SPRING与设计模式--策略模式与模板方法模式

SPRING与设计模式--策略模式与模板方法模式

模板方法模式在一个方法中定义一个算法的骨架，而将一些步骤延迟到子类中。模板方法使得子类可以在不改变算法接口的情况下，重新定义算法的某些步骤。

策略模式定义了算法族，分别封装起来，让他们之间可以互相体会。此模式让算法的变化独立于使用算法的客户。

开源框架为了保证灵活性，方便开发者进行二次开发，经常使用这两种模式。

springsecurity4中身份验证的算法被分割成多个步骤，每个步骤都可以由开发者进行自定义。如果读者需要自己实现定制化的验证方式，那么就可以继承AbstractAuthenticationProcessingFilter，覆盖其中的步骤完成定制化。

身份验证功能由AbstractAuthenticationProcessingFilter实现算法骨架，子类来进行自定义其中的步骤完成最终需要的效果，见类图：

代码中核心的操作步骤见上图，核心源码如下：

public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain)
			throws IOException, ServletException {

		HttpServletRequest request = (HttpServletRequest) req;
		HttpServletResponse response = (HttpServletResponse) res;

		if (!requiresAuthentication(request, response)) {
			chain.doFilter(request, response);

			return;
		}

		if (logger.isDebugEnabled()) {
			logger.debug("Request is to process authentication");
		}

		Authentication authResult;

		try {
			authResult = attemptAuthentication(request, response);
			if (authResult == null) {
				// return immediately as subclass has indicated that it hasn't completed
				// authentication
				return;
			}
			sessionStrategy.onAuthentication(authResult, request, response);
		}
		catch (InternalAuthenticationServiceException failed) {
			logger.error(
					"An internal error occurred while trying to authenticate the user.",
					failed);
			unsuccessfulAuthentication(request, response, failed);

			return;
		}
		catch (AuthenticationException failed) {
			// Authentication failed
			unsuccessfulAuthentication(request, response, failed);

			return;
		}

		// Authentication success
		if (continueChainBeforeSuccessfulAuthentication) {
			chain.doFilter(request, response);
		}

		successfulAuthentication(request, response, chain, authResult);
	}

其中的SessionAuthenticationStrategy使用策略模式，作用描述见源码注释，有两个实现类，见上图

/**
 * Allows pluggable support for HttpSession-related behaviour when an authentication
 * occurs.
 * 

 * Typical use would be to make sure a session exists or to change the session Id to guard
 * against session-fixation attacks.
 *
 * @author Luke Taylor
 * @since
 */
public interface SessionAuthenticationStrategy {

   /**
    * Performs Http session-related functionality when a new authentication occurs.
    *
    * @throws SessionAuthenticationException if it is decided that the authentication is
    * not allowed for the session. This will typically be because the user has too many
    * sessions open at once.
    */
   void onAuthentication(Authentication authentication, HttpServletRequest request,
         HttpServletResponse response) throws SessionAuthenticationException;

}