详解Tomcat常用的过滤器

Tomcat中的过滤器可以在请求被目标servlet或JSP之前或之后执行某些操作，如修改请求、响应或扩展请求所需的功能。在Web开发中，常用的过滤器有字符编码过滤器、登录验证过滤器、权限控制过滤器等。下面将详细介绍常用的Tomcat过滤器。

字符编码过滤器

字符编码过滤器可以设置HttpServletRequest和HttpServletResponse的编码格式，避免乱码的产生。示例代码如下：

public class EncodingFilter implements Filter {
    private String encoding;

    public void init(FilterConfig config) throws ServletException {
        encoding = config.getInitParameter("encoding");
    }

    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
                throws IOException, ServletException {
        HttpServletRequest req = (HttpServletRequest) request;
        HttpServletResponse resp = (HttpServletResponse) response;
        req.setCharacterEncoding(encoding);
        resp.setCharacterEncoding(encoding);
        chain.doFilter(req, resp);
    }

    public void destroy() {}
}

在web.xml中配置过滤器：

<filter>
    <filter-name>EncodingFilter</filter-name>
    <filter-class>com.example.EncodingFilter</filter-class>
    <init-param>
        <param-name>encoding</param-name>
        <param-value>UTF-8</param-value>
    </init-param>
</filter>
<filter-mapping>
    <filter-name>EncodingFilter</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>

登录验证过滤器

登陆验证过滤器通过检查用户是否已经登录，如果没有则重定向到登录页面。示例代码如下：

public class LoginFilter implements Filter {
    private String loginPage;

    public void init(FilterConfig config) throws ServletException {
        loginPage = config.getInitParameter("login-page");
    }

    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {
        HttpServletRequest req = (HttpServletRequest) request;
        HttpServletResponse resp = (HttpServletResponse) response;
        HttpSession session = req.getSession();
        if (session.getAttribute("currentUser") == null) {
            resp.sendRedirect(req.getContextPath() + loginPage);
        } else {
            chain.doFilter(req, resp);
        }
    }

    public void destroy() {}
}

在web.xml中配置过滤器：

<filter>
    <filter-name>LoginFilter</filter-name>
    <filter-class>com.example.LoginFilter</filter-class>
    <init-param>
        <param-name>login-page</param-name>
        <param-value>/login.jsp</param-value>
    </init-param>
</filter>
<filter-mapping>
    <filter-name>LoginFilter</filter-name>
    <url-pattern>/admin/*</url-pattern>
</filter-mapping>

此配置将LoginFilter映射到/webapp/admin/*路径，只有当用户在登录后才可以访问该界面。

总结

以上是常用的Tomcat过滤器的简单介绍。使用过滤器可以很好地保证Web应用的安全性和可靠性，提高Web应用的质量和效率。