@RequestParam @RequestBody @PathVariable 等参数绑定注解详解（）

转载自：http://blog.csdn.net/walkerjong/article/details/7946109

学习了下，对@RequestBody，@SessionAttributes,@ModelAttribute还不是很了解，继续学习，也望知道的能告诉我一下，谢谢。

简介：

handler method 参数绑定常用的注解,我们根据他们处理的Request的不同内容部分分为四类：（主要讲解常用类型）

A、处理requet uri 部分（这里指uri template中variable，不含queryString部分）的注解：   @PathVariable;

B、处理request header部分的注解：   @RequestHeader, @CookieValue;

C、处理request body部分的注解：@RequestParam,  @RequestBody;

D、处理attribute类型是注解： @SessionAttributes, @ModelAttribute;

1、 @PathVariable

当使用@RequestMapping URI template 样式映射时， 即 someUrl/{paramId}, 这时的paramId可通过 @Pathvariable注解绑定它传过来的值到方法的参数上。

示例代码：

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1. @Controller
2. @RequestMapping("/owners/{ownerId}")
3. publicclass RelativePathUriTemplateController
   {
4. @RequestMapping("/pets/{petId}")
5. publicvoid findPet(@PathVariable String
   ownerId, @PathVariable String petId,
   Model model) {
6. // implementation
   omitted
7. }
8. }

@Controller
@RequestMapping("/owners/{ownerId}")
public class RelativePathUriTemplateController {

  @RequestMapping("/pets/{petId}")
  public void findPet(@PathVariable String ownerId, @PathVariable String petId, Model model) {
    // implementation omitted
  }
}

上面代码把URI template 中变量 ownerId的值和petId的值，绑定到方法的参数上。若方法参数名称和需要绑定的uri template中变量名称不一致，需要在@PathVariable("name")指定uri template中的名称。

2、 @RequestHeader、@CookieValue

@RequestHeader 注解，可以把Request请求header部分的值绑定到方法的参数上。

示例代码：

这是一个Request 的header部分：

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1. Host                    localhost:8080
2. Accept                  text/html,application/xhtml+xml,application/xml;q=0.9
3. Accept-Language         fr,en-gb;q=0.7,en;q=0.3
4. Accept-Encoding         gzip,deflate
5. Accept-Charset          ISO-8859-1,utf-8;q=0.7,*;q=0.7
6. Keep-Alive              300

Host                    localhost:8080
Accept                  text/html,application/xhtml+xml,application/xml;q=0.9
Accept-Language         fr,en-gb;q=0.7,en;q=0.3
Accept-Encoding         gzip,deflate
Accept-Charset          ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive              300

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1. @RequestMapping("/displayHeaderInfo.do")
2. publicvoid displayHeaderInfo(@RequestHeader("Accept-Encoding")
   String encoding,
3. @RequestHeader("Keep-Alive") long keepAlive) 
   {
4. //...
5. }

@RequestMapping("/displayHeaderInfo.do")
public void displayHeaderInfo(@RequestHeader("Accept-Encoding") String encoding,
                              @RequestHeader("Keep-Alive") long keepAlive)  {

  //...

}

上面的代码，把request header部分的 Accept-Encoding的值，绑定到参数encoding上了， Keep-Alive header的值绑定到参数keepAlive上。

@CookieValue 可以把Request header中关于cookie的值绑定到方法的参数上。

例如有如下Cookie值：

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1. JSESSIONID=415A4AC178C59DACE0B2C9CA727CDD84

JSESSIONID=415A4AC178C59DACE0B2C9CA727CDD84

参数绑定的代码：

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1. @RequestMapping("/displayHeaderInfo.do")
2. publicvoid displayHeaderInfo(@CookieValue("JSESSIONID")
   String cookie)  {
3. //...
4. }

@RequestMapping("/displayHeaderInfo.do")
public void displayHeaderInfo(@CookieValue("JSESSIONID") String cookie)  {

  //...

