java - 数据库项目中线程 "main"java.lang.NullPointerException 中的异常

Question

我正在尝试查看数据库中的所有数据或向数据库中插入数据。两种方法都给出相同的响应。我的 pojo 类:

public class Title {

private Integer id;
private String name;
private String code;

public Integer getId() {
    return id;
}

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

public String getName() {
    return name;
}

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

public String getCode() {
    return code;
}

public void setCode(String code) {
    this.code = code;
}

}

我的添加和显示方法的服务类:

public abstract class TitleService {

 public static void addTitle(Title title) throws Exception {

    Connection c = Global.createConnection();
    //Title title=new Title();
    PreparedStatement ps = c.prepareStatement(
            "insert into Title (id, name, code) values (?, ?, ?)");
    ps.setInt(1, title.getId());
    ps.setString(2, title.getName());   
    ps.setString(3, title.getCode());

    ps.executeUpdate();
    c.commit();
    ps.close();
    c.close();
  }
public static List<Title> allTitles() throws SQLException {
    Connection c = Global.createConnection();
    Statement st = c.createStatement();
    ResultSet rs = st.executeQuery("SELECT * FROM Title");
    List <Title> titles = new ArrayList<>();
    Title title;
        while (rs.next()) {
            title = new Title();
            title.setId(rs.getInt(1));
            title.setName(rs.getString(2));
            title.setCode(rs.getString(3));
            titles.add(title);
        }

    st.close();
    c.close();
    return titles;
    }
}

测试类:

public class ServiceTest {


public static void main(String[] arga) throws SQLException, Exception {

    Title t = new Title();
    t.setId(1);
    t.setCode("Prof");
    t.setName("Profesor");

    TitleService.addTitle(t);

    //TitleService.allTitles();

}

}

当我运行它给出的测试类时，

Exception in thread "main" java.lang.NullPointerException

at service.TitleService.addTitle(TitleService.java:22)

at test.ServiceTest.main(ServiceTest.java:26)

Java Result: 1

TitleService.java:22 ---> PreparedStatement ps = c.prepareStatement(
                "insert into Title (id, name, code) values (?, ?, ?)");

ServiceTest.java:26)---> TitleService.addTitle(t);
this exception.

不知道是什么问题

我的全局类敌人数据库连接:

公共(public)抽象类全局{

public static Connection createConnection() {
     Connection conn;
    System.out.println("Checking if Driver is registered with DriverManager.");
try{
        Class.forName("org.postgresql.Driver");
} catch (ClassNotFoundException e){
        System.out.println("Couldn't find the driver!");
        System.out.println("exit.");
        System.exit(1);
}
System.out.println("make a connection.");
try{
        conn = DriverManager.getConnection("jdbc:postgresql://localhost:5432/uni_db", "postgres", "postgres");
        System.out.println("Success.");
} catch(SQLException e){
        System.out.println("Couldn't connect: exit.");
        System.exit(1);}
    return null;
}    

Answer 1

您正在从 createConnection() 方法返回 null 值您应该返回 Connection 对象。

 return conn;