Working with J2EE Libraries 
In many situations, you want to avoid certain functions that are encapsulated in a JAR being packed into an application. For example, you may want to use a JAR in a Web project, but not pack it into the EAR of the application. Instead, only a reference to this JAR should be included in the used project during the build. This means that you must add the JAR to be used to the Java build path of your application. To make a JAR file available in the J2EE engine, it must be packed as a J2EE library (or : library, for short). These libraries group reusable functions together and are also available to other libraries or applications under one logical name.
Depending on whether or not your application projects are based on the SAP component model, there are two different forms of library:
Libraries
The Developer Studio allows you to define libraries in special projects. A library project can contain one or more JARs or references to existing server components (libraries, interfaces, services). A library project defines exactly one library, which is made available in the J2EE engine under a logical name.
Whenever you do not use the SAP component model in your J2EE applications, you work with normal library projects.
Library DCs
You can also define a library as a development component (DC). This is particularly useful if your application projects are based on the SAP component model and therefore make use of the SAP development infrastructure.
Topics
Defining Library Development Components
Referencing Libraries in Applications