A Java instance is a unit in the AS Java cluster, which can be started, stopped, and monitored separately. The cluster elements that form an instance run on one physical machine. It is also possible to run several instances on one physical machine. An instance is identified by the system ID (SID) and the instance number.
Central Services are a special example of a Java instance.
Another special instance is the one that runs the SDM ( Software Deployment Manager). This one usually runs with the database and Central Services on the same machine and is then indicated as the central instance.
A Java instance contains at least one Server Process.
Normally it comprises one Java Dispatcher and several server processes.
A Java instance is started and stopped by the Java Startup and Control Framework.
There could be Java instances in mixed ABAP/J2EE systems, as well as in Java-only systems. For more information, see SAP NetWeaver Application Server with ABAP and Java.
An HTTP request to execute a servlet runs through several layers in the J2EE Engine. The Java dispatcher receives the request, selects a server process for the processing and establishes the connection from the client to the server.