Deserialize JSON into transient field

Question

I have a class with these fields:

private transient List<Peer> peers;
private final String name;
private final int points;
private final int size;

Using Gson I want to deserialize this JSON String request:

{
    "name": "game1",
    "points": "11",
    "size": "10",
    "peers": [
        {
            "address": "localhost",
            "port": 1234,
            "fullAddress": "localhost:1234"
        }
    ]
}

My problem is that the Peer object does not get deserialized into the peers List unless I don't declare the field as transient.

Is there a way, with Gson, to have some field transient only during serialization but not during deserialization?

Answer 1

You have two options.

excludeFieldsWithoutExposeAnnotation()

Gson provides @Expose that serves the exact purpose. The only caveat here is that you have to annotate every field:

private static final Gson gson = new GsonBuilder()
        .excludeFieldsWithoutExposeAnnotation()
        .create();

@Expose(serialize = false) final List<Peer> peers;
@Expose final String name;
@Expose final int points;
@Expose final int size;

addSerializationExclusionStrategy(...)

Say, you can easily introduce something like this:

@Target(FIELD)
@Retention(RUNTIME)
@interface ReadOnly {
}

Now, once this one is declared, you can register a strategy to the Gson instance:

private static final Gson gson = new GsonBuilder()
        .addSerializationExclusionStrategy(new ExclusionStrategy() {
            @Override
            public boolean shouldSkipField(final FieldAttributes f) {
                return f.getAnnotation(ReadOnly.class) != null;
            }

            @Override
            public boolean shouldSkipClass(final Class<?> clazz) {
                return false;
            }
        })
        .create();

@ReadOnly final List<Peer> peers;
final String name;
final int points;
final int size;

You can easily use @Expose for the option #2 by just handling it with something like f.getAnnotation(Expose.class) != null && !f.getAnnotation(Expose.class).serialize() in the strategy, but I find @ReadOnly somewhat more convenient.

For both options, the following code

public static void main(final String... args)
        throws IOException {
    try ( final JsonReader jsonReader = getPackageResourceJsonReader(Q43893428.class, "foo.json") ) {
        final Foo foo = gson.fromJson(jsonReader, Foo.class);
        for ( final Peer peer : foo.peers ) {
            System.out.println(peer.fullAddress);
        }
        System.out.println(gson.toJson(foo));
    }
}

produces the following result:

localhost:1234
{"name":"game1","points":11,"size":10}