✎ javacard:jcre:6.2.4.3_client_applet_b_requests_services_from_applet_a [wiki]

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=== 6.2.4.3 Client Applet B Requests Services from Applet A ===

1. Applet B can request service from applet A by invoking one of the shareable interface methods of SIO. During the invocation the Java Card VM performs a context switch. The original currently active context (B) is saved on a stack and the context of the owner (A) of the actual object (O) becomes the new currently active context. A's implementation of the shareable interface method (SI method) executes in A's context. 

2.  The SI method can determine the AID of its client (B) via the **JCSystem.getPreviousContextAID** method. This is described in [[6.2.5_Determining the Previous Context| Section 6.2.5, Determining the Previous Context]]. The method determines whether or not it will perform the service for applet B. 

3.  Because of the context switch, the firewall allows the SI method to access all the fields and methods of object O and any other object in the context of A. At the same time, the firewall prevents the method from accessing non-shared objects in the context of B. 

4.  The SI method can access the parameters passed by B and can provide a return value to B. 

5.  During the return, the Java Card VM performs a restoring context switch. The original currently active context (B) is popped from the stack, and again becomes the currently active context. 

6.  Because of the context switch, the firewall again allows B to access any of its objects and prevents B from accessing non-shared objects in the context of A.

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