HttpServletRequest 没有提供批改/删除的Api
HttpServletRequest中定义的对Header的操作全是只读，没有批改。
public interface HttpServletRequest extends ServletRequest { ... public long getDateHeader(String name); public String getHeader(String name); public Enumeration<String> getHeaders(String name); public Enumeration<String> getHeaderNames(); public int getIntHeader(String name); ...}HttpServletRequest 只是一个接口，实现由Servlet容器提供。不论是任何容器，实现类，必定是要把申请的Header存储在某个中央，于是能够通过反射来对存储Header的容器进行增删。
先定义一个测试的Controller
这个Controller很简略，把客户端的所有Header，以JSON形似响应给客户端。
import java.util.ArrayList;import java.util.Enumeration;import java.util.LinkedHashMap;import java.util.List;import java.util.Map;import javax.servlet.http.HttpServletRequest;import org.springframework.web.bind.annotation.GetMapping;import org.springframework.web.bind.annotation.RequestMapping;import org.springframework.web.bind.annotation.RestController;@RestController@RequestMapping("/demo")public class DemoController { // 遍历所有申请Header，响应给客户端。 map<string, string[]> @GetMapping public Object demo (HttpServletRequest request) { Map<String, List<String>> headers = new LinkedHashMap<>(); Enumeration<String> nameEnumeration = request.getHeaderNames(); while (nameEnumeration.hasMoreElements()) { String name = nameEnumeration.nextElement(); List<String> values = headers.get(name); if (values == null) { values = new ArrayList<>(); headers.put(name, values); } Enumeration<String> valueEnumeration = request.getHeaders(name); while (valueEnumeration.hasMoreElements()) { values.add(valueEnumeration.nextElement()); } } return headers; }}应用Tomcat作为容器
Tomcat对HttpServletRequest的实现
Tomcat应用了外观模式（Facade），这个实现略微有一点点简单
org.apache.catalina.connector.RequestFacade|-org.apache.catalina.connector.Request |-org.apache.coyote.Request |-org.apache.tomcat.util.http.MimeHeaders首先是 org.apache.catalina.connector.RequestFacade 实现，它有一个org.apache.catalina.connector.Request 的对象。这个对象又有一个org.apache.coyote.Request的对象，这个对象又有一个org.apache.tomcat.util.http.MimeHeaders 字段，它就是存储了客户端申请头的容器，只有通过反射获取到这个MimeHeaders，对它进行批改即可。
org.apache.catalina.connector.RequestFacade
public class RequestFacade implements HttpServletRequest { protected org.apache.catalina.connector.Request request = null; ...}org.apache.catalina.connector.Request
public class Request implements HttpServletRequest { protected org.apache.coyote.Request coyoteRequest; ...}org.apache.coyote.Request coyoteRequest
public final class Request { private final org.apache.tomcat.util.http.MimeHeaders headers = new MimeHeaders();}在Filter中通过反射对申请Header进行增删
假如的场景是，须要对申请Header对立增加一个x-request-id，通过这个ID来从日志中定位每一个申请。import java.io.IOException;import java.lang.reflect.Field;import java.util.UUID;import javax.servlet.FilterChain;import javax.servlet.ServletException;import javax.servlet.annotation.WebFilter;import javax.servlet.http.HttpFilter;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import org.apache.catalina.connector.Request;import org.apache.catalina.connector.RequestFacade;import org.apache.tomcat.util.http.MimeHeaders;import org.springframework.core.annotation.Order;import org.springframework.stereotype.Component;import org.springframework.util.ReflectionUtils;@WebFilter(urlPatterns = "/*")@Component@Order(-999)public class RequestIdGenFilter extends HttpFilter { /** * */ private static final long serialVersionUID = 1787347739651657706L; @Override protected void doFilter(HttpServletRequest req, HttpServletResponse res, FilterChain chain) throws IOException, ServletException { try { // 从 RequestFacade 中获取 org.apache.catalina.connector.Request Field connectorField = ReflectionUtils.findField(RequestFacade.class, "request", Request.class); connectorField.setAccessible(true); Request connectorRequest = (Request) connectorField.get(req); // 从 org.apache.catalina.connector.Request 中获取 org.apache.coyote.Request Field coyoteField = ReflectionUtils.findField(Request.class, "coyoteRequest", org.apache.coyote.Request.class); coyoteField.setAccessible(true); org.apache.coyote.Request