This sample guides you through hello-jni, a minimal C/C++ application built with the NDK. This sample is in the hello-jnidirectory of ndk-samples repo, inside android-mk branch.
The following two lines provide the name of the native source file, along
with the name of the shared library to build. The full name of the built
libhello-jni.so, once the build system adds the
lib prefix and the
LOCAL_SRC_FILES := hello-jni.c LOCAL_MODULE := hello-jni
For more information about what the
Android.mk file does, and how to use it, see
This line tells the build system the CPU and architecture against which to build. In this example, the build system builds for all supported architectures.
APP_ABI := all
For more information about the
Application.mk file, and how to use it, see
helloJNI.java file is located in
hellojni/src/com/example/hellojni/. It calls
a function to retrieve a string from the native side, then displays it on the screen.
The source code contains three lines of particular interest to the NDK user. They are presented here in the order in which they are used, rather than by line order.
This function call loads the
.so file upon application startup.
native keyword in this method declaration tells the
virtual machine that the function is in the shared library (that is, implemented on the native
external fun stringFromJNI(): String
public native String stringFromJNI();
The Android framework calls the function loaded and declared in the previous steps, displaying the string on the screen.
tv.text = stringFromJNI()
tv.setText( stringFromJNI() );
hello-jni.c file is located in
hello-jni/jni/. It contains a function that
returns a string that the Java side requested). The function declaration is as
JNIEXPORT jstring JNICALL Java_com_example_hellojni_HelloJni_stringFromJNI( JNIEnv* env, jobject thiz )
This declaration corresponds to the native function declared in the
Java source code. The return type,
jstring, is a data type defined
Interface Specification. It is not actually a string, but a
pointer to a Java string.
jstring comes the function name, which is based on the
Java function name and the path to the file containing it. Construct it
according to the following rules:
- Describe the filepath relative to the top-level source directory.
- Use underscores in place of forward slashes.
- Omit the
- After the last underscore, append the function name.
Following these rules, this example uses the function name
Java_com_example_hellojni_HelloJni_stringFromJNI. This name refers to a Java
stringFromJNI(), which resides in
JNIEnv* is the pointer to the VM, and
jobject is a pointer to the implicit
this object passed from
the Java side.
The following line calls the VM API
(*env), and passes it a return value:
that is, the string that the function on the Java side had requested.
return (*env)->NewStringUTF(env, "Hello from JNI ! Compiled with ABI " ABI ".");