[置顶] Android数据库GreenDAO3.2.2的使用（四、数据库升级）

使用场景：使用GreenDao操作数据库，必然涉及到数据库版本升级：增加表、增加字段。

greendao {
schemaVersion 1//数据库版本号
}

数据库版本升级就是提升“数据库版本号”，当数据库版本号提升以后，悲剧发生了，以前的数据全部清空了，这可怎么办呢？最终在stackoverflow  https://stackoverflow.com/找到一个完美的解决方案，一行代码即可处理数据库升级，现在为大家分享一下。
他的实现思路是：

1、创建临时表temporary，将原先表数据迁移过去
2、删除所有原表
3、创建新表
4、将temporary表数据复制到新表，最后将temporary表删除

创建一个类MigrationHelper：

/**
*
* please call {@link #migrate(SQLiteDatabase, Class[])}
*
*/
public final class MigrationHelper {

public static boolean DEBUG = false;
private static String TAG = "MigrationHelper";
private static final String SQLITE_MASTER = "sqlite_master";
private static final String SQLITE_TEMP_MASTER = "sqlite_temp_master";

public static void migrate(SQLiteDatabase db, Class<? extends AbstractDao<?, ?>>... daoClasses) {
Database database = new StandardDatabase(db);

printLog("【The Old Database Version】" + db.getVersion());
printLog("【Generate temp table】start");
generateTempTables(database, daoClasses);
printLog("【Generate temp table】complete");

dropAllTables(database, true, daoClasses);
createAllTables(database, false, daoClasses);

printLog("【Restore data】start");
restoreData(database, daoClasses);
printLog("【Restore data】complete");
}

private static void generateTempTables(Database db, Class<? extends AbstractDao<?, ?>>... daoClasses) {
for (int i = 0; i < daoClasses.length; i++) {
String tempTableName = null;

DaoConfig daoConfig = new DaoConfig(db, daoClasses[i]);
String tableName = daoConfig.tablename;
if (!isTableExists(db, false, tableName)) {
printLog("【New Table】" + tableName);
continue;
}
try {
tempTableName = daoConfig.tablename.concat("_TEMP");
StringBuilder dropTableStringBuilder = new StringBuilder();
dropTableStringBuilder.append("DROP TABLE IF EXISTS ").append(tempTableName).append(";");
db.execSQL(dropTableStringBuilder.toString());

StringBuilder insertTableStringBuilder = new StringBuilder();
insertTableStringBuilder.append("CREATE TEMPORARY TABLE ").append(tempTableName);
insertTableStringBuilder.append(" AS SELECT * FROM ").append(tableName).append(";");
db.execSQL(insertTableStringBuilder.toString());
printLog("【Table】" + tableName +"\n ---Columns-->"+getColumnsStr(daoConfig));
printLog("【Generate temp table】" + tempTableName);
} catch (SQLException e) {
Log.e(TAG, "【Failed to generate temp table】" + tempTableName, e);
}
}
}

private static boolean isTableExists(Database db, boolean isTemp, String tableName) {
if (db == null || TextUtils.isEmpty(tableName)) {
return false;
}
String dbName = isTemp ? SQLITE_TEMP_MASTER : SQLITE_MASTER;
String sql = "SELECT COUNT(*) FROM " + dbName + " WHERE type = ? AND name = ?";
Cursor cursor=null;
int count = 0;
try {
cursor = db.rawQuery(sql, new String[]{"table", tableName});
if (cursor == null || !cursor.moveToFirst()) {
return false;
}
count = cursor.getInt(0);
} catch (Exception e) {
e.printStackTrace();
} finally {
if (cursor != null)
cursor.close();
}
return count > 0;
}

private static String getColumnsStr(DaoConfig daoConfig) {
if (daoConfig == null) {
return "no columns";
}
StringBuilder builder = new StringBuilder();
for (int i = 0; i < daoConfig.allColumns.length; i++) {
builder.append(daoConfig.allColumns[i]);
builder.append(",");
}
if (builder.length() > 0) {
builder.deleteCharAt(builder.length() - 1);
}
return builder.toString();
}

private static void dropAllTables(Database db, boolean ifExists, @NonNull Class<? extends AbstractDao<?, ?>>... daoClasses) {
reflectMethod(db, "dropTable", ifExists, daoClasses);
printLog("【Drop all table】");
}

private static void createAllTables(Database db, boolean ifNotExists, @NonNull Class<? extends AbstractDao<?, ?>>... daoClasses) {
reflectMethod(db, "createTable", ifNotExists, daoClasses);
printLog("【Create all table】");
}

