使用 Jackson 將 Json 反序列化為其他類層次結構

Question

現在我正在和傑克遜一起工作，我對此有一些疑問。

首先。 我有兩個服務，第一個是數據收集和發送服務，第二個是接收這些數據，例如，將其記錄到文件中。

因此，第一個服務具有如下類層次結構：

         +----ConcreteC
         |
Base ----+----ConcreteA
         |
         +----ConcreteB

第二個服務有這樣的類層次結構：

ConcreteAAdapter extends ConcreteA implements Adapter {}
ConcreteBAdapter extends ConcreteB implements Adapter {}
ConcreteCAdapter extends ConcreteC implements Adapter {}

第一個服務對ConcreteXAdapter一無所知。

我在第一個服務上發送數據的方式：

Collection<Base> data = new LinkedBlockingQueue<Base>()
JacksonUtils utils = new JacksonUtils();
data.add(new ConcreteA());
data.add(new ConcreteB());
data.add(new ConcreteC());
...
send(utils.marshall(data));
...

public class JacksonUtils {

    public byte[] marshall(Collection<Base> data) throws IOException {
        ByteArrayOutputStream out = new ByteArrayOutputStream() {
            @Override
            public byte[] toByteArray() {
                return buf;
            }
        };

        getObjectMapper().writeValue(out, data);
        return out.toByteArray();
    }
    protected ObjectMapper getObjectMapper() {
        return new ObjectMapper();
    }

    public Object unmarshall(byte[] json) throws IOException {
        return getObjectMapper().readValue(json, Object.class);
    }

    public <T> T unmarshall(InputStream source, TypeReference<T> typeReference) throws IOException {
        return getObjectMapper().readValue(source, typeReference);
    }

    public <T> T unmarshall(byte[] json, TypeReference<T> typeReference) throws IOException {
        return getObjectMapper().readValue(json, typeReference);
    }
}

所以，我想desirialize JSON成集ConcreteXAdapter ，不進的收藏ConcreteX （ ConcreteA -> ConcreteAAdapter, ConcreteB -> ConcreteBAdapter, ConcreteC -> ConcreteCAdapter ）。 在我描述的情況下，我想得到：

Collection [ConcreteAAdapter, ConcreteBAdapter, ConcreteCAdapter]

我怎樣才能做到這一點？

Answer 1

為此，您需要在 JSON 中傳遞其他信息：

@JsonTypeInfo(use=JsonTypeInfo.Id.NAME, 
      include=JsonTypeInfo.As.PROPERTY, property="@type")
class Base {
...
}

然后在序列化時它會添加@type 字段：

objectMapper.registerSubtypes(
            new NamedType(ConcreteAAdapter.class, "ConcreteA"),
            new NamedType(ConcreteBAdapter.class, "ConcreteB"),
            new NamedType(ConcreteCAdapter.class, "ConcreteC")
            );

// note, that for lists you need to pass TypeReference explicitly
objectMapper.writerWithType(new TypeReference<List<Base>>() {})
     .writeValueAsString(someList);


    {
      "@type" : "ConcreteA",
      ...
    }

在反序列化時，它將是：

    objectMapper.registerSubtypes(
            new NamedType(ConcreteA.class, "ConcreteA"),
            new NamedType(ConcreteB.class, "ConcreteB"),
            new NamedType(ConcreteC.class, "ConcreteC")
            );
    objectMapper.readValue(....)

更多信息在這里

Answer 2

我是如何解決這個問題的。 下面是一個示例項目的類圖：

所以我希望得到ConcreteAAdapter形式ConcreteA反序列化后。

我的解決方案是擴展ClassNameIdResolver以添加將基類對象反序列化為子類型類對象的功能（子類型類不添加額外的功能和附加字段）。

這是一個為反序列化創建ObjectMapper的代碼：

protected ObjectMapper getObjectMapperForDeserialization() {
        ObjectMapper mapper = new ObjectMapper();

