http://developer.android.com/guide/topics/providers/content-provider-basics.html
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Content Provider Basics
Together, providers and provider clients offer a consistent, standard interface to data that also handles inter-process communication and secure data access.
Overview
A content provider presents data to external applications as one or more tables that are similar to the tables found in a relational database. A row represents an instance of some type of data the provider collects, and each column in the row represents an individual piece of data collected for an instance.
Accessing a provider
An application accesses the data from a content provider with a ContentResolver client object. This object has methods that call identically-named methods in the provider object, an instance of one of the concrete subclasses of ContentProvider. The ContentResolver methods provide the basic "CRUD" (create, retrieve, update, and delete) functions of persistent storage.
Note:To access a provider, your application usually has to request specific permissions in its manifest file. This is described in more detail in the section Content Provider Permissions
Content URIs
A content URI is a URI that identifies data in a provider. Content URIs include the symbolic name of the entire provider (its authority) and a name that points to a table (a path). When you call a client method to access a table in a provider, the content URI for the table is one of the arguments.
content://user_dictionary/words
Retrieving Data from the Provider
Requesting read access permission
To retrieve data from a provider, your application needs
read access permission for the provider. You can't request this permission at run-time; instead, you have to specify that you need this permission in your manifest, using the
element and the exact permission name defined by the provider.
Constructing the query
In the next snippet, if the user doesn't enter a word, the selection clause is set to null, and the query returns all the words in the provider. If the user enters a word, the selection clause is set to
UserDictionary.Words.WORD + " = ?" and the first element of selection arguments array is set to the word the user enters.
/*
* This defines a one-element String array to contain the selection argument.
*/
String[] mSelectionArgs = {""};
// Gets a word from the UI
mSearchString = mSearchWord.getText().toString();
// Remember to insert code here to check for invalid or malicious input.
// If the word is the empty string, gets everything
if (TextUtils.isEmpty(mSearchString)) {
// Setting the selection clause to null will return all words
mSelectionClause = null;
mSelectionArgs[0] = "";
} else {
// Constructs a selection clause that matches the word that the user entered.
mSelectionClause = UserDictionary.Words.WORD + " = ?";
// Moves the user's input string to the selection arguments.
mSelectionArgs[0] = mSearchString;
}
// Does a query against the table and returns a Cursor object
mCursor = getContentResolver().query(
UserDictionary.Words.CONTENT_URI, // The content URI of the words table
mProjection, // The columns to return for each row
mSelectionClause // Either null, or the word the user entered
mSelectionArgs, // Either empty, or the string the user entered
mSortOrder); // The sort order for the returned rows
// Some providers return null if an error occurs, others throw an exception
if (null == mCursor) {
/*
* Insert code here to handle the error. Be sure not to use the cursor! You may want to
* call android.util.Log.e() to log this error.
*
*/
// If the Cursor is empty, the provider found no matches
} else if (mCursor.getCount() < 1) {
/*
* Insert code here to notify the user that the search was unsuccessful. This isn't necessarily
* an error. You may want to offer the user the option to insert a new row, or re-type the
* search term.
*/
} else {
// Insert code here to do something with the results
}
This query is analogous to the SQL statement:
SELECT _ID, word, locale FROM words WHERE word = <userinput> ORDER BY word ASC;
Protecting against malicious input
If the data managed by the content provider is in an SQL database, including external untrusted data into raw SQL statements can lead to
SQL injection.
To avoid this problem, use a selection clause that uses ? as a replaceable parameter and a separate array of selection arguments. When you do this, the user input is bound directly to the query rather than being interpreted as part of an SQL statement. Because it's not treated as SQL, the user input can't inject malicious SQL.
Displaying query results
The ContentResolver.query() client method always returns a Cursor containing the columns specified by the query's projection for the rows that match the query's selection criteria. A Cursor object provides random read access to the rows and columns it contains. Using Cursor methods, you can iterate over the rows in the results, determine the data type of each column, get the data out of a column, and examine other properties of the results. Some Cursor implementations automatically update the object when the provider's data changes, or trigger methods in an observer object when the Cursor changes, or both.
