设计模式-拦截过滤器

2020-11-27

简介

拦截过滤器模式(Intercepting Filter Pattern)用于对应用程序的请求或响应做一些预处理/后处理. 定义过滤器, 并在把请求传给实际目标应用程序之前应用在请求上. 过滤器可以做认证/授权/记录日志, 或者跟踪请求, 然后把请求传给相应的处理程序. 以下是这种设计模式的实体.

  • 过滤器(Filter): 过滤器在请求处理程序执行请求之前或之后, 执行某些任务.
  • 过滤器链(Filter Chain): 过滤器链带有多个过滤器, 并在 Target 上按照定义的顺序执行这些过滤器.
  • 目标(Target): 目标对象是请求处理程序.
  • 过滤管理器(Filter Manager): 过滤管理器管理过滤器和过滤器链.
  • 客户端(Client): Client 是向 Target 对象发送请求的对象.

类图

类图

和过滤器模式(Filter Pattern)的区别

  1. 过滤器模式中没有FilterChain这个概念, 无法阻止对目标的调用. 而拦截过滤器模式由于有这个概念, 在每个Filter执行时选择是否执行FilterChain里的下一个Filter. 如果不执行, 则不会对目标进行调用

  2. 过滤器模式是对已有对象列表过滤. 而拦截过滤器模式是对调用目标的单个请求的过滤

例子

拦截过滤器模式在 javax.servlet 中的应用

Filter接口

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public interface Filter {
    ...
    public void doFilter(ServletRequest request, ServletResponse response,
            FilterChain chain) throws IOException, ServletException;
    ...

FilterChain接口

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public interface FilterChain {
    public void doFilter(ServletRequest request, ServletResponse response)
            throws IOException, ServletException;
}

FilterChain实现类ApplicationFilterChain

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final class ApplicationFilterChain implements FilterChain {

}

1. Filter初始化过程

  1. ContextConfig#webConfig方法中扫描web.xml文件, 构建WebXml对象

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    /**
     * Scan the web.xml files that apply to the web application and merge them
     * using the rules defined in the spec. For the global web.xml files,
     * where there is duplicate configuration, the most specific level wins. ie
     * an application's web.xml takes precedence over the host level or global
     * web.xml file.
     */
    protected void webConfig() {
        ...
        WebXml webXml = createWebXml();
        ...
        configureContext(webXml);
        ...
    }

  2. ContextConfig#configureContext方法将WebXml中解析到的FilterMap列表放入StandardContext中

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    private void configureContext(WebXml webxml) {
        ...
        for (FilterMap filterMap : webxml.getFilterMappings()) {
            context.addFilterMap(filterMap);
        }
        ...
    }

2. Filter执行过程

  1. ApplicationDispatcher#doDispatch方法处理请求

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    private void doDispatch(ServletRequest request, ServletResponse response) throws ServletException, IOException {
        ...
        invoke(state.outerRequest, state.outerResponse, state);
    }

  2. ApplicationDispatcher#invoke调用方法创建FilterChain, 执行FilterChain的doFilter方法

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    private void invoke(ServletRequest request, ServletResponse response,
        State state) throws IOException, ServletException {
        ...
        ApplicationFilterChain filterChain = ApplicationFilterFactory.      createFilterChain(request, wrapper, servlet);
        try {
            // for includes/forwards
            if ((servlet != null) && (filterChain != null)) {
               filterChain.doFilter(request, response);
             }
            // Servlet Service Method is called by the FilterChain
        }
        ...
    }

  3. ApplicationFilterFactory#createFilterChain提取StandardContext中的Filter列表放入新创建的FilterChain中

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    public static ApplicationFilterChain createFilterChain(ServletRequest request, Wrapper wrapper, Servlet servlet) {
        ...
        filterChain = new ApplicationFilterChain();
        ...
        // Add the relevant path-mapped filters to this filter chain
        for (int i = 0; i < filterMaps.length; i++) {
            if (!matchDispatcher(filterMaps[i] ,dispatcher)) {
                continue;
            }
            if (!matchFiltersURL(filterMaps[i], requestPath))
                continue;
            ApplicationFilterConfig filterConfig = (ApplicationFilterConfig)
                context.findFilterConfig(filterMaps[i].getFilterName());
            if (filterConfig == null) {
                // FIXME - log configuration problem
                continue;
            }
            filterChain.addFilter(filterConfig);
        }
        ...
    }

  4. FilterChain的doFilter方法中获取第一个Filter并执行其doFilter方法

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    public void doFilter(ServletRequest request, ServletResponse response)
        throws IOException, ServletException {
        ...
        internalDoFilter(request,response);
        ...
    }

    private void internalDoFilter(ServletRequest request, ServletResponse response){
        ...
        filter.doFilter(request, response, this);
        ...
    }

应用

业务代码中应用的可行性探讨

参数校验上使用的可行性

通过拦截过滤器模式进行参数校验. 满足过滤器条件的继续执行, 不满足的直接返回.

使用注解关联调用类和过滤器