libjnivm

Pseudo Java Native Interface implement Java functions in C++

Dependencies

Only the c++14 or c++17 standard library. To run Unit Tests this project automatically downloads Google Test during configuration of cmake.

Building

You need g++, clang++ or msvc with at least c++14 support and cmake to build and use it.

Get the Source

git clone https://github.com/ChristopherHX/libjnivm.git
cd libjnivm

Configure

Choose zero or more configuration options to change the behaviour of this library.

CMake Option	Values	Default	Description
JNIVM_ENABLE_TRACE	ON, OFF	OFF	activate logging of called function to help Troubleshooting errors
JNIVM_ENABLE_GC	ON, OFF	ON	deprecated option, previous version had only a experimental GC used to disable it enitirely
JNIVM_ENABLE_DEBUG	ON, OFF	ON	enables additional debugging features like a stub code generator for faster reverse engineering. Use together with void jnivm::VM::GenerateClassDump(const char * path); to generate the stubs to a file (c++) with the specified path, you may need to create the parent folder of the path. You will get different generated code if you change the value of the configuration option JNIVM_USE_FAKE_JNI_CODEGEN
JNIVM_USE_FAKE_JNI_CODEGEN	ON, OFF	OFF	choose to generate FakeJni compatible stubs instead of the default syntax of this library. Depends on JNIVM_ENABLE_DEBUG=ON to work. Use together with Baron::Jvm::printStatistics() to print the stubs to stdout
JNIVM_ENABLE_RETURN_NON_ZERO	ON, OFF	OFF	contruct objects which are default_contructible with a parameterless contructor or classes without a native type as an empty jnivm::Object and returns these instead of returning a nullptr. Use together with JNIVM_ENABLE_TRACE=ON, to see if a method wasn't found, but a return value was constructed
JNIVM_FAKE_JNI_MINECRAFT_LINUX_COMPAT	ON, OFF	OFF	It is unclear how the fake-jni interface should handle static functions and static fields, based on the original sample from https://github.com/dukeify/fake-jni/blob/16b82688cb9a8794580293253fbe313f550eb00c/examples/src/main.cpp it seems, it should promote them to instance functions. To intercept this behavior add JNIVM_FAKE_JNI_MINECRAFT_LINUX_COMPAT=ON, to keep them static if they are not explicitly set to static like { Function<&staticFunction>, "staticFunction", JMethodID::Static }
JNIVM_ENABLE_TESTS	ON, OFF	OFF	enables and build gtest tests
JNIVM_BUILD_EXAMPLES	ON, OFF	OFF	Enable jnivm / fake-jni (compat) examples

create and change to a build directory

mkdir build
cd build

Add the configure options like -D<Option>=<value> per option, add a space between each one. Then place them after cmake .. . For example two options shown here:

cmake .. -DJNIVM_ENABLE_TRACE=ON -DJNIVM_BUILD_EXAMPLES=ON

Now build the library and optionally it's Example and / or Tests

cmake --build .

Fake-jni / Baron Compatibility

Probably not all features of Fake-jni / Baron are implemented right now, open an issue to increase the chance to improve the compatibility. As of 3.2021 the original has some broken features, they will close all issues without notice.

• FakeJni::JString provided in this implementation correctly handles modified utf8 to utf16 like the Java native interface specification requires, but the original fake-jni has a strange things going on see here dukeify/fake-jni#105
• FakeJni::Array<T> (T is a FakeJni Class) has a similiar class hirachy as the type T
  • So you can write std::shared_ptr<FakeJni::Array<Base>> b = std::make_shared<FakeJni::Array<Derived>>(); without hitting undefined behavior or compile time errors, if Derived has the super class Base and you passed the type into the DEFINE_CLASS_NAME("Derived", Base) macro
• This implementation does not aim to provide a jvmti implementation at all, original FakeJni has only stubs as of 3.2021
• This implementation has a Garbage collector based on std::shared_ptr, all object have to be wrapped within one, if they aren't a jni type
  • You write std::shared_ptr<FakeJni::JObject> obj; instead of something like FakeJni::JObject * obj; or FakeJni::JObject obj;
  • For Arrays you only need to wrap the outer array, like std::shared_ptr<FakeJni::Array<FakeJni::JString>> for a array of strings
  • Multi dimension Arrays are defined like std::shared_ptr<FakeJni::Array<FakeJni::Array<FakeJni::JString>>>
  • This is the same syntax as the changes to original FakeJni by minecraft-linux
• FakeJni sometimes fabricates Object's instead of returning 0, to mimic this add -DJNIVM_ENABLE_RETURN_NON_ZERO=ON
• Add -DJNIVM_ENABLE_RETURN_NON_ZERO=ON, -DJNIVM_FAKE_JNI_MINECRAFT_LINUX_COMPAT=ON to share your source code between minecraft-linux and this implementation
• The samples of Baron's Readme will compile with this implementation
  • We provide void Baron::Jvm::printStatistics() while the original doesn't even declare it

Examples

You find some examples inside src/examples. Please open one or more issue's if you have any additional questions or found a Bug not covered by a Test.

Similar Open Source Projects

• https://github.com/minecraft-linux/fake-jni + https://github.com/minecraft-linux/baron
• https://github.com/dukeify/fake-jni + https://github.com/dukeify/baron Unmaintained since 4 Apr 2020

History

• Initially developed for https://github.com/ChristopherHX/mcpelauncher-manifest, fall 2019
• Now optionally used by https://github.com/minecraft-linux/mcpelauncher-manifest/tree/ng to replace https://github.com/minecraft-linux/baron, fall 2020
• Version 1.0 spring 2021
  • Increased Test Line Coverage to ca. 70%
  • Increased FakeJni / Baron compatibility
  • Fixed String conversions, correctly handle and use Modified UTF8
  • Implemented a lot of missing things
    • exception handling
    • WeakGlobal's
    • distinguish Global from Local References
  • Removed unsafe type casts, a jobject is now always jnivm::Object* then casted via dynamic_cast to the right type