The following snippet continues the code from the previous snippet. It creates a SimpleCursorAdapter object containing the Cursor retrieved by the query, and sets this object to be the adapter for a ListView:
// Defines a list of columns to retrieve from the Cursor and load into an output row
String[] mWordListColumns =
{
UserDictionary.Words.WORD, // Contract class constant containing the word column name
UserDictionary.Words.LOCALE // Contract class constant containing the locale column name
};
// Defines a list of View IDs that will receive the Cursor columns for each row
int[] mWordListItems = { R.id.dictWord, R.id.locale};
// Creates a new SimpleCursorAdapter
mCursorAdapter = new SimpleCursorAdapter(
getApplicationContext(), // The application's Context object
R.layout.wordlistrow, // A layout in XML for one row in the ListView
mCursor, // The result from the query
mWordListColumns, // A string array of column names in the cursor
mWordListItems, // An integer array of view IDs in the row layout
0); // Flags (usually none are needed)
// Sets the adapter for the ListView
mWordList.setAdapter(mCursorAdapter);
Getting data from query results
// Determine the column index of the column named "word"
int index = mCursor.getColumnIndex(UserDictionary.Words.WORD);
/*
* Only executes if the cursor is valid. The User Dictionary Provider returns null if
* an internal error occurs. Other providers may throw an Exception instead of returning null.
*/
if (mCursor != null) {
/*
* Moves to the next row in the cursor. Before the first movement in the cursor, the
* "row pointer" is -1, and if you try to retrieve data at that position you will get an
* exception.
*/
while (mCursor.moveToNext()) {
// Gets the value from the column.
newWord = mCursor.getString(index);
// Insert code here to process the retrieved word.
...
// end of while loop
}
} else {
// Insert code here to report an error if the cursor is null or the provider threw an exception.
}
Cursor implementations contain several "get" methods for retrieving different types of data from the object. For example, the previous snippet uses getString(). They also have a getType() method that returns a value indicating the data type of the column.
Content Provider Permissions
To get the permissions needed to access a provider, an application requests them with a
element in its manifest file. When the Android Package Manager installs the application, a user must approve all of the permissions the application requests. If the user approves all of them, Package Manager continues the installation; if the user doesn't approve them, Package Manager aborts the installation.
Inserting, Updating, and Deleting Data
Inserting data
To insert data into a provider, you call the ContentResolver.insert() method. This method inserts a new row into the provider and returns a content URI for that row. This snippet shows how to insert a new word into the User Dictionary Provider:
// Defines a new Uri object that receives the result of the insertion
Uri mNewUri;
...
// Defines an object to contain the new values to insert
ContentValues mNewValues = new ContentValues();
/*
* Sets the values of each column and inserts the word. The arguments to the "put"
* method are "column name" and "value"
*/
mNewValues.put(UserDictionary.Words.APP_ID, "example.user");
mNewValues.put(UserDictionary.Words.LOCALE, "en_US");
mNewValues.put(UserDictionary.Words.WORD, "insert");
mNewValues.put(UserDictionary.Words.FREQUENCY, "100");
mNewUri = getContentResolver().insert(
UserDictionary.Word.CONTENT_URI, // the user dictionary content URI
mNewValues // the values to insert
);
The data for the new row goes into a single ContentValues object, which is similar in form to a one-row cursor. The columns in this object don't need to have the same data type, and if you don't want to specify a value at all, you can set a column to null using ContentValues.putNull().
The snippet doesn't add the _ID column, because this column is maintained automatically. The provider assigns a unique value of _ID to every row that is added. Providers usually use this value as the table's primary key.
The content URI returned in newUri identifies the newly-added row, with the following format:
content://user_dictionary/words/<id_value>
Updating data
To update a row, you use a ContentValues object with the updated values just as you do with an insertion, and selection criteria just as you do with a query. The client method you use is ContentResolver.update(). You only need to add values to the ContentValues object for columns you're updating. If you want to clear the contents of a column, set the value to null.
The following snippet changes all the rows whose locale has the language "en" to a have a locale of null. The return value is the number of rows that were updated:
// Defines an object to contain the updated values
ContentValues mUpdateValues = new ContentValues();
// Defines selection criteria for the rows you want to update
String mSelectionClause = UserDictionary.Words.LOCALE + "LIKE ?";
String[] mSelectionArgs = {"en_%"};
// Defines a variable to contain the number of updated rows
int mRowsUpdated = 0;
...
