Datastorageand exchangein Android - Laurentianweb.cs.laurentian.ca/ppawluk/assets/documents/lecture5-data.pdf · Przemyslaw Pawluk Data storage and exchange in Android. Outline Introduction

OutlineIntroduction

Shared PreferencesSQLite Database

Content ProvidersFile system

Data storage and exchange in AndroidMobile App Development

Przemyslaw Pawluk

Przemyslaw Pawluk Data storage and exchange in Android

OutlineIntroduction



1 IntroductionOverview

2 Shared Preferences

3 SQLite DatabaseSQLite – OverviewImplementation

4 Content ProvidersOverviewMethods to implementURI like SQL

5 File systemInternal storageExternal storage


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Overview







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Overview

Options we have

SharedPreferences

Implements named maps ofname/value pairs that can bepersisted across sessions andshared among applicationcomponents running within thesame application sandbox.

SQLite

Is a SQL implementation.

Content provider

An interface used betweenapplications. The serverapplication that hosts the datamanages it through basiccreate, read, update, anddelete (CRUD) operations.

Files

Local files stored in shared orapp specific space


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Overview

Options we have

SharedPreferences


SQLite


Content provider


Files



OutlineIntroduction



Overview

Options we have

SharedPreferences


SQLite


Content provider


Files



OutlineIntroduction



Overview

Options we have

SharedPreferences


SQLite


Content provider


Files



OutlineIntroduction









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Shared Preferences

How it works?

Shared Preferences are stored within the application’s sandbox, sothey can be shared between application’s components but aren’tavailable to other applications.

How to use it?

To modify a Shared Preference, use theSharedPreferences.Editor class. Get the Editor object bycalling edit on the Shared Preferences object you want to change.Use editor.putXXXX([name], [val]) to add preferences andeditor.getXXXX([name]) to retrieve.


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Shared Preferences

How it works?

Shared Preferences are stored within the application’s sandbox, sothey can be shared between application’s components but aren’tavailable to other applications.

How to use it?

To modify a Shared Preference, use theSharedPreferences.Editor class. Get the Editor object bycalling edit on the Shared Preferences object you want to change.Use editor.putXXXX([name], [val]) to add preferences andeditor.getXXXX([name]) to retrieve.


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User Preferences

Use built-in PreferencesActivity

Creating your own Activity to control user preferences is consideredbad practice. Android Studio provides you with a standard settingsscreen using the Settings Activity.

Question

Why is building custom settings activity a bad practice?


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User Preferences

Use built-in PreferencesActivity

Creating your own Activity to control user preferences is consideredbad practice. Android Studio provides you with a standard settingsscreen using the Settings Activity.

Question

Why is building custom settings activity a bad practice?


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SQLite – OverviewImplementation







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SQLite

Android uses the SQLite database engine:

self-contained,

transactional,

requires no separate server process,

actively developed by a large community


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What is SQLite?

SQLite is an international, open source project implementing atransactional, self-contained relational database.It is not a Google project, but Google has contributed to it.


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How it is stored?

With SQLite, the database is a simple single disk file. All of thedata structures making up a relational databasetables, views,indexes, etc.are within this file.


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Before we start

Good practices

It is a good practice to separate database specific code from therest of your application and encapsulate it in a single class. Otherclasses should use standard Java classes or Cursors and be unawareof how the data is actually stored.


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Schema and Contract

Schema

A formal declaration of how the database is organized. Theschema is reflected in the SQL statements that you use to createyour database.

Contract

A companion class, which explicitly specifies the layout of yourschema in a systematic and self-documenting way.


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Why do we need a Contract class?

A contract class is a container for constants that define namesfor URIs, tables, and columns. The contract class allows youto use the same constants across all the other classes in thesame package. This lets you change a column name in oneplace and have it propagate throughout your code.

A good way to organize a contract class is to put definitionsthat are global to your whole database in the root level of theclass. Then create an inner class for each table thatenumerates its columns.


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Note:BaseColumns

Note

By implementing the BaseColumns interface, your inner class caninherit a primary key field called _ID that some Android classessuch as cursor adaptors will expect it to have. It’s not required,but this can help your database work harmoniously with theAndroid framework.


