为什么conn为空public List<Bean_01_Student> select (){ List<Bean_01_Student> list=null; Bean_01_Student bean=null; try { conn=DBUtils.getConnection(); String sql="select * from student"; ps=conn.prepareStatement(sql); rs=ps.executeQuery(); while(rs.next()) { int stuid=rs.getInt(1); String stuname=rs.getString(2); String stusex=rs.getString(3); int stuage=rs.getInt(4); String sturemark=rs.getString(5); bean=new Bean_01_Student(stuid,stuname,stusex,stuage,sturemark); list.add(bean); } } catch (SQLException e) { // TODO Auto-generated catch block e.printStackTrace(); }finally { DBUtils.closeAll(conn, ps, rs); } return list; }

因为在方法中没有对list进行初始化，所以它的值为null。在使用list之前应该先进行初始化，例如将它初始化为一个空的ArrayList：

List<Bean_01_Student> list=new ArrayList<>();

相关问题

创建Bean查询数据库代码实现

以下是一个简单的Java代码示例，用于创建一个Bean类并从数据库中查询数据：

1.创建Bean类

public class Student {
private int id;
private String name;
private int age;
private String gender;

// 构造函数
public Student(int id, String name, int age, String gender) {
this.id = id;
this.name = name;
this.age = age;
this.gender = gender;
}

// Getter和Setter方法
public int getId() {
return id;
}

public void setId(int id) {
this.id = id;
}

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

public int getAge() {
return age;
}

public void setAge(int age) {
this.age = age;
}

public String getGender() {
return gender;
}

public void setGender(String gender) {
this.gender = gender;
}
}

2.查询数据库

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;

public class StudentDao {
// JDBC驱动器和数据库URL
static final String JDBC_DRIVER = "com.mysql.jdbc.Driver";
static final String DB_URL = "jdbc:mysql://localhost:3306/testdb";

// 数据库用户名和密码
static final String USER = "root";
static final String PASS = "123456";

public List<Student> getAllStudents() {
Connection conn = null;
PreparedStatement stmt = null;
ResultSet rs = null;
List<Student> students = new ArrayList<Student>();
try {
// 注册JDBC驱动器
Class.forName(JDBC_DRIVER);

// 打开一个连接
conn = DriverManager.getConnection(DB_URL, USER, PASS);

// 执行查询
String sql = "SELECT id, name, age, gender FROM students";
stmt = conn.prepareStatement(sql);
rs = stmt.executeQuery();

// 处理结果集
while (rs.next()) {
int id = rs.getInt("id");
String name = rs.getString("name");
int age = rs.getInt("age");
String gender = rs.getString("gender");
Student student = new Student(id, name, age, gender);
students.add(student);
}
} catch (SQLException se) {
se.printStackTrace();
} catch (Exception e) {
e.printStackTrace();
} finally {
// 清理资源
try {
if (rs != null) {
rs.close();
}
if (stmt != null) {
stmt.close();
}
if (conn != null) {
conn.close();
}
} catch (SQLException se) {
se.printStackTrace();
}
}
return students;
}
}

3.使用查询

public class Test {
public static void main(String[] args) {
StudentDao dao = new StudentDao();
List<Student> students = dao.getAllStudents();
for (Student student : students) {
System.out.println(student.getId() + ", " + student.getName() + ", " + student.getAge() + ", " + student.getGender());
}
}
}

以上代码将从数据库中检索所有学生，并将其打印到控制台。您可以根据需要进行修改以满足您的具体要求。