path: root/docs/html/guide/topics/providers/content-providers.jd

page.title=Content Providers
@jd:body

<div id="qv-wrapper">
<div id="qv">
<h2>Key classes</h2>
<ol>
<li>{@link android.content.ContentProvider}</li>
<li>{@link android.content.ContentResolver}</li>
<li>{@link android.database.Cursor}</li>
</ol>

<h2>In this document</h2>
<ol>
<li><a href="#basics">Content provider basics</a></li>
<li><a href="#querying">Querying a content provider</a></li>
<li><a href="#modifying">Modifying data in a provider</a></li>
<li><a href="#creating">Creating a content provider</a></li>
<li><a href="#urisum">Content URI summary</a></li>
</ol>
</div>
</div>

<p>
Content providers store and retrieve data and make it accessible to all 
applications.  They're the only way to share data across applications; there's 
no common storage area that all Android packages can access.
</p>   

<p>
Android ships with a number of content providers for common data types 
(audio, video, images, personal contact information, and so on).  You can 
see some of them listed in the {@link android.provider android.provider} 
package.  You can query these providers for the data they contain (although,
for some, you must acquire the proper permission to read the data).
</p>   

<p>
If you want to make your own data public, you have two options:  You can 
create your own content provider (a {@link android.content.ContentProvider} 
subclass) or you can add the data to an existing provider &mdash; if there's 
one that controls the same type of data and you have permission to write to it. 
</p>   

<p>
This document is an introduction to using content providers.  After a 
brief discussion of the fundamentals, it explores how to query a content 
provider, how to modify data controlled by a provider, and how to create
a content provider of your own.
</p>   


<h2><a name="basics"></a>Content Provider Basics</h2>

<p>
How a content provider actually stores its data under the covers is 
up to its designer.  But all content providers implement a common interface 
for querying the provider and returning results &mdash; as well as for 
adding, altering, and deleting data.
</p>   

<p>
It's an interface that clients use indirectly, most generally through 
{@link android.content.ContentResolver} objects.  You get a ContentResolver 
by calling <code>{@link android.content.Context#getContentResolver 
getContentResolver()}</code> from within the implementation of an Activity 
or other application component:
</p>   

<pre>ContentResolver cr = getContentResolver();</pre>

<p>
You can then use the ContentResolver's methods to interact with whatever 
content providers you're interested in.
</p>   

<p>
When a query is initiated, the Android system identifies the content provider 
that's the target of the query and makes sure that it is up and running.  
The system instantiates all ContentProvider objects; you never need to do it 
on your own.  In fact, you never deal directly with ContentProvider objects 
at all.  Typically, there's just a single instance of each type of 
ContentProvider.  But it can communicate with multiple ContentResolver objects 
in different applications and processes.  The interaction between processes is 
handled by the ContentResolver and ContentProvider classes.
</p>   


<h3>The data model</h3>

<p>
Content providers expose their data as a simple table on a database model, 
where each row is a record and each column is data of a particular type 
and meaning.  For example, information about people and their phone numbers 
might be exposed as follows: 
</p>   

<table>
   <tr>
      <th scope="col">_ID</th>
      <th scope="col">NUMBER</th>
      <th scope="col">NUMBER_KEY</th>
      <th scope="col">LABEL</th>
      <th scope="col">NAME</th>
      <th scope="col">TYPE</th>
   </tr>
   <tr>
      <td>13</td>
      <td>(425) 555 6677</td>
      <td>425 555 6677</td>
      <td>Kirkland office</td>
      <td>Bully Pulpit</td>
      <td>{@code TYPE_WORK}</td>
   </tr>
   <tr>
      <td>44</td>
      <td>(212) 555-1234</td>
      <td>212 555 1234</td>
      <td>NY apartment</td>
      <td>Alan Vain</td>
      <td>{@code TYPE_HOME}</td>
   </tr>
   <tr>
      <td>45</td>
      <td>(212) 555-6657</td>
      <td>212 555 6657</td>
      <td>Downtown office</td>
      <td>Alan Vain</td>
      <td>{@code TYPE_MOBILE}</td>
   </tr>
   <tr>
      <td>53</td>
      <td>201.555.4433</td>
      <td>201 555 4433</td>
      <td>Love Nest</td>
      <td>Rex Cars</td>
      <td>{@code TYPE_HOME}</td>
   </tr>
</table>

