Compiling Native Code for Android Apps using Gradle and NDK

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Alright, something that’ll make your Android apps run faster than Usain Bolt on juice: compiling native code using Gradle and NDK! Now, before you start hyperventilating at the thought of all those confusing acronyms, hear me out.

First, let’s break down what we’re dealing with here. Gradle is a build automation tool that helps us manage our project dependencies and compile code more efficiently. NDK stands for Native Development Kit, which allows us to write native C/C++ code in our Android apps. So basically, we’re going to use Gradle to handle the Java side of things while also incorporating some sweet, sweet native goodness into our app.

Now that you have a basic understanding of what we’re working with, let’s dive right in! First, make sure your project is set up properly by adding the following lines to your build.gradle file:

// This script is used to set up the project properly by adding necessary lines to the build.gradle file.

// Dependencies section is used to declare external libraries or modules that the project depends on.
dependencies {
    def ndkVersion = "r16b" // Declaring a variable to store the NDK version.
    def ndkDir = "/path/to/your/ndk-bundle/" // Declaring a variable to store the path to NDK folder on the machine.

    // ExternalNativeBuild section is used to configure native build settings.
    externalNativeBuild {
        // CMake is a cross-platform build system used for building native code.
        cmake {
            // Arguments section is used to pass arguments to the CMake build.
            arguments '-DCMAKE_TOOLCHAIN_FILE=', "${ndkDir}/build/cmake/android.toolchain.cmake" // Setting the path to the CMake toolchain file.
            arguments '-DANDROID_NDK:PATH=' + ndkVersion // Setting the path to the NDK version.
        }
    }
}

This code tells Gradle to use the CMake build system and set up our project for native development using the specified NDK version. Next, create a new folder in your app’s directory called “jni” (without quotes) where we will store all of our native source files. Inside that folder, add a file named Android.mk with the following contents:

“`c++
// Set the module name and include path for C header files
// The LOCAL_PATH variable stores the current directory path
// The CLEAR_VARS macro clears all previously set variables
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
// The LOCAL_MODULE variable specifies the name of the module to be built
LOCAL_MODULE := yourModuleName
// The LOCAL_SRC_FILES variable specifies the source file(s) to be compiled
LOCAL_SRC_FILES := yourSourceFile.cpp
// The LOCAL_CFLAGS variable adds any additional compiler flags
// Here, we are setting the C++ version to be used
LOCAL_CFLAGS += -std=c++14
// The LOCAL_LDLIBS variable adds any additional libraries needed by the code
// Here, we are adding the log library
LOCAL_LDLIBS += -llog
// The BUILD_SHARED_LIBRARY macro builds a shared library using the specified variables
include $(BUILD_SHARED_LIBRARY)



Replace "yourModuleName" and "yourSourceFile.cpp" with the appropriate values for your project. This file tells CMake how to compile our native source files into a shared library that can be used in our Java code. Finally, add the following lines to your app's build.gradle file:

groovy
// This script is used to configure the build process for a project that uses native source files.
// It sets the source directory for the native libraries and creates tasks for generating a sources jar and javadoc.

// Set the source directory for the native libraries
sourceSets {
main {
jniLibs.srcDir ‘jni/obj/’ // replace with path where CMake will generate object files
}
}

// Create a task for generating a sources jar
task sourcesJar(type: Jar) {
classifier = ‘sources’
from sourceSets.main.allJava // include all java source files in the jar
}

// Create a task for generating javadoc
task javadoc(type: Javadoc) {
options.add ‘-sourcepath’, sourceSets.main.allSource // set the sourcepath for javadoc to include all source files
}
“`

This code tells Gradle to include the generated object files in our APK and create a sources jar for debugging purposes. And that’s it! Now you can compile your native code using Gradle and NDK, which will result in faster app performance and happier users.

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