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Introduction (part I): Java and Android basics
Alexey Golubev, Dmitry Lukashev
What is the plan?
Part I Part II Part III
Android UI
Layouts, ListView,
Menu, Dialog, Widgets, Tips & tricks, etc.
Android in Action
Screen rotation, Memory analyze, AIDL, SAX, Debug,
Wakelock, etc.
Java +Android (basics)
JVM, GC, Threads, SDK, NDK, Activity,
Code style, etc.
Part I: Introduction
What is this about?
• Introduction to Java world
• JVM, Dalvik VM, JIT
• GC, multithreading, collections, etc.
• Android Basics
• Architecture
• Building blocks, SDK, NDK, etc.
• NDK demo
Java overview
• Sun VM, Dalvik VM, JIT
• Garbage collection
• References
• Exceptions
• Threads
• Synchronization
• Collections
• Wrapper classes
• Java and Android code style
Where to start
• http://developer.android.com/
• http://source.android.com/
• http://android.git.kernel.org/
• http://android-developers.blogspot.com/
Java overview
• Sun VM, Dalvik VM, JIT
• Garbage collection
• References
• Exceptions
• Threads
• Synchronization
• Collections
• Wrapper classes
• Java and Android code style
JVM
The pc register (program counter)
JVM Stack
Native methods stacks
Heap
Method Area VM
Thread
Сlass Runtime Constant Pool
class files Class loader subsystem
native method libraries
Execution engine
Native method interface
Dalvik VM
• Was written by Dan Bornstein
• Transform class files into DEX
• It is VM… – integrated with Linux
– uses shared memory, mmap
– for OS without swap space
– while powered by a battery
– zygote
• The Dalvik VM is register-based: fewer instructions, code units, instructions
• Verification & optimization at installation time
Give me your huddled bytecodes
yearning to run free. And I lift
the light beside the coder’s door
DEX file – shared constant pool
.jar file
.dex file heterogeneous constant pool
other data
.class file
heterogeneous constant pool
other data
.class file
heterogeneous constant pool
other data
.class file
string_ids constant pool
type_ids constant pool
proto_ids constant pool
field_ids constant pool
method_ids constant pool
other data
“Hello World”
“Lcom/data/Data”
int
String[ ]
String fn()
void fn(int)
String.offset
Integer.MAX_VALUE
PrintStream.println(…)
Collection.size()
DEX file – memory saving
• minimal repetition
• per-type pools (implicit typing)
• implicit labeling
common system libraries
(U) 21445320 — 100% (uncompressed jar file)
(J) 10662048 — 50% (compressed jar file)
(D) 10311972 — 48% (uncompressed dex file)
web browser app
(U) 470312 — 100%
(J) 232065 — 49%
(D) 209248 — 44%
• Google claims that Dalvik bytecode is more efficient than Sun’s stack
bytecode
JIT (since Android 2.2)
• Translates byte code to optimized native code at run time
• Part of the open source
• Trace JIT vs Method JIT
• Trace JIT – Minimizing memory usage critical for mobile devices (100K)
– Important to deliver performance boost quickly
– Trace request is built during interpretation
– Compiled traces chained together in translation cache
– Per process translation cache
• Leave open the possibility of supplementing with method-based JIT
Java overview
• Sun VM, Dalvik VM, JIT
• Garbage collection
• References
• Exceptions
• Threads
• Synchronization
• Collections
• Wrapper classes
• Java and Android code style
Garbage collection
• GC is under control of the JVM • An object is eligible for garbage
collection when no live thread can access it
• System.gc(); Runtime.getRuntime().gc()
• static – variables with the longest life time
public class Island {
Island i;
public static void main(String[] args) {
Island i2 = new Island();
Island i3 = new Island();
Island i4 = new Island();
i2.n = i3; i3.n = i4; i4.n = i2;
i2 = null; i3 = null; i4 = null;
Island x = new Island(); x = null;
}
}
i4.n
i3.n
i2.n
i2
i3
i4
The heap
x
“Islands of Isolation”
Mark and Sweep example
• Obtain locks and suspend threads
• Mark phase
– Process of identifying all objects reachable from the root set.
– All “live” objects are marked by setting a mark bit in the mark bit vector.
• Sweep phase
– Sweep phase identifies all the
objects that have been allocated,
but no longer referenced.
• Compaction (optional)
– Once garbage has been removed,
we consider compacting the
resulting set of objects to remove
spaces between them.
