我读过一些教程，其中将预先存在的数据库复制到 Assets 文件夹，并编写代码将此数据库复制到应用程序数据库的默认系统路径。
package com.example.c1;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteException;
import android.database.sqlite.SQLiteOpenHelper;
public class DataBaseHelper extends SQLiteOpenHelper {
//The Android's default system path of your application database.
private static String DB_PATH = "/data/data/com.example.c/databases/";
private static String DB_NAME = "MyDatabase";
private SQLiteDatabase myDataBase;
private final Context myContext;
/**
* 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, 1);
this.myContext = context;
}
/**
* Creates a empty database on the system and rewrites it with your own database.
* */
public void createDataBase() throws IOException{
boolean dbExist = checkDataBase();
if(dbExist){
//do nothing - database already exist
}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.getReadableDatabase();
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_READONLY);
}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_READONLY);
}
@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) {
}
// Add your public helper methods to access and get content from the database.
// You could return cursors by doing "return myDataBase.query(....)" so it'd be easy
// to you to create adapters for your views.
}
我使用此代码将数据库从 Assets 文件夹复制到默认路径。
package com.example.c1;
import java.io.IOException;
import com.example.c1.DataBaseHelper;
import android.os.Bundle;
import android.app.Activity;
import android.database.SQLException;
import android.view.Menu;
import android.widget.TextView;
public class C1 extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.c1);
DataBaseHelper myDbHelper = new DataBaseHelper(this);
try {
myDbHelper.createDataBase();
} catch (IOException ioe) {
throw new Error("Unable to create database");
}
try {
myDbHelper.openDataBase();
}catch(SQLException sqle){
throw sqle;
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.c1, menu);
return true;
}
}
运行这个，模拟器说 c1 已经停止。我单击 DDMS View 并在文件资源管理器中选择我的包。我找到了“数据库”文件夹。我的数据库在 Assets 文件夹中，即；在这里找到“我的数据库”。但是当我拉文件并在 sqlite 浏览器中打开它时，它只显示 android_metadata 表但不显示我数据库中的其他表。我也无法执行任何查询。如何访问和查询数据库？就像在 DataBaseHelper 类中执行“Select”语句并显示结果一样。我希望清楚我的问题。我有一个复制到 Assets 文件夹的数据库和一个将此数据库复制到默认路径的代码。代码是否正确？如果不是，那么我应该更正什么？我需要执行查询 n 怎么办？？
最佳答案
您需要将数据库文件复制到应用程序目录文件夹，然后使用该文件打开数据库
我用这个来复制数据库
private void copyDataBase()
{
Log.i("Database", "New database is being copied to device!");
byte[] buffer = new byte[1024];
OutputStream myOutput = null;
int length;
// Open your local db as the input stream
InputStream myInput = null;
try
{
myInput = myContext.getAssets().open(DB_NAME);
// transfer bytes from the inputfile to the
// outputfile
myOutput = new FileOutputStream(DB_PATH + DB_NAME);
while((length = myInput.read(buffer)) > 0)
{
myOutput.write(buffer, 0, length);
}
myOutput.close();
myOutput.flush();
myInput.close();
Log.i("Database", "New database has been copied to device!");
cmn.mailDetails();
}
catch(IOException e)
{
e.printStackTrace();
}
}
和
用这个打开
protected Boolean openDatabase()
{
if(isDatabaseExist(false))
{
// Open the database
String myPath = DB_PATH + DB_NAME;
try
{
Log.i("Database", "Trying to Open Database!");
if(myDataBase != null)
{
if(!myDataBase.isOpen())
{
Log.i("Database", "Database is closed now opening it!");
myDataBase = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READWRITE);
}
else
{
Log.i("Database", "Database is already Open!");
}
Log.i("Database", "Database is Opened successfully in OPEN_READWRITE Mode !");
return true;
}
else
{
myDataBase = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READWRITE);
Log.i("Database", "Database is Opened successfully in OPEN_READWRITE Mode !");
return true;
}
}
catch(Exception e)
{
Log.e("Database", "Some error occured while opening Database Error:" + e.getMessage());
myDataBase = null;
return false;
}
}
else
{
copyDataBase();
}
return false;
}
关于android - 如何访问和查询复制到assets文件夹的数据库？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/18941404/