Android 4 学习（16）：Database and Content Providers

参考《Professional Android 4 Development》

Database and Content Providers

Android Database简介

Android使用SQLite数据库和ContentProvider来实现结构化数据的读写。在Android中，SQLite是以lib的形式存在的，每个应用程序含有自己的SQLite lib，减少了数据库层的耦合，并且提升了安全性。SQLite的数据文件默认存储在这个目录：

/data/data/<package_name>/databases

Content Provider使用URI的方式提供了一个统一的数据访问接口，Schema是content://，与SQLite不同，Content Provider可以跨应用程序使用，可以通过发布自己的Content Provider供他人使用。

Content Values和Cursors

Content Values用于插入数据，Cursor则是返回的查询结果。Content Values对应数据中的一行数据。Cursor类提供了移动游标的方法，常用的有这些：

moveToFirst
moveToNext
moveToPrevious
getCount
getColumnIndexOrThrow
getColumnName
getColumnNames
moveToPosition
getPosition

SQLiteOpenHelper

SQLiteOpenHelper是一个抽象类，使用它可以更方便地进行操作数据库。SQLiteOpenHelper会缓存数据库实例，提升访问效率，也正是因为这种缓存策略，用户只有在不再访问数据库的时候才需要关闭数据库连接。下面是一个SQLiteOpenHelper的实现类例子：

private static class HoardDBOpenHelper extends SQLiteOpenHelper {
　　private static final String DATABASE_NAME = “myDatabase.db";
　　private static final String DATABASE_TABLE = “GoldHoards";
　　private static final int DATABASE_VERSION = 1;
　　// SQL Statement to create a new database.
　　private static final String DATABASE_CREATE = “create table “ + DATABASE_TABLE + “ (“ + KEY_ID +
　　　　　　“ integer primary key autoincrement, “ +
　　　　　　KEY_GOLD_HOARD_NAME_COLUMN + “ text not null, “ +
　　　　　　KEY_GOLD_HOARDED_COLUMN + “ float, “ +
　　　　　　KEY_GOLD_HOARD_ACCESSIBLE_COLUMN + “ integer);";
　　public HoardDBOpenHelper(Context context, String name, CursorFactory factory, int version) {
　　　　super(context, name, factory, version);
　　}
　　// Called when no database exists in disk and the helper class needs to create a new one.
　　@Override
　　public void onCreate(SQLiteDatabase db) {
　　　　db.execSQL(DATABASE_CREATE);
　　}
　　// Called when there is a database version mismatch meaning that the version of the database on disk needs to be upgraded to
　　// the current version.
　　@Override
　　public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
　　　　// Log the version upgrade.
　　　　Log.w(“TaskDBAdapter", “Upgrading from version “ + oldVersion + “ to “ + newVersion + “, which will destroy all old data");
　　　　// Upgrade the existing database to conform to the new version. Multiple previous versions can be handled by comparing oldVersion and newVersion values.
　　　　// The simplest case is to drop the old table and create a new one.
　　　　db.execSQL(“DROP TABLE IF IT EXISTS “ + DATABASE_TABLE);
　　　　// Create a new one.
　　　　onCreate(db);
　　}
}

调用SQLiteOpenHelper的getWritableDatabase或者getReadableDatabase方法可以获得一个数据库实例，若不存在数据库实例，SQLiteOpenHelper的内部实现是使用onCreate()方法来创建一个数据库实例并返回。

使用其他方式创建或打开SQLite数据库

SQLiteDatabase db = context.openOrCreateDatabase(DATABASE_NAME, Context.MODE_PRIVATE, null);

使用上面的代码可以获得数据库，onCreate和onUpdate方法中的逻辑需要手动来完成。

Android数据库设计需要注意的事情

1. 文件不要直接存在数据库中。更好的方法是将文件路径放到数据库中。

2. 表中加入auto-increment的id字段。

数据库查询

使用query方法，传入下面这些参数，即可实现数据库查询：

Boolean类型的参数，用于指示是否包含重复数据。
要查询的数据库表名。
Projection，指定要返回的列。
Where，可以用？做占位符。
Where参数。
Group by。
Having。
字符串，指定返回行的顺序。
字符串，用于指定返回的最大行数

