Dao造函数中jdbcTemplate的NullPointerException

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1

我是Spring Boot的新手，正试图为Spring Boot JDBC实现以下代码。在运行应用程序时，我在UserDao构造函数中的this.jdbcTemplate得到一个NullPointerException。我认为这个错误是因为UserDao构造函数是在JdbcTemplate实例化之前被调用的，这对吗？到现在为止我还没有遇到过这种错误，@Autowired不能解决依赖关系。谁能解释一下这个错误的原因？

SpringBootDatabaseApplication.java

package com.mrityu.springbootdatabase;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class SpringBootDatabaseApplication {
    
    @Autowired
    private UserDao userDao;
    public static void main(String[] args) {
        SpringApplication.run(SpringBootDatabaseApplication.class, args);
    }

}

UserDao.java

package com.mrityu.springbootdatabase;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.jdbc.core.JdbcTemplate;
import org.springframework.stereotype.Repository;

@Repository
public class UserDao {
   @Autowired
   private JdbcTemplate jdbcTemplate;

   public UserDao() {
        String query = "create table if not exists User(id int primary key, name varchar(200))";
        int update = this.jdbcTemplate.update(query);
        System.out.println("Constructor Called: " + update);
   }


}

StackOverflow：spring - NullPointerException of JDBC Template in DAO Constructor - Stack Overflow