<p>
Every record includes a numeric {@code _ID} field that uniquely identifies 
the record within the table.  IDs can be used to match records in related 
tables &mdash; for example, to find a person's phone number in one table 
and pictures of that person in another.
</p>   

<p>
A query returns a {@link android.database.Cursor} object that can move from 
record to record and column to column to read the contents of each field.  
It has specialized methods for reading each type of data.  So, to read a field, 
you must know what type of data the field contains.  (There's more on query 
results and Cursor objects later.)
</p>   


<h3><a name="uri"></a>URIs</h3>

<p>
Each content provider exposes a public URI (wrapped as a {@link android.net.Uri} 
object) that uniquely identifies its data set.  A content provider that controls 
multiple data sets (multiple tables) exposes a separate URI for each one.  All 
URIs for providers begin with the string "{@code content://}".  The {@code content:} 
scheme identifies the data as being controlled by a content provider.
</p>   

<p>
If you're defining a content provider, it's a good idea to also define a 
constant for its URI, to simplify client code and make future updates cleaner.  
Android defines {@code CONTENT_URI} constants for all the providers that come 
with the platform.  For example, the URI for the table that matches 
phone numbers to people and the URI for the table that holds pictures of 
people (both controlled by the Contacts content provider) are:
</p>   

<p>
<p style="margin-left: 2em">{@code android.provider.Contacts.Phones.CONTENT_URI}
<br/>{@code android.provider.Contacts.Photos.CONTENT_URI}
</p>   

<p>
Similarly, the URIs for the table of recent phone calls and the table 
of calendar entries are:
</p>   

<p>
<p style="margin-left: 2em">{@code android.provider.CallLog.Calls.CONTENT_URI}
<br/>{@code android.provider.Calendar.CONTENT_URI}
</p>   

<p>
The URI constant is used in all interactions with the content provider. 
Every {@link android.content.ContentResolver} method takes the URI 
as its first argument.  It's what identifies which provider the ContentResolver 
should talk to and which table of the provider is being targeted.
</p>   


<h2><a name="querying"></a>Querying a Content Provider</h2>

<p>
You need three pieces of information to query a content provider:
</p>   

<ul>
<li>The URI that identifies the provider</li>
<li>The names of the data fields you want to receive</li>
<li>The data types for those fields</li>
</ul>

<p>
If you're querying a particular record, you also need the ID for that record.
</p>   


<h3>Making the query</h3>

<p>
To query a content provider, you can use either the 
<code>{@link android.content.ContentResolver#query ContentResolver.query()}</code> 
method or the <code>{@link  android.app.Activity#managedQuery 
Activity.managedQuery()}</code> method. 
Both methods take the same set of arguments, and both return a 
Cursor object.  However, {@code managedQuery()} 
causes the activity to manage the life cycle of the Cursor.  A managed Cursor 
handles all of the niceties, such as unloading itself when the activity pauses, 
and requerying itself when the activity restarts.  You can ask an Activity to 
begin managing an unmanaged Cursor object for you by calling 
<code>{@link android.app.Activity#startManagingCursor 
Activity.startManagingCursor()}</code>. 
</p>   

<p>
The first argument to either <code>{@link android.content.ContentResolver#query query()}</code> 
or <code>{@link android.app.Activity#managedQuery managedQuery()}</code> is the provider URI 
&mdash; the {@code CONTENT_URI} constant that identifies a particular 
ContentProvider and data set (see <a href="#uri">URIs</a> earlier).
</p>   

<p>
To restrict a query to just one record, you can append the {@code _ID} value for 
that record to the URI &mdash; that is, place a string matching the ID as the 
last segment of the path part of the URI.  For example, if the ID is 23, 
the URI would be:
</p>   

