Client.getInstance() and getRawClient() do not perform safe lazy initialization

The static methods Client.getInstance() and getRawClient() do not perform safe lazy initialization: https://github.com/shannah/Java-Objective-C-Bridge/blob/473221aeef8f12ca9a659d899f3ca7092aed27ef/src/main/java/ca/weblite/objc/Client.java#L46-L51

To correct this perform these steps:

Make instance and rawClient volatile
In the access methods, do double checked locking:
1. Check if instance == null (for increased performance store current value in local var)
2. If null:
  1. Synchronize
  2. Check again if instance == null (and overwrite local var, see 2.1)
  3. If null: Initialize (and update local var, see 2.1)
3. Return instance (or local var) value

E.g.:

private static volatile Client instance;

public Client getInstance() {
    Client localInstance = instance;
    if (localInstance == null) {
        // Could also use dedicated lock for `instance` and `rawClient`, but likely not worth it
        synchronized (Client.class) {
            localInstance = instance;
            if (localInstance == null) {
                instance = localInstance = new Client();
            }
        }
    }
    return localInstance;
}

Or alternatively making these methods synchronized would also suffice and the performance decrease (if any) would likely be negligible. See also "LCK10-J. Use a correct form of the double-checked locking idiom"

For sanity I would also recommend:

Implementing a private constructor, currently Client has the public default constructor
Making fields coerceInputs and coerceOutputs final and initializing them from the constructor

shannah / Java-Objective-C-Bridge

Client.getInstance() and getRawClient() do not perform safe lazy initialization #25