Spring JDBCTemplate.空指针异常 [英] Spring JDBCTemplate. Null pointer exception

问题描述

我正试图从头开始建立一个SpringMVC网站，但是我走到了穷途末路.

I'm trying to set up a SpringMVC website from scratch, but I've hit a dead end.

我正在使用自动装配来实例化具有数据源的JdbcTemplate，但是不知何故我遇到了Null指针异常.非常感谢您的帮助.

I'm using autowiring to instanciate JdbcTemplate with a DataSource, but somehow I'm getting a Null pointer exception. I'd appreciate your help with this.

下一个是我的AppConfig:

My AppConfig is the next:

@Configuration
@ComponentScan
public class AppConfig {
    @Bean
    public DriverManagerDataSource dataSource() {
        DriverManagerDataSource dataSource = new DriverManagerDataSource();
        dataSource.setDriverClassName("com.mysql.jdbc.Driver");
        dataSource.setUrl("jdbc:mysql://localhost:3306/onlinelibrary");
        dataSource.setUsername("root");
        dataSource.setPassword("root");
        return dataSource;
    }

    @Bean
    public JdbcTemplate jdbcTemplate(DataSource dataSource) {
        return new JdbcTemplate(dataSource);
    }
    /*Deleted this code, still doesn't work
    @Bean
    public Book Book() {
        return new Book();
    }
    */
}

我的书"类如下:

@Component
public class Book {
    private JdbcTemplate jdbcTemplate;
    private String title;
    private String author;
    private String isbn;

    public Book() {

    }

    @Autowired
    public Book(JdbcTemplate jdbcTemplate) {
        this.jdbcTemplate = jdbcTemplate;
    }

    public ModelMap getBooks() {
        ModelMap model = new ModelMap();
        String sql = "SELECT * FROM Books";
        model.put("data", jdbcTemplate.queryForList(sql));
        return model;
    }
}

这是臭名昭著的NullPointer异常:

And this is the infamous NullPointer Exception:

任何帮助将不胜感激.我可能忘了做某事，但我自己无法解决，而且我在StackOverflow上也找不到任何对我有帮助的东西(尽管到目前为止我已经阅读了许多文章).

Any help would be highly appreciated. I probably forgot to do something, but I can't solve it myself, and I can't find anything on StackOverflow that helps me, either (although I've read many articles by now).

更新更多数据:

我的项目结构是下一个:

My project structure is the next:

我正在此控制器中使用Book对象:

And I'm using the Book object in this controller:

@Controller
public class BookController {
    @RequestMapping(value = "/", method = RequestMethod.GET)
    public String getBookData(Book book, ModelMap model) {
        model.put("data", book.getBooks());
        return "BookView";
    }
}

推荐答案

当您在该类上拥有@Component时，这意味着Spring将为您创建一个Bean，前提是您的组件扫描程序正在扫描 Book 班级.您不需要

When you have @Component over the class, it means Spring will create a Bean for you provided your component scanner is scanning Book class. You don't need

@Bean
public Book Book() {
    return new Book();
}

由于此bean没有注入jdbcTemplate而抛出NullPointerException.

It's because of this bean that doesn't have jdbcTemplate injected which is throwing NullPointerException.

更新:

您对弹簧注射的理解是错误的.我已经更新了应该工作的控制器代码.

Your understanding about spring injection is wrong. I have updated the controller code that should work.

@Controller
public class BookController {

    @Autowired
    Book book;

    @RequestMapping(value = "/", method = RequestMethod.GET)
    public String getBookData(ModelMap model) {
        model.put("data", book.getBooks());
        return "BookView";
    }
}

更新:组件扫描

@ComponentScan(basePackages = "models")
public class AppConfig {

这篇关于Spring JDBCTemplate.空指针异常的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！