        StdTypeResolverBuilder typeResolverBuilder = new ObjectMapper.DefaultTypeResolverBuilder(ObjectMapper.DefaultTyping.OBJECT_AND_NON_CONCRETE);
        typeResolverBuilder = typeResolverBuilder.inclusion(JsonTypeInfo.As.PROPERTY);
        typeResolverBuilder.init(JsonTypeInfo.Id.CLASS, new ClassNameIdResolver(SimpleType.construct(Base.class), TypeFactory.defaultInstance()) {
            private HashMap<Class, Class> classes = new HashMap<Class, Class>() {
                {
                    put(ConcreteA.class, ConcreteAAdapter.class);
                    put(ConcreteB.class, ConcreteBAdapter.class);
                    put(ConcreteC.class, ConcreteCAdapter.class);
                }
            };

            @Override
            public String idFromValue(Object value) {
                return (classes.containsKey(value.getClass())) ? value.getClass().getName() : null;
            }

            @Override
            public JavaType typeFromId(String id) {
                try {
                    return classes.get(Class.forName(id)) == null ? super.typeFromId(id) : _typeFactory.constructSpecializedType(_baseType, classes.get(Class.forName(id)));
                } catch (ClassNotFoundException e) {
                    // todo catch the e
                }
                return super.typeFromId(id);
            }
        });
        mapper.setDefaultTyping(typeResolverBuilder);
        return mapper;
    }

這是一個為序列化創建ObjectMapper的代碼：

protected ObjectMapper getObjectMapperForSerialization() {
    ObjectMapper mapper = new ObjectMapper();

    StdTypeResolverBuilder typeResolverBuilder = new ObjectMapper.DefaultTypeResolverBuilder(ObjectMapper.DefaultTyping.OBJECT_AND_NON_CONCRETE);
    typeResolverBuilder = typeResolverBuilder.inclusion(JsonTypeInfo.As.PROPERTY);
    typeResolverBuilder.init(JsonTypeInfo.Id.CLASS, new ClassNameIdResolver(SimpleType.construct(Base.class), TypeFactory.defaultInstance()));
    mapper.setDefaultTyping(typeResolverBuilder);

    return mapper;
}

測試代碼：

public static void main(String[] args) throws IOException {
    JacksonUtils JacksonUtils = new JacksonUtilsImpl();

    Collection<Base> data = new LinkedBlockingQueue<Base>();
    data.add(new ConcreteA());
    data.add(new ConcreteB());
    data.add(new ConcreteC());

    String json = JacksonUtils.marshallIntoString(data);

    System.out.println(json);

    Collection<? extends Adapter> adapters = JacksonUtils.unmarshall(json, new TypeReference<ArrayList<Adapter>>() {});

    for (Adapter adapter : adapters) {
        System.out.println(adapter.getClass().getName());
    }
}

JacksonUtils 類的完整代碼：

public class JacksonUtilsImpl implements JacksonUtils {

    @Override
    public byte[] marshall(Collection<Base> data) throws IOException {
        ByteArrayOutputStream out = new ByteArrayOutputStream() {
            @Override
            public byte[] toByteArray() {
                return buf;
            }
        };

        getObjectMapperForSerialization().writerWithType(new TypeReference<Collection<Base>>() {}).writeValue(out, data);
        return out.toByteArray();
    }

    @Override
    public String marshallIntoString(Collection<Base> data) throws IOException {
        return getObjectMapperForSerialization().writeValueAsString(data);
    }

    protected ObjectMapper getObjectMapperForSerialization() {
        ObjectMapper mapper = new ObjectMapper();

        StdTypeResolverBuilder typeResolverBuilder = new ObjectMapper.DefaultTypeResolverBuilder(ObjectMapper.DefaultTyping.OBJECT_AND_NON_CONCRETE);
        typeResolverBuilder = typeResolverBuilder.inclusion(JsonTypeInfo.As.PROPERTY);
        typeResolverBuilder.init(JsonTypeInfo.Id.CLASS, new ClassNameIdResolver(SimpleType.construct(Base.class), TypeFactory.defaultInstance()));
        mapper.setDefaultTyping(typeResolverBuilder);

        return mapper;
    }

    protected ObjectMapper getObjectMapperForDeserialization() {
        ObjectMapper mapper = new ObjectMapper();