<p style="margin-left: 2em">{@code content://. . . ./23}</p>   

<p>
There are some helper methods, particularly 
<code>{@link android.content.ContentUris#withAppendedId 
ContentUris.withAppendedId()}</code> and <code>{@link 
android.net.Uri#withAppendedPath Uri.withAppendedPath()}</code>, 
that make it easy to append an ID to a URI.  Both are static methods that return 
a Uri object with the ID added.  So, for example, if you were looking for record 
23 in the database of people contacts, you might construct a query as follows:
</p>   

<pre>
import android.provider.Contacts.People;
import android.content.ContentUris;
import android.net.Uri;
import android.database.Cursor;

// Use the ContentUris method to produce the base URI for the contact with _ID == 23.
Uri myPerson = ContentUris.withAppendedId(People.CONTENT_URI, 23);

// Alternatively, use the Uri method to produce the base URI.
// It takes a string rather than an integer.
Uri myPerson = Uri.withAppendedPath(People.CONTENT_URI, "23");

// Then query for this specific record:
Cursor cur = managedQuery(myPerson, null, null, null, null);
</pre>

<p>
The other arguments to the <code>{@link android.content.ContentResolver#query query()}</code> 
and <code>{@link android.app.Activity#managedQuery managedQuery()}</code> methods delimit 
the query in more detail.  They are:
</p>   

<ul>
<li>The names of the data columns that should be returned.  A {@code null} 
value returns all columns.  Otherwise, only columns that are listed by name
are returned.  All the content providers that come with the platform define 
constants for their columns.  For example, the 
{@link android.provider.Contacts.Phones android.provider.Contacts.Phones} class 
defines constants for the names of the columns in the phone table illustrated 
earlier &mdash {@code _ID}, {@code NUMBER}, {@code NUMBER_KEY}, {@code NAME},
and so on.</li>

<li><p>A filter detailing which rows to return, formatted as an SQL {@code WHERE} 
clause (excluding the {@code WHERE} itself).  A {@code null} value returns 
all rows (unless the URI limits the query to a single record).</p></li>

<li><p>Selection arguments.</p></li>

<li><p>A sorting order for the rows that are returned, formatted as an SQL 
{@code ORDER BY} clause (excluding the {@code ORDER BY} itself).  A {@code null} 
value returns the records in the default order for the table, which may be
unordered.</p></li>  
</ul>

<p>
Let's look at an example query to retrieve a list of contact names and their 
primary phone numbers:
</p>

<pre>
import android.provider.Contacts.People;
import android.database.Cursor;

// Form an array specifying which columns to return. 
String[] projection = new String[] {
                             People._ID,
                             People._COUNT,
                             People.NAME,
                             People.NUMBER
                          };

// Get the base URI for the People table in the Contacts content provider.
Uri contacts =  People.CONTENT_URI;

// Make the query. 
Cursor managedCursor = managedQuery(contacts,
                         projection, // Which columns to return 
                         null,       // Which rows to return (all rows)
                         null,       // Selection arguments (none)
                         // Put the results in ascending order by name
                         People.NAME + " ASC");
</pre>

<p>
This query retrieves data from the People table of the Contacts content 
provider.  It gets the name, primary phone number, and unique record ID for
each contact.  It also reports the number of records that are returned as 
the {@code _COUNT} field of each record.
</p>

<p>
The constants for the names of the columns are defined in various interfaces 
&mdash; {@code _ID} and {@code _COUNT} in 
{@link android.provider.BaseColumns BaseColumns}, {@code NAME} in {@link android.provider.Contacts.PeopleColumns PeopleColumns}, and {@code NUMBER} 
in {@link android.provider.Contacts.PhonesColumns PhoneColumns}.  The 
{@link android.provider.Contacts.People Contacts.People} class implements 
each of these interfaces, which is why the code example above could refer 
to them using just the class name. 
</p>


<h3>What a query returns</h3>

<p>
A query returns a set of zero or more database records.  The names of the 
columns, their default order, and their data types are specific to each 
content provider. 
But every provider has an {@code _ID} column, which holds a unique numeric 
ID for each record.  Every provider can also report the number
of records returned as the {@code _COUNT} column; its value 
is the same for all rows. 
</p>