/*
* Sets the updated value and updates the selected words.
*/
mUpdateValues.putNull(UserDictionary.Words.LOCALE);
mRowsUpdated = getContentResolver().update(
UserDictionary.Words.CONTENT_URI, // the user dictionary content URI
mUpdateValues // the columns to update
mSelectionClause // the column to select on
mSelectionArgs // the value to compare to
);
Deleting data
Deleting rows is similar to retrieving row data: you specify selection criteria for the rows you want to delete and the client method returns the number of deleted rows. The following snippet deletes rows whose appid matches "user". The method returns the number of deleted rows.
// Defines selection criteria for the rows you want to delete
String mSelectionClause = UserDictionary.Words.APP_ID + " LIKE ?";
String[] mSelectionArgs = {"user"};
// Defines a variable to contain the number of rows deleted
int mRowsDeleted = 0;
...
// Deletes the words that match the selection criteria
mRowsDeleted = getContentResolver().delete(
UserDictionary.Words.CONTENT_URI, // the user dictionary content URI
mSelectionClause // the column to select on
mSelectionArgs // the value to compare to
);
Provider Data Types
Content providers can offer many different data types. The User Dictionary Provider offers only text, but providers can also offer the following formats:
- integer
- long integer (long)
- floating point
- long floating point (double)
Another data type that providers often use is Binary Large OBject (BLOB) implemented as a 64KB byte array. You can see the available data types by looking at the Cursor class "get" methods.
Alternative Forms of Provider Access
Batch access
Batch access to a provider is useful for inserting a large number of rows, or for inserting rows in multiple tables in the same method call, or in general for performing a set of operations across process boundaries as a transaction (an atomic operation).
To access a provider in "batch mode", you create an array of ContentProviderOperation objects and then dispatch them to a content provider with ContentResolver.applyBatch(). You pass the content provider's authority to this method, rather than a particular content URI. This allows each ContentProviderOperation object in the array to work against a different table. A call to ContentResolver.applyBatch() returns an array of results.
// Prepare contact creation request
//
// Note: We use RawContacts because this data must be associated with a particular account.
// The system will aggregate this with any other data for this contact and create a
// coresponding entry in the ContactsContract.Contacts provider for us.
ArrayList<ContentProviderOperation> ops = new ArrayList<ContentProviderOperation>();
ops.add(ContentProviderOperation.newInsert(ContactsContract.RawContacts.CONTENT_URI)
.withValue(ContactsContract.RawContacts.ACCOUNT_TYPE, mSelectedAccount.getType())
.withValue(ContactsContract.RawContacts.ACCOUNT_NAME, mSelectedAccount.getName())
.build());
ops.add(ContentProviderOperation.newInsert(ContactsContract.Data.CONTENT_URI)
.withValueBackReference(ContactsContract.Data.RAW_CONTACT_ID, 0)
.withValue(ContactsContract.Data.MIMETYPE,
ContactsContract.CommonDataKinds.StructuredName.CONTENT_ITEM_TYPE)
.withValue(ContactsContract.CommonDataKinds.StructuredName.DISPLAY_NAME, name)
.build());
ops.add(ContentProviderOperation.newInsert(ContactsContract.Data.CONTENT_URI)
.withValueBackReference(ContactsContract.Data.RAW_CONTACT_ID, 0)
.withValue(ContactsContract.Data.MIMETYPE,
ContactsContract.CommonDataKinds.Phone.CONTENT_ITEM_TYPE)
.withValue(ContactsContract.CommonDataKinds.Phone.NUMBER, phone)
.withValue(ContactsContract.CommonDataKinds.Phone.TYPE, phoneType)
.build());
ops.add(ContentProviderOperation.newInsert(ContactsContract.Data.CONTENT_URI)
.withValueBackReference(ContactsContract.Data.RAW_CONTACT_ID, 0)
.withValue(ContactsContract.Data.MIMETYPE,
ContactsContract.CommonDataKinds.Email.CONTENT_ITEM_TYPE)
.withValue(ContactsContract.CommonDataKinds.Email.DATA, email)
.withValue(ContactsContract.CommonDataKinds.Email.TYPE, emailType)
.build());
// Ask the Contact provider to create a new contact
Log.i(TAG,"Selected account: " + mSelectedAccount.getName() + " (" +
mSelectedAccount.getType() + ")");
Log.i(TAG,"Creating contact: " + name);
try {
getContentResolver().applyBatch(ContactsContract.AUTHORITY, ops);
} catch (Exception e) {
// Display warning
Context ctx = getApplicationContext();
CharSequence txt = getString(R.string.contactCreationFailure);
int duration = Toast.LENGTH_SHORT;
Toast toast = Toast.makeText(ctx, txt, duration);
toast.show();
// Log exception
Log.e(TAG, "Exceptoin encoutered while inserting contact: " + e);
}
Asynchronous queries
You should do queries in a separate thread. One way to do this is to use a CursorLoader object. The examples in the Loaders guide demonstrate how to do this
Data access via intents
Although you can't send an intent directly to a provider, you can send an intent to the provider's application, which is usually the best-equipped to modify the provider's data.
