无法从资产文件夹中复制Android的SQLite数据库 [英] Can't copy sqlite database from assets folder in Android

问题描述

我一直在试图从资产文件夹复制SQLite数据库文件，所以我创建了一个SQLite数据库，并在我的程序中使用它，这里是code： -

I have been trying to copy a sqlite database file from assets folder so that i create a sqlite database and use it in my program, here is the code:-

首先 SqliteOpenHelperClass

public class DBHandler extends SQLiteOpenHelper {
protected static final String DATABASE_NAME_PRODUCTION = "productionComments.db";

public DBHandler(Context context, String name, SQLiteDatabase.CursorFactory factory,
                     int version) {
        super(context, name, factory, DATABASE_VERSION);
    }

       //copy database from assets folder (.sqlite) file to an empty database
        public void copyDataBase() throws IOException{

        //Open your local db as the input stream
        InputStream myInput = context.getAssets().open("prod.db");

        // Path to the just created empty db
        String outFileName = "/data/data/com.qarun.qpcbeta/databases/"+DATABASE_NAME_PRODUCTION;

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

        //transfer bytes from the input file to the output file
        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();
    }

现在在哪里我应该叫 copyDatabase（）法活动： -

Now the activity where i am supposed to call the copyDatabase() method:-

public class ProductionCommentsActivity extends Activity {
private DBHandler dbHandler;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_production_comments);
    dbHandler = new DBHandler(this, dbHandler.DATABASE_NAME_PRODUCTION, null, 1);
    try {
        dbHandler.copyDataBase();
        //Toast.makeText(this,"Works",Toast.LENGTH_LONG).show();
    } catch (IOException e) {
        e.printStackTrace();
        Log.d("copydb",e.getMessage());
        //Toast.makeText(this,e.getMessage(),Toast.LENGTH_LONG).show();
    }
}

下面是在logcat的错误： -

Here is the error in the logcat:-

Process: com.qarun.qpcbeta, PID: 1271
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.qarun.qpcbeta/com.qarun.qpcbeta.ProductionCommentsActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.AssetManager android.content.Context.getAssets()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
        at android.app.ActivityThread.access$800(ActivityThread.java:151)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5257)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.AssetManager android.content.Context.getAssets()' on a null object reference
        at com.qarun.qpcbeta.DBHandler.copyDataBase(DBHandler.java:100)
        at com.qarun.qpcbeta.ProductionCommentsActivity.onCreate(ProductionCommentsActivity.java:24)


   at android.app.Activity.performCreate(Activity.java:5990)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
            at android.app.ActivityThread.access$800(ActivityThread.java:151)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5257)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

该应用程序停止工作（崩溃）当我打开 ProductionCommentsActivity

推荐答案

请您DbHelper这样，

Make your DbHelper like this,

public class DBHandler extends SQLiteOpenHelper {
protected static final String DATABASE_NAME_PRODUCTION = "productionComments.db";
private Context context;

public DBHandler(Context context, String name, SQLiteDatabase.CursorFactory factory,
                     int version) {
        super(context, name, factory, DATABASE_VERSION);
        this.context = context;
    }

       //copy database from assets folder (.sqlite) file to an empty database
        public void copyDataBase() throws IOException{

        //Open your local db as the input stream
        InputStream myInput = context.getAssets().open("prod.db");

        // Path to the just created empty db
        String outFileName = "/data/data/com.qarun.qpcbeta/databases/"+DATABASE_NAME_PRODUCTION;

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

        //transfer bytes from the input file to the output file
        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();
    }

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