java - 如何使用 Jackson 和注释以不同方式序列化关联对象？

Question

给定以下类层次结构，我希望 Foo 根据它在我的类层次结构中使用的上下文进行不同的序列化。

public class Foo {
    public String bar;
    public String biz;
}

public class FooContainer {
    public Foo fooA;
    public Foo fooB;
}

当我序列化 FooContainer 时，我希望 biz 属性不出现在 fooB 中。所以输出看起来像下面这样。

{
  "fooA": {"bar": "asdf", "biz": "fdsa"},
  "fooB": {"bar": "qwer"}
}

我打算使用一些 JsonView，但必须在类的所有实例的映射器层应用它，并且这是依赖于上下文的。

---

在 Jackson 用户邮件列表上，Tatu 给出了最简单的解决方案(适用于 2.0)，我现在可能最终会使用它。将赏金授予 jlabedo，因为答案是如何使用自定义注释扩展 Jackson 的绝佳示例。

public class FooContainer {
    public Foo fooA;

    @JsonIgnoreProperties({ "biz" })
    public Foo fooB;
}

Answer 1

您可以使用 JsonViews 将自定义序列化程序与自定义属性过滤器结合使用。这是一些使用 Jackson 2.0 的代码

定义自定义注解:

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface FilterUsingView {
    Class<?>[] value();
}

定义一些 View :

// Define your views here
public static class Views {
    public class Public {};
    public class Internal extends Public{};
}

然后你可以这样写你的实体。请注意，您可以定义自己的注释而不是使用 @JsonView :

public class Foo {
    @JsonView(Views.Public.class)
    public String bar;
    @JsonView(Views.Internal.class)
    public String biz;
}

public class FooContainer {
    public Foo fooA;
    @FilterUsingView(Views.Public.class)
    public Foo fooB;
}

然后，这里是代码开始的地方:)首先是您的自定义过滤器:

public static class CustomFilter extends SimpleBeanPropertyFilter {

    private Class<?>[] _nextViews;

    public void setNextViews(Class<?>[] clazz){
        _nextViews = clazz;
    }

    @Override
    public void serializeAsField(Object bean, JsonGenerator jgen,
            SerializerProvider prov, BeanPropertyWriter writer)
            throws Exception {

        Class<?>[] propViews = writer.getViews();

        if(propViews != null && _nextViews != null){
            for(Class<?> propView : propViews){
                System.out.println(propView.getName());
                for(Class<?> currentView : _nextViews){
                    if(!propView.isAssignableFrom(currentView)){
                        // Do the filtering!
                        return;
                    }
                }
            }
        }
        // The property is not filtered
        writer.serializeAsField(bean, jgen, prov);
    }
}

然后一个自定义的 AnnotationIntrospector 将做两件事:

1. 为任何 bean 启用您的自定义过滤器...除非在您的类中定义了另一个过滤器(所以您不能同时使用它们，如果您明白我的意思的话)
2. 如果他发现一个 @FilterUsingView 注释，启用你的 CustomSerializer。

这是代码

public class CustomAnnotationIntrospector extends AnnotationIntrospector {
    @Override
    public Version version() {
        return DatabindVersion.instance.version();
    }

    @Override
    public Object findFilterId(AnnotatedClass ac) {
      // CustomFilter is used for EVERY Bean, unless another filter is defined
      Object id = super.findFilterId(ac);
      if (id == null) {
        id = "CustomFilter";
      }
      return id;
    }

    @Override
    public Object findSerializer(Annotated am) {
        FilterUsingView annotation = am.getAnnotation(FilterUsingView.class);
        if(annotation == null){
            return null;
        }
        return new CustomSerializer(annotation.value());
    }
}

这是您的自定义序列化程序。它所做的唯一一件事就是将注释的值传递给自定义过滤器，然后让默认的序列化程序完成这项工作。

public class CustomSerializer extends JsonSerializer<Object> {

    private Class<?>[] _activeViews;

    public CustomSerializer(Class<?>[] view){
        _activeViews = view;
    }

    @Override
    public void serialize(Object value, JsonGenerator jgen,
            SerializerProvider provider) throws IOException,
            JsonProcessingException {

        BeanPropertyFilter filter = provider.getConfig().getFilterProvider().findFilter("CustomFilter");
        if(filter instanceof CustomFilter){
            CustomFilter customFilter = (CustomFilter) filter;

            // Tell the filter that we will filter our next property
            customFilter.setNextViews(_activeViews);

            provider.defaultSerializeValue(value, jgen);

            // Property has been filtered and written, do not filter anymore
            customFilter.setNextViews(null);
        }else{
            // You did not define a CustomFilter ? Well this serializer is useless...
            provider.defaultSerializeValue(value, jgen);
        }
    }
}

终于!让我们把这些放在一起:

public class CustomModule extends SimpleModule {

    public CustomModule() {
        super("custom-module", new Version(0, 1, 0, "", "", ""));
    }

    @Override
    public void setupModule(SetupContext context) {
        super.setupModule(context);
        AnnotationIntrospector ai = new CustomAnnotationIntrospector();
        context.appendAnnotationIntrospector(ai);
    }

}



@Test
public void customField() throws Exception {
    FooContainer object = new FooContainer();
    object.fooA = new Foo();
    object.fooA.bar = "asdf";
    object.fooA.biz = "fdsa";
    object.fooB = new Foo();
    object.fooB.bar = "qwer";
    object.fooB.biz = "test";

    ObjectMapper mapper = new ObjectMapper();
    mapper.registerModule(new CustomModule());

    FilterProvider fp = new SimpleFilterProvider().addFilter("CustomFilter", new CustomFilter());
    StringWriter writer = new StringWriter();

    mapper.writer(fp).writeValue(writer, object);

    String expected = "{\"fooA\":{\"bar\":\"asdf\",\"biz\":\"fdsa\"},\"fooB\":{\"bar\":\"qwer\"}}";

    Assert.assertEquals(expected, writer.toString());
}