Spring MVC 4.1 支持jsonp

使用ResponseBodyAdvice支持jsonp

ResponseBodyAdvice是一个接口，接口描述，

package org.springframework.web.servlet.mvc.method.annotation;

/**
* Allows customizing the response after the execution of an {@code @ResponseBody}
* or an {@code ResponseEntity} controller method but before the body is written
* with an {@code HttpMessageConverter}.
*
* <p>Implementations may be may be registered directly with
* {@code RequestMappingHandlerAdapter} and {@code ExceptionHandlerExceptionResolver}
* or more likely annotated with {@code @ControllerAdvice} in which case they
* will be auto-detected by both.
*
* @author Rossen Stoyanchev
* @since 4.1
*/
public interface ResponseBodyAdvice<T> {

/**
* Whether this component supports the given controller method return type
* and the selected {@code HttpMessageConverter} type.
* @param returnType the return type
* @param converterType the selected converter type
* @return {@code true} if {@link #beforeBodyWrite} should be invoked, {@code false} otherwise
*/
boolean supports(MethodParameter returnType, Class<? extends HttpMessageConverter<?>> converterType);

/**
* Invoked after an {@code HttpMessageConverter} is selected and just before
* its write method is invoked.
* @param body the body to be written
* @param returnType the return type of the controller method
* @param selectedContentType the content type selected through content negotiation
* @param selectedConverterType the converter type selected to write to the response
* @param request the current request
* @param response the current response
* @return the body that was passed in or a modified, possibly new instance
*/
T beforeBodyWrite(T body, MethodParameter returnType, MediaType selectedContentType,
Class<? extends HttpMessageConverter<?>> selectedConverterType,
ServerHttpRequest request, ServerHttpResponse response);

}

作用：

Allows customizing the response after the execution of an {@code @ResponseBody} or an {@code ResponseEntity} controller method but before the body is written

with an {@code HttpMessageConverter}.

其中一个方法就是 beforeBodyWrite 在使用相应的HttpMessageConvert 进行write之前会被调用，就是一个切面方法。

和jsonp有关的实现类是AbstractJsonpResponseBodyAdvice，如下是 beforeBodyWrite 方法的实现，

@Override
public final Object beforeBodyWrite(Object body, MethodParameter returnType,
MediaType contentType, Class<? extends HttpMessageConverter<?>> converterType,
ServerHttpRequest request, ServerHttpResponse response) {

MappingJacksonValue container = getOrCreateContainer(body);
beforeBodyWriteInternal(container, contentType, returnType, request, response);
return container;
}

位于AbstractJsonpResponseBodyAdvice的父类中，而beforeBodyWriteInternal是在AbstractJsonpResponseBodyAdvice中实现的 ，如下，

@Override
protected void beforeBodyWriteInternal(MappingJacksonValue bodyContainer, MediaType contentType,
MethodParameter returnType, ServerHttpRequest request, ServerHttpResponse response) {

HttpServletRequest servletRequest = ((ServletServerHttpRequest) request).getServletRequest();

for (String name : this.jsonpQueryParamNames) {
String value = servletRequest.getParameter(name);
if (value != null) {
MediaType contentTypeToUse = getContentType(contentType, request, response);
response.getHeaders().setContentType(contentTypeToUse);
bodyContainer.setJsonpFunction(value);
return;
}
}
}

就是根据callback 请求参数或配置的其他参数来确定返回jsonp协议的数据。

如何实现jsonp？

首先继承AbstractJsonpResponseBodyAdvice ，如下，

package com.usoft.web.controller.jsonp;

import org.springframework.web.bind.annotation.ControllerAdvice;
import org.springframework.web.servlet.mvc.method.annotation.AbstractJsonpResponseBodyAdvice;

/**
*
*/
@ControllerAdvice(basePackages = "com.usoft.web.controller.jsonp")
public class JsonpAdvice extends AbstractJsonpResponseBodyAdvice {
public JsonpAdvice() {
super("callback", "jsonp");
}
}

super("callback", "jsonp");的意思就是当请求参数中包含callback 或 jsonp参数时，就会返回jsonp协议的数据。其value就作为回调函数的名称。

这里必须使用@ControllerAdvice注解标注该类，并且配置对哪些Controller起作用。关于注解@ControllerAdvice 的作用这里不做描述。

Controller实现jsonp，

package com.usoft.web.controller.jsonp;

