Reference jars inside a jar

Question

I have a jar whose content looks as shown below     Below is my manifest file  Manifest-Version  1 0 Ant-Version  Apache Ant 1 8 3 Created-By  1 7 0 06-b24  Oracle Corporation  Main-Class  org sai com DerbyDemo Class-Path  derby jar derbyclient jar derbynet jar derbytools jar   When i try to run the jar  it has thrown a ClassNotFoundExcception meaning it isn t referencing the jars inside the outer jar   In the Class-Path attribute  how can I reference jars  derby jar  etc  inside the actual jar

User · Answer

in eclipse  right click project  select RunAs -  Run Configuration and save your run configuration  this will be used when you next export as Runnable JARs

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You will need a custom class loader for this  have a look at One Jar       One-JAR lets you package a Java application together with its dependency Jars into a single executable Jar file    It has an ant task which can simplify the building of it as well   REFERENCE  from background      Most developers reasonably assume that putting a dependency Jar file into their own Jar file  and adding a Class-Path attribute to the META-INF MANIFEST will do the trick     jarname jar    META-INF      MANIFEST MF        Main-Class  com mydomain mypackage Main        Class-Path  commons-logging jar    com mydomain mypackage      Main class   commons-logging jar      Unfortunately this is does not work  The Java Launcher AppClassLoader does not know how to load classes from a Jar inside a Jar with this kind of Class-Path  Trying to use jar file jarname jar  commons-logging jar also leads down a dead-end  This approach will only work if you install  i e  scatter  the supporting Jar files into the directory where the jarname jar file is installed

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In Eclipse you have option to export executable jar   You have an option to package all project related jars into generated jar and in this way eclipse add custom class loader which will refer to you integrated jars within new jar

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Default implementations of the classloader cannot load from a jar-within-a-jar  in order to do so  the entire  sub-jar  would have to be loaded into memory  which defeats the random-access benefits of the jar format  reference pending - I ll make an edit once I find the documentation supporting this    I recommend using a program such as JarSplice to bundle everything for you into one clean executable jar   Edit  Couldn t find the source reference  but here s an un-resolved RFE off the Sun website describing this exact  problem   http   bugs sun com bugdatabase view bug do bug id 4648386  Also  you could  test  that your program works by placing the library jar files in a  lib sub-directory of your classes directory  then running from the command line  In other words  with the following directory structure   classes org sai com DerbyDemo class classes org sai com OtherClassFiles class classes lib derby jar classes lib derbyclient jar   From the command line  navigate to the above-mentioned  classes  directory  and type   java -cp   lib   org sai com DerbyDemo

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You can t  From the official tutorial      By using the Class-Path header in the manifest  you can avoid having   to specify a long -classpath flag when invoking Java to run the your   application       Note  The Class-Path header points to classes or JAR files on the   local network  not JAR files within the JAR file or classes accessible   over internet protocols  To load classes in JAR files within a JAR   file into the class path  you must write custom code to load those   classes  For example  if MyJar jar contains another JAR file called   MyUtils jar  you cannot use the Class-Path header in MyJar jar s   manifest to load classes in MyUtils jar into the class path

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Add the jar files to your library if using netbeans  and modify your manifest s file classpath as follows   Class-Path  lib derby jar lib derbyclient jar lib derbynet jar lib derbytools jar   a similar answer exists here

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if you do not want to create a custom class loader  You can read the jar file stream  And transfer it to a File object  Then you can get the url of the File  Send it to the URLClassLoader  you can load the jar file as you want  sample   InputStream resourceAsStream   this getClass   getClassLoader   getResourceAsStream  example     jar    final File tempFile   File createTempFile  temp     jar    tempFile deleteOnExit        you can delete the temp file or not try  FileOutputStream out   new FileOutputStream tempFile         IOUtils copy resourceAsStream  out     IOUtils closeQuietly resourceAsStream   URL url   tempFile toURI   toURL    URLClassLoader urlClassLoader   new URLClassLoader new URL   url    urlClassLoader loadClass

[java] Reference jars inside a jar

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