我正在尝试从资产文件夹中读取现有的sqlite数据库！但我收到此错误：java.lang.runtimeexception unable to instantiate activity componentinfo
请帮帮我！
这是我的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.example.sqltest/databases/";
private static String DB_NAME = "test.sqlite";
private SQLiteDatabase myDataBase;
private final Context myContext;
/**`enter code here`
* 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();
this.close();
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.
}
这是MainActivity类：
public class MainActivity extends Activity {
DataBaseHelper myDbHelper;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
public MainActivity(Context context) {
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;
}
}
}
谁能提供最简单的代码来显示我现有的sqlite数据库的数据？
最佳答案
我想不出有什么很好的理由在这个活动子类的constructor中做任何事情（通常情况下）。通常，您从不直接构造一个活动，我们经常使用Intent，我想这是您的问题代码。使用onCreate()代替其构造函数，并执行所有常规的静态设置-创建视图，将数据绑定到列表，数据库等等，这些均在onCreate（）中。
编辑1：
您可以轻松解决java.lang.runtimeexception unable to instantiate activity componentinfo，只需更改MainActivity：
public class MainActivity extends Activity {
DataBaseHelper myDbHelper;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
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;
}
}
}
编辑2：
确保将
test.sqlite放置在项目的资产文件夹中，并且项目包的名称为com.example.sqltest。如果您的代码没有其他问题，它将正常工作。这意味着您的数据库已成功复制并且可以使用。现在您必须访问数据库并从中获取内容。请在课程末尾注意注释：
添加您的公共帮助程序方法以访问和获取内容
数据库。您可以通过执行“返回”来返回游标
myDataBase.query（....）“，因此您很容易为它创建适配器
您的意见。
如果看到黑页，则可能是因为布局为空。可以使用数据库为视图创建适配器，因此活动不会为空。
您可以在
SQLiteOpenHelper类中查看这些页面以获取更多详细信息：Android SQLite Database and ContentProvider - Tutorial
Android SQLite Database Tutorial
关于android - 无法实例化sqlite数据库的 Activity componentinfo，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/12150052/