0%

# Filter, Interceptor, Aop, 啥, 啥, 啥? 这些都是啥?

Posted on 2020-08-22 Edited on 2020-08-23 In Java , Spring , Filter , Interceptor - AOP , Servlet , Interceptor , AOP Views: Views: Disqus:

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
    2
    3
    4
    5
    6
    7
    8
    9
    10
    11
    12
    13
    14
    15
    16
    17
    18
    19
    20
    21
    22
    23
    24
    25
    26
    27
    28
    29
    30
    31
    32
    33
    34
    35
    36
    37
    38
    39
    40
    41
    42
    43
    44
    45
    46
    47
    48
    49
    50
    51
    52
    53
    54
    55
    56
    57
    58
    59
    60
    61
    62
    63
    64
    65
    66
    67
    68
    69
    70
    71
    72
    73
    74
    75
    76
    77
    78
    79
    80
    81
    82
    83
    84
    85
    86
    87
    88
    89
    90
    91
    92
    93
    94
    95
    96
    97
    98
    99
    100
    101
    102
    103
    104
    105
    106
    107
    108
    109
    110
    111
    112
    113
    114
    115
    116
    117
    118
    119
    120
    121
    122
    123
    124
    125
    126
    127
    128
    129
    130
    131
    132
    133
    134
    135
    136
    137
    138
    139
    140
    141
    142
    143
    144
    145
    146
    147
    148
    149
    150
    151
    152
    153
    154
    155
    156
    157
    158
    159
    160
    161
    162
    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
    2
    3
    4
    5
    6
    7
    8
    9
    10
    11
    12
    13
    14
    15
    16
    17
    18
    19
    20
    21
    22
    23
    24
    25
    26
    27
    28
    29
    30
    31
    32
    33
    34
    35
    36
    37
    38
    39
    40
    41
    42
    43
    44
    45
    46
    47
    48
    49
    50
    51
    52
    53
    54
    55
    56
    57
    58
    59
    60
    61
    62
    63
    64
    65
    66
    67
    68
    69
    70
    71
    72
    73
    74
    75
    76
    77
    78
    79
    80
    81
    82
    83
    84
    85
    86
    87
    88
    89
    90
    91
    92
    93
    94
    95
    96
    97
    98
    99
    100
    101
    102
    103
    104
    105
    106
    107
    108
    109
    110
    111
    112
    113
    114
    115
    116
    117
    118
    119
    120
    121
    122
    // ---------------------------------------------------- FilterChain Methods

        /**
         * Invoke the next filter in this chain, passing the specified request
         * and response.  If there are no more filters in this chain, invoke
         * the <code>service()</code> method of the servlet itself.
         *
         * @param request The servlet request we are processing
         * @param response The servlet response we are creating
         *
         * @exception IOException if an input/output error occurs
         * @exception ServletException if a servlet exception occurs
         */
        @Override
        public void doFilter(ServletRequest request, ServletResponse response)
            throws IOException, ServletException {

            if( Globals.IS_SECURITY_ENABLED ) {
                final ServletRequest req = request;
                final ServletResponse res = response;
                try {
                    java.security.AccessController.doPrivileged(
                        new java.security.PrivilegedExceptionAction<Void>() {
                            @Override
                            public Void run()
                                throws ServletException, IOException {
                                internalDoFilter(req,res);
                                return null;
                            }
                        }
                    );
                } catch( PrivilegedActionException pe) {
                    Exception e = pe.getException();
                    if (e instanceof ServletException)
                        throw (ServletException) e;
                    else if (e instanceof IOException)
                        throw (IOException) e;
                    else if (e instanceof RuntimeException)
                        throw (RuntimeException) e;
                    else
                        throw new ServletException(e.getMessage(), e);
                }
            } else {
                internalDoFilter(request,response);
            }
        }
        private void internalDoFilter(ServletRequest request,
                                      ServletResponse response)
            throws IOException, ServletException {

            // Call the next filter if there is one
            if (pos < n) {
                ApplicationFilterConfig filterConfig = filters[pos++];
                try {
                    Filter filter = filterConfig.getFilter();

                    if (request.isAsyncSupported() && "false".equalsIgnoreCase(
                            filterConfig.getFilterDef().getAsyncSupported())) {
                        request.setAttribute(Globals.ASYNC_SUPPORTED_ATTR, Boolean.FALSE);
                    }
                    if( Globals.IS_SECURITY_ENABLED ) {
                        final ServletRequest req = request;
                        final ServletResponse res = response;
                        Principal principal =
                            ((HttpServletRequest) req).getUserPrincipal();

