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Servlet与设计模式

news 2025/7/7 20:40:14

1 过滤器和包装器

过滤器可以拦截请求及控制响应，而servlet对此毫无感知。过滤器有如下作用：

1）请求过滤器：完成安全检查、重新格式化请求首部或体、建立请求审计日志。

2）响应过滤器：压缩响应流、追加或修改响应流、创建一个完全不同的响应。

1.1 声明和确定过滤器顺序

当多个过滤器映射到一个给定资源时，容器会使用先URL模式后servlet-name的规则。

<filter-mapping>

<filter-name>filter3</filter-name>

<servlet-name>helloServlet</servlet-name>

</filter-mapping>

<filter-mapping>

<filter-name>filter1</filter-name>

<url-pattern>/*</url-pattern>

</filter-mapping>

<filter-mapping>

<filter-name>filter2</filter-name>

<url-pattern>*.action</url-pattern>

</filter-mapping>

过滤器的顺序为： filter1、filter2、filter3

1.2 过滤器实战

需求：1）项目只接受/hello 请求的GET方法，对于其他请求则返回“404没有找到对应资源”。2）对所有响应，返回内容结尾都追加“filter实战”。

public class HelloServlet extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {PrintWriter writer = resp.getWriter();writer.print("你好 Servlet");writer.close();}}public class RequestFilter implements Filter {@Overridepublic void init(FilterConfig filterConfig) throws ServletException {System.out.println("拦截器初始化");}@Overridepublic void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {HttpServletRequest request = (HttpServletRequest) servletRequest;String method = request.getMethod();String servletPath = request.getServletPath();HttpServletResponse response = (HttpServletResponse) servletResponse;CustomResponseWrapper responseWrapper = new CustomResponseWrapper(response);responseWrapper.setContentType("text/html;charset=UTF-8");if ("GET".equalsIgnoreCase(method) && "/hello".equalsIgnoreCase(servletPath)) {filterChain.doFilter(request,responseWrapper);} else {PrintWriter writer = responseWrapper.getWriter();writer.print("404没有找到对应资源");writer.close();}}@Overridepublic void destroy() {System.out.println("拦截器销毁");}
}public class CustomResponseWrapper extends HttpServletResponseWrapper {private CustomPrintWriter pw;public CustomResponseWrapper(HttpServletResponse response) {super(response);}@Overridepublic PrintWriter getWriter() throws IOException {if (pw == null) pw = new CustomPrintWriter(super.getWriter());return pw;}
}public class CustomPrintWriter extends PrintWriter {public CustomPrintWriter(Writer out) {super(out);}@Overridepublic void print(String s) {super.print(s + "——filter实战");}
}

<web-app xmlns="http://java.sun.com/xml/ns/j2ee"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"version="2.4"><servlet><servlet-name>helloServlet</servlet-name><servlet-class>HelloServlet</servlet-class></servlet><servlet-mapping><servlet-name>helloServlet</servlet-name><url-pattern>/hello</url-pattern></servlet-mapping><filter><filter-name>reqFilter</filter-name><filter-class>RequestFilter</filter-class></filter><filter-mapping><filter-name>reqFilter</filter-name><url-pattern>/*</url-pattern></filter-mapping>
</web-app>

web.xml