GreenDao3.2 版本升级数据迁移

参考：https://stackoverflow.com/questions/13373170/greendao-schema-update-and-data-migration/30334668#30334668

GreenDao升级时，默认删除所有数据
看源码可以发现原因：

public class DaoMaster extends AbstractDaoMaster {......public static class DevOpenHelper extends OpenHelper {public DevOpenHelper(Context context, String name) {super(context, name);}public DevOpenHelper(Context context, String name, CursorFactory factory) {super(context, name, factory);}@Overridepublic void onUpgrade(Database db, int oldVersion, int newVersion) {Log.i("greenDAO", "Upgrading schema from version " + oldVersion + " to " + newVersion + " by dropping all tables");//把表都删除了dropAllTables(db, true);onCreate(db);}}}

想要保留数据，就要对 DevOpenHelper 进行修改，数据迁移思路先说明下。

第一步：先创建临时表，把老数据存入

第二步：把旧的表删除，新建数据库表

第三步：把临时表数据，存入新数据库

具体实现请看 MigrationHelper ：

public class MigrationHelper {private static final String CONVERSION_CLASS_NOT_FOUND_EXCEPTION = "MIGRATION HELPER - CLASS DOESN'T MATCH WITH THE CURRENT PARAMETERS";private static MigrationHelper instance;public static MigrationHelper getInstance() {if(instance == null) {instance = new MigrationHelper();}return instance;}/*** 迁移数据库* @param db* @param daoClasses*/public void migrate(Database db, Class<? extends AbstractDao<?, ?>>... daoClasses) {generateTempTables(db, daoClasses);DaoMaster.dropAllTables(db, true);DaoMaster.createAllTables(db, false);restoreData(db, daoClasses);}/*** 创建临时表* @param db* @param daoClasses*/private void generateTempTables(Database db, Class<? extends AbstractDao<?, ?>>... daoClasses) {for(int i = 0; i < daoClasses.length; i++) {DaoConfig daoConfig = new DaoConfig(db, daoClasses[i]);String divider = "";String tableName = daoConfig.tablename;String tempTableName = daoConfig.tablename.concat("_TEMP");ArrayList<String> properties = new ArrayList<>();StringBuilder createTableStringBuilder = new StringBuilder();createTableStringBuilder.append("CREATE TABLE ").append(tempTableName).append(" (");for(int j = 0; j < daoConfig.properties.length; j++) {String columnName = daoConfig.properties[j].columnName;if(getColumns(db, tableName).contains(columnName)) {properties.add(columnName);String type = null;try {type = getTypeByClass(daoConfig.properties[j].type);} catch (Exception exception) {
//                        Crashlytics.logException(exception);exception.printStackTrace();}createTableStringBuilder.append(divider).append(columnName).append(" ").append(type);if(daoConfig.properties[j].primaryKey) {createTableStringBuilder.append(" PRIMARY KEY");}divider = ",";}}createTableStringBuilder.append(");");db.execSQL(createTableStringBuilder.toString());StringBuilder insertTableStringBuilder = new StringBuilder();insertTableStringBuilder.append("INSERT INTO ").append(tempTableName).append(" (");insertTableStringBuilder.append(TextUtils.join(",", properties));insertTableStringBuilder.append(") SELECT ");insertTableStringBuilder.append(TextUtils.join(",", properties));insertTableStringBuilder.append(" FROM ").append(tableName).append(";");db.execSQL(insertTableStringBuilder.toString());}}/*** 恢复表* @param db* @param daoClasses*/private 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");ArrayList<String> properties = new ArrayList();for (int j = 0; j < daoConfig.properties.length; j++) {String columnName = daoConfig.properties[j].columnName;if(getColumns(db, tempTableName).contains(columnName)) {properties.add(columnName);}}StringBuilder insertTableStringBuilder = new StringBuilder();insertTableStringBuilder.append("INSERT INTO ").append(tableName).append(" (");insertTableStringBuilder.append(TextUtils.join(",", properties));insertTableStringBuilder.append(") SELECT ");insertTableStringBuilder.append(TextUtils.join(",", properties));insertTableStringBuilder.append(" FROM ").append(tempTableName).append(";");StringBuilder dropTableStringBuilder = new StringBuilder();dropTableStringBuilder.append("DROP TABLE ").append(tempTableName);db.execSQL(insertTableStringBuilder.toString());db.execSQL(dropTableStringBuilder.toString());}}private String getTypeByClass(Class<?> type) throws Exception {if(type.equals(String.class)) {return "TEXT";}if(type.equals(Long.class) || type.equals(Integer.class) || type.equals(Date.class) || type.equals(long.class) || type.equals(int.class) ) {return "INTEGER";}if(type.equals(Boolean.class)) {return "BOOLEAN";}Exception exception = new Exception(CONVERSION_CLASS_NOT_FOUND_EXCEPTION.concat(" - Class: ").concat(type.toString()));
//        Crashlytics.logException(exception);throw exception;}private static List<String> getColumns(Database db, String tableName) {List<String> columns = new ArrayList<>();Cursor cursor = null;try {cursor = db.rawQuery("SELECT * FROM " + tableName + " limit 1", null);if (cursor != null) {columns = new ArrayList<>(Arrays.asList(cursor.getColumnNames()));}} catch (Exception e) {Log.v(tableName, e.getMessage(), e);e.printStackTrace();} finally {if (cursor != null)cursor.close();}return columns;}
}

怎么使用这个工具类呢，需要继承DaoMaster.DevOpenHelper，重写onUpgrade方法

public class GmTechOpenHelper extends DaoMaster.DevOpenHelper {public GmTechOpenHelper(Context context, String name, SQLiteDatabase.CursorFactory factory) {super(context, name, factory);}public GmTechOpenHelper(Context context, String name) {super(context, name);}@Overridepublic void onUpgrade(Database db, int oldVersion, int newVersion) {if (oldVersion < newVersion){MigrationHelper.getInstance().migrate(db, TestDao.class);}Log.d("GmTechOpenHelper", "oldVersion: "+oldVersion+",newVersion: "+newVersion);}
}

需要在greenDao初始化时，替换自定义的DaoMaster.DevOpenHelper

        //创建数据库mydb.dbDaoMaster.DevOpenHelper helper = new GmTechOpenHelper(BaseApplication.getContext(),"数据库名");//获取可写数据库SQLiteDatabase database = helper.getWritableDatabase();//获取数据库对象DaoMaster daoMaster = new DaoMaster(database);//获取Dao对象管理者daoSession = daoMaster.newSession();

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