Android 如果读取asserts 中的db文件，并进行查询操作

    前言：前篇已经讲了，如何将excel表格转化成db文件使用。不清楚的朋友可以看下我上篇的文章。然而在这里，主要说下怎么操作assserts目录下的db文件，进行对数据库的查询操作。废话不多说，开始吧！

      步骤：

             1，在项目中创建一个asserts 目录，有很多人都是手动添加文件夹，命名asserts,其实studio可以直接创建，如图

         2，将创建好的.db文件拷贝到asserts目录，需要将db文件通过流的形式写入手机中，便于打开数据库，具体代码如下：

         这里是将文件拷贝到手机里，以及查询数据库的操作

public class DBManager {
    private String DB_NAME = "demo.db";
    private Context mContext;
    /*选择题的集合*/
    public List mBeanLists = new ArrayList();
    public DBManager(Context mContext) {
        this.mContext = mContext;
    }
    //把assets目录下的db文件复制到dbpath下
    public SQLiteDatabase DBManager(String packName) {
        String dbPath = Environment.getExternalStorageDirectory().getAbsolutePath() + "/databases/" + DB_NAME;
        if (!new File(dbPath).exists()) {
            try {
                boolean flag = new File(Environment.getExternalStorageDirectory().getAbsolutePath() + "/databases/").mkdirs();
                boolean newFile = new File(dbPath).createNewFile();
                try {
                    FileOutputStream out = new FileOutputStream(dbPath);
                    InputStream in = mContext.getAssets().open("demo.db");
                    byte[] buffer = new byte[1024];
                    int readBytes = 0;
                    while ((readBytes = in.read(buffer)) != -1)
                        out.write(buffer, 0, readBytes);
                    in.close();
                    out.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return SQLiteDatabase.openOrCreateDatabase(dbPath, null);
    }
    //查询选择题
    public List query(SQLiteDatabase sqliteDB, String[] columns, String selection, String[] selectionArgs) {
        Bean bean= null;
        try {
            String table = "bank";
            Cursor cursor = sqliteDB.query(table, columns, selection, selectionArgs, null, null, null);
            while (cursor.moveToNext()) {
                String title = cursor.getString(cursor.getColumnIndex("title"));
                String answer = cursor.getString(cursor.getColumnIndex("answer"));
                String daan1 = cursor.getString(cursor.getColumnIndex("daan1"));
                String daan2 = cursor.getString(cursor.getColumnIndex("daan2"));
                String daan3 = cursor.getString(cursor.getColumnIndex("daan3"));
                String daan4 = cursor.getString(cursor.getColumnIndex("daan4"));
                bean= new Bean();
                bean.setTitle(title);
                bean.setAnswer(answer);
                bean.setDaan1(daan1);
                bean.setDaan2(daan2);
                bean.setDaan3(daan3);
                bean.setDaan4(daan4);
                mBeanLists.add(bean);
            }
            cursor.close();
            return mCityLists;
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }

         3，在主Activity中怎么去查询数据库，以及展示数据呢？需要注意的是（ 具体的查询语句，操作语句，耗时操作要放在子线程里处理）

new Thread(new Runnable() {
            @Override
            public void run() {
                DBManager dbManager = new DBManager(NextActivity.this);
                SQLiteDatabase sqLiteDatabase = dbManager.DBManager("com.yoyo.myapplication");
                String[] columns = new String[]{"title", "answer","daan1","daan2","daan3","daan4"};
                String selection = "title like '"+"%"+mKey+"%'";
                List mCityLists = dbManager.query(sqLiteDatabase, columns, selection, null);
                Message message = new Message();
                message.what = DAN_XUAN;
                message.obj = mCityLists;
                handler.sendMessage(message);
                sqLiteDatabase.close();
            }
        }).start();

        4，查询完毕，回到handler里面做需求操作

Handler handler = new Handler(){
        @Override
        public void handleMessage(Message msg) {
            super.handleMessage(msg);
            switch (msg.what){
                case DUI_CUO:
                    List mDuiCuoLists = (List) msg.obj;
                    Toast.makeText(NextActivity.this, "共" + mDuiCuoLists.size() + "题对错题", Toast.LENGTH_SHORT).show();
                    DuiCuoAdapter duiCuoAdapter = new DuiCuoAdapter(NextActivity.this, mDuiCuoLists);
                    mLv.setAdapter(duiCuoAdapter);
                    break;
                case DAN_XUAN:
                    List mDanxuanLists = (List) msg.obj;
                    Toast.makeText(NextActivity.this, "共" + mDanxuanLists.size() + "题选择题", Toast.LENGTH_SHORT).show();
                    XuanZheAdapter adapter = new XuanZheAdapter(NextActivity.this, mDanxuanLists);
                    mLv.setAdapter(adapter);
                    break;
            }
        }
    };