Getting access with temporary permissions
You can access data in a content provider, even if you don't have the proper access permissions, by sending an intent to an application that does have the permissions and receiving back a result intent containing "URI" permissions. These are permissions for a specific content URI that last until the activity that receives them is finished. The application that has permanent permissions grants temporary permissions by setting a flag in the result intent:
- Read permission: FLAG_GRANT_READ_URI_PERMISSION
- Write permission: FLAG_GRANT_WRITE_URI_PERMISSION
For example, you can retrieve data for a contact in the Contacts Provider, even if you don't have the READ_CONTACTS permission. You might want to do this in an application that sends e-greetings to a contact on his or her birthday. Instead of requesting READ_CONTACTS, which gives you access to all of the user's contacts and all of their information, you prefer to let the user control which contacts are used by your application. To do this, you use the following process:
- Your application sends an intent containing the action ACTION_PICK and the "contacts" MIME type CONTENT_ITEM_TYPE, using the method startActivityForResult().
- Because this intent matches the intent filter for the People app's "selection" activity, the activity will come to the foreground.
- In the selection activity, the user selects a contact to update. When this happens, the selection activity calls setResult(resultcode, intent) to set up a intent to give back to your application. The intent contains the content URI of the contact the user selected, and the "extras" flags FLAG_GRANT_READ_URI_PERMISSION. These flags grant URI permission to your app to read data for the contact pointed to by the content URI. The selection activity then calls finish() to return control to your application.
- Your activity returns to the foreground, and the system calls your activity's onActivityResult() method. This method receives the result intent created by the selection activity in the People app.
- With the content URI from the result intent, you can read the contact's data from the Contacts Provider, even though you didn't request permanent read access permission to the provider in your manifest. You can then get the contact's birthday information or his or her email address and then send the e-greeting.
Using another application
A simple way to allow the user to modify data to which you don't have access permissions is to activate an application that has permissions and let the user do the work there.
For example, the Calendar application accepts an ACTION_INSERT intent, which allows you to activate the application's insert UI. You can pass "extras" data in this intent, which the application uses to pre-populate the UI. Because recurring events have a complex syntax, the preferred way of inserting events into the Calendar Provider is to activate the Calendar app with an ACTION_INSERT and then let the user insert the event there.
Contract Classes
A contract class defines constants that help applications work with the content URIs, column names, intent actions, and other features of a content provider. Contract classes are not included automatically with a provider; the provider's developer has to define them and then make them available to other developers. Many of the providers included with the Android platform have corresponding contract classes in the package android.provider.
MIME Type Reference
Content providers can return standard MIME media types, or custom MIME type strings, or both.
For example, the well-known MIME type text/html has the text type and the html subtype. If the provider returns this type for a URI, it means that a query using that URI will return text containing HTML tags.
Custom MIME type strings, also called "vendor-specific" MIME types, have more complex type and subtype values. The type value is always
vnd.android.cursor.dir
for multiple rows, or
vnd.android.cursor.item
for a single row.