• Release locks and resume threads
Parallel mark bits in Dalvik VM (separate from other heap memory)
Generational GC
• Most recently created objects – most likely to become unreachable quickly
Young Generation Old Generation
• Minor Collection – takes place only in the young generation,
normally done through direct copying – very efficient
• Major Collection – takes place in the new and old generation
and uses the normal mark/sweep (+compact) algorithm
Strings
• Strings are immutable in Java
The heap
“Hi”
x
String x = “Hi";
x = x.concat(" GTUG!");
The heap
“Hi”
x
“Hi GTUG”
String s1 = "abc"; // 1 object, 1 reference
String s2 = "abc";
// s1 == s2
String s3 = new String("abc"); // 2
objects, 1 reference
String s4 = new String("abc");
// s3 != s4
// optimization by interning strings
s3 = s3.intern();
s4 = s4.intern();
• Use StringBuffer, StringBuilder
• Set initial capacity of StringBuilder
Quiz
class Foo {
int[] mArray = {
1, 2, 3, 4, 5
};
}
public class Test {
public static void main(String[] args) {
Foo[] fa = new Foo[3];
fa[0] = new Foo();
Foo f = new Foo();
fa[1] = f;
f = null;
fa[1] = null;
// how many objects were created at this point?
// how many are eligible for gc()?
}
}
[ ref0, ref1, ref2]
[ ] Foo()
[ ] Foo()
Performance
• Do not allocate memory as much as you can
• GC is slow (~x00 ms on Android device)
• Decrease number of objects – GC will work faster (i.e. StringBuffer vs String)
• Use primitive types for arrays (int vs Integer)
• Use special methods: String.indexOf(), substring(), etc.
• Use exact class types instead of interfaces (i.e. HashMap instead of Map) on devices without JIT
• Use static where possible
• Avoid internal getters/setters
• Use enhanced for-loop syntax
ArrayList<MyClass> mList = new
ArrayList<MyClass>();
...
for (MyClass next : mList) {
...
}
Java overview
• Sun VM, Dalvik VM, JIT
• Garbage collection
• References
• Exceptions
• Threads
• Synchronization
• Collections
• Wrapper classes
• Java and Android code style
References
• Strong reference: Object obj = new Object();
• SoftReference – soft references will be cleared before the JVM reports an out-of-memory condition (memory-sensitive cache)
• WeakReference – gc frees weakly reachable object when it founds it, avoid memory leaks (e.g. http://en.wikipedia.org/wiki/Flyweight_pattern)
• PhantomReference – is useful only to track the impending collection of the referring object. Must be used only with the ReferenceQueue class. Most often used for scheduling pre-mortem cleanup actions in a more flexible way than is possible with the Java finalization mechanism
SoftReference: simple cache
private final HashMap<String, SoftReference<T>> mCache;
public put(String key, T value) {
mCache.put(key, new SoftReference<T>(value));
}
public T get(String key, ValueBuilder builder) {
T value = null;
SoftReference<T> reference = mCache.get(key);
if (reference != null) {
value = reference.get();
}
// Not in cache or gc'd
if (value == null) {
value = builder.build(key);
mCache.put(key, new SoftReference<T>(value));
}
return value;
}
package com.android.mms.model;
public class ImageModel extends RegionMediaModel {
private static final String TAG = "Mms/image";
private static final int THUMBNAIL_BOUNDS_LIMIT = 480;
private SoftReference<Bitmap> mBitmapCache = new SoftReference<Bitmap>(null);
...
private Bitmap internalGetBitmap(Uri uri) {
Bitmap bm = mBitmapCache.get();
if (bm == null) {
try {
bm = createThumbnailBitmap(THUMBNAIL_BOUNDS_LIMIT, uri);
if (bm != null) {
mBitmapCache = new SoftReference<Bitmap>(bm);
}
} catch (OutOfMemoryError ex) {
// fall through and return a null bitmap.
