Differences between .aar and .jar files on Android: Complete guide

If you program applications for Android or develop libraries for this ecosystem, you've surely encountered .jar and .aar files more than once. You may even have doubts about when to use each format, what the real differences are between the two, and what implications their choice has for your project. Although both are compressed files designed to distribute reusable code and resources, their structure, purpose, and capabilities vary significantly in the context of Android. This article aims to answer all your questions on the subject, and to do so, we'll go into depth, with clear and comprehensive explanations, but also with a natural and practical approach, ideal whether you're just starting out or are already a veteran of Android development.

You will discover in detail What is a JAR file, what characterizes an AAR and how both formats are used in the different phases of Android development. We'll review their advantages, limitations, internal structure, and how they influence the distribution and use of libraries, both in our own and third-party projects. We'll also examine their role in modern build systems like Gradle and Bazel, and you'll learn key tips for getting the most out of them while avoiding typical problems. Everything is explained in Spanish, with very clear examples and highlighting key points so you can easily refer to them whenever you need to.

What are JAR files?

The JAR (Java ARchive) file is, in essence, a compressed file containing files .class (already compiled Java code), resources such as images, properties files and configuration files, as well as meta information in the file META-INFThe JAR was designed to simplify the distribution of Java applications and their libraries by allowing all necessary components to be packaged into a single, easy-to-transport and manage file. In classic Java, .jar files are the basic unit for distributing both complete applications and reusable modules.

On Android, JARs still exist, and are often used to encapsulate pure Java logic, such as utilities, algorithms, or platform-independent libraries. But be careful: a JAR can never contain Android resources (layouts, drawables, etc.), nor can it contain an AndroidManifest.xml. It also doesn't include native code (C/C++). Therefore, when your code needs to interact with Android system components, define visual resources, or manage permissions, the JAR falls short and the AAR enters the scene.

What is an AAR file?

The AAR (Android ARchive) file is the Android-specific evolution of the JAR concept. An AAR is also a compressed file. (basically a ZIP), but with a structure and content designed to meet all the needs of a modern Android library. An AAR can contain:

• Compiled Java code (.class, bundled in .jar)
• Archives AndroidManifest.xml specific to the library
• Android Features: layouts, drawables, strings, icons, etc.
• Native code C / C ++ (.so files, optionally)
• Proguard files, Lint resources, and more

This makes the AAR is the recommended format to distribute libraries and plugins on Android, since it encapsulates everything necessary for a bookstore to function as it is in any project, managing both the code and the associated resources and configurations.

Structural differences between JAR and AAR

Let's get down to business with a clear comparison of what each format can and can't do. Just to give you an overview:

While a JAR is limited to packaging pure Java code and metadata files for any type of Java project, the AAR is designed to hold the entire Android ecosystem: resources, manifest, native code, and specific rules that may be needed when integrating the library with an app or other libraries.

What are JAR files used for on Android today?

Using JARs on Android still makes a lot of sense in certain contexts. Pure Java libraries (no resources, no manifest, no Android-specific code) are usually distributed as JARs, as this format is universally accepted by any Java project, whether desktop, backend, or Android. If you have string handling utilities, mathematical logic, third-party libraries developed before the rise of Android, annotation processors, or standalone modules, the JAR is a valid and optimal option due to its simplicity.

In addition, many annotation processors and tools in the Java ecosystem (such as those used with kapt o annotationProcessor) are distributed as JARs, since their function is limited to compilation time and they do not require additional resources.

When is it essential to use AAR instead of JAR?

Every time your bookstore needs Android resources —whether layouts, drawables, menus, strings, styles— or your own manifesto, you must package it as AARIf, for example, you're developing a visual component (a custom button, a snippet, a widget) or need to include a permissions block, services, receivers, or native code... the JAR is no longer useful.

The AAR allows the bookstore to:

• Inject resources into the consuming application, making it possible for them to be used layouts and drawables defined in the library.
• Keep a AndroidManifest.xml separate, which the compilation system will then merge with the final app.
• Retail .so files with native code, necessary for certain advanced functionalities.
• Define Proguard/Lint rules associated with the library to protect the code or add automatic checks.

Ultimately, the AAR format is the de facto standard for any reusable code that needs to be fully integrated into an Android project, facilitating modularity and proper dependency management.

