对HttpServletRequest中的Header进行增删

HttpServletRequest 没有提供修改/删除的 Api

HttpServletRequest中定义的对 Header 的操作全是只读，没有修改。代码实现如下：

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。不管是任何容器，实现类，肯定是要把请求的 Header 存储在某个地方，于是可以通过反射来对存储Header的容器进行增删。

先定义一个测试的 Controller

这个 Controller 很简单，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 {

	// 遍历所有请求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;
	}
}

使用 Tomcat 作为容器

Tomcat 对 HttpServletRequest 的实现

Tomcat 使用了外观模式（Facade），这个实现稍微有一点点复杂

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请求头的容器，只要通过反射获取到这个MimeHeaders，对它进行修改即可。

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 中通过反射对请求 Header 进行增删

假设的场景是，需要对请求 Header 统一添加一个x-request-id，通过这个 ID 来从日志中定位每一个请求。

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) {
		// 添加一个Header，随机生成请求ID
		mimeHeaders.addValue("x-request-id").setString(UUID.randomUUID().toString());;
		// 移除一个header
		mimeHeaders.removeHeader("User-Agent");
	}
}

请求 Controller 获取响应结果

可以看到成功添加了x-request-id header，并且删除了User-Agent header。

使用 Undertow 作为容器

越来越多人使用 Undertow 作为 Servlet 容器，据说性能比 Tomcat 要好很多

SpringBoot 替换 Tomcat 为 Undertow

只需要把spring-boot-starter-web中的spring-boot-starter-tomcat排除，然后手动添加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，结构如下：

io.undertow.servlet.spec.HttpServletRequestImpl
|-io.undertow.server.HttpServerExchange
  |-io.undertow.util.HeaderMap

io.undertow.servlet.spec.HttpServletRequestImpl实现类中有一个属性对象

io.undertow.server.HttpServerExchange，这个属性对象又包含了一个io.undertow.util.HeaderMap，HeaderMap就是请求 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 中通过反射对请求 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) {
		// 添加Header
		requestHeaderMap.add(new HttpString("x-request-id"), UUID.randomUUID().toString());
		// 移除Header
		requestHeaderMap.remove("User-Agent");
	}
}