Slide: 1 Copyright © 2013 AdaCore

José F. Ruiz

FOSDEM 2013, Brussels

Senior Software Engineer AdaCore

Programming Android in Ada

3 February 2013

Slide: 2 Copyright © 2013 AdaCore

Outline

•  The execution platform

•  How to build applications –  Using the virtual machine –  Interfacing the virtual machine

–  Ada compilers and related technologies

•  The show

–  Or how to use it

Slide: 3 Copyright © 2013 AdaCore

Android

•  Linux-based operating system –  Open source

•  Targets smartphones and tablets –  But not only

•  Applications implemented mostly in Java –  Available SDK, APIs, IDE

•  Main hardware platform is ARM –  But also x86, MIPS

•  There are nice emulators to facilitate development and testing

Slide: 4 Copyright © 2013 AdaCore

•  Linux kernel

•  Middleware

•  Libraries

•  APIs

•  Application framework –  Includes Java-compatible libraries –  Uses the Dalvik virtual machine with just-in-time compilation

–  Run Dalvik 'dex-code' (Dalvik Executable) –  Usually translated from Java bytecode

Android software architecture

Slide: 5 Copyright © 2013 AdaCore

Android SDK

•  Android Software Development Kit (SDK) –  API libraries and developer tools necessary to build, test, and debug apps for Android –  Includes simulator

–  http://developer.android.com/sdk/index.html

•  Android Native Development Kit (NDK) –  Implement parts of your app using native-code languages (C, C++, Ada,…) –  http://developer.android.com/tools/sdk/ndk/index.html

Slide: 6 Copyright © 2013 AdaCore

The strategy for Ada

•  Use the Android SDK for what it handles well –  Structure of the application, graphical interface, … –  Java at this point

–  On the Dalvik VM

•  Use GNATbench to create a mixed Java/Ada application –  Write the Ada subprogram as usual

–  Automatic creation of the glue code for the interface to Java

–  Use the generated Java code from the Android application

package Hello is function Msg return String; end Hello;

import com.example.Hello.Hello_Package; public class MainActivity extends Activity { … view.SetText (Hello_Package.Msg ().toString ()); … }

Slide: 7 Copyright © 2013 AdaCore

Interfacing Ada and Java

•  Goal: applications not written entirely in Java –  Standard Java class library missing platform-dependent features –  Reuse already existing code –  Implement time-critical part of the application

•  A communication layer between the two languages –  Java Native Interface (JNI) –  Low level interface between the JVM and the native environment –  Expensive and error-prone to use manually

•  An interface in the target language –  Binding generation tool (Ada2Java)

•  A builder for the whole application –  GPRbuild

•  A debugger for the whole application –  Eclipse (with GDB & Java debugger underneath)

Slide: 8 Copyright © 2013 AdaCore

How it works

I have code written in Ada I want to use this code in a Java project

JNI

procedure Proc (P : Rec); void Proc (Rec_Bound P);

On a single machine…

Slide: 9 Copyright © 2013 AdaCore

Ada2Java

•  Takes a set of Ada packages as input –  The interface that we want to make available to Java

•  Process the public specifications, using ASIS

•  Generates Ada and Java connection to JNI

•  Generates a Java high level and type safe view of the Ada package

•  The user does not deal with JNI directly

Slide: 10 Copyright © 2013 AdaCore

Example of binding generation

package Test is function Addition (A, B : Integer) return Integer; end Test;

package Test; public class Test_Package { static public int Addition (int A, int B){ // Code … } }

public static void main (String [] argv) { int r = Test_Package.Addition(10, 20); System.out.println ("10 + 20 = " + r); }

Slide: 11 Copyright © 2013 AdaCore

Ada part of the toolset

•  GNATbench –  Official Android IDE is Eclipse –  Eclipse plug-in to build Ada libraries on Android

•  Ada-Java Interfacing Suite (AJIS) –  Java applications on Android can invoke Ada code using interfaces automatically

generated by AJIS

•  GNAT for Android

–  For the part of the application running on Linux/Android (outside the VM)

–  Configured for ARM/Linux

Slide: 12 Copyright © 2013 AdaCore

Ada tasks on Android

•  Ada task runs on top of operating system thread –  One-to-one correspondence

•  Task dispatching policy –  Can be selected with pragma Dispatching_Policy

–  SCHED_OTHER by default

–  Preemptive priority scheduling –  pragma Dispatching_Policy (FIFO_Within_Priorities)

•  pthread_setschedparam (Thread, SCHED_FIFO, Param’Access) –  Means run until blocked (or preempted), no time slicing –  Reduces non-determinism

•  Multiprocessing –  Unfortunately, the thread affinity interface does not seem to be there

Slide: 13 Copyright © 2013 AdaCore

Synchronization

•  A protected type is a data object with locks –  Data encapsulation, accessible through interface (locked access routines)

–  functions (read the data with read lock)

–  procedures (read/write the data with write lock)

–  entries (wait until some condition is met, then read/write the data with write lock)

•  Priority inheritance –  Guard against priority inversion

–  low priority task grabs resource X

–  high priority task needs resource X, waits –  medium priority task preempts low priority task, and runs for a long time, holding up high priority task

–  Solution, while high priority task is waiting, lend high priority to low priority task

•  Implementation –  Mutual exclusion

–  Mutex

–  Waiting / Signaling operations –  Conditional variable

Slide: 14 Copyright © 2013 AdaCore

System programming

•  Clock and delay –  Clock uses gettimeofday –  Delay operations use timed conditional variables

–  We can wakeup the task before expiration if needed

•  Interrupt handling –  Underlying signal mechanism –  A server task per interrupt served

–  With specific mask to serve the required signal

–  Call to sigwait

Slide: 15 Copyright © 2013 AdaCore

Demo

•  Lunar Lander –  Sample provided by the Android SDK

•  Reimplement the computation of physics behavior in Ada –  Position and speed of the spacecraft

Slide: 16 Copyright © 2013 AdaCore

Conclusion

•  Simple way to benefit from the Android SDK and Ada

•  Do as much as is convenient with the Android SDK

•  Then use Ada for –  CPU intensive, critical, portable, … parts

•  The trick is –  To realize that the Android platform is a Linux platform on with there is a JVM

–  To use the automatic generation of Ada/Java bindings

•  Nice integration with Eclipse to facilitate usage