Calling jar file from jsp - serious problem !

Question

Hi there, I'm trying to call a jar from a jsp file. I need an import clause in my jsp, to the class I want to use.
I'm using eclipse, and while i"m writing "<%= " and the begining of my class name , he autocompletes it and automatically puts an import of my class in the top of the html. It is very convenient , but the problem is that he underling the class name (in the impot: <%@page import="myClass"%>), and I get "The import myClass cannot be resolved" error.
I got an advice to put my class in a package, since it was in a default package. I did it. But now I have 2 problems:
1. I can't run my jar in command window: java - jar myJar.jar. in gives me: Exception in thread "main" java.lang.NoClassDefFoundError....
2. I still get an error - "The import myClass cannot be resolved" in the jsp, although I threw the old jar (that his class I want to use was out of package), and brought a new jar after creating it.
I hope I managed to clarify myself. I'll be thankful of any help !!!

Answer 1

Hi MSoldier.
There are two ways to do this:
1) You use the java.lang.Runtime classes to start the java application in your JAR file, just like you would have done it from the command line. The Runtime class has three different exec() methods that can be used to run any program just like you would from the command line. Remember, using this method you will have to write everything just like if you run your Java application from the command line, incl. classpath etc. This way your Java application will be run as a separate process. When the application finishes the process is shutdown and the resources (RAM etc.) taken by it freed, just like when you shutdown any other application.
2) You can open the JAR file yourself from inside the JSP page, look inside the META-INF file to find the main class, and then calls the main class main method. There is an example of how to do this in the Java Tutorial available from Sun under the JAR files track. Check these links :
The Java Tutorial at Sun
http://java.sun.com/docs/books/tutorial/
The JAR File Tutorial at Sun
http://java.sun.com/docs/books/tutorial/jar/index.html
Opening JAR Files via the Java JAR File API
http://java.sun.com/docs/books/tutorial/jar/api/index.html
Note however, if you have just created some classes that you want to use from inside your JSP pages, and not a separate application that needs to be run separately, you should just include the JAR file with your custom classes inside the WEB-INF/lib directory. These classes are then automatically available to your JSP pages, just like the rest of the Java API.
I hope this answers your questions,
Kind Regards,
Jakob Jenkov
Software Architect
Jenkov Development
www.jenkov.com