// Specify the result column projection. Return the minimum set of columns required to satisfy your requirements.
String[] result_columns = new String[] {KEY_ID, KEY_GOLD_HOARD_ACCESSIBLE_COLUMN, KEY_GOLD_HOARDED_COLUMN };
// Specify the where clause that will limit our results.
String where = KEY_GOLD_HOARD_ACCESSIBLE_COLUMN + “=" + 1;
// Replace these with valid SQL statements as necessary.
String whereArgs[] = null;
String groupBy = null;
String having = null;
String order = null;
SQLiteDatabase db = hoardDBOpenHelper.getWritableDatabase();
Cursor cursor = db.query(HoardDBOpenHelper.DATABASE_TABLE, result_columns, where, whereArgs, groupBy, having, order);

从Cursor中提取数据

从Cursor中提取数据需要两步。首先使用前面介绍的moveTo<loction>等方法首先移动到某行，然后使用get<type>方法获取某列的值。使用getColumnIndexOrThrow或getColumnIndex方法可以通过列名获得该列在Cursor中的index。若找不到该列，getColumnIndex方法将返回-1，而getColumnIndexOrThrow将抛出异常。

int columnIndex = cursor.getColumnIndex(KEY_COLUMN_1_NAME);
if (columnIndex > -1) {
　　String columnValue = cursor.getString(columnIndex);
　　// Do something with the column value.
}
else {
　　// Do something else if the column doesn’t exist.
}

下面是一个从Cursor中读取数据的完整示例：

float totalHoard = 0f;
float averageHoard = 0f;
// Find the index to the column(s) being used.
int GOLD_HOARDED_COLUMN_INDEX = cursor.getColumnIndexOrThrow(KEY_GOLD_HOARDED_COLUMN);
// Iterate over the cursors rows.
// The Cursor is initialized at before first, so we can check only if there is a “next" row available. If the
// result Cursor is empty this will return false.
while (cursor.moveToNext()) {
　　float hoard = cursor.getFloat(GOLD_HOARDED_COLUMN_INDEX);
　　totalHoard += hoard;
}
// Calculate an average -- checking for divide by zero errors.
float cursorCount = cursor.getCount();
averageHoard = cursorCount > 0 ? (totalHoard / cursorCount) : Float.NaN;
// Close the Cursor when you’ve finished with it.
cursor.close();

增、改、删

插入数据：

// Create a new row of values to insert.
ContentValues newValues = new ContentValues();
// Assign values for each row.
newValues.put(KEY_GOLD_HOARD_NAME_COLUMN, hoardName);
newValues.put(KEY_GOLD_HOARDED_COLUMN, hoardValue);
newValues.put(KEY_GOLD_HOARD_ACCESSIBLE_COLUMN, hoardAccessible);
// [ ... Repeat for each column / value pair ... ]
// Insert the row into your table
SQLiteDatabase db = hoardDBOpenHelper.getWritableDatabase();
db.insert(HoardDBOpenHelper.DATABASE_TABLE, null, newValues);

更新数据：

// Create the updated row Content Values.
ContentValues updatedValues = new ContentValues();
// Assign values for each row.
updatedValues.put(KEY_GOLD_HOARDED_COLUMN, newHoardValue);
// [ ... Repeat for each column to update ... ]
// Specify a where clause the defines which rows should be updated. Specify where arguments as necessary.
String where = KEY_ID + “=" + hoardId;
String whereArgs[] = null;
// Update the row with the specified index with the new values.
SQLiteDatabase db = hoardDBOpenHelper.getWritableDatabase();
db.update(HoardDBOpenHelper.DATABASE_TABLE, updatedValues, where, whereArgs);

删除数据：

// Specify a where clause that determines which row(s) to delete.
// Specify where arguments as necessary.
String where = KEY_GOLD_HOARDED_COLUMN + “=" + 0;
String whereArgs[] = null;
// Delete the rows that match the where clause.
SQLiteDatabase db = hoardDBOpenHelper.getWritableDatabase();
db.delete(HoardDBOpenHelper.DATABASE_TABLE, where, whereArgs);

