应用使用了GreenDao数据库，在版本升级的时候需要更改dao的字段，新增、修改、删除字段操作，如果直接删除原来的表的话那用户原来的一些数据就没有了，所以在更新数据库的时候需要做一次封装，把原来的数据保存后再更新数据库版本；

代码是用的网上的大神的，经过一些修改。利用这个类替换在Application里面的Helper类就可以完成数据的迁移了。

import android.database.Cursor;import android.text.TextUtils;import android.util.Log;import com.example.bob.testlistener.database.greendao.DaoMaster;import org.greenrobot.greendao.AbstractDao;import org.greenrobot.greendao.database.Database;import org.greenrobot.greendao.internal.DaoConfig;import java.util.ArrayList;import java.util.Arrays;import java.util.List;/** * Created by pokawa on 18/05/15. * 数据迁移类（数据库版本升级的时候使用这个类，会保留先前版本的数据） */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
     >... daoClasses) {        generateTempTables(db, daoClasses);        DaoMaster.dropAllTables(db, true);        DaoMaster.createAllTables(db, false);        restoreData(db, daoClasses);    }    private void generateTempTables(Database db, Class
     >... 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
      >... 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;    }}
        
       
      
     

这是在application里面的初始化GreenDao方法

/**     * 初始化GreenDao     */    private void initGreenDao() {        MyGreenHelper helper = new MyGreenHelper(this,"test_user.db");        SQLiteDatabase db = helper.getWritableDatabase();        DaoMaster daoMaster = new DaoMaster(db);        daoSession = daoMaster.newSession();    }

 

完成上面的修改后，就能够在entity里面更改数据库的字段了，然后make一下工程，生成新的daoMaster；

最后在gradle文件里面更改数据库版本号

greendao {    schemaVersion 2 //数据库版本号    daoPackage 'com.example.bob.testlistener.database.greendao'// 设置DaoMaster、DaoSession、Dao 包名    targetGenDir 'src/main/java'//设置DaoMaster、DaoSession、Dao目录}

 

数据库更新后原来的数据的新增或者更改字段的值都是null,需要在代码里面做相应的处理。