}

即把JSESSIONID的值绑定到参数cookie上。

3、@RequestParam, @RequestBody

@RequestParam

A） 常用来处理简单类型的绑定，通过Request.getParameter() 获取的String可直接转换为简单类型的情况（ String--> 简单类型的转换操作由ConversionService配置的转换器来完成）；因为使用request.getParameter()方式获取参数，所以可以处理get 方式中queryString的值，也可以处理post方式中 body data的值；

B）用来处理Content-Type: 为 application/x-www-form-urlencoded编码的内容，提交方式GET、POST；

C) 该注解有两个属性： value、required； value用来指定要传入值的id名称，required用来指示参数是否必须绑定；

示例代码：

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1. @Controller
2. @RequestMapping("/pets")
3. @SessionAttributes("pet")
4. publicclass EditPetForm
   {
5. // ...
6. @RequestMapping(method
   = RequestMethod.GET)
7. public String
   setupForm(@RequestParam("petId") int petId,
   ModelMap model) {
8. Pet pet = this.clinic.loadPet(petId);
9. model.addAttribute("pet",
   pet);
10. return"petForm";
11. }
12. // ...

@Controller
@RequestMapping("/pets")
@SessionAttributes("pet")
public class EditPetForm {

    // ...

    @RequestMapping(method = RequestMethod.GET)
    public String setupForm(@RequestParam("petId") int petId, ModelMap model) {
        Pet pet = this.clinic.loadPet(petId);
        model.addAttribute("pet", pet);
        return "petForm";
    }

    // ...

@RequestBody

该注解常用来处理Content-Type: 不是application/x-www-form-urlencoded编码的内容，例如application/json, application/xml等；

它是通过使用HandlerAdapter 配置的HttpMessageConverters来解析post data body，然后绑定到相应的bean上的。

因为配置有FormHttpMessageConverter，所以也可以用来处理 application/x-www-form-urlencoded的内容，处理完的结果放在一个MultiValueMap<String, String>里，这种情况在某些特殊需求下使用，详情查看FormHttpMessageConverter api;

示例代码：

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1. @RequestMapping(value
   = "/something", method = RequestMethod.PUT)
2. publicvoid handle(@RequestBody String
   body, Writer writer) throws IOException
   {
3. writer.write(body);
4. }

@RequestMapping(value = "/something", method = RequestMethod.PUT)
public void handle(@RequestBody String body, Writer writer) throws IOException {
  writer.write(body);
}

4、@SessionAttributes, @ModelAttribute

@SessionAttributes:

该注解用来绑定HttpSession中的attribute对象的值，便于在方法中的参数里使用。

该注解有value、types两个属性，可以通过名字和类型指定要使用的attribute 对象；

示例代码：

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1. @Controller
2. @RequestMapping("/editPet.do")
3. @SessionAttributes("pet")
4. publicclass EditPetForm
   {
5. // ...
6. }

@Controller
@RequestMapping("/editPet.do")
@SessionAttributes("pet")
public class EditPetForm {
    // ...
}

@ModelAttribute

该注解有两个用法，一个是用于方法上，一个是用于参数上；

用于方法上时：  通常用来在处理@RequestMapping之前，为请求绑定需要从后台查询的model；

用于参数上时： 用来通过名称对应，把相应名称的值绑定到注解的参数bean上；要绑定的值来源于：

A） @SessionAttributes 启用的attribute 对象上；

B） @ModelAttribute 用于方法上时指定的model对象；

C） 上述两种情况都没有时，new一个需要绑定的bean对象，然后把request中按名称对应的方式把值绑定到bean中。

用到方法上@ModelAttribute的示例代码：

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1. // Add one attribute
2. // The return value
   of the method is added to the model under the name "account"
3. // You can customize
   the name via @ModelAttribute("myAccount")
4. @ModelAttribute
5. public Account
   addAccount(@RequestParam String number)
   {
6. return accountManager.findAccount(number);
7. }

// Add one attribute
// The return value of the method is added to the model under the name "account"
// You can customize the name via @ModelAttribute("myAccount")

@ModelAttribute
public Account addAccount(@RequestParam String number) {
    return accountManager.findAccount(number);
}

这种方式实际的效果就是在调用@RequestMapping的方法之前，为request对象的model里put（“account”， Account）；

用在参数上的@ModelAttribute示例代码：

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1. @RequestMapping(value="/owners/{ownerId}/pets/{petId}/edit",
   method = RequestMethod.POST)
2. public String
   processSubmit(@ModelAttribute Pet pet)
   {
3. }

@RequestMapping(value="/owners/{ownerId}/pets/{petId}/edit", method = RequestMethod.POST)
public String processSubmit(@ModelAttribute Pet pet) {

}

首先查询 @SessionAttributes有无绑定的Pet对象，若没有则查询@ModelAttribute方法层面上是否绑定了Pet对象，若没有则将URI template中的值按对应的名称绑定到Pet对象的各属性上。

补充讲解：

问题： 在不给定注解的情况下，参数是怎样绑定的？

通过分析AnnotationMethodHandlerAdapter和RequestMappingHandlerAdapter的源代码发现，方法的参数在不给定参数的情况下：

若要绑定的对象时简单类型：  调用@RequestParam来处理的。

若要绑定的对象时复杂类型：  调用@ModelAttribute来处理的。

这里的简单类型指java的原始类型(boolean, int 等)、原始类型对象（Boolean, Int等）、String、Date等ConversionService里可以直接String转换成目标对象的类型；

下面贴出AnnotationMethodHandlerAdapter中绑定参数的部分源代码：

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1. private Object[]
   resolveHandlerArguments(Method handlerMethod, Object handler,
2. NativeWebRequest webRequest, ExtendedModelMap implicitModel) throws Exception
   {
3. Class[] paramTypes = handlerMethod.getParameterTypes();
4. Object[] args = new Object[paramTypes.length];
5. for (int i
   = 0; i < args.length; i++) {
6. MethodParameter methodParam = new MethodParameter(handlerMethod,
   i);
7. methodParam.initParameterNameDiscovery(this.parameterNameDiscoverer);
8. GenericTypeResolver.resolveParameterType(methodParam, handler.getClass());
9. String paramName = null;
10. String headerName = null;
11. boolean requestBodyFound
    = false;
12. String cookieName = null;
13. String pathVarName = null;
14. String attrName = null;
15. boolean required
    = false;
16. String defaultValue = null;
17. boolean validate
    = false;
18. Object[] validationHints = null;
19. int annotationsFound
    = 0;
20. Annotation[] paramAnns = methodParam.getParameterAnnotations();
21. for (Annotation
    paramAnn : paramAnns) {
22. if (RequestParam.class.isInstance(paramAnn))
    {
23. RequestParam requestParam = (RequestParam) paramAnn;
24. paramName = requestParam.value();
25. required = requestParam.required();
26. defaultValue = parseDefaultValueAttribute(requestParam.defaultValue());
27. annotationsFound++;
28. }
29. elseif (RequestHeader.class.isInstance(paramAnn))
    {
30. RequestHeader requestHeader = (RequestHeader) paramAnn;
31. headerName = requestHeader.value();
32. required = requestHeader.required();
33. defaultValue = parseDefaultValueAttribute(requestHeader.defaultValue());
34. annotationsFound++;
35. }
36. elseif (RequestBody.class.isInstance(paramAnn))
    {
37. requestBodyFound = true;
38. annotationsFound++;
39. }
40. elseif (CookieValue.class.isInstance(paramAnn))
    {
41. CookieValue cookieValue = (CookieValue) paramAnn;
42. cookieName = cookieValue.value();
43. required = cookieValue.required();
44. defaultValue = parseDefaultValueAttribute(cookieValue.defaultValue());
45. annotationsFound++;
46. }
47. elseif (PathVariable.class.isInstance(paramAnn))
    {
48. PathVariable pathVar = (PathVariable) paramAnn;
49. pathVarName = pathVar.value();
50. annotationsFound++;
51. }
52. elseif (ModelAttribute.class.isInstance(paramAnn))
    {
53. ModelAttribute attr = (ModelAttribute) paramAnn;
54. attrName = attr.value();
55. annotationsFound++;
56. }
57. elseif (Value.class.isInstance(paramAnn))
    {
58. defaultValue = ((Value) paramAnn).value();
59. }
60. elseif (paramAnn.annotationType().getSimpleName().startsWith("Valid"))
    {
61. validate = true;
62. Object value = AnnotationUtils.getValue(paramAnn);
63. validationHints = (value instanceof Object[]
    ? (Object[]) value : new Object[]
    {value});
64. }
65. }
66. if (annotationsFound
    > 1) {
67. thrownew IllegalStateException("Handler
    parameter annotations are exclusive choices - " +
68. "do
    not specify more than one such annotation on the same parameter: " + handlerMethod);
69. }
70. if (annotationsFound
    == 0) {//
    若没有发现注解
71. Object argValue = resolveCommonArgument(methodParam, webRequest);    //判断WebRquest是否可赋值给参数
72. if (argValue
    != WebArgumentResolver.UNRESOLVED) {
73. args[i] = argValue;
74. }
75. elseif (defaultValue
    != null)
    {
76. args[i] = resolveDefaultValue(defaultValue);
77. }
78. else {
79. Class<?> paramType = methodParam.getParameterType();
80. if (Model.class.isAssignableFrom(paramType)
    || Map.class.isAssignableFrom(paramType))
    {
81. if (!paramType.isAssignableFrom(implicitModel.getClass()))
    {
82. thrownew IllegalStateException("Argument
    [" + paramType.getSimpleName() + "]
    is of type " +
83. "Model
    or Map but is not assignable from the actual model. You may need to switch " +
84. "newer
    MVC infrastructure classes to use this argument.");
85. }
86. args[i] = implicitModel;
87. }
88. elseif (SessionStatus.class.isAssignableFrom(paramType))
    {
89. args[i] = this.sessionStatus;
90. }
91. elseif (HttpEntity.class.isAssignableFrom(paramType))
    {
92. args[i] = resolveHttpEntityRequest(methodParam, webRequest);
93. }
94. elseif (Errors.class.isAssignableFrom(paramType))
    {
95. thrownew IllegalStateException("Errors/BindingResult
    argument declared " +
96. "without
    preceding model attribute. Check your handler method signature!");
97. }
98. elseif (BeanUtils.isSimpleProperty(paramType))
    {// 判断是否参数类型是否是简单类型，若是在使用@RequestParam方式来处理,否则使用@ModelAttribute方式处理
99. paramName = "";
100. }
101. else {
102. attrName = "";
103. }
104. }
105. }
106. if (paramName
     != null)
     {
107. args[i] = resolveRequestParam(paramName, required, defaultValue, methodParam, webRequest, handler);
108. }
109. elseif (headerName
     != null)
     {
110. args[i] = resolveRequestHeader(headerName, required, defaultValue, methodParam, webRequest, handler);
111. }
112. elseif (requestBodyFound)
     {
113. args[i] = resolveRequestBody(methodParam, webRequest, handler);
114. }
115. elseif (cookieName
     != null)
     {
116. args[i] = resolveCookieValue(cookieName, required, defaultValue, methodParam, webRequest, handler);
117. }
118. elseif (pathVarName
     != null)
     {
119. args[i] = resolvePathVariable(pathVarName, methodParam, webRequest, handler);
120. }
121. elseif (attrName
     != null)
     {
122. WebDataBinder binder =
123. resolveModelAttribute(attrName, methodParam, implicitModel, webRequest, handler);
124. boolean assignBindingResult
     = (args.length > i + 1 && Errors.class.isAssignableFrom(paramTypes[i
     + 1]));
125. if (binder.getTarget()
     != null)
     {
126. doBind(binder, webRequest, validate, validationHints, !assignBindingResult);
127. }
128. args[i] = binder.getTarget();
129. if (assignBindingResult)
     {
130. args[i + 1]
     = binder.getBindingResult();
131. i++;
132. }
133. implicitModel.putAll(binder.getBindingResult().getModel());
134. }
135. }
136. return args;
137. }