<p> 
Here is an example result set for the query in the previous section:
</p>

<table border="1">
   <tbody>
      <tr>
         <th scope="col">_ID</th>
         <th scope="col">_COUNT</th>
         <th scope="col">NAME</th>
         <th scope="col">NUMBER</th>     
      </tr>
      <tr>
         <td>44</td>
         <td>3</td>
         <td>Alan Vain</td>
         <td>212 555 1234</td>   
      </tr>
      <tr>
         <td>13</td>
         <td>3</td>
         <td>Bully Pulpit</td>
         <td>425 555 6677</td>   
      </tr>
      <tr>
         <td>53</td>
         <td>3</td>
         <td>Rex Cars</td>
         <td>201 555 4433</td>
      </tr>
   </tbody>
</table>

<p>
The retrieved data is exposed by a {@link android.database.Cursor Cursor} 
object that can be used to iterate backward or forward through the result 
set.  You can use this object only to read the data.  To add, modify, or 
delete data, you must use a ContentResolver object.
</p>


<h3>Reading retrieved data</h3>

<p>
The Cursor object returned by a query provides access to a recordset of
results.  If you have queried for a specific record by ID, this set will
contain only one value.  Otherwise, it can contain multiple values.  
(If there are no matches, it can also be empty.)  You 
can read data from specific fields in the record, but you must know the 
data type of the field, because the Cursor object has a separate method
for reading each type of data &mdash; such as <code>{@link 
android.database.Cursor#getString getString()}</code>, <code>{@link 
android.database.Cursor#getInt getInt()}</code>, and <code>{@link 
android.database.Cursor#getFloat getFloat()}</code>.  
(However, for most types, if you call the method for reading strings, 
the Cursor object will give you the String representation of the data.)  
The Cursor lets you request the column name from the index of the column, 
or the index number from the column name.
</p>

<p>
The following snippet demonstrates reading names and phone numbers from
the query illustrated earlier:
</p>

<pre>
import android.provider.Contacts.People;

private void getColumnData(Cursor cur){ 
    if (cur.moveToFirst()) {

        String name; 
        String phoneNumber; 
        int nameColumn = cur.getColumnIndex(People.NAME); 
        int phoneColumn = cur.getColumnIndex(People.NUMBER);
        String imagePath; 
    
        do {
            // Get the field values
            name = cur.getString(nameColumn);
            phoneNumber = cur.getString(phoneColumn);
           
	    // Do something with the values. 
            ... 

        } while (cur.moveToNext());

    }
}
</pre>

<p>
If a query can return binary data, such as an image or sound, the data 
may be directly entered in the table or the table entry for that data may be
a string specifying a {@code content:} URI that you can use to get the data.  
In general, smaller amounts of data (say, from 20 to 50K or less) are most often 
directly entered in the table and can be read by calling 
<code>{@link android.database.Cursor#getBlob Cursor.getBlob()}</code>.  
It returns a byte array.
</p>
  
<p>
If the table entry is a {@code content:} URI, you should never try to open 
and read the file directly (for one thing, permissions problems can make this 
fail).  Instead, you should call 
<code>{@link android.content.ContentResolver#openInputStream 
ContentResolver.openInputStream()}</code> to get an 
{@link java.io.InputStream} object that you can use to read the data.  
</p>


<h2><a name="modifying"></a>Modifying Data</h2>

<p>
Data kept by a content provider can be modified by:
</p>

<ul>
<p><li>Adding new records</li>
<li>Adding new values to existing records</li>
<li>Batch updating existing records</li>
<li>Deleting records</li>
</ul>

<p>
All data modification is accomplished using {@link android.content.ContentResolver}
methods.  Some content providers require a more restrictive permission for writing
data than they do for reading it.  If you don't have permission to write to a 
content provider, the ContentResolver methods will fail.
</p>