                        Object[] args = new Object[]{req, res, this};
                        SecurityUtil.doAsPrivilege ("doFilter", filter, classType, args, principal);
                    } else {
                        filter.doFilter(request, response, this);
                    }
                } catch (IOException | ServletException | RuntimeException e) {
                    throw e;
                } catch (Throwable e) {
                    e = ExceptionUtils.unwrapInvocationTargetException(e);
                    ExceptionUtils.handleThrowable(e);
                    throw new ServletException(sm.getString("filterChain.filter"), e);
                }
                return;
            }

            // We fell off the end of the chain -- call the servlet instance
            try {
                if (ApplicationDispatcher.WRAP_SAME_OBJECT) {
                    lastServicedRequest.set(request);
                    lastServicedResponse.set(response);
                }

                if (request.isAsyncSupported() && !servletSupportsAsync) {
                    request.setAttribute(Globals.ASYNC_SUPPORTED_ATTR,
                            Boolean.FALSE);
                }
                // Use potentially wrapped request from this point
                if ((request instanceof HttpServletRequest) &&
                        (response instanceof HttpServletResponse) &&
                        Globals.IS_SECURITY_ENABLED ) {
                    final ServletRequest req = request;
                    final ServletResponse res = response;
                    Principal principal =
                        ((HttpServletRequest) req).getUserPrincipal();
                    Object[] args = new Object[]{req, res};
                    SecurityUtil.doAsPrivilege("service",
                                               servlet,
                                               classTypeUsedInService,
                                               args,
                                               principal);
                } else {
                    servlet.service(request, response);
                }
            } catch (IOException | ServletException | RuntimeException e) {
                throw e;
            } catch (Throwable e) {
                e = ExceptionUtils.unwrapInvocationTargetException(e);
                ExceptionUtils.handleThrowable(e);
                throw new ServletException(sm.getString("filterChain.servlet"), e);
            } finally {
                if (ApplicationDispatcher.WRAP_SAME_OBJECT) {
                    lastServicedRequest.set(null);
                    lastServicedResponse.set(null);
                }
            }
        }

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
    2
    3
    4
    5
    6
    7
    8
    9
    10
    11
    12
    13
    14
    15
    16
    17
    18
    19
    20
    21
    22
    23
    24
    25
    26
    27
    28
    29
    30
    31
    32
    33
    34
    35
    36
    37
    38
    39
    40
    41
    42
    43
    44
    45
    46
    47
    48
    49
    50
    51
    52
    53
    54
    55
    56
    57
    58
    59
    60
    61
    62
    63
    64
    65
    66
    67
    68
    69
    70
    71
    protected 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 filter

    1
    2
    3
    4
    5
    6
    7
    8
    9
    10
    11
    12
    13
    14
    15
    16
    17
    18
    19
    20
    21
    public class DemoFilter extends HttpServlet implements Filter {
        @Override
        public void init(FilterConfig filterConfig) throws ServletException {
            System.out.println("==>DemoFilter启动");
        }

        @Override
        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");
        }

        @Override
        public void destroy() {

        }
    }

因为用的springboot，所以就不写 web.xml 了，写个配置类

    1
    2
    3
    4
    5
    6
    7
    8
    9
    10
    11
    12
    13
    14
    15
    16
    17
    18
    19
    @Configuration
    public class FilterConfiguration {
        @Bean
        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
    2
    3
    4
    5
    6
    7
    8
    9
    10
    11
    12
    13
    14
    15
    16
    17
    18
    19
    20
    21
    22
    23
    24
    25
    26
    27
    28
    29
    30
    31
    32
    33
    34
    35
    36
    37
    38
    39
    40
    public class DemoInterceptor extends HandlerInterceptorAdapter {
        @Override
        public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
            System.out.println("preHandle test");
            return true;
        }

        @Override
        public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception {
            System.out.println("postHandle test");
        }
    }
    @Aspect
    @Component
    public class DemoAspect {

        @Pointcut("execution( public * com.nicksxs.springbootdemo.demo.DemoController.*())")
        public void point() {

        }

        @Before("point()")
        public void doBefore(){
            System.out.println("==doBefore==");
        }

        @After("point()")
        public void doAfter(){
            System.out.println("==doAfter==");
        }
    }
    @RestController
    public class DemoController {

        @RequestMapping("/hello")
        @ResponseBody
        public String hello() {
            return "hello world";
        }
    }

好了，请求一下，看看 stdout，
搞定完事儿~

Related Posts

- 聊聊 Dubbo 的 SPI 续之自适应拓展
- Apollo 的 value 注解是怎么自动更新的
- Comparator使用小记
- Maven实用小技巧
- mybatis 的 $ 和 # 是有啥区别

请我喝杯咖啡

WeChat Pay

Alipay

- Post author: Nicksxs
- Post link: https://nicksxs.me/2020/08/22/Filter-Intercepter-Aop-啥-啥-啥-这些都是啥/
- Copyright Notice: All articles in this blog are licensed under BY-NC-SA unless stating additionally.