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

import com.usoft.web.controller.JsonMapper;
import com.usoft.web.controller.Person;

/**
* jsonp
*/
@Controller
public class JsonpController {

/**
* callback({"id":1,"age":12,"name":"lyx"})
*
* @param args
*/
public static void main(String args[]) {
Person person = new Person(1, "lyx", 12);
System.out.println(JsonMapper.nonNullMapper().toJsonP("callback",
person));
}

@RequestMapping("/jsonp1")
public Person jsonp1() {
return new Person(1, "lyx", 12);
}

@RequestMapping("/jsonp2")
@ResponseBody
public Person jsonp2() {
return new Person(1, "lyx", 12);
}

@RequestMapping("/jsonp3")
@ResponseBody
public String jsonp3() {
return JsonMapper.nonNullMapper().toJsonP("callback",
new Person(1, "lyx", 12));
}
}

jsonp2 方法就是 一个jsonp协议的调用。http://localhost:8081/jsonp2?callback=test可以直接调用这个方法，并且返回jsonp协议的数据。

通过debug代码，我们来看一下他是怎么返回jsonp协议的数据的。

正因为我们前面在 该Controller 上配置了 JsonpAdvice 的 ControllerAdvice，在调用 MappingJackson2HttpMessageConverter的write（）方法往回写数据的时候，首先会调用

beforeBodyWrite，具体的代码如下，

@Override
protected void beforeBodyWriteInternal(MappingJacksonValue bodyContainer, MediaType contentType,
MethodParameter returnType, ServerHttpRequest request, ServerHttpResponse response) {

HttpServletRequest servletRequest = ((ServletServerHttpRequest) request).getServletRequest();

for (String name : this.jsonpQueryParamNames) {
String value = servletRequest.getParameter(name);
if (value != null) {
MediaType contentTypeToUse = getContentType(contentType, request, response);
response.getHeaders().setContentType(contentTypeToUse);
bodyContainer.setJsonpFunction(value);
return;
}
}
}

当请求参数中含有配置的相应的回调参数时，就会bodyContainer.setJsonpFunction(value);这就标志着 返回的数据时jsonp格式的数据。

然后接下来就到了 MappingJackson2HttpMessageConverter 的write（）方法真正写数据的时候了。看他是怎么写数据的，相关的代码如下，

@Override
protected void writeInternal(Object object, HttpOutputMessage outputMessage)
throws IOException, HttpMessageNotWritableException {

JsonEncoding encoding = getJsonEncoding(outputMessage.getHeaders().getContentType());
JsonGenerator generator = this.objectMapper.getFactory().createGenerator(outputMessage.getBody(), encoding);
try {
writePrefix(generator, object);
Class<?> serializationView = null;
Object value = object;
if (value instanceof MappingJacksonValue) {
MappingJacksonValue container = (MappingJacksonValue) object;
value = container.getValue();
serializationView = container.getSerializationView();
}
if (serializationView != null) {
this.objectMapper.writerWithView(serializationView).writeValue(generator, value);
}
else {
this.objectMapper.writeValue(generator, value);
}
writeSuffix(generator, object);
generator.flush();

}
catch (JsonProcessingException ex) {
throw new HttpMessageNotWritableException("Could not write content: " + ex.getMessage(), ex);
}
}
@Override
protected void writePrefix(JsonGenerator generator, Object object) throws IOException {
if (this.jsonPrefix != null) {
generator.writeRaw(this.jsonPrefix);
}
String jsonpFunction =
(object instanceof MappingJacksonValue ? ((MappingJacksonValue) object).getJsonpFunction() : null);
if (jsonpFunction != null) {
generator.writeRaw(jsonpFunction + "(");
}
}
@Override
protected void writeSuffix(JsonGenerator generator, Object object) throws IOException {
String jsonpFunction =
(object instanceof MappingJacksonValue ? ((MappingJacksonValue) object).getJsonpFunction() : null);
if (jsonpFunction != null) {
generator.writeRaw(");");
}
}

代码非常清晰。看我们jsonp调用的结果。
http://localhost:8081/jsonp2?callback=test
响应消息如下，

HTTP/1.1 200 OK

Server: Apache-Coyote/1.1

Content-Type: application/javascript

Transfer-Encoding: chunked

Date: Sun, 19 Jul 2015 13:01:02 GMT

test({"id":1,"age":12,"name":"lyx"});

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