private Object[] resolveHandlerArguments(Method handlerMethod, Object handler,
			NativeWebRequest webRequest, ExtendedModelMap implicitModel) throws Exception {

		Class[] paramTypes = handlerMethod.getParameterTypes();
		Object[] args = new Object[paramTypes.length];

		for (int i = 0; i < args.length; i++) {
			MethodParameter methodParam = new MethodParameter(handlerMethod, i);
			methodParam.initParameterNameDiscovery(this.parameterNameDiscoverer);
			GenericTypeResolver.resolveParameterType(methodParam, handler.getClass());
			String paramName = null;
			String headerName = null;
			boolean requestBodyFound = false;
			String cookieName = null;
			String pathVarName = null;
			String attrName = null;
			boolean required = false;
			String defaultValue = null;
			boolean validate = false;
			Object[] validationHints = null;
			int annotationsFound = 0;
			Annotation[] paramAnns = methodParam.getParameterAnnotations();

			for (Annotation paramAnn : paramAnns) {
				if (RequestParam.class.isInstance(paramAnn)) {
					RequestParam requestParam = (RequestParam) paramAnn;
					paramName = requestParam.value();
					required = requestParam.required();
					defaultValue = parseDefaultValueAttribute(requestParam.defaultValue());
					annotationsFound++;
				}
				else if (RequestHeader.class.isInstance(paramAnn)) {
					RequestHeader requestHeader = (RequestHeader) paramAnn;
					headerName = requestHeader.value();
					required = requestHeader.required();
					defaultValue = parseDefaultValueAttribute(requestHeader.defaultValue());
					annotationsFound++;
				}
				else if (RequestBody.class.isInstance(paramAnn)) {
					requestBodyFound = true;
					annotationsFound++;
				}
				else if (CookieValue.class.isInstance(paramAnn)) {
					CookieValue cookieValue = (CookieValue) paramAnn;
					cookieName = cookieValue.value();
					required = cookieValue.required();
					defaultValue = parseDefaultValueAttribute(cookieValue.defaultValue());
					annotationsFound++;
				}
				else if (PathVariable.class.isInstance(paramAnn)) {
					PathVariable pathVar = (PathVariable) paramAnn;
					pathVarName = pathVar.value();
					annotationsFound++;
				}
				else if (ModelAttribute.class.isInstance(paramAnn)) {
					ModelAttribute attr = (ModelAttribute) paramAnn;
					attrName = attr.value();
					annotationsFound++;
				}
				else if (Value.class.isInstance(paramAnn)) {
					defaultValue = ((Value) paramAnn).value();
				}
				else if (paramAnn.annotationType().getSimpleName().startsWith("Valid")) {
					validate = true;
					Object value = AnnotationUtils.getValue(paramAnn);
					validationHints = (value instanceof Object[] ? (Object[]) value : new Object[] {value});
				}
			}

			if (annotationsFound > 1) {
				throw new IllegalStateException("Handler parameter annotations are exclusive choices - " +
						"do not specify more than one such annotation on the same parameter: " + handlerMethod);
			}

