This document summarizes different approaches for using the Go programming language to build Android applications. It discusses building a pure Go app using the NativeActivity class, integrating Go as a library using bindings, and spawning separate Go processes from an Android app using inter-process communication. Code examples are provided for each approach. While Go support for Android is still unstable, these techniques allow Go to be used for simple Android projects as an alternative to Java.

1. HelloAndroid.go
SeongJae Park <sj38.park@gmail.com>

2. This work by SeongJae Park is licensed under the Creative
Commons Attribution-ShareAlike 3.0 Unported License. To
view a copy of this license, visit http://creativecommons.
org/licenses/by-sa/3.0/.

3. This slides were presented during
3rd GDG Korea Android Conference
(http://event.android.gdg.kr/3rd-GKAC/)
The talk video is available at:
https://www.youtube.com/watch?v=vMZFjDipaK8&index=2&list=PL_WJkTbDHdBl5QXy6N_bMMBYlKLna5RER

4. Nice To Meet You
SeongJae Park
sj38.park@gmail.com
golang newbie programmer

5. Warning
● This speech could be useless for you
○ This is just for fun
http://m.c.lnkd.licdn.com/mpr/mpr/p/4/005/095/091/210ae8c.jpg

6. Warning
● This speech could be useless for you
○ This is just for fun
● Don’t try this at office
○ The code is not stable yet
http://m.c.lnkd.licdn.com/mpr/mpr/p/4/005/095/091/210ae8c.jpg

7. golang: Programming Language
● For simple, reliable, and efficient software.
http://blog.golang.org/5years/gophers5th.jpg

8. golang: Programming Language
● For simple, reliable, and efficient software.
● Could it be used for simple, reliable, and
efficient Android application?
http://blog.golang.org/5years/gophers5th.jpg

9. Golang and Android
● Golang supports Android from v1.4
○ Though it’s still unstable

10. Golang and Android
● Golang supports Android from v1.4
○ Though it’s still unstable
● https://github.com/golang/mobile
○ Packages and build tools for using Go on Android

11. Goal of This Speak
● Showing how we can use golang on Android
○ By exploring example code

12. Goal of This Speak
● Showing how we can use golang on Android
○ By exploring example code
● Just for fun, rather than profit

13. Pre-requisites
● Basic development environment(vim, git,
gcc, java, …)
http://www.theprospect.net/wp-content/uploads/2014/02/one-does-not-simply-skip-a-step.jpg

14. Pre-requisites
● Basic development environment(vim, git,
gcc, java, …)
● Android SDK & NDK
http://www.theprospect.net/wp-content/uploads/2014/02/one-does-not-simply-skip-a-step.jpg

15. Pre-requisites
● Basic development environment(vim, git,
gcc, java, …)
● Android SDK & NDK
● Golang 1.4 or higher cross-compiled for
GOOS=android
http://www.theprospect.net/wp-content/uploads/2014/02/one-does-not-simply-skip-a-step.jpg

16. Pure Golang Android App
NO JAVA!

17. Main Idea: Use NDK
● c / c++ only apk is available
using NativeActivity
○ Golang be compiled to native binary, too. Why not?
http://www.android.pk/images/android-ndk.jpg

18. Main Idea: Use NDK
● c / c++ only apk is available
using NativeActivity
○ Golang be compiled to native binary, too. Why not?
Plan is...
● Build golang program as .so file
○ ELF shared object
● Implement every callbacks using OpenGL
http://www.android.pk/images/android-ndk.jpg

19. Main Idea: Use NDK
● c / c++ only apk is available
using NativeActivity
○ Golang be compiled to native binary, too. Why not?
Plan is...
● Build golang program as .so file
○ ELF shared object
● Implement every callbacks using OpenGL
● Build NativeActivity apk using NDK / SDK
http://www.android.pk/images/android-ndk.jpg

20. Example Code
https://github.
com/golang/mobile/tree/master/example/basic

21. Example Code
https://github.
com/golang/mobile/tree/master/example/basic
$ tree
.
├── all.bash
├── all.bat
├── AndroidManifest.xml
├── build.xml
├── jni
│ └── Android.mk
├── main.go
├── make.bash
└── make.bat
1 directory, 8 files

22. main.go: Register Callbacks
Register callbacks from golang entrypoint
func main() {
app.Run(app.Callbacks{
Start: start,
Stop: stop,
Draw: draw,
Touch: touch,
})
}
(mobile/example/basic/main.go)

23. main.go: Use OpenGL
func draw() {
gl.ClearColor(1, 0, 0, 1)
...
green += 0.01
if green > 1 {
green = 0
}
gl.Uniform4f(color, 0, green, 0, 1)
...
debug.DrawFPS()
}
(mobile/example/basic/main.go)

