View Resolvers / 视图解析器

View Resolvers

MVC 配置简化了视图解析器的注册。

下面的 Java 配置例子通过使用 JSP 和 Jackson 作为 JSON 渲染的默认视图，配置了内容协商视图解析：

@Configuration
@EnableWebMvc
public class WebConfig implements WebMvcConfigurer {
    @Override
    public void configureViewResolvers(ViewResolverRegistry registry) {
        registry.enableContentNegotiation(new MappingJackson2JsonView());
        registry.jsp();
    }
}

对应的 XML

<mvc:view-resolvers>
    <mvc:content-negotiation>
        <mvc:default-views>
            <bean class="org.springframework.web.servlet.view.json.MappingJackson2JsonView"/>
        </mvc:default-views>
    </mvc:content-negotiation>
    <mvc:jsp/>
</mvc:view-resolvers>

然而，请注意，FreeMarker、Tiles、Groovy Markup 和 script templates（脚本模板）也需要配置底层视图技术。

MVC 命名空间提供了专门的元素。下面的例子适用于 FreeMarker：

<mvc:view-resolvers>
    <mvc:content-negotiation>
        <mvc:default-views>
            <bean class="org.springframework.web.servlet.view.json.MappingJackson2JsonView"/>
        </mvc:default-views>
    </mvc:content-negotiation>
    <mvc:freemarker cache="false"/>
</mvc:view-resolvers>
<mvc:freemarker-configurer>
    <mvc:template-loader-path location="/freemarker"/>
</mvc:freemarker-configurer>

对应的 Java 配置

@Configuration
@EnableWebMvc
public class WebConfig implements WebMvcConfigurer {
    @Override
    public void configureViewResolvers(ViewResolverRegistry registry) {
        registry.enableContentNegotiation(new MappingJackson2JsonView());
        registry.freeMarker().cache(false);
    }
    @Bean
    public FreeMarkerConfigurer freeMarkerConfigurer() {
        FreeMarkerConfigurer configurer = new FreeMarkerConfigurer();
        configurer.setTemplateLoaderPath("/freemarker");
        return configurer;
    }
}

例子

这个内容协商很强悍：客户端需要什么格式，就给你返回什么格式

package cn.mrcode.study;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.servlet.config.annotation.EnableWebMvc;
import org.springframework.web.servlet.config.annotation.ViewResolverRegistry;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
import org.springframework.web.servlet.view.json.MappingJackson2JsonView;
/**
 * @author mrcode
 */
@EnableWebMvc
@ComponentScan("cn.mrcode.study.springdocsread")
@Configuration
public class AppConfig implements WebMvcConfigurer {
    @Override
    public void configureViewResolvers(ViewResolverRegistry registry) {
        registry.enableContentNegotiation(new MappingJackson2JsonView());
    }
}

然后有一个控制器

package cn.mrcode.study.springdocsread.web;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.ResponseBody;
import java.util.Date;
import javax.validation.Valid;
/**
 * @author mrcode
 */
@Controller
@RequestMapping("/demo")
public class Demo2Controller {
    @GetMapping("/accept")
    @ResponseBody
    public Pet data2() {
        final Pet pet = new Pet();
        pet.setDate(new Date());
        pet.setName("张三");
        return pet;
    }
}

在请求的时候，就可以通过请求头 Accept 来让返回的 Pet 格式化成你这里想要的格式，比如

### 默认情况
GET http://localhost:8080/demo/accept
// 响应信息
HTTP/1.1 200 
Vary: Origin
Vary: Access-Control-Request-Method
Vary: Access-Control-Request-Headers
Content-Type: application/xml;charset=UTF-8
Transfer-Encoding: chunked
Date: Mon, 11 Apr 2022 07:39:41 GMT
Keep-Alive: timeout=60
Connection: keep-alive
<Pet>
    <name>张三</name>
    <date>1649662781992</date>
</Pet>

### 可以要求想要 json 格式数据
GET http://localhost:8080/demo/accept
Accept: application/json
// 
HTTP/1.1 200 
Vary: Origin
Vary: Access-Control-Request-Method
Vary: Access-Control-Request-Headers
Content-Type: application/json
Transfer-Encoding: chunked
Date: Mon, 11 Apr 2022 07:43:19 GMT
Keep-Alive: timeout=60
Connection: keep-alive
{
  "name": "张三",
  "date": 1649662999369
}