GreenDao的更新，不会使数据删除所导致重新创建新表的示例

首先导入依赖
再Projict下的gradle中添加

dependencies {    classpath 'org.greenrobot:greendao-gradle-plugin:3.2.2'}

在Module下的gradle

 在apply plugin: 'com.android.application'下添加apply plugin: 'org.greenrobot.greendao'然后在android{greendao {        schemaVersion 1 //版本        daoPackage '项目的包名.dao' //app包名+生成文件的文件夹名        targetGenDir 'src/main/java'  //文件生成路径}}最后添加依赖： implementation 'org.greenrobot:greendao:3.2.2'

创建数据库

public class MyDb {    private static MyDb myDb;    private final DbBeanDao dbBeanDao;    public MyDbHelpre() {        DaoMaster.DevOpenHelper devOpenHelper = new DaoMaster.DevOpenHelper(MyApp.getMyApp(), "MySQLite.db");                //在这里呢，用来更新的工具类创建好之后，上边创建的可以注释掉，然后使用下边这个        MyOpen myOpen = new MyOpen(MyApp.getMyApp(), "MySQLite.db");        DaoMaster daoMaster = new DaoMaster(myOpen.getWritableDatabase());        只需要改这两行代码就可以进行更新了                DaoMaster daoMaster = new DaoMaster(devOpenHelper.getWritableDatabase());        DaoSession daoSession = daoMaster.newSession();        dbBeanDao = daoSession.getDbBeanDao();    }    public static MyDbHelpre myDbHelpre() {        if (myDbHelpre == null) {            synchronized (MyDbHelpre.class) {                if (myDbHelpre == null) {                    myDbHelpre = new MyDbHelpre();                }            }        }        return myDbHelpre;    }    public boolean has(DbBean bean) {        List list = dbBeanDao.queryBuilder().where(DbBeanDao.Properties.Name.eq(bean.getName())).list();        if (list.size() > 0) {            return true;        }        return false;    }    //增    public void insert(DbBean bean) {        if (has(bean)) {            return;        }        dbBeanDao.insert(bean);    }    //删    public void delete(DbBean bean) {        dbBeanDao.delete(bean);    }    //改    public void update(DbBean dbBean) {        dbBeanDao.update(dbBean);    }   //查    public List query() {        return dbBeanDao.queryBuilder().list();    }}

在Activity中初始化数据

public class MainActivity extends AppCompatActivity implements View.OnClickListener {    private Button btn_1;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        initView();    }    private void initView() {        btn_1 = (Button) findViewById(R.id.btn_1);        btn_1.setOnClickListener(this);        for (int i = 0; i < 10; i++) {            DbBean dbBean = new DbBean(null, "张三" + i, "男" + i);            MyDb.myDb().insert(dbBean);        }    }    @Override    public void onClick(View v) {        switch (v.getId()) {            case R.id.btn_1:                List query = MyDb.myDb().query();                for (int i = 0; i < query.size(); i++) {                    Toast.makeText(this, query.toString(), Toast.LENGTH_SHORT).show();                }                break;        }    }}

在实体类中定义

@Entitypublic class Bean{@Idprivate Long id;//这里注意，id一定要是Long类型，不能是longprivate String name;private String sex;}然后在studio中点击Make Project

创建完毕之后，一个简单的数据库就可以运行了
这个工具类的作用是用来创建一个临时表，把旧表的数据存入临时表，当新表创建完成之后，再将临时表中的数据再存入新表，这样就可以保证GreenDao升级不会删除之前的数据
工具类代码：

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;    }    public void migrate(Database db, Class<? extends AbstractDao<?, ?>>... daoClasses) {        generateTempTables(db, daoClasses);        DaoMaster.dropAllTables(db, true);        DaoMaster.createAllTables(db, false);        restoreData(db, 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 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) {                        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());        }    }    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 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(long.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 getColumns(Database db, String tableName) {        List 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;    }}

public class MyOpen extends DaoMaster.DevOpenHelper {    public MyOpen(Context context, String name) {        super(context, name);    }    @Override    public void onUpgrade(Database db, int oldVersion, int newVersion) {        super.onUpgrade(db, oldVersion, newVersion);        MigrationHelper.getInstance().migrate(db,DbBeanDao.class);    }}

创建好这两个工具类之后，就可以进行数据库的更新了，只需要版本的升级就可以了
以上就是数据库更新版本并且数据不会丢失的步骤了

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