理解Servlet与Filter的关系与设计思路

朴灿烈づ我的快乐病毒、 2023-06-23 11:55 10阅读 0赞

## 什么是Servlet ##

对一个HTTP请求的正常的处理流程是：

1.  发送HTTP请求
2.  服务端的HTTP服务器收到请求
3.  调用业务逻辑
4.  返回HTTP响应

产生了下面3个问题：

1.  HTTP 服务器怎么知道要调用哪个业务逻辑，也就是 Java 类的哪个方法呢？
    
    HTTP服务器可以被设计成收到请求后，接续寻找该请求的处理逻辑，但是这样就会使得HTTP 服务器的代码跟业务逻辑耦合在一起。
2.  如果问题1交给HTTP服务器，如何解决HTTP 服务器的代码跟业务逻辑耦合在一起？
    
    根据职责单一原则，HTTP服务器的功能就应当设计成接收请求、发送响应，具体的处理逻辑可以交给一个第三方，由第三方去寻找处理逻辑所在的Java类。这个第三方就是Servlet容器。
3.  Servlet容器是否需要知道具体的业务逻辑？
    
    不需要，只需要给到基本的数据（`ServletRequest`、`ServletResponse`）给到业务逻辑处理单位即可，不用关注业务逻辑的具体实现。

所以Servlet被设计成了一个接口：

public interface Servlet {
        
        // Servlet容器在加载Servlet类的时候会调用
        void init(ServletConfig config) throws ServletException;
        // 在web.xml给改Servlet配置的参数
        ServletConfig getServletConfig();
        // 业务逻辑处理入口
        void service(ServletRequest req, ServletResponse res）throws ServletException, IOException;
        
        String getServletInfo();
        // 在卸载的时候会调用
        void destroy();
    }

其处理的流程：

![format_png][]

HTTP 服务器不直接调用业务类，而是把请求交给容器来处理，容器通过 Servlet 接口调用业务类。因此 Servlet 接口和 Servlet 容器的出现，达到了 HTTP 服务器与业务类解耦的目的。

**Servlet 接口和 Servlet 容器这一整套规范叫作 Servlet 规范**。Tomcat按照 Servlet 规范的要求实现了 Servlet 容器，同时它们也具有 HTTP 服务器的功能。

## 什么是Filter ##

过滤器。这个接口允许你对请求和响应做一些统一的定制化处理，比如你可以根据请求的频率来限制访问，或者根据国家地区的不同来修改响应内容。过滤器的工作原理是这样的：Web 应用部署完成后，Servlet 容器需要实例化 Filter 并把 Filter 链接成一个 FilterChain。当请求进来时，获取第一个 Filter 并调用 doFilter 方法，doFilter 方法负责调用这个 FilterChain 中的下一个 Filter。

它在Java中的变现为一个接口：

/**
 * A filter is an object that performs filtering tasks on either the request to
 * a resource (a servlet or static content), or on the response from a resource,
 * or both. 
 *
 * Filters perform filtering in the <code>doFilter</code> method. Every Filter
 * has access to a FilterConfig object from which it can obtain its
 * initialization parameters, a reference to the ServletContext which it can
 * use, for example, to load resources needed for filtering tasks.
 */
 public interface Filter {
 
 
 /**
 * Called by the web container to indicate to a filter that it is being
 * placed into service. The servlet container calls the init method exactly
 * once after instantiating the filter. The init method must complete
 * successfully before the filter is asked to do any filtering work.
 */
 public default void init(FilterConfig filterConfig) throws ServletException {
 }
 
