我正在使用这个 DataBaseHelper.class 并且我被困在 onUpgrade() 方法上。我不知道如何弄清楚数据库的版本号是多少。我可以将版本设置为 1，第一次发布它时，当我发布更新时，我可以将版本设置为 2 (myDataBase.setVersion(2);)。但只要应用程序正在运行，它就只会是 2。下次启动时它又是 1。 private static int DATABASE_VERSION 也是如此。我正在考虑将版本号存储在一个额外的表中，但在我看来这似乎不是最佳做法。

那么如何确保版本号在升级后增加并保持不变(分配给 private static int DATABASE_VERSION 或 myDataBase.getVersion() 的值;)?

DataBaseHelper 类:

public class DataBaseHelper extends SQLiteOpenHelper {

    //The Android's default system path of your application database.
    private static String DB_PATH = "/data/data/com.mydatabase.db/databases/";

    private static String DB_NAME = "database.sl3";

    private SQLiteDatabase myDataBase;

    private final Context myContext;


    // Do you need this?
    private static int DATABASE_VERSION = 2;
    // or is this correct:
    // private static int DATABASE_VERSION = myDataBase.getVersion();


    /**
     * Constructor
     * Takes and keeps a reference of the passed context in order to access to
     * the application assets and resources.
     * 
     * @param context
     */
    public DataBaseHelper(Context context) {

        super(context, DB_NAME, null, DATABASE_VERSION);
        this.myContext = context;
    }

    /**
     * Creates an empty database on the system and rewrites it with your own
     * database.
     * */
    public void
        createDataBase() throws IOException {
        boolean dbExist = checkDataBase();

        if (dbExist) {


        }
        else {

            //By calling this method and empty database will be created into the default system path
            //of your application so we are gonna be able to overwrite that database with our database.


            this.getWritableDatabase();


            try {

               copyDataBase();

            } catch (IOException e) {

                throw new Error("Error copying database");

            }
        }

    }

    /**
     * Check if the database already exist to avoid re-copying the file each
     * time you open the application.
     * 
     * @return true if it exists, false if it doesn't
     */
    private boolean
        checkDataBase() {

        SQLiteDatabase checkDB = null;

        try {
            String myPath = DB_PATH + DB_NAME;
            checkDB = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READWRITE);

        } catch (SQLiteException e) {

            //database does't exist yet.

        }

        if (checkDB != null) {

            checkDB.close();

        }
        return checkDB != null ? true : false;
    }

    /**
     * Copies your database from your local assets-folder to the just created
     * empty database in the
     * system folder, from where it can be accessed and handled.
     * This is done by transfering bytestream.
     * */
    private void
        copyDataBase() throws IOException {

        //Open your local db as the input stream
        InputStream myInput = myContext.getAssets().open(DB_NAME);


        // Path to the just created empty db
        String outFileName = DB_PATH + DB_NAME;


        //Open the empty db as the output stream
        OutputStream myOutput = new FileOutputStream(outFileName);


        //transfer bytes from the inputfile to the outputfile
        byte[] buffer = new byte[1024];
        int length;
        while ((length = myInput.read(buffer)) > 0) {
            myOutput.write(buffer, 0, length);
        }


        //Close the streams
        myOutput.flush();
        myOutput.close();
        myInput.close();

    }

    public void 
       openDataBase() throws SQLException {

        //Open the database
        String myPath = DB_PATH + DB_NAME;
        myDataBase = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READWRITE);



        // Which parameters do I have to pass?
        onUpgrade(myDataBase, DATABASE_VERSION, 2);
    }

    @Override
    public synchronized void
        close() {

        if (myDataBase != null)
            myDataBase.close();

        super.close(); 

    }

    @Override
    public void
        onCreate(SQLiteDatabase db) {

    }

   @Override
   public void
        onUpgrade(  SQLiteDatabase db,
                int oldVersion,
                int newVersion) {

    Log.d ("onUpgrade first log", Integer.toString(myDataBase.getVersion()));




    if (oldVersion == 1) {

        // do something


        // And then do this!?
        DATABASE_VERSION = 2;
        // or do this
        myDataBase.setVersion(2);
        Log.d ("onUpgrade sedond log", Integer.toString(myDataBase.getVersion()));

    }

    else {
        Log.d("onUpgrade", "else-clause: Already upgraded!");
    }



}

最佳答案

// Do you need this?
private static int DATABASE_VERSION = 2;

是的，你需要这个。 (更好的是，也让它成为 final。)

这告诉数据库助手最新版本的数据库模式是什么。这应该在您的应用程序代码中固定，并在您更改架构时递增。

当您的应用程序启动时，帮助程序会在运行时检查您的代码的最新版本与上次创建或升级数据库时的 Activity 版本相同。 (这就是 db.getVersion() 的用途。)如果数字不匹配，那么助手就知道存储的数据库相对于您的应用程序代码已经过时，并且所以它运行升级例程。

看起来您并不是从头开始创建数据库，而是从您的 Assets 中导入现有数据库。当您执行此初始导入时，这是确保存储的版本与您的代码版本相匹配的时间；要么将它直接应用于 Assets 中的数据库文件，要么，如果您确定 Assets 中的数据库文件与代码匹配，则调用 setVersion(DATABASE_VERSION)。

在任何情况下，您都不应该尝试在 onUpgrade() 例程中修改版本号。仅当版本不匹配时才会调用此方法，您在这里要做的就是进行必要的更改以使数据库保持最新。升级完成后，助手将管理新版本号的存储。

关于java - onUpgrade 数据库 - o​​ldVersion - newVersion，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/7173896/