我将我的数据库从资产复制到设备，但它无法从这里的数据库中读取数据代码
package ta.ex;
import java.io.IOException;
import android.app.Activity;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.widget.ListView;
import android.widget.SimpleCursorAdapter;
public class MainActivity extends Activity {
/** Called when the activity is first created. */
MyDatabase mydb;
ListView listtema;
SQLiteDatabase db;
Cursor cursor;
SimpleCursorAdapter adapter;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
mydb = new MyDatabase(this);
try {
mydb.createDataBase();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
db = db.openDatabase("data/data/ta.ex/databases/ensiklopedia.sqlite", null, db.OPEN_READONLY);
listtema = (ListView)findViewById(R.id.listTema);
String[] columns = new String[]{"_id","tema"};
cursor = db.query("tema", columns, null, null, null, null, null);
if (cursor != null){
adapter = new SimpleCursorAdapter(this,
android.R.layout.simple_list_item_1,cursor,
new String[]{"tema"},new int[]{android.R.id.text1});
listtema.setAdapter(adapter);
}
}
}
我的数据库.java
package ta.ex;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteException;
import android.database.sqlite.SQLiteOpenHelper;
public class MyDatabase extends SQLiteOpenHelper{
private static final String DB_PATH = "/data/data/ta.ex/databases/";
private static final String DB_NAME = "ensiklopedia.sqlite";
private SQLiteDatabase sqldb;
private final Context myContext;
public MyDatabase(Context context) {
super(context,DB_NAME,null,1 );
// TODO Auto-generated constructor stub
this.myContext = context;
}
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
*/
boolean checkDataBase(){
SQLiteDatabase checkDB = null;
try{
String myPath = DB_PATH + DB_NAME;
checkDB = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READONLY);
}
catch(SQLiteException e){
//database doesn'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
File outFileName = myContext.getDatabasePath(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 boolean isDatabaseExist() {
File dbFile = new File(DB_PATH+DB_NAME);
return dbFile.exists();
}
public void openDataBase(){
//Open the database
String myPath = DB_PATH + DB_NAME;
sqldb = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READWRITE);
}
@Override
public synchronized void close() {
if(sqldb != null)
sqldb.close();
super.close();
}
@Override
public void onCreate(SQLiteDatabase arg0) {
// TODO Auto-generated method stub
}
@Override
public void onUpgrade(SQLiteDatabase arg0, int arg1, int arg2) {
// TODO Auto-generated method stub
}
}
原木猫
11-16 19:18:12.182: ERROR/AndroidRuntime(5818): java.lang.RuntimeException: Unable to start activity ComponentInfo{ta.ex/ta.ex.MainActivity}: java.lang.NullPointerException
11-16 19:18:12.182: ERROR/AndroidRuntime(5818): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
11-16 19:18:12.182: ERROR/AndroidRuntime(5818): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
11-16 19:18:12.182: ERROR/AndroidRuntime(5818): at android.app.ActivityThread.access$2300(ActivityThread.java:125)
11-16 19:18:12.182: ERROR/AndroidRuntime(5818): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
11-16 19:18:12.182: ERROR/AndroidRuntime(5818): at android.os.Handler.dispatchMessage(Handler.java:99)
11-16 19:18:12.182: ERROR/AndroidRuntime(5818): at android.os.Looper.loop(Looper.java:123)
11-16 19:18:12.182: ERROR/AndroidRuntime(5818): at android.app.ActivityThread.main(ActivityThread.java:4627)
11-16 19:18:12.182: ERROR/AndroidRuntime(5818): at java.lang.reflect.Method.invokeNative(Native Method)
11-16 19:18:12.182: ERROR/AndroidRuntime(5818): at java.lang.reflect.Method.invoke(Method.java:521)
11-16 19:18:12.182: ERROR/AndroidRuntime(5818): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
11-16 19:18:12.182: ERROR/AndroidRuntime(5818): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
11-16 19:18:12.182: ERROR/AndroidRuntime(5818): at dalvik.system.NativeStart.main(Native Method)
11-16 19:18:12.182: ERROR/AndroidRuntime(5818): Caused by: java.lang.NullPointerException
11-16 19:18:12.182: ERROR/AndroidRuntime(5818): at ta.ex.MainActivity.onCreate(MainActivity.java:33)
11-16 19:18:12.182: ERROR/AndroidRuntime(5818): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
11-16 19:18:12.182: ERROR/AndroidRuntime(5818): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)
当我使用复制到系统的 createDatabase() 数据库时，但每当我尝试打开数据库时，它总是强制关闭 java.lang.nullpointerexception。请帮忙