一個內容提供者訪問數據的中央資源庫。提供者是應用程序的一部分，提供自己的操作數據的UI。然而，內容提供者主要是被其他應用程序引用，通過提供者客戶對象訪問提供者。提供者和提供者客戶端為數據提供一個一致的，標準的接口，也處理進程間的聯系和數據安全訪問。

本文討論下面幾個方面的基礎：

：

內容提供類如何工作。

從內容提供者獲取數據的API。

向內容提供者你插入、更新、刪除數據的API。

便利的使用內容提供者的API。

Overview

提供者向應用程序呈現數據就像一個或多張表，就像是在關系數據庫里一樣。一行顯示一些數據類型的實例，列的每行顯示實例數據集合的獨立數據。

例如：一個在android平臺里內建的提供者是用戶詞典，用來存儲用戶想保存的非標準的拼寫。表一說明了在提供者的表里數據是如何的:

Table 1: Sample user dictionary table.

word

app id

frequency

locale

_ID

mapreduce

user1

100

en_US

1

precompiler

user14

200

fr_FR

2

applet

user2

225

fr_CA

3

const

user1

255

pt_BR

4

int

user5

100

en_UK

5

在表1里，每行的單詞都是在標準詞典里找不到的。沒列有一些和單詞相關的數據，比如：它所屬的區域。列頭是列的名字。引用列的區域，你可以引用它的locale列。對于提供者來講，自動認為列的_ID是“主鍵”。

注意：提供者并不是一定需要主鍵，它不需要使用_ID 作為主鍵的列名如果已經存在。然而，如果你想綁定一個提供者到一個ListView，一個列名是需要_ID的。對于這個需要的解釋，在這里：Displaying query results。

訪問一個提供者

一個應用程序通過客戶類ContentResolver訪問內容提供者的數據。這個函數有一個在提供者對象里同名的函數，是一個ContentProvider子類的實例。函數ContentResolver提供”CRUD”的基礎(創建，獲取，更新，刪除)函數。

擁有提供者的應用程序進程的ContentResolver對象和ContentProvider對象可以自動處理進程間的數據交換。

注意：訪問一個提供者，應用程序通常在manifest文件里請求指定的權限。在Content Provider Permissions一節里有更多細節。

例如，從User Dictionary Provider獲取單詞的一列和語言環境，你調用ContentResolver.query().query()函數調用定義的函數[ ContentProvider.query()]。下面的代碼顯示的是一個[ ContentResolver.query()]調用：

// Queries the user dictionary and returns results

mCursor = getContentResolver().query(

UserDictionary.Words.CONTENT_URI, // The content URI of the words table

mProjection, // The columns to return for each row

mSelectionClause // Selection criteria

mSelectionArgs, // Selection criteria

mSortOrder); // The sort order for the returned rows

Table 2 shows how the arguments to

Table 2: Query() compared to SQL query.

query() argument

SELECT keyword/parameter

Notes

Uri

FROM table_name

Uri maps to the table in the provider named table_name.

projection

col,col,col,...

projection is an array of columns that should be included for each row

retrieved.

selection

WHERE col = value

selection specifies the criteria for selecting rows.

selectionArgs

(No exact equivalent. Selection arguments replace ? placeholders in the

selection clause.)

sortOrder

ORDER BY col,col,...

sortOrder specifies the order in which rows appear in the returned

Content URIs

一個內容URI是一個定于數據的URI。內容URI包含一個提供者的符號名(它的權限)和一個指向一個表(一個路徑)的名字。當你調用客戶端函數來訪問表的時候，這個URI是參數的一個。

上面的代碼，常量CONTENT_URI包含user dictionary的單詞表的內容URI。對象ContentResolver 解析URI的權限，使用它。ContentResolver能分派查詢參數來更正提供者。

