Develop Reference

Why combine all jars together?

I have create RESTful web service based on the JAX-RS and used Jersey embedded web server. My ant script compiles code successfully while it gives me error ClassNotFoundException when I run my main class. So after doing research I came up with solution & here it goes java build ant file with external jar files . What I did was created a bundled jar file try to execute that & it works perfectly fine. I want to know the reason behind :
why this solution works ?
Why I should combine all jar file ?
Is it similar to war file which we create following J2EE architecture otherwise war will not be extracted by server ( say TOMCAT ) & in my case jar file for Jersey embedded HTTP server?
EDIT:
Here is my ant build.xml file
<property name="lib.dir" value="${user.dir}/lib"/>
<property name="build.dir" value="${user.dir}/build"/>
<property name="build.lib.dir" value="${build.dir}/lib"/>
<property name="build.classes.dir" value="${build.dir}/classes"/>
<property name="src.dir" value="${user.dir}/src/main/java"/>
<property name="main.class" value="com.assignment.ConsoleServer"/>
<path id="classpath">
<fileset dir="${lib.dir}" includes="**/*.jar"/>
</path>
<target name="clean">
<delete dir="${build.dir}"/>
</target>
<target name="init" depends="clean">
<!-- Create the build directory structure used by compile -->
<mkdir dir="${build.dir}"/>
<mkdir dir="${build.classes.dir}"/>
</target>
<target name="copy_jars" depends="init" >
<copy todir="${build.lib.dir}" >
<fileset dir="${lib.dir}">
<include name="**/*.jar" />
</fileset>
</copy>
</target>
<target name="compile" depends="copy_jars">
<javac srcdir="${src.dir}" destdir="${build.classes.dir}" classpathref="classpath" includeantruntime="false"/>
</target>
<target name="jar" depends="compile">
<jar destfile="${build.dir}/${ant.project.name}.jar" basedir="${build.classes.dir}">
<manifest>
<attribute name="Main-Class" value="${main.class}"/>
</manifest>
<zipgroupfileset dir="${lib.dir}" includes="*.jar"/>
</jar>
</target>
<target name="run" depends="jar">
<java fork="true" classname="${main.class}">
<classpath>
<path refid="classpath"/>
<path location="${build.dir}/${ant.project.name}.jar"/>
</classpath>
</java>
</target>
Here is my folder structure
P.S. I am not java expert so pardon me if this question is stupid.

Why this solution works?
In your particular case, you probably didn't include all of the necessary dependencies in your deployment in your previous. (It is not clear from your question how you were originally doing the deployment.)
Now you have put all of the application and dependent class files, etc into one JAR file, and presumably you are deploying / running that file. It works because now it has everything that it needs to run ... which it didn't before.
Why I should combine all jar file?
In your case I suspect that it was not strictly necessary. There was probably a way to "deploy" all of the dependencies without combining them into a single JAR file.
However, there is one case where a "uber-jar" has advantages. That is when the JAR is intended to be an "executable" JAR, and you want to be able to distribute / install it as a single file. (And executable JAR
file can refer to external JARs, etc, but the way that you have to do
it is "fragile".)
Is it similar to war file ... ?
Sort of, though a WAR file contains JAR files ... and typically other kinds of resources that the web-container understands.

The solution works because you packed all you service classes and depending libraries in one jar. That jar and everything inside will be in the class path and visible to your execution virtual machines class loader.
If you leave your depending libraries out your Jersey Web server needs to have them on it's class path, then you wouldn't get ClassNotFoundExcpetion
You shouldn't pack web application in single jar. You should crate war file where you dependencies will be placed inside WEB-INF/lib. You would easily then deploy that war on any application server. Switching to Maven instead of Ant can help a lot.
EDIT: After you added more details to description and ant
If you don't want to use fat-jar you can either
modify your antjava task to specify classpath that will reference
all external libraries (basically telling ant how to build
-classpath parameter for java -jar command
even better, modify your javac ant task by making complete Manifest file that specifies Class-Path correctly, take a better
look at the solution (at the bottom) of the answer you linked (java build ant file with external jar files)
For completness reference on Manifest here

How to add external .jar to android project? (with certain conditions)

