如何在Java中创建自定义JsonDeserializer？ [英] how to create a custom JsonDeserializer in Java?

问题描述

我有地图< A，B> fieldOfC 作为类C的字段。当我尝试用Jackson反序列化C时，会抛出异常，因为它找不到Map的密钥A的反序列化器。所以，我想解决方案是扩展StdJsonDeserializer并手动完成。

我的问题是我找不到一个如何使用解析器和我必须实现的方法反序列化的上下文的例子。

I have a Map<A,B> fieldOfC as a field of a class C. When I try to deserialize C with Jackson, an Exception is thrown because it can't find a Deserializer for Map's key A. So, I guess the solution is to extend StdJsonDeserializer and do it manually.
My problem is that I can't find an example on how to use the parser and the context of the method "deserialize" that I have to implement.

任何人都可以编写这个简单示例的代码，这样我就可以用它作为开始构建我真正的反序列化器吗？

Can anyone write the code for this simple example so I can use it as a start to build my real deserializer?

public class A{
  private String a1;
  private Integer a2;
}

public class B{
  private String b1;
}

public class C{
  @JsonDeserialize(keyUsing=ADeserializer.class)
  //also tried this: @JsonDeserialize(keyAs=A.class) without success
  private Map<A,B> fieldOfC;
  private String c1;
}

public class ADeserializer extends StdKeyDeserializer {

  protected ADeserializer(Class<A> cls) {
    super(cls);
  }

  protected Object _parse(String key, DeserializationContext ctxt) throws Exception {
    ObjectMapper mapper = new ObjectMapper();
    return mapper.readValue(key, A.class);
  }
}

提前致谢

编辑：谷歌搜索，我发现测试我遇到的同样问题。这正是我的问题

EDIT: googling, I found a test of the same problem I have. This is exactly my problem

编辑：当我读到 here in method findKeyDeserializer（org.codehaus.jackson。 map.DeserializationConfig，org.codehaus.jackson.type.JavaType，org.codehaus.jackson.map.BeanProperty）

EDIT: changed extended class from StdDeserializer to StdKeyDeserializer as I read here in method findKeyDeserializer(org.codehaus.jackson.map.DeserializationConfig, org.codehaus.jackson.type.JavaType, org.codehaus.jackson.map.BeanProperty)

编辑：解决了这个问题后，我得到了这个相关的。

EDIT: After solving this issue I got this one that is related.

推荐答案

我是杰克逊的新手，但下面的内容适用于我。

I am a complete newbie with Jackson, but the following works for me.

首先我添加一个JsonCreator方法到A：

First I add a JsonCreator method to A:

public class A {
    private String a1;
    private Integer a2;
    public String getA1() { return a1; }
    public Integer getA2() { return a2; }
    public void setA1(String a1) { this.a1 = a1; }
    public void setA2(Integer a2) { this.a2 = a2; }

    @JsonCreator
    public static A fromJSON(String val) throws JsonParseException, JsonMappingException, IOException {
        ObjectMapper mapper = new ObjectMapper();
        A a = mapper.readValue(val,A.class);
        return a;
    }
} 

这就解决了反序列化问题。对我来说更难的部分是键的正确序列化。我在那里做的是定义一个密钥序列化器，它将命名类序列化为JSON序列化，如下所示：

That alone solves the deserialization problem. The harder part for me was the correct serialization of the keys. What I did there was to define a key serializer that serializes named classes as there JSON serialization, like this:

public class KeySerializer extends SerializerBase<Object> {
    private static final SerializerBase<Object> DEFAULT = new StdKeySerializer();

    private Set<Class<?>> objectKeys_ = Collections.synchronizedSet(new HashSet<Class<?>>());

    protected KeySerializer(Class<?>... objectKeys) {
        super(Object.class);
        for(Class<?> cl:objectKeys) {
            objectKeys_.add(cl);
        }
    }


    @Override
    public JsonNode getSchema(SerializerProvider provider, Type typeHint) throws JsonMappingException {
        return DEFAULT.getSchema(provider, typeHint);
    }


    @Override
    public void serialize(Object value, JsonGenerator jgen,
            SerializerProvider provider) throws IOException,
            JsonGenerationException {
        if (objectKeys_.contains(value.getClass())) {
            ObjectMapper mapper = new ObjectMapper();
            StringWriter writer = new StringWriter();
            mapper.writeValue(writer, value);
            jgen.writeFieldName(writer.toString());
        } else {
            DEFAULT.serialize(value, jgen, provider);
        }
    }
}

然后证明它有效，序列化和反序列化类C的实例：

Then to prove it works, serializing and deserializing an instance of class C:

    ObjectMapper mapper = new ObjectMapper();
    StdSerializerProvider provider = new StdSerializerProvider();
    provider.setKeySerializer(new KeySerializer(A.class));
    mapper.setSerializerProvider(provider);

    StringWriter out = new StringWriter();
    mapper.writeValue(out, c);
    String json = out.toString();
    System.out.println("JSON= "+json);

    C c2 = mapper.readValue(json, C.class);
    System.out.print("C2= ");
    StringWriter outC2 = new StringWriter();
    mapper.writeValue(outC2, c2);
    System.out.println(outC2.toString());

对我来说，这产生了输出：

For me this produced the output:

JSON= {"c1":"goo","map":{"{\"a1\":\"1ccf\",\"a2\":7376}":{"b1":"5ox"},"{\"a1\":\"1cd2\",\"a2\":7379}":{"b1":"5p0"},"{\"a1\":\"1cd5\",\"a2\":7382}":{"b1":"5p3"},"{\"a1\":\"1cd8\",\"a2\":7385}":{"b1":"5p6"}}}
C2= {"c1":"goo","map":{"{\"a1\":\"1ccf\",\"a2\":7376}":{"b1":"5ox"},"{\"a1\":\"1cd2\",\"a2\":7379}":{"b1":"5p0"},"{\"a1\":\"1cd5\",\"a2\":7382}":{"b1":"5p3"},"{\"a1\":\"1cd8\",\"a2\":7385}":{"b1":"5p6"}}}

我觉得有应该是一个更好的方法来说明如何通过使用注释来序列化密钥，但我无法解决它。

I feel there ought to have been a better way of doing saying how to serialize the key by using annotations, but I could not work it out.

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