SAP JCo Scenario: Java/ABAP Communication in the Application Server 
In the SAP NW Application Server (SAP_BASIS), you can work both in a Java environment and in an ABAP environment. Communication between these components is established by the SAP JCo. Figure 1 shows an overview of the architecture of the SAP Application Server with the SAP Java Connector as the interface between the Java and the ABAP environment:
Figure 1: Java/ABAP Coupling in the SAP Application Server using SAP JCo
Data transfer between SAP JCo and the ABAP environment is performed by means of RFC (with gateway communication by means of CPI-C):
Figure 2: Communication between ABAP and Java using CPI-C (Application Server)
More Information
You can find an overview of the architecture of AS Java in: