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Binding a Java library

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The Android community offers many Java libraries that you may want to use in your app. These Java libraries are often packaged in .JAR (Java Archive) or .AAR (Android Archive) format, but you can package a .JAR/.AAR in a Java Bindings Library so that its functionality is available to .NET for Android applications. The purpose of the Java Bindings library is to make the APIs in the .JAR/.AAR file available to C# code through automatically-generated code wrappers.

Tip

.NET 9 introduces support for automatically downloading and binding a Java library from a Maven repository. See the Binding a Java Library from Maven documentation to simplify this scenario.

Walkthrough

In this walkthrough, we are going to bind version 3.1.0 of CircleImageView, a library that displays an image in a circular canvas.

From the Maven repository, download circleimageview-3.1.0.aar locally to be bound.

Creating the bindings library

First, create a new Bindings Library project. This can be done with the "Android Java Binding Library" project template available in Visual Studio or via the dotnet command line with:

dotnet new android-bindinglib

Copy the circleimageview-3.1.0.aar file into the project directory.

Like .NET SDK style projects, .NET for Android binding projects automatically include any .JAR/.AAR files in the project directory as an AndroidLibrary type file, so no additional configuration is needed.

Now build the project using Visual Studio's Build command, or from the command line:

dotnet build

This Java library has now been bound and it ready to be referenced by a .NET for Android application project or published to NuGet for public consumption.

Additional options

Skip managed bindings

By default, C# bindings will be created for any .JAR/.AAR placed in the project. However C# bindings can be tricky to create and are not necessary if you do not intend to call the Java API from C#.

This is especially the case when the Java library is simply a dependency of another Java library and does not need to be called from C# directly. In this case, the Bind="false" attribute can be used to only include the Java dependency but not bind it.

<ItemGroup>
  <AndroidLibrary Update="circleimageview-3.1.0.aar" Bind="false" />
</ItemGroup>

Note if using automatic imports you will need to use Update to change the automatically imported file instead of adding an additional copy with Include.

Java dependencies

A Java library may depend on other Java libraries which will be required to be packaged with your application in order for your application to work. This information is traditionally provided in a .POM file, and it is your responsibility to ensure that any needed dependencies are properly referenced, usually via a NuGet package or by bundling the needed .JAR/.AAR files in your project.

In .NET 9, the Java Dependency Verification feature was added. By providing the .POM file, the binding tooling can help ensure you have met all required Java dependencies.

To enable Java Dependency Verification for our walkthrough, ensure you are using .NET 9+ and your project targets net9.0-android or greater.

From the Maven repository, download circleimageview-3.1.0.pom locally and place it in your project folder. Note that .POM files will not get detected automatically because they need to be associated with the correct .JAR/.AAR.

Update the automatically imported AndroidLibrary to specify the location of the .POM file with Manifest attribute. Additionally, specify the JavaArtifact and JavaVersion of the Java library:

<!-- JavaArtifact format is {GroupId}:{ArtifactId} -->
<ItemGroup>
  <AndroidLibrary
    Update="circleimageview-3.1.0.aar"
    Manifest="circleimageview-3.1.0.pom"
    JavaArtifact="de.hdodenhof:circleimageview"
    JavaVersion="3.1.0" />
</ItemGroup>

This library is trivial and does not have any Java dependencies, but if did and they were unmet, error like this would be emitted:

error XA4241: Java dependency 'androidx.collection:collection:1.0.0' is not satisfied.
error XA4242: Java dependency 'org.jetbrains.kotlin:kotlin-stdlib:1.9.0' is not satisfied. Microsoft maintains the NuGet package 'Xamarin.Kotlin.StdLib' that could fulfill this dependency.

Additional information on configuring Java Dependency Verification and how to satisfy dependencies can by found in the documentation.

Next steps

  • Customizing Bindings with Metadata - The Java library bound in the walkthrough is trivial and the binding tooling was able to fully automatically convert it to a C# API. Unfortunately this is often not the case and there will often be compile errors. These errors must be fixed with "metadata" to manually instruct the binding tooling how to resolve differences between the Java and C# languages.

  • Changing Namespaces - The types in the walkthrough end up in the .NET namespace DE.Hdodenhof.Circleimageview. Java package names tend to be more verbose than .NET namespaces and it may be more desirable to change it, for example to CircleImageViewLibrary using an AndroidNamespaceReplacement:

<ItemGroup>
  <AndroidNamespaceReplacement Include='DE.Hdodenhof.Circleimageview' Replacement='CircleImageViewLibrary' />
</ItemGroup>