创建Content Providers

Content Provider是一个接口，Content Resolver可以通过这个接口读取数据。通过继承ContentProvider类，即可创建一个Content Provider：

public class MyContentProvider extends ContentProvider

注册Content Provider

将Content Provider加到Manifest后，Content Resolver才能找到并使用。每个Content Provider标签中都有authorities属性，这个属性标识Content Provider。

<provider android:name=".MyContentProvider"　android:authorities="com.paad.skeletondatabaseprovider" />

发布Content Provider

每个ContentProvider都应该还有一个静态字符串变量CONTENT_URI，这个变量由该ContentProvider的authorities和data path组成，如：

public static final Uri CONTENT_URI =　Uri.parse(“content://com.paad.skeletondatabaseprovider/elements”);

在CONTENT_URI后面加上行号，即可获得某一行数据的URI，如：

content://com.paad.skeletondatabaseprovider/elements/5

在实际使用中，最好将这两种URI都发布出来，并配合UriMatcher使用：

// Create the constants used to differentiate between the different URI requests.
private static final int ALLROWS = 1;
private static final int SINGLE_ROW = 2;
private static final UriMatcher uriMatcher;
// Populate the UriMatcher object, where a URI ending in ‘elements’ will correspond to a request for all items,
// and ‘elements/[rowID]’ represents a single row.
static {
　　uriMatcher = new UriMatcher(UriMatcher.NO_MATCH);
　　uriMatcher.addURI(“com.paad.skeletondatabaseprovider”, “elements”, ALLROWS);
　　uriMatcher.addURI(“com.paad.skeletondatabaseprovider”, “elements/#”, SINGLE_ROW);
}

UriMatcher经常和SQLiteQueryBuilder配合使用：

SQLiteQueryBuilder queryBuilder = new SQLiteQueryBuilder();
// If this is a row query, limit the result set to the passed in row.
switch (uriMatcher.match(uri)) {
　　case SINGLE_ROW :
　　　　String rowID = uri.getPathSegments().get(1);
　　　　queryBuilder.appendWhere(KEY_ID + “=” + rowID);
　　default: break;
}

创建Content Provider的数据库

使用SQLiteOpenHelper实现ContentProvider的onCreate方法：

private MySQLiteOpenHelper myOpenHelper;
@Override
public boolean onCreate() {
　　// Construct the underlying database.
　　// Defer opening the database until you need to perform a query or transaction.
　　myOpenHelper = new MySQLiteOpenHelper(getContext(),　MySQLiteOpenHelper.DATABASE_NAME, null, 　MySQLiteOpenHelper.DATABASE_VERSION);
　　return true;
}

实现Content Provider查询

@Override
public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) {
　　// Open the database.
　　SQLiteDatabase db;
　　try {
　　　　db = myOpenHelper.getWritableDatabase();
　　} catch (SQLiteException ex) {
　　　　db = myOpenHelper.getReadableDatabase();
　　}
　　// Replace these with valid SQL statements if necessary.
　　String groupBy = null;
　　String having = null;
　　// Use an SQLite Query Builder to simplify constructing the database query.
　　SQLiteQueryBuilder queryBuilder = new SQLiteQueryBuilder();
　　// If this is a row query, limit the result set to the passed in row.
　　switch (uriMatcher.match(uri)) {
　　　　case SINGLE_ROW :
　　　　　　String rowID = uri.getPathSegments().get(1);
　　　　　　queryBuilder.appendWhere(KEY_ID + “=” + rowID);
　　　　default: 
　　　　　　break;
　　}
　　// Specify the table on which to perform the query. This can be a specific table or a join as required.
　　queryBuilder.setTables(MySQLiteOpenHelper.DATABASE_TABLE);
　　// Execute the query.
　　Cursor cursor = queryBuilder.query(db, projection, selection, selectionArgs, groupBy, having, sortOrder);
　　// Return the result Cursor.
　　return cursor;
}

