Обсуждение: Servlet problems

Hi all.

I'm a bit confused here. I'm using Tomcat JSP/Servlet engine. I have a database in PostgreSQL, which holds a table
describingan n-tree structure (hierarchy). I would like my JSP page to able to display (if parameter "print" is
present)a complete sub-tree of a given node. This is a simple recursion. What I'm trying to do is make our coroporate
E-Mailaddress book, so I have organizational units and users. 

Without recursion everything works fine, all the direct subnodes are displayed. When I use recusrion to include all
subnodes,I get strange errors. 

First, it was killing the response in the middile of a table, in the middle of a row. Now it dies with
"NullPointerException:next()". In other words, in one of the "ResultSet.next()" methods, it runs into a non existant
ResultSet.

I have tried putting a "breakpoint" (throwing a ServletException) on all lines of my JSP page. When there is a
"breakpoint"I do not run into a NullPointerException. When I remove it, I have exception. 

My conclusion was that I get the error AFTER my JSP page starts emerging from the recursion (if it was inside
recursion,it would have been stopped by the "breakpoint"). So, after a page calls itself once, the "insider" does it's
thingand then exits and releases control to it's caller, the the exception occurs. 

I must admit that JSP page closes it's JDBC objects, I thought it would be OK, since the page creates them.

This is the flow of events:

OUDisplay.jsp [instance 1]:
----------------------------
  DriverManager.getConnection()
  Connection.createStatement()
  Statement.executeQuery()
  while( ResultSet.next() ) {
    ...
    <jsp.include page="OUDisplay.jsp" />

    OUDisplay.jsp [instance 2]:
    ----------------------------
      DriverManager.getConnection()
      Connection.createStatement()
      Statement.executeQuery()
      while( ResultSet.next() ) {
        ...
      }
      ResultSet.close()
      Statement.close()
      Connection.close()
    --------------[instance 2]-

  }
  ResultSet.close()
  Statement.close()
  Connection.close()
--------------[instance 1]-

The problem occurs inside the outer "next() {...}", which leads me to believe that Some objects in the "instance 1" get
screwedup, but I don't know how. 

Any insight?

Nix.

Nikola,

I suspect that you are correct, you would have to look at the compiled
code to see what is going on. It is possible that it is using the same
result set? Either way you would be better reading the categories in a
class. In my servlet code I read the categories into a static tree. The
assumptions are

1) the category tree doesn't change a lot.
2) it is very costly to recurse through the tree

Dave

-----Original Message-----
From: pgsql-jdbc-owner@postgresql.org
[mailto:pgsql-jdbc-owner@postgresql.org] On Behalf Of Nikola Milutinovic
Sent: Monday, December 17, 2001 9:32 AM
To: PostgreSQL JDBC
Subject: [JDBC] Servlet problems


Hi all.

I'm a bit confused here. I'm using Tomcat JSP/Servlet engine. I have a
database in PostgreSQL, which holds a table describing an n-tree
structure (hierarchy). I would like my JSP page to able to display (if
parameter "print" is present) a complete sub-tree of a given node. This
is a simple recursion. What I'm trying to do is make our coroporate
E-Mail address book, so I have organizational units and users.

Without recursion everything works fine, all the direct subnodes are
displayed. When I use recusrion to include all subnodes, I get strange
errors.

First, it was killing the response in the middile of a table, in the
middle of a row. Now it dies with "NullPointerException: next()". In
other words, in one of the "ResultSet.next()" methods, it runs into a
non existant ResultSet.

I have tried putting a "breakpoint" (throwing a ServletException) on all
lines of my JSP page. When there is a "breakpoint" I do not run into a
NullPointerException. When I remove it, I have exception.

My conclusion was that I get the error AFTER my JSP page starts emerging
from the recursion (if it was inside recursion, it would have been
stopped by the "breakpoint"). So, after a page calls itself once, the
"insider" does it's thing and then exits and releases control to it's
caller, the the exception occurs.

I must admit that JSP page closes it's JDBC objects, I thought it would
be OK, since the page creates them.

This is the flow of events:

OUDisplay.jsp [instance 1]:
----------------------------
  DriverManager.getConnection()
  Connection.createStatement()
  Statement.executeQuery()
  while( ResultSet.next() ) {
    ...
    <jsp.include page="OUDisplay.jsp" />

    OUDisplay.jsp [instance 2]:
    ----------------------------
      DriverManager.getConnection()
      Connection.createStatement()
      Statement.executeQuery()
      while( ResultSet.next() ) {
        ...
      }
      ResultSet.close()
      Statement.close()
      Connection.close()
    --------------[instance 2]-

  }
  ResultSet.close()
  Statement.close()
  Connection.close()
--------------[instance 1]-

The problem occurs inside the outer "next() {...}", which leads me to
believe that Some objects in the "instance 1" get screwed up, but I
don't know how.

