第三章 spring-bean之SimpleAliasRegistry（2）

前言

AliasRegistry的實現類，嗯，嗯，嗯，一個很簡單的實現。只有一個map。

源碼解讀

接口

AliasRegistry

public interface AliasRegistry {

	void registerAlias(String name, String alias);

	void removeAlias(String alias);

	boolean isAlias(String name);

	String[] getAliases(String name);
}

實現

SimpleAliasRegistry

public class SimpleAliasRegistry implements AliasRegistry {

	private final Map<String, String> aliasMap = new ConcurrentHashMap<String, String>(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");
		if (alias.equals(name)) {
			this.aliasMap.remove(alias);
		}
		else {
			String registeredName = this.aliasMap.get(alias);
			if (registeredName != null) {
				if (registeredName.equals(name)) {
					// An existing alias - no need to re-register
					return;
				}
				if (!allowAliasOverriding()) {
					throw new IllegalStateException("Cannot register alias '" + alias + "' for name '" + name + "': It is already registered for name '" + registeredName + "'.");
				}
			}
			checkForAliasCircle(name, alias);
			this.aliasMap.put(alias, name);
		}
	}


	protected boolean allowAliasOverriding() {
		return true;
	}


	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) {
		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);
	}

	@Override
	public String[] getAliases(String name) {
		List<String> result = new ArrayList<String>();
		synchronized (this.aliasMap) {
			retrieveAliases(name, result);
		}
		return StringUtils.toStringArray(result);
	}


	private void retrieveAliases(String name, List<String> result) {
		for (Map.Entry<String, String> entry : this.aliasMap.entrySet()) {
			String registeredName = entry.getValue();
			if (registeredName.equals(name)) {
				String alias = entry.getKey();
				result.add(alias);
				retrieveAliases(alias, result);
			}
		}
	}


	public void resolveAliases(StringValueResolver valueResolver) {
		Assert.notNull(valueResolver, "StringValueResolver must not be null");
		synchronized (this.aliasMap) {
			Map<String, String> aliasCopy = new HashMap<String, String>(this.aliasMap);
			for (String alias : aliasCopy.keySet()) {
				String registeredName = aliasCopy.get(alias);
				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);
							break;
						}
						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);
				}
			}
		}
	}


	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");
		}
	}


	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;
	}

}

內部使用

ConfigurationClassBeanDefinitionReader

private void loadBeanDefinitionsForBeanMethod(BeanMethod beanMethod) {
	........
	AnnotationAttributes bean = AnnotationConfigUtils.attributesFor(metadata, Bean.class);
	List<String> names = new ArrayList<String>(Arrays.asList(bean.getStringArray("name")));
	String beanName = (names.size() > 0 ? names.remove(0) : methodName);

	// Register aliases even when overridden
	for (String alias : names) {
		this.registry.registerAlias(beanName, alias);
	}
	........
}

bean(註解)

public @interface Bean {

	@AliasFor("name")
	String[] value() default {};

	@AliasFor("value")
	String[] name() default {};
}

解析xml的bean標籤

DefaultBeanDefinitionDocumentReader

protected void processAliasRegistration(Element ele) {
		String name = ele.getAttribute(NAME_ATTRIBUTE);
		String alias = ele.getAttribute(ALIAS_ATTRIBUTE);// 得到alias屬性
		boolean valid = true;
		if (!StringUtils.hasText(name)) {
			getReaderContext().error("Name must not be empty", ele);
			valid = false;
		}
		if (!StringUtils.hasText(alias)) {
			getReaderContext().error("Alias must not be empty", ele);
			valid = false;
		}
		if (valid) {
			try {
				getReaderContext().getRegistry().registerAlias(name, alias);
			}
			catch (Exception ex) {
				getReaderContext().error("Failed to register alias '" + alias +
						"' for bean with name '" + name + "'", ele, ex);
			}
			getReaderContext().fireAliasRegistered(name, alias, extractSource(ele));
		}
	}

從BeanDefinitionHolder得到

BeanDefinitionReaderUtils

public static void registerBeanDefinition(
			BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
			throws BeanDefinitionStoreException {

		// Register bean definition under primary name.
		String beanName = definitionHolder.getBeanName();
		registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());

		// Register aliases for bean name, if any.
		String[] aliases = definitionHolder.getAliases();
		if (aliases != null) {
			for (String alias : aliases) {
				registry.registerAlias(beanName, alias);
			}
		}
}

dome

總結

• 一個bean能夠對應多個alias
• 經過beanName獲得alias須要遍歷整個map，不少操做都會遍歷整個map
• 別名是否覆蓋，若是不容許覆蓋，就報異常
• 從別人的獲取方式看來，alias基本沒有做用。@bean基本不會用name，xml已經被淘汰了。BeanDefinitionHolder更加不會又人用了。因此AliasRegistry與SimpleAliasRegistry基本無用。