Jni¶

Implement a new jni function¶

Add the following flags to the cmake command like cmake -DJNIVM_ENABLE_TRACE=ON -DJNIVM_USE_FAKE_JNI_CODEGEN=ON, this doesn’t work if you have configured to use FakeJni ( JNI_USE_JNIVM=OFF )

Edit this file https://github.com/minecraft-linux/mcpelauncher-client/blob/eb2465a1f59422b1cf11d14b9838eeb5d5c32237/src/jni/jni_support.cpp#L254 and add

vm.printStatistics();

Running mcpelauncher-client now, will print almost every function call from the game to the fake java native interface.
Closing the gamewindow now should log compileable stubs of the called functions without class inherence
Put the class or method declaration into a new header file in https://github.com/minecraft-linux/mcpelauncher-client/blob/eb2465a1f59422b1cf11d14b9838eeb5d5c32237/src/jni.
Put the stubs of the class / methods you want to implement into a new cpp file
Put the class descriptor into https://github.com/minecraft-linux/mcpelauncher-client/blob/eb2465a1f59422b1cf11d14b9838eeb5d5c32237/src/jni/jni_descriptors.cpp
Put vm.registerClass<MyClass>() into https://github.com/minecraft-linux/mcpelauncher-client/blob/eb2465a1f59422b1cf11d14b9838eeb5d5c32237/src/jni/jni_support.cpp#L25

Access FakeJni::JString¶

call FakeJni::JString::asStdString() to get a std::string
create a new jni string with std::make_shared<FakeJni::JString>("mycstr")

Register a native jni static function¶

To call native functions you have to register them first. Put

registerNatives(BrowserLaunchActivity::getDescriptor(), {
    {"somefunction", "(JLjava/lang/String;)V"},
}, symResolver);

BrowserLaunchActivity is the c++ class name of the jni class
"(JLjava/lang/String;)V" is the jni descriptor of this native function describes the parameter and return value of the function, see the jdk jni documentation
"somefunction" is the jni name of this function

into https://github.com/minecraft-linux/mcpelauncher-client/blob/eb2465a1f59422b1cf11d14b9838eeb5d5c32237/src/jni/jni_support.cpp#L91

Call a native jni instance function from an instance method¶

auto method = getClass().getMethod("(JLjava/lang/String;)V", "somefunction");
FakeJni::LocalFrame frame;
method->invoke(frame.getJniEnv(), this, callback, frame.getJniEnv().createLocalReference(std::make_shared<FakeJni::JString>("mystr")));

frame.getJniEnv().createLocalReference(std::make_shared<FakeJni::JString>("mystr")) creates a new jni reference for calling a native jni method
FakeJni::LocalFrame frame; reads the current attached fake jvm for this thread and creates a new jni reference frame, it’s destructor releases all local references of this frame. Call the contructor with a vm object to create a frame on detached threads.
"(JLjava/lang/String;)V" is the jni descriptor of this native function describes the parameter and return value of the function, see the jdk jni documentation
"somefunction" is the jni name of this function

Call a native jni static function from an instance method¶

auto method = getClass().getMethod("(JLjava/lang/String;)V", "somefunction");
FakeJni::LocalFrame frame;
method->invoke(frame.getJniEnv(), &getClass(), callback, frame.getJniEnv().createLocalReference(std::make_shared<FakeJni::JString>("mystr")));

frame.getJniEnv().createLocalReference(std::make_shared<FakeJni::JString>("mystr")) creates a new jni reference for calling a native jni method
FakeJni::LocalFrame frame; reads the current attached fake jvm for this thread and creates a new jni reference frame, it’s destructor releases all local references of this frame. Call the contructor with a vm object to create a frame on detached threads.
"(JLjava/lang/String;)V" is the jni descriptor of this native function describes the parameter and return value of the function, see the jdk jni documentation
"somefunction" is the jni name of this function

Call an arbitary native jni static function from an instance method¶

auto method = BrowserLaunchActivity::getDescriptor()->getMethod("(JLjava/lang/String;)V", "somefunction");
FakeJni::LocalFrame frame;
method->invoke(frame.getJniEnv(), BrowserLaunchActivity::getDescriptor().get(), callback, frame.getJniEnv().createLocalReference(std::make_shared<FakeJni::JString>("mystr")));

frame.getJniEnv().createLocalReference(std::make_shared<FakeJni::JString>("mystr")) creates a new jni reference for calling a native jni method
FakeJni::LocalFrame frame; reads the current attached fake jvm for this thread and creates a new jni reference frame, it’s destructor releases all local references of this frame. Call the contructor with a vm object to create a frame on detached threads.
"(JLjava/lang/String;)V" is the jni descriptor of this native function describes the parameter and return value of the function, see the jdk jni documentation
"somefunction" is the jni name of this function

Call an arbitary native jni static function from anywhere¶

// A reference to the jvm, you might have to manually pass them to your function or class
Baron::Jvm vm;
//...
FakeJni::LocalFrame frame(&vm);
auto method = BrowserLaunchActivity::getDescriptor()->getMethod("(JLjava/lang/String;)V", "somefunction");
method->invoke(frame.getJniEnv(), BrowserLaunchActivity::getDescriptor().get(), callback, frame.getJniEnv().createLocalReference(std::make_shared<FakeJni::JString>("mystr")));

frame.getJniEnv().createLocalReference(std::make_shared<FakeJni::JString>("mystr")) creates a new jni reference for calling a native jni method
FakeJni::LocalFrame frame; reads the current attached fake jvm for this thread and creates a new jni reference frame, it’s destructor releases all local references of this frame. Call the contructor with a vm object to create a frame on detached threads.
"(JLjava/lang/String;)V" is the jni descriptor of this native function describes the parameter and return value of the function, see the jdk jni documentation
"somefunction" is the jni name of this function

Create a global jni reference of a c++ object¶

// A reference to the jvm, you might have to manually pass them to your function or class
Baron::Jvm vm;
//...
auto activity = std::make_shared<MainActivity>();
auto activityRef = vm.createGlobalReference(activity);