有一个连接到apache derby数据库的java类。突然停止工作,我不知道为什么。Java resultset.next()不返回结果
基本上客户端连接到服务器,服务器调用“联系人”类,然后联系人类查询数据库并返回结果。
下面是从Contact类代码:
import java.sql.*;
//Contact Class. Used to get information from the contact database.
//Takes the database connection and the student ID as arguments
public class Contact {
//Define the original variables
String student = null;
Connection db = null;
PreparedStatement selectStatement = null;
ResultSet resultSet = null;
StringBuilder result = null;
//Constructor method. Used to set passed arguments to class variables.
public Contact(Connection conn, String studentNumber)
{
this.student = studentNumber;
this.db = conn;
}
//getResult method used to query the database and return result.
public String getResult()
{
//Wrap it all in a try loop to catch sql errors.
try {
//Set up the statement, prepare the statement and set the variable.
String selectSQL = "SELECT * FROM Contact WHERE STUID = ?";
selectStatement = db.prepareStatement(selectSQL);
selectStatement.setString(1, this.student);
//Execute the query
resultSet = selectStatement.executeQuery();
//If there is no results, set up a string to return letting the user know.
this.result = new StringBuilder();
if(!resultSet.next())
{
result.append("No record for ");
result.append(this.student);
result.append(".");
}
else
{
//If there are results, loop through the result and build a string
//To be able to return to the user with the correct details.
System.out.println("FOUND A RESULT");
while(resultSet.next())
{
System.out.println("TEST");
System.out.println(resultSet.getString("STUID"));
result.append(resultSet.getString("STUID"));
result.append(" ");
result.append(resultSet.getString("STUNAME"));
result.append(" ");
result.append(resultSet.getString("ADDRESS"));
result.append(" ");
result.append(resultSet.getString("POSTCODE"));
result.append(" ");
result.append(resultSet.getString("EMAIL"));
}
}
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
//Return the built string with correct information.
return this.result.toString();
}
}
如果我输入一个STUID,是不是在数据库中,它成功地让我知道通过返回“没有记录ID”。但是,如果我输入数据库中的一个,它会打印“FOUND A RESULT”行(只是一个测试行),但从未真正进入“TEST”输出 - 因此从不会生成任何带有结果的字符串。
我知道这不是数据库,因为我测试了我的线程类里面的一些代码,查询同一个数据库(调用这个类接触前)工作原理:
Statement s = null;
ResultSet rs = null;
try{
s = dbConnection.createStatement();
rs = s.executeQuery("SELECT * FROM Contact");
while(rs.next())
{
System.out.println(rs.getString("STUID"));
}
}catch(SQLException e)
{
e.printStackTrace();
}
该测试代码的工作。所以我真的很困惑,为什么它可以成功地查询数据库,并找出没有结果(通过使用if(!resultSet.next()),但如果它实际上存在,实际上有因此,它不能管理遍历给我详细的任何帮助将不胜感激
这适用于一个结果 - 将其交换为if(resultSet.next())。 如果我返回了多个结果,我该怎么办? – CynePhoba12
每次调用ResultSet.Next()时,都会移至结果集中的下一行。 –