CSE 486/586, Spring 2012

CSE 486/586 Distributed Systems

Recitation

CSE 486/586, Spring 2012

Android Emulator Networking

• Note: this is my (Steve’s) best guess; actual details might be different. But it should be good enough for your understanding.

• Each emulator instance is attached to a virtual router.• The virtual router runs on top of the host (probably as

an application).• The virtual router forwards connections originated

from its emulator instance.• However, the virtual router acts as a firewall; it blocks

all connection attempts going into its emulator instance from somewhere else.

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CSE 486/586, Spring 2012

Android Emulator Networking

• What if we want to allow others (in general) to connect to an app running inside an emulator instance?– Android provides redirection as a solution.

3

emu0

App

connect()

VR0

Host

emu1

App

connect()

VR1

emu2

App

connect()

VR2

CSE 486/586, Spring 2012

Android Emulator Networking

• Redirection Basics– In order to allow connection attempts going into the

emulator instance, we need to set up redirection from the virtual router to its emulator instance.

– Since everything happens on top of the host, the only way to enable any kind of networking is to borrow the host’s IP and ports.

– Redirection forces the virtual router to listen on a host port and forward all traffic coming to the host port to the emulator instance.

• Process– Pick a host port to borrow (that others will use to connect to)– Set up redirection from that host port to the emulator port

that the app is listening on– E.g., redir add tcp:<host port>:<emulator port>

» Meaning, forward all traffic from the host port to the emulator port

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CSE 486/586, Spring 2012

Android Emulator Networking

• On emu0: redir add tcp:10000:8080

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emu0

App

VR0

Host

VR0 starts listening on 10000 & forwards all traffic to emu0’s 8080.

App may or may not listen on 8080 (not required for redirection).

8080

10000

CSE 486/586, Spring 2012

Android Emulator Networking

• On emu0: redir add tcp:10000:8080

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emu0

App

VR0

Host

VR0 starts listening on 10000 & forwards all traffic to emu0’s 8080.

App may or may not listen on 8080 (not required for redirection).

8080

10000

For any connect() attempt to <host IP:10000> (either from outside or from an app running on the host), it will succeed.

CSE 486/586, Spring 2012

Android Emulator Networking

• On emu0: redir add tcp:10000:8080

7

emu0

App

VR0

Host

VR0 starts listening on 10000 & forwards all traffic to emu0’s 8080.

App may or may not listen on 8080 (not required for redirection).

8080

10000

For any connect() attempt to <host IP:10000> (either from outside or from an app running on the host), it will succeed.

This (port 8080) is not visible to anyone outside of VR0. All others can only see port 10000. This is why you can use the same server port number in your app.

CSE 486/586, Spring 2012

Android Emulator Networking

• On emu0: redir add tcp:10000:8080• On emu1: redir add tcp:20000:8080

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emu0

App

VR0

Host

8080

10000

emu1

App

VR1

8080

20000

Since we’re borrowing the host’s port numbers, this has to be different from VR0’s port number.

CSE 486/586, Spring 2012

Android Emulator Networking

• Now all connect() should be to <host IP>:<redirected port>

• E.g., connect() to <host IP>:10000 will be forwarded to emu0:8080

9

emu0

App

VR0

Host

8080

10000

emu1

App

VR1

8080

20000

CSE 486/586, Spring 2012

Android Emulator Networking

• IP Addresses– A virtual router assigns an IP address to its emulator

instance (10.0.2.15) and to the host (10.0.2.2) from a private IP range (10.0.0.0/8).

– It assigns an IP address to the host mainly for convenience (I think…).

• In the previous example, if emu1 wants to talk to emu0, it can use:– connect() to 10.0.2.2:10000

• Effectively, we are identifying different emulators using host port numbers we’re borrowing instead of their IP:port pairs.

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CSE 486/586, Spring 2012

Content Providers

• A content provider provides a table view of data.• If you write a content provider, any client application

with the permission can enter/read/update/delete data items in your content provider.

• A client application (that uses your content provider) uses ContentResolver to interact with your content provider.

• You need to extend ContentProvider and implement necessary methods.

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CSE 486/586, Spring 2012

How a Client Interacts

• Table identification URI (android.net.Uri)– E.g., content://user_dictionary/words

• Insert– public final Uri ContentResolver.insert (Uri url,

ContentValues values)

• Update– public final int ContentResolver.update (Uri uri,

ContentValues values, String where, String[] selectionArgs)

• Query– public final Cursor ContentResolver.query (Uri uri, String[]

projection, String selection, String[] selectionArgs, String sortOrder)

• Delete– public final int ContentResolver.delete (Uri url, String where,

String[] selectionArgs)

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CSE 486/586, Spring 2012

How to Write a Content Provider

1. Declare in AndroidManifest.xml2. Define a URI that client apps will use3. Define permissions4. Implement necessary methods in ContentProvider5. When implementing ContentProvider, use either the

Android file system or SQLite as the actual data storage.

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CSE 486/586, Spring 2012

Declare in AndroidManifest.xml

<manifest ... > ... <application ... > <provider android:name=".ExampleProvider" /> ... </application></manifest>

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CSE 486/586, Spring 2012

Defining a URI

• Typical format– content://<authority>/<table name>– Authority: a global (Android-wide) name for the provider

» E.g., edu.buffalo.cse.cse486.proj1.provider– Table name: the name of a table that the provider exposes

» Note: a provider can expose more than one table.