coyoteRequest = (org.apache.coyote.Request) coyoteField.get(connectorRequest); // 从 org.apache.coyote.Request 中获取 MimeHeaders Field mimeHeadersField = ReflectionUtils.findField(org.apache.coyote.Request.class, "headers", MimeHeaders.class); mimeHeadersField.setAccessible(true); MimeHeaders mimeHeaders = (MimeHeaders) mimeHeadersField.get(coyoteRequest); this.mineHeadersHandle(mimeHeaders); } catch (Exception e) { throw new RuntimeException(e); } super.doFilter(req, res, chain); } protected void mineHeadersHandle (MimeHeaders mimeHeaders) { // 增加一个Header，随机生成申请ID mimeHeaders.addValue("x-request-id").setString(UUID.randomUUID().toString());; // 移除一个header mimeHeaders.removeHeader("User-Agent"); }}申请Controller获取响应后果
能够看到胜利增加了x-request-id header，并且删除了User-Agent header。
SpringBoot默认的Servlet容器就是Tomcat
应用Undertow作为容器
越来越多人应用Undertow作为Servlet容器，据说性能比Tomcat要好很多
SpringBoot替换Tomcat为Undertow
只须要把spring-boot-starter-web中的spring-boot-starter-tomcat排除，而后手动增加spring-boot-starter-undertow即可
<dependencies> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-web</artifactId> <exclusions> <exclusion> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-tomcat</artifactId> </exclusion> </exclusions> </dependency> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-undertow</artifactId> </dependency> </dependencies>Undertow中的HttpServletRequest实现
它的实现就比较简单
io.undertow.servlet.spec.HttpServletRequestImpl|-io.undertow.server.HttpServerExchange |-io.undertow.util.HeaderMapio.undertow.servlet.spec.HttpServletRequestImpl实现类中有一个属性对象io.undertow.server.HttpServerExchange，这个属性对象又蕴含了一个io.undertow.util.HeaderMap，HeaderMap就是申请Header的存储容器，反射获取它就行。
io.undertow.servlet.spec.HttpServletRequestImpl
public final class HttpServletRequestImpl implements HttpServletRequest { private final io.undertow.server.HttpServerExchange exchange;}io.undertow.server.HttpServerExchange
public final class HttpServerExchange extends AbstractAttachable { private final HeaderMap requestHeaders;}在Filter中通过反射对申请Header进行增删
import java.io.IOException;import java.lang.reflect.Field;import java.util.UUID;import javax.servlet.FilterChain;import javax.servlet.ServletException;import javax.servlet.annotation.WebFilter;import javax.servlet.http.HttpFilter;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import org.springframework.core.annotation.Order;import org.springframework.stereotype.Component;import org.springframework.util.ReflectionUtils;import io.undertow.server.HttpServerExchange;import io.undertow.servlet.spec.HttpServletRequestImpl;import io.undertow.util.HeaderMap;import io.undertow.util.HttpString;@WebFilter(urlPatterns = "/*")@Component@Order(-999)public class RequestIdGenFilter extends HttpFilter { /** * */ private static final long serialVersionUID = 1787347739651657706L; @Override protected void doFilter(HttpServletRequest req, HttpServletResponse res, FilterChain chain) throws IOException, ServletException { try { // 从HttpServletRequestImpl中获取HttpServerExchange Field exchangeField = ReflectionUtils.findField(HttpServletRequestImpl.class, "exchange", HttpServerExchange.class); exchangeField.setAccessible(true); HttpServerExchange httpServerExchange = (HttpServerExchange) exchangeField.get(req); // 从HttpServerExchange中获取HeaderMap Field headerMapField = ReflectionUtils.findField(HttpServerExchange.class, "requestHeaders", HeaderMap.class); headerMapField.setAccessible(true); HeaderMap requestHeaderMap = (HeaderMap) headerMapField.get(httpServerExchange); this.handleRequestHeaderMap(requestHeaderMap); } catch (Exception e) { throw new RuntimeException(e); } super.doFilter(req, res, chain); } private void handleRequestHeaderMap(HeaderMap requestHeaderMap) { // 增加Header requestHeaderMap.add(new HttpString("x-request-id"), UUID.randomUUID().toString()); // 移除Header requestHeaderMap.remove("User-Agent"); }}申请Controller获取后果
最初
还有其余的Servlet容器，例如Jetty。只有相熟根本的反射，实现对申请头的增删，都很简略。
原文：https://springboot.io/t/topic...