基于Spring MVC的Web应用开发(7) - Headers

本文接上一篇文章，介绍@RequestMapping中的headers属性，并进一步研究produces属性以及和它配对的consumes属性。

首先看看讲解用到的类：

package org.springframework.samples.mvc.simple;

import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.ResponseBody;

@Controller
public class SimpleControllerRevisited {

    @RequestMapping(value="/simple/revisited", method=RequestMethod.GET, headers="Accept=text/plain")
    public @ResponseBody String simple() {
        return "Hello world revisited!";
    }

}

package org.springframework.samples.mvc.simple;

import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.ResponseBody;

@Controller
public class SimpleControllerRevisited {

	@RequestMapping(value="/simple/revisited", method=RequestMethod.GET, headers="Accept=text/plain")
	public @ResponseBody String simple() {
		return "Hello world revisited!";
	}

}

header是HTTP协议中的一个概念，一般翻译成头域，为了不陷入讲解理论的泥潭，我们使用Firefox的firebug看一下header到底是什么，还记得HelloWorld那一篇文章的例子么？访问http://localhost:8080/web/simple，浏览器显示Hello World!，与本例唯一的区别就是@RequestMapping没有加headers， 打开firebug，在"网络"这一栏看到有一行记录URL "GET simple"，Status "200 OK"，Domain "localhost:8080"等信息，展开，有三列数据，分别为Headers，Response，HTML，看Headers这列，

Request Headers：

Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Encoding: gzip, deflate
Accept-Language: en-us,en;q=0.5
Connection: keep-alive
Host: localhost:8080
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:10.0.2) Gecko/20100101 Firefox/10.0.2

Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Encoding: gzip, deflate
Accept-Language: en-us,en;q=0.5
Connection: keep-alive
Host: localhost:8080
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:10.0.2) Gecko/20100101 Firefox/10.0.2

 Response Headers：

Content-Length: 12
Content-Type: text/html
Server: Jetty(7.2.0.v20101020)

Content-Length: 12
Content-Type: text/html
Server: Jetty(7.2.0.v20101020)

Request Headers中有个Accept，它是由浏览器发出的，表示浏览器认为它可以支持的格式，并不是真的只能支持这些格式，而Response Headers中有个Content-Type，这个值是服务端（如Servlet）封装到Respose的Headers中的。

访问例子程序的URL：http://localhost:8080/web/simple/revisited，

Request Headers：

Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Encoding: gzip, deflate
Accept-Language: en-us,en;q=0.5
Connection: keep-alive
Host: localhost:8080
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:10.0.2) Gecko/20100101 Firefox/10.0.2

Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Encoding: gzip, deflate
Accept-Language: en-us,en;q=0.5
Connection: keep-alive
Host: localhost:8080
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:10.0.2) Gecko/20100101 Firefox/10.0.2

 Response Headers：

Content-Length: 22
Content-Type: text/plain
Server: Jetty(7.2.0.v20101020)

Content-Length: 22
Content-Type: text/plain
Server: Jetty(7.2.0.v20101020)

Response Headers中的Content-Type变成了text/plain，因为RequestMapping的headers设置了Accept＝text/plain。

注意这个Accept和Request Headers中的Accept没有半点关系，它跟Response Headers的Content-Type有关。

我们已经数次提及Content-Type了，其实@RequestMapping的headers属性除了Accept外也有Content-Type：

@RequestMapping(value="/simple/revisited2", method=RequestMethod.GET, headers="Content-Type=text/plain")
public @ResponseBody String simple2() {
    return "Hello world 2 revisited!";
}

	@RequestMapping(value="/simple/revisited2", method=RequestMethod.GET, headers="Content-Type=text/plain")
	public @ResponseBody String simple2() {
		return "Hello world 2 revisited!";
	}

访问http://localhost:8080/web/simple/revisited2，发现浏览器上返回HTTP状态码为415的出错页面：

HTTP ERROR 415

Problem accessing /web/simple/revisited2. Reason:

    Unsupported Media Type

Powered by Jetty://

HTTP ERROR 415

Problem accessing /web/simple/revisited2. Reason:

    Unsupported Media Type

Powered by Jetty://

这是因为headers="Content-Type=text/plain"的含义是

它必须要求Request Headers里的Content-Type为"text/plain"才能执行该方法，

看firebug的Request Headers：

Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Encoding: gzip, deflate
Accept-Language: en-us,en;q=0.5
Connection: keep-alive
Host: localhost:8080
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:10.0.2) Gecko/20100101 Firefox/10.0.2

Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Encoding: gzip, deflate
Accept-Language: en-us,en;q=0.5
Connection: keep-alive
Host: localhost:8080
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:10.0.2) Gecko/20100101 Firefox/10.0.2

