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, Classextends 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, Classextends 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 Classextends AbstractDao, ?>>... daoClasses) {
        reflectMethod(db, "dropTable", ifExists, daoClasses);
        printLog("【Drop all table】");
    }

    private static void createAllTables(Database db, boolean ifNotExists, @NonNull Classextends 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 Classextends 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, Classextends 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 columns = getColumns(db, tempTableName);
                ArrayList 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 getColumns(Database db, String tableName) {
        List 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。