JDBC-ResultSet 在 while-Loop 中关闭 [英] JDBC-ResultSet is closed in while-Loop
问题描述
我在使用 ResultSet 时遇到了非常糟糕的情况,它在迭代此 ResultSet 的 while 循环中关闭.我知道 ResultSet 关闭的确切行,但我不知道为什么.
I'm having a really bad time with a ResultSet, which is closed within a while-Loop for iterating this ResultSet. I have know the exact line in which the ResultSet is closed, but i have no idea why.
public LinkedList<Athlet> alleAbrufen () throws SQLException {
LinkedList<Athlet> alleAthleten = new LinkedList<Athlet>();
String abrufenAthleten = "SELECT * FROM Athlet ORDER BY athlet_id";
ResultSet athleten_rs = stmt.executeQuery(abrufenAthleten);
while (athleten_rs.next()) {
long id = athleten_rs.getInt(1);
String name = athleten_rs.getString(2);
LinkedList<Leistung> alleLeistungen = alleAbrufen((int) (id)); //after this line the ResultSet gets closed
alleAthleten.add(new Athlet(id, name, alleLeistungen));
}
return alleAthleten;
}
public LinkedList<Leistung> alleAbrufen(int athlet_id) throws SQLException {
LinkedList<Leistung> alleLeistungen = new LinkedList<Leistung>();
String selectLeistungen = "SELECT * FROM Leistung WHERE athlet_id="+athlet_id;
ResultSet rs = stmt.executeQuery(selectLeistungen);
while (rs.next()) {
long id = rs.getInt(1);
String bezeichnung = rs.getString(2);
String datum = rs.getString(3);
double geschwindigkeit = rs.getDouble(4);
boolean selectedForSlopeFaktor = rs.getBoolean(5);
int strecke_id = rs.getInt(7);
long longAthlet_id = (long) athlet_id;
Leistung leistung = new Leistung(strecke_id, longAthlet_id, bezeichnung, datum, geschwindigkeit);
leistung.setLeistungID(id);
leistung.setIsUsedForSlopeFaktor(selectedForSlopeFaktor);
alleLeistungen.add(leistung);
}
return alleLeistungen;
}
我用注释标记了 ResultSet
结束后的那一行.Alle
上面例子中使用的其他方法、构造函数等都经过测试可以正常工作.有谁知道为什么调用第二个方法会关闭第一个方法中的 ResultSet
?
I marked the line after which the ResultSet
is closed with a comment. Alle
other methods, constructors, etc used in the above example are tested an working properly. Does anyone have a clue why calling the second method closes the ResultSet
in the first method?
推荐答案
问题是Statement
只能维护一组ResultSet
的每个执行的语句.由于您为两个方法共享相同的 Statement stmt
,在 alleAbrufen
中,Statement
执行另一个语句,这将破坏对先前 <代码>结果集代码>.
The problem is that the Statement
can only maintain a single group of ResultSet
s per executed statement. Since you share the same Statement stmt
for your two methods, in alleAbrufen
the Statement
executes another statement, which will break the reference to the prior ResultSet
.
这种情况的最佳解决方案是为每个语句执行创建一个 Statement
.也就是说,每个方法都应该包含其唯一的Statement
和相关的ResultSet
.
The best solution for this case is to create a Statement
per statement execution. This is, every method should contain its unique Statement
and related ResultSet
s.
public LinkedList<Athlet> alleAbrufen () throws SQLException {
LinkedList<Athlet> alleAthleten = new LinkedList<Athlet>();
String abrufenAthleten = "SELECT * FROM Athlet ORDER BY athlet_id";
//here
Statement stmtAlleAbrufen = con.createStatement();
ResultSet athleten_rs = stmtAlleAbrufen.executeQuery(abrufenAthleten);
while (athleten_rs.next()) {
long id = athleten_rs.getInt(1);
String name = athleten_rs.getString(2);
LinkedList<Leistung> alleLeistungen = alleAbrufen((int) (id)); //after this line the ResultSet gets closed
alleAthleten.add(new Athlet(id, name, alleLeistungen));
}
return alleAthleten;
}
public LinkedList<Leistung> alleAbrufen(int athlet_id) throws SQLException {
LinkedList<Leistung> alleLeistungen = new LinkedList<Leistung>();
//here again, but since you need to use parameters in your query
//use PreparedStatement instead
//note that I commented the current query
//String selectLeistungen = "SELECT * FROM Leistung WHERE athlet_id="+athlet_id;
//this is how a query with parameters look like
String selectLeistungen = "SELECT * FROM Leistung WHERE athlet_id=?";
//the connection prepares the statement
PreparedStatement pstmt = con.prepareStatement(selectLeistungen);
//then we pass the parameters
pstmt.setInt(1, athlet_id);
ResultSet rs = pstmt.executeQuery();
while (rs.next()) {
long id = rs.getInt(1);
String bezeichnung = rs.getString(2);
String datum = rs.getString(3);
double geschwindigkeit = rs.getDouble(4);
boolean selectedForSlopeFaktor = rs.getBoolean(5);
int strecke_id = rs.getInt(7);
long longAthlet_id = (long) athlet_id;
Leistung leistung = new Leistung(strecke_id, longAthlet_id, bezeichnung, datum, geschwindigkeit);
leistung.setLeistungID(id);
leistung.setIsUsedForSlopeFaktor(selectedForSlopeFaktor);
alleLeistungen.add(leistung);
}
return alleLeistungen;
}
使用后不要忘记关闭资源,Statement
和 ResultSet
.
Don't forget to close the resources, Statement
and ResultSet
, after using them.
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