Can't cast JsonNull to JsonObject

问题

I want to copy one primitve property from one JsonObject to another

JsonObject propertyToBeCopied = source.getAsJsonObject(propertyName);

but I always run into this exception:

com.google.gson.JsonNull cannot be cast to com.google.gson.JsonObject

According to the documentation it should be possible to do the cast, or am I wrong?

回答1:

According to the docsJsonNull is a JsonElement but not a JsonObject (which is itself a JsonElement). Using

JsonElement element = source.get(propertyName);
if (!(element instanceof JsonNull)) {
    JsonObject propertyToBeCopied = (JsonObject) element;
}

would return a JsonElement that is casted to JsonObject if it is not of the type JsonNull.

回答2:

According to the API reference, JsonNull derives from JsonElement and not JsonObject, so I don't see how that cast could ever be valid.

And have you considered using json-simple instead of gson? As a general rule I find it much more convenient to work with than other json frameworks, although of course it doesn't have a lot of the extra features that gson offers. But if all you're doing with gson is parsing json, it might be worth switching to the simpler library.

回答3:

JsonElement element = source.get(propertyName);
if (!(element.isJsonNull())) {
    JsonObject propertyToBeCopied = (JsonObject) element;
}

Which one will have better performance isJsonNull or using instanceOf operator ?

回答4:

If output do not have a Response array then there is a chance why this error is thrown.

JsonObject playListObject = (JsonObject) parser.parse(output);

JsonArray playListInformationArray = playListObject.getAsJsonArray("Response");

来源：https://stackoverflow.com/questions/12379405/cant-cast-jsonnull-to-jsonobject