没有Content-Type属性，不光这个，前面的那个Request Headers也没有Content-Type，其实GET方式访问的URL的Request Header里压根就没有Content-Type（我原来也不知道，直到我搜索到了这篇文章）！ 所以我们只能使用POST方式测试这个特性，很遗憾，我暂时没有找到如何通过浏览器地址栏的方式来POST提交，但是，想到了上一篇文章附录中的RestTemplate，可以写个Java小程序模拟一下，在SimpleControllerRevisited中增加一个方法：

@RequestMapping(value="/simple/revisited3", method=RequestMethod.POST, headers="Content-Type=text/plain")
public @ResponseBody String simple3() {
    return "Hello world 3 revisited!";
}

	@RequestMapping(value="/simple/revisited3", method=RequestMethod.POST, headers="Content-Type=text/plain")
	public @ResponseBody String simple3() {
		return "Hello world 3 revisited!";
	}

写个RestTemplate的小程序：

package org.springframework.samples.mvc.simple;

import org.springframework.http.HttpStatus;
import org.springframework.http.MediaType;
import org.springframework.http.ResponseEntity;
import org.springframework.web.client.RestTemplate;

public class SimpleControllerRevisitedTest {

    public static void main(String[] args) {
        testPost();
    }

    public static void testPost() {
        RestTemplate template = new RestTemplate();
        ResponseEntity<String> entity = template.postForEntity(
                "http://localhost:8080/web/simple/revisited3", null, String.class);
        String body = entity.getBody();
        MediaType contentType = entity.getHeaders().getContentType();
        System.out.println("contentType:[" + contentType + "]");
        HttpStatus statusCode = entity.getStatusCode();
        System.out.println("statusCode:[" + statusCode + "]");
    }
}

package org.springframework.samples.mvc.simple;

import org.springframework.http.HttpStatus;
import org.springframework.http.MediaType;
import org.springframework.http.ResponseEntity;
import org.springframework.web.client.RestTemplate;

public class SimpleControllerRevisitedTest {

	public static void main(String[] args) {
		testPost();
	}
	
	public static void testPost() {
		RestTemplate template = new RestTemplate();
		ResponseEntity<String> entity = template.postForEntity(
				"http://localhost:8080/web/simple/revisited3", null, String.class);
		String body = entity.getBody();
		MediaType contentType = entity.getHeaders().getContentType();
		System.out.println("contentType:[" + contentType + "]");
		HttpStatus statusCode = entity.getStatusCode();
		System.out.println("statusCode:[" + statusCode + "]");
	}
}

 执行，报错：

DEBUG: org.springframework.web.client.RestTemplate - Created POST request for "http://localhost:8080/web/simple/revisited3"
DEBUG: org.springframework.web.client.RestTemplate - Setting request Accept header to [text/plain, */*]
WARN : org.springframework.web.client.RestTemplate - POST request for "http://localhost:8080/web/simple/revisited3" resulted in 415 (Unsupported Media Type); invoking error handler
Exception in thread "main" org.springframework.web.client.HttpClientErrorException: 415 Unsupported Media Type
    at org.springframework.web.client.DefaultResponseErrorHandler.handleError(DefaultResponseErrorHandler.java:76)
    at org.springframework.web.client.RestTemplate.handleResponseError(RestTemplate.java:486)
    at org.springframework.web.client.RestTemplate.doExecute(RestTemplate.java:443)
    at org.springframework.web.client.RestTemplate.execute(RestTemplate.java:401)
    at org.springframework.web.client.RestTemplate.postForEntity(RestTemplate.java:302)
    at org.springframework.samples.mvc.simple.SimpleControllerRevisitedTest.testPost(SimpleControllerRevisitedTest.java:16)
    at org.springframework.samples.mvc.simple.SimpleControllerRevisitedTest.main(SimpleControllerRevisitedTest.java:11)

DEBUG: org.springframework.web.client.RestTemplate - Created POST request for "http://localhost:8080/web/simple/revisited3"
DEBUG: org.springframework.web.client.RestTemplate - Setting request Accept header to [text/plain, */*]
WARN : org.springframework.web.client.RestTemplate - POST request for "http://localhost:8080/web/simple/revisited3" resulted in 415 (Unsupported Media Type); invoking error handler
Exception in thread "main" org.springframework.web.client.HttpClientErrorException: 415 Unsupported Media Type
	at org.springframework.web.client.DefaultResponseErrorHandler.handleError(DefaultResponseErrorHandler.java:76)
	at org.springframework.web.client.RestTemplate.handleResponseError(RestTemplate.java:486)
	at org.springframework.web.client.RestTemplate.doExecute(RestTemplate.java:443)
	at org.springframework.web.client.RestTemplate.execute(RestTemplate.java:401)
	at org.springframework.web.client.RestTemplate.postForEntity(RestTemplate.java:302)
	at org.springframework.samples.mvc.simple.SimpleControllerRevisitedTest.testPost(SimpleControllerRevisitedTest.java:16)
	at org.springframework.samples.mvc.simple.SimpleControllerRevisitedTest.main(SimpleControllerRevisitedTest.java:11) 

