Filter, Interceptor, Aop, 啥, 啥, 啥? 这些都是啥?
本来是想取个像现在那些公众号转了又转的文章标题，”面试官再问你xxxxx，就把这篇文章甩给他看”这种标题，但是觉得实在太 low 了，还是用一部我比较喜欢的电影里的一句台词，《人在囧途》里王宝强对着那张老板给他的欠条，看不懂字时候说的那句，这些都是些啥（第四声）
当我刚开始面 Java 的时候，其实我真的没注意这方面的东西，实话说就是不知道这些是啥，开发中用过 Interceptor和 Aop，了解 aop 的实现原理，但是不知道 Java web 中的 Filter 是怎么回事，知道 dubbo 的 filter，就这样，所以被问到了的确是回答不出来，可能就觉得这个渣渣，这么简单的都不会，所以还是花点时间来看看这个是个啥，为了避免我口吐芬芳，还是耐下性子来简单说下这几个东西
首先是 servlet，怎么去解释这个呢，因为之前是 PHPer，所以比较喜欢用它来举例子，在普通的 PHP 的 web 应用中一般有几部分组成，接受 HTTP 请求的是前置的 nginx 或者 apache，但是这俩玩意都是只能处理静态的请求，远古时代 PHP 和 HTML 混编是通过 apache 的 php module，跟后来 nginx 使用 php-fpm 其实道理类似，就是把请求中需要 PHP 处理的转发给 PHP，在 Java 中呢，是有个比较牛叉的叫 Tomcat 的，它可以把请求转成 servlet，而 servlet 其实就是一种实现了特定接口的 Java 代码，1
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package javax.servlet;
import java.io.IOException;
/**
* Defines methods that all servlets must implement.
*
* <p>
* A servlet is a small Java program that runs within a Web server. Servlets
* receive and respond to requests from Web clients, usually across HTTP, the
* HyperText Transfer Protocol.
*
* <p>
* To implement this interface, you can write a generic servlet that extends
* <code>javax.servlet.GenericServlet</code> or an HTTP servlet that extends
* <code>javax.servlet.http.HttpServlet</code>.
*
* <p>
* This interface defines methods to initialize a servlet, to service requests,
* and to remove a servlet from the server. These are known as life-cycle
* methods and are called in the following sequence:
* <ol>
* <li>The servlet is constructed, then initialized with the <code>init</code>
* method.
* <li>Any calls from clients to the <code>service</code> method are handled.
* <li>The servlet is taken out of service, then destroyed with the
* <code>destroy</code> method, then garbage collected and finalized.
* </ol>
*
* <p>
* In addition to the life-cycle methods, this interface provides the
* <code>getServletConfig</code> method, which the servlet can use to get any
* startup information, and the <code>getServletInfo</code> method, which allows
* the servlet to return basic information about itself, such as author,
* version, and copyright.
*
* @see GenericServlet
* @see javax.servlet.http.HttpServlet
*/
public interface Servlet {
/**
* Called by the servlet container to indicate to a servlet that the servlet
* is being placed into service.
*
* <p>
* The servlet container calls the <code>init</code> method exactly once
* after instantiating the servlet. The <code>init</code> method must
* complete successfully before the servlet can receive any requests.
*
* <p>
* The servlet container cannot place the servlet into service if the
* <code>init</code> method
* <ol>
* <li>Throws a <code>ServletException</code>
* <li>Does not return within a time period defined by the Web server
* </ol>
*
*
* @param config
* a <code>ServletConfig</code> object containing the servlet's
* configuration and initialization parameters
*
* @exception ServletException
* if an exception has occurred that interferes with the
* servlet's normal operation
*
* @see UnavailableException
* @see #getServletConfig
*/
public void init(ServletConfig config) throws ServletException;
/**
*
* Returns a {@link ServletConfig} object, which contains initialization and
* startup parameters for this servlet. The <code>ServletConfig</code>
* object returned is the one passed to the <code>init</code> method.
*
* <p>
* Implementations of this interface are responsible for storing the
* <code>ServletConfig</code> object so that this method can return it. The
* {@link GenericServlet} class, which implements this interface, already
* does this.
*
* @return the <code>ServletConfig</code> object that initializes this
* servlet
*
* @see #init
*/
public ServletConfig getServletConfig();
/**
* Called by the servlet container to allow the servlet to respond to a
* request.
*
* <p>
* This method is only called after the servlet's <code>init()</code> method
* has completed successfully.
*
* <p>
* The status code of the response always should be set for a servlet that
* throws or sends an error.
*
*
* <p>
* Servlets typically run inside multithreaded servlet containers that can
* handle multiple requests concurrently. Developers must be aware to
* synchronize access to any shared resources such as files, network
* connections, and as well as the servlet's class and instance variables.
* More information on multithreaded programming in Java is available in <a
* href
* ="http://java.sun.com/Series/Tutorial/java/threads/multithreaded.html">
* the Java tutorial on multi-threaded programming</a>.
