全链路追踪关键技术-HTTP协议Header添加

蔚落 2023-01-17 03:22 83阅读 0赞

Filter 中能够获取到的 [javax.servlet.http.HttpServletRequest][] 暴露的接口，除了从其父接口 [javax.servlet.ServletRequest ][javax.servlet.ServletRequest]接口继承的`setCharacterEncoding`方法和`setAttribute`方法之外，没有`setter`方法。因此 HttpServletRequest类型对象是只读的。

我们只能使用装饰器模式，扩展 HttpServletRequest 的一个实现类 [javax.servlet.http.HttpServletRequestWrapper][]，并使用装饰的对象`MutableHttpServletRequest`代替原始对象，以下为示例代码：

import java.util.Collections;
    import java.util.Enumeration;
    import java.util.HashMap;
    import java.util.HashSet;
    import java.util.Map;
    import java.util.Set;
    
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletRequestWrapper;
    
    final class MutableHttpServletRequest extends HttpServletRequestWrapper {
        // holds custom header and value mapping
        private final Map<String, String> customHeaders;
    
        public MutableHttpServletRequest(HttpServletRequest request){
            super(request);
            this.customHeaders = new HashMap<String, String>();
        }
    
        public void putHeader(String name, String value){
            this.customHeaders.put(name, value);
        }
    
        public String getHeader(String name) {
            // check the custom headers first
            String headerValue = customHeaders.get(name);
    
            if (headerValue != null){
                return headerValue;
            }
            // else return from into the original wrapped object
            return ((HttpServletRequest) getRequest()).getHeader(name);
        }
    
        public Enumeration<String> getHeaderNames() {
            // create a set of the custom header names
            Set<String> set = new HashSet<String>(customHeaders.keySet());
    
            // now add the headers from the wrapped request object
            @SuppressWarnings("unchecked")
            Enumeration<String> e = ((HttpServletRequest) getRequest()).getHeaderNames();
            while (e.hasMoreElements()) {
                // add the names of the request headers into the list
                String n = e.nextElement();
                set.add(n);
            }
    
            // create an enumeration from the set and return
            return Collections.enumeration(set);
        }
    }

有了这个装饰对象，就可以在 Filter 中使用 MutableHttpServletRequest 类来拦截传入的HTTP 请求以添加一些自定义 Header，代码如下：

import java.io.IOException;
    
    import javax.servlet.FilterChain;
    import javax.servlet.FilterConfig;
    import javax.servlet.ServletException;
    import javax.servlet.ServletRequest;
    import javax.servlet.ServletResponse;
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;
    
    public class SecurityFilter implements javax.servlet.Filter {
    
        @Override
        public void destroy() {
    
        }
    
        @Override
        public void doFilter(ServletRequest request, ServletResponse response,
                FilterChain chain) throws IOException, ServletException {
            HttpServletRequest req = (HttpServletRequest) request;
            MutableHttpServletRequest mutableRequest = new MutableHttpServletRequest(req);
            ...
            mutableRequest.putHeader("x-custom-header", "custom value");
            chain.doFilter(mutableRequest, response);
        }
    
        @Override
        public void init(FilterConfig filterConfig) throws ServletException {
    
        }
    }

[javax.servlet.http.HttpServletRequest]: https://docs.oracle.com/javaee/6/api/javax/servlet/http/HttpServletRequest.html
[javax.servlet.ServletRequest]: https://docs.oracle.com/javaee/6/api/javax/servlet/ServletRequest.html
[javax.servlet.http.HttpServletRequestWrapper]: https://docs.oracle.com/javaee/6/api/javax/servlet/http/HttpServletRequestWrapper.html