I need to add some external .jar files to my android project BUT
1) without using Eclipse or any other IDE (so no .classpath)
2) without copying them to /libs folder, i need them to stay where they are
How can i accomplish that? I know that ant should have this functionality but and cant find it. It would be great if there is something like adding "android.library.reference" to project.properties but for .jar instead of library project.

i am not sure that it will work for you but you can use this
<path id="class.path">
<fileset dir="jar_folder">
<include name="**/*.jar" />
</fileset>
</path>
and you can use it in javac command as
<javac executable="${env.JAVA_HOME}/bin/javac" encoding="utf-8" srcdir="src/" destdir="temp/" debug="on" debuglevel="lines,source,var" fork="true" memoryInitialSize="256m" memoryMaximumSize="1024m">
<classpath refid="class.path" />
</javac>

When are Ant path references set?

When I use a path reference ID, Ant seems to evaluate any variables inside the definition before any tasks run. For example, ${common.dist} and ${common.lib} below seem to be evaluated before any tasks run.
<path id="compile.classpath">
<fileset dir="lib">
<include name="*.jar" />
</fileset>
<fileset dir="${common.dist}">
<include name="*.jar" />
</fileset>
<fileset dir="${common.lib}">
<include name="*.jar" />
</fileset>
</path>
In the Ant output I see something like this:
Adding reference: compile.classpath
Property "common.dist" has not been set
Property "common.lib" has not been set
...
Build sequence for target(s) `package' is [...]
Complete build sequence is [...]
This makes it seem like the path reference is being processed before any targets are run.
I have a compile target like this:
<target name="compile" depends="init,common">
<javac destdir="build/classes" debug="true" deprecation="true" optimize="true">
<src path="src/java" />
<classpath>
<path refid="compile.classpath" />
</classpath>
</javac>
</target>
If I copy the guts of the path reference into the classpath element inside the compile target, things seem to work fine.

Any tasks outside a target are executed on every build, in order of appearance in the build.xml, before any targets are run. If you want to use properties in a <path> defined outside a target then you need to put the <property> task that defines the properties also outside a target, and before the <path>. If you need to load the properties within a target then you'll have to put the <path> definition inside a target too (either the same one or one that runs after the one defining the properties).
See this question (and my answer) for more details.

The answer is in the Ant manual - path like structures:
By default a path like structure will re-evaluate all nested resource
collections whenever it is used, which may lead to unnecessary
re-scanning of the filesystem ...
I think you maybe forgot to set the ${common.dist} and ${common.lib} properties. They should be outside any target:
<property name="common.dist" location="dist"/>
<property name="common.lib" location="lib"/>

Eclipse: Ant script to export User Library/Libraries for project

I am new to Java programming. I initially started with NetBeans but have moved to Eclipse given the advice from a friend.
In NetBeans, a pre-written ant build script for the project would generate a Project.jar file and place all required libraries/jars in a lib/ folder.
However, in Eclipse it appears that I need to write my own ant script. I have written a few lines to generate the jar file:
<target name="compile">
<mkdir dir="${build.dir}"/>
<javac srcdir="${src.dir}" destdir="${build.dir}" classpathref="jars" debug="on"/>
</target>
How do I write a command to copy all of the jars in my User Library to a ${build.dir}/lib/ folder?
Thanks.

Use the copy task
like so, with the appropriate include or exclude pattern
<copy todir="${build.dir}/lib/">
<fileset dir="src_dir">
<include name="**/*.jar"/>
</fileset>
</copy>
<copy todir="${build.dir}/lib/">
<fileset dir="src_dir" excludes="**/*.java"/>
</copy>

If you are new to Java take the chance to have a look at maven. It is a build tool like ant with a lot of predefined 'goals' and a fully developed dependency (to other libraries) handling. You will find a eclipse plugin which will be very useful.
Maven projects have a special directory layout which is kind of best practise and helpful for beginners. If you work on a maven project you can just use the command
mvn dependency:copy-dependencies
as a console command (or eclipse run configuration) to copy your project dependencies (libraries) to the <project>\target\dependency directory.