实现query方法之后，还需要提供返回数据的MIME信息。因此要重写getType方法并返回一个字符串来描述你的数据类型。数据类型有两种，一种是单条数据，另一种是集合数据：

单条数据：vnd.android.cursor.item/vnd.<companyname>.<contenttype>

所有数据：vnd.android.cursor.dir/vnd.<companyname>.<contenttype>

@Override
public String getType(Uri uri) {
　　// Return a string that identifies the MIME type for a Content Provider URI
　　switch (uriMatcher.match(uri)) {
　　　　case ALLROWS:
　　　　　　return “vnd.android.cursor.dir/vnd.paad.elemental”;
　　　　case SINGLE_ROW:
　　　　　　return “vnd.android.cursor.item/vnd.paad.elemental”;
　　　　default:
　　　　　　throw new IllegalArgumentException(“Unsupported URI: “ + uri);
　　}
}

Content Provider Transactions

在修改完数据后，需要调用Content Resolver的notifyChange方法，这样Content Observer即可获得数据修改通知。下面是一段比较全面的示例代码：

@Override
public int delete(Uri uri, String selection, String[] selectionArgs) {
　　// Open a read / write database to support the transaction.
　　SQLiteDatabase db = myOpenHelper.getWritableDatabase();
　　// If this is a row URI, limit the deletion to the specified row.
　　switch (uriMatcher.match(uri)) {
　　　　case SINGLE_ROW :
　　　　　　String rowID = uri.getPathSegments().get(1);
　　　　　　selection = KEY_ID + “=” + rowID + (!TextUtils.isEmpty(selection) ?“ AND (“ + selection + ‘)’ : “”);
　　default: break;
}
// To return the number of deleted items you must specify a where clause. To delete all rows and return a value pass in “1”.
if (selection == null)
　　selection = “1”;
　　// Perform the deletion.
　　int deleteCount = db.delete(MySQLiteOpenHelper.DATABASE_TABLE, selection, selectionArgs);
　　// Notify any observers of the change in the data set.
　　getContext().getContentResolver().notifyChange(uri, null);
　　// Return the number of deleted items.
　　return deleteCount;
}
@Override
public Uri insert(Uri uri, ContentValues values) {
　　// Open a read / write database to support the transaction.
　　SQLiteDatabase db = myOpenHelper.getWritableDatabase();
　　// To add empty rows to your database by passing in an empty
　　// Content Values object you must use the null column hack parameter to specify the name of the column that can be set to null.
　　String nullColumnHack = null;
　　// Insert the values into the table
　　long id = db.insert(MySQLiteOpenHelper.DATABASE_TABLE, nullColumnHack, values);
　　// Construct and return the URI of the newly inserted row.
　　if (id > -1) {
　　　　// Construct and return the URI of the newly inserted row.
　　　　Uri insertedId = ContentUris.withAppendedId(CONTENT_URI, id);
　　　 // Notify any observers of the change in the data set.
　　　　getContext().getContentResolver().notifyChange(insertedId, null);
　　　　return insertedId;
　　}else
　　　　return null;
}
@Override
public int update(Uri uri, ContentValues values, String selection, String[] selectionArgs) {
　　// Open a read / write database to support the transaction.
　　SQLiteDatabase db = myOpenHelper.getWritableDatabase();
　　// If this is a row URI, limit the deletion to the specified row.
　　switch (uriMatcher.match(uri)) {
　　　　case SINGLE_ROW :
　　　　　　String rowID = uri.getPathSegments().get(1);
　　　　　　selection = KEY_ID + “=” + rowID + (!TextUtils.isEmpty(selection) ?“ AND (“ + selection + ‘)’ : “”);
　　　　default: break;
　　}
　　// Perform the update.
　　int updateCount = db.update(MySQLiteOpenHelper.DATABASE_TABLE, values, selection, selectionArgs);
　　// Notify any observers of the change in the data set.
　　getContext().getContentResolver().notifyChange(uri, null);
　　return updateCount;
}

