如何在自定义序列化程序中访问默认杰克逊序列化

Question

我想创建一个自定义序列化程序，它会做一些工作，然后将其余部分留给默认序列化。

例如：

@JsonSerialize(using = MyClassSerializer.class)
public class MyClass {
  ...
}

public class MyClassSerializer extends JsonSerializer<MyClass> {
    @Override
    public void serialize(MyClass myClass, JsonGenerator generator, 
                          SerializerProvider provider) 
            throws JsonGenerationException, IOException {
        if (myClass.getSomeProperty() == someCalculationResult) {
            provider.setAttribute("special", true);
        }
        generator.writeObject(myClass);
    }  
}

考虑为聚合对象创建其他自定义序列化程序，这些聚合对象的行为基于“特殊”属性值而不同。但是，上面的代码不起作用，因为它毫无疑问地进入了无限递归。

一旦我设置了属性，有没有办法告诉杰克逊使用默认序列化？我真的不想像许多自定义序列化程序那样枚举所有属性，因为该类相当复杂，而且我不想每次更改类时都必须对序列化程序进行双重维护。

Answer 1

BeanSerializerModifier 将为您提供对默认序列化的访问权限。

将默认序列化程序注入自定义序列化程序

public class MyClassSerializer extends JsonSerializer<MyClass> {
    private final JsonSerializer<Object> defaultSerializer;

    public MyClassSerializer(JsonSerializer<Object> defaultSerializer) {
        this.defaultSerializer = checkNotNull(defaultSerializer);
    }

    @Override
    public void serialize(MyClass myclass, JsonGenerator gen, SerializerProvider provider) throws IOException {
        if (myclass.getSomeProperty() == true) {
            provider.setAttribute("special", true);
        }
        defaultSerializer.serialize(myclass, gen, provider);
    }
}

为MyClass 创建一个BeanSerializerModifier

public class MyClassSerializerModifier extends BeanSerializerModifier {
    @Override
    public JsonSerializer<?> modifySerializer(SerializationConfig config, BeanDescription beanDesc, JsonSerializer<?> serializer) {
        if (beanDesc.getBeanClass() == MySpecificClass.class) {
            return new MyClassSerializer((JsonSerializer<Object>) serializer);
        }
        return serializer;
    }
}

注册序列化器修饰符

ObjectMapper om = new ObjectMapper()
        .registerModule(new SimpleModule()
                .setSerializerModifier(new MyClassSerializerModifier()));

Answer 2

@JsonSerialize(using = MyClassSerializer.class)
public class MyClass {
...
}

public class MyClassSerializer extends JsonSerializer<MyClass> {
    @Override
     public void serialize(MyClass myClass, JsonGenerator generator, 
                      SerializerProvider provider) 
        throws JsonGenerationException, IOException {
        if (myClass.getSomeProperty() == someCalculationResult) {
            provider.setAttribute("special", true);
        } else {
            provider.defaultSerializeValue(myClass, generator);
        }
    }  
}

如果你只是像往常一样写一个对象，使用上面的

Answer 3

如果这是您想要进行的唯一更改，您可以使用 @JsonGetter 而不是使用自定义序列化程序。

public class MyClass{

    @JsonGetter("special")
    protected boolean getSpecialForJackson() {
        return myClass.getSomeProperty() == someCalculationResult;
    }

}

Answer 4

要添加到所选答案，序列化程序实现可能还必须实现ContextualSerializer 和ResolvableSerializer 接口。请在此处查看相关问题 https://github.com/FasterXML/jackson-dataformat-xml/issues/259

public class MyClassSerializer extends JsonSerializer<MyClass>
    implements ContextualSerializer, ResolvableSerializer {
private final JsonSerializer<Object> defaultSerializer;

public MyClassSerializer(JsonSerializer<Object> defaultSerializer) {
    this.defaultSerializer = checkNotNull(defaultSerializer);
}

@Override
public void serialize(MyClass myclass, JsonGenerator gen, SerializerProvider provider)
        throws IOException {
    if (myclass.getSomeProperty() == true) {
        provider.setAttribute("special", true);
    }
    defaultSerializer.serialize(myclass, gen, provider);
}

@Override
public JsonSerializer<?> createContextual(SerializerProvider prov, BeanProperty property)
        throws JsonMappingException {
    if (defaultSerializer instanceof ContextualSerializer) {
        JsonSerializer<?> contextual = ((ContextualSerializer)defaultSerializer).createContextual(prov, property);
        return new MyClassSerializer((JsonSerializer<Object>)contextual);
    }
    return new MyClassSerializer(defaultSerializer);
}

@Override
public void resolve(SerializerProvider provider) throws JsonMappingException {
    if (defaultSerializer instanceof ResolvableSerializer) {
        ((ResolvableSerializer)defaultSerializer).resolve(provider);
    }
}

}