// The callers can handle a null
// result and show R.drawable.ic_missing_thumbnail_picture
}
}
return bm;
}
}
git://android.git.kernel.org/platform/packages/apps/Mms.git/src/com/android/mms/model/ImageModel.java
SoftReference
package android.app;
public class Dialog implements DialogInterface, Window.Callback,
KeyEvent.Callback, OnCreateContextMenuListener {
...
private static final class ListenersHandler extends Handler {
private WeakReference<DialogInterface> mDialog;
public ListenersHandler(Dialog dialog) {
mDialog = new WeakReference<DialogInterface>(dialog);
}
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case DISMISS:
((OnDismissListener) msg.obj).onDismiss(mDialog.get());
break;
case CANCEL:
((OnCancelListener) msg.obj).onCancel(mDialog.get());
break;
case SHOW:
((OnShowListener) msg.obj).onShow(mDialog.get());
break;
}
}
}
}
git://android.git.kernel.org/platform/frameworks/base.git/core/java/android/app/Dialog.java
WeakReference
finalize()
• The finalize() method is guaranteed to run once and only once before GC deletes an object
• GC makes no guarantees, finalize() may never run
• You can uneligibilize an object for GC within finalize()
• Do not use it for resource closing
Java overview
• Sun VM, Dalvik VM, JIT
• Garbage collection
• References
• Exceptions
• Threads
• Synchronization
• Collections
• Wrapper classes
• Java and Android code style
Exceptions
try {
// do stuff
} catch (SomeException e) {
// do exception handling
} finally {
// clean up
}
try {
// do stuff
} finally {
// clean up
}
Object
Throwable
Exception Error
RuntimeException AssertionError StackOverflowError OutOfMemoryError NullPointerException IndexOfBoundException IOException SQLiteException
try/catch/finally (1)
class Test {
private String data = "test";
private int test() {
try {
System.out.println(data.length());
return 0;
} catch (NullPointerException e) {
System.out.println("exception");
return 1;
} finally {
System.out.println("finally");
return 2;
}
}
public static void main(String[] args) {
Test t = new Test();
System.out.println("result = " + t.test());
}
}
4
finally
result = 2
try/catch/finally (2)
class Test {
private String data;
private int test() {
try {
System.out.println(data.length());
return 0;
} catch (NullPointerException e) {
System.out.println("exception");
return 1;
} finally {
System.out.println("finally");
return 2;
}
}
public static void main(String[] args) {
Test t = new Test();
System.out.println("result = " + t.test());
}
}
exception
finally
result = 2
DefaultExceptionHandler
public class DefaultExceptionHandler implements UncaughtExceptionHandler {
private UncaughtExceptionHandler mDefaultExceptionHandler;
public DefaultExceptionHandler(UncaughtExceptionHandler pDefaultExceptionHandler)
{
mDefaultExceptionHandler = pDefaultExceptionHandler;
}
public void uncaughtException(Thread t, Throwable e) {
mDefaultExceptionHandler.uncaughtException(t, e);
t.getThreadGroup().destroy();
}
}
Thread.setDefaultUncaughtExceptionHandler(new
DefaultExceptionHandler(Thread.getDefaultUncaughtExceptionHandler()));
Java overview
• Sun VM, Dalvik VM, JIT
• Garbage collection
• References
• Exceptions
• Threads
• Synchronization
• Collections
• Wrapper classes
• Java and Android code style
Threads
• When it comes to threads, very little is guaranteed
• Create thread: extend java.lang.Thread or implement Runnable interface
• A thread is done being a thread when its target run() method completes
• Once a thread has been started, it can never be started again
• The order in which runnable threads are chosen is not guaranteed
• sleep(long ms), yield(), join(), setPriority(int priority), wait(), notify(), notifyAll()
Thread states
New Runnable Running
Waiting/
blocking
Dead
not alive alive not alive
run() completes start()
wait, sleep, join, lock
yield
Java overview
• Sun VM, Dalvik VM, JIT
• Garbage collection
• References
• Exceptions
• Threads
• Synchronization
• Collections
• Wrapper classes
• Java and Android code style
Synchronization
• Every Object in java has build-in lock
• Only methods or blocks can be synchronized public static synchronized int getCount() {
return count;
}
public static int getCount() {
synchronized(MyClass.class) { // Class c = Class.forName(“MyClass”);
return count;
}
}
• notify(), join(), sleep(), yield() keep locks, wait() gives up lock
• synchronized keyword is not automatically inherited when
subclasses override superclass method
Volatile
public class Task extends Thread {
private volatile boolean mIsStop;
public void run() {
while (!mIsStop) {
// do some stuff...
}
}
public void stopMe() {
mIsStop = true;
}
}
volatile:
• Every thread accessing the field will read its current value before continuing, instead of (potentially) using a cached value
• Statements accessing the variable will be executed in the order they are written (Java 5 or later)
class VolatileExample {
int x = 0;
volatile boolean v = false;
public void writer() {
x = 42;
v = true;
}
public void reader() {
if (v == true) {
// uses x - guaranteed to see 42.
