Android JNI Development Guide155

Java Native Interface (JNI) is a powerful feature of Android that allows you to call native C/C++ code from your Java code. This can be useful for performance-critical tasks, accessing hardware-specific features, or integrating with existing C/C++ libraries.

Prerequisites

Before you start developing JNI code, you'll need to have the following:
Android Studio
NDK (Native Development Kit)
C/C++ compiler

Creating a New JNI Project
Open Android Studio and create a new project.
In the "Configure your project" window, select "C++ Support" and click "Next".
In the "Edit Native Build System" window, select "CMake" and click "Finish".

Writing Native Code

Native code is written in C/C++ and placed in the src/main/cpp directory. The following code demonstrates how to create a simple JNI function:```cpp
#include
JNIEXPORT jstring JNICALL
Java_com_example_myapplication_MainActivity_getStringFromJNI(
JNIEnv *env,
jobject /* this */) {
return (*env)->NewStringUTF(env, "Hello from JNI!");
}
```

In this code:* #include includes the JNI header file.
* JNIEXPORT jstring JNICALL defines the function signature.
* Java_com_example_myapplication_MainActivity_getStringFromJNI is the fully qualified name of the Java method that will call this function.
* env is a pointer to the JNI environment.
* (*env)->NewStringUTF(env, "Hello from JNI!") creates a new Java string with the specified value.

Registering Native Functions

To make native functions accessible to Java code, you need to register them using the RegisterNatives function. Add the following code to the onCreate method of your main activity:```java
("native-lib");
// Get the JNI environment
JNIEnv env = (JNIEnv) ().getJNIEnv();
// Get the class of MainActivity
jclass cls = ("com/example/myapplication/MainActivity");
// Register the native function
(cls, methods, methodCount);
```

In this code:* ("native-lib") loads the native library containing your JNI code.
* ().getJNIEnv() retrieves the JNI environment.
* ("com/example/myapplication/MainActivity") gets the class of MainActivity.
* (cls, methods, methodCount) registers the native functions with the specified class.

Calling Native Code from Java

To call a native function from Java, use the Java_ prefix followed by the fully qualified name of the native function. For example, to call the getStringFromJNI function, you would use the following code:```java
String result = ();
```

Debugging JNI Code

Debugging JNI code can be challenging. Here are some tips:* Use logging statements to trace the execution of your code.
* Set breakpoints in your native code and use the LLDB debugger.
* Use JNIAttachThread to debug native code from the Java debugger.

Conclusion

JNI is a powerful tool that can extend the capabilities of your Android applications. By following these steps, you can create and use JNI code effectively.

2024-12-11

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