Java Native Interface

This chapter introduces the JNI-Plugin, which is responsible for supporting the Java when using the ApertusVR library, and is used by the Android apps to cooperate with ApertusVR.

The Java Native Interface

The Java Native Interface (JNI) is a native language support, which allows Java code (running on JVM) to interoperate with applications and libraries written in C and C++. JNI provides a one-way access to call C/C++ functions from Java. Therefore it helps to use ApertusVR's features from Java codes, which is indispensably important, when programming on Android.

To learn more about the JNI interface see the official documentation on Oracle's website. For an easy to follow tutorial check NTU Signapore's guide.

Overview of the plugin

ApertusVR uses the JNI language support as a plugin, which helps distribute the events, and implements wrapper functionality for ApertusVR entities. The JNI-plugin consists of three layers:

1. The C++ layer (apeJNIPlugin shared library)

2. Java static native functions (ApertusJNI.java)

3. Wrapper classes (ape<CppClassName>.java)

1. The C++ layer

This layer is an ordinary ape::IPlugin implementation, which gather the events into its event queue, then when somebody calls the appropriate function in Java (ApertusJNI.processEventDoubleQueue()), the plugin fires the events on the Java side, using the JNI's callback possibilities.

2. ApertusJNI

This is a static Java class, which contains the Java "headers" of the C++ JNI functions, and everything else needed to use the plugin in Java.

3. Wrapper classes

For each subclass of ape::Entity in ApertusVR, there is a corresponding Java class (the ape::Entity is also included), which provides an interface to access, query, and modify an object on the C++ side. These wrapper classes contains only a String, and a apeEntity.Type member variables, which makes it available to identify the entity related to the interface object. Then it calls the static nativefunctions in ApertusJNI, to access the entity in C++.