 /**
 * The <code>doFilter</code> method of the Filter is called by the container
 * each time a request/response pair is passed through the chain due to a
 * client request for a resource at the end of the chain. The FilterChain
 * passed in to this method allows the Filter to pass on the request and
 * response to the next entity in the chain.
 * 
 * A typical implementation of this method would follow the following
 * pattern:- 
 * 1. Examine the request 
 * 2. Optionally wrap the request object with a custom implementation to
 * filter content or headers for input filtering 
 * 3. Optionally wrap the response object with a custom implementation to
 * filter content or headers for output filtering 
 * 4. a) Either invoke the next entity in the chain using
 * the FilterChain object (<code>chain.doFilter()</code>), 
 * 4. b) or not pass on the request/response pair to the
 * next entity in the filter chain to block the request processing 
 * 5. Directly set headers on the response after invocation of the next
 * entity in the filter chain.
 */
 public void doFilter(ServletRequest request, ServletResponse response,
 FilterChain chain) throws IOException, ServletException;
 /**
 * Called by the web container to indicate to a filter that it is being
 * taken out of service. This method is only called once all threads within
 * the filter's doFilter method have exited or after a timeout period has
 * passed. After the web container calls this method, it will not call the
 * doFilter method again on this instance of the filter.
 *
 * This method gives the filter 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 filter's current
 * state in memory.
 */
 public default void destroy() {
 }
 }

### 什么是FilterChain ###

/**
     * A FilterChain is an object provided by the servlet container to the developer
     * giving a view into the invocation chain of a filtered request for a resource.
     * Filters use the FilterChain to invoke the next filter in the chain, or if the
     * calling filter is the last filter in the chain, to invoke the resource at the
     * end of the chain.
     **/
    public interface FilterChain {
        
        /**
         * Causes the next filter in the chain to be invoked, or if the calling
         * filter is the last filter in the chain, causes the resource at the end of
         * the chain to be invoked.
         */
        // doFilter方法负责调用这个FilterChain中的下一个Filter
        public void doFilter(ServletRequest request, ServletResponse response)
                throws IOException, ServletException;
    }

`FilterChain`是一个接口，Tomcat里面有个一实现该接口的final类`ApplicationFilterChain.java`。

![format_png 1][]

其中含有一个`ApplicationFilterConfig.java`

private ApplicationFilterConfig[] filters = new ApplicationFilterConfig[0];
    /**
    	* The int which is used to maintain the current position in the filter chain.
    	*/
    private int pos = 0;
    /**
       * The int which gives the current number of filters in the chain.
       */
    private int n = 0;

实现了`FilterChain`的`doFilter(ServletRequest request, ServletResponse response)`方法，其实就是调用了该类中的`internalDoFilter(ServletRequest request, ServletResponse response)`。调用完毕后，pos++。如果下次使用该`FilterChain`的`doFilter()`方法，会调用下一个`Filter`（如果存在）

private void internalDoFilter(ServletRequest request,
 ServletResponse response) throws IOException, ServletException {
 
 
 // Call the next filter if there is one
 if (pos < n) {
 
 // 注意pos++
 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 {
 
 // 实际调用Filter的doFilter()方法
 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;
 }

其中有一个对`Filter`的包装类`ApplicationFilterConfig`，其中持有一个`Filter`对象，可通过`getFilter()`方法去获取。上述类之间的UML图关系如下：

![format_png 2][]

所以如果在`Filter`的`doFilte()`方法中，调用`filterChain.doFilter(servletRequest, servletResponse);`的话，会将流程交给`chainFilter`。因此也不难理解项目中`Filter`的使用如下：

@Order(1)
    @WebFilter(urlPatterns = "/*", filterName = "requestWrapperFilter")
    public class RequestWrapperFilter implements Filter {
        
    
        @Override
        public void init(FilterConfig filterConfig) throws ServletException {
         }
    
        @Override
        public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        
            // 添加traceId
            String traceId = ((HttpServletRequest) servletRequest).getHeader(Constants.REQUEST_HEADER_TRACEID);
            if (StringUtils.isBlank(traceId)) {
        
                traceId = StringUtils.replace(UUID.randomUUID().toString(), "-", "");
            }
            MDC.put(Constants.REQUEST_HEADER_TRACEID, traceId);
    
            try {
        
                // multipart/form-data请求不添加traceId
                if (ServletFileUpload.isMultipartContent((HttpServletRequest) servletRequest)) {
        
                    filterChain.doFilter(servletRequest, servletResponse);
                } else {
        
                    filterChain.doFilter(new RequestWrapper((HttpServletRequest) servletRequest), servletResponse);
                }
            } finally {
        
                // 清除 traceId
                MDC.remove(Constants.REQUEST_HEADER_TRACEID);
            }
        }
    
        @Override
        public void destroy() {
        }
    }

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