}
}
}
Singleton pattern example (1)
public class Singleton {
private static final Singleton INSTANCE = new Singleton();
private Singleton() {}
public static Singleton getInstance() {
return INSTANCE;
}
} thread-safe
public class Singleton {
private static Singleton sInstance;
private Singleton() {}
public static Singleton getInstance() {
if (sInstance == null) {
sInstance = new Singleton();
}
return sInstance;
}
} non-thread-safe
lazy init
Singleton pattern example (2)
public class Singleton {
private static Singleton sInstance;
private Singleton() {}
public synchronized static Singleton getInstance() {
if (sInstance == null) {
sInstance = new Singleton();
}
return sInstance;
}
}
lazy init
thread-safe
low performance
Singleton pattern example (3)
public class Singleton {
private static Singleton sInstance;
private Singleton() {}
public static Singleton getInstance() {
if (sInstance == null) {
synchronized (Singleton.class) {
if (sInstance == null) {
sInstance = new Singleton();
}
}
}
return sInstance;
}
}
if (sInstance == null) { // “Double-Checked Locking” idiom
}
Singleton pattern example (4)
public class Singleton {
private volatile static Singleton sInstance;
private Singleton() {}
public static Singleton getInstance() {
if (sInstance == null) {
synchronized (Singleton.class) {
if (sInstance == null) {
sInstance = new Singleton();
}
}
}
return sInstance;
}
} lazy init & thread-safe (Java 5 or later)
Singleton pattern example (5)
public class Singleton {
private Singleton() {}
private static class SingletonHolder {
private static final Singleton INSTANCE = new Singleton();
}
public static Singleton getInstance() {
return SingletonHolder.INSTANCE;
}
}
lazy init & thread-safe (for all Java version)
Quiz
public class Foo {
private static final int DELTA = 6;
private static Foo sInstance = new Foo();
private static int sBase = 7;
private int mX;
private Foo() {
mX = sBase + DELTA;
}
public static void main(String[] args) {
System.out.println(Foo.sInstance.mX);
}
}
What is the print? 6
Java overview
• Sun VM, Dalvik VM, JIT
• Garbage collection
• References
• Exceptions
• Threads
• Synchronization
• Collections
• Wrapper classes
• Java and Android code style
Collection
Set List Queue
HashSet
LinkedHashSet
SortedSet
NavigableSet
TreeSet
ArrayList Vector LinkedList PriorityQueue
Map
SortedMap
NavigableMap
TreeMap LinkedHashMap
HashMap HashTable
Collections
1.6
1.6
WeakHashMap ConcurrentHashMap CopyOnWriteArraySet etc…
Alex Alex
Hash table
• Choosing a good hash function
• Collision resolution: separate chaining, open addressing
• Load factor
Key Hashcode Algorithm Hashcode
Alex A(1)+L(12)+E(5)+X(24) 42
Bob B(2)+O(15)+B(2) 19
Dirk D(4)+I(9)+R(18)+K(11) 42
Fred F(6)+R(18)+E(5)+D(4) 33
19
33
Bob
Fred
42 Dirk
19
33
Bob
Fred
42
Dirk 43
• Find the right bucket (using hashCode())
• Search the bucket for the right element (using equals())
Contracts
The equals() contract
• Reflexive – x.equals(x)=true
• Symmetric – x.equals(y)=true -> y.equals(x)=true
• Transitive – x.equals(y)=true, y.equals(z)=true -> x.equals(z)=true
• Consistent – multiple invocations
• x.equals(null)=false
The hashCode() contract
• Consistent during the app execution
• If x.equals(y) -> hash must be the same
@Override
public int hashCode() {
return 1;
}
Java overview
int
• Sun VM, Dalvik VM, JIT
• Garbage collection
• References
• Exceptions
• Threads
• Synchronization
• Collections
• Wrapper classes
• Java and Android code style
Wrapper Classes and Boxing (1)
Integer x = 100;
x++;
• The wrapper classes correlate to primitive types
• Wrappers have two main functions: – To wrap primitives so that they can be handled like objects
– To provide utility methods for primitives (conversions)
• Wrapper constructors can take String (except Character)
• As of Java 5, boxing allows to convert primitives to wrappers or vice versa automatically.