			if (annotationsFound == 0) {// 若没有发现注解
				Object argValue = resolveCommonArgument(methodParam, webRequest);    //判断WebRquest是否可赋值给参数
				if (argValue != WebArgumentResolver.UNRESOLVED) {
					args[i] = argValue;
				}
				else if (defaultValue != null) {
					args[i] = resolveDefaultValue(defaultValue);
				}
				else {
					Class<?> paramType = methodParam.getParameterType();
					if (Model.class.isAssignableFrom(paramType) || Map.class.isAssignableFrom(paramType)) {
						if (!paramType.isAssignableFrom(implicitModel.getClass())) {
							throw new IllegalStateException("Argument [" + paramType.getSimpleName() + "] is of type " +
									"Model or Map but is not assignable from the actual model. You may need to switch " +
									"newer MVC infrastructure classes to use this argument.");
						}
						args[i] = implicitModel;
					}
					else if (SessionStatus.class.isAssignableFrom(paramType)) {
						args[i] = this.sessionStatus;
					}
					else if (HttpEntity.class.isAssignableFrom(paramType)) {
						args[i] = resolveHttpEntityRequest(methodParam, webRequest);
					}
					else if (Errors.class.isAssignableFrom(paramType)) {
						throw new IllegalStateException("Errors/BindingResult argument declared " +
								"without preceding model attribute. Check your handler method signature!");
					}
					else if (BeanUtils.isSimpleProperty(paramType)) {// 判断是否参数类型是否是简单类型，若是在使用@RequestParam方式来处理,否则使用@ModelAttribute方式处理
						paramName = "";
					}
					else {
						attrName = "";
					}
				}
			}

			if (paramName != null) {
				args[i] = resolveRequestParam(paramName, required, defaultValue, methodParam, webRequest, handler);
			}
			else if (headerName != null) {
				args[i] = resolveRequestHeader(headerName, required, defaultValue, methodParam, webRequest, handler);
			}
			else if (requestBodyFound) {
				args[i] = resolveRequestBody(methodParam, webRequest, handler);
			}
			else if (cookieName != null) {
				args[i] = resolveCookieValue(cookieName, required, defaultValue, methodParam, webRequest, handler);
			}
			else if (pathVarName != null) {
				args[i] = resolvePathVariable(pathVarName, methodParam, webRequest, handler);
			}
			else if (attrName != null) {
				WebDataBinder binder =
						resolveModelAttribute(attrName, methodParam, implicitModel, webRequest, handler);
				boolean assignBindingResult = (args.length > i + 1 && Errors.class.isAssignableFrom(paramTypes[i + 1]));
				if (binder.getTarget() != null) {
					doBind(binder, webRequest, validate, validationHints, !assignBindingResult);
				}
				args[i] = binder.getTarget();
				if (assignBindingResult) {
					args[i + 1] = binder.getBindingResult();
					i++;
				}
				implicitModel.putAll(binder.getBindingResult().getModel());
			}
		}

		return args;
	}

RequestMappingHandlerAdapter中使用的参数绑定，代码稍微有些不同，有兴趣的同仁可以分析下，最后处理的结果都是一样的。

示例：

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1. @RequestMapping ({"/", "/home"})
2. public String
   showHomePage(String key){
3. logger.debug("key="+key);
4. return"home";
5. }

@RequestMapping ({"/", "/home"})
	public String showHomePage(String key){

		logger.debug("key="+key);

		return "home";
	}

这种情况下，就调用默认的@RequestParam来处理。

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1. @RequestMapping (method
   = RequestMethod.POST)
2. public String
   doRegister(User user){
3. if(logger.isDebugEnabled()){
4. logger.debug("process
   url[/user], method[post] in "+getClass());
5. logger.debug(user);
6. }
7. return"user";
8. }

	@RequestMapping (method = RequestMethod.POST)
	public String doRegister(User user){
		if(logger.isDebugEnabled()){
			logger.debug("process url[/user], method[post] in "+getClass());
			logger.debug(user);
		}

		return "user";
	}

这种情况下，就调用@ModelAttribute来处理。

参考文档：

1、 Spring Web Doc：

spring-3.1.0/docs/spring-framework-reference/html/mvc.html