Competitive advantages of the AAR over the JAR

On a practical level, the main Advantages of AAR in Android projects regarding the JAR are:

• Complete distribution of functionalitiesBy allowing you to include resources, manifests, and native code, AAR makes it possible to package any type of functionality, from third-party SDKs to modules that extend your own app.
• Easy and centralized updates: Using remote repository systems (such as MavenCentral or the Google repository), AAR libraries can be published and updated quickly, affecting all projects that use them.
• Integration with modern build systemsGradle and Bazel, the most widely used build systems on Android, offer direct support for AARs and automatically manage the merging of resources, manifests, and build rules derived from the file.
• Support for modularity and testingModern Android apps are often divided into multiple modules and libraries, many of which require their own resources or manifests. AAR facilitates these schemes and allows you to write tests (unit and instrumentation) tightly coupled to the module.

The JAR is still useful for pure Java logic or annotation processors, but for full-featured, functional Android modules that include resources and integration with the operating system, the AAR is practically unbeatable.

Limitations and considerations to take into account

Not everything is an advantage, of course. The JAR format is universal: Any Java environment can consume it, from servers to desktop applications. But the AAR only makes sense in Android projects and is designed to be consumed by Gradle or Bazel, not by other environments. Furthermore, Packaging resources in AAR may require extra adjustments (such as proper namespace management, resource and manifest merging, and dependency versioning). Therefore, if you're distributing cross-platform modules or your logic is completely independent of Android, consider packaging that part of your code in a JAR.

Another key aspect: JARs cannot include Android resourcesIf you need them, you'll have to manually duplicate them in each consuming project, losing all the benefits of modularity, centralized maintenance, and automatic AAR updates.

How JARs and AARs integrate into build systems (Gradle and Bazel)

Android Studio's build system (based on Gradle) and advanced systems like Bazel have specific rules for handling both JARs and AARs.

In Gradle

When you add a JAR dependency to your Android module, you use a notation like the following in your build.gradle:

dependencies { implementation files('libs/mimil.jar') }

Esto includes the Java code in your APK, but it doesn't give you access to Android resources. For AARs, you simply add the reference to the AAR artifact (usually from a Maven repository or the Google repository), and Gradle takes care of merging resources, manifest, and native code:

dependencies { implementation 'com.mycompany:milibreria-android:1.0.0' }

If the artifact is an AAR, Gradle automatically detects, unpacks, merges, and resolves it, and your resources and manifest are seamlessly integrated into the final app.

Related article:

File Hierarchy and Structure in Android: A Complete Guide to Understanding and Optimizing the File System

In Bazel

Bazel, increasingly popular in large projects due to its speed and modularity, defines clear rules for working with both formats:

• java_library: compiles Java sources and produces an output JAR.
• android_library: same as java_library, but also produces an AAR (with manifest and resources).
• aar_import: allows you to directly consume an AAR file, which can then be used in android_binary or android_library rules.

Basic example of integrating an AAR in Bazel:

aar_import( name = "google-vr-sdk", aar = "gvr-android-sdk/libraries/sdk-common-1.10.0.aar", ) android_binary( name = "app", manifest = "AndroidManifest.xml", srcs = glob(), deps = , )

With this, the AAR is accessible for any module or app that needs it, merging with its own resources and manifests.

What happens if I include Android resources in a JAR?

If you try to put Android resource files (XML layouts, images, strings…) into a JAR and then depend on that JAR in your project, those resources will never be available in your app when building. Gradle and Bazel only process resources included in the app tree and in AAR modules. Therefore, access to R.layout.tulayout will fail at compile time and/or run time, generating errors that are difficult to debug.

The internal structure of an AAR file

Let's see how a typical .aar file is organized internally. If you unzip it (remember, it's a .zip), you'll usually find these files and folders:

• classes.jar: the compiled Java code
• AndroidManifest.xml: specific manifesto of the bookstore
• res /: Android resource folders (layouts, drawables, values…)
• assets /: binary resources or extra data
• jni/: native code for different architectures
• R.txt: definition of exported resources
• proguard.txt, lint.jar: optional rules for Proguard or Lint

All of this content is seamlessly merged by the build system, ensuring that your app can directly use resources, manifests, and native code defined in the AAR.