*
*
* @param req
* the <code>ServletRequest</code> object that contains the
* client's request
*
* @param res
* the <code>ServletResponse</code> object that contains the
* servlet's response
*
* @exception ServletException
* if an exception occurs that interferes with the servlet's
* normal operation
*
* @exception IOException
* if an input or output exception occurs
*/
public void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException;
/**
* Returns information about the servlet, such as author, version, and
* copyright.
*
* <p>
* The string that this method returns should be plain text and not markup
* of any kind (such as HTML, XML, etc.).
*
* @return a <code>String</code> containing servlet information
*/
public String getServletInfo();
/**
* Called by the servlet container to indicate to a servlet that the servlet
* is being taken out of service. This method is only called once all
* threads within the servlet's <code>service</code> method have exited or
* after a timeout period has passed. After the servlet container calls this
* method, it will not call the <code>service</code> method again on this
* servlet.
*
* <p>
* This method gives the servlet an opportunity to clean up any resources
* that are being held (for example, memory, file handles, threads) and make
* sure that any persistent state is synchronized with the servlet's current
* state in memory.
*/
public void destroy();
}
重点看 servlet 的 service方法，就是接受请求，处理完了给响应，不说细节，不然光 Tomcat 的能说半年，所以呢再进一步去理解，其实就能知道，就是一个先后的问题，盗个图
filter 跟后两者最大的不一样其实是一个基于 servlet，在非常外层做的处理，然后是 interceptor 的 prehandle 跟 posthandle，接着才是我们常规的 aop，就这么点事情，做个小试验吧(还是先补段代码吧)
Filter
1 | // ---------------------------------------------------- FilterChain Methods |
注意看这一行filter.doFilter(request, response, this);
是不是看懂了，就是个 filter 链，但是这个代码在哪呢，org.apache.catalina.core.ApplicationFilterChain#doFilter
然后是interceptor，1
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71protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
boolean multipartRequestParsed = false;
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
try {
try {
ModelAndView mv = null;
Object dispatchException = null;
try {
processedRequest = this.checkMultipart(request);
multipartRequestParsed = processedRequest != request;
mappedHandler = this.getHandler(processedRequest);
if (mappedHandler == null) {
this.noHandlerFound(processedRequest, response);
return;
}
HandlerAdapter ha = this.getHandlerAdapter(mappedHandler.getHandler());
String method = request.getMethod();
boolean isGet = "GET".equals(method);
if (isGet || "HEAD".equals(method)) {
long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
if ((new ServletWebRequest(request, response)).checkNotModified(lastModified) && isGet) {
return;
}
}
/**
* 看这里看这里‼️
*/
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
this.applyDefaultViewName(processedRequest, mv);
/**
* 再看这里看这里‼️
*/
mappedHandler.applyPostHandle(processedRequest, response, mv);
} catch (Exception var20) {
dispatchException = var20;
} catch (Throwable var21) {
dispatchException = new NestedServletException("Handler dispatch failed", var21);
}
this.processDispatchResult(processedRequest, response, mappedHandler, mv, (Exception)dispatchException);
} catch (Exception var22) {
this.triggerAfterCompletion(processedRequest, response, mappedHandler, var22);
} catch (Throwable var23) {
this.triggerAfterCompletion(processedRequest, response, mappedHandler, new NestedServletException("Handler processing failed", var23));
}
} finally {
if (asyncManager.isConcurrentHandlingStarted()) {
if (mappedHandler != null) {
mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
}
} else if (multipartRequestParsed) {
this.cleanupMultipart(processedRequest);
}
}
}
代码在哪呢，org.springframework.web.servlet.DispatcherServlet#doDispatch，然后才是我们自己写的 aop，是不是差不多明白了，嗯，接下来是例子
写个 filter1
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21public class DemoFilter extends HttpServlet implements Filter {
public void init(FilterConfig filterConfig) throws ServletException {
System.out.println("==>DemoFilter启动");
}
public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
// 将请求转换成HttpServletRequest 请求
HttpServletRequest req = (HttpServletRequest) servletRequest;
HttpServletResponse resp = (HttpServletResponse) servletResponse;
System.out.println("before filter");
filterChain.doFilter(req, resp);
System.out.println("after filter");
}
public void destroy() {
}
}
因为用的springboot，所以就不写 web.xml 了，写个配置类1
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public class FilterConfiguration {
public FilterRegistrationBean filterDemo4Registration() {
FilterRegistrationBean registration = new FilterRegistrationBean();
//注入过滤器
registration.setFilter(new DemoFilter());
//拦截规则
registration.addUrlPatterns("/*");
//过滤器名称
registration.setName("DemoFilter");
//是否自动注册 false 取消Filter的自动注册
registration.setEnabled(true);
//过滤器顺序
registration.setOrder(1);
return registration;
}
}
然后再来个 interceptor 和 aop,以及一个简单的请求处理1
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40public class DemoInterceptor extends HandlerInterceptorAdapter {
public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
System.out.println("preHandle test");
return true;
}
public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception {
System.out.println("postHandle test");
}
}
public class DemoAspect {
public void point() {
}
public void doBefore(){
System.out.println("==doBefore==");
}
public void doAfter(){
System.out.println("==doAfter==");
}
}
public class DemoController {
public String hello() {
return "hello world";
}
}
好了，请求一下，看看 stdout，
搞定完事儿~
