带有条件隐藏成员的Jackson自定义序列化程序生成无效的JSON

Question

我需要创建一个自定义序列化程序，有条件地跳过字段。与使用jackson序列化时有条件跳过对象中描述的情况不同，我的类包含一个POJO成员。PersonalInfo有一个成员地址。如果地址被隐藏，结果JSON仍然有“Address”标记，但没有值。我想不出怎么解决这个问题。

在ObjectMapper上创建自定义序列化程序（参见3. athttp://www.baeldung.com/jackson-custom-serialization）会导致完全相同的结果。

以下是参考问题中显示问题的改编代码：

public class JacksonHide {
    @JsonIgnoreProperties("hidden")
    public static interface IHideable {
        boolean isHidden();
    }

    public static class Address implements IHideable {
        public final String city;
        public final String street;
        public final boolean hidden;

        public Address(String city, String street, boolean hidden) {
            this.city = city;
            this.street = street;
            this.hidden = hidden;
        }

        @Override
        public boolean isHidden() {
            return hidden;
        }
    }

    public static class PersonalInfo implements IHideable {
        public final String name;
        public final int age;
        public final Address address;
        public final boolean hidden;

        public PersonalInfo(String name, int age, Address address, boolean hidden) {
            this.name = name;
            this.age = age;
            this.address = address;
            this.hidden = hidden;
        }

        @Override
        public boolean isHidden() {
            return hidden;
        }
    }

    private static class MyBeanSerializerModifier extends BeanSerializerModifier {
        @Override
        public JsonSerializer<?> modifySerializer(SerializationConfig config, BeanDescription beanDesc, JsonSerializer<?> serializer) {
            if (IHideable.class.isAssignableFrom(beanDesc.getBeanClass())) {
                return new MyIHideableJsonSerializer((JsonSerializer<IHideable>) serializer);
            }
            return super.modifySerializer(config, beanDesc, serializer);
        }

        private static class MyIHideableJsonSerializer extends JsonSerializer<IHideable> {
            private final JsonSerializer<IHideable> serializer;

            public MyIHideableJsonSerializer(JsonSerializer<IHideable> serializer) {
                this.serializer = serializer;
            }

            @Override
            public void serialize(IHideable value, JsonGenerator jgen, SerializerProvider provider) throws IOException {
                if (!value.isHidden()) {
                    serializer.serialize(value, jgen, provider);
                }

            }
        }
    }

    public static void main(String[] args) throws JsonProcessingException {
        ObjectMapper mapper = new ObjectMapper();
        SimpleModule module = new SimpleModule();
        module.setSerializerModifier(new MyBeanSerializerModifier());
        mapper.registerModule(module);

        PersonalInfo p1 = new PersonalInfo("John", 30, new Address("A", "B", false), false);
        PersonalInfo p2 = new PersonalInfo("Ivan", 20, new Address("C", "D", true), true);
        PersonalInfo p3 = new PersonalInfo("Mary", 40, new Address("C", "D", true), false);
        Address a1 = new Address("A", "B", false);
        Address a2 = new Address("C", "D", true);

        System.out.println(mapper.writerWithDefaultPrettyPrinter().writeValueAsString(Arrays.asList(p1, p2, p3, a1, a2)));
    }

}

更新：感谢反馈，我现在有一个基于@JSONFilter的版本，给我至少有效的JSON。不幸的是，节点仍然存在，但现在是空的（{}）。我怎样才能彻底摆脱他们？

public class JacksonFilterHide {

    @JsonFilter("HiddenFilter")
    @JsonIgnoreProperties("hidden")
    public static interface IHideable {
        boolean isHidden();
    }

    public static class Address implements IHideable {
        public final String city;
        public final String street;
        public final boolean hidden;

        public Address(String city, String street, boolean hidden) {
            this.city = city;
            this.street = street;
            this.hidden = hidden;
        }

        @Override
        public boolean isHidden() {
            return hidden;
        }
    }

    public static class PersonalInfo implements IHideable {
        public final String name;
        public final int age;
        public final Address address;
        public final boolean hidden;

        public PersonalInfo(String name, int age, Address address, boolean hidden) {
            this.name = name;
            this.age = age;
            this.address = address;
            this.hidden = hidden;
        }

        @Override
        public boolean isHidden() {
            return hidden;
        }
    }

    static final PropertyFilter hiddenFilter = new SimpleBeanPropertyFilter() {
        @Override
        public void serializeAsField(Object pojo, JsonGenerator jgen, SerializerProvider provider, PropertyWriter writer) throws Exception {
            if (include(writer)) {
                if (pojo instanceof IHideable && ((IHideable) pojo).isHidden()) {
                    return;
                } else {
                    writer.serializeAsField(pojo, jgen, provider);
                    return;
                }
            } else if (!jgen.canOmitFields()) { // since 2.3
                writer.serializeAsOmittedField(pojo, jgen, provider);
            }
        }

        @Override
        protected boolean include(BeanPropertyWriter writer) {
            return true;
        }

        @Override
        protected boolean include(PropertyWriter writer) {
            return true;
        }
    };

    public static void main(String[] args) throws JsonProcessingException {
        ObjectMapper mapper = new ObjectMapper();
        // ObjectMapper mapper = UserInteractionModel.getMapper();
        FilterProvider filters = new SimpleFilterProvider().addFilter("HiddenFilter", hiddenFilter);
        mapper.setFilters(filters);
        mapper.enable(SerializationFeature.INDENT_OUTPUT);

        PersonalInfo p1 = new PersonalInfo("John", 30, new Address("A", "B", false), false);
        PersonalInfo p2 = new PersonalInfo("Ivan", 20, new Address("C", "D", true), true);
        PersonalInfo p3 = new PersonalInfo("Mary", 40, new Address("C", "D", true), false);
        Address a1 = new Address("A", "B", false);
        Address a2 = new Address("C", "D", true);

        System.out.println(mapper.writeValueAsString(Arrays.asList(p1, p2, p3, a1, a2)));
    }

}

现在的输出是：

[{姓名：约翰，年龄：30，地址：{城市：A，街道：B" } }, { }, { "姓名：玛丽，年龄：40，地址": { } }, { "城市：A，“街道”：“B" }, { } ]

预期：

[{姓名：约翰，年龄：30，地址：{城市：A，街道：B" } }, { "姓名：玛丽，年龄：40, }, { "城市：A，街道：B}]

Update2通过遍历树并删除空节点进行临时修复。很丑，但现在管用。仍在寻找更好的答案。

private void removeEmptyNodes(JSONObject json) {
    Iterator<String> iter = json.keys();
    while (iter.hasNext()) {
        String key = iter.next();
        JSONObject node;
        try {
            node = json.getJSONObject(key);
        } catch (JSONException e) {
            continue;
        }
        if (node.length() == 0) {
            iter.remove();
        } else {
            removeEmptyNodes(node);
        }

    }
}

受这个问题启发的解决方案：如何使用Jackson删除Java中的空json节点？

Answer 1

序列化程序已损坏：如果请求，它无法选择不写入值。在写入属性值的情况下，调用方已经写出了属性名，所以不写入该值确实会中断输出。这要么导致抛出异常（理想情况下），要么导致输出中断（不理想情况下）；不管怎样，JsonSerializer都不允许尝试决定是否正在写入值。

要排除正在序列化的属性，有效选项包括：

1. 静态属性注释，比如总是排除特定命名属性的@JsonIgnore和@JsonIgnoreProperties