.AAR and .JAR files

Abhinay Gupta
4 min readJun 11, 2023

.AAR Files

An AAR (Android Archive) file is a binary distribution format for Android libraries. It is similar to a JAR (Java Archive) file used in Java projects but specifically designed for Android development. An AAR file contains compiled code, resources, and metadata required to use a library module in an Android application.

When you create a library module in Android Studio, the build process generates an AAR file as the output. This file encapsulates the library’s code, resources, and dependencies, making it easier to distribute and reuse in other Android projects. The AAR file provides a convenient way to package an entire Android library into a single archive.

Components of an AAR file typically include:

1. Compiled Code: The AAR file contains the compiled bytecode of the library module. This includes the Java or Kotlin classes that make up the library’s functionality.

2. Resources: The AAR file includes any resources used by the library, such as layouts, drawables, strings, and other assets.

3. Manifest File: The AAR file contains the manifest file of the library module. The manifest defines the library’s package name, permissions, activities, services, and other important metadata.

4. ProGuard/R8 Configuration: If the library module applies code shrinking or obfuscation using ProGuard or R8, the AAR file may include the corresponding configuration files.

5. Library Dependencies: The AAR file may include information about the dependencies required by the library module. This allows the consuming application to resolve and include those dependencies when using the library.

Using an AAR file, developers can easily import and include external libraries into their Android projects. They can add the AAR file as a dependency in the project’s build.gradle file and access the library’s code, resources, and functionality.

To create an AAR file in Android Studio, you can create a library module, build the project, and find the generated AAR file in the `build/outputs/aar` directory of the library module.

Overall, an AAR file provides a convenient and standardized way to distribute Android libraries, allowing developers to share and reuse code across different projects efficiently.

.JAR files

A JAR (Java Archive) file is a file format used to store multiple Java classes, associated metadata, and resources into a single compressed file. JAR files are commonly used to distribute and package Java libraries, applications, or applets.

A JAR file can contain the following components:

1. Compiled Java Classes: The JAR file contains compiled bytecode (.class files) of Java classes. These classes implement the functionality provided by the library or application.

2. Resources: The JAR file can include various resources such as images, XML files, property files, configuration files, or any other files required by the library or application.

3. Manifest File: The JAR file contains a manifest file named `META-INF/MANIFEST.MF`. The manifest file contains metadata about the JAR file, including information about the main class to be executed, classpath entries, version information, and more.

4. Libraries and Dependencies: JAR files can include other JAR files or libraries that the application or library depends on. These dependencies can be specified in the manifest file or included directly in the JAR file.

5. Security Features: JAR files can include security-related features such as digital signatures to verify the authenticity and integrity of the contents.

JAR files provide a convenient way to package and distribute Java applications or libraries. They can be executed directly using the `java -jar` command or included as dependencies in other Java projects. JAR files are platform-independent, making them portable and compatible with different Java Virtual Machines (JVMs) on various operating systems.

To create a JAR file, you can use the `jar` command-line tool provided by the Java Development Kit (JDK) or use build tools like Apache Maven or Gradle, which automate the JAR creation process.

Overall, JAR files play a crucial role in the Java ecosystem, enabling the distribution, sharing, and reuse of Java code and resources in a compact and organized manner.

Both .jar and .aar files can be extracted and their contents can be modified, but the ability to modify them depends on various factors.

1. .jar Files: JAR files are typically used to distribute Java libraries or applications. They are essentially ZIP archives that contain compiled Java classes and resources. JAR files can be opened with archive tools, and you can extract the contents to view and modify them. However, modifying the compiled .class files directly may not be practical or straightforward, especially if you don’t have access to the original source code. Additionally, if the JAR file is digitally signed, modifying its contents may invalidate the digital signature.

2. .aar Files: AAR (Android Archive) files are specifically used for distributing Android libraries. They contain compiled code, resources, and metadata. Similar to JAR files, AAR files are also ZIP archives that can be extracted. You can modify the resources or make changes to the metadata, but modifying the compiled code (Java or Kotlin classes) directly may not be practical unless you have access to the original source code. If the AAR file includes ProGuard or R8 obfuscation, the code may be obfuscated, making it challenging to modify and understand.

It’s important to note that modifying the contents of a .jar or .aar file directly is not recommended in most cases. It is generally preferable to obtain the original source code, make the necessary modifications, and rebuild the library or application using the appropriate build tools. This ensures that the modifications are properly integrated and maintain the integrity of the code.

Additionally, when using libraries or dependencies, it is considered good practice to adhere to the licensing terms and conditions specified by the library’s author or provider. Modifying the code without proper authorization or violating the license terms may have legal implications.

In summary, while .jar and .aar files can be extracted and their contents can be modified, it is generally recommended to work with the original source code and follow proper software development practices to make modifications and enhancements to libraries and applications.