java-error-No converter found for return value of type

java-error-No converter found for return value of type

问题描述 ：
日志如下 ：

14-Jul-2023 15:27:46.747 严重 [http-nio-8080-exec-5] org.apache.catalina.core.StandardWrapperValve.invoke 在路径为[]的上下文中，Servlet[action]的Servlet.service（）引发了具有根本原因的异常[Request processing failed; nested exception is java.lang.IllegalArgumentException: No converter found for return value of type: class com.xbm.domain.Student]
	java.lang.IllegalArgumentException: No converter found for return value of type: class com.xbm.domain.Student
		at org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:187)
		at org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.handleReturnValue(RequestResponseBodyMethodProcessor.java:174)
		at org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:81)
		at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:132)
		at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:827)
		at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:738)
		at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85)
		at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:963)
		at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:897)
		at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970)
		at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:861)
		at javax.servlet.http.HttpServlet.service(HttpServlet.java:529)
		at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846)
		at javax.servlet.http.HttpServlet.service(HttpServlet.java:623)
		at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:209)
		at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:153)
		at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:51)
		at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:178)
		at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:153)
		at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:197)
		at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
		at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:178)
		at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:153)
		at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:167)
		at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:90)
		at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:481)
		at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:130)
		at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:93)
		at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:673)
		at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74)
		at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343)
		at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:390)
		at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:63)
		at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:926)
		at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1791)
		at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:52)
		at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191)
		at org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659)
		at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
		at java.base/java.lang.Thread.run(Thread.java:833)

问题分析：为什么这里会报这个错？？
先看测试代码 ：

package com.xbm.controler;

import com.xbm.domain.Student;
import com.xbm.services.StrudentService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.ResponseBody;
import org.springframework.web.bind.annotation.RestController;

import javax.servlet.http.HttpServletRequest;

@RestController
public class RestStudentController {

    @Autowired
    private StrudentService service;
    /**
     * http://127.0.0.1:8080/getStudent.do?id=101
     * @param id
     * @param request
     * @return
     */
    @RequestMapping("/getStudent")
    @ResponseBody
    public Student getStudent(String id, HttpServletRequest request) {
        Student student = service.getStudent(id);
        return student;
    }
}

从上面的代码可以看出我的目的是为了通过请求返回student 这个Bean 的信息，但是却报了No converter found for return value of type: class com.xbm.domain.Student 这个问题 ，从字面意思很好理解，就是缺少转换器，没有办法将 com.xbm.domain.Student 转化为json。

因为要解决这个问题，方案就是给出一个转换器即可!
了解springMVC的 老铁都知道，SpringMVC 是有实现这个功能的 ，spring mvc默认使用的是用jackson来转换json的。如果没有的话，就会去找其他自定义的实现方法，这里既没有默认的又没有自定义的，所以就报错了。

解决方案：
方案1：
在pom.xml 中 引入默认的jackson 相关包

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-annotations</artifactId>
    <version>2.12.2</version>
</dependency>



<!-- https://mvnrepository.com/artifact/com.fasterxml.jackson.core/jackson-core 
Core Jackson processing abstractions (aka Streaming API), implementation for JSON  -->
<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-core</artifactId>
    <version>2.12.2</version>
</dependency>


<!-- https://mvnrepository.com/artifact/com.fasterxml.jackson.core/jackson-databind 
General data-binding functionality for Jackson: works on core streaming API  -->
<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.12.2</version>
</dependency>


<!-- https://mvnrepository.com/artifact/com.fasterxml.jackson.module/jackson-module-paranamer -->
<dependency>
    <groupId>com.fasterxml.jackson.module</groupId>
    <artifactId>jackson-module-paranamer</artifactId>
    <version>2.12.2</version>
</dependency>

亲测，引入后问题解决 ！

方案二：就是自己写一个将对象转换为json 的方法，然后引入。