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這是我的CreateQuery.java類。在postgres上運行JDBC時出錯
package DbConnect;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
public class CreateQuery {
Connection conn;
public CreateQuery() throws ClassNotFoundException, SQLException, IOException {
conn=new DbAccess().returnDatabaseConnection();
}
public int addNewLayertoDB(){
try {
PreparedStatement statement = null;
//String table_name = feature_name + "_" + shape;
String query = "SELECT the_geom from bbmp ";
statement = conn.prepareStatement(query);
//statement.setString(1, feature_name);
ResultSet rs = statement.executeQuery();
rs.close();
return 1;
} catch (SQLException ex) {
System.out.println("Sql exception");
return 0;
}
}
public void closeConn() throws SQLException {
if (conn != null) {
this.conn.close();
}
}
public static void main(String args[]) throws FileNotFoundException, IOException, ClassNotFoundException, SQLException{
CreateQuery cq = new CreateQuery();
cq.addNewLayertoDB();
cq.closeConn();
}
}
這是我的數據庫連接類
package DbConnect;
import java.sql.DriverManager;
import java.sql.Connection;
import java.sql.SQLException;
public class DbAccess{
public static void main(String[] argv) {
System.out.println("-------- PostgreSQL " +
"JDBC Connection Testing ------------");
try {
Class.forName("org.postgresql.Driver");
} catch (ClassNotFoundException e) {
System.out.println("Where is your PostgreSQL JDBC Driver? " +
"Include in your library path!");
e.printStackTrace();
return;
}
System.out.println("PostgreSQL JDBC Driver Registered!");
Connection connection = null;
try {
connection = DriverManager.getConnection(
"jdbc:postgresql://127.0.0.1:5432/Ethermap","postgres", "*******");
} catch (SQLException e) {
System.out.println("Connection Failed! Check output console");
e.printStackTrace();
return;
}
if (connection != null){
System.out.println("You made it, take control your database now!");
}else{
System.out.println("Failed to make connection!");
}
}
public Connection returnDatabaseConnection() {
System.out.println("DB not connected");
return null;
}
}
我得到的錯誤，當我運行的createQuery是
DB not connected
Exception in thread "main" java.lang.NullPointerException
at DbConnect.CreateQuery.addNewLayertoDB(CreateQuery.java:24)
at DbConnect.CreateQuery.main(CreateQuery.java:45)
什麼錯誤？我如何調試它？