int x2 = x.intValue();
x2++;
x = new Integer(x2);
Wrapper Classes and Boxing (2)
Integer i1 = 1000;
Integer i2 = 1000;
if (i1 != i2) System.out.println("i1!=i2");
if (i1 == i2) System.out.println("i1==i2");
Integer i1 = 100;
Integer i2 = 100;
if (i1 != i2) System.out.println("i1!=i2");
if (i1 == i2) System.out.println("i1==i2");
JVM saves memory for:
• Boolean
• Byte
• Character from \u0000 to \u007f
• Short and Integer from -128 to 127
i1!=i2
I1==i2
Java overview
• Sun VM, Dalvik VM, JIT
• Garbage collection
• References
• Exceptions
• Threads
• Synchronization
• Collections
• Wrapper classes
• Java and Android code style
Java Code Style: why?
• 80% of the lifetime cost of a piece of software goes to maintenance
• Hardly any software is maintained for its whole life by the original author
• Code conventions improve the readability of the software, allowing engineers to understand new code more quickly and thoroughly
• Makes it easier to conduct code reviews
http://developers.sun.com/sunstudio/products/archive/whitepapers/java-style.pdf
Comments, TODOs, etc.
• Comments
• TODO, FIXME, XXX
– Use TODO comments for code that is temporary, a short-term solution, or good-enough but not perfect.
• Copyrights
Android Code Style (1)
• Import ordering – Android imports – Imports from third parties (com, junit, net, org) – java and javax
• Indentation – We use 4 space indents for blocks. We never use tabs. When in doubt,
be consistent with code around you – We use 8 space indents for line wraps, including function calls and
assignments
• Field Names – Non-public, non-static field names start with m. – Static field names start with s. – Other fields start with a lower case letter. – Public static final fields (constants) are ALL_CAPS_WITH_UNDERSCORES.
Android Code Style (2)
• Braces
– Braces do not go on their own line; they go on the same line as the code before them
• Line length
– Each line of text in your code should be at most 100 characters long.
• Annotations, Logging
http://source.android.com/submit-patches/code-style-guide
if (condition) {
body; // ok
}
if (condition) body; // ok - 1 line
if (condition)
body; // bad
BAD Code Style sample (1)
for(int i=0;i<amount;++i)
db.execSQL(sqls[i]);
public static final int sMaxSQLsInTransaction= 1000;
if (delayed == true) {
...
} else {
...
}
Good:
XmlHttpRequest getCustomerId
class Html
long id
Bad:
XMLHTTPRequest getCustomerID
class HTML
long ID
BAD Code Style sample (2)
public static final boolean PushValues(String pattern,
ArrayList<Integer> patternIds,/*OUT */ArrayList<String> values)
{
int idsAmount= (null != patternIds ? patternIds.size() : 0);
if(0<idsAmount){
final int patternLen=(null!=pattern ? pattern.length():0);
if(0<patternLen){
for(int i=0; i<idsAmount; ++i){
if(!pushValues(pattern,patternLen,patternIds.get(i),values)){
return (false);
}
}
return (true);
}
}
return (false);
}
BAD Code Style sample (2)
public static final ArrayList<String> getValues(final String pattern,
final ArrayList<Integer> patternIds) {
ArrayList<Integer> result = new ArrayList<Integer>();
if (pattern != null && pattern.length() > 0 && patternIds != null) {
for (int patternId : patternIds) {
ArrayList<Integer> values = getValuesForPatternId(pattern, patternId);
if (values == null) {
return null;
}
result.addAll(values);
}
}
return result;
}
Let’s Have a Break! 10 min
Android basics
• Architecture Overview
• Building Blocks
• Manifest
• System Services
• Platform versions
• Saving Application State
• NDK
• Make your code better
Android
• Architecture Overview
• Building Blocks
• Manifest
• System Services
• Platform versions
• Saving Application State
• NDK
• Make your code better
Android History
Sep 2008
1.0 Mar 2009
1.1 1.5
May 2009
1.6
Sep 2009
2.0
Oct 2009
2.0.1
Dec 2009
2.1
Jan 2010
2.2
May 2010
Cupcake Donut
Éclair
Froyo
Gingerbread
• Jul 2005 – Google acquired Android Inc. (co-founded by Andy Rubin)
• Nov 2007 – Google Android presentation and “early look” release
Android Architecture Overview
Linux Kernel
Display Driver
