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SpringMVC教程(十)
发表时间：2020-06-02 16:41:32
返回json

在前后端分离开发的大背景下，SpringMVC的Controller更多的是充当接口的作用，返回json数据给前端。

Controller如何返回json呢？

1.@Controller+@ResponseBody

我先看最简单的一种情况，返回纯文本。例如：

package com.controller;

import com.entity.Users;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.ModelAttribute;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.ResponseBody;
import javax.servlet.http.HttpSession;

@Controller
@RequestMapping("users")
public class UsersController {

    @RequestMapping(value="login",method ={RequestMethod.GET,RequestMethod.POST})
    @ResponseBody
    public String login(Users user){
        System.out.println("执行用户登录...");
        System.out.println(user);
        return "login_success";
    }

}

使用PostMan测试 /users/login动作，返回视图结果如下：

下来，我们尝试返回一个json字符串。改写如下：

package com.controller;

import com.entity.Users;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.ModelAttribute;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.ResponseBody;
import javax.servlet.http.HttpSession;

@Controller
@RequestMapping("users")
public class UsersController {

    @RequestMapping(value="login",method ={RequestMethod.GET,RequestMethod.POST})
    @ResponseBody
    public Users login(Users user){
        System.out.println("执行用户登录...");
        System.out.println(user);
        return user;
    }

}

使用PostMan测试 /users/login动作，返回视图结果如下： 

HTTP ERROR 500
Problem accessing /springmvctest/users/login. Reason:

    Server Error
Caused by:
org.springframework.http.converter.HttpMessageNotWritableException: No converter found for return value of type: class com.entity.Users
    at org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:230)
    at org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.handleReturnValue(RequestResponseBodyMethodProcessor.java:181)
    at org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:82)
    at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:123)
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:879)
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:793)
    at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87)
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1040)
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:943)
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006)
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:898)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:687)
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
    at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:865)
    at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1623)
...

出现这个错误的原因是SpringMVC不知道如何把实体类Users转换为json格式，这里需要添加Spring默认的jackson依赖实现json格式的转换。在pom.xml中添加以下依赖：

<?xml version="1.0" encoding="UTF-8"?>

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.simoniu</groupId>
    <artifactId>springmvcdemo</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>war</packaging>
    <name>springmvcdemo Maven Webapp</name>

    <properties>
        <spring.version>5.2.5.RELEASE</spring.version>
        <servlet-version>4.0.1</servlet-version>
        <jackson-versoin>2.13.2</jackson-versoin>
    </properties>

    <dependencies>
        <!-- https://mvnrepository.com/artifact/org.springframework/spring-core -->
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-core</artifactId>
            <version>${spring.version}</version>
        </dependency>

        <!-- https://mvnrepository.com/artifact/org.springframework/spring-context -->
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-context</artifactId>
            <version>${spring.version}</version>
        </dependency>

        <!-- https://mvnrepository.com/artifact/org.springframework/spring-web -->
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-web</artifactId>
            <version>${spring.version}</version>
        </dependency>

        <!-- https://mvnrepository.com/artifact/org.springframework/spring-webmvc -->
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-webmvc</artifactId>
            <version>${spring.version}</version>
        </dependency>

        <!-- https://mvnrepository.com/artifact/junit/junit -->
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.12</version>
            <scope>test</scope>
        </dependency>

        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-test</artifactId>
            <version>${spring.version}</version>
            <scope>test</scope>
        </dependency>

        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>javax.servlet-api</artifactId>
            <version>${servlet-version}</version>
            <scope>compile</scope>
        </dependency>

        <!-- https://mvnrepository.com/artifact/com.fasterxml.jackson.core/jackson-core -->
        <!--添加jackson依赖-->
        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-core</artifactId>
            <version>${jackson-versoin}</version>
        </dependency>

        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-databind</artifactId>
            <version>${jackson-versoin}</version>
        </dependency>

        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-annotations</artifactId>
            <version>${jackson-versoin}</version>
        </dependency>

    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <configuration>
                    <source>1.8</source>
                    <!-- 此处的 1.8 指的是 JDK 的版本，目前一般为 1.7 或 1.8 -->
                    <target>1.8</target>
                </configuration>
            </plugin>
            <!--servlet容器 jetty插件-->
            <plugin>
                <groupId>org.eclipse.jetty</groupId>
                <artifactId>jetty-maven-plugin</artifactId>
                <version>9.4.18.v20190429</version>
                <configuration>
                    <webApp>
                        <!--项目的上下文配置-->
                        <contextPath>/springmvctest</contextPath>
                    </webApp>
                    <!--项目的端口号和服务器配置-->
                    <httpConnector>
                        <port>8888</port>
                        <host>localhost</host>
                    </httpConnector>
                    <scanIntervalSeconds>1</scanIntervalSeconds>
                </configuration>
            </plugin>
        </plugins>
    </build>

</project>

再次使用PostMan测试 /users/login动作，返回视图结果如下：

2.使用@RestController 把上面的UsersController改写如下：

package com.controller;
import com.entity.Users;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.*;
import javax.servlet.http.HttpSession;

@RestController
@RequestMapping("users")
public class UsersController {

    @RequestMapping(value="login",method ={RequestMethod.GET,RequestMethod.POST})
    public Users login(Users user){
        System.out.println("执行用户登录...");
        System.out.println(user);
        return user;
    }

}

使用PostMan测试 /users/login动作，返回视图结果如下：

3.@RestController与@Controller对比

@RestController注解，相当于@Controller+@ResponseBody两个注解的结合，

返回json数据不需要在方法前面加@ResponseBody注解了，但使用@RestController这个注解，就不能返回jsp,html页面，视图解析器无法解析jsp,html页面。