GreenDao 3.0的基本用法以及正确的数据库版本升级

1: Gradle配置

在项目根目录中的build.gradle和主model(app)工程下的build.gradle分别加入如下配置:

:
dependencies {
classpath 'com.android.tools.build:gradle:3.1.3'
classpath 'org.greenrobot:greendao-gradle-plugin:3.2.2' // add plugin
}

greendao {
schemaVersion 5 //数据库版本号
daoPackage 'com.gx.greendao' //设置DaoMaster.DaoSession,Dao包名
targetGenDir 'src/main/java' //设置daoPackage所在的文件路径
}

dependencies {
implementation fileTree(include: ['*.jar'], dir: 'libs')
implementation 'com.android.support:appcompat-v7:27.1.0'
implementation 'com.android.support.constraint:constraint-layout:1.0.2'
implementation 'com.android.support:recyclerview-v7:27.1.0'
implementation 'com.android.support:support-v4:27.1.0'
compile 'com.android.support:design:27.1.0'

implementation 'org.greenrobot:greendao:3.2.2' // add library

}

2: 项目实战运用

新建User的java实体类,如下:
@Entity
public class User {
@Id(autoincrement = true)
private Long id;
private String name;
private int age;
private String pic;
private String content;
}

然后执行Build->Make Model app ,会自动根据配置在项目中对应的com.gx.greendao目录下生成DaoMasterDaoSessionUserDao三个文件

6999348-bcbaff168caa1cb6.png

在项目的application中完成相应的初始化操作,如下:
private String DATA_BASE_NAME = "GxDb";
private static DaoSession mDaoSession;

@Override
public void onCreate() {
    super.onCreate();
    setUpDataBase(this);
}

private void setUpDataBase(Context context) {
    DaoMaster.DevOpenHelper openHelper = new DaoMaster.DevOpenHelper(context, DATA_BASE_NAME);
    Database db = openHelper.getWritableDb();
    DaoMaster daoMaster = new DaoMaster(db);


    mDaoSession = daoMaster.newSession();
}

public static DaoSession getDaoSession() {
    return mDaoSession;
}

增删改查操作:
//插入
User user = new User(null, "GexYY" + new Random().nextInt(100), new Random().nextInt(40) + 20,"content","csd");
mUserDaoSession.getUserDao().insert(user);
mQueryBtn.performClick();

            //删除
            mUserDaoSession.getUserDao().deleteByKey(1l);
            mQueryBtn.performClick();

            //更新
            User user1 = mUserDaoSession.getUserDao().load(1l);
            if (user1 != null) {
                user1.setName("GexYY_Update");
                mUserDaoSession.getUserDao().update(user1);
            }
           
            //查询
            List<User> userList = mUserDaoSession.getUserDao().loadAll();
            StringBuilder builder = new StringBuilder();
            for (User entity : userList) {
                builder.append(entity.toString()).append("\n");
            }
            mContent.setText(builder.toString());

3: 数据库升级

1GreenDao默认帮我们处理来升级操作，只需要在build.gradle中修改一下schemaVersion版本号即可

greendao { schemaVersion 5 //数据库版本号
daoPackage 'com.gx.greendao' //设置
DaoMaster.DaoSession,Dao包名
targetGenDir 'src/main/java' //设置daoPackage所在的文件路径 }

但是我们查看DaoMaster.DevOpenHelper的代码中的onUpgragde()方法如下:
@Override
public void onUpgrade(Database db, int oldVersion, int newVersion) {
Log.i("greenDAO", "Upgrading schema from version " + oldVersion + " to " + newVersion + " by dropping all tables");
dropAllTables(db, true);
onCreate(db);
}

发现每次升级都是把老的数据库删除掉，包括数据，然后再重新创建相关的数据库和表，这样就导致了数据库升级之后数据的丢失.
那么如何保证升级后数据的不丢失呢？
这里就用到了一个大神写的MigrationHelper
https://stackoverflow.com/questions/13373170/greendao-schema-update-and-data-migration/30334668#30334668
使用方法如下：
@Override
public void onUpgrade(Database db, int oldVersion, int newVersion) {
if (oldVersion < newVersion) {
DbBackUpHelper.getInstance().migrate(db, UserDao.class);
}
}
2修改要增加的字段类重新运行

但是由于DaoMaster.DevOpenHelper是编译自动生成的，所以不建议直接在其里边去实现，这样每次编译都会把相关代码还原删除，
我们应该自定义一个openHelper,并继承'DaoMaster.DevOpenHelper',如下:
public class DbOpenHelper extends DaoMaster.DevOpenHelper {
public DbOpenHelper(Context context, String name) {
super(context, name);
}

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

@Override
public void onUpgrade(Database db, int oldVersion, int newVersion) {
   //切记不要调用super.onUpgrade(db,oldVersion,newVersion)
    if (oldVersion < newVersion) {
        DbBackUpHelper.getInstance().migrate(db, UserDao.class);
    }
}

}

切记不要调用super.onUpgrade(db,oldVersion,newVersion),不然依然会删除数据记录

然后在修改一下初始化地方的代码如下:
private void setUpDataBase(Context context) {
DbOpenHelper openHelper = new DbOpenHelper(context, DATA_BASE_NAME, null);
Database db = openHelper.getWritableDb();
DaoMaster daoMaster = new DaoMaster(db);
mDaoSession = daoMaster.newSession();
}

升级辅助类：
public class DbOpenHelper extends DaoMaster.DevOpenHelper {
public DbOpenHelper(Context context, String name) {
super(context, name);
}

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

@Override
public void onUpgrade(Database db, int oldVersion, int newVersion) {
    //切记不要调用super.onUpgrade(db,oldVersion,newVersion)
    if (oldVersion < newVersion) {
        MigrationHelper.migrate(db, testUserDao.class);
    }
}

}