Keypad Driver
Camera Driver
WiFi Driver
Flash Memory Driver
Audio Drivers
Binder (IPC) Driver
Power Management
Libraries
Surface Manager Media Framework
OpenGL | ES FreeType
SQLite
WebKit
SGL SSL libc
Android Runtime
Core Libraries
Dalvik VM
Application Framework
Activity Manager
Package Manager
Window Manager
Telephony Manager
Content Providers
Notification Manager
View System
Location Manager
Resource Manager
Applications
Home Contacts Phone Browser …
ND
K /
JN
I
Android
• Architecture Overview
• Building Blocks
• Manifest
• System Services
• Platform versions
• Saving Application State
• NDK
• Make your code better
Android Application Building Blocks
• Activity – Presents UI – Each Activity is independent screen – Activity: setContentView(View)
• Service – Used for background operations
• BroadcastReceiver – Receive and process broadcast system or user events
• ContentProvider – Share application data with others – Data can be stored in FS, DB or even in memory – Communicate via ContentResolver
• Intent – Used to activate Activities, Services & BroadcastReceivers
http://developer.android.com/guide/topics/fundamentals.html
Android Application
• Controls global Application state
• Extend Application class (android.app.Application)
– onCreate()
– onLowMemory()
– onTerminate()
– getApplicationContext() – to use it in classes, where is no Context
• Point custom Application class in AndroidManifest.xml
Android Context
Application Context • Non UI Context
• startActivity(Intent)
• start /stopService(Intent)
• sendBroadcast(Intent)
• register / unregisterReciever()
• Application FS, Preferences, Resources
• getSystemService
Activity Context
• Same as for Application, but specific
to current Activity
• startActivityForResult(Intent) /
finish()
• bindService(Intent)
• UI: setContentView(View),
findViewById()
• User events handling (Keys,
Touches, Trackball)
Android
• Architecture Overview
• Building Blocks
• Manifest
• System Services
• Platform versions
• Saving Application State
• NDK
• Make your code better
AndroidManifest.xml (1)
<?xml version="1.0" encoding="utf-8"?>
<manifest
xmlns:android="http://schemas.android.com/apk/res/android"
package="com.my.example"
android:versionName="1.0 beta" android:versionCode="2">
<application android:name=".MyApplication"
android:label="..." android:icon="...">
<activity android:name=".MyActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.DEFAULT" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<service ... />
<receiver ... />
<provider ... />
</application>
</manifest>
AndroidManifest.xml (2)
<activity android:name=".MyActivity"
android:launchMode="singleTask"
android:theme="@style/Theme.MyDialog" />
<service android:name=".MyService" android:process="new"/>
<receiver android:name=".MyReceiver" >
<intent-filter>
<action android:name= "android.intent.action.PACKAGE_REMOVED" />
<category android:name= "android.intent.category.DEFAULT" />
<data android:scheme= "package" />
</intent-filter>
</receiver>
<provider android:name=".MyProvider"
android:authorities="com.my.provider" />
AndroidManifest.xml (3)
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="..." package="com.my.example"
android:versionName="1.0 beta" android:versionCode="2">
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.BLUETOOTH" />
<uses-sdk
android:minSdkVersion="3"
android:targetSdkVersion="4"/>
<supports-screens
android:largeScreens="true"
android:normalScreens="true"
android:smallScreens="true"
android:resizeable="true"
android:anyDensity="true" />
<application ...> ... </application>
</manifest>
AndroidManifest.xml (4)
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="..." package="com.my.example"
android:versionName="1.0 beta" android:versionCode="2">
<uses-configuration android:reqHardKeyboard="true"
android:reqTouchScreen="true" />
<uses-feature android:name="android.software.live_wallpaper" />
<uses-feature android:name="android.hardware.telephony" />
<uses-feature android:name="android.hardware.telephony.cdma" />
...