<h3>Adding records</h3>

<p>
To add a new record to a content provider, first set up a map of key-value pairs 
in a {@link android.content.ContentValues} object, where each key matches 
the name of a column in the content provider and the value is the desired 
value for the new record in that column.  Then call <code>{@link 
android.content.ContentResolver#insert ContentResolver.insert()}</code> and pass 
it the URI of the provider and the ContentValues map.  This method returns 
the full URI of the new record &mdash; that is, the provider's URI with 
the appended ID for the new record.  You can then use this URI to query and 
get a Cursor over the new record, and to further modify the record.  
Here's an example:
</p>

<pre>
import android.provider.Contacts.People;
import android.content.ContentResolver;
import android.content.ContentValues; 

ContentValues values = new ContentValues();

// Add Abraham Lincoln to contacts and make him a favorite.
values.put(People.NAME, "Abraham Lincoln");
// 1 = the new contact is added to favorites
// 0 = the new contact is not added to favorites
values.put(People.STARRED, 1);

Uri uri = getContentResolver().insert(People.CONTENT_URI, values);
</pre>


<h3>Adding new values</h3>

<p>
Once a record exists, you can add new information to it or modify 
existing information.  For example, the next step in the example above would 
be to add contact information &mdash; like a phone number or an IM or e-mail 
address &mdash; to the new entry.  
</p>

<p>
The best way to add to a record in the Contacts database is to append 
the name of the table where the new data goes to the URI for the 
record, then use the amended URI to add the new data values.  Each
Contacts table exposes a name for this purpose as a {@code 
CONTENT_DIRECTORY} constant.  The following code continues the previous 
example by adding a phone number and e-mail address for the record
just created:
</p>

<pre>
Uri phoneUri = null;
Uri emailUri = null;

// Add a phone number for Abraham Lincoln.  Begin with the URI for
// the new record just returned by insert(); it ends with the _ID
// of the new record, so we don't have to add the ID ourselves.
// Then append the designation for the phone table to this URI,
// and use the resulting URI to insert the phone number.
phoneUri = Uri.withAppendedPath(uri, People.Phones.CONTENT_DIRECTORY);

values.clear();
values.put(People.Phones.TYPE, People.Phones.TYPE_MOBILE);
values.put(People.Phones.NUMBER, "1233214567");
getContentResolver().insert(phoneUri, values);

// Now add an email address in the same way.
emailUri = Uri.withAppendedPath(uri, People.ContactMethods.CONTENT_DIRECTORY);

values.clear();
// ContactMethods.KIND is used to distinguish different kinds of
// contact methods, such as email, IM, etc. 
values.put(People.ContactMethods.KIND, Contacts.KIND_EMAIL);
values.put(People.ContactMethods.DATA, "test@example.com");
values.put(People.ContactMethods.TYPE, People.ContactMethods.TYPE_HOME);
getContentResolver().insert(emailUri, values);   
</pre>

<p>
You can place small amounts of binary data into a table by calling 
the version of <code>{@link android.content.ContentValues#put 
ContentValues.put()}</code> that takes a byte array.  
That would work for a small icon-like image or a short audio clip, for example.  
However, if you have a large amount of binary data to add, such as a photograph
or a complete song, put a {@code content:} URI for the data in the table and call
<code>{@link android.content.ContentResolver#openOutputStream 
ContentResolver.openOutputStream()}</code> 
with the file's URI.  (That causes the content provider to store the data 
in a file and record the file path in a hidden field of the record.)
</p>

<p>
In this regard, the {@link android.provider.MediaStore} content 
provider, the main provider that dispenses image, audio, and video 
data, employs a special convention:  The same URI that is used with 
{@code query()} or {@code managedQuery()} to get meta-information 
about the binary data (such as, the caption of a photograph or the
date it was taken) is used with {@code openInputStream()} 
to get the data itself.  Similarly, the same URI that is used with
{@code insert()} to put meta-information into a MediaStore record 
is used with {@code openOutputStream()} to place the binary data there.
The following code snippet illustrates this convention:
</p>

<pre>
import android.provider.MediaStore.Images.Media;
import android.content.ContentValues;
import java.io.OutputStream;

