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This article introduces the knowledge of "the difference and usage of filter Filter and interceptor HandlerIntercepter". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

Recently, I feel dizzy when I see a colleague using both interceptors and filters for login verification in the code. It seems that both of them can meet business requirements, but which approach is better, and what's the difference between the two?

Principle 1. Filter:

Depending on the servlet container, you can filter almost all requests based on function callback in implementation. A filter instance can only be called when the container is initialized. It is started with the startup of your web application. It can be initialized only once, and you can intercept related requests later. Only when your web application is stopped or redeployed can it be destroyed.

The purpose of using the filter is to do some filtering operations to get the data we want, such as: modify the character encoding in the filter; modify some parameters of HttpServletRequest in the filter, including: filtering vulgar text, dangerous characters and so on.

two。 Interceptor:

Rely on the web framework, which in SpringMVC is dependent on the SpringMVC framework. The reflection mechanism based on Java is an application of aspect-oriented programming (AOP).

Because the interceptor is based on the invocation of the web framework, you can use Spring's dependency injection (DI) to do some business operations without modifying handler's own implementation. However, the disadvantage is that it can only intercept controller requests, but there is no way to intercept other requests such as direct access to static resources.

3. The difference between filter and interceptor:

The ① interceptor is based on java's reflection mechanism, while the filter is based on function callbacks.

The ② interceptor does not depend on the servlet container and the filter depends on the servlet container.

③ interceptors can only work on action requests, while filters can work on almost all requests.

④ interceptors can access objects in the action context and value stack, but filters cannot.

⑤ in the life cycle of action, the interceptor can be called many times (here, it means that the three methods in the interceptor can be called at different times in an action, fine-grained control), while the filter can only filter one request at a time, and cannot work in different life cycles.

It is important that the ⑥ interceptor can get the individual bean in the IOC container, but not the filter, by injecting a service into the interceptor to invoke the business logic.

Second, code description 1. Filter: three methods are defined in the javax.servlet.Filter interface: void init (FilterConfig filterConfig) is used to initialize the filter, void destroy () is used to complete the recovery of some resources before the filter is destroyed, void doFilter (ServletRequest request, ServletResponse response,FilterChain chain) is used to implement the filtering function. This method adds additional processing public class FilterUtil implements Filter {@ SuppressWarnings ("unused") private FilterConfig filterConfig to each request. @ Override public void init (FilterConfig filterConfig) throws ServletException {this.filterConfig = filterConfig; System.out.println ("filter Filter initialization");} @ Override public void doFilter (ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {if (! (request instanceof HttpServletRequest) | |! (response instanceof HttpServletResponse) {throw new ServletException ("FilterUtil just supports HTTP requests");} HttpServletRequest httpRequest = (HttpServletRequest) request HttpServletResponse httpResponse = (HttpServletResponse) response; httpRequest.setCharacterEncoding (this.filterConfig.getInitParameter ("encoding")); httpResponse.setCharacterEncoding (this.filterConfig.getInitParameter ("encoding")); chain.doFilter (httpRequest, httpResponse);} @ Override public void destroy () {System.out.println ("filter Filter destruction");}}

Web.xml configuration:

EncodingFilter com.cn.util.FilterUtil true encoding UTF-8 encodingFilter / *

The call to the method chain.doFilter (request, response) serves as a watershed. In fact, the doService () method of Servlet is called in the method chain.doFilter (request, response).

two。 Interceptor: preHandle () this method is executed one step before the filter's chain.doFilter (request, response) method. After the postHandle () method and before the return ModelAndView, you can manipulate the ModelAndView content of the Controller. The afterCompletion () method is executed before the filter is returned to the front end, that is, in [chain.doFilter (request, response)] [System.out.println ("after...")] In between.

Note: redirect: after the execution of the current page code, jump to the specified page to execute other code. Forward: after the code on this page executes the forwarding statement, it jumps to the specified page to execute other code, and then returns the code after the forwarding statement is executed.

/ * * user authentication interceptor * / public class LoginInterceptor implements HandlerInterceptor {@ Value ("${TT_TOKEN_KEY}") private String TT_TOKEN_KEY; @ Value ("${SSO_URL}") private String SSO_URL; @ Autowired private UserLoginService loginservice / / execute @ Override public boolean preHandle (HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {/ / user authentication before entering the target method to verify / / 1. Take token / / 1 in cookie. Get the user's token String token = CookieUtils.getCookieValue (request, TT_TOKEN_KEY) from cookie; / / 2. To determine whether token exists, if (StringUtils.isEmpty (token)) {/ / 3. If it does not exist, you are not logged in-"redirect to the login page / / request.getRequestURL () .toString (): the visited URL localhost:8092/order/order-cart.html response.sendRedirect (SSO_URL+" / page/login?redirect= "+ request.getRequestURL () .toString ()); return false } / / 4. If token exists, the service that calls SSO queries the user's information (to see if the user has expired) TAotaoresult result = loginservice.getUserByToken (token); if (result.getStatus ()! = 200) {/ / 5. User has expired-"redirect to login page response.sendRedirect (SSO_URL+" / page/login?redirect= "+ request.getRequestURL (). ToString ()); return false;} / / 6. User does not expire (indicates login)-"release / / set user information to request, the request of the target method can obtain user information request.setAttribute (" USER_INFO ", result.getData ()); return true } / / after entering the target method, perform some settings of the / / shared variable before returning modelandview. @ Override public void postHandle (HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception {/ / TODO Auto-generated method stub} / / after returning modelandview, before rendering to the page / / exception handling Clean up work @ Override public void afterCompletion (HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {/ / TODO Auto-generated method stub} 3. Execution order

The mechanism of SpringMVC is that the same Servlet distributes requests to different Controller, but this step is actually performed in the service () method of Servlet. So the order of execution of the filter, interceptor, service () method, and dispatche'r () method is as follows

IV. Suggestions for use

Interceptors are more suitable for fine-grained Handler control, especially some common processing code, authorization checking, etc., and filters are more suitable for processing request content and view content, such as multipart forms, GZIP compression and so on. Looking back at the previous confusion, the best solution is to use an interceptor for login verification.

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