• Should be added to AndroidManifest.xml– E.g., <provider

android:authorities=“edu.buffalo.cse.cse486.proj1.provider” …>…</provider>

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CSE 486/586, Spring 2012

Define Permissions

• Should define permissions (for others) in AndroidManifest.xml

• android:permission: Single provider-wide read/write permission.– E.g., <provider

android:permission=“edu.buffalo.cse.cse486.proj1.provider.permission.USE_PROJ1_PROVIDER” …>…</provider>

• android:readPermission: Provider-wide read permission.

• android:writePermission: Provider-wide write permission.

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CSE 486/586, Spring 2012

Necessary Methods

• query()– Retrieve data from your provider.

• insert()– Insert a new row into your provider.

• update()– Update existing rows in your provider.

• delete()– Delete rows from your provider.

• getType()– Return the MIME type corresponding to a content URI.

• onCreate()– Initialize your provider. The Android system calls this method

immediately after it creates your provider. Notice that your provider is not created until a ContentResolver object tries to access it.

• These need to handle concurrent accesses (need to be thread-safe)

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CSE 486/586, Spring 2012

Storage Options

• Internal storage: file system, private to the app• External storage: file system, open to everybody• SQLite: database, private to the app• Read:

http://developer.android.com/guide/topics/data/data-storage.html

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CSE 486/586, Spring 2012

Internal Storage

• Saving files directly on the device's internal storage.• User uninstallation files are removed.• To create and write a private file to the internal

storage:– Call openFileOutput() with the name of the file and the

operating mode. This returns a FileOutputStream.– Write to the file with write().– Close the stream with close().

• E.g.,String FILENAME = "hello_file";String string = "hello world!”;FileOutputStream fos = openFileOutput(FILENAME,

Context.MODE_PRIVATE);fos.write(string.getBytes());fos.close();

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CSE 486/586, Spring 2012

Internal Storage

• MODE_PRIVATE create the file (or replace a file of the same name) and make it private to your application.

• Other modes available are: – MODE_APPEND, MODE_WORLD_READABLE, and

MODE_WORLD_WRITEABLE.

• To read a file from internal storage:– Call openFileInput() and pass it the name of the file to read.

This returns a FileInputStream.– Read bytes from the file with read().– Then close the stream with close().

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CSE 486/586, Spring 2012

External Storage

• Shared "external storage”– E.g., a removable storage media (such as an SD card) or an

internal (non-removable) storage.

• Files saved to the external storage are:– World-readable– Can be modified by the user when they enable USB mass

storage to transfer files on a computer.

• Checking media availability– Before you do any work with the external storage, you

should always call getExternalStorageState() to check whether the media is available.

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CSE 486/586, Spring 2012

External Storage

• Accessing files on external storage (API Level 8 or greater)– Use getExternalFilesDir() to open a File that represents the

external storage directory where you should save your files.– This method takes a type parameter that specifies the type

of subdirectory you want, such as DIRECTORY_MUSIC and DIRECTORY_RINGTONES (pass null to receive the root of your application's file directory). This method will create the appropriate directory if necessary.

– If the user uninstalls your application, this directory and all its contents will be deleted.

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CSE 486/586, Spring 2012

External Storage

• Saving files that should be shared– For files not specific to your application and that should not

be deleted when your application is uninstalled– Save them to one of the public directories on the external

storage.– These directories lay at the root of the external storage,

such as Music/, Pictures/, Ringtones/, and others.

• (API Level 8 or greater)– Use getExternalStoragePublicDirectory(), passing it the type

of public directory you want, such as DIRECTORY_MUSIC, DIRECTORY_PICTURES, DIRECTORY_RINGTONES, or others. This method will create the appropriate directory if necessary.

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CSE 486/586, Spring 2012

Services

• A service runs in the background with no UI for long-running operations.– Playing music, sending/receiving network messages, …– Subclass of android.app.Service

• Started service– A service is "started" when an application component (such

as an activity) starts it by calling startService(). Once started, a service can run in the background indefinitely, even if the component that started it is destroyed.

• Bound service– A service is "bound" when an application component binds

to it by calling bindService(). A bound service offers a client-server interface that allows components to interact with the service, send requests, get results, and even do so across processes with interprocess communication (IPC).

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CSE 486/586, Spring 2012

How to Write a Service

• Declare in AndroidManifest.xml• Implement necessary methods in Service

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CSE 486/586, Spring 2012

Declare in AndroidManifest.xml

<manifest ... > ... <application ... > <service android:name=".ExampleService" /> ... </application></manifest>

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CSE 486/586, Spring 2012

Necessary Methods

• onStartCommand()– The system calls this method when another component,

such as an activity, requests that the service be started, by calling startService().

• onBind()– The system calls this method when another component

wants to bind with the service (such as to perform RPC), by calling bindService().

• onCreate()– The system calls this method when the service is first

created, to perform one-time setup procedures (before it calls either onStartCommand() or onBind()).

• onDestroy()– The system calls this method when the service is no longer

used and is being destroyed.

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CSE 486/586, Spring 2012

Service Lifecycle

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