24. NativeActivity
NativeActivity only application doesn’t need
JAVA
<application android:label="Basic" android:hasCode="false">
<activity android:name="android.app.NativeActivity"
android:label="Basic"
android:configChanges="orientation|keyboardHidden">
<meta-data android:name="android.app.lib_name" android:value="basic" />
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
(mobile/example/basic/AndroidManifest.xml)

25. NativeActivity
NativeActivity only application doesn’t need
JAVA
<application android:label="Basic" android:hasCode="false">
<activity android:name="android.app.NativeActivity"
android:label="Basic"
android:configChanges="orientation|keyboardHidden">
<meta-data android:name="android.app.lib_name" android:value="basic" />
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
(mobile/example/basic/AndroidManifest.xml)

26. NativeActivity
NativeActivity only application doesn’t need
JAVA
<application android:label="Basic" android:hasCode="false">
<activity android:name="android.app.NativeActivity"
android:label="Basic"
android:configChanges="orientation|keyboardHidden">
<meta-data android:name="android.app.lib_name" android:value="basic" />
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
(mobile/example/basic/AndroidManifest.xml)

27. Build Process
Build golang code into ELF shared object for
ARM
mkdir -p jni/armeabi
CGO_ENABLED=1 GOOS=android GOARCH=arm GOARM=7
go build -ldflags="-shared" -o jni/armeabi/libbasic.so .
ndk-build NDK_DEBUG=1
ant debug
(mobile/example/basic/make.bash)

28. Build Process
Build golang code into ELF shared object for
ARM
NDK to add the so file
mkdir -p jni/armeabi
CGO_ENABLED=1 GOOS=android GOARCH=arm GOARM=7
go build -ldflags="-shared" -o jni/armeabi/libbasic.so .
ndk-build NDK_DEBUG=1
ant debug
(mobile/example/basic/make.bash)

29. Build Process
Build golang code into ELF shared object for
ARM
NDK to add the so file
SDK to build apk file
mkdir -p jni/armeabi
CGO_ENABLED=1 GOOS=android GOARCH=arm GOARM=7
go build -ldflags="-shared" -o jni/armeabi/libbasic.so .
ndk-build NDK_DEBUG=1
ant debug
(mobile/example/basic/make.bash)

30. Pure Golang Android App
Pros: No more JAVA! Yay!!!
Cons: Should I learn OpenGL to show a cat?

31. Golang as a Library
Cooperate Java and Golang

32. Java and C language connected via JNI
Main Idea: Use JNI-like way
Java
CPP
JNI

33. Main Idea: Use JNI-like way
Java and C language connected via JNI
C language and Golang connected via cgo
Java
CPP
GO
JNI CGO

34. Main Idea: Use JNI-like way
Java and C language connected via JNI
C language and Golang connected via cgo
...But, JNI and then cgo looks tedious
Java
CPP
GO
JNI CGO

35. Main Idea: Use JNI-like way
Java and C language connected via JNI
C language and Golang connected via cgo
...But, JNI and then cgo looks tedious
Golang supports Java-Golang bind
Java
CPP
GO
JNI CGO
bind/seq

36. Example Code
https://github.
com/golang/mobile/tree/
master/example/libhello

37. Example Code
https://github.
com/golang/mobile/tree/
master/example/libhello
$ tree
.
├── all.bash
├── all.bat
├── AndroidManifest.xml
├── build.xml
├── hi
│ ├── go_hi
│ │ └── go_hi.go
│ └── hi.go
├── main.go
├── make.bash
├── make.bat
├── README
└── src
├── com
│ └── example
│ └── hello
│ └── MainActivity.java
└── go
└── hi
└── Hi.java
8 directories, 12 files

38. Callee in Go
Golang code is implementing Hello() function
func Hello(name string) {
fmt.Printf("Hello, %s!n", name)
}
(mobile/example/libhello/hi/hi.go)

39. Caller in JAVA
Java code is calling Golang function, Hello()
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Go.init(getApplicationContext());
Hi.Hello("world");
}
(mobile/example/libhello/src/com/example/hello/MainActivity.java)

40. gobind
generate language bindings that make it
possible to call Go code and pass objects from
Java
go install golang.org/x/mobile/cmd/gobind
gobind -lang=go github.com/libhello/hi > hi/go_hi/go_hi.go
gobind -lang=java github.com/libhello/hi > src/go/hi/Hi.java

41. gobind: Generated .java
Provides wrapper function for golang calling
code
public static void Hello(String name) {
go.Seq _in = new go.Seq();
go.Seq _out = new go.Seq();
_in.writeUTF16(name);
Seq.send(DESCRIPTOR, CALL_Hello, _in, _out);
}
private static final int CALL_Hello = 1;
private static final String DESCRIPTOR = "hi";
(mobile/example/libhello/src/go/hi/Hi.java)

