1# JNI 2As a general rule, jarjar every static library dependency used in a mainline module into the 3modules's namespace (especially if it is also used by other modules) 4 5Fully-qualified name of java class needs to be hard-coded into the JNI .so, because JNI_OnLoad 6does not take any parameters. This means that there needs to be a different .so target for each 7post-jarjared package, so for each module. 8 9This is the guideline to provide JNI library shared with modules: 10 11* provide a common java library in frameworks/libs/net with the Java class (e.g. BpfMap.java). 12 13* provide a common native library in frameworks/libs/net with the JNI and provide the native 14 register function with class_name parameter. See register_com_android_net_module_util_BpfMap 15 function in frameworks/libs/net/common/native/bpfmapjni/com_android_net_module_util_BpfMap.cpp 16 as an example. 17 18When you want to use JNI library from frameworks/lib/net: 19 20* Each module includes the java library (e.g. net-utils-device-common-bpf) and applies its jarjar 21 rules after build. 22 23* Each module creates a native library in their directory, which statically links against the 24 common native library (e.g. libnet_utils_device_common_bpf), and calls the native registered 25 function by hardcoding the post-jarjar class_name. Linkage *MUST* be static because common 26 functions in the file (e.g., `register_com_android_net_module_util_BpfMap`) will appear in the 27 library (`.so`) file, and different versions of the library loaded in the same process by 28 different modules will in general have different versions. It's important that each of these 29 libraries loads the common function from its own library. Static linkage should guarantee this 30 because static linkage resolves symbols at build time, not runtime.