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Eclipse Java Build Path Jar Library classes not found even if available in jar

Recently I wanted to add the nebula shelf to my eclipse plugin. Therefore I added the org.eclipse.nebula.widgets.pshelf.source_1.1.0.201701302244.jar the build path of my eclipse project.
Sadly eclipse can't find the class files inside of the jar, even after cleaning and rebuilding the whole project.
Picture:
Checking the same jar file with jd-gui results in:
Picture:
I exported the *.java files with jd-gui and copied them to my project. Using this method my project built successfully and worked as expected.
Can someone tell me how I can use the jar file without exporting all of the java files manually? Thanks for your help! :-)

Do not use source jars. You can tell eclipse while debugging to use the source jar.
Add the jar to build path: right click on jar -> build path -> add to build path

Don't try to add Eclipse plugins directly to the build path of another plugin as it won't work.
Instead add the plugin to your target platform or import it in to your workspace.
Then add the plugin to your plugin's Dependencies. In the MANIFEST.MF editor you do this on the 'Dependencies' tab in the 'Required Plug-ins' list.

Including external jars permanently in NetBeans

I'm building a Java Swing Application. This project requires a jar file called JCalender. I've added this to Libraries folder and everything works fine on my computer in NetBeans. But when i open the same project in my friends NetBeans, it doesn't recognizes this library. I've to manually select the file placed inside the projects dist/lib folder. How to avoid this? please help!

But when i open the same project in my friends NetBeans, it doesn't
recognizes this library. I've to manually select the file placed
inside the projects dist/lib folder. How to avoid this?
You can't avoid this because it's not a problem actually. To compile and run a project you need to have access to the external libraries involved in the development, so if you open your NetBeans project in a different computer than yours you will definitely need to resolve the reference to the external libraries. There's no way for the IDE to do it automatically as far as I know.
Can't i give the relative path to the lib folder for that specific
library somewhere in project properties?
You could just give it a try. IMHO if the real goal is to share a project with other developers then I'd change the strategy. I'd create a Library (Tools -> Libraries) and tell my mates to create the very same library including the JCalendar JAR files in the library's classpath. I'd include this library in the project properties and finally I'd use a versioning tool like Git or SVN to share the project.
By doing this your mates still need a copy of the JAR file wrapped in a NetBeans Library, but the project properties won't point to a fixed/relative path looking for a JAR file but wil include a reference to a given Library. The Library itself will resolve the dependency to the JAR file. If you take a look to the project.properties file you'll see something like this:
javac.classpath=\
${file.reference.jcalendar-1.4.jar}
But if you as I've suggested then you'll see something like this:
javac.classpath=\
${libs.JCalendar.classpath}
Here libs.JCalendar.classpath will resolve the dependency so your mates can have the actual JAR file located in whatever folder they like and the project should compile just fine.
Another option is using Maven to manage the projects dependencies but honestly I'm not a Maven expert so I can't help you in this path.

You need to do a "clean and build" and your jar will be in the dist folder. It will include the external jars
You can read more about it here

I think your problem is due to you are providing absolute path of jar file.
while choosing jar on write side of filechooser there is option of
Relative path and Absolute path there you should click on Absolute path.

i am new to java , but anyway , i have faced the same problem and found a solution for my project
If you are in Netbeans , its would be very easy for you
Let you project name is ABC and all your dependent jar file is under the
the folder MyResourceCollection
now we need to permanently import all jars under this folder ,
So from Netbeans ,
right click on your project name
go to properties
go to library
in the library page , check the "libraries folder" label
you can find a browse button at the right side of the label
click browse and select the MyResourceCollection folder
a new window will come , just press Next-->Next--->Finish
all is done , now check yourself by moving the folder into different location

Add .jars in alternative folder

I am new to Eclipse and Java, and I know that .jar files I need to add into libs derectory so that compiler could recognize them. How I can add .jars not only in libs folder but also in other folder too. How can I configure Eclipse to do that?

It's easiest if you have the jar files visible in the package explorer to start with, i.e. within your project directory. At that point, you can just right-click on the jar file, go to the "Build path" section of the context menu, then select "Add to build path".
If the jar file is elsewhere and you don't want to move it, you can right-click on the project, and under the "Build path" part of the context menu, select "Add external archives...". Then find the jar file in the file browser, and hit OK.
All of this can also be done from the project properties dialog, in the Java Build Path section.

Right Click project
Select Properties
Select Java Build Path
Under Tab Libraries Click on add jars or add external jar to add the required jars

The Java build path is used while compiling a Java project to discover dependent classes . It is made up of the following items:
Code in the source folders
Jars and classes folder associated with the project
Classes and libraries exported by projects referenced by this
project
Our goal is to feed our classes with the dependent classes present in the jars during compile time. Eclipse provies with number of easy ways to do it .Here you can find a good article about how to add the jars in the projects with screenshorts attached to it http://www.wikihow.com/Add-JARs-to-Project-Build-Paths-in-Eclipse-(Java)

why eclipse not detect the jar file from lib folder

I am trying to develop java desktop application using eclipse and using some external jar file in project.
I have put that all jar file in /lib folder (this is desktop application so here /WEB-INF does not exist) now the problem is that the application cannot detect external jar file what is the problem ? and how I can solve this problem?

You need to add external jar file as :-
Click Properties > Java Build Path > Libraries.
Click Add External Jars and browse to the JAR file that contains the look and feel, then click OK.
Now, you are ready to apply the new Look and Feel to your application.

You have add it to classpath as well. right click on project and click add to build project.
click libraries tab and click external jar files. select your jar files. It will be added to classpath.

I think you have to inserti it. Rightclick in project properties build path, add external jar and browser to it.

just put jar file into the libs folder and abb to build path thats it. Updated eclipse not getting jar or configure file from the lib they get jar file from the libs folder.

You can right click on your jar file and choose Build Path and then choose add to build path

Where the heck is the Android app's lib folder?

I've been trying to find the so-called lib folder, to install some 3rd-party tools (this) there. They tell you to: "place it (downloaded jar file) in your Android app’s libs/ folder". But, in Eclipse, I could not find the lib folder even after expanding all of the directories shown in the navigator area. Will someone tell me where this lib folder is? Thanks.

If you use Eclipse do the following:
Right-click your project and hit 'Project Properties'.
Choose 'Java Build Path'.
Choose the 'Libraries' tab.
Hit 'Add External JARs...'.
Navigate to your desired .jar-file and hit 'Open'.
Click 'OK'.
Then you should be able to reference your external library.

You have to create it if it's not already there. It should be in the top level of your project folder, at the same level as the src and res folders.

Your lib folder is indeed used for storing external libraries.
This is commonly used to hold external .jars for external libraries, similar to how a .dll works.
You need to place the files in this folder yourself, then add them to the project as an existing archive to use them.
Let me know if this is unclear or you have any more questions!
EDIT:
You need to make sure you're putting this in your imports:
import myLib.fullyQualifiedClassName.*;
Hope this helps!