在Content Provider中存储文件

文件存储的最佳设计模式是将文件路径存储在数据库中，使用Content Provider的接口方法读写。文件路径应该存放在数据库表中的_data列中，并重写Content Provider的openFile方法，返回ParcelFileDescriptor：

@Override
public ParcelFileDescriptor openFile(Uri uri, String mode) throws FileNotFoundException {
　　// Find the row ID and use it as a filename.
　　String rowID = uri.getPathSegments().get(1);
　　// Create a file object in the application’s external files directory.
　　String picsDir = Environment.DIRECTORY_PICTURES;
　　File file = new File(getContext().getExternalFilesDir(picsDir), rowID);
　　// If the file doesn’t exist, create it now.
　　if (!file.exists()) {
　　　　try {
　　　　　　file.createNewFile();
　　　　} catch (IOException e) {
　　　　　　Log.d(TAG, “File creation failed: “ + e.getMessage());
　　　　}
　　}
　　// Translate the mode parameter to the corresponding Parcel File Descriptor open mode.
　　int fileMode = 0;
　　if (mode.contains(“w”))
　　　　fileMode |= ParcelFileDescriptor.MODE_WRITE_ONLY;
　　if (mode.contains(“r”))
　　　　fileMode |= ParcelFileDescriptor.MODE_READ_ONLY;
　　if (mode.contains(“+”))
　　　　fileMode |= ParcelFileDescriptor.MODE_APPEND;
　　// Return a Parcel File Descriptor that represents the file.
　　return ParcelFileDescriptor.open(file, fileMode);
}

