Question

 <target name="run" depends="jar">
        <java fork="true" classname="${main-class}">
          <classpath>
                <path refid="classpath"/>
                <path location="${jar.dir}/${ant.project.name}.jar"/>
        </classpath>
        </java>
    </target>

Why I cannot use something like

   <java jar="build/jar/HelloWorld.jar" fork="true" >
          <classpath>
             <path refid="classpath"/>  
            <path location="${jar.dir}/${ant.project.name}.jar"/>   
  </classpath> 
   </java> 

I received the error like

 [java] java.lang.NoClassDefFoundError: org/apache/log4j/Logger
 [java]     at oata.HelloWorld.(Unknown Source)
 [java] Caused by: java.lang.ClassNotFoundException: org.apache.log4j.Logger

Answer 1

According to the java task documentation, your second example should just work as expected (except that the classpath will be ignored).

Edit: The information you added to your question implies a few things:

1. the .jar file is found
2. it has the correct Main-Class entry in its manifest
3. your application requires log4j
4. it does not have a correct Class-Path entry pointing to a log4j.jar file

Basically the java ant task has the same requirements as using java -jar on the command line: if your .jar file doesn’t correctly run using java -jar myApp.jar on the command line, then the java Ant task won’t work like that either.

Edit 2: Whenever you invoke a .jar file directly in Java, then no further classpath can be specified in any way. That’s true when you use java -jar myApp.jar and it’s also true when you use the java Ant task with the jar attribute. That means that the .jar file itself must have a correct Class-Path entry.