Tools for writing great JNI code
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README.md

JniHelpers

JniHelpers is a library designed to facilitate writing JNI code with C++. It provides the following features:

  • Registry for cached class info (ie, jclass, jmethodID and jfieldID) lookups
  • "Automatic" conversion between Java <--> C++ objects. Note the ironic quotes around "automatic", which means:
    • Conversion must be enabled for the given class. This is as a feature, not a limitation, and means that classes may be represented by a Java/C++ counterpart, but you may not necessarily care to copy data from them in every JNI call. JniHelpers can do this for you, but you can also do it manally if performance is a concern.
    • Conversions are not performed automatically in every native callback. However, getting a C++ instance of a Java class or creating a Java instance of a C++ class is trivial.
    • JniHelpers prefers to copy memory, thus avoiding tricky issues that arise when trying to mix the very different memory management models of the Java garbage collector and C++ RAII paradigm.
  • Conversion between Java String objects and stl::string.
  • Conversion between Java byte arrays and C void* arrays.
  • Persisting native objects through Java without copying data. Persisted objects are created on the heap and leaked; their memory address is stored in a corresponding Java instance in a long field. If you want to access the native instance again, a call is provided to fetch the native instance from the field. This is very useful for creating long-lived native objects without having to make them singletons.
  • Easy registration of native methods. Rather than having to memorize the complex rules of creating JNI type signatures, a simple varargs interface is provided to do the grunt work for you.
  • Conveniences for common JNI functions, including:
    • Lookup Java classes with a given classloader
    • Attach/detach native threads to Java
    • Throw Java exceptions from native code, with printf-style string formatting
    • RAII template classes for JNI local/global/weak global references.
  • Android-friendly. JniHelpers was designed to work well on Android, and although it also runs fine on desktop Java, the CMake configuration is meant to be easily adapted for an Android build.
  • Tested! There is a test suite provided which is executed from Java that will run the native tests and prove communication across JNI works as expected.
  • Documented! Extensive class documentation is provided.

Examples

The best examples for JniHelpers is the test suite itself. See the classes within src/test/cpp, they are documented and explain correct usage of the library.

What JniHelpers is NOT

JniHelpers is not SWIG. It does not attempt to automatically create wrappers around native code.

JniHelpers is also not a complete wrapper around JNI. It does not attempt to completely shield the programmer from all JNI calls. Rather, it just makes the trickiest parts of JNI a bit friendlier to work with.

Building

JniHelpers contains both Java and C++ code, and is built with Gradle and CMake, respectively. To build JniHelpers as a static library, just run the following commands:

$ mkdir build
$ cd build
$ cmake ..
$ make

The Java part of JniHelpers can now be built as a JAR file:

$ ./gradlew assemble

To use JniHelpers in your project, it is not necessary to build standalone artifacts. Rather, you can use this project as a submodule in your code and include the CMake/Gradle script from your project. This is the preferred method for using the library.

Building JniHelpers on Windows has not been tested and may not work. The project is known to build for desktop Linux, Mac OS X, and Android.

Testing

JniHelpers has test cases, which are implemented in JUnit. If you wish to submit a pull request to this project, please add tests for any new code or bugfixes, and make sure that the existing tests continue to pass before sending the request. Pull requests with failing tests will not be merged!

Unfortunately, there is not yet integration between the two Gradle and CMake build system, so one must run them independently to execute the tests (we aim to fix this in a future release so that Gradle can execute the CMake build on its own). First, one must build the C++ parts with CMake as detailed above in the "Building" section. After the JniHelpers libraries have been built, one can run the JUnit tests from the top-level project directory like so:

$ ./gradlew test

If all goes well, Gradle should report "BUILD SUCCESSFUL" and no other output. Any test failures will be reported to the command line.

License

JniHelpers is made available under the Apache 2.0 license. For more information, please see the LICENSE.txt file distributed with JniHelpers.