Jar, Jni and .so files

Great question! This touches on the core of Android's native interface.

Let’s break this down step-by-step from the very basics to how .so files, jniLibs, and JNI work in an Android project — especially in the context of using a .jar file that relies on native code.

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🔹 What is JNI?

JNI stands for Java Native Interface.

It is a bridge that allows Java/Kotlin code to call functions written in native code — typically C or C++.

This is useful when:

• You need high performance (e.g., image processing, cryptography).

• You want to reuse existing C/C++ libraries in your Android app.

• You're interfacing with hardware or OS-level features.

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🔹 What is a .so file?

• .so = Shared Object file (Linux equivalent of .dll in Windows).

• It is a compiled binary that contains native (C/C++) functions.

• Android apps use .so files for native libraries.

For example:

libmynative.so

is a native library your app might need to run some C/C++ code from Java/Kotlin using JNI.

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🔹 What is jniLibs/?

jniLibs/ is a special directory in Android projects where you put your native libraries (.so files).

Structure:

app/
├── src/
│   └── main/
│       └── jniLibs/
│           ├── armeabi-v7a/
│           │   └── libmylib.so
│           ├── arm64-v8a/
│           │   └── libmylib.so
│           └── x86/
│               └── libmylib.so

Each subfolder corresponds to a CPU architecture (ABI). Android supports multiple ABIs, including:

• armeabi-v7a → 32-bit ARM

• arm64-v8a → 64-bit ARM

• x86 → Intel (emulators)

• x86_64 → 64-bit Intel

So you need to provide .so files for each ABI your app supports.

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🔄 How JAR + .so Works with JNI

Let’s say:

• You import a JAR file: libnativeclient.jar

• Inside the JAR, it contains Java code like this:

public class NativeBridge {
    static {
        System.loadLibrary("nativeclient");
    }

    public native String getSerialNumber();
}

This means:

• System.loadLibrary("nativeclient") tells the JVM to load libnativeclient.so

• getSerialNumber() is a native function implemented in C/C++

You must ensure:

• The .so file libnativeclient.so is present in your project’s jniLibs/<abi>/ directory.

• The method is implemented in your .cpp file (e.g., nativeclient.cpp).

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🔥 What Happens at Runtime

At app launch:

1. The app loads the class.

2. System.loadLibrary("nativeclient") executes → looks for libnativeclient.so in:

   • /data/app/<your.package>/lib/<abi>/

   • Which comes from the jniLibs directory during the build.

3. The native function getSerialNumber() is now callable.

If the .so file is missing: You’ll get this error:

java.lang.UnsatisfiedLinkError: dlopen failed: library "libnativeclient.so" not found

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🔧 How to Add .so Files to Your Project

Step 1: Place .so files in the right folder

app/
└── src/
    └── main/
        └── jniLibs/
            ├── armeabi-v7a/
            │   └── libnativeclient.so
            └── arm64-v8a/
                └── libnativeclient.so

Step 2: Update build.gradle

Gradle picks up native libraries automatically from jniLibs, but make sure you don't strip them:

android {
    sourceSets {
        main {
            jniLibs.srcDirs = ['src/main/jniLibs']
        }
    }
}

Optional (but recommended) in build.gradle:

packagingOptions {
    pickFirst 'lib/**/libnativeclient.so'
}

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✅ Summary Table

Concept	Description
JNI	A bridge to call C/C++ code from Java/Kotlin
.so file	Shared library file with compiled native code
jniLibs/	Folder to place .so files for Android builds
System.loadLibrary("x")	Loads libx.so from native libs at runtime
Error if missing	UnsatisfiedLinkError: dlopen failed

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Would you like a small working example of Java + native C/C++ JNI code in Android? Or how to link it with Gradle and verify loading works?