我正在尝试在单个数据库连接中进行不同的查询，但我找不到方法。 查询是根据之前结果的wery的函数。 这是代码：
String query_disco="SELECT * FROM disco";
String name = "";
String adress="";
String web="";
int disco_id=0;
String query_disco_var="SELECT * FROM disco_var";
float price=0;
String style="";
String hin="";
String hout="";
String party_name="";
int day=0;
String docType="";
String docTable="";
String docBody="";
Calendar calendar = Calendar.getInstance();
try {
DAO a= new DAO();
ResultSet b =a.executarSQL(query_disco);
day = calendar.get(Calendar.DAY_OF_WEEK);
docType ="<!DOCTYPE HTML PUBLIC\"--�//W3C//DTD HTML 4.0 " +"Transitional//EN\">\n";
docBody =
"<HTML>\n"+
"<HEAD><TITLE>HelloWorld!</TITLE></HEAD>\n"+
"<BODY BGCOLOR=\"#FDF5E6\">\n"+
"<H1>Disco</H1>\n"+
"<table border=1>"+
"<tr> <th> name </th> <th> adress </th> <th> web </th> <th> price </th>" +
"<th> style </th> <th> horari </th> <th> party name </th> </tr>";
while(b.next()){
disco_id=b.getInt("disco_id");
name=b.getString("name");
adress=b.getString("adress");
web=b.getString("web");
query_disco_var = query_disco_var + " WHERE disco_id = " + disco_id + " AND day = " + day;
DAO d= new DAO();
ResultSet c = d.executarSQL(query_disco_var);
price=c.getFloat("price");
style=c.getString("style");
hin=c.getString("hin");
hout=c.getString("hout");
party_name=c.getString("party_name");
docTable = docTable + "<tr> <td> "+name+" </td> <td> "+adress+" </td> <th>" +web+ "</td> </tr>"+
price+ "</td> <td>" +style+ "</td> <td>" +hin+ "-" +hout+ "</td> <td>" +party_name+ "</td> </tr>";
query_disco_var="SELECT * FROM disco_var";
}
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
out.println(docType+docBody+docTable+
"</table>" +
"</BODY></HTML>");
}
/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
}
}
与de db的连接适用于此脚本：
public class DAO {
private Connection connexio;
private Statement sentencia;
/**
*
* @throws Exception
*/
public DAO() throws Exception {
String user="trmcfunt";
String password="44445555";
Class.forName("com.mysql.jdbc.Driver").newInstance();
connexio=
DriverManager.getConnection("jdbc:mysql://localhost/iguanaparty_db_test?user="+user+"&password="+password);
sentencia=connexio.createStatement();
}
/**
*
* @param query
* @return
* @throws SQLException
*/
public ResultSet executarSQL(String query)throws SQLException{
return sentencia.executeQuery(query);
}
/**
*
* @throws SQLException
*/
public void desconnectarBD() throws SQLException {
sentencia.close();
connexio.close();
}
}
我正在接受这个问题：
java.sql.SQLException: Before start of result set
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1074)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:988)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:974)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:919)
at com.mysql.jdbc.ResultSetImpl.checkRowPos(ResultSetImpl.java:854)
at com.mysql.jdbc.ResultSetImpl.getStringInternal(ResultSetImpl.java:5772)
at com.mysql.jdbc.ResultSetImpl.getString(ResultSetImpl.java:5692)
at com.mysql.jdbc.ResultSetImpl.getFloat(ResultSetImpl.java:2615)
at com.mysql.jdbc.ResultSetImpl.getFloat(ResultSetImpl.java:2635)
at controllers.HelloWorld.doGet(HelloWorld.java:87)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:222)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:123)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:171)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:99)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:953)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1023)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:589)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:310)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:724)
问候。
答案 0 :(得分：2)
ResultSet c = d.executarSQL(query_disco_var);
price = c.getFloat("price");
在从结果集中获取值之前，必须调用next（），将光标放在第一行上。否则，如异常中所述，您在结果集开始之前：
ResultSet c = d.executarSQL(query_disco_var);
if (c.next()) {
price = c.getFloat("price");
另外，了解prepared statements，因为通过连接参数将参数传递给查询会使您的代码容易受到SQL注入攻击，广告只会在一个字符串参数包含单个参数时立即中断报价。
答案 1 :(得分：0)
您的ResultSet c永远不会移动到第一个结果（如果有的话）。你应该使用类似
if (c.next()) {
// your original code here
} else {
// whatever you want to do in this case
}
此外，如果找到多个结果，您可能需要考虑该怎么做。
答案 2 :(得分：0)
执行任何SQL后，光标位于之前第一行。这就是为什么你得到了execpetion，因为没有可用的数据。
执行第二个SQL查询后尝试c.next();。这会将光标定位在第一个结果上。请记住，如果没有返回数据，此调用将返回false，并且任何后续的列提取都将失败，因此您应该抓住它。