The Context.java Java example source code
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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*
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
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* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
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package com.sun.tools.javac.util;
import java.util.*;
/**
* Support for an abstract context, modelled loosely after ThreadLocal
* but using a user-provided context instead of the current thread.
*
* <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>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>{@code
* 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>{@code
* 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>
* NewPhase.preRegister(context);
* </pre>
*
* <p>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.</b>
*/
public class Context {
/** The client creates an instance of this class for each key.
*/
public static class Key<T> {
// note: we inherit identity equality from Object.
}
/**
* The client can register a factory for lazy creation of the
* instance.
*/
public static interface Factory<T> {
T make(Context c);
};
/**
* The underlying map storing the data.
* We maintain the invariant that this table contains only
* mappings of the form
* {@literal Key<T> -> T }
* or
* {@literal Key<T> -> Factory }
*/
private Map<Key ht = new HashMap,Object>();
/** Set the factory for the key in this context. */
public <T> void put(Key key, Factory fac) {
checkState(ht);
Object old = ht.put(key, fac);
if (old != null)
throw new AssertionError("duplicate context value");
checkState(ft);
ft.put(key, fac); // cannot be duplicate if unique in ht
}
/** Set the value for the key in this context. */
public <T> void put(Key key, T data) {
if (data instanceof Factory<?>)
throw new AssertionError("T extends Context.Factory");
checkState(ht);
Object old = ht.put(key, data);
if (old != null && !(old instanceof Factory<?>) && old != data && data != null)
throw new AssertionError("duplicate context value");
}
/** Get the value for the key in this context. */
public <T> T get(Key key) {
checkState(ht);
Object o = ht.get(key);
if (o instanceof Factory<?>) {
Factory<?> fac = (Factory)o;
o = fac.make(this);
if (o instanceof Factory<?>)
throw new AssertionError("T extends Context.Factory");
Assert.check(ht.get(key) == o);
}
/* The following cast can't fail unless there was
* cheating elsewhere, because of the invariant on ht.
* Since we found a key of type Key<T>, the value must
* be of type T.
*/
return Context.<T>uncheckedCast(o);
}
public Context() {}
/**
* The table of preregistered factories.
*/
private Map<Key> ft = new HashMap,Factory>();
public Context(Context prev) {
kt.putAll(prev.kt); // retain all implicit keys
ft.putAll(prev.ft); // retain all factory objects
ht.putAll(prev.ft); // init main table with factories
}
/*
* The key table, providing a unique Key<T> for each Class.
*/
private Map<Class> kt = new HashMap, Key>();
private <T> Key key(Class clss) {
checkState(kt);
Key<T> k = uncheckedCast(kt.get(clss));
if (k == null) {
k = new Key<T>();
kt.put(clss, k);
}
return k;
}
public <T> T get(Class clazz) {
return get(key(clazz));
}
public <T> void put(Class clazz, T data) {
put(key(clazz), data);
}
public <T> void put(Class clazz, Factory fac) {
put(key(clazz), fac);
}
/**
* TODO: This method should be removed and Context should be made type safe.
* This can be accomplished by using class literals as type tokens.
*/
@SuppressWarnings("unchecked")
private static <T> T uncheckedCast(Object o) {
return (T)o;
}
public void dump() {
for (Object value : ht.values())
System.err.println(value == null ? null : value.getClass());
}
public void clear() {
ht = null;
kt = null;
ft = null;
}
private static void checkState(Map<?,?> t) {
if (t == null)
throw new IllegalStateException();
}
}
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