spring中自定义过滤器

OncePerRequestFilter

 而Spring的OncePerRequestFilter类实际上是一个实现了Filter接口的抽象类。spring对Filter进行了一些封装处理

OncePerRequestFilter，顾名思义，它能够确保在一次请求中只通过一次filter，而需要重复的执行。大家常识上都认为，一次请求本来就只filter一次，为什么还要由此特别限定呢，往往我们的常识和实际的实现并不真的一样，经过一番资料的查阅，此方法是为了兼容不同的web container，也就是说并不是所有的container都入我们期望的只过滤一次，servlet版本不同，执行过程也不同，因此，为了兼容各种不同运行环境和版本，默认filter继承OncePerRequestFilter是一个比较稳妥的选择

源码说明：

public abstract class OncePerRequestFilter extends GenericFilterBean {
        //一个标记，后面会用到
	public static final String ALREADY_FILTERED_SUFFIX = ".FILTERED";
 
	@Override
	public final void doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain)
			throws ServletException, IOException {
 
		if (!(request instanceof HttpServletRequest) || !(response instanceof HttpServletResponse)) {
			throw new ServletException("OncePerRequestFilter just supports HTTP requests");
		}
		HttpServletRequest httpRequest = (HttpServletRequest) request;
		HttpServletResponse httpResponse = (HttpServletResponse) response;
		//这里获取一个名称，该名称后面会被用于放到request当作key
		String alreadyFilteredAttributeName = getAlreadyFilteredAttributeName();
		//检测当前请求是否已经拥有了该标记，如果拥有该标记则代表该过滤器执行过了（后面注释有说明）
		boolean hasAlreadyFilteredAttribute = request.getAttribute(alreadyFilteredAttributeName) != null;
 
		if (skipDispatch(httpRequest) || shouldNotFilter(httpRequest)) {
 
			// Proceed without invoking this filter...
			filterChain.doFilter(request, response);
		}
		//如果此过滤器已经被执行过则执行如下的逻辑
		else if (hasAlreadyFilteredAttribute) {
 
			if (DispatcherType.ERROR.equals(request.getDispatcherType())) {
				doFilterNestedErrorDispatch(httpRequest, httpResponse, filterChain);
				return;
			}
 
			// Proceed without invoking this filter...
			filterChain.doFilter(request, response);
		}
		//走到这里说明该过滤器没有被执行过
		else {
			// Do invoke this filter...
			// 在当前请求里面设置一个标记，key就是前面拼接的那个变量，value是true，这个标记如果在request存在则在前面会被检测到并改变hasAlreadyFilteredAttribute的值
			request.setAttribute(alreadyFilteredAttributeName, Boolean.TRUE);
			try {
                                // 这个方法是一个抽象方法需要子类去实现具体的过滤逻辑
				doFilterInternal(httpRequest, httpResponse, filterChain);
			}
			finally {
				// Remove the "already filtered" request attribute for this request.
				// 执行完毕之后移除该标记
				request.removeAttribute(alreadyFilteredAttributeName);
			}
		}
	}
    //其余代码略
}

自定义代码案例：

@Component
@Order(-1)
@Slf4j
public class SessionContextFilter extends OncePerRequestFilter implements Ordered {
 
    /**
     * 请求拦截器
     */
    @Override
    protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain) throws ServletException, IOException {
 
        Thread thread = Thread.currentThread();
        Long userId = request.getHeader("userId") == null ? 0L : Long.valueOf(request.getHeader("userId"));
        String userName = request.getHeader("userName") == null ? "admin" : request.getHeader("userName");
        ContextHolder.setContext(new OperationVO(userId,userName));
        try {
            OperationVO operationVO = ContextHolder.getContext();
            // 除这行代码外，其他均为自定义业务代码
            filterChain.doFilter(request,response);
        } finally {
            ContextHolder.clearContext();
            OperationVO operationVO = ContextHolder.getContext();
        }
    }
 
    @Override
    public int getOrder() {
        return 0;
    }
}

filterChain.doFilter

实现filterchain的dofilter方法采用 责任链设计模式，把自身接收到的请求request对象和response对象和自身对象即filterchain

作为下一个过滤器的dofilter的三个形参传递过去，这样才能使得过滤器传递下去，当然这个方法中还存在一些判断if等机制

用来判断现在的这个过滤器是不是最后一个，是的话就可以把请求和响应对象传递给浏览器请求的页面

第一个疑问是该过滤器链里面的过滤器源于哪里？

答案是该类里面包含了一个ApplicationFilterConfig对象，而该对象则是个filter容器

当web容器启动是ApplicationFilterConfig自动实例化，它会从该web工程的web.xml文件中读取配置的filter信息，然后装进该容器
 下个疑问是它如何执行该过滤器容器里面的filter呢?

答案是通过pos它来标识当前ApplicationFilterChain（当前过滤器链）执行到哪个过滤器

 ApplicationFilterChain采用责任链设计模式达到对不同过滤器的执行
首先ApplicationFilterChain 会调用它重写FilterChain的doFilter方法，然后doFilter里面会调用
 internalDoFilter(request,response)方法;该方法使过滤器容器拿到每个过滤器，然后调用它们重写Filter接口里面的dofilter方法

以下是ApplicationFilterChain 里面重写FilterChain里面的doFilter方法的描述

/** 
 * Invoke the next filter in this chain, passing the specified request 
 * and response.  If there are no more filters in this chain, invoke 
 * the service() 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) 

以下是internalDoFilter的部分代码

 // Call the next filter if there is one  
if (pos < n) {  
	//先拿到下个过滤器，将指针向下移动一位  
	ApplicationFilterConfig filterConfig = filters[pos++];  
	Filter filter = null;  
	try {  
		//获取当前指向的filter实例  
		filter = filterConfig.getFilter();  
		support.fireInstanceEvent(InstanceEvent.BEFORE_FILTER_EVENT,  
								  filter, request, response);  
		 
		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调用doFilter(request, response, this)方法  
			//ApplicationFilterChain里面的filter都实现了filter
			filter.doFilter(request, response, this);  
		}
	}
}		 

以下是Filter接口doFilter定义如下
 public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain)

过滤器链里面的filter在调用dofilter完成后，会继续调用chain.doFilter(request,response)方法，而这个chain其实就是applicationfilterchain,所以调用过程又回到了上面调用dofilter和调用internalDoFilter方法，这样执行直到里面的过滤器全部执行

当filte都调用完成后，它就会初始化相应的servlet，（例如jsp资源，默认它会开启一个 JspServlet对象）

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