Develop Reference

Generated JAR throws ClassNotFoundException for main class

I'm using IntelliJ IDEA to create a JAR. I selected "JAR from modules with dependencies" and "extract to the target JAR" for library JARs - the generated JAR looks just fine:
myJar.jar
|
+- META-INF
| +- MANIFEST.MF
+- com
| +- my
| +- package
| +- Main.class
+- some dependencies...
I checked twice: all the needed dependencies are present. The Main-Class field in MANIFEST.MF points to the correct main class (com.my.package.Main in this example). I've opened the file in my archive tool and inspected it using
jar tf myJar.jar
and both show that the classes needed are available (also Main.class). I've left the Classpath field in IntelliJ's wizard empty, as I do not need any external libraries. However, when I issue
java -jar myJar.jar
it throws the following exception:
Error: could not find or load main class com.my.package.Main
Caused by: java.lang.ClassNotFoundException: com.my.package.Main
I've followed this guide by JetBrains
, and IntelliJ automatically added all dependencies from the Maven pom.xml to be extracted into the JAR, which is correct.
Am I missing something I should configure here and how comes that this does not work?
(Note: Generating a JAR from that project also does not work when I choose "copy to output folder and link via manifest" for the library JARs.)
(Note 2: Using maven-assembly-plugin is sadly not an option since I referenced other IntelliJ workspace modules which would then not get included.)
Update: That it should work as-is shows also the following phenominum: When I unzip my JAR and do
java -cp . com.my.package.Main
in the created directory, it works without any problems, which is strange given that Java refuses to load it...

In my case, my JAR did not work as expected due to some signed JAR dependencies.
These JARs bring a signature and key file, which also gets extracted when you embed the signed JARs. There is basically no risk in removing them, but they are one of the many possible reasons your JAR may not work.
How to remove such signatures from a JAR?
I'm describing the following steps for Linux/Darwin, but I think that there is a similar way for Windows.
Unpack you JAR.
Since JARs are nothing more than simple ZIP archives, you can use unzip:
mkdir temporaryDirectory
unzip myJar.jar -d temporaryDirectory/
The -d option is optional, but it helps keeping your directory structure clean because it sets the target directory.
Locate the signature files.
The signature files (or keys) are located in the META-INF/ directory, so change there:
cd temporaryDirectory/META-INF/
Next, we need to find the troubling files. They have the file extensions .SF (for the signature) and .DSA for the key file:
ll | grep '.DSA\|.SF'
Delete (or rename) the signature and key files.
Renaming these files bares the benefit that you can restore them later (for whatever reason), deleting them also does the trick but is a bit more risky:
Deleting:
rm signature.DSA signature.SF
# Either enter the name of the files
# instead of 'signature' or use * to delete any:
# rm *.DSA *.SF
Renaming:
rename 's/\.DSA$/.DSA.old/' * # Append ".old" to all .DSA files
rename 's/\.SF$/.SF.old/' * # Append ".old" to all .SF files
Repacking your JAR.
Still in temporaryDirectory/, we can repack our JARs to make them work:
cd ../ # If you're still in temporaryDirectory/META-INF
jar cfm ../myWorkingJar.jar ./META-INF/MANIFEST.MF -C ./ .
Explanation:
jar cfm
jar is Java's built-in JAR builder. Invoking it with c means that we want to create a JAR, f stands for the output file (which we specify next) and m is for the MANIFEST.MF we want to use. If you omit m, jar will write an empty MANIFEST.MF to the JAR.
../myWorkingJar.jar
This is the path we want to output our JAR to. It belongs to f which we specified earlier.
./META-INF/MANIFEST.MF
This is the manifest file we want to use. It belongs to the m of cfm.
-C ./
This means that our .class files are located in this directory (.).
. (last argument)
This specifies that we want to add this directory to the JAR.
If you want a detailed description of what jar is doing, you can issue the command from above with cvfm instead of cfm (v stands for verbose).
Verify that it works.
All set, now you can check that your JAR is working as intended by issuing
java -jar myWorkingJar.jar
Removing the temporary directory.
Since you're finished repairing your JAR, you can safely delete the temporary directory we created (and/or the "broken" JAR).
I've created a simple BASH script which "automates" this process a little bit:
#!/bin/bash
JARNAME=myJar.jar # enter your JAR's name here
OUT_JARNAME=myJar_out.jar # enter your output JAR's name here
# Creating the directory, unpacking the JAR, entering META-INF/
mkdir temp
unzip $JARNAME -d temp
cd temp/META-INF
# Renaming the troublemakers.
rename 's/\.DSA$/.DSA.old/' *
rename 's/\.SF$/.SF.old/' *
# Reassembling the JAR
cd ../
jar cfm ../$OUT_JARNAME ./META-INF/MANIFEST.MF -C ./ .
cd ../
# Uncomment this line if you wish to delete the temp directory upon finish.
# rm -r temp
I hope this helps people also encountering this issue.

