java - Servlet 与过滤器

Question

Servlet 和 Filter 有什么区别？您建议使用什么来授权页面？

Answer 1

当您想根据特定条件过滤和/或修改请求时，请使用 Filter。当您想要控制、预处理和/或后处理请求时，请使用 Servlet。

Java EE tutorial提到以下有关过滤器的内容:

A filter is an object that can transform the header and content (or both) of a request or response. Filters differ from web components in that filters usually do not themselves create a response. Instead, a filter provides functionality that can be “attached” to any kind of web resource. Consequently, a filter should not have any dependencies on a web resource for which it is acting as a filter; this way it can be composed with more than one type of web resource.

The main tasks that a filter can perform are as follows:

• Query the request and act accordingly.
• Block the request-and-response pair from passing any further.
• Modify the request headers and data. You do this by providing a customized version of the request.
• Modify the response headers and data. You do this by providing a customized version of the response.
• Interact with external resources.

对于授权，Filter 是最合适的。以下是过滤器如何检查登录用户请求的基本启动示例:

public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws ServletException, IOException {
    if (((HttpServletRequest) request).getSession().getAttribute("user") == null) {
        // User is not logged in. Redirect to login page.
        ((HttpServletResponse) response).sendRedirect("login");
    } else {
        // User is logged in. Just continue with request.
        chain.doFilter(request, response);
    }
}