使用內容URI的部分路徑來選擇想訪問的表。提供者的每個外在的表都有一個路徑。

之前的代碼，“words”表完整的URI是：

content://user_dictionary/words

字符串user_dictionary是提供者的權限，字符串words是表的路徑。字符串content://(配置)通常指示標識這項是一個內容URI。

提供者允許你通過附加在URI后的一個ID值來訪問表的一行。例如，從獲取_ID是4的一行，你可以使用內容URI：

Uri singleUri = ContentUri.withAppendedId(UserDictionary.Words.CONTENT_URI,4);

通常獲取一個行集合的時候使用id并且想要更新或者刪除它們中的一個。

注意：類Uri和Uri.Builder包含便利的函數來從一個字符串格式化Uri對象。ContetnUris包含一個便利的函數withAppendedId()來向URI最近一個id。之前的片段是用來追加id到Userdictionary。

從提供者里獲取數據

從提供者里獲取數據，例子使用User Dictionary Provider。

為了清晰，這一節的代碼段在“UI線程”里調用ContentResolver.query()。實際的代碼，然而，你需要在分開的線程里做異步查詢。可以使用CursorLoader實現，更多的信息在Loaders 指南。代碼很短；他們沒有顯示一個完整的應用。

獲取數據，有以下的兩個步驟：

1、需要提供者允許讀訪問。

2、發送一個query到提供者的代碼。

請求讀訪問權限

從提供者獲取數據，應用程序需要“讀許可”。不可以在運行的時候申請；在你的manifest文件里聲明，使用元素擴展權限名，它是提供者定義的。

當你在manifest里聲明元素，實際上你就是申請權限。當用戶安裝引用程序，也就隱式的授予了請求。

為你使用的提供者查找確切的讀訪問權限的名字，就像其它提供者的訪問權限的名字一樣，參考提供者文檔。

在Content Provider Permissions一節，有關于訪問提供者的權限的更多信息。

User Dictionary Provider在manifest文件里定義android.permission.READ_USER_DICTIONARY權限，應用程序想從提供者讀取需要這個權限。

構造查詢

下一步獲取數據時提供者構造一個查詢。第一個片段為訪問User Dictionary Provider定義了一些變量：

// A "projection" defines the columns that will be returned for each row

String[] mProjection =

{

UserDictionary.Words._ID, // Contract class constant for the _ID column name

UserDictionary.Words.WORD, // Contract class constant for the word column name

UserDictionary.Words.LOCALE // Contract class constant for the locale column name

};

// Defines a string to contain the selection clause

String mSelectionClause = null;

// Initializes an array to contain selection arguments

String[] mSelectionArgs = {""};

下面的代碼顯示如何使用ContentResolver.query()，例子使用。提供者客戶端查詢是類似SQL查詢，它返回一個列的集合，選擇標準集，排序命令。

查詢需要返回的列的集合被疑個Projection(變量mProjection)調用。知道獲取行的表達式是分到一個選擇語句和一個選擇參數里的。選擇語句是一個邏輯、布爾值、列名、值(變量mSelection)復合表達式。如果你指定替換參數？來代替一個值，查詢函數從選擇參數數列里獲取值(變量mSelectionArgs)。

下面的片段，如果用戶不輸入一個單詞，設置為null，查詢返回提供者里所有的單詞。如果用戶輸入一個單詞，設置 UserDictionary.Words.Word + " = ?" 并且選擇參數數組的第一個元素設置為用戶輸入的。

/*

* 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

}

這個查詢和SQL語句類似。

SELECT _ID, word, frequency, locale FROM words WHERE word = ORDER BY word ASC;

這個SQL語句，實際的列名用于替代合約類的常量。

防止惡意插入

如果通過提供者管理的數據在SQL數據庫里，包括外部不可信的數據進入原始的SQL語句會導致SQL注入。 考慮這種情況：

Consider this selection clause:

// Constructs a selection clause by concatenating the user's input to the column name

String mSelectionClause = "var = " + mUserInput;

如果你這樣做，你就允許用戶串聯惡意的SQL到你的SQL語句里。例如：用戶可以為mUserInput輸入“nothing; DROP TABLE *;”結果選擇語句var = nothing; DROP TABLE *;