I recommend to use ant4eclipse library for ant based eclipse projects. When you use it, you can access eclipse workspace/project settings, and can iterate tought eclipse project class path in ant.
See the example code bellow:
<path id="ant.classpath">
<fileset dir="${lib.dir}/ant4eclipse">
<include name="*.jar" />
</fileset>
<taskdef resource="net/sf/antcontrib/antlib.xml" />
<taskdef resource="net/sf/ant4eclipse/antlib.xml" />
<targetPlatform
<target name="copy_jars">
<getEclipseClasspath workspace="${basedir}/.."
projectname="TestProject"
targetPlatformLocation="c:/eclipse"
property="classpath"
relative="false"
runtime="true"
pathseparator="#" />
<!-- iterate over all classpath entries -->
<foreach list="${classpath}" delimiter="#"
target="copy_jar_file" param="classpath.entry" />
</target>
<target name="copy_jar_file">
<!-- check if current is a .jar-file ... -->
<if>
<isfileselected file="${classpath.entry}">
<filename name="**/*.jar" />
</isfileselected>
<then>
<!-- copy the jar file to a destination directory -->
<copy file="${classpath.entry}" tofile="${dest.dir}"/>
</then>
</if>
</target>
If you would like to use user libraries, you can define it by userlibraries command.

How can include multiple jars in the classpath using ant?

I have a bunch of .java files in a "src" folder that depend on three jars in a "lib" folder. I have the following build.xml file:
<?xml version="1.0"?>
<project name="MyProj" basedir=".">
<property name="src" value="src"/>
<property name="build" value="build"/>
<property name="lib" value="lib"/>
<path id="master-classpath">
<fileset dir="${lib}">
<include name="activemq-all-5.1-SNAPSHOT.jar"/>
<include name="geronimo-jms_1.1_spec-1.1.1.jar"/>
<include name="activemq-core-5.3.0.jar"/>
</fileset>
</path>
<javac destdir="${build}">
<src path="${src}"/>
<classpath refid="master-classpath"/>
</javac>
</project>
This compiles fine, but when I try and run I get
"java.lang.NoClassDefFoundError:
javax/jms/Destination"
This program runs and compiles fine when I include the jars in the buildpath using Eclipse, though.
EDIT: So I copied the jars into the folder that has the compiled classes. The class with the main method is NDriver.class. When I try:
java -classpath
./geronimo-jms_1.1_spec-1.1.1.jar:./activemq-core-5.3.0.jar:./activemq-all-5.1-SNAPSHOT.jar
NDriver
This gives:
Exception in thread "main"
java.lang.NoClassDefFoundError:
NDriver
I'd appreciate any help.

You need to put the jars used at compile time on the classpath when running the application. Sadly, you didn't provide any detail on how you are actually running it so its hard to provide more guidance.
UPDATE: The directory containing the compiled classes needs to be added to the classpath too. If you launch java from the directory containing the compiled classes, then you can use . to designate the current directory. Add it to the classpath as shown below to tell java to look for classes there too (I've added . right after activemq-all-5.1-SNAPSHOT.jar):
java -classpath ./geronimo-jms_1.1_spec-1.1.1.jar:./activemq-core-5.3.0.jar:./activemq-all-5.1-SNAPSHOT.jar:. NDriver

One way (slightly different variables than yours)
<path id="classpath">
<fileset dir="${lib.dir}" includes="**/*.jar"/>
</path>
<manifestclasspath property="manifest.classpath" jarfile="${jarfile}">
<classpath refid="classpath"/>
</manifestclasspath>
<target name="jar" depends="compile" description="create the jar">
<jar destfile="${jarfile}" basedir="${build.dir}">
<manifest>
<attribute name="Manifest-Version" value="${manifest-version}"/>
<attribute name="Created-By" value="${ant.java.version}"/>
<attribute name="Main-Class" value="${main-class}"/>
<attribute name="Class-Path" value="${manifest.classpath}"/>
</manifest>
</jar>
</target>
Of course here I'm assuming that you are creating a jar and running it (including the classpath there). Another option would be to have a run target which use the <java> tag and explicitly use the classpath there.

Are the library jars included in the classpath when you run the program? Eclipse automatically add these, but you need to specifying them when you run the program from the command line.

From my experience it seems Eclipse will often include classes and jars in the classpath without explicitly using the classpath declaration. Indeed it can sometimes be quite hard to remove classes from Eclipse's build (they have to be deleted or clean'ed).