Any insight?

Nix.

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Hi!

I'm afraid I don't speak JSP ;-) but I may siggest something.

Can you try to flush every output you send?

For every println(), if you use them, just place a flush() immediately
after it. That WILL NOT solve your problem, and WILL degrade
performance, but MAY help you find the source for your trouble. Once
your problem is solved (I let you do the hard work), you can safely take
them away.

Good luck!

Antonio


Nikola Milutinovic wrote:

>Hi all.
>
>I'm a bit confused here. I'm using Tomcat JSP/Servlet engine. I have a database in PostgreSQL, which holds a table
describingan n-tree structure (hierarchy). I would like my JSP page to able to display (if parameter "print" is
present)a complete sub-tree of a given node. This is a simple recursion. What I'm trying to do is make our coroporate
E-Mailaddress book, so I have organizational units and users. 
>
>Without recursion everything works fine, all the direct subnodes are displayed. When I use recusrion to include all
subnodes,I get strange errors. 
>
>First, it was killing the response in the middile of a table, in the middle of a row. Now it dies with
"NullPointerException:next()". In other words, in one of the "ResultSet.next()" methods, it runs into a non existant
ResultSet.
>
>I have tried putting a "breakpoint" (throwing a ServletException) on all lines of my JSP page. When there is a
"breakpoint"I do not run into a NullPointerException. When I remove it, I have exception. 
>
>My conclusion was that I get the error AFTER my JSP page starts emerging from the recursion (if it was inside
recursion,it would have been stopped by the "breakpoint"). So, after a page calls itself once, the "insider" does it's
thingand then exits and releases control to it's caller, the the exception occurs. 
>
>I must admit that JSP page closes it's JDBC objects, I thought it would be OK, since the page creates them.
>
>This is the flow of events:
>
>OUDisplay.jsp [instance 1]:
>----------------------------
>  DriverManager.getConnection()
>  Connection.createStatement()
>  Statement.executeQuery()
>  while( ResultSet.next() ) {
>    ...
>    <jsp.include page="OUDisplay.jsp" />
>
>    OUDisplay.jsp [instance 2]:
>    ----------------------------
>      DriverManager.getConnection()
>      Connection.createStatement()
>      Statement.executeQuery()
>      while( ResultSet.next() ) {
>        ...
>      }
>      ResultSet.close()
>      Statement.close()
>      Connection.close()
>    --------------[instance 2]-
>
>  }
>  ResultSet.close()
>  Statement.close()
>  Connection.close()
>--------------[instance 1]-
>
>The problem occurs inside the outer "next() {...}", which leads me to believe that Some objects in the "instance 1"
getscrewed up, but I don't know how. 
>
>Any insight?
>
>Nix.
>
>---------------------------(end of broadcast)---------------------------
>TIP 3: if posting/reading through Usenet, please send an appropriate
>subscribe-nomail command to majordomo@postgresql.org so that your
>message can get through to the mailing list cleanly
>
>.
>

Nikola-

I have a thought which doesn't match your description precisely, but might
still be worth a try if you haven't already thought of it. I'm reminded of
it because I had a similar mystery once where my jsp would die mysteriously
unless I interrupted it before completion with a debug statement.

In my case the problem was that totmcat starts you off with a fairly small
chunk of memory to write each page to. I don't remember the exact number,
but I'm thinking it was 8K or 16K. The problem was that I overran this
buffer, resulting in an error that was misleading every time the page grew
to a certain size. A sizable email directory might be big enough to do this.

The fix was to insert this line at the top:

<%@ page buffer="64kb" %>

If this doesn't help, I'd suggest looking in Tomcat's work directory for the
java code that gets produced & posting a copy of it to the list. Although
many folks on the list don't speak jsp, they'll all be able to grok the java
version.

