Simple JDBC problem - Connection throws Nullpointerexception
Adam Haglund
Greenhorn
Posts: 7
posted 8 years ago
Hi guys,
I'm am currently trying to learn Java and want to connect to a Postgresql database.
I am using a simple JSP page and a bean which contains all database functions. This is set up in Tomcat and JDK/1.6.0. Everything works fine but the connection itself, for some reason it throws a null value.
I suspect that the problem is the connection configuration, in tomcat/conf/context.xml.
Connecting to the database from terminal works fine, using psql -h etc...
This is what I have:
tomcat/conf/context.xml
tomcat/webapps/ROOT/WEB-INF/web.xml
tomcat/webapps/ROOT/WEB-INF/classes/bean/db.java
tomcat/webapps/ROOT/login.jsp
Running login.jsp returns error here:
Error output:
What am I doing wrong? Help is very much appreciated.
Thank you.
I'm am currently trying to learn Java and want to connect to a Postgresql database.
I am using a simple JSP page and a bean which contains all database functions. This is set up in Tomcat and JDK/1.6.0. Everything works fine but the connection itself, for some reason it throws a null value.
I suspect that the problem is the connection configuration, in tomcat/conf/context.xml.
Connecting to the database from terminal works fine, using psql -h etc...
This is what I have:
tomcat/conf/context.xml
tomcat/webapps/ROOT/WEB-INF/web.xml
tomcat/webapps/ROOT/WEB-INF/classes/bean/db.java
tomcat/webapps/ROOT/login.jsp
Running login.jsp returns error here:
Error output:
What am I doing wrong? Help is very much appreciated.
Thank you.
Adam Haglund
Greenhorn
Posts: 7
Adam Haglund
Greenhorn
Posts: 7
posted 8 years ago
Hi again,
I did not get the e.printStacktrace() to work (compilation error, missing symbol), but this code compiled fine:
However, the only difference from the previous error is that the jsp error row (out.println(db.loginUser("adam","test")); ) wasn't mentioned.
Still getting this stacktrace:
The NullpointerException is still pointing to the conn.createStatement(); row.
What can I do?
Thank you for your patience.
I did not get the e.printStacktrace() to work (compilation error, missing symbol), but this code compiled fine:
However, the only difference from the previous error is that the jsp error row (out.println(db.loginUser("adam","test")); ) wasn't mentioned.
Still getting this stacktrace:
The NullpointerException is still pointing to the conn.createStatement(); row.
What can I do?
Thank you for your patience.
posted 8 years ago
Use e.printStackTrace() outside System.out.println , as it returns nothing..
OK , Add the Proper Exceptions handlers in both of you methods.
Its because is conn object is null. So should look in constructor itself where the conn is intialised. Add Exception as below.
Please post complete Sys out log on whats happening before the exception.
Refer this this for checking basic setup
I did not get the e.printStacktrace() to work (compilation error, missing symbol), but this code compiled fine:
Use e.printStackTrace() outside System.out.println , as it returns nothing..
OK , Add the Proper Exceptions handlers in both of you methods.
The NullpointerException is still pointing to the conn.createStatement(); row.
Its because is conn object is null. So should look in constructor itself where the conn is intialised. Add Exception as below.
Please post complete Sys out log on whats happening before the exception.
Refer this this for checking basic setup
Adam Haglund
Greenhorn
Posts: 7
posted 8 years ago
I tried the code with the new catch statements. The strange thing is that it no longer returns an error at all, the page loads fine and the function returns false. (It should return true)
This is the complete code for db.java now:
I don't know if there is a way to make the function print my query, but System.out.println did not work (nothing was printed).
Though, I am pretty sure the sql is ok, because when I changed the last return to true, the whole function returned true.
Btw, the reference looked good, I have read the part about Postgresql but I don't think I have missed anything in the setup.
This is the complete code for db.java now:
I don't know if there is a way to make the function print my query, but System.out.println did not work (nothing was printed).
Though, I am pretty sure the sql is ok, because when I changed the last return to true, the whole function returned true.
Btw, the reference looked good, I have read the part about Postgresql but I don't think I have missed anything in the setup.
posted 8 years ago
BTW , did you call close() to close the connection. Otherwise you get such abnormal behaviour
Is it not in the console or catalina.out ? where did you check.. did you route to different log ?
IMO this is not the correct design at all. you can have db.java as utility class for serving up connections and what ever you use in jsp:usebean should be a bean which follows bean standards.
BTW , did you call close() to close the connection. Otherwise you get such abnormal behaviour
don't know if there is a way to make the function print my query, but System.out.println did not work (nothing was printed).
Is it not in the console or catalina.out ? where did you check.. did you route to different log ?
IMO this is not the correct design at all. you can have db.java as utility class for serving up connections and what ever you use in jsp:usebean should be a bean which follows bean standards.
Adam Haglund
Greenhorn
Posts: 7
posted 8 years ago
I added the finally clause to the function like this:
This returned an error at conn.close(); as conn is null.
I have also attached the last log entry in catalina.out to this post, maybe you could translate to english and see what is wrong
Catalina.out
To me, it still looks like a configuration problem, like if it doesnt find the database host.
This returned an error at conn.close(); as conn is null.
I have also attached the last log entry in catalina.out to this post, maybe you could translate to english and see what is wrong
Catalina.out
To me, it still looks like a configuration problem, like if it doesnt find the database host.
posted 8 years ago
Thats 100% right. but we have been trying to catch the error and found out whats the misconfiguration. I wonder how your code manages to escape exceptions and show error on different line on different run..
I m not able to donwload the file Catalina.out. Did you look in to that ? found anything ? Is it printing SOPs ? Unless the i see the SOP or log , cant predict anything..
To me, it still looks like a configuration problem, like if it doesnt find the database host.
Thats 100% right. but we have been trying to catch the error and found out whats the misconfiguration. I wonder how your code manages to escape exceptions and show error on different line on different run..
I m not able to donwload the file Catalina.out. Did you look in to that ? found anything ? Is it printing SOPs ? Unless the i see the SOP or log , cant predict anything..
Adam Haglund
Greenhorn
Posts: 7
posted 8 years ago
context.xml is not properly "well -formed" Check out how to configure context.xml and meanwhile post your complete context.xml
SEVERE: Parse error in context.xml for /manager
context.xml is not properly "well -formed" Check out how to configure context.xml and meanwhile post your complete context.xml
posted 8 years ago
Adam,
A few problems:
1) You are ignoring the SQL Exception thrown on creating the connection. It's not valid to continue so you might as well take the user to an error page when this happens.
2) It's traditional to use an if statement in the finally block.
3) You need to close the connection when done with it not after the first query.
A few problems:
1) You are ignoring the SQL Exception thrown on creating the connection. It's not valid to continue so you might as well take the user to an error page when this happens.
2) It's traditional to use an if statement in the finally block.
3) You need to close the connection when done with it not after the first query.
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