Content Provider的Skeleton示例

import android.content.ContentProvider;
import android.content.ContentUris;
import android.content.ContentValues;
import android.content.Context;
import android.content.UriMatcher;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteDatabase.CursorFactory;
import android.database.sqlite.SQLiteOpenHelper;
import android.database.sqlite.SQLiteQueryBuilder;
import android.net.Uri;
import android.text.TextUtils;
import android.util.Log;
public class MyContentProvider extends ContentProvider {
　　public static final Uri CONTENT_URI = Uri.parse(“content://com.paad.skeletondatabaseprovider/elements”);
　　// Create the constants used to differentiate between the different URI requests.
　　private static final int ALLROWS = 1;
　　private static final int SINGLE_ROW = 2;
　　private static final UriMatcher uriMatcher;
　　// Populate the UriMatcher object, where a URI ending in ‘elements’ will correspond to a request for all
　　// items, and ‘elements/[rowID]’ represents a single row.
　　static {
　　　　uriMatcher = new UriMatcher(UriMatcher.NO_MATCH);
　　　　uriMatcher.addURI(“com.paad.skeletondatabaseprovider”, “elements”, ALLROWS);
　　　　uriMatcher.addURI(“com.paad.skeletondatabaseprovider”, “elements/#”, SINGLE_ROW);
　　}
　　// The index (key) column name for use in where clauses.
　　public static final String KEY_ID = “_id”;
　　// The name and column index of each column in your database.
　　// These should be descriptive.
　　public static final String KEY_COLUMN_1_NAME = “KEY_COLUMN_1_NAME”;
　　// TODO: Create public field for each column in your table.
　　// SQLite Open Helper variable
　　private MySQLiteOpenHelper myOpenHelper;
　　@Override
　　public boolean onCreate() {
　　　　// Construct the underlying database.
　　　　// Defer opening the database until you need to perform a query or transaction.
　　　　myOpenHelper = new MySQLiteOpenHelper(getContext(), MySQLiteOpenHelper.DATABASE_NAME, null, MySQLiteOpenHelper.DATABASE_VERSION);
　　　　return true;
　　}
　　@Override
　　public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) {
　　// Open the database.
　　SQLiteDatabase db = myOpenHelper.getWritableDatabase();
　　// Replace these with valid SQL statements if necessary.
　　String groupBy = null;
　　String having = null;
　　SQLiteQueryBuilder queryBuilder = new SQLiteQueryBuilder();
　　queryBuilder.setTables(MySQLiteOpenHelper.DATABASE_TABLE);
　　// If this is a row query, limit the result set to the passed in row.
　　switch (uriMatcher.match(uri)) {
　　　　case SINGLE_ROW :
　　　　　　String rowID = uri.getPathSegments().get(1);
　　　　　　queryBuilder.appendWhere(KEY_ID + “=” + rowID);
　　　　default: break;
　　}
　　Cursor cursor = queryBuilder.query(db, projection, selection, selectionArgs, groupBy, having, sortOrder);
　　return cursor;
}
@Override
public int delete(Uri uri, String selection, String[] selectionArgs){
　　// Open a read / write database to support the transaction.
　　SQLiteDatabase db = myOpenHelper.getWritableDatabase();
　　// If this is a row URI, limit the deletion to the specified row.
　　switch (uriMatcher.match(uri)) {
　　　　case SINGLE_ROW :
　　　　　　String rowID = uri.getPathSegments().get(1);
　　　　　　selection = KEY_ID + “=” + rowID + (!TextUtils.isEmpty(selection) ?“ AND (“ + selection + ‘)’ : “”);
　　　　default: break;
　　}
　　// To return the number of deleted items, you must specify a where clause. To delete all rows and return a value, pass in “1”.
　　if (selection == null)　　selection = “1”;
　　// Execute the deletion.
　　int deleteCount = db.delete(MySQLiteOpenHelper.DATABASE_TABLE, selection, selectionArgs);
　　// Notify any observers of the change in the data set.
　　getContext().getContentResolver().notifyChange(uri, null);
　　return deleteCount;
}
@Override
public Uri insert(Uri uri, ContentValues values) {
　　// Open a read / write database to support the transaction.
　　SQLiteDatabase db = myOpenHelper.getWritableDatabase();
　　// To add empty rows to your database by passing in an empty
　　// Content Values object, you must use the null column hack parameter to specify the name of the column that can be set to null.
　　String nullColumnHack = null;
　　// Insert the values into the table
　　long id = db.insert(MySQLiteOpenHelper.DATABASE_TABLE, nullColumnHack, values);
　　if (id > -1) {
　　// Construct and return the URI of the newly inserted row.
　　Uri insertedId = ContentUris.withAppendedId(CONTENT_URI, id);
　　// Notify any observers of the change in the data set.
　　getContext().getContentResolver().notifyChange(insertedId, null);
　　return insertedId;
}else
　　return null;
}
@Override
public int update(Uri uri, ContentValues values, String selection, String[] selectionArgs) {
　　// Open a read / write database to support the transaction.
　　SQLiteDatabase db = myOpenHelper.getWritableDatabase();
　　// If this is a row URI, limit the deletion to the specified row.
　　switch (uriMatcher.match(uri)) {
　　　　case SINGLE_ROW :
　　　　　　String rowID = uri.getPathSegments().get(1);
　　　　　　selection = KEY_ID + “=” + rowID + (!TextUtils.isEmpty(selection) ?“ AND (“ + selection + ‘)’ : “”);
　　　　default: break;
　　}
　　// Perform the update.
　　int updateCount = db.update(MySQLiteOpenHelper.DATABASE_TABLE, values, selection, selectionArgs);
　　// Notify any observers of the change in the data set.
　　getContext().getContentResolver().notifyChange(uri, null);
　　return updateCount;
}
@Override
public String getType(Uri uri) {
　　// Return a string that identifies the MIME type for a Content Provider URI
　　switch (uriMatcher.match(uri)) {
　　　　case ALLROWS:
　　　　　　return “vnd.android.cursor.dir/vnd.paad.elemental”;
　　　　case SINGLE_ROW:
　　　　　　return “vnd.android.cursor.item/vnd.paad.elemental”;
　　　　default:
　　　　　　throw new IllegalArgumentException(“Unsupported URI: “ + uri);
　　}
}
private static class MySQLiteOpenHelper extends SQLiteOpenHelper {
　　// [ ... SQLite Open Helper Implementation ... ]
}
}