How to convert the following json string to java object?

No need to go with GSON for this; Jackson can do either plain Maps/Lists:
ObjectMapper mapper = new ObjectMapper();
Map map = mapper.readValue(json, Map.class);

or more convenient JSON Tree:
JsonNode rootNode = mapper.readTree(json);

By the way, there is no reason why you could not actually create Java classes and do it (IMO) more conveniently:
public class Library {
@JsonProperty(“libraryname”)
public String name;

@JsonProperty(“mymusic”)
public List songs;
}
public class Song {
@JsonProperty(“Artist Name”) public String artistName;
@JsonProperty(“Song Name”) public String songName;
}

Library lib = mapper.readValue(jsonString, Library.class);

Check out Google’s Gson: http://code.google.com/p/google-gson/
From their website:
Gson gson = new Gson(); // Or use new GsonBuilder().create();
MyType target2 = gson.fromJson(json, MyType.class); // deserializes json into target2

You would just need to make a MyType class (renamed, of course) with all the fields in the json string. It might get a little more complicated when you’re doing the arrays, if you prefer to do all of the parsing manually (also pretty easy) check out http://www.json.org/ and download the Java source for the Json parser objects.