求助：在其他的檔案里創建了dealDatabase用來處理資料庫，然后打包成jar匯入到idea創建的javaweb專案中，但是在tomcat運行時idea一直報錯
顯示java.lang.NullPointerException: Cannot invoke "java.sql.Connection.prepareStatement(String)" because "conn" is null
但是在原來的其他檔案中一直都能夠運行
tomcat的lib中也加入了mysql-connector.jar
使用tomcat版本8.5.61，jdk15,mysql5.7
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.StringTokenizer;
public class dealDatabase {
static final String JDBC_Driver = "com.mysql.jdbc.Driver";
static final String DB_URL = "jdbc:mysql://localhost:3306/webdata?serverTimezone=UTC&useSSL=false";
static final String USER = "root";
static final String PASS = "39393939";
public dealDatabase() {
}
public static void main(String[] args) {
dealDatabase hhh = new dealDatabase();
System.out.println(hhh.getMaxFrame(1));
}
//向資料庫中增加內容
public void addOnestep(int step, int[] matrix, int[] numberOfNote, int remark, double[] degree, double avR, double cc, int noteNum, int sideNum) {
Connection conn = null;
String query = "insert into graph(id,matrix,note,remarks,degree,avR,cc,noteNum,sideNum) values(?,?,?,?,?,?,?,?,?)";
try {
conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/webdata?serverTimezone=UTC&useSSL=false", "root", "39393939");
} catch (SQLException var17) {
var17.printStackTrace();
}
try {
PreparedStatement preparedstmt = conn.prepareStatement(query);
preparedstmt.setInt(1, step);
preparedstmt.setString(2, this.toStringMethod(matrix));
preparedstmt.setString(3, this.toStringMethod(numberOfNote));
preparedstmt.setInt(4, remark);
preparedstmt.setString(5, this.toStringMethod(degree));
preparedstmt.setDouble(6, avR);
preparedstmt.setDouble(7, cc);
preparedstmt.setInt(8, noteNum);
preparedstmt.setInt(9, sideNum);
preparedstmt.executeUpdate();
preparedstmt.close();
} catch (Exception var16) {
var16.printStackTrace();
}
try {
conn.close();
} catch (SQLException var15) {
var15.printStackTrace();
}
}
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