/**
* dao class already define the sql exec method, so just invoke it
*/
private static void reflectMethod(Database db, String methodName, boolean isExists, @NonNull Class<? extends AbstractDao<?, ?>>... daoClasses) {
if (daoClasses.length < 1) {
return;
}
try {
for (Class cls : daoClasses) {
Method method = cls.getDeclaredMethod(methodName, Database.class, boolean.class);
method.invoke(null, db, isExists);
}
} catch (NoSuchMethodException e) {
e.printStackTrace();
} catch (InvocationTargetException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
}
}

private static void restoreData(Database db, Class<? extends AbstractDao<?, ?>>... daoClasses) {
for (int i = 0; i < daoClasses.length; i++) {
DaoConfig daoConfig = new DaoConfig(db, daoClasses[i]);
String tableName = daoConfig.tablename;
String tempTableName = daoConfig.tablename.concat("_TEMP");

if (!isTableExists(db, true, tempTableName)) {
continue;
}

try {
// get all columns from tempTable, take careful to use the columns list
List<String> columns = getColumns(db, tempTableName);
ArrayList<String> properties = new ArrayList<>(columns.size());
for (int j = 0; j < daoConfig.properties.length; j++) {
String columnName = daoConfig.properties[j].columnName;
if (columns.contains(columnName)) {
properties.add(columnName);
}
}
if (properties.size() > 0) {
final String columnSQL = TextUtils.join(",", properties);

StringBuilder insertTableStringBuilder = new StringBuilder();
insertTableStringBuilder.append("INSERT INTO ").append(tableName).append(" (");
insertTableStringBuilder.append(columnSQL);
insertTableStringBuilder.append(") SELECT ");
insertTableStringBuilder.append(columnSQL);
insertTableStringBuilder.append(" FROM ").append(tempTableName).append(";");
db.execSQL(insertTableStringBuilder.toString());
printLog("【Restore data】 to " + tableName);
}
StringBuilder dropTableStringBuilder = new StringBuilder();
dropTableStringBuilder.append("DROP TABLE ").append(tempTableName);
db.execSQL(dropTableStringBuilder.toString());
printLog("【Drop temp table】" + tempTableName);
} catch (SQLException e) {
Log.e(TAG, "【Failed to restore data from temp table 】" + tempTableName, e);
}
}
}

private static List<String> getColumns(Database db, String tableName) {
List<String> columns = null;
Cursor cursor = null;
try {
cursor = db.rawQuery("SELECT * FROM " + tableName + " limit 0", null);
if (null != cursor && cursor.getColumnCount() > 0) {
columns = Arrays.asList(cursor.getColumnNames());
}
} catch (Exception e) {
e.printStackTrace();
} finally {
if (cursor != null)
cursor.close();
if (null == columns)
columns = new ArrayList<>();
}
return columns;
}

private static void printLog(String info){
if(DEBUG){
Log.d(TAG, info);
}
}

}

自定义一个OpentHelper类继承于DaoMaster.OpenHelper类：

/**
* 自定义一个OpenHelper类，为以后数据库升级时使用
* Created by KingsleyCheng on 2017/5/18.
*/

public class MySQLiteOpenHelper extends DaoMaster.OpenHelper {

public MySQLiteOpenHelper(Context context, String name) {
super(context, name);
}

public MySQLiteOpenHelper(Context context, String name,
                            SQLiteDatabase.CursorFactory factory) {
super(context, name, factory);
}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
MigrationHelper.migrate(db,
ProjectListDao.classUserListDao.class);//数据版本变更才会执行
}

}

在DaoMaster类的onUpgrade方法添加如下代码即可，参数为所有的Dao类：

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
MigrationHelper.migrate(db,“所有的Dao类”);//数据版本变更才会执行
}

这样数据库升级后以前的数据就不会丢失了，当然如果数据库升级后数据初始化，重新获取，不影响项目功能，可以不用考虑此方法。