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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

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 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * 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).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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 */

package jdk.internal.loader;

import java.util.Objects;

import java.util.function.BiFunction;

/**

 * root-ClassLoaderValue. Each instance defines a separate namespace for

 * associated values.

 * <p>

 * ClassLoaderValue allows associating a

 * {@link #computeIfAbsent(ClassLoader, BiFunction) computed} non-null value with

 * a {@code (ClassLoader, keys...)} tuple. The associated value, as well as the

 * keys are strongly reachable from the associated ClassLoader so care should be

 * taken to use such keys and values that only reference types resolvable from

 * the associated ClassLoader. Failing that, ClassLoader leaks are inevitable.

 * <p>

 * Example usage:

 * <pre>{@code

 * // create a root instance which represents a namespace and declares the type of

 * // associated values (Class instances in this example)

 * static final ClassLoaderValue<Class<?>> proxyClasses = new ClassLoaderValue<>();

 *

 * // create a compound key composed of a Module and a list of interfaces

 * Module module = ...;

 * List<Class<?>> interfaces = ...;

 * ClassLoaderValue<Class<?>>.Sub<Module>.Sub<List<Class<?>>> key =

 *     proxyClasses.sub(module).sub(interfaces);

 *

 * // use the compound key together with ClassLoader to lazily associate

 * // the value with tuple (loader, module, interfaces) and return it

 * ClassLoader loader = ...;

 * Class<?> proxyClass = key.computeIfAbsent(loader, (ld, ky) -> {

 *     List<Class<?>> intfcs = ky.key();

 *     Module m = ky.parent().key();

 *     Class<?> clazz = defineProxyClass(ld, m, intfcs);

 *     return clazz;

 * });

 * }</pre>

 * <p>

 * {@code classLoaderValue.<operation>(classLoader, ...)} represents an operation

 * to {@link #get}, {@link #putIfAbsent}, {@link #computeIfAbsent} or {@link #remove}

 * a value associated with a (classLoader, classLoaderValue) tuple. ClassLoader

 * instances and root-{@link ClassLoaderValue} instances are compared using

 * identity equality while {@link Sub sub}-ClassLoaderValue instances define

 * {@link #equals(Object) equality} in terms of equality of its

 * {@link Sub#parent() parent} ClassLoaderValue and its

 * {@link #key() key} component.

 *

 * @param <V> the type of value(s) associated with the root-ClassLoaderValue and

 *            all its {@link #sub(Object) descendants}.

 * @author Peter Levart

 * @since 9

 */

public final class ClassLoaderValue<V>

    extends AbstractClassLoaderValue<ClassLoaderValue<V>, V> {

    /**

     * Constructs new root-ClassLoaderValue representing its own namespace.

     */

    public ClassLoaderValue() {}

    /**

     * @return the key component of this root-ClassLoaderValue (itself).

     */

    @Override

    public ClassLoaderValue<V> key() {

        return this;

    }

    /**

     * root-ClassLoaderValue can only be equal to itself and has no predecessors.

     */

    @Override

    public boolean isEqualOrDescendantOf(AbstractClassLoaderValue<?, V> clv) {

        return equals(Objects.requireNonNull(clv));

    }

}