Jackson deserializing into Map with an Enum Key, POJO Value

问题

I am trying to deserialize JSON into a Java POJO using Jackson. Without giving away confidential information, here is an example stack trace when ObjectMapper's deserialization fails:

org.codehaus.jackson.map.JsonMappingException: Can not construct Map key of type com.example.MyEnum from String "coins": not a valid representation: Can not construct Map key of type com.example.MyEnum from String "coins": not one of values for Enum class

My JSON looks like this:

"foo": {
    "coins": null,
    ...
}

And the class I want to deserialize into has this field:

private Map<MyEnum, MyPojo> foo;

And my enum type looks like this:

public enum MyEnum {
    COINS("coins"),
    ...
}

I do realize that I am trying to deserialize a null value. But I believe this should still work: the result of the deserialization should be equivalent to having a Map with foo.put(MyEnum.COINS, null), which is indeed a valid Java instruction. Help is much appreciated, thanks in advance.

回答1:

In addition to one good solution presented (factory method), there are 2 other ways:

• If 'MyEnum.toString()' would return "coins", you can make Jackson use "toString()" over "name()"with ObjectMapper.enable(DeserializationFeature.READ_ENUMS_USING_TO_STRING)
• You could add some other method to return id to use, and mark that method with @JsonValue annotation (you can actually use that on toString() as well, instead of enabling above feature) -- if that annotation exists, value returned by that method is used as the id.

回答2:

GRR! Figured it out.

Solution for me was to create a static method, annotated with @JsonCreator, in my enum that creates an instance of the enum, based on a String parameter. It looked like this:

@JsonCreator
public static MyEnum create(String str) {
    // code to return an enum based on the String parameter
}

回答3:

Provide a static factory method in your enumeration class that constructs enum by string and annotate it with @JsonCreator:

@JsonCreator
public static MyEnum fromValue(String v) {
    for (MyEnum myEnum : values()) {
        if (myEnum.text.equals(v)) {
            return myEnum;
        }
    }
    throw new IllegalArgumentException("invalid string value passed: " + v);
}

来源：https://stackoverflow.com/questions/13406568/jackson-deserializing-into-map-with-an-enum-key-pojo-value