</manifest>
Android
• Architecture Overview
• Building Blocks
• Manifest
• System Services
• Platform versions
• Saving Application State
• NDK
• Make your code better
Android System Services
Object getApplicationContext().getSystemService(String serviceName)
serviceName value (contsants in Context)
Service Class name Description
WINDOW_SERVICE WindowManager Controls on-screen windows and their parameters
LAYOUT_INFLATER_SERVICE LayoutInflater Inflates layout resources
POWER_SERVICE PowerManager Controls power management
NOTIFICATION_SERVICE NotificationManager Status bar notifications
CONNECTIVITY_SERVICE ConnectivityManager Handling network connections
WIFI_SERVICE WifiManager Handling Wi-Fi network status
TELEPHONY_SERVICE TelephonyManager Handling phone calls states
LOCATION_SERVICE LocationManager Controls location (GPS) updates
SENSOR_SERVICE SensorManager Controls sensor
… … …
Android
• Architecture Overview
• Building Blocks
• Manifest
• System Services
• Platform versions
• Saving Application State
• NDK
• Make your code better
Android Platform Versions
• May 2009 – platform ver. 1.5
• May 2010 – platform ver. 2.2
• Six platform releases in a year (platform ver.1.1 is hard to find on real phones)
• Platforms are back-compatible
• It is good point to be compatible with dominated platforms (1.5, 1.6, 2.0.1 and 2.1)
• To use advantages of the latest platform versions, Java Reflection mechanism should be used
Android Platform Fragmentation in the World
December 2009
June 2010
Android Platform Fragmentation in Russia
• Data gathered by Flurry service in real application
• About 0.5% of 2.1 in Russia are custom ROMs
Android 1.5 ; 43,6%
Android 2.1; 33,6%
Android 1.6 ; 21,7%
Android 2.0.1 ; 0,6% Android 2.2 ; 0,5% Android 2.0 ; 0,1%
Java Reflection
• Originally used to inspect classes, interfaces, fields and methods at runtime, without knowing the names of them at compile time. It can be used for observing and/or modifying program execution at runtime
• Classes: Class, Method, Field, Constructor, etc.
// Without reflection
Foo foo = new Foo();
foo.hello();
// With reflection
Class cls = Class.forName("Foo");
Object foo = cls.newInstance();
Method method = cls.getMethod("hello", null);
method.invoke(foo, null);
Java Reflection and Android
public class ClassHolder {
private static NewClass foo;
public static void tryNewClass() {
foo = new NewClass();
}
}
try {
ClassHolder.tryNewClass();
// NewClass available
} catch (VerifyError e) {}
try {
Method methodName = ClassName.class.getMethod(...);
methodName.invoke(Object obj, Object... args);
} catch (NoSuchMethodException e) {}
try {
Field fieldName = ClassName.class.getField(...);
fieldName.getType(Object obj);
} catch (NoSuchFieldException e) {}
Java Reflection and Android (Example)
private static Field densityDpiField;
private static Field densityDpiLowResValue;
private static boolean sIsLowRes = false;
private static boolean sIsNormalRes = true;
static {
try {
densityDpiField = DisplayMetrics.class.getField("densityDpi");
densityDpiLowResValue = DisplayMetrics.class.getField("DENSITY_LOW");
} catch (NoSuchFieldException nfe) {
// Old device - no density Field
}
}
Java Reflection and Android (Example) (Cont.)
static {
if (densityDpiField != null) {
DisplayMetrics dMetrics = sContext.getResources().getDisplayMetrics();
int densityDpi = 0;
try {
densityDpi = densityDpiField.getInt(dMetrics);
sIsLowRes = (densityDpi == densityDpiLowResValue.getInt(dMetrics));
sIsNormalRes =(densityDpi==densityDpiMediumResValue.getInt(dMetrics));
} catch (Exception e) {
sIsLowRes = false;
sIsNormalRes = true;
}
}
}
Java Reflection and Android (Example) (Cont.)
