000在SimpleModule中只注册带有DecimalFormat输出模式的DoubleSerializer,输出为:

{
  "double1" : 1234.5678,
  "Double1" : {
    "value" : "91,011.121"
  },
  "student1" : {
    "double2" : 1920.2122,
    "Double2" : {
      "value" : "2,324.253"
    }
  }
}

您可以看到POJO序列化区分double和Double,使用DoubleSerialzer for Doubles并使用常规String格式表示双精度.

4)注册DoubleSerializer和ClassroomSerializer,不带StudentSerializer.我们期望输出是这样的,如果我们将double写为对象,它的行为就像一个Double,如果我们将Double写为数字,它的行为就像一个double.Student实例变量应该写为POJO并遵循上面的模式,因为它没有注册.

{
  "double1-Object" : {
    "value" : "1,234.568"
  },
  "double1-Number" : 1234.5678,
  "Double1-Object" : {
    "value" : "91,011.121"
  },
  "Double1-Number" : 91011.1213,
  "student1" : {
    "double2" : 1920.2122,
    "Double2" : {
      "value" : "2,324.253"
    }
  }
}

5)注册所有序列化程序.输出是:

{
  "double1-Object" : {
    "value" : "1,234.568"
  },
  "double1-Number" : 1234.5678,
  "Double1-Object" : {
    "value" : "91,011.121"
  },
  "Double1-Number" : 91011.1213,
  "student1" : {
    "double2-Object" : {
      "value" : "1,920.212"
    },
    "double2-Number" : 1920.2122,
    "Double2-Object" : {
      "value" : "2,324.253"
    },
    "Double2-Number" : 2324.2526
  }
}

完全符合预期.

另一个重要注意事项:如果为同一个模块注册的同一个类有多个序列化程序,则模块将选择最近添加到列表中的该类的序列化程序.这不应该被使用 - 这令人困惑,我不确定这是多么一致

道德:如果要自定义对象中基元的序列化,则必须为对象编写自己的序列化程序.你不能依赖POJO Jackson系列化.

Answer 6

杰克逊的JSON视图可能是一种更简单的方法来满足您的要求,特别是如果您的JSON格式具有一定的灵活性.

如果{"id":7, "itemNr":"TEST", "createdBy":{id:3}}是可接受的表示,那么使用非常少的代码就可以很容易地实现.

您只需将User的名称字段注释为视图的一部分,并在序列化请求中指定不同的视图(默认情况下将包含未注释的字段)

例如:定义视图:

public class Views {
    public static class BasicView{}
    public static class CompleteUserView{}
}

注释用户:

public class User {
    public final int id;

    @JsonView(Views.CompleteUserView.class)
    public final String name;

    public User(int id, String name) {
        this.id = id;
        this.name = name;
    }
}

并序列化请求不包含您要隐藏的字段的视图(默认情况下序列化未注释的字段):

objectMapper.getSerializationConfig().withView(Views.BasicView.class);

Answer 7

在我的例子中(Spring 3.2.4和Jackson 2.3.1),自定义序列化器的XML配置:

<mvc:annotation-driven>
    <mvc:message-converters register-defaults="false">
        <bean class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter">
            <property name="objectMapper">
                <bean class="org.springframework.http.converter.json.Jackson2ObjectMapperFactoryBean">
                    <property name="serializers">
                        <array>
                            <bean class="com.example.business.serializer.json.CustomObjectSerializer"/>
                        </array>
                    </property>
                </bean>
            </property>
        </bean>
    </mvc:message-converters>
</mvc:annotation-driven>

以无法解释的方式被某种东西覆盖回默认值.

这对我有用:

CustomObject.java

@JsonSerialize(using = CustomObjectSerializer.class)
public class CustomObject {

    private Long value;

    public Long getValue() {
        return value;
    }

    public void setValue(Long value) {
        this.value = value;
    }
}

CustomObjectSerializer.java

public class CustomObjectSerializer extends JsonSerializer<CustomObject> {

    @Override
    public void serialize(CustomObject value, JsonGenerator jgen,
        SerializerProvider provider) throws IOException,JsonProcessingException {
        jgen.writeStartObject();
        jgen.writeNumberField("y", value.getValue());
        jgen.writeEndObject();
    }

    @Override
    public Class<CustomObject> handledType() {
        return CustomObject.class;
    }
}

<mvc:message-converters>(...)</mvc:message-converters>我的解决方案中不需要XML配置().

Answer 8

我写了一个自定义示例 Timestamp.class序列化/反序列化，但是您可以将其用于任何所需的操作。

创建对象映射器时，请执行以下操作：

public class JsonUtils {

    public static ObjectMapper objectMapper = null;

    static {
        objectMapper = new ObjectMapper();
        SimpleModule s = new SimpleModule();
        s.addSerializer(Timestamp.class, new TimestampSerializerTypeHandler());
        s.addDeserializer(Timestamp.class, new TimestampDeserializerTypeHandler());
        objectMapper.registerModule(s);
    };
}

例如，java ee您可以使用以下代码对其进行初始化：

import java.time.LocalDateTime;

import javax.ws.rs.ext.ContextResolver;
import javax.ws.rs.ext.Provider;

import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.module.SimpleModule;

@Provider
public class JacksonConfig implements ContextResolver<ObjectMapper> {

    private final ObjectMapper objectMapper;

    public JacksonConfig() {
        objectMapper = new ObjectMapper();
        SimpleModule s = new SimpleModule();
        s.addSerializer(Timestamp.class, new TimestampSerializerTypeHandler());
        s.addDeserializer(Timestamp.class, new TimestampDeserializerTypeHandler());
        objectMapper.registerModule(s);
    };

    @Override
    public ObjectMapper getContext(Class<?> type) {
        return objectMapper;
    }
}

序列化器应该是这样的：

import java.io.IOException;
import java.sql.Timestamp;

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.SerializerProvider;

public class TimestampSerializerTypeHandler extends JsonSerializer<Timestamp> {

    @Override
    public void serialize(Timestamp value, JsonGenerator jgen, SerializerProvider provider) throws IOException, JsonProcessingException {
        String stringValue = value.toString();
        if(stringValue != null && !stringValue.isEmpty() && !stringValue.equals("null")) {
            jgen.writeString(stringValue);
        } else {
            jgen.writeNull();
        }
    }

    @Override
    public Class<Timestamp> handledType() {
        return Timestamp.class;
    }
}

和反序列化器是这样的：

import java.io.IOException;
import java.sql.Timestamp;

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonDeserializer;
import com.fasterxml.jackson.databind.SerializerProvider;

public class TimestampDeserializerTypeHandler extends JsonDeserializer<Timestamp> {

    @Override
    public Timestamp deserialize(JsonParser jp, DeserializationContext ds) throws IOException, JsonProcessingException {
        SqlTimestampConverter s = new SqlTimestampConverter();
        String value = jp.getValueAsString();
        if(value != null && !value.isEmpty() && !value.equals("null"))
            return (Timestamp) s.convert(Timestamp.class, value);
        return null;
    }

    @Override
    public Class<Timestamp> handledType() {
        return Timestamp.class;
    }
}