问题
I've a strange situation with a little application in Java using a JDBC-OBDC. I'm inspecting a Database using the DatabaseMetaData Class. When I execute the program, everything works without anyproblem. But when I want to debug to see the values inside the Resulset containing the DatabaseMetaData a java.sql.SQLException is thrown only if I put a breakpoint within the while. Here's my code:
DatabaseMetaData patrol = con.getMetaData();
ResultSet answer = patrol.getTables(null, null, null, null);
while(answer.next()) {
if (answer.wasNull() == false) {
tableNamesAsOne = tableNamesAsOne + answer.getString("TABLE_NAME") + " ";
}
}
answer.close();
Why I cannot put my breakpoint in this section of code??
This is the printStackTrace.
Exception in thread "main" java.sql.SQLException: No data found
at sun.jdbc.odbc.JdbcOdbc.standardError(Unknown Source)
at sun.jdbc.odbc.JdbcOdbc.SQLGetDataString(Unknown Source)
at sun.jdbc.odbc.JdbcOdbcResultSet.getDataString(Unknown Source)
at sun.jdbc.odbc.JdbcOdbcResultSet.getString(Unknown Source)
at sun.jdbc.odbc.JdbcOdbcResultSet.getString(Unknown Source)
at Snooper.inspect(Snooper.java:56)
at Snooper.<init>(Snooper.java:26)
at Snooper.createAndShowGUI(Snooper.java:112)
at Snooper.main(Snooper.java:125)
Line Snooper.java:56 in my code refers to
tableNamesAsOne = tableNamesAsOne + answer.getString("TABLE_NAME") + " ";
Thanks.
回答1:
I have installed SQL Server to reproduce your problem and verify it.
Short explanation
You must read the values ONLY ONCE and in the ORDER they appear in the SELECT. JdbcOdbc sucks. While debugging, you're reading them multiple times.
Long explanation
What you are doing is inspecting a stateful object in the debugger, which leads to dynamic results.
In this case it's the sun.jdbc.odbc.JdbcOdbcResultSet and executing the expression resultSet.getString(...) multiple times. The first time, it will work (in case your breakpoint suspends the Thread before the resultSet is asked). Then, the second time you (or your debugger) inspects the value of the expression again, the getString() method is called again and this method changes the internal state of the ResultSet object.
Although the method's name suggests it's a simple getter, it's not. It does more than that. It may actually retrieve the data from the database, change its internal position counters etc. You cannot execute the getter methods multiple times.
The ODBC Driver is a very bad thing and of low quality. Expect strange behavior and other dragons. You can get some debug information by enabling JdbcOdbc Tracing. That is done by setting a LogWriter on the DriverManager, before the JdbcOdbc-Bridge is activated:
java.sql.DriverManager.setLogWriter(new PrintWriter(System.out));
Then, you will get verbose debugging output of the JdbcOdbc-Driver like the following. It may help you to debug the problem you have. When debugging, just ensure to store the data retrieved from the ResultSet objects in local objects, so you can inspect them multiple times in the debugger.
DriverManager.getConnection("jdbc:odbc:testdbodbc")
JdbcOdbcDriver class loaded
registerDriver: driver[className=sun.jdbc.odbc.JdbcOdbcDriver,sun.jdbc.odbc.JdbcOdbcDriver@7b479feb]
DriverManager.initialize: jdbc.drivers = null
JDBC DriverManager initialized
trying driver[className=sun.jdbc.odbc.JdbcOdbcDriver,sun.jdbc.odbc.JdbcOdbcDriver@7b479feb]
*Driver.connect (jdbc:odbc:testdbodbc)
JDBC to ODBC Bridge: Checking security
No SecurityManager present, assuming trusted application/applet
JDBC to ODBC Bridge 2.0001
Current Date/Time: Wed Jan 26 00:31:27 CET 2011
Loading JdbcOdbc library
Allocating Environment handle (SQLAllocEnv)
hEnv=115724512
Allocating Connection handle (SQLAllocConnect)
hDbc=116219184
Connecting (SQLDriverConnect), hDbc=116219184, szConnStrIn=DSN=testdbodbc
RETCODE = 1
WARNING - Generating SQLWarning...
SQLWarning: reason([Microsoft][ODBC SQL Server Driver][SQL Server]Changed database context to 'master'.) SQLState(01000) vendor code(5701)
SQLWarning: reason([Microsoft][ODBC SQL Server Driver][SQL Server]Changed language setting to us_english.) SQLState(01000) vendor code(5703)
*Connection.getMetaData
*DatabaseMetaData.getDriverName
Get connection info string (SQLGetInfo), hDbc=116219184, fInfoType=6, len=300
SQLSRV32.DLL
*DatabaseMetaData.getDriverVersion
Get connection info string (SQLGetInfo), hDbc=116219184, fInfoType=7, len=300
06.01.7600
*DatabaseMetaData.getDriverName
Get connection info string (SQLGetInfo), hDbc=116219184, fInfoType=6, len=300
SQLSRV32.DLL
Driver name: JDBC-ODBC Bridge (SQLSRV32.DLL)
*DatabaseMetaData.getDriverVersion
P.S. And this was the reproduced exception, including line numbers of the Sun code for JDK 1.6.0_22. As you can see in the first line, this is what is printed out on the console when I inspected the getString() method.
Get string data (SQLGetData), hStmt=108067024, column=3, maxLen=257
RETCODE = 100
ERROR - No data found
java.sql.SQLException: No data found
at sun.jdbc.odbc.JdbcOdbc.standardError(JdbcOdbc.java:7138)
at sun.jdbc.odbc.JdbcOdbc.SQLGetDataString(JdbcOdbc.java:3907)
at sun.jdbc.odbc.JdbcOdbcResultSet.getDataString(JdbcOdbcResultSet.java:5698)
at sun.jdbc.odbc.JdbcOdbcResultSet.getString(JdbcOdbcResultSet.java:354)
at sun.jdbc.odbc.JdbcOdbcResultSet.getString(JdbcOdbcResultSet.java:411)
at sandbox.DatabaseMetadataTest.testDBMetadata(DatabaseMetadataTest.java:27)
回答2:
Yeah, the debugger runs in a different thread than the metadata obtained by con.getMetaData();... so, you know, it's a different transaction with a different metadata.
Well, ok, that would be my 1st guess. I have not looked into Obdc driver source code to confirm.
Edit: thanks for mhaller excellent remark a made a 2nd look/guess: you call wasNull() prematurely, it has meaning after some get operation of the ResultSet. Copy from javadoc
* Note that you must first call one of the getter methods * on a column to try to read its value and then call * the method <code>wasNull</code> to see if the value read was * SQL <code>NULL</code>.
phew, me sucks tonight!
来源：https://stackoverflow.com/questions/4795784/why-i-cannot-debug-a-databasemetadata