Neil’s point-free blog

Lets be clear about one thing: the Android platform is not Java. It is an entirely new platform which, at best, has a compatibility layer for Java code.

Android uses the Dalvik virtual machine, which is a register-based VM whereas the Java VM is stack-based. Neither a specification nor the source code for Dalvik are available yet, but it’s clear that the bytecodes are quite different from Java bytecodes. There is a translation tool from JVM bytecode (.class files) to Dalvik bytecode (.dex files) but it is a build-time tool only. JVM bytecodes are never loaded onto an Android device.

Despite this, Java is used to develop Android applications, with the help of the Eclipse-based SDK. The key to this apparent contradiction is that somewhere on their way to deployment, the applications cease to be Java. The build pipeline uses a standard Java compiler to produce .class files but then Android’s “dx” tool transforms and repackages them into .dex files. In addition, a subset of the standard JRE libraries are available, although these have been similarly transformed. Because of this compatibility layer, it is possible to compile “most” standard Java source code to run on Android, so long as one avoids the parts of the standard library that have been omitted. Incidentally, I believe that Google have used the class library from Apache Harmony to achieve this. If they had taken the class library from OpenJDK, then they would have been required to adopt the GPL license.

Why use a custom VM instead of the JVM? Dalvik is apparently optimized to allow multiple instances of the VM to run simultaneously, even on devices with meagre memory. Each Android application runs in a separate Linux process, which is how the overall runtime achieves dynamic installation, activation and deactivation of applications. However this could have been achieved with a single JVM process using OSGi. Specifically, the Application Admin Service specification from the OSGi Mobile Expert Group addresses exactly this requirement.

It’s my suspicion that Google are only temporarily leveraging the Java language in order to gain developer mindshare and make use of existing tools such as Eclipse JDT, and they may well be planning to launch an efficient, Dalvik-specific language at some point in the future.