public static int scaleToDensity(int dp_size) {
if (dp_size == 0 || sIsNormalRes) {
return dp_size;
}
return (int)(dp_size *
sContext.getResources().getDisplayMetrics().density + 0.5f);
}
Android
• Architecture Overview
• Building Blocks
• Manifest
• System Services
• Platform versions
• Saving Application State
• NDK
• Make your code better
Saving State of Android Application
• Shared Preferences are stored in the application private data space and can be shared only inside this Application, but between launches and versions
• Instance of SharedPreferences class should be obtained:
• To read:
• To write:
• SharedPreferences.OnSharedPreferenceChangeListener
SharedPreferences.Editor editor = mPreferences.edit();
editor.putType(String key, T value);
editor.commit();
Activity.getPreferences()
PreferenceManager.getDefaultSharedPreferences(Context ctx)
Context.getSharedPreferences(String name, int mode)
mPreferences.getType(String key, T defValue);
Backup Application Data (android.app.backup – 2.2)
• Perform backup arbitrary data to remote “cloud” storage • Easily perform backup of SharedPreferences and files • Restore the data saved to remote storage • Controlled by Android Backup Manager
– Extend class BackupAgent and override onBackup() & onRestore() OR – Extend BackupAgentHelper to backup/restore SharedPreferences and
files from internal storage – Add your agent to AndroidManifest.xml
• BackupManager.dataChanged()/requestRestore() • New bmgr tool for testing
http://developer.android.com/guide/developing/tools/bmgr.html • Full guide with examples:
http://developer.android.com/guide/topics/data/backup.html
<application android:backupAgent=".MyBackupAgent" >
Android
• Architecture Overview
• Building Blocks
• Manifest
• System Services
• Platform versions
• Saving Application State
• NDK
• Make your code better
Android NDK (Native Development Kit)
• Provides ability and tools to embed components that make use of native code in Android applications
• Supports only restricted set of native libraries:
– libc (C library) headers
– libm (math library) headers
– JNI interface headers
– libz (Zlib compression) headers
– liblog (Android logging) header
– OpenGL ES 1.1 (since 1.6) and OpenGL ES 2.0 (3D graphics libraries, since 2.0) headers
– libjnigraphics (Pixel buffer access) header (since 2.2)
– A Minimal set of headers for C++ support
• For Windows Cygwin 1.7 (or higher) is needed
Android NDK Demo
Android
• Architecture Overview
• Building Blocks
• Manifest
• System Services
• Platform versions
• Saving Application State
• NDK
• Make your code better
Designing for Performance
• Use final for constants
• Avoid enums
• Avoid float
• Use local variables for access to object fields inside loops (or more than once)
• Don’t use private scope with Inner Classes
Action Time
Add a local variable 1
Add a member variable 4
Call String.length() 5
Call empty static native method 5
Call empty static method 12
Call empty virtual method 12.5
Call empty interface method 15
Call Iterator:next() on a HashMap 165
Call put() on a HashMap 600
Inflate 1 View from XML 22,000
Inflate 1 LinearLayout containing 1 TextView
25,000
Inflate 1 LinearLayout containing 6 TextView objects
135,000
Launch an empty activity 3000,000
Designing for Responsiveness
• Application Not Responding (ANR) dialog: – No response to an input event (e.g. key press, screen
touch) within 5 seconds – A BroadcastReceiver hasn't finished executing within 10
seconds
• Quick Activity life-cycle methods (onCreate, onResume) and all other methods working on UI thread
• Use new Thread for long-time operations (i.e. network/database operations, expensive computations, bitmap resizing, etc.)
• Use ProgressBar or ProgressDialog to show “Loading…” or use splash screen when starting application
• To avoid long-time operations in BroadcastReceiver – start Service (not Activity!)
Designing for Seamlessness
• React correctly on system events (i.e. phone call) • onSaveInstanceState/onResumeInstanceState • onPause/onResume • TelephonyManager, etc.
• Share data using ContentProvider • Use NotificationManager and Notifications from background
processes (Service, BroadcastReceiver) • Don’t overload a single Activity screen – use several Activities for
correct dealing with history and Android “backstack” model • Use Threads for long-time operations to avoid ANR • Remember about multiple screen resolutions and multiple Android
platform versions • Assume the Network is slow (change speed in Eclipse DDMS for
Emulator) • Don’t assume Touchscreen or Keyboard • Remember about battery life
Partners
Contacts
Dmitry Lukashev
http://ru.linkedin.com/in/dmitrylukashev
Blog - http://android.amberfog.com/
Alexey Golubev
http://ru.linkedin.com/in/golubevalexey
Future reading
• http://developer.android.com/guide/index.html
• http://developer.android.com/resources/index.html
• Soft, weak, phantom references: http://www.ibm.com/developerworks/library/j-refs/ http://www.ibm.com/developerworks/java/library/j-jtp11225/index.html http://www.ibm.com/developerworks/java/library/j-jtp01246.html
• JVM specification: http://java.sun.com/docs/books/jvms/second_edition/html/VMSpecTOC.doc.html
• The Java Memory Model: http://www.cs.umd.edu/~pugh/java/memoryModel/
• Dalvik VM: http://sites.google.com/site/io/dalvik-vm-internals
• Reflection: http://java.sun.com/docs/books/tutorial/reflect/TOC.html http://tutorials.jenkov.com/java-reflection/index.html
Thank You!
Questions?