java中的Filter使用示例详解

public class AuthFilter implements Filter {
    @Override
    public void init(FilterConfig config) { /* 初始化资源 */ }
    @Override
    public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) 
        throws IOException, ServletException {
        // 请求处理逻辑（如权限检查）
        chain.doFilter(req, res); // 放行
        // 响应处理逻辑（如日志记录）
    }
    @Override
    public void destroy() { /* 释放资源 */ }
}

<filter>
    <filter-name>AuthFilter</filter-name>
    <filter-class>com.example.AuthFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>AuthFilter</filter-name>
    <url-pattern>/secure/*</url-pattern>
    <dispatcher>REQUEST</dispatcher> <!-- 拦截请求类型 -->
</filter-mapping>

@WebFilter(urlPatterns = {"/secure/*"}, initParams = {@WebInitParam(name = "param", value = "value")})
public class AuthFilter implements Filter { /* ... */ }

请求 → Filter1 → Filter2 → Servlet → Filter2 → Filter1 → 响应

public class EncodingFilter implements Filter {
    @Override
    public void doFilter(...) {
        request.setCharacterEncoding("UTF-8");
        response.setCharacterEncoding("UTF-8");
        chain.doFilter(request, response);
    }
}

public class LoginFilter implements Filter {
    @Override
    public void doFilter(...) {
        HttpSession session = ((HttpServletRequest) request).getSession();
        if (session.getAttribute("user") == null) {
            ((HttpServletResponse) response).sendRedirect("/login");
        } else {
            chain.doFilter(request, response);
        }
    }
}

public class LogFilter implements Filter {
    @Override
    public void doFilter(...) {
        long startTime = System.currentTimeMillis();
        chain.doFilter(request, response);
        long duration = System.currentTimeMillis() - startTime;
        System.out.println("请求耗时：" + duration + "ms");
    }
}