Servlet与设计模式

news2024/11/24 0:57:13

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 {

    @Override
    protected 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 {

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("拦截器初始化");
    }

    @Override
    public 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();
        }
    }

    @Override
    public void destroy() {
        System.out.println("拦截器销毁");
    }
}

public class CustomResponseWrapper extends HttpServletResponseWrapper {

    private CustomPrintWriter pw;

    public CustomResponseWrapper(HttpServletResponse response) {
        super(response);
    }

    @Override
    public 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);
    }

    @Override
    public 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