maven-core/src/main/java/org/apache/maven/session/scope/internal/SessionScope.java

package org.apache.maven.session.scope.internal;

/*
 * 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.
 */

import java.util.Collection;
import java.util.List;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.CopyOnWriteArrayList;

import com.google.inject.Key;
import com.google.inject.OutOfScopeException;
import com.google.inject.Provider;
import com.google.inject.Scope;

/**
 * SessionScope
 */
public class SessionScope
    implements Scope
{

    private static final Provider<Object> SEEDED_KEY_PROVIDER = () ->
    {
        throw new IllegalStateException();
    };

    /**
     * ScopeState
     */
    protected static final class ScopeState
    {
        private final Map<Key<?>, CachingProvider<?>> provided = new ConcurrentHashMap<>();

        public <T> void seed( Class<T> clazz, Provider<T> value )
        {
            provided.put( Key.get( clazz ), new CachingProvider<>( value ) );
        }

        @SuppressWarnings( "unchecked" )
        public <T> Provider<T> scope( Key<T> key, Provider<T> unscoped )
        {
            Provider<?> provider = provided.computeIfAbsent( key, k -> new CachingProvider<>( unscoped ) );
            return ( Provider<T> ) provider;
        }

        public Collection<CachingProvider<?>> providers()
        {
            return provided.values();
        }
    }

    private final List<ScopeState> values = new CopyOnWriteArrayList<>();

    public void enter()
    {
        values.add( 0, new ScopeState() );
    }

    protected ScopeState getScopeState()
    {
        if ( values.isEmpty() )
        {
            throw new OutOfScopeException( "Cannot access session scope outside of a scoping block" );
        }
        return values.get( 0 );
    }

    public void exit()
    {
        if ( values.isEmpty() )
        {
            throw new IllegalStateException();
        }
        values.remove( 0 );
    }

    public <T> void seed( Class<T> clazz, Provider<T> value )
    {
        getScopeState().seed( clazz, value );
    }

    public <T> void seed( Class<T> clazz, final T value )
    {
        seed( clazz, ( Provider<T> ) () -> value );
    }

    public <T> Provider<T> scope( final Key<T> key, final Provider<T> unscoped )
    {
        // Lazy evaluating provider
        return () -> getScopeState().scope( key, unscoped ).get();
    }

    /**
     * A provider wrapping an existing provider with a cache
     * @param <T> the provided type
     */
    protected static class CachingProvider<T> implements Provider<T>
    {
        private final Provider<T> provider;
        private volatile T value;

        CachingProvider( Provider<T> provider )
        {
            this.provider = provider;
        }

        public T value()
        {
            return value;
        }

        @Override
        public T get()
        {
            if ( value == null )
            {
                synchronized ( this )
                {
                    if ( value == null )
                    {
                        value = provider.get();
                    }
                }
            }
            return value;
        }
    }

    @SuppressWarnings( { "unchecked" } )
    public static <T> Provider<T> seededKeyProvider()
    {
        return (Provider<T>) SEEDED_KEY_PROVIDER;
    }
}