有一個連接到apache derby數據庫的java類。突然停止工作,我不知道爲什麼。Java resultset.next()不返回結果
基本上客戶端連接到服務器,服務器調用「聯繫人」類,然後聯繫人類查詢數據庫並返回結果。
下面是從Contact類代碼:
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();
}
}
如果我輸入一個STUID,是不是在數據庫中,它成功地讓我知道通過返回「沒有記錄ID」。但是,如果我輸入數據庫中的一個,它會打印「FOUND A RESULT」行(只是一個測試行),但從未真正進入「TEST」輸出 - 因此從不會生成任何帶有結果的字符串。
我知道這不是數據庫,因爲我測試了我的線程類裏面的一些代碼,查詢同一個數據庫(調用這個類接觸前)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();
}
該測試代碼的工作。所以我真的很困惑,爲什麼它可以成功地查詢數據庫,並找出沒有結果(通過使用if(!resultSet.next()),但如果它實際上存在,實際上有因此,它不能管理遍歷給我詳細的任何幫助將不勝感激
這適用於一個結果 - 將其交換爲if(resultSet.next())。 如果我返回了多個結果,我該怎麼辦? – CynePhoba12
每次調用ResultSet.Next()時,都會移至結果集中的下一行。 –