當選擇語句被認為是一個SQL語句，就會引起提供者擦除SQLite數據庫里所有的表(除非提供者設置捕獲SQL injection 的語句)。

為了解決這個問題，使用一個有？作為可替換參數的選擇語句和一個分開的選擇參數數組。這樣做，用戶輸入一個**到查詢而不是作為SQL語句的一部分被中斷。因為，它沒有被視為是SQL，用戶輸入不可以注入惡意的SQL。使用如下的選擇語句，而不是使用級聯用戶輸入的語句。

// Constructs a selection clause with a replaceable parameter

String mSelectionClause = "var = ?";

設置選擇參數數組如下：

// Defines an array to contain the selection arguments

String[] selectionArgs = {""};

設置一個選擇參數數組就如下：

// Sets the selection argument to the user's input

selectionArgs[0] = mUserInput;

？是可替換參數，選擇查詢數組首選的方法是指定一節，甚至提供者不基于一個SQL數據庫。

顯示查詢結果

ContentResolver.query() 函數總是返回一個Cursor ，它包含查詢的Projection指定的列，行是匹配查詢選擇標準的。一個Cursor 對象提供隨機的讀行權限還有它包含的列。使用Cursor 函數，你可以在結果里遍歷行，決定每列的數據類型，獲取列外的數據，使用結果的其它屬性。一些Cursor 實現當提供者數據變化時自動的更新。或當Cursor 改變是觸發觀察對象的方法，或者兩者都有。

注意：一個提供者可能限制訪問列基于對象的屬性來生成查詢。例如：合約提供者訪問一些列來同步適配器，因此它不返回到Activity或一個服務。

如果沒有匹配選擇標準的行，提供者返回一個Cursor對象，它的Cursor.getCount() 是0(一個空的cursor)。

如果內部發生一個錯誤，查詢結果依靠指定的提供者。可能選擇返回null，或者拋出異常。

如果Cursor是行的列表，顯示Cursor內容的方法是把它通過SimpleCursorAdapter和一個ListView連接。

下面的代碼是前面代碼的繼續。它創建一個對象SimpleCursorAdapter包含通過查詢獲取的Cursor，設置這個對象作為ListView的適配器。

// 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);

注意：備份一個ListView和一個Cursor，curso需要包含一個列名叫_ID。因為，查詢顯示之前返回的單詞表的_ID列，甚至ListView不顯示。這也解釋了為什么每個表的列有一個_ID。

從查詢結果里獲取數據

不是簡單顯示你查詢的結果，你可使用它們做別的任務。例如：你可以從user dictionary獲取拼寫并且在其它提供者里查詢。可以在Cursor里遍歷行：

// 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實現一系列的“get”函數，為從對象獲取不同類型的數據。例如：前面的代碼使用函數getString()。getType()函數返回一個表示數據類型的值。

內容提供者權限

一個內容提供者可以指明其它需要訪問數據的程序的權限。權限確保用戶知道程序想要訪問的數據。基于提供者的請求，程序請求權限以此訪問提供者。當安裝應用程序的時候，終端用戶看到請求權限。 如果提供者的程序不指明權限，其它應用程序不可以訪問數據。然而，提供者程序的組件擁有所有的讀寫權，不管有沒有指定。

之前提到的，User Dictionary Provider請求android.permission.READ_USER_DICTIONARY 權限來獲取數據。提供者為插入、更新、刪除數據分開android.permission.WRITE_USER_DICTIONARY 權限。

獲取訪問提供者訪問的權限，一個應用程序需要在manifest文件里有元素。當Android包管理器安裝應用程序，用戶必須同意程序請求的所有權限。如果用戶允許，包管理器繼續安裝；如果用戶不允許，包管理器終止安裝。 下面的元素請求User Dictionary Provider讀訪問。

關于提供者權限的影響，更多的信息在Security and Permissions 。

插入，更新，刪除數據

一些你從提供者獲取數據的方法，你使用提供者客戶端和提供者的ContentProvider 來修改數據。你調用函數**。提供者和提供者客戶端自動處理安全和進程間通信。

插入數據

向一個提供者插入數據，你可以調用。這個函數插入一個新的行，返回該行的內容URI。以下代碼顯示如何向一個User Dictionary Provider插入新行：

這里有代碼

代碼段不需要添加-ID列，因為列的維護是自動的。如果提供者給每個添加的行分配一個獨一無二的-ID值。提供者使用這個值作為表的主鍵。

newUri 返回的內容URI指明新增加的行，使用如下格式：

content://user_dictionary/words/

The is the contents of _ID for the new row.

