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| <html> |
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| <meta name="author" content="Jonathan Gibbons"> |
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| <title>com.sun.tools.javac.comp.Enter</title> |
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| |
| <h3>com.sun.tools.javac.util.Context</h3> |
| |
| <p>Contexts provides a way to share data between the different parts of |
| the compiler.</p> |
| |
| <p>They provide support for an abstract context, modelled loosely after |
| ThreadLocal but using a user-provided context instead of the current |
| thread.</p> |
| |
| <p>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.</p> |
| |
| <p>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:</p> |
| |
| <pre> |
| 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 intitialization follows... |
| } |
| } |
| </pre> |
| |
| <p>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:</p> |
| |
| <pre> |
| 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); |
| } |
| }); |
| } |
| } |
| </pre> |
| |
| <p>And is registered early in the extended compiler like this:</p> |
| |
| <pre> |
| NewPhase.preRegister(context); |
| </pre> |
| |
| |
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