42. gobind: Generated .go
Provides proxy and registering for the exported
function
func proxy_Hello(out, in *seq.Buffer) {
param_name := in.ReadUTF16()
hi.Hello(param_name)
}
func init() {
seq.Register("hi", 1, proxy_Hello)
}
(mobile/example/libhello/hi/go_hi/go_hi.go)

43. Golang as a Library
Pros: JAVA for UI, Golang for background
(Looks efficient enough)

44. Golang as a Library
Pros: JAVA for UI, Golang for background
(Looks efficient enough)
Cons: bind/seq is not so efficient, yet

45. Inter-Process
Communication
Philosophy of Unix

46. Main Idea: Android is a Linux
http://www.kitguru.net/wp-content/uploads/2012/03/linux-android.png
Android is a variant of Linux system with ARM
x86 Androids exist, though...

47. Main Idea: Android is a Linux
http://www.kitguru.net/wp-content/uploads/2012/03/linux-android.png
Android is a variant of Linux system with ARM
x86 Androids exist, though...
Go supports ARM & Linux Officially
with static-linking

48. Main Idea: Android is a Linux
http://www.kitguru.net/wp-content/uploads/2012/03/linux-android.png
Android is a variant of Linux system with ARM
x86 Androids exist, though...
Go supports ARM & Linux Officially
with static-linking
Remember the philosophy of Unix

49. Example Code
https://github.com/sjp38/goOnAndroid
https://github.com/sjp38/goOnAndroidFA

50. Example Code
https://github.com/sjp38/goOnAndroid
https://github.com/sjp38/goOnAndroidFA
Were demonstrated by live-coding from
GDG Korea DevFair 2014 (http://devfair2014.
gdg.kr/) and
GDG Golang Seoul Meetup 2015 (https:
//developers.google.
com/events/5381849181323264/)

51. Golang Process on Android: Plan
1. Cross compile Go program as ARM / Linux

52. Golang Process on Android: Plan
1. Cross compile Go program as ARM / Linux
2. Include the binary in assets/ of Android app

53. Golang Process on Android: Plan
1. Cross compile Go program as ARM / Linux
2. Include the binary in assets/ of Android app
3. Copy the binary in private space of the app

54. Golang Process on Android: Plan
1. Cross compile Go program as ARM / Linux
2. Include the binary in assets/ of Android app
3. Copy the binary in private space of the app
4. Give execute permission to the binary
/data/data/com.example.goRunner/files # ls -al
-rwxrwxrwx u0_a55 u0_a55 4512840 2014-11-28 17:45 gobin

55. Golang Process on Android: Plan
1. Cross compile Go program as ARM / Linux
2. Include the binary in assets/ of Android app
3. Copy the binary in private space of the app
4. Give execute permission to the binary
5. Execute it
/data/data/com.example.goRunner/files # ls -al
-rwxrwxrwx u0_a55 u0_a55 4512840 2014-11-28 17:45 gobin

56. Go bin Loading
Load golang program from assets to private dir
private void copyGoBinary() {
String dstFile = getBaseContext().getFilesDir().getAbsolutePath() + "/verChecker.bin";
try {
InputStream is = getAssets().open("go.bin");
FileOutputStream fos = getBaseContext().openFileOutput(
"verChecker.bin", MODE_PRIVATE);
byte[] buf = new byte[8192];
int offset;
while ((offset = is.read(buf)) > 0) {
fos.write(buf, 0, offset);
}
Runtime.getRuntime().exec("chmod 0777 " + dstFile);
} catch (IOException e) { }
}

57. Execute Go process
Spawn new process for the program and
communicates using stdio
ProcessBuilder pb = new ProcessBuilder();
pb.command(goBinPath());
pb.redirectErrorStream(false);
goProcess = pb.start();
new CopyToAndroidLogThread("stderr",
goProcess.getErrorStream())
.start();

58. Inter Process Communication
Pros: Just normal unix way

59. Inter Process Communication
Pros: Just normal unix way
Golang team is using this for Camlistore
(https://github.com/camlistore/camlistore)

60. Inter Process Communication
Pros: Just normal unix way
Golang team using this for Camlistore
(https://github.com/camlistore/camlistore)
Cons: Hacky, a little

61. Summary
You can use Golang for Android
though it’s not stable yet
Just for fun

62. This work by SeongJae Park is licensed under the
Creative Commons Attribution-ShareAlike 3.0 Unported
License. To view a copy of this license, visit http:
//creativecommons.org/licenses/by-sa/3.0/.