libnativehelper is a collection of JNI related utilities used in Android.
There are several header and binary libraries here and not all of the functionality fits together well. The header libraries are mostly C++ based. The binary libraries are entirely written in C with no C++ dependencies. This is by design as the code here can be distributed in multiple ways, including mainline modules, so keeping the size down benefits everyone with smaller downloads and a stable ABI.
This is a header library that contains provides the API represented in the JNI Specification 1.6. Any project in Android that depends on
jni.h should depend on this.
These headers provide utilities that defined entirely within the headers. There are scoped resource classes that make common JNI patterns of acquiring and releasing resources safer to use than the JNI specification equivalents. Examples being
ScopedLocalRef to manage the lifetime of local references and
ScopedUtfChars to manage the lifetime of Java strings in native code and provide access to utf8 characters.
jni_macros.h header provide compile time checking of JNI methods implemented in C++. They ensure the C++ method declaration match the Java signature they are associated with.
A shared library distributed in the ART module that provides helper routines built on Java APIs. This library depends on details that are private to libcore and use should be restricted to platform code and within the ART module.
This library also contains the JNI invocation API from the JNI Specification and the glue that connects the ART runtime to the API implementation. The glue logic is platform only as it is used with the Zygote and the standalone dalvikvm.
This shared and static library contains a subset of the helper routines in libnativehelper based on public Java API. This code can be statically linked as the Java APIs it depends on are considered stable. The name of this library is a misnomer since it contains no C++ code.