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Example

1 p u b l i c f i n a l c l a s s FeedReaderCont rac t {2 // To p r e v en t someone from a c c i d e n t a l l y i n s t a n t i a t i n g

the c o n t r a c t c l a s s ,3 // g i v e i t an empty c o n s t r u c t o r .4 p u b l i c FeedReaderCont rac t ( ) {}5

6 /∗ I n n e r c l a s s t ha t d e f i n e s the t a b l e c on t en t s ∗/7 p u b l i c s t a t i c a b s t r a c t c l a s s FeedEntry implements

BaseColumns {8 p u b l i c s t a t i c f i n a l S t r i n g TABLE NAME = ” en t r y ” ;9 p u b l i c s t a t i c f i n a l S t r i n g COLUMN NAME ENTRY ID = ”

e n t r y i d ” ;10 p u b l i c s t a t i c f i n a l S t r i n g COLUMN NAME TITLE = ”

t i t l e ” ;11 p u b l i c s t a t i c f i n a l S t r i n g COLUMN NAME SUBTITLE = ”

s u b t i t l e ” ;12 . . .13 }14 }


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Database interface

We will implement a class MyDatabaseManager serving as aninterface to the database. It extends the abstractSQLiteOpenHelper class, and therefore must override theonCreate and onUpgrade methods.


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onCreate

Automatically called when the application starts for the first time;its job is to create the database

onUpgrade

Used to update a database on the phone when new version of theapplication is shipped


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General elements of the class

Constants

DATABASE NAME – holds the filename of the database

DATABASE VERSION – defines the database version understoodby the software that defines the constant. If the version of thedatabase on the machine is less than DATABASE VERSION, theapplication should run onUpgrade


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General elements of the class

Constructor

call constructor from super

store a context object (usually an activity that opens thedatabase)


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onCreate

You can use two approaches to the implementation that are :

Embedded SQL

SQL commands are embeddedinto Java. The code is in asingle file

Separated SQL

SQL command are stored intoXML file strings.xml andrecalled when needed. Codedistributed among two(potentially more) files


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Reading Data from the Database

Create an SQL statement that describes the data that youneed to retrieve.

Execute that statement against the database.

Map the resulting SQL data into data structures that thelanguage you’re working in can understand.


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Better approach

Android offers slightly better approach – custom cursors.

Coursor

It is a class encapsulating a query results. Query returns a simplecursor, while custom cursor provides (in addition) methods dealingwith specific structure of the data


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OverviewMethods to implementURI like SQL







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Content Provider – Overview

What is Content Provider

It is a mechanism enabling data sharing across applications. Ingeneral reading from other application is not allowed. Both readerand writer (producer/consumer) uses an API to provide or accessdata


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Nine steps to success

1 Extend the ContentProviderclass.

2 Define the CONTENT URI foryour content provider.

3 Create the data storage foryour content.

4 Create the column names forcommunication with clients.

5 Define the process by whichbinary data is returned to theclient.

6 Declare public static Stringsthat clients use to specifycolumns.

7 Implement the CRUD methodsof a Cursor to return to theclient.

8 Update theAndroidManifest.xml file todeclare your < provider >.

9 Define MIME types for anynew data types.


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onCreate

This method is called during the content provider’s startup. Anycode you want to run just once, such as making a databaseconnection, should reside in this method.

getType

This method, when given a URI, returns the MIME type of thedata that this content provider provides at that URI. The URIcomes from the client application interested in accessing the data.


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insert/create

This method is called when the client code wishes to insert datainto the database your content provider is serving. Normally, theimplementation for this method will either directly or indirectlyresult in a database insert operation.

query/read

This method is called whenever a client wishes to read data fromthe content provider’s database. It is normally called throughContentProvider’s managedQuery method. Normally, here youretrieve data using a SQL SELECT statement and return a cursorcontaining the requested data.


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update

This method is called when a client wishes to update one or morerows in the ContentProvider’s database. It translates to a SQLUPDATE statement.

delete

This method is called when a client wishes to delete one or morerows in the ContentProvider’s database. It translates to a SQLDELETE statement.


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URI

content://authority/path/id

where:

content:// is the standard required prefix.

authority is the name of the provider. Using your fullyqualified package name is recommended to prevent namecollisions.

path is a virtual directory within the provider that identifiesthe kind of data being requested.

id is the primary key of a specific record being requested. Torequest all records of a particular type, omit this and thetrailing slash.


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Default content provided by Android

content://browser

content://contacts

content://media

content://settings


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Internal storageExternal storage







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Storage types

Internal storage

It’s always available.

No special permissionrequired

Files saved here areaccessible by only your appby default.

When the user uninstallsyour app, the systemremoves all your app’s filesfrom internal storage.

External storage

It’s not always available(e.g. removed USB storageor SD card)

It’s world-readable, so filessaved here may be readoutside of your control.

When the user uninstallsyour app, the systemremoves your app’s files onlyfromgetExternalFilesDir().


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Which one should I choose?

Internal storage

Internal storage is best when youwant to be sure that neither theuser nor other apps can accessyour files.

External storage

External storage is the best placefor files that don’t require accessrestrictions and for files that youwant to share with other apps orallow the user to access with acomputer.


