Is it OK to use Gson instance as a static field in

Question

Here's the model I implemented:

public class LoginSession {
    private static final Gson gson = new Gson();

    private String id;
    private String name;
    private long timestamp;

    public LoginSession(String id, String name) {
        this.id = id;
        this.name = name;
        this.timestamp = System.currentTimeMillis();
    }

    public String toJson() {
        return gson.toJson(this);
    }

    public static LoginSession fromJson(String json) {
        checkArgument(!isNullOrEmpty(json));
        return gson.fromJson(json, LoginSession.class);
    }
}

I thought it's useless to create new Gson instance for every LoginSession instance.

But what I'm worried about is thread-safety issues. Approximately 1000+ instances/sec will be created.

Is it OK to use Gson instance as static field?

Thanks for any advices/corrections.

Answer 1

The core Gson class is thread-safe. I just encountered a thread-safety issue that was supposedly with GSON. The issue happened when using a custom JsonDeserializer and JsonSerializer for Date parsing and formatting. As it turned out, the thread-safety issue was with my method's use of a static SimpleDateFormat instance which is not thread-safe. Once I wrapped the static SimpleDateFormat in a ThreadLocal instance, everything worked out fine.

Answer 2

According to the comments the existing unit test does not really test much, be careful with anything related to thread safety...

There is a unit test checking for thread safety:

/**
 * Tests for ensuring Gson thread-safety.
 *
 * @author Inderjeet Singh
 * @author Joel Leitch
 */
public class ConcurrencyTest extends TestCase {
  private Gson gson;
  ...

You may wonder if this unit test is sufficient to find every possible problem on every possible machine configuration ? Any comments on this ?

There is also this sentence in the docs:

The Gson instance does not maintain any state while invoking Json operations. So, you are free to reuse the same object for multiple Json serialization and deserialization operations.

Answer 3

It seems just fine to me. There is nothing in the GSON instance that makes it related to a specific instance of LoginSession, so it should be static.

GSON instances should be thread-safe, and there was a bug regarding that which was fixed.