AliasRegistry源码阅读

public interface AliasRegistry
{
//对指定的名称注册别名
    public abstract void registerAlias(String s, String s1);
//从当前容器移除指定别名
    public abstract void removeAlias(String s);
//判断指定名称是否为别名
    public abstract boolean isAlias(String s);
//返回指定名称的全部别名
    public abstract String[] getAliases(String s);
}

AliasRegistry接口的功能：注册、移除、判断、获取

/*
 * 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
 *
 *      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.springframework.core;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

import org.springframework.util.Assert;
import org.springframework.util.StringUtils;
import org.springframework.util.StringValueResolver;

/**
 * Simple implementation of the {@link AliasRegistry} interface.
 * Serves as base class for
 * {@link org.springframework.beans.factory.support.BeanDefinitionRegistry}
 * implementations.
 *
 * @author Juergen Hoeller
 * @since 2.5.2
 */
public class SimpleAliasRegistry implements AliasRegistry {

	/** Map from alias to canonical name */
	private final Map<String, String> aliasMap = new ConcurrentHashMap<>(16);


	@Override
	public void registerAlias(String name, String alias) {
		Assert.hasText(name, "'name' must not be empty");
		Assert.hasText(alias, "'alias' must not be empty");
		synchronized (this.aliasMap) {
//若是名称和别名相同，则移除别名
			if (alias.equals(name)) {
				this.aliasMap.remove(alias);
			}
			else {
				String registeredName = this.aliasMap.get(alias);
//若是别名已经存在，那么不用再注册，直接返回
				if (registeredName != null) {
					if (registeredName.equals(name)) {
						// An existing alias - no need to re-register
						return;
					}
//若是别名不容许被覆盖则抛异常，
					if (!allowAliasOverriding()) {
						throw new IllegalStateException("Cannot register alias '" + alias + "' for name '" +
								name + "': It is already registered for name '" + registeredName + "'.");
					}
				}
//递归检查别名
				checkForAliasCircle(name, alias);
//注册别名
				this.aliasMap.put(alias, name);
			}
		}
	}

	/**
	 * Return whether alias overriding is allowed.
	 * Default is {@code true}.
	 */
	protected boolean allowAliasOverriding() {
		return true;
	}

/**
*以A->B为例，A为名称，B为别名，需为A注册别名B
*checkForAliasCircle(A, B);
*hasAlias(B, A);
*检查B是否有别名，若是B无其余别名，hasAlias(B, A)==false
*若是B有别名C，则检查C是否与A相同，若是相同，则hasAlias(B, A)==true，说明B->A->B，即存在环形指向关系
*若是A与C不相同，则递归调用hasAlias(C, A);
*检查C是否有别名，若是C无其余别名，hasAlias(C, A)==false，hasAlias(B, A)==false
*若是C有别名D，且D与A相同，则hasAlias(C, A)==true，hasAlias(B, A)==true，说明C->A->B->C，即存在环形指向关系
*/
	/**
	 * Determine whether the given name has the given alias registered.
	 * @param name the name to check
	 * @param alias the alias to look for
	 * @since 4.2.1
	 */
	public boolean hasAlias(String name, String alias) {
		for (Map.Entry<String, String> entry : this.aliasMap.entrySet()) {
			String registeredName = entry.getValue();
			if (registeredName.equals(name)) {
				String registeredAlias = entry.getKey();
				return (registeredAlias.equals(alias) || hasAlias(registeredAlias, alias));
			}
		}
		return false;
	}

	@Override
	public void removeAlias(String alias) {
		synchronized (this.aliasMap) {
			String name = this.aliasMap.remove(alias);
			if (name == null) {
				throw new IllegalStateException("No alias '" + alias + "' registered");
			}
		}
	}

	@Override
	public boolean isAlias(String name) {
		return this.aliasMap.containsKey(name);
	}

//根据指定名称获取全部别名，递归检索，其中包含直接指向的别名和间接指向的别名
	@Override
	public String[] getAliases(String name) {
		List<String> result = new ArrayList<>();
		synchronized (this.aliasMap) {
			retrieveAliases(name, result);
		}
		return StringUtils.toStringArray(result);
	}

//根据指定名称获取别名列表
	/**
	 * Transitively retrieve all aliases for the given name.
	 * @param name the target name to find aliases for
	 * @param result the resulting aliases list
	 */
	private void retrieveAliases(String name, List<String> result) {
		this.aliasMap.forEach((alias, registeredName) -> {
			if (registeredName.equals(name)) {
				result.add(alias);
				retrieveAliases(alias, result);
			}
		});
	}

	/**
	 * Resolve all alias target names and aliases registered in this
	 * factory, applying the given StringValueResolver to them.
	 * <p>The value resolver may for example resolve placeholders
	 * in target bean names and even in alias names.
	 * @param valueResolver the StringValueResolver to apply
	 */
	public void resolveAliases(StringValueResolver valueResolver) {
		Assert.notNull(valueResolver, "StringValueResolver must not be null");
		synchronized (this.aliasMap) {
			Map<String, String> aliasCopy = new HashMap<>(this.aliasMap);
			aliasCopy.forEach((alias, registeredName) -> {
				String resolvedAlias = valueResolver.resolveStringValue(alias);
				String resolvedName = valueResolver.resolveStringValue(registeredName);
				if (resolvedAlias == null || resolvedName == null || resolvedAlias.equals(resolvedName)) {
					this.aliasMap.remove(alias);
				}
				else if (!resolvedAlias.equals(alias)) {
					String existingName = this.aliasMap.get(resolvedAlias);
					if (existingName != null) {
						if (existingName.equals(resolvedName)) {
							// Pointing to existing alias - just remove placeholder
							this.aliasMap.remove(alias);
							return;
						}
						throw new IllegalStateException(
								"Cannot register resolved alias '" + resolvedAlias + "' (original: '" + alias +
								"') for name '" + resolvedName + "': It is already registered for name '" +
								registeredName + "'.");
					}
					checkForAliasCircle(resolvedName, resolvedAlias);
					this.aliasMap.remove(alias);
					this.aliasMap.put(resolvedAlias, resolvedName);
				}
				else if (!registeredName.equals(resolvedName)) {
					this.aliasMap.put(alias, resolvedName);
				}
			});
		}
	}
//由于SimpleAliasRegistry根据名称获取别名列表时，会将直接、间接关系的别名均列出来，因此注册别名时不容许名称与别名见存在环形指向关联关系。
//检查给定的名称是否已经指向给定的别名做为另外一个方向的别名，在前面捕获一个循环引用并引起相应IllegalstateException。

	/**
	 * Check whether the given name points back to the given alias as an alias
	 * in the other direction already, catching a circular reference upfront
	 * and throwing a corresponding IllegalStateException.
	 * @param name the candidate name
	 * @param alias the candidate alias
	 * @see #registerAlias
	 * @see #hasAlias
	 */
	protected void checkForAliasCircle(String name, String alias) {
		if (hasAlias(alias, name)) {
			throw new IllegalStateException("Cannot register alias '" + alias +
					"' for name '" + name + "': Circular reference - '" +
					name + "' is a direct or indirect alias for '" + alias + "' already");
		}
	}

	/**
	 * Determine the raw name, resolving aliases to canonical names.
	 * @param name the user-specified name
	 * @return the transformed name
	 */
	public String canonicalName(String name) {
		String canonicalName = name;
		// Handle aliasing...
		String resolvedName;
		do {
			resolvedName = this.aliasMap.get(canonicalName);
			if (resolvedName != null) {
				canonicalName = resolvedName;
			}
		}
		while (resolvedName != null);
		return canonicalName;
	}

}