Support for an abstract context, modelled loosely after ThreadLocal but using a user-provided context instead of the current thread.

Within the compiler, a single Context is used for each invocation of the compiler. The context is then used to ensure a single copy of each compiler phase exists per compiler invocation.

The context can be used to assist in extending the compiler by extending its components. To do that, the extended component must be registered before the base component. We break initialization cycles by (1) registering a factory for the component rather than the component itself, and (2) a convention for a pattern of usage in which each base component registers itself by calling an instance method that is overridden in extended components. A base phase supporting extension would look something like this:

public class Phase {
    protected static final Context.Key<Phase> phaseKey =
        new Context.Key<Phase>();

    public static Phase instance(Context context) {
        Phase instance = context.get(phaseKey);
        if (instance == null)
            // the phase has not been overridden
            instance = new Phase(context);
        return instance;
    }

    protected Phase(Context context) {
        context.put(phaseKey, this);
        // other initialization follows...
    }
}

In the compiler, we simply use Phase.instance(context) to get the reference to the phase. But in extensions of the compiler, we must register extensions of the phases to replace the base phase, and this must be done before any reference to the phase is accessed using Phase.instance(). An extended phase might be declared thus:

public class NewPhase extends Phase {
    protected NewPhase(Context context) {
        super(context);
    }
    public static void preRegister(final Context context) {
        context.put(phaseKey, new Context.Factory<Phase>() {
            public Phase make() {
                return new NewPhase(context);
            }
        });
    }
}

And is registered early in the extended compiler like this

    NewPhase.preRegister(context);

This is NOT part of any supported API. If you write code that depends on this, you do so at your own risk. This code and its internal interfaces are subject to change or deletion without notice.