还是报415 Unsupported Media Type，

这说明Content-Type并不是text-plain，那么到底是什么呢？新写一个方法，去掉headers属性，仅保留POST：

@RequestMapping(value="/simple/revisited4", method=RequestMethod.POST)
public @ResponseBody String simple4() {
    return "Hello world 4 revisited!";
}

	@RequestMapping(value="/simple/revisited4", method=RequestMethod.POST)
	public @ResponseBody String simple4() {
		return "Hello world 4 revisited!";
	}

 将SimpleControllerRevisitedTest中的URL改为http://localhost:8080/web/simple/revisited4， 查看服务端打印的日志（注意访问前一个URL时，服务端是不打印此日志的，因为Request Header中的Content-Type就不满足要求，直接拒掉了）：

DEBUG: org.springframework.web.servlet.mvc.method.annotation.HttpEntityMethodProcessor - Reading [[B] as "application/x-www-form-urlencoded" using [org.springframework.http.converter.ByteArrayHttpMessageConverter@1d05c9a1]

DEBUG: org.springframework.web.servlet.mvc.method.annotation.HttpEntityMethodProcessor - Reading [[B] as "application/x-www-form-urlencoded" using [org.springframework.http.converter.ByteArrayHttpMessageConverter@1d05c9a1]

发现使用POST方式，Request Headers中的Content-Type为"application/x-www-form-urlencoded"，

那么再增加一个方法：

@RequestMapping(value="/simple/revisited5", method=RequestMethod.POST, headers="Content-Type=application/x-www-form-urlencoded")
public @ResponseBody String simple5() {
    return "Hello world 5 revisited!";
}

	@RequestMapping(value="/simple/revisited5", method=RequestMethod.POST, headers="Content-Type=application/x-www-form-urlencoded")
	public @ResponseBody String simple5() {
		return "Hello world 5 revisited!";
	}

 将SimpleControllerRevisitedTest中的URL改为http://localhost:8080/web/simple/revisited5，

DEBUG: org.springframework.web.client.RestTemplate - Created POST request for "http://localhost:8080/web/simple/revisited5"
DEBUG: org.springframework.web.client.RestTemplate - Setting request Accept header to [text/plain, */*]
DEBUG: org.springframework.web.client.RestTemplate - POST request for "http://localhost:8080/web/simple/revisited5" resulted in 200 (OK)
DEBUG: org.springframework.web.client.RestTemplate - Reading [java.lang.String] as "text/plain;charset=ISO-8859-1" using [org.springframework.http.converter.StringHttpMessageConverter@5f326484]
contentType:[text/plain;charset=ISO-8859-1]
statusCode:[200]

DEBUG: org.springframework.web.client.RestTemplate - Created POST request for "http://localhost:8080/web/simple/revisited5"
DEBUG: org.springframework.web.client.RestTemplate - Setting request Accept header to [text/plain, */*]
DEBUG: org.springframework.web.client.RestTemplate - POST request for "http://localhost:8080/web/simple/revisited5" resulted in 200 (OK)
DEBUG: org.springframework.web.client.RestTemplate - Reading [java.lang.String] as "text/plain;charset=ISO-8859-1" using [org.springframework.http.converter.StringHttpMessageConverter@5f326484]
contentType:[text/plain;charset=ISO-8859-1]
statusCode:[200]

从程序运行结果看，一切OK（注意不要被这个日志中的contentType弄混了，这个contentType是Response的Headers里的）。

总结一下，@RequestMapping中的headers有两种写法，

一种是：headers="Accept=text/plain",

另一种是：headers="Content-Type=application/x-www-form-urlencoded"，

Accept指定的格式为Response Headers的Content-Type的格式，

Content-Type指定的格式为Reqeust Headers必须要满足的格式，

否则将被SpringMVC直接拒绝掉，并返回415的HTTP状态码。

大家可能会想，@RequestMapping的headers属性搞这么复杂干嘛？Spring团队应该是听到了社区群众的呼声，或者他们自己都觉得晕？因此在Spring 3.1 M2发布的时候，他们做了改进，SpringSource的Team Blog上有一篇文章 SPRING 3.1 M2: SPRING MVC ENHANCEMENTS，说道了Spring 3.1 M2版本针对以前版本的编程模型改进（programming model improvement），将第四点和第五点简而言之就是：

@RequestMapping中

headers="Content-Type=application/json"被consumes="application/json"替换了，

headers="Accept=application/json"被produces="application/json"替换了。

现在应该对上一篇文章中，我写道

后者通过指定@RequestMapping的produces属性为text/plain，字符集为UTF-8，可以避免乱码，这个是可以理解的。

 理解了，任何一个框架都是不断的在发展和完善的，如果能在学习的过程中，知道一点框架的变迁史，将会对我们的理解产生莫大的好处。