// Save the name and description of an image in a ContentValues map.  
ContentValues values = new ContentValues(3);
values.put(Media.DISPLAY_NAME, "road_trip_1");
values.put(Media.DESCRIPTION, "Day 1, trip to Los Angeles");
values.put(Media.MIME_TYPE, "image/jpeg");

// Add a new record without the bitmap, but with the values just set.
// insert() returns the URI of the new record.
Uri uri = getContentResolver().insert(Media.EXTERNAL_CONTENT_URI, values);

// Now get a handle to the file for that record, and save the data into it.
// Here, sourceBitmap is a Bitmap object representing the file to save to the database.
try {
    OutputStream outStream = getContentResolver().openOutputStream(uri);
    sourceBitmap.compress(Bitmap.CompressFormat.JPEG, 50, outStream);
    outStream.close();
} catch (Exception e) {
    Log.e(TAG, "exception while writing image", e);
}
</pre>


<h3>Batch updating records</h3>

<p>
To batch update a group of records (for example, to change "NY" to "New York" 
in all fields), call the <code>{@link 
android.content.ContentResolver#update ContentResolver.update()}</code> 
method with the columns and values to change.
</p>


<h3><a name="deletingrecord"></a>Deleting a record</h3>

<p>
To delete a single record, call {<code>{@link 
android.content.ContentResolver#delete ContentResolver.delete()}</code> 
with the URI of a specific row.
</p>

<p>
To delete multiple rows, call <code>{@link 
android.content.ContentResolver#delete ContentResolver.delete()}</code> 
with the URI of the type of record to delete (for example, {@code android.provider.Contacts.People.CONTENT_URI}) and an SQL {@code WHERE} 
clause defining which rows to delete.  (<i><b>Caution</b>: 
Be sure to include a valid {@code WHERE} clause if you're deleting a general 
type, or you risk deleting more records than you intended!</i>).
</p>


<h2><a name="creating"></a>Creating a Content Provider</h2>

<p>
To create a content provider, you must:
</p>

<ul>
<li>Set up a system for storing the data.  Most content providers 
store their data using Android's file storage methods or SQLite databases, 
but you can store your data any way you want.  Android provides the
{@link android.database.sqlite.SQLiteOpenHelper SQLiteOpenHelper}
class to help you create a database and {@link 
android.database.sqlite.SQLiteDatabase SQLiteDatabase} to manage it.</li>

<li><p>Extend the {@link android.content.ContentProvider} class to provide 
access to the data.</p></li>

<li><p>Declare the content provider in the manifest file for your 
application (AndroidManifest.xml).</p></li>
</ul>

<p>
The following sections have notes on the last two of these tasks.
</p>


<h3>Extending the ContentProvider class</h3>

<p>
You define a {@link android.content.ContentProvider} subclass to 
expose your data to others using the conventions expected by 
ContentResolver and Cursor objects.  Principally, this means 
implementing six abstract methods declared in the ContentProvider class:
</p>

<p style="margin-left: 2em">{@code query()}
<br/>{@code insert()}
<br/>{@code update()}
<br/>{@code delete()}
<br/>{@code getType()}
<br/>{@code onCreate()}</p>

<p>
The {@code query()} method must return a {@link android.database.Cursor} object 
that can iterate over the requested data.  Cursor itself is an interface, but
Android provides some ready-made Cursor objects that you can use.  For example,
{@link android.database.sqlite.SQLiteCursor} can iterate over data stored in
an SQLite database.  You get the Cursor object by calling any of the {@link 
android.database.sqlite.SQLiteDatabase SQLiteDatabase} class's {@code query()}
methods.  There are other Cursor implementations &mdash; such as {@link 
android.database.MatrixCursor} &mdash; for data not stored in a database.
</p>

<p>
Because these ContentProvider methods can be called from 
various ContentResolver objects in different processes and threads, 
they must be implemented in a thread-safe manner. 
</p>

