api/maven-api-core/src/main/java/org/apache/maven/api/SessionData.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
package org.apache.maven.api;

import java.util.Objects;
import java.util.function.Supplier;

import org.apache.maven.api.annotations.Experimental;
import org.apache.maven.api.annotations.Nonnull;
import org.apache.maven.api.annotations.Nullable;
import org.apache.maven.api.annotations.Provider;
import org.apache.maven.api.annotations.ThreadSafe;

/**
 * A container for data that is specific to a session.
 * All components may use this storage to associate arbitrary data with a session.
 * <p>
 * Unlike a cache, this session data is not subject to purging. For this same reason, session data should also not be
 * abused as a cache (i.e. for storing values that can be re-calculated) to avoid memory exhaustion.
 * <p>
 * <strong>Note:</strong> Actual implementations must be thread-safe.
 *
 * @see Session#getData()
 * @since 4.0.0
 */
@Experimental
@ThreadSafe
@Provider
public interface SessionData {

    /**
     * Associates the specified session data with the given key.
     *
     * @param key the key under which to store the session data, must not be {@code null}
     * @param value the data to associate with the key, may be {@code null} to remove the mapping
     */
    <T> void set(@Nonnull Key<T> key, @Nullable T value);

    /**
     * Associates the specified session data with the given key if the key is currently mapped to the given value. This
     * method provides an atomic compare-and-update of some key's value.
     *
     * @param key the key under which to store the session data, must not be {@code null}
     * @param oldValue the expected data currently associated with the key, may be {@code null}
     * @param newValue the data to associate with the key, may be {@code null} to remove the mapping
     * @return {@code true} if the key mapping was successfully updated from the old value to the new value,
     *         {@code false} if the current key mapping didn't match the expected value and was not updated.
     */
    <T> boolean replace(@Nonnull Key<T> key, @Nullable T oldValue, @Nullable T newValue);

    /**
     * Gets the session data associated with the specified key.
     *
     * @param key the key for which to retrieve the session data, must not be {@code null}
     * @return the session data associated with the key or {@code null} if none
     */
    @Nullable
    <T> T get(@Nonnull Key<T> key);

    /**
     * Retrieve of compute the data associated with the specified key.
     *
     * @param key the key for which to retrieve the session data, must not be {@code null}
     * @param supplier the supplier will compute the new value
     * @return the session data associated with the key
     */
    @Nullable
    <T> T computeIfAbsent(@Nonnull Key<T> key, @Nonnull Supplier<T> supplier);

    /**
     * Create a key using the given class as an identifier and as the type of the object.
     */
    static <T> Key<T> key(Class<T> clazz) {
        return new Key<>(clazz, clazz);
    }

    /**
     * Create a key using the given class and id.
     */
    static <T> Key<T> key(Class<T> clazz, Object id) {
        return new Key<>(clazz, id);
    }

    /**
     * Key used to query the session data
     * @param <T> the type of the object associated to this key
     */
    final class Key<T> {

        private final Class<T> type;
        private final Object id;

        private Key(Class<T> type, Object id) {
            this.type = type;
            this.id = id;
        }

        public Class<T> type() {
            return type;
        }

        @Override
        public boolean equals(Object o) {
            if (this == o) {
                return true;
            }
            if (o == null || getClass() != o.getClass()) {
                return false;
            }
            Key<?> key = (Key<?>) o;
            return Objects.equals(id, key.id) && Objects.equals(type, key.type);
        }

        @Override
        public int hashCode() {
            return Objects.hash(id, type);
        }
    }
}