NoClassDefFoundError when runnable jar

Hi Guys I have included the Webcam-Capture API in my project.
When I run it in Netbeans everything works fine. But when i compile everything to a runnable jar i get this message trying to run it by cmd line.
can anyone of you help me?
i already tried to unbound and rebound all jars and changing jdks but its not working

add -classpath flag in the command line ,pointing to the path where Webcam-Capture API exists in your file system, unless you want to create a single package executable.In your case It should be something like below
java -classpath YOURJAR.jar;folder_of_dependant_jar/*;. com.awesome.pagackage.Starter
Where YOURJAR.jar contains the com.awesome.pagackage.Starter.main(String args[])
You also mentioned that your jar is a runnable jar it also means that while exporting/building you can do one of the following way.( NOTE , this feature is in eclipse , but you would get the idea ).Each of the following options you see in the library handling does specific things.
The first option: Extracts the dependent jar into your target jar as java packaging.This means if your package is com.awesome.package and the dependent jar has package logic.package; , after the runnable jar is build you could find both these package exists in your jar file.
The second option: I think it is more on eclipse specific since eclipse adds few classes of its own , of runnable generation, so I am not explaining it here.
The third option : is the most interesting one. it creates folder stucture like below
ndon_lib\external.jar ( external jar file )
ndon.jar ( your jar file )
This time the manifest.mf file contains something like below.
Class-Path: . ndon_lib/external.jar
Main-Class: com.awesome.pagackage.Starter

You should set the classpath
java -cp "your.jar" "yourclass"

Running jar doesn't work

We are trying to make a jar out of our class files but somehow it does not work. The creation of the jar-file works fine with
jar -cvfm client.jar Mainfest.txt /home/pi/Desktop/Client/*.class
But when we try to run it comes with a classnodefferror. Our application is using 2 property files - one for database and one for log4j.
The directory and the one subdirectory included looks like this:
See link - image nr. 1 and 2
When we try to execute the jar file it shows this error:
(See link image 3)
Normally when we run it, we type (see link image 4)
And the manifest-file looks like this (see link image 5)
We have tried different solutions like changing paths, leaving out environment (-D) etc. and we really can't figure out what we do wrong.

There are several problems here:
It is better to explicitly mention each included JAR file in the manifest - wildcards like * isnt working too well
You can't use absolute paths in a JAR file - use relative paths instead
So, 1) change Manifest.txt to
Main-Class: Client
Class-Path: . includes/log4j-1.2.17.jar includes/mysql-connector-java-5.1.33-bin.jar includes/sqlitejdbc-v056.jar includes/RXTXcomm-2.2pre2.jar includes/..... (repeat for all pi4j JAR files)
And, 2) copy ALL the JAR files you need (including RXTXcomm-2.2pre2.jar and the pi4j JARs) into the include/ subdirectory, and change command to
jar -cvfm client.jar Manifest.txt *.class *.properties includes/*

Jar executable with sqlite driver

I made a code that connects to my sqlite driver which is in the CLASSPATH and reads some database file. I want to create an executable which can be used on computers that don't have the sqlite driver.
If I do:
jar cvfe exec.jar main_class
I will get "class not found: org.sqlite.JDBC" when running with
java -jar exec.jar
What should I do to make the executable work?
Edit:
I don't know if it makes any difference, but this is the JDBC driver I use:
https://bitbucket.org/xerial/sqlite-jdbc

You need to include the library inside the JAR. Maybe you don't know this, but JAR files are just ZIP files, so you can change their contents easily. Here are some quick instructions on how to do it. Assuming your JAR file is named exec.jar, and the JAR of the library you want to include (the JAR you downloaded) is driver.jar
Change your file name from exec.jar to exec.zip.
Extract all the contents of exec.zip into folder exec/
Change your library file name from driver.jar to driver.zip
Extract all the contents of driver.zip into folder driver/
Copy the contents of driver/ into exec/, but do not copy the META-INF folder. If a pop-up asks if it's ok to merge the folders, click yes.
Compress all files in exec/ into exec.zip
Rename exec.zip to exec.jar (replace the original).
You can include any java library inside a JAR using this method.

Here is the doc:
C:\Windows\System32>jar /?
Illegal option: /
Usage: jar {ctxui}[vfmn0Me] [jar-file] [manifest-file] [entry-point] [-C dir] files ...
And so I think the command you need is:
jar cvfe exec.jar main_class main_class

Reference jars inside a jar

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?

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.

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.

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.

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

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()
...

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

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