Question

我有一个Map<A,B> fieldOfC作为C类的字段。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，我猜解决方案是扩展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。

任何人都可以为这个简单的例子编写代码，这样我就可以用它作为开始构建真正的反序列化器吗？

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，我发现了test我遇到的同样问题。这正是我的问题

编辑：当我在方法findKeyDeserializer(org.codehaus.jackson.map.DeserializationConfig, org.codehaus.jackson.type.JavaType, org.codehaus.jackson.map.BeanProperty)

中阅读here时，将扩展类从StdDeserializer更改为StdKeyDeserializer

编辑：解决此问题后，我得到了this one相关信息。

Answer 1

我是杰克逊的新手，但下面的内容适用于我。

首先，我将一个JsonCreator方法添加到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;
    }
}

仅此解决了反序列化问题。对我来说更难的部分是正确的键序列化。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：

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C类实例：

    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：

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"}}}

我觉得应该用一种更好的方法来说明如何使用注释来序列化密钥，但我无法解决这个问题。

如何在Java中创建自定义JsonDeserializer？

1 个答案: