问题：这只发生在某些页面上。当我按回app停止。我收到这个错误。我无法弄清楚确切的问题。我是android的新手。这种情况发生在很多页面上。非常感谢任何帮助。
logcat错误：
InputEventSender:Exception dispatching finished signal.E/MessageQueue-JNI:Exception in MessageQueue callback:handleReceiveCallbackE/MessageQueue-JNI:java.lang.NullPointerException: Attempt to invoke virtual method 'void android.database.sqlite.SQLiteDatabase.close()' on a null object reference at twik.in.DBHelper.close(DBHelper.java:102)at twik.in.ActivityMenuDetail.onBackPressed(ActivityMenuDetail.java:285)at android.app.Activity.onKeyUp(Activity.java:2477)at android.view.KeyEvent.dispatch(KeyEvent.java:2664)at android.app.Activity.dispatchKeyEvent(Activity.java:2730)
错误地提到了这两个类，所以我在这里发布它的代码
public class ActivityMenuDetail extends Activity {
ImageView imgPreview;
TextView txtText, txtSubText;
WebView txtDescription;
Button btnAdd;
ScrollView sclDetail;
ProgressBar prgLoading;
TextView txtAlert;
// declare dbhelper object
static DBHelper dbhelper;
// declare ImageLoader object
ImageLoader imageLoader;
// declare variables to store menu data
String Menu_image, Menu_name, Menu_serve, Menu_description;
double Menu_price;
int Menu_quantity;
long Menu_ID;
String MenuDetailAPI;
int IOConnect = 0;
// create price format
DecimalFormat formatData = new DecimalFormat("#.##");
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.menu_detail);
ActionBar bar = getActionBar();
bar.setBackgroundDrawable(new ColorDrawable(getResources().getColor(R.color.header)));
bar.setTitle("Service Centre Detail");
bar.setDisplayHomeAsUpEnabled(true);
bar.setHomeButtonEnabled(true);
imgPreview = (ImageView) findViewById(R.id.imgPreview);
txtText = (TextView) findViewById(R.id.txtText);
txtSubText = (TextView) findViewById(R.id.txtSubText);
txtDescription = (WebView) findViewById(R.id.txtDescription);
btnAdd = (Button) findViewById(R.id.btnAdd);
//btnShare = (Button) findViewById(R.id.btnShare);
sclDetail = (ScrollView) findViewById(R.id.sclDetail);
prgLoading = (ProgressBar) findViewById(R.id.prgLoading);
txtAlert = (TextView) findViewById(R.id.txtAlert);
// get screen device width and height
DisplayMetrics dm = new DisplayMetrics();
getWindowManager().getDefaultDisplay().getMetrics(dm);
int wPix = dm.widthPixels;
int hPix = wPix / 2 + 50;
// change menu image width and height
LinearLayout.LayoutParams lp = new LinearLayout.LayoutParams(wPix, hPix);
imgPreview.setLayoutParams(lp);
imageLoader = new ImageLoader(ActivityMenuDetail.this);
dbhelper = new DBHelper(this);
// get menu id that sent from previous page
Intent iGet = getIntent();
Menu_ID = iGet.getLongExtra("menu_id", 0);
// Menu detail API url
MenuDetailAPI = Constant.MenuDetailAPI+"?accesskey="+Constant.AccessKey+"&menu_id="+Menu_ID;
// call asynctask class to request data from server
new getDataTask().execute();
// event listener to handle add button when clicked
btnAdd.setOnClickListener(new OnClickListener() {
public void onClick(View arg0) {
// TODO Auto-generated method stub
// show input dialog
addtoCart();
}
});
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_detail, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
switch (item.getItemId()) {
case R.id.cart:
// refresh action
Intent iMyOrder = new Intent(ActivityMenuDetail.this, ActivityCart.class);
startActivity(iMyOrder);
overridePendingTransition (R.anim.open_next, R.anim.close_next);
return true;
case android.R.id.home:
// app icon in action bar clicked; go home
this.finish();
overridePendingTransition(R.anim.open_main, R.anim.close_next);
return true;
default:
return super.onOptionsItemSelected(item);
}
}
// method to show number of order form
void addtoCart(){
// open database first
try{
dbhelper.openDataBase();
}catch(SQLException sqle){
throw sqle;
}
if(dbhelper.isDataExist(Menu_ID)){
dbhelper.updateData(Menu_ID, 1, (Menu_price));
}else{
dbhelper.addData(Menu_ID, Menu_name, 1, (Menu_price));
}
startActivity(new Intent(ActivityMenuDetail.this,ActivityCart.class));
};
// asynctask class to handle parsing json in background
public class getDataTask extends AsyncTask<Void, Void, Void>{
// show progressbar first
getDataTask(){
if(!prgLoading.isShown()){
prgLoading.setVisibility(0);
txtAlert.setVisibility(8);
}
}
@Override
protected Void doInBackground(Void... arg0) {
// TODO Auto-generated method stub
// parse json data from server in background
parseJSONData();
return null;
}
@Override
protected void onPostExecute(Void result) {
// TODO Auto-generated method stub
// when finish parsing, hide progressbar
prgLoading.setVisibility(8);
// if internet connection and data available show data
// otherwise, show alert text
if((Menu_name != null) && IOConnect == 0){
sclDetail.setVisibility(0);
imageLoader.DisplayImage(Constant.AdminPageURL+Menu_image, imgPreview);
txtText.setText(Menu_name);
txtSubText.setText("Price : " +Menu_price+" "+ActivityMenuList.Currency+"\n"+"Status : "+Menu_serve+"\n"+"Empty Slots : "+Menu_quantity);
txtDescription.loadDataWithBaseURL("", Menu_description, "text/html", "UTF-8", "");
txtDescription.setBackgroundColor(Color.parseColor("#e7e7e7"));
}else{
txtAlert.setVisibility(0);
}
}
}
// method to parse json data from server
public void parseJSONData(){
try {
// request data from menu detail API
HttpClient client = new DefaultHttpClient();
HttpConnectionParams.setConnectionTimeout(client.getParams(), 15000);
HttpConnectionParams.setSoTimeout(client.getParams(), 15000);
HttpUriRequest request = new HttpGet(MenuDetailAPI);
HttpResponse response = client.execute(request);
InputStream atomInputStream = response.getEntity().getContent();
BufferedReader in = new BufferedReader(new InputStreamReader(atomInputStream));
String line;
String str = "";
while ((line = in.readLine()) != null){
str += line;
}
// parse json data and store into tax and currency variables
JSONObject json = new JSONObject(str);
JSONArray data = json.getJSONArray("data"); // this is the "items: [ ] part
for (int i = 0; i < data.length(); i++) {
JSONObject object = data.getJSONObject(i);
JSONObject menu = object.getJSONObject("Menu_detail");
Menu_image = menu.getString("Menu_image");
Menu_name = menu.getString("Menu_name");
Menu_price = Double.valueOf(formatData.format(menu.getDouble("Price")));
Menu_serve = menu.getString("Serve_for");
Menu_description = menu.getString("Description");
Menu_quantity = menu.getInt("Quantity");
}
} catch (MalformedURLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IOException e) {
// TODO Auto-generated catch block
IOConnect = 1;
e.printStackTrace();
} catch (JSONException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
// close database before back to previous page
@Override
public void onBackPressed() {
// TODO Auto-generated method stub
super.onBackPressed();
dbhelper.close();
finish();
overridePendingTransition(R.anim.open_main, R.anim.close_next);
}
@Override
protected void onDestroy() {
// TODO Auto-generated method stub
//imageLoader.clearCache();
super.onDestroy();
}
@Override
public void onConfigurationChanged(final Configuration newConfig)
{
// Ignore orientation change to keep activity from restarting
super.onConfigurationChanged(newConfig);
}
}
和debhelper课程在这里
public class DBHelper extends SQLiteOpenHelper{
String DB_PATH;
private final static String DB_NAME = "db_order";
public final static int DB_VERSION = 1;
public static SQLiteDatabase db;
private final Context context;
private final String TABLE_NAME = "tbl_order";
private final String ID = "id";
private final String MENU_NAME = "Menu_name";
private final String QUANTITY = "Quantity";
private final String TOTAL_PRICE = "Total_price";
public DBHelper(Context context) {
super(context, DB_NAME, null, DB_VERSION);
this.context = context;
DB_PATH = Constant.DBPath;
}
public void createDataBase() throws IOException{
boolean dbExist = checkDataBase();
SQLiteDatabase db_Read = null;
if(dbExist){
//do nothing - database already exist
}else{
db_Read = this.getReadableDatabase();
db_Read.close();
try {
copyDataBase();
} catch (IOException e) {
throw new Error("Error copying database");
}
}
}
private boolean checkDataBase(){
File dbFile = new File(DB_PATH + DB_NAME);
return dbFile.exists();
}
private void copyDataBase() throws IOException{
InputStream myInput = context.getAssets().open(DB_NAME);
String outFileName = DB_PATH + DB_NAME;
OutputStream myOutput = new FileOutputStream(outFileName);
byte[] buffer = new byte[1024];
int length;
while ((length = myInput.read(buffer))>0){
myOutput.write(buffer, 0, length);
}
myOutput.flush();
myOutput.close();
myInput.close();
}
public void openDataBase() throws SQLException{
String myPath = DB_PATH + DB_NAME;
db = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READWRITE);
}
@Override
public void close() {
db.close();
}
@Override
public void onCreate(SQLiteDatabase db) {
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
}
/** this code is used to get all data from database */
public ArrayList<ArrayList<Object>> getAllData(){
ArrayList<ArrayList<Object>> dataArrays = new ArrayList<ArrayList<Object>>();
Cursor cursor = null;
try{
cursor = db.query(
TABLE_NAME,
new String[]{ID, MENU_NAME, QUANTITY, TOTAL_PRICE},
null,null, null, null, null);
cursor.moveToFirst();
if (!cursor.isAfterLast()){
do{
ArrayList<Object> dataList = new ArrayList<Object>();
dataList.add(cursor.getLong(0));
dataList.add(cursor.getString(1));
dataList.add(cursor.getString(2));
dataList.add(cursor.getString(3));
dataArrays.add(dataList);
}
while (cursor.moveToNext());
}
cursor.close();
}catch (SQLException e){
Log.e("DB Error", e.toString());
e.printStackTrace();
}
return dataArrays;
}
/** this code is used to get all data from database */
public boolean isDataExist(long id){
boolean exist = false;
Cursor cursor = null;
try{
cursor = db.query(
TABLE_NAME,
new String[]{ID},
ID +"="+id,
null, null, null, null);
if(cursor.getCount() > 0){
exist = true;
}
cursor.close();
}catch (SQLException e){
Log.e("DB Error", e.toString());
e.printStackTrace();
}
return exist;
}
/** this code is used to get all data from database */
public boolean isPreviousDataExist(){
boolean exist = false;
Cursor cursor = null;
try{
cursor = db.query(
TABLE_NAME,
new String[]{ID},
null,null, null, null, null);
if(cursor.getCount() > 0){
exist = true;
}
cursor.close();
}catch (SQLException e){
Log.e("DB Error", e.toString());
e.printStackTrace();
}
return exist;
}
public void addData(long id, String menu_name, int quantity, double total_price){
// this is a key value pair holder used by android's SQLite functions
ContentValues values = new ContentValues();
values.put(ID, id);
values.put(MENU_NAME, menu_name);
values.put(QUANTITY, quantity);
values.put(TOTAL_PRICE, total_price);
// ask the database object to insert the new data
try{db.insert(TABLE_NAME, null, values);}
catch(Exception e)
{
Log.e("DB ERROR", e.toString());
e.printStackTrace();
}
}
public void deleteData(long id){
// ask the database manager to delete the row of given id
try {db.delete(TABLE_NAME, ID + "=" + id, null);}
catch (Exception e)
{
Log.e("DB ERROR", e.toString());
e.printStackTrace();
}
}
public void deleteAllData(){
// ask the database manager to delete the row of given id
try {db.delete(TABLE_NAME, null, null);}
catch (Exception e)
{
Log.e("DB ERROR", e.toString());
e.printStackTrace();
}
}
public void updateData(long id, int quantity, double total_price){
// this is a key value pair holder used by android's SQLite functions
ContentValues values = new ContentValues();
values.put(QUANTITY, quantity);
values.put(TOTAL_PRICE, total_price);
// ask the database object to update the database row of given rowID
try {db.update(TABLE_NAME, values, ID + "=" + id, null);}
catch (Exception e)
{
Log.e("DB Error", e.toString());
e.printStackTrace();
}
}}
答案 0 :(得分：1)
logcat表示在执行dbhelper
close()对象为空
@Override
public void onBackPressed() {
// TODO Auto-generated method stub
super.onBackPressed();
if (dbhelper != null)
dbhelper.close();
finish();
overridePendingTransition(R.anim.open_main, R.anim.close_next);
}
那应该解决问题
答案 1 :(得分：0)
从您提供的错误跟踪中，似乎＆＃34; db＆＃34; DBHelper中的实例为null。因此，当您尝试通过调用dbHelper.close（）来关闭db时，在onBackPressed（）中，它会抛出NullPointerException。添加空检查将有所帮助。
//在返回上一页之前关闭数据库 @override
public void onBackPressed() {
// TODO Auto-generated method stub
super.onBackPressed();
dbhelper.close();
finish();
overridePendingTransition(R.anim.open_main, R.anim.close_next);
}
上述函数最终将调用以下函数：
@Override
public void close() {
db.close();
}
为＆＃34; db＆＃34;
添加空检查@Override
public void close() {
if (db != null) {
db.close();
}
}
快速浏览一下代码，似乎只是在＆＃34;添加到购物车＆＃34;等操作上打开数据库。
void addtoCart(){
// open database first
try{
dbhelper.openDataBase();
}catch(SQLException sqle){
throw sqle;
}
if(dbhelper.isDataExist(Menu_ID)){
dbhelper.updateData(Menu_ID, 1, (Menu_price));
}else{
dbhelper.addData(Menu_ID, Menu_name, 1, (Menu_price));
}
startActivity(new Intent(ActivityMenuDetail.this,ActivityCart.class));
};
如果您尝试在不执行此类操作的情况下关闭屏幕，则DB对象将无法使用。
答案 2 :(得分：0)
根据logcat，你的dbhelper是null，所以它给出了一个NULL POINTER EXCEPTION。
@Override
public void onBackPressed() {
// TODO Auto-generated method stub
super.onBackPressed();
dbhelper.close();
finish();
overridePendingTransition(R.anim.open_main, R.anim.close_next);
}
在上面的方法中，在关闭数据库之前首先调用super.onBackPressed();，这不是一个好习惯。
将其更改为，
@Override
public void onBackPressed() {
if (dbhelper != null)
dbhelper.close();
finish();
overridePendingTransition(R.anim.open_main, R.anim.close_next);
super.onBackPressed();
}
同样改变，
public void close() {
db.close();
}
到
public void close() {
if(db!=null)
db.close();
}
在Dbhelper.class中
答案 3 :(得分：0)
这是因为它可能会获得dbhelper的空对象引用，并且它也不需要使用finish（）;当onBackPressed（）调用活动自动完成时。
if (dbhelper != null)
dbhelper.close();
finish();
overridePendingTransition(R.anim.open_main, R.anim.close_next);
super.onBackPressed();