-Nick

> -----Original Message-----
> From: pgsql-jdbc-owner@postgresql.org
> [mailto:pgsql-jdbc-owner@postgresql.org]On Behalf Of Nikola Milutinovic
> Sent: Monday, December 17, 2001 9:32 AM
> To: PostgreSQL JDBC
> Subject: [JDBC] Servlet problems
>
>
> Hi all.
>
> I'm a bit confused here. I'm using Tomcat JSP/Servlet engine. I
> have a database in PostgreSQL, which holds a table describing an
> n-tree structure (hierarchy). I would like my JSP page to able to
> display (if parameter "print" is present) a complete sub-tree of
> a given node. This is a simple recursion. What I'm trying to do
> is make our coroporate E-Mail address book, so I have
> organizational units and users.
>
> Without recursion everything works fine, all the direct subnodes
> are displayed. When I use recusrion to include all subnodes, I
> get strange errors.
>
> First, it was killing the response in the middile of a table, in
> the middle of a row. Now it dies with "NullPointerException:
> next()". In other words, in one of the "ResultSet.next()"
> methods, it runs into a non existant ResultSet.
>
> I have tried putting a "breakpoint" (throwing a ServletException)
> on all lines of my JSP page. When there is a "breakpoint" I do
> not run into a NullPointerException. When I remove it, I have exception.
>
> My conclusion was that I get the error AFTER my JSP page starts
> emerging from the recursion (if it was inside recursion, it would
> have been stopped by the "breakpoint"). So, after a page calls
> itself once, the "insider" does it's thing and then exits and
> releases control to it's caller, the the exception occurs.
>
> I must admit that JSP page closes it's JDBC objects, I thought it
> would be OK, since the page creates them.
>
> This is the flow of events:
>
> OUDisplay.jsp [instance 1]:
> ----------------------------
>   DriverManager.getConnection()
>   Connection.createStatement()
>   Statement.executeQuery()
>   while( ResultSet.next() ) {
>     ...
>     <jsp.include page="OUDisplay.jsp" />
>
>     OUDisplay.jsp [instance 2]:
>     ----------------------------
>       DriverManager.getConnection()
>       Connection.createStatement()
>       Statement.executeQuery()
>       while( ResultSet.next() ) {
>         ...
>       }
>       ResultSet.close()
>       Statement.close()
>       Connection.close()
>     --------------[instance 2]-
>
>   }
>   ResultSet.close()
>   Statement.close()
>   Connection.close()
> --------------[instance 1]-
>
> The problem occurs inside the outer "next() {...}", which leads
> me to believe that Some objects in the "instance 1" get screwed
> up, but I don't know how.
>
> Any insight?
>
> Nix.
>
> ---------------------------(end of broadcast)---------------------------
> TIP 3: if posting/reading through Usenet, please send an appropriate
> subscribe-nomail command to majordomo@postgresql.org so that your
> message can get through to the mailing list cleanly
>

Use try/catch/finally blocks. Make sure that you close your connection
in finally block.

  try {
    all ur code goes here
  }
  catch (SQLException e) {
    print exception
  }
  catch (Exception e) {
    print exception
  }
  finally {
    try {
     close resultset
     close statement
     close connection
    catch (SQLException e) {
    }
  }

Jayesh

-----Original Message-----
From: Antonio Fiol Bonnín [mailto:fiol@w3ping.com]
Sent: Monday, December 17, 2001 6:55 AM
To: Nikola Milutinovic
Cc: PostgreSQL JDBC
Subject: Re: [JDBC] Servlet problems


Hi!

I'm afraid I don't speak JSP ;-) but I may siggest something.

Can you try to flush every output you send?

For every println(), if you use them, just place a flush() immediately
after it. That WILL NOT solve your problem, and WILL degrade
performance, but MAY help you find the source for your trouble. Once
your problem is solved (I let you do the hard work), you can safely take

them away.

Good luck!

Antonio


Nikola Milutinovic wrote:

>Hi all.
>
>I'm a bit confused here. I'm using Tomcat JSP/Servlet engine. I have a
database in PostgreSQL, which holds a table describing an n-tree
structure (hierarchy). I would like my JSP page to able to display (if
parameter "print" is present) a complete sub-tree of a given node. This
is a simple recursion. What I'm trying to do is make our coroporate
E-Mail address book, so I have organizational units and users.
>
>Without recursion everything works fine, all the direct subnodes are
displayed. When I use recusrion to include all subnodes, I get strange
errors.
>
>First, it was killing the response in the middile of a table, in the
middle of a row. Now it dies with "NullPointerException: next()". In
other words, in one of the "ResultSet.next()" methods, it runs into a
non existant ResultSet.
>
>I have tried putting a "breakpoint" (throwing a ServletException) on
all lines of my JSP page. When there is a "breakpoint" I do not run into
a NullPointerException. When I remove it, I have exception.
>
>My conclusion was that I get the error AFTER my JSP page starts
emerging from the recursion (if it was inside recursion, it would have
been stopped by the "breakpoint"). So, after a page calls itself once,
the "insider" does it's thing and then exits and releases control to
it's caller, the the exception occurs.
>
>I must admit that JSP page closes it's JDBC objects, I thought it would
be OK, since the page creates them.
>
>This is the flow of events:
>
>OUDisplay.jsp [instance 1]:
>----------------------------
>  DriverManager.getConnection()
>  Connection.createStatement()
>  Statement.executeQuery()
>  while( ResultSet.next() ) {
>    ...
>    <jsp.include page="OUDisplay.jsp" />
>
>    OUDisplay.jsp [instance 2]:
>    ----------------------------
>      DriverManager.getConnection()
>      Connection.createStatement()
>      Statement.executeQuery()
>      while( ResultSet.next() ) {
>        ...
>      }
>      ResultSet.close()
>      Statement.close()
>      Connection.close()
>    --------------[instance 2]-
>
>  }
>  ResultSet.close()
>  Statement.close()
>  Connection.close()
>--------------[instance 1]-
>
>The problem occurs inside the outer "next() {...}", which leads me to
believe that Some objects in the "instance 1" get screwed up, but I
don't know how.
>
>Any insight?
>
>Nix.
>
>---------------------------(end of
broadcast)---------------------------
>TIP 3: if posting/reading through Usenet, please send an appropriate
>subscribe-nomail command to majordomo@postgresql.org so that your
>message can get through to the mailing list cleanly
>
>.
>




