Executing java class in JAR from a java program

Question

My Question is:

I have a test.java class which is the main class. There is one more class(non-main class) which is present in JAR file. Now i want to compile and execute demo class present in JAR from the main class present outside the JAR. Please explain why.

public class test
{   
public static void main(String args[])
{
    demo d1 = new demo();
    demo d2 = new demo("message passed from main class");
}
} 

public class demo
{
demo()}
System.out.println("message in demo class");
}
demo(String str)
{
System.out.println(str);
}}

Just a word on design, your classes should always start with a capita letter, so public class Test, public class Demo — David, Feb 07 '12 at 08:43
Agree. I didn't much attention to that because I wanted to post sample code here. — old_soul_on_the_run, Sep 22 '20 at 23:41

casablanca · Answer 1 · 2012-02-07T04:45:48.200

3

Have a look at this: Packaging Programs in JAR Files

Once you have both your main class and JAR file ready, you can simply run the main class with the JAR file on the classpath and Java will find it. Assuming the demo class is in demo.jar and your main class is test (as in your example):

java -cp demo.jar test

edited Feb 07 '12 at 04:45

answered Feb 07 '12 at 04:30

casablanca

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Its giving an exception saying exception in thread main. java.lang.Noclassdeffounderror:TEST. – old_soul_on_the_run Feb 07 '12 at 04:55
in my demo.jar i have demo.class and demo.java. Outside i have test.java. I compiled test by "JAVAC TEST.JAVA -CLASSPATH DEMO.JAR". It was compiled successfully. Now i m trying to execute TEST by "JAVA -CP DEMO.JAR TEST" and now its giving this exception. – old_soul_on_the_run Feb 07 '12 at 04:56
Java is case-sensitive. If you defined the class as `test`, you need to run it as `test` and not `TEST`. – casablanca Feb 07 '12 at 05:21

score 1 · Accepted Answer · answered Feb 07 '12 at 05:16

I guess the proper answer us 'use ant', but:

javac test.java
javac demo.java
jar cf test.jar test.class demo.class
jar ufe test.jar test

and then,

java -jar test.jar

If you intend to expand on this, and continue not to use ant, you'll likely want your build system to stop on an error, and will notice that 'javac' doesn't return non-zero on error. Pipe its stderr to a temporary file and then quit depending on grep's non-zero return of that file. e.g., from an on-device build script that cannot use ant:

cd src
for F in R.java Maze.java EmptyActivity.java; do
  javac -d ../build/classes $P/$F 2> ~/tmp/javac.out
  echo -en "\033[1m"; cat ~/tmp/javac.out; echo -en "\033[0m"
  grep error ~/tmp/javac.out && exit
done
cd ..

score 0 · Answer 3 · answered Feb 07 '12 at 05:08

Simply if you want to use any class in java then that class must be present in class-path because when you run your program then all class needed for that is always looking in class path..
So you have to set class-path of your demo.jar
There is no of ways to set class path take a look of this How set class path

or simply you can do another option use java parameter -cp

java -cp /your jar path/demo.jar test

Executing java class in JAR from a java program

3 Answers3