Android App因错误doInBackground而崩溃

Question

I am new in Android development. I came across an issue in a particular scenario. When I open my app, it starts AsyncTask for db query. As soon as the app opens I tap on the back button to terminate the app, the app crashes with this error:

java.lang.RuntimeException: An error occurred while executing doInBackground()
   at android.os.AsyncTask$3.done(AsyncTask.java:299)
   at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:352)
   at java.util.concurrent.FutureTask.setException(FutureTask.java:219)
   at java.util.concurrent.FutureTask.run(FutureTask.java:239)
   at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:230)
   at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1080)
   at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:573)
   at java.lang.Thread.run(Thread.java:838)
Caused by: java.lang.NullPointerException
   at sarsat.lokate.Activity.MainActivity.Tab1$LongOperation.doInBackground(Tab1.java:62)
   at sarsat.lokate.Activity.MainActivity.Tab1$LongOperation.doInBackground(Tab1.java:58)
   at android.os.AsyncTask$2.call(AsyncTask.java:287)
   at java.util.concurrent.FutureTask.run(FutureTask.java:234)
   at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:230)
   at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1080)
   at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:573)
   at java.lang.Thread.run(Thread.java:838)

And this is my code:

 @Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    Fabric.with(this, new Crashlytics());
    setContentView(R.layout.activity_main);
    toolbar = (Toolbar) findViewById(R.id.tool_bar);
    setSupportActionBar(toolbar);
    initImageLoader(getApplicationContext());
    // Initializing
    dataList = new ArrayList<DrawerItem>();
    mTitle = mDrawerTitle = getTitle();
    mDrawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
    mDrawerList = (ListView) findViewById(R.id.left_drawer);
    mDrawerList.setDivider(null);
    mDrawerList.setDividerHeight(0);

    int width = ((getResources().getDisplayMetrics().widthPixels)) - ((getResources().getDisplayMetrics().widthPixels)/5);
    DrawerLayout.LayoutParams params = (DrawerLayout.LayoutParams) mDrawerList.getLayoutParams();
    params.width = width;
    mDrawerList.setLayoutParams(params);

    addDrawerItems(dataList);
    drawerAdapter = new CustomDrawerAdapter(this, R.layout.custom_drawer_item,
            dataList);

    mDrawerList.setAdapter(drawerAdapter);
    mDrawerList.setOnItemClickListener(new AdapterView.OnItemClickListener() {

        @Override
        public void onItemClick(AdapterView<?> arg0, View v, int position, long arg3) {
            //Object o = mDrawerList.getItemAtPosition(position);
            //String str=(String)o; //As you are using Default String Adapter
            //Toast.makeText(getApplicationContext(),"Hello",Toast.LENGTH_SHORT).show();
            com.rey.material.widget.CheckBox checkBox = (com.rey.material.widget.CheckBox) v.findViewById(R.id.checkbox);
            if(checkBox.isChecked()){
                checkBox.setChecked(false);
            }
            else{
                checkBox.setChecked(true);
            }
        }
    });

    getSupportActionBar().setDisplayHomeAsUpEnabled(true);
    getSupportActionBar().setHomeButtonEnabled(true);


    mDrawerToggle = new ActionBarDrawerToggle(
            MainActivity.this,                    /* host Activity */
            mDrawerLayout,                    /* DrawerLayout object */
            R.string.drawer_open,             /* "open drawer" description for accessibility */
            R.string.drawer_close  /* "close drawer" description for accessibility */
    ) {
        public void onDrawerClosed(View view) {
            getSupportActionBar().setTitle(mTitle);
            invalidateOptionsMenu(); // creates call to
            // onPrepareOptionsMenu()
            mDrawerToggle.syncState();
        }

        public void onDrawerOpened(View drawerView) {
            getSupportActionBar().setTitle(mDrawerTitle);
            invalidateOptionsMenu(); // creates call to
            // onPrepareOptionsMenu()
            mDrawerToggle.syncState();
        }
    };
    mDrawerToggle.setDrawerIndicatorEnabled(true);
    mDrawerToggle.syncState();
    mDrawerLayout.setDrawerListener(mDrawerToggle);
//        getSupportActionBar().setLogo(R.drawable.ic_drawer);
    mDrawerLayout.post(new Runnable() {
        @Override
        public void run() {
            mDrawerToggle.syncState();
        }
    });

    // Creating The Toolbar and setting it as the Toolbar for the activity

    // Creating The ViewPagerAdapter and Passing Fragment Manager, Titles fot the Tabs and Number Of Tabs.
    adapter =  new ViewPagerAdapter(getSupportFragmentManager(),Titles,Numboftabs);
    // Assigning ViewPager View and setting the adapter
    pager = (ViewPager) findViewById(R.id.pager);
    pager.setAdapter(adapter);
    // Assiging the Sliding Tab Layout View
    tabs = (SlidingTabLayout) findViewById(R.id.tabs);
    tabs.setDistributeEvenly(true); // To make the Tabs Fixed set this true, This makes the tabs Space Evenly in Available width
    // Setting Custom Color for the Scroll bar indicator of the Tab View

    tabs.setCustomTabColorizer(new SlidingTabLayout.TabColorizer() {
        @Override
        public int getIndicatorColor(int position) {
            return getResources().getColor(R.color.tabsScrollColor);
        }
    });
    // Setting the ViewPager For the SlidingTabsLayout
    tabs.setViewPager(pager);
    //seedData();
    LongOperation operation = new LongOperation();
    operation.execute();
}

And in this code:

LongOperation operation = new LongOperation();
    operation.execute();

This is what happening:

private class LongOperation extends AsyncTask<String, Void, String> {

    @Override
    protected String doInBackground(String... params) {
        seedData();
        return "Executed";
    }

    @Override
    protected void onPostExecute(String result) {
    }

    @Override
    protected void onPreExecute() {}

    @Override
    protected void onProgressUpdate(Void... values) {

    }
}

How can I handle this issue in AsyncTask so that the app does not crash anymore.

EDITED :

seedData method:

public void seedData(){

    CouchBase db = new CouchBase(getApplicationContext());
    db.DeleteAllDocs();

    //Add J. Ad
    doc = new Doc();
    doc.vendorTitle = "Wear J. this summer - upto 50% off";
    doc.vendorHandler = "@XYZ";

    db.createAd(doc);
}

Answer 1

What you will need to do when you figure out where your NullPointerException lies is design it to break out when the AsyncTask has been cancelled.

Your calling method for the task must keep a reference to it and within the onPause call asyncTask.cancel() .

You'll have to handle the break properly within your AsyncTask as well checking if the task has been cancelled before continuing what you are doing.

protected Object doInBackground(Ojbect... objs) {
     //...
     while (completingYourTask) {
         // Do your task but check before your NPException when needed
         // Escape early if cancel() is called
         if (isCancelled()) break;
     }
     return null;
 }