java - 如何使用 Jackson 反序列化 Map ？

Question

我正在尝试序列化/反序列化 Map<?, ?>使用任意对象作为 Jackson 2.8 版的键。 JSON 对应物应该是一组对，即给定

public class Foo {
    public String foo;
    public Foo(String foo) {
        this.foo = foo;
    }
}

public class Bar {
    public String bar;
    public Bar(String bar) {
        this.bar = bar;
    }
}

然后

Map<Foo, Bar> map;
map.put(new Foo("foo1"), new Bar("bar1"));
map.put(new Foo("foo2"), new Bar("bar2"));

应该由这个 JSON 表示

[
    [ { "foo": "foo1" }, { "bar": "bar1" } ],
    [ { "foo": "foo2" }, { "bar": "bar2" } ]
]

所以我做了序列化器部分

public class MapToArraySerializer extends JsonSerializer<Map<?, ?>> {

    @Override
    public void serialize(Map<?, ?> value, JsonGenerator gen, SerializerProvider serializers)
        throws IOException, JsonProcessingException {
        gen.writeStartArray();
        for (Map.Entry<?, ?> entry : value.entrySet()) {
            gen.writeStartArray();
            gen.writeObject(entry.getKey());
            gen.writeObject(entry.getValue());
            gen.writeEndArray();
        }
        gen.writeEndArray();
    }

}

但我不知道如何写 JsonDeserializer做相反的工作。有什么建议？

注意:我需要 [ [ "key1", "value1" ], [ "key2", "value2" ] ]能够在 JavaScript 中使用该 JSON 的表示法 new Map( ... )和 JSON.stringify(map)也会产生那个符号(见 https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Map)。

澄清一下，这样的 map 将是其他类的字段，例如

public class Baz {

    @JsonSerialize(using = MapToArraySerializer.class)
    @JsonDeserialize(using = ArrayToMapDeserializer.class, keyAs = Foo.class, contentAs = Bar.class)
    Map<Foo, Bar> map;

}

和 ArrayToMapDeserializer extends JsonDeserializer<Map<?, ?>>是我寻求帮助的地方。

Answer 1

我想出了这个解决方案:

public class ArrayToMapDeserializer extends JsonDeserializer<SortedMap<Object, Object>>
    implements ContextualDeserializer {

    private Class<?> keyAs;

    private Class<?> contentAs;

    @Override
    public Map<Object, Object> deserialize(JsonParser p, DeserializationContext ctxt)
        throws IOException, JsonProcessingException {
        return this.deserialize(p, ctxt, new HashMap<>());
    }

    @Override
    public Map<Object, Object> deserialize(JsonParser p, DeserializationContext ctxt,
        Map<Object, Object> intoValue) throws IOException, JsonProcessingException {
        JsonNode node = p.readValueAsTree();
        ObjectCodec codec = p.getCodec();
        if (node.isArray()) {
            node.forEach(entry -> {
                try {
                    JsonNode keyNode = entry.get(0);
                    JsonNode valueNode = entry.get(1);
                    intoValue.put(keyNode.traverse(codec).readValueAs(this.keyAs),
                        valueNode.traverse(codec).readValueAs(this.contentAs));
                } catch (NullPointerException | IOException e) {
                    // skip entry
                }
            });
        }
        return intoValue;
    }

    @Override
    public JsonDeserializer<?> createContextual(DeserializationContext ctxt, BeanProperty property)
        throws JsonMappingException {
        JsonDeserialize jsonDeserialize = property.getAnnotation(JsonDeserialize.class);
        this.keyAs = jsonDeserialize.keyAs();
        this.contentAs = jsonDeserialize.contentAs();
        return this;
    }

}

可以这样使用:

public class Baz {

    @JsonSerialize(using = MapToArraySerializer.class)
    @JsonDeserialize(using = ArrayToMapDeserializer.class,
        keyAs = Foo.class, contentAs = Bar.class)
    Map<Foo, Bar> map;

}