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Java integration with Xamarin.Android

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The Java ecosystem includes a diverse and immense collection of components. Many of these components can be used to reduce the time it takes to develop an Android application. This document will introduce and provide a high-level overview of some of the ways that developers can use these existing Java components to improve their Xamarin.Android application development experience.

Overview

Given the extent of the Java ecosystem, it is very likely that any given functionality required for a Xamarin.Android application has already been coded in Java. Because of this, it is appealing to try and reuse these existing libraries when creating a Xamarin.Android application.

There are three possible ways to reuse Java libraries in a Xamarin.Android application:

  • Create a Java Bindings Library – With this technique, a Xamarin.Android project is used to create C# wrappers around the Java types. A Xamarin.Android application can then reference the C# wrappers created by this project, and then use the .jar file.

  • Java Native Interface – The Java Native Interface (JNI) is a framework that allows non-Java code (such as C++ or C#) to call or be called by Java code running inside a JVM.

  • Port the Code – This method involves taking the Java source code, and then converting it to C#. This can be done manually, or by using an automated tool such as Sharpen.

At the core of the first two techniques is the Java Native Interface (JNI). JNI is a framework that allows applications not written in Java to interact with Java code running in a Java Virtual Machine. Xamarin.Android uses JNI to create bindings for C# code.

The first technique is a more automated, declarative approach to binding Java libraries. It involves using either Visual Studio for Mac or a Visual Studio project type that is provided by Xamarin.Android – the Java Bindings Library. To successfully create these bindings, a Java Bindings Library may still require some manual modifications, but not as many as would a pure JNI approach. See Binding a Java Library for more information about Java Binding libraries.

The second technique, using JNI, works at a much lower level, but can provide for finer control and access to Java methods that would not normally be accessible through a Java Binding Library.

The third technique is radically different from the previous two: porting the code from Java to C#. Porting code from one language to another can be a very laborious process, but it is possible to reduce that effort with the help of a tool called Sharpen. Sharpen is an open source tool that is a Java-to-C# converter.

Summary

This document provided a high-level overview of some of the different ways that libraries from Java can be reused in a Xamarin.Android application. It introduced the concepts of bindings and managed callable wrappers, and discussed options for porting Java code to C#.