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/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed 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
 *
 *      https://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.springframework.core.env;

import java.util.Collection;
import java.util.Collections;
import java.util.Map;
import java.util.Set;

import org.springframework.lang.Nullable;

/**
 * Read-only {@code Map} implementation that is backed by system
 * properties or environment variables.
 *
 * Used by {@link AbstractEnvironment} when a {@link SecurityManager} prohibits
 * access to {@link System#getProperties()} or {@link System#getenv()}. It is for this
 * reason that the implementations of {@link #keySet()}, {@link #entrySet()}, and
 * {@link #values()} always return empty even though {@link #get(Object)} may in fact
 * return non-null if the current security manager allows access to individual keys.
 *
 * @author Arjen Poutsma
 * @author Chris Beams
 * @since 3.0
 */
abstract class ReadOnlySystemAttributesMap implements Map {

	@Override
	public boolean containsKey(Object key) {
		return (get(key) != null);
	}

	/**
	 * Returns the value to which the specified key is mapped, or {@code null} if this map
 	 * contains no mapping for the key.
	 * @param key the name of the system attribute to retrieve
	 * @throws IllegalArgumentException if given key is non-String
	 */
	@Override
	@Nullable
	public String get(Object key) {
		if (!(key instanceof String)) {
			throw new IllegalArgumentException(
					"Type of key [" + key.getClass().getName() + "] must be java.lang.String");
		}
		return getSystemAttribute((String) key);
	}

	@Override
	public boolean isEmpty() {
		return false;
	}

	/**
	 * Template method that returns the underlying system attribute.
	 * Implementations typically call {@link System#getProperty(String)} or {@link System#getenv(String)} here.
	 */
	@Nullable
	protected abstract String getSystemAttribute(String attributeName);


	// Unsupported

	@Override
	public int size() {
		throw new UnsupportedOperationException();
	}

	@Override
	public String put(String key, String value) {
		throw new UnsupportedOperationException();
	}

	@Override
	public boolean containsValue(Object value) {
		throw new UnsupportedOperationException();
	}

	@Override
	public String remove(Object key) {
		throw new UnsupportedOperationException();
	}

	@Override
	public void clear() {
		throw new UnsupportedOperationException();
	}

	@Override
	public Set keySet() {
		return Collections.emptySet();
	}

	@Override
	public void putAll(Map map) {
		throw new UnsupportedOperationException();
	}

	@Override
	public Collection values() {
		return Collections.emptySet();
	}

	@Override
	public Set> entrySet() {
		return Collections.emptySet();
	}

}