HttpMessageNotWritableException：未找到类型为java的返回值的转换器。util。Spring MVC中的HashMap

Question

我试图在控制器中的一个方法中使用这样的HashMap返回json

//list all users
    @RequestMapping(value = "/user",method = RequestMethod.GET)
    public ResponseEntity<Map<String,Object>> handleRequest() throws Exception{
        List<User> users = userService.list();          
        Map<String, Object> message = new HashMap<String, Object>();

        message.put("severity", "info");
        message.put("user", users);
        message.put("summary", "Not successfully.");
        message.put("code", 200);

        Map<String, Object> json = new HashMap<String, Object>();
        json.put("success", true);
        json.put("message", message);

        return new ResponseEntity<Map<String,Object>>(message,HttpStatus.CREATED);
    }

当我在PostMan中点击这个方法时，我得到了这个错误

org.springframework.http.converter.HttpMessageNotWritableException: No converter found for return value of type: class java.util.HashMap
org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:226)
org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.handleReturnValue(RequestResponseBodyMethodProcessor.java:180)
org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:82)
org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:119)
org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:877)
org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:783)
org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87)
org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:991)
org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:925)
org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:974)
org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:866)
javax.servlet.http.HttpServlet.service(HttpServlet.java:635)
org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:851)
javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

这就是我设置应用程序的方式

@Configuration
@ComponentScan("com.fyt.fyt")
@EnableTransactionManagement
public class ApplicationContextConfig {


    @Bean
    public InternalResourceViewResolver getViewResolver() {
        InternalResourceViewResolver internalResourceViewResolver = new InternalResourceViewResolver();
        internalResourceViewResolver.setPrefix("/WEB-INF/views/");
        internalResourceViewResolver.setSuffix(".jsp");
        return internalResourceViewResolver;
    }

    @Bean
    public DataSource getDataSource() {
        BasicDataSource dataSource = new BasicDataSource();
        dataSource.setDriverClassName("com.mysql.jdbc.Driver");
        dataSource.setUrl("jdbc:mysql://localhost:3306/fyt");
        dataSource.setUsername("root");
        dataSource.setPassword("");
        return dataSource;
    }

    @Bean
    public LocalSessionFactoryBean getSessionFactory() {

        LocalSessionFactoryBean factoryBean = new LocalSessionFactoryBean();
        factoryBean.setDataSource(getDataSource());
        factoryBean.setHibernateProperties(getHibernateProperties());
        factoryBean.setAnnotatedClasses(User.class);
        return factoryBean;

    }

    private Properties getHibernateProperties() {
        Properties properties = new Properties();
        properties.put("hibernate.show_sql", "true");
        properties.put("hibernate.dialect", "org.hibernate.dialect.MySQLDialect");
        return properties;
    }

    @Bean
    public HibernateTransactionManager getTransactionManager() {
        HibernateTransactionManager transactionManager = new HibernateTransactionManager();
        transactionManager.setSessionFactory(getSessionFactory().getObject());
        return transactionManager;
    }

    @Bean
    public UserDao userDao() {
        return new UserDaoImpl();
    }
}

错误说，它不能将列表转换为httpMessges，所以我需要任何转换器或类似的东西吗？我错过了什么？

Answer 1

我认为您需要定制httpmessageconvertor来实现这一点
添加依赖项

<dependency>
     <groupId>com.fasterxml.jackson.core</groupId>
     <artifactId>jackson-databind</artifactId>
     <version>2.5.0</version>
</dependency>

如果这不起作用，请尝试在配置文件中配置将Jackson映射到HttpMessage转换器的自定义配置。

Answer 2

对于有类似问题的人：

如果返回的类的对象没有Getter和Setter方法，则可能会发生这种情况。

例如：如果您有以下设置

public class Employee {
    private int code;
    private String name;
}

并以某种方式使用它：

@GetMapping("")
public ResponseEntity<Employee> getEmployee(@RequestParam("id") Integer id) {
    Employee employee;
    /* CODE. . . 
                */
    return new ResponseEntity<>(employee, HttpStatus.OK);
 }

你可能会得到一个错误

org.springframework.http.converter.HttpMessageNotWritableException：未找到返回值类型：类com.dummy.springWeb. ExceptionHandler.员工的转换器

要解决这个问题，只需在employee类中添加getter和setter，它就能解决这个问题！

Answer 3

在控制器类级别提供@RestController，在方法级别提供@ResponseBody。确保所有与jackson相关的JAR都在类路径中。

在pom中添加与jackson相关的依赖项。xml

@RequestMapping(value = "/user",method = RequestMethod.GET)
@ResponseBody
    public ResponseEntity<Map<String,Object>> handleRequest() throws Exception{
        List<User> users = userService.list();          
        Map<String, Object> message = new HashMap<String, Object>();

        message.put("severity", "info");
        message.put("user", users);
        message.put("summary", "Not successfully.");
        message.put("code", 200);

        Map<String, Object> json = new HashMap<String, Object>();
        json.put("success", true);
        json.put("message", message);

        return new ResponseEntity<Map<String,Object>>(message,HttpStatus.CREATED);
    }

jackson的pom依赖性示例。

<dependency>
        <groupId>com.fasterxml.jackson.core</groupId>
        <artifactId>jackson-core</artifactId>
        <version>2.8.1</version>
    </dependency>
    <dependency>
        <groupId>com.fasterxml.jackson.core</groupId>
        <artifactId>jackson-databind</artifactId>
        <version>2.8.1</version>
    </dependency>
    <dependency>
        <groupId>org.codehaus.jackson</groupId>
        <artifactId>jackson-mapper-asl</artifactId>
        <version>1.9.13</version>
        </dependency>