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Storage and App location

App installation

Although apps are installed onto the internal storage by default,you can specify the android:installLocation attribute in yourmanifest so your app may be installed on external storage. Usersappreciate this option when the APK size is very large and theyhave an external storage space that’s larger than the internalstorage.


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Required permissions for external

1 <uses−p e rm i s s i o n and ro i d : name=” and ro i d . p e rm i s s i o n .READ EXTERNAL STORAGE” />

1 <uses−p e rm i s s i o n and ro i d : name=” and ro i d . p e rm i s s i o n .WRITE EXTERNAL STORAGE” />

Note

You need only one of the above becauseWRITE_EXTERNAL_STORAGE implicitly gives you a READpermission.


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Internal Storage

Your app’s internal storage directory is specified by your app’spackage name in a special location of the Android file system.

Another app can read your internal files if you set the filemode to be readable but

Other apps cannot browse your internal directories and do nothave read or write access unless you explicitly set the files tobe readable or writable.

If you use MODE_PRIVATE for your files on the internalstorage, they are never accessible to other apps.


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Create a file

1 F i l e f i l e = new F i l e ( c on t e x t . g e t F i l e s D i r ( ) , f i l e n ame ) ;

Figure: Create empty file


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Create a file

1 S t r i n g f i l e n ame = ” my f i l e ” ;2 S t r i n g s t r i n g = ” He l l o wor ld ! ” ;3 F i l eOutputSt ream outputStream ;4 t r y {5 outputStream = openF i l eOutput ( f i l e name , Context .

MODE PRIVATE) ;6 outputStream . w r i t e ( s t r i n g . ge tBy te s ( ) ) ;7 outputStream . c l o s e ( ) ;8 } ca tch ( Excep t i on e ) {9 e . p r i n t S t a c kT r a c e ( ) ;

10 }

Figure: Alternatively, you can write to a file in your internal directoryusing a stream.


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Create a file

1 p u b l i c F i l e getTempFi le ( Context contex t , S t r i n g u r l ) {2 F i l e f i l e ;3 t r y {4 S t r i n g f i l eName = Ur i . p a r s e ( u r l ) . getLastPathSegment

( ) ;5 f i l e = F i l e . c r ea t eTempF i l e ( f i l eName , n u l l , c on t e x t .

ge tCacheDi r ( ) ) ;6 ca tch ( IOExcept i on e ) {7 // E r r o r wh i l e c r e a t i n g f i l e8 }9 r e t u r n f i l e ;

10 }

Figure: Create a temporary file e.g, to cache some data


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External storage

Because the external storage may be unavailable–such as when theuser has mounted the storage to a PC or has removed the SD cardthat provides the external storage–you should always verify thatthe volume is available before accessing it. You can query theexternal storage state


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Storage state

Checks if external storage is available to at least read

1 p u b l i c boo l ean i s E x t e r n a l S t o r a g eR e a d a b l e ( ) {2 S t r i n g s t a t e = Envi ronment . g e t E x t e r n a l S t o r a g e S t a t e ( ) ;3 i f ( Env i ronment .MEDIA MOUNTED. equa l s ( s t a t e ) | |4 Envi ronment .MEDIA MOUNTED READ ONLY. e qua l s ( s t a t e ) ) {5 r e t u r n t r u e ;6 }7 r e t u r n f a l s e ;8 }


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Storage state

Checks if external storage is available for read and write

1 p u b l i c boo l ean i s E x t e r n a l S t o r a g eW r i t a b l e ( ) {2 S t r i n g s t a t e = Envi ronment . g e t E x t e r n a l S t o r a g e S t a t e ( ) ;3 i f ( Env i ronment .MEDIA MOUNTED. equa l s ( s t a t e ) ) {4 r e t u r n t r u e ;5 }6 r e t u r n f a l s e ;7 }


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File management in exernal storage

Public files

Files that should be freelyavailable to other apps and to theuser. When the user uninstallsyour app, these files shouldremain available to the user.

photos

downloaded files

Private files

Files that rightfully belong toyour app and should be deletedwith the app. These files areaccessible by the user and otherapps but they are files that don’tprovide value to the user outsideyour app.

additional resources

temporary media files


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Query free space

Query

If you know ahead of time how much data you’re saving, you canfind out whether sufficient space is available without causing anIOException by calling getFreeSpace() or getTotalSpace()

Warning

The system does not guarantee that you can write as many bytesas are indicated by getFreeSpace().


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Datastorageand exchangein Android - Laurentianweb.cs.laurentian.ca/ppawluk/assets/documents/lecture5-data.pdf · Przemyslaw Pawluk Data storage and exchange in Android. Outline Introduction