Re: Calling Java from RPG

[Scott Klement <rpg400-l@xxxxxxxxxxxxxxxx>]

Hi Jon,

Java supports overloading. Overloading means that you can have multiple 
Java methods (equiv to RPG subprocedures) that have the exact same name, 
but have different parameters. In other words, you could have 2 
different Java methods named CGProCLI within the same object, and each 
one can be called independently.

How does that work?  When the system looks up the Java method name, it 
doesn't only look for the name itself, but it also looks for a 
"signature".  The signature is basically a code that describes the 
parameters and return value of the method.   Both the signature and the 
method name have to match.

Let's look at the error method you described:

    Java exception "java.lang.NoClassDefFoundError: CGProCLI" when
    calling method "<init>" with signature "()V" in class
    "CGProCLI".

<init> means it's calling the constructor method (*CONSTRUCTOR in RPG).
Signature ()V means that it's trying to call a Java method which has a 
return value of VOID (meaning nothing is returned), and that accepts no 
parameters.  If there were parameters, they'd be described in between 
the parenthesis.

However, when I look at your Java code, I see this:

   public CGProCLI(String host,int port,String user,String password)

While that is a "void" method (it doesn't return anything) it certainly 
DOES accept parameters.   I'm telling you this so that you'll understand 
the error message next time you get it.   The error isn't that you 
spelled the method name wrong -- it's that you aren't passing the right 
parameters to it!!

In Java, when you code a constructor method, you name the method the 
same as the class it's a part of.   That's how the Java compiler knows 
that it's a constructor -- the names match.

In RPG, when you call a constructor (which, again, is done when the 
class & method names match) you don't code the method name on the 
prototype.  Instead, you supply *CONSTRUCTOR to tell it to call the 
constructor method.  RPG knows that constructors have the same name as 
the class, you don't have to specify the class name.

This seems to be where you went wrong.  You decided to call it twice, 
Once using *CONSTRUCTOR (but without any parms) and again with a method 
name that matches the class name.   So what happens?

a)  When you call the *CONSTRUCTOR one, it bombs because you didn't pass 
the parameters, and it therefore can't find a class that has the right 
name and parameters.

b)  The one that does call it with the correct class isn't reached 
(because your program blows up) but if it were, it also wouldn't work 
because you didn't use the special value *CONSTRUCTOR.  (And, also, you 
have an error in that you've defined the PORT as a string instead of an 
integer)

So this is what you need to do:

a) Eliminate the superfluous CGConnect2

b) Add the parameters that you placed on CGConnect2 to the original 
new_CGConnect() prototype.

c) Change the "port" parameter to an Integer  (10i 0 by VALUE in RPG).

I hope that helps you both understand what you need to do, and what the 
error messages are trying to tell you.



Jon S wrote:
> All,
>  
> I this is my first attempt at calling Java from RPG and I am about to pull my hair out.
> I have a .jar for from a third party vendor that I am trying to communicate with named CGProCLI.jar.
> I tried my own directory at first but the .jar currently resides in the QIBM/UserData/Java400/ext directory and it’s my understanding that I don’t even need to set the class path to there as it’s a default, but I set the classpath anyway.
> I have spent quite some time reading and looking at various examples on the web and even created my own java program on the 400 and compiled it in above directory and got it to work. Every time I run this I get the dreaded java.lang.NoClassDefFoundError error. I have included my prototype, RPG, the java code from the third party and my errors. I am sure this is something really silly on my part, but any help would be appreciated.
>  
> Thanks, Jon 
>  
>  
> *  Connect method
> D CGConnect       S               O   CLASS(*JAVA:'CGProCLI')
>  *  Connect method
> D new_CGConnect...
> D                 PR                  Like(CGConnect)
> D                                     ExtProc(*JAVA:'CGProCLI'
> D                                     :*CONSTRUCTOR)
> D CGConnect2...
> D                 PR
> D                                     ExtProc(*JAVA:'CGProCLI'
> D                                     :'CGProCLI')
>  *  method parameters
> D  host                           O   Class(*JAVA:'java.lang.String')
> D                                     Const
> D  port                           O   Class(*JAVA:'java.lang.String')
> D                                     Const
> D  login                          O   Class(*JAVA:'java.lang.String')
> D                                     Const
>  D  password                   O   Class(*JAVA:'java.lang.String')
>  D                                     Const
>  
>  
> And finally the code:
>  
>  
> D Conn            S                   like(CGConnect)
>  *
>  /Free
>                     shost  = newString(%trim(host));
>                     sport  = newString(%trim(port));
>                     slogin = newString(%trim(login));
>                     spassword = newString(%trim(password));
>  
>                     Conn = new_CGConnect(); This is where it blows.
>  
>                     RtnCode = CGConnect2(Conn:sHost:sPort+
>                                                   sLogin:sPassword);
>  
>  /End-Free
>  
> The java code:
>  
> public CGProCLI(String host,int port,String user,String password) throws CGProException {
>     openStream(host,port);
>     connect(user,password);        
>   }
>   
> And the ever too popular error:
>  
> Cause . . . . . :   RPG procedure CG0001 in program GDSBILLBLD/CG0001 received
>   Java exception "java.lang.NoClassDefFoundError: CGProCLI" when calling
>   method "<init>" with signature "()V" in class "CGProCLI".
>  
> When I run:
>  
> echo $CLASSPATH | tr ":" "\n"
>  
> I get:
> '.
> /QIBM/UserData/Java400/ext/CGProCLI.jar
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