<p>
As a courtesy, you might also want to call <code>{@link android.content.ContentResolver#notifyChange(android.net.Uri,android.database.ContentObserver)
ContentResolver.notifyChange()}</code> to notify listeners when there are 
modifications to the data. 
</p>

<p>
Beyond defining the subclass itself, there are other steps you should take
to simplify the work of clients and make the class more accessible: 
</p>

<ul>
<li>Define a {@code public static final} {@link android.net.Uri} 
named {@code CONTENT_URI}.  This is the string that represents the full 
{@code content:} URI that your content provider handles.  You must define a 
unique string for this value.  The best solution is to use the fully-qualified 
class name of the content provider (made lowercase).  So, for example, the 
URI for a TransportationProvider class could be defined as follows:

<pre>public static final Uri CONTENT_URI = 
               Uri.parse("content://com.example.codelab.transportationprovider");</pre>

<p>
If the provider has subtables, also define {@code CONTENT_URI} constants for
each of the subtables.  These URIs should all have the same authority (since
that identifies the content provider), and be distinguished only by their paths. 
For example:
</p>

<p style="margin-left: 2em">{@code content://com.example.codelab.transportationprovider/train} 
<br/>{@code content://com.example.codelab.transportationprovider/air/domestic}
<br/>{@code content://com.example.codelab.transportationprovider/air/international}</p>

<p>
For an overview of {@code content:} URIs, see the <a href="#urisum">Content URI 
Summary</a> at the end of this document.
</p></li>

<li><p>Define the column names that the content provider will return to clients. 
If you are using an underlying database, these column names are typically 
identical to the SQL database column names they represent.  Also define
{@code public static} String constants that clients can use to specify 
the columns in queries and other instructions.
</p>

<p>
Be sure to include an integer column named "{@code _id}" 
(with the constant {@code _ID}) for 
the IDs of the records.  You should have this field whether or not you have 
another field (such as a URL) that is also unique among all records.  If 
you're using the SQLite database, the {@code _ID} field should be the 
following type:
</p>

<p style="margin-left: 2em">{@code INTEGER PRIMARY KEY AUTOINCREMENT}</p>

<p>
The {@code AUTOINCREMENT} descriptor is optional.  But without it, SQLite
increments an ID counter field to the next number above the largest
existing number in the column.  If you delete the last row, the next row added
will have the same ID as the deleted row.  {@code AUTOINCREMENT} avoids this 
by having SQLite increment to the next largest value whether deleted or not.
</p>
</li>

<li><p>Carefully document the data type of each column.  Clients need this
information to read the data.</p></li>

<li><p>If you are handling a new data type, you must define a new MIME type 
to return in your implementation of <code>{@link 
android.content.ContentProvider#getType ContentProvider.getType()}</code>.  
The type depends in part on whether or not the {@code content:} URI submitted 
to {@code getType()} limits the request to a specific record.  There's one 
form of the MIME type for a single record and another for multiple records.  
Use the {@link android.net.Uri Uri} methods to help determine what is being 
requested.  Here is the general format for each type:</p></li>

<ul>
<li><p>For a single record:&nbsp;&nbsp;&nbsp; {@code vnd.android.cursor.item/vnd.<em>yourcompanyname.contenttype</em}</p> 

<p>For example, a request for train record 122, like this URI,</p>
<p style="margin-left: 2em">{@code content://com.example.transportationprovider/trains/122}</p>

<p>might return this MIME type:</p>
<p style="margin-left: 2em">{@code vnd.android.cursor.item/vnd.example.rail}</p>
</li>

<li><p>For multiple records:&nbsp;&nbsp;&nbsp; {@code vnd.android.cursor.dir/vnd.<em>yourcompanyname.contenttype</em>}</p>

<p>For example, a request for all train records, like the following URI,</p>
<p style="margin-left: 2em">{@code content://com.example.transportationprovider/trains}</p>

<p>might return this MIME type:</p>
<p style="margin-left: 2em">{@code vnd.android.cursor.dir/vnd.example.rail}</p>
</li>
</ul>

