One quick way would be to drag the library on to the sketch, which will place it in a "code" folder in the sketchpath.  It should work in the full path you described "libraries -> TestLib -> library -> TestLib.jar".  Note that a restart of Processing is required to pick up new additions to the libraries folder, once there you can update it without restarting.  You'll also want to make sure your import TestLib.*; is correct based on the internal structure of the package.

If you haven't read the wiki, it's a good resource for guidance.
http://wiki.processing.org/w/Create_Libraries

Thanks Jeff.

I successfully tried the first solution, but this copies the jar-file, right? (see the edit in my first posting...)

The other solution does not work, even after restarting processing.

"You'll also want to make sure your import TestLib.*; is correct based on the internal structure of the package."

Sorry, i don't understand what this means. I have learned to call packages like "mydomain.packagename", Processing does not accept these kind of packagenames...

Okay i'll read the wiki...

Okay, i now renamed the package-name inside eclipse to a name without dots and exported it again with the same name.

I have put this jar file into the libraries folder as described, and now it works.

But, i mean, it is very common to have package-names with dots in java, isn't it possible to use them?

Dots should work fine, which should mirror the directory structure in the project and the package name.

package com.mystuff.myprogram;

Should follow a directory structure on the disk of src/com/mystuff/myprogram, which Eclipse should package as com.mystuff.myprogram.

Then
import com.mystuff.myprogram.*;

Jeff, please excuse my dullness, i don't get it. Perhaps i did not understand some basics about java-packages.

The package is named like this:
com.mystuff.myprogramms.jar

But where i have to put it in the processing-directory?

Processing -> libraries -> com -> mystuff -> myprogramms -> com.mystuff.myprogramms.jar
(would make sense somehow but does not work)

or

Processing -> libraries -> com.mystuff.myprogramms -> library -> com.mystuff.myprogramms.jar

???

In processing, it should just be libraries/myprogramms/library/myprogramms.jar

You're package name should not have .jar on the end, unless jar is the name of your program.  jar is a packaging of class files.  If you were to rename your myprogramms.jar to myprogramms.zip and open it up, you should see a directory structure of com/mystuff/myprogramms/  which contains all your class files.

- Jeff

Many thanks, Jeff, now i got it!

Again i have some troubles with using external java-libraries in Processing.

I have my own library which i can use inside Processing.

But some of my methods are using the apache commons-lang3-3.0.1 library.
when i call methods which do so, i get (for example):

"Exception in thread "Animation Thread" java.lang.NoClassDefFoundError: org/apache/commons/lang3/math/NumberUtils"

Actually i expected, that Eclipse would export all the resources i use within my project.
Under Preferences-> Java Build Path -> User Libraries i had embedded the apache-commons as a user library.
At the properties of my Eclipse-Project, i have added this user library with
Java Build Path -> Libraries -> Add Librariy -> User Library ...

This works fine inside Eclipse, everything demanded is there.

Also at the project properties at
Java Build Path -> Order and Export -> i have checked the apache commons.

Question 1: What else do i have to do to export my library with all the resources i use in it?

Question 2: Alternatively i could put the apache commons into the Procesing libraries folder. But i don't manage this!
The exact filename of the apache commons is:
"commons-lang3-3.0.1.jar"
but when i put this folder into the Processings libraries folder:
commons-lang3-3.0.1/library/commons-lang3-3.0.1.jar
Processing says:
"The library commons-lang..... cannot be used. Library Names msut contain only basic letters and numbers......"
But when i rename the folder and the file like this:
apache/library/apache.jar
... i cannot use the library.

???

Question 3:
Jeff, you wrote "You're package name should not have .jar on the end, unless jar is the name of your program."-
I don't understand this. All the external libs in Processing have this .jar ending, like controlP5.jar. Whats wrong with this?

Thanks for reading + hopefully patience.
Hope this sounds not too confusing or confused!

EDIT:
okay, now i found out: i have:
apache/library/apache.jar
then i have to write inside the code of the skech:
import org.apache.commons.lang3.*;

and it works. Anyhow, i don't understand why this is neccesary.

Any comments on my confusion will be highly appreciated!

" Anyhow, i don't understand why this is neccesary"
Perhaps because thus it is easier for Processing to find the libraries (having the folder of same name than the file).

" I don't understand this. All the external libs in Processing have this .jar ending, like controlP5.jar"
There is indeed a little confusion here. Package names are different from name of the archive where the library lives.
.jar is just a file extension, like .zip. Actually, .jar files are .zip files. It is about files.
A package name is just a way to order the classes. This allows, among other things, to have two classes named List in several packages: in the standard Java, there is one in java.util and one in java.awt, for example. It is a common name, and we want to avoid clashes.
When you create a package org.philhosoft.util, it actually corresponds to path:
/some/path/to/the/program/org/philhosoft/util and the source file is there.
So, no .jar at the end of the package name (well, you can, it just corresponds to a folder of this name at the end of th path).

Have you tried the Processing Library Template? When you get it set up, among other things, it automatically writes a copy of your library in your sketchbook libraries folder.

thanks...