        StdTypeResolverBuilder typeResolverBuilder = new ObjectMapper.DefaultTypeResolverBuilder(ObjectMapper.DefaultTyping.OBJECT_AND_NON_CONCRETE);
        typeResolverBuilder = typeResolverBuilder.inclusion(JsonTypeInfo.As.PROPERTY);
        typeResolverBuilder.init(JsonTypeInfo.Id.CLASS, new ClassNameIdResolver(SimpleType.construct(Base.class), TypeFactory.defaultInstance()) {
            private HashMap<Class, Class> classes = new HashMap<Class, Class>() {
                {
                    put(ConcreteA.class, ConcreteAAdapter.class);
                    put(ConcreteB.class, ConcreteBAdapter.class);
                    put(ConcreteC.class, ConcreteCAdapter.class);
                }
            };

            @Override
            public String idFromValue(Object value) {
                return (classes.containsKey(value.getClass())) ? value.getClass().getName() : null;
            }

            @Override
            public JavaType typeFromId(String id) {
                try {
                    return classes.get(Class.forName(id)) == null ? super.typeFromId(id) : _typeFactory.constructSpecializedType(_baseType, classes.get(Class.forName(id)));
                } catch (ClassNotFoundException e) {
                    // todo catch the e
                }
                return super.typeFromId(id);
            }
        });
        mapper.setDefaultTyping(typeResolverBuilder);
        return mapper;
    }

    @Override
    public Object unmarshall(byte[] json) throws IOException {
        return getObjectMapperForDeserialization().readValue(json, Object.class);
    }

    @Override
    public <T> T unmarshall(InputStream source, TypeReference<T> typeReference) throws IOException {
        return getObjectMapperForDeserialization().readValue(source, typeReference);
    }

    @Override
    public <T> T unmarshall(byte[] json, TypeReference<T> typeReference) throws IOException {
        return getObjectMapperForDeserialization().readValue(json, typeReference);
    }

    @Override
    public <T> Collection<? extends T> unmarshall(String json, Class<? extends Collection<? extends T>> klass) throws IOException {
        return getObjectMapperForDeserialization().readValue(json, klass);
    }


    @Override
    public <T> Collection<? extends T> unmarshall(String json, TypeReference typeReference) throws IOException {
        return getObjectMapperForDeserialization().readValue(json, typeReference);
    }
}

Answer 3

我發現程序員布魯斯的方法是最清晰、最容易上手的（下面的例子）。 我從他對相關問題的回答中得到了信息： https : //stackoverflow.com/a/6339600/1148030和相關的博客文章： http : //programmerbruce.blogspot.fi/2011/05/deserialize-json-with -jackson-into.html

另請查看這個友好的 wiki 頁面（也在 Eugene Retunsky 的回答中提到）： https : //github.com/FasterXML/jackson-docs/wiki/JacksonPolymorphicDeserialization

另一個不錯的維基頁面： https : //github.com/FasterXML/jackson-docs/wiki/JacksonMixInAnnotations

這是一個簡短的示例，可以讓您了解這個想法：

像這樣配置 ObjectMapper：

    mapper.getDeserializationConfig().addMixInAnnotations(Base.class, BaseMixin.class);
    mapper.getSerializationConfig().addMixInAnnotations(Base.class, BaseMixin.class);

示例 BaseMixin 類（易於定義為內部類。）

@JsonTypeInfo(use=JsonTypeInfo.Id.NAME, include=JsonTypeInfo.As.PROPERTY, property="type")
@JsonSubTypes({
    @JsonSubTypes.Type(value=ConcreteA.class, name="ConcreteA"),
    @JsonSubTypes.Type(value=ConcreteB.class, name="ConcreteB")
})  
private static class BaseMixin {
}

在第二個服務中，您可以像這樣定義 BaseMixin：

@JsonTypeInfo(use=JsonTypeInfo.Id.NAME, include=JsonTypeInfo.As.PROPERTY, property="type")
@JsonSubTypes({
    @JsonSubTypes.Type(value=ConcreteAAdapter.class, name="ConcreteA"),
    @JsonSubTypes.Type(value=ConcreteBAdapter.class, name="ConcreteB")
})  
private static class BaseMixin {
}