Importing Existing C++ Libraries (.a or .so) into NDK Android
Introduction
This article guides you on importing existing C++ libraries in .a or .so format into your Android NDK project. These libraries can be external dependencies, pre-compiled modules, or your own reusable C++ code.
Understanding the Concepts
C++ Libraries
* **Static Libraries (.a):** These libraries are linked directly into your application binary during compilation. They’re typically smaller and faster, but can bloat the final app size if used extensively.
* **Shared Libraries (.so):** These libraries are loaded dynamically at runtime, providing flexibility and allowing code sharing across multiple applications. They tend to be larger than static libraries.
Android NDK
* **Android Native Development Kit (NDK):** Enables you to use native languages like C and C++ to develop portions of your Android application.
* **JNI (Java Native Interface):** Allows communication between Java code (running on the Android VM) and native code (compiled by the NDK).
Steps for Importing Libraries
1. Prepare the Library
* **Verify Compatibility:** Ensure your library is compiled for the appropriate Android architecture (ARM, x86, etc.).
* **Check Dependencies:** Make sure your library doesn’t have external dependencies that aren’t already present in your NDK setup.
2. Include the Library in your Project
* **Create a “jniLibs” folder:** Within your Android project’s “src/main” folder, create a folder named “jniLibs.”
* **Organize by Architecture:** Inside “jniLibs,” create subfolders for each supported architecture (e.g., armeabi-v7a, x86).
* **Place the Library:** Move your .a or .so file to the corresponding architecture folder.
3. Configure the Build System
* **Android.mk:** Create an “Android.mk” file in the root of your project’s “jni” folder.
* **Include Library:** Add the following lines to your “Android.mk” to incorporate your library:
“`
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := your_library_name
LOCAL_SRC_FILES := your_library_file.a
# Or LOCAL_SRC_FILES := your_library_file.so
LOCAL_LDLIBS += -L$(LOCAL_PATH)/../jniLibs -lyour_library_name
# Replace “your_library_name” with the actual library name
include $(BUILD_SHARED_LIBRARY)
“`
* **Application.mk:** Ensure you have an “Application.mk” file in the same location.
* **Specify ABI:** Set the “APP_ABI” variable to include the architectures your library supports (e.g., armeabi-v7a, x86).
“`
APP_ABI := armeabi-v7a x86
“`
4. Utilize the Library
* **Create a Header File:** If your library provides functions, create a header file (.h) that defines them.
* **Write JNI Bridge Code:** Implement JNI functions in your Java code that will act as an interface to call the library’s functionalities.
* **Example:**
“`java
public class NativeLib {
static {
System.loadLibrary(“your_library_name”);
// Replace “your_library_name” with the actual library name
}
public native int add(int x, int y);
}
“`
5. Build and Run
* **Clean and Rebuild:** Clean your project and perform a fresh build to incorporate the changes.
* **Run on Device:** Launch your application on a device or emulator.
* **Verify Functionality:** Test your application to ensure that the imported library works correctly.
Table: Static vs. Shared Libraries
| Feature | Static Libraries (.a) | Shared Libraries (.so) |
|————–|———————–|————————-|
| Linking | At compile time | At runtime |
| Code Size | Smaller, but larger app | Larger, but smaller app |
| Performance | Faster | Potentially slower |
| Flexibility | Less | More |
| Code Sharing | Not possible | Possible |
Example: Using a Pre-built Math Library
Let’s assume you have a pre-built math library “libMath.a” (for ARM architecture) that provides a function `calculateSum(int, int)` to add two integers.
Project Structure:
“`
MyAndroidProject/
|
+– src/
|
+– main/
|
+– java/
| +– com/
| +– example/
| +– MainActivity.java
|
+– jni/
|
+– Android.mk
+– Application.mk
+– MathLib.cpp
+– MathLib.h
+– jniLibs/
|
+– armeabi-v7a/
|
+– libMath.a
“`
1. Create “jniLibs” folder and place the library:
* Create a “jniLibs” folder in “src/main” folder.
* Create a subfolder “armeabi-v7a” inside “jniLibs”.
* Place “libMath.a” inside “armeabi-v7a”.
2. Configure Android.mk:
* Create an “Android.mk” file in the “jni” folder with the following content:
“`
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := MathLib
LOCAL_SRC_FILES := MathLib.cpp
LOCAL_LDLIBS += -L$(LOCAL_PATH)/../jniLibs -lMath
include $(BUILD_SHARED_LIBRARY)
“`
3. Create MathLib.h:
* Create a “MathLib.h” file in the “jni” folder:
“`c++
#ifndef MATHLIB_H
#define MATHLIB_H
extern “C” {
int calculateSum(int a, int b);
}
#endif
“`
4. Create MathLib.cpp:
* Create a “MathLib.cpp” file in the “jni” folder:
“`c++
#include
#include “MathLib.h”
extern “C” JNIEXPORT jint JNICALL
Java_com_example_MainActivity_calculateSum(JNIEnv *env, jobject thiz, jint a, jint b) {
return calculateSum(a, b);
}
“`
5. Update MainActivity.java:
* Add the following code to “MainActivity.java”:
“`java
public class MainActivity extends AppCompatActivity {
static {
System.loadLibrary(“MathLib”);
}
public native int calculateSum(int a, int b);
// …rest of your code…
}
“`
6. Build and Run
* Clean and rebuild your Android project.
* Run your application. You should be able to call `calculateSum` from your Java code and get the result.
Conclusion
By following these steps, you can successfully incorporate existing C++ libraries into your Android NDK project. Remember to ensure compatibility and adapt the configurations based on your library’s specific requirements. This approach enables code reusability, reduces development time, and simplifies the integration of pre-built components into your Android projects.