<li><p>If you are exposing byte data that's too big to put in the table itself
&mdash; such as a large bitmap file &mdash; the field that exposes the
data to clients should actually contain a {@code content:} URI string.
This is the field that gives clients access to the data file.  The record 
should also have another field, named "{@code _data}" that lists the exact file 
path on the device for that file.  This field is not intended to be read by 
the client, but by the ContentResolver.  The client will call <code>{@link 
android.content.ContentResolver#openInputStream ContentResolver.openInputStream()}</code> 
on the user-facing field holding the URI for the item.  The ContentResolver 
will request the "{@code _data}" field for that record, and because
it has higher permissions than a client, it should be able to access
that file directly and return a read wrapper for the file to the client.</p></li>

</ul>

<p>
For an example of a private content provider implementation, see the 
NodePadProvider class in the Notepad sample application that ships with the SDK.
</p>


<h3>Declaring the content provider</h3>

<p>
To let the Android system know about the content provider you've developed, 
declare it with a {@code &lt;provider&gt;} element in the application's 
AndroidManifest.xml file.  Content providers that are not declared in the
manifest are not visible to the Android system
</p>

<p>
The {@code name} attribute is the fully qualified name of the ContentProvider
subclass.  The {@code authorities} attribute is the authority part of the 
{@code content:} URI that identifies the provider.
For example if the ContentProvider subclass is AutoInfoProvider, the 
{@code &lt;provider&gt;} element might look like this:
</p>

<pre>
&lt;provider name="com.example.autos.AutoInfoProvider"
          authorities="com.example.autos.autoinfoprovider" 
          . . . /&gt
&lt;/provider&gt;
</pre>

<p>
Note that the {@code authorities} attribute omits the path part of a 
{@code content:} URI.  For example, if AutoInfoProvider controlled subtables
for different types of autos or different manufacturers,
</p>

<p style="margin-left: 2em">{@code content://com.example.autos.autoinfoprovider/honda}
<br/>{@code content://com.example.autos.autoinfoprovider/gm/compact}
<br/>{@code content://com.example.autos.autoinfoprovider/gm/suv}</p>

<p>
those paths would not be declared in the manifest.  The authority is what 
identifies the provider, not the path; your provider can interpret the path 
part of the URI in any way you choose.
</p>

<p>
Other {@code &lt;provider&gt;} attributes can set permissions to read and 
write data, provide for an icon and text that can be displayed to users, 
enable and disable the provider, and so on.  Set the {@code multiprocess} 
attribute to "{@code true}" if data does not need to be synchronized between 
multiple running versions of the content provider.  This permits an instance 
of the provider to be created in each client process, eliminating the need 
to perform IPC. 
</p>


<h2><a name="urisum"></a>Content URI Summary</h2>

<p>
Here is a recap of the important parts of a content URI:
</p>

<p>
<img src="{@docRoot}images/content_uri.png" alt="Elements of a content URI" 
height="80" width="528">
</p>

<ol type="A">
<li>Standard prefix indicating that the data is controlled by a
content provider. It's never modified.</li>

<li><p>The authority part of the URI; it identifies the content provider. 
For third-party applications, this should be a fully-qualified class name 
(reduced to lowercase) to ensure uniqueness.  The authority is declared in 
the {@code &lt;provider&gt;} element's {@code authorities} attribute:</p>

<pre>&lt;provider name=".TransportationProvider"
          authorities="com.example.transportationprovider"
          . . .  &gt;</pre></li>

<li><p>The path that the content provider uses to determine what kind of data is
being requested.  This can be zero or more segments long.  If the content provider
exposes only one type of data (only trains, for example), it can be absent.
If the provider exposes several types, including subtypes, it can be several 
segments long &mdash; for example, "{@code land/bus}", "{@code land/train}", 
"{@code sea/ship}", and "{@code sea/submarine}" to give four possibilities.</p></li>

<li><p>The ID of the specific record being requested, if any.  This is the 
{@code _ID} value of the requested record.  If the request is not limited to
a single record, this segment and the trailing slash are omitted:</p>

<p style="margin-left: 2em">{@code content://com.example.transportationprovider/trains}</p>
</li>
</ol>