Android Application Framework

Android applications runs in the Dalvik virtual machine. This isn’t the Oracle’s Java Virtual Machine. This is the Google’s virtual machine optimized for mobile devices.

The Android SDK provides the tools and APIs necessary for developing applications on the Android platform using the Java programming language. In a standard project Java (from Oracle) is necessary to include the JDK (Java Development Kit) adding some jar in our project as rt.jar, resources.jar, among others, while in an Android project must include the Android SDK, only the android.jar, in our project.

Although this may sound strange at first, almost all the core classes of Oracle SDK are present in the Android SDK (totally rewritten in spite of it must have the same behavior). For example, the class String.java from Oracle’s Java is inside the rt.jar in the package java.lang, while the String.java from Google’s Java is inside android.jar in the same package structure, java.lang.

Beyond the core we find in Android SDK some classes that gives developers the ability to take advantage of the device hardware, such as access location information, background services, alarms, notifications, among others. The android.jar is a compilation of all the APIs you can use to develop your applications using Java.

An interesting note on Android SDK is that the code in android.jar is stubbed out. This because the code inside the jar is never runs, so there’s no reason to make the SDK bigger because of this. To better undersand, if you look at these classes using a decompiler (with jad for example) you will only see: public void something(…){ throw new RuntimeException(“Stub!”); }. This means that you use android.jar only for the purpose of compiling the code, use autocomplete and imports in Eclipse.

Moreover, android.jar contains only the APIs exposed for the level of applications development. It’s a promise from Google that if you compile your application using this jar, you are guaranteed your application will run in future versions of Android. There is a part of the compilation process in the Android system that removes methods and classes of API. It’s used internally by the framework and should not be used for an Android developer. Why does Google do it? Well, I believe it’s because Google needs to write too much code to get API exposed. A lot of code starting in the hardware until to get a high-level API, which can be changed from one version to another, even codes that may no longer exist in future versions.

But, how the Android is an open source project, we can download the code at: http://source.android.com/source/download.html

So, you can find, for example, the source of Activity.java into {base-code}/frameworks/base/core/java/android/app/. After compiling the source code, you can see a folder called “out” is created. For example, there is the Android system image in out/target/product/generic (files system.img, userdata.img and ramdisk.img). But what interests us, the library of the SDK, is in {base-code}/prebuilt/sdk/. You can see that was built a stub Activity class in out/target/common/obj/JAVA_LIBRARIES/android_stubs_current_intermediates/src/android/app/Activity.java.

To be continued…