---------------------------(end of broadcast)---------------------------
TIP 4: Don't 'kill -9' the postmaster

Nikola,

I hope that my comments will help. I included some code examples on what
to do and what will not work. Didn't you say you like the "Find the
differences" game? You have a good example of it below. Take your time
to read it, especially because I did not take a long time to write it
and there may even be errors in it. Sorry if there are some.

Good luck!


>>I'm afraid I don't speak JSP ;-) but I may suggest something.
>>
>
>The question is not really JSP related, I could test it in a Java standalone. I would hate to do so... I'm running low
ontime/nerves. 
>
We all suffer from that :-(


>>Can you try to flush every output you send?
>>
>
>Perhaps... So, JSP would print as far as it goes and I will see where it crashes? Hmm, not bad. The other alternative
Iwas thinking of was Log4J package to do some logging along the way. 
>
Try logging on the web page itself. This will help you MATCH the log
messages to the web page output. It is dirty, ugly, and you must take
the debugging out for any production tests, but it helped me find really
ugly mistakes.

Something else tou may try is logging as much information as possible
about the state prior to an exception, in your catch blocks. Well, I
suppose you knew that already...

I am not sure whether Log4J is not a bit overkill for your present
problem, I never used it yet. Maybe System.err.println() is easier, but
that will probably be implementation-dependent.

>>For every println(), if you use them, just place a flush() immediately
>>after it. That WILL NOT solve your problem, and WILL degrade
>>performance, but MAY help you find the source for your trouble. Once
>>your problem is solved (I let you do the hard work), you can safely take
>>them away.
>>
>
>I'll try it.
>
>My dilema was whether JDBC calls were thread/recursion safe, I guess.
>
Stated that way, the answer is YES.

Supposing you will be using a single Connecion object, as soon as you
use different Statement objects for all your SELECT queries, which will
give you different ResultSet objects, it is thread/recursion safe.

*WHAT YOU SHOULD DO* :

Statement stmt1 = conn.createStatement();
ResultSet rs1 = stmt1.executeQuery(query1);
while(rs1.next()) {
    Statement stmt2 = conn.createStatement();
    ResultSet rs2 = stmt2.executeQuery(query2);
    ...
    rs2.close();
    stmt2.close();
}


*YOU MAY ALSO DO* :

Statement stmt1 = conn.createStatement();
Statement stmt2 = conn.createStatement();
ResultSet rs1 = stmt1.executeQuery(query1);
while(rs1.next()) {
    ResultSet rs2 = stmt2.executeQuery(query2);
    ...
    rs2.close();
}


*BUT YOU SHOULD NEVER TRY* :

Statement stmt1 = conn.createStatement();
ResultSet rs1 = stmt1.executeQuery(query1);
while(rs1.next()) {
    rs1 = stmt1.executeQuery(query2);
    ...
    rs1.close();
}



*THIS WILL NOT WORK EITHER* :

Statement stmt1 = conn.createStatement();
ResultSet rs1 = stmt1.executeQuery(query1);
while(rs1.next()) {
    ResultSet rs2 = stmt1.executeQuery(query2);
    ...
    rs2.close();
}

This last example will not work because stmt1.executeQuery(query2) WILL
IMPLICITLY CLOSE rs1.

In case you close a ResultSet implicitly, I strongly believe that the
data set is destroyed, and thus ResultSet.next() will throw a
NullPointerException.

This kind of problem is always hard to find. Do not despair...

Hope that helps,


Antonio Fiol
W3ping