Most providers can detect this form of content URI automatically and then perform the requested

operation on that particular row.

To get the value of _ID from the returned

Updating data

To update a row, you use a 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

);

You should also sanitize user input when you call

Protecting against malicious input.

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

);

You should also sanitize user input when you call

Protecting against malicious input.

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

The data type for each column in a provider is usually listed in its documentation.

The data types for the User Dictionary Provider are listed in the reference documentation

for its contract class Contract Classes).

You can also determine the data type by calling

Providers also maintain MIME data type information for each content URI they define. You can

use the MIME type information to find out if your application can handle data that the

provider offers, or to choose a type of handling based on the MIME type. You usually need the

MIME type when you are working with a provider that contains complex

data structures or files. For example, the

The section MIME Type Reference describes the

syntax of both standard and custom MIME types.

Alternative Forms of Provider Access

Three alternative forms of provider access are important in application development:

Batch access: You can create a batch of access calls with methods in

the

Asynchronous queries: You should do queries in a separate thread. One way to do this is to

use a 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.

Batch access and modification via intents are described in the following sections.

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 authority to this

method, rather than a particular content URI, which allows each

The description of the Contact Manager

sample application contains an example of batch access in its ContactAdder.java

source file.

Displaying data using a helper app

If your application does have access permissions, you still may want to use an

intent to display data in another application. For example, the Calendar application accepts an

Calendar Provider guide.

The application to which you send the intent doesn't have to be the application

associated with the provider. For example, you can retrieve a contact from the

Contact Provider, then send an

Data access via intents

Intents can provide indirect access to a content provider. You allow the user to access

data in a provider even if your application doesn't have access permissions, either by

getting a result intent back from an application that has permissions, or by activating an

application that has permissions and letting the user do work in it.

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:

Note: These flags don't give general read or write access to the provider

whose authority is contained in the content URI. The access is only for the URI itself.

A provider defines URI permissions for content URIs in its manifest, using the

Security and Permissions guide,

in the section "URI Permissions".

For example, you can retrieve data for a contact in the Contacts Provider, even if you don't

have the

Your application sends an intent containing the action

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

Your activity returns to the foreground, and the system calls your activity's

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

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

For example, the User Dictionary Provider has a contract class

String[] mProjection =

{

UserDictionary.Words._ID,

UserDictionary.Words.WORD,

UserDictionary.Words.LOCALE

};

Another contract class is

MIME Type Reference

Content providers can return standard MIME media types, or custom MIME type strings, or both.

MIME types have the format

type/subtype

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.

The subtype is provider-specific. The Android built-in providers usually have a simple

subtype. For example, the when the Contacts application creates a row for a telephone number,

it sets the following MIME type in the row:

vnd.android.cursor.item/phone_v2

Notice that the subtype value is simply phone_v2.

Other provider developers may create their own pattern of subtypes based on the provider's

authority and table names. For example, consider a provider that contains train timetables.

The provider's authority is com.example.trains, and it contains the tables

Line1, Line2, and Line3. In response to the content URI

content://com.example.trains/Line1

for table Line1, the provider returns the MIME type

vnd.android.cursor.dir/vnd.example.line1

In response to the content URI

content://com.example.trains/Line2/5

for row 5 in table Line2, the provider returns the MIME type

vnd.android.cursor.item/vnd.example.line2

Most content providers define contract class constants for the MIME types they use. The

Contacts Provider contract class

Content URIs for single rows are described in the section

Content URIs.

總結

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