Why does android not fall with the error "CalledFromWrongThreadException"?

Question

When I run a code below, application does not fall:

@Override
protected void onCreate(Bundle savedInstanceState) {
    ...
    final Button b = (Button)findViewById(R.id.button1);
    new Mt(b).start();

}

And this code falls (when i click on the button1 with error "CalledFromWrongThreadException"):

@Override
protected void onCreate(Bundle savedInstanceState) {
    ....
    final Button b = (Button)findViewById(R.id.button1);
    b.setOnClickListener(new Button.OnClickListener() {         
            @Override
            public void onClick(View v) {
                new Mt(b).start();
            }
        });

}

Where Mt class is

class Mt extends Thread{
    Button b;
    Mt(Button b){
        this.b=b;
    }
    @Override
    public void run() {
        b.setText("4");
    }
}

Why the first example does not fall with error "CalledFromWrongThreadException" ?

score 0 · Answer 1 · answered Oct 19 '13 at 10:40

0

Thread will run in non UI Thread. UI related operations must be done in UI thread.. Modify the code in thread.run() method

public void run() {
    runOnUiThread(new Runnable() {

       @Override
       public void run() {
       // TODO Auto-generated method stub
           b.setText("4");  
       }
   });

}

answered Oct 19 '13 at 10:40

Jagadesh Seeram

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So why does the first example work? – Simon Oct 19 '13 at 10:41
Because you are changing the text of the button in OnClick. Until unless you click on the button, thread will not start. – Jagadesh Seeram Oct 19 '13 at 10:45
2

Still, both should fail. The access to the button happens on a separate thread regardless. – Delyan Oct 19 '13 at 10:51

score 0 · Answer 2 · answered Oct 19 '13 at 10:59

This is just a hunch, but the first example does not fail because there is no UI during the execution of onCreate. The check for thread access does not occur in the setText method until it actually plans on changing something that is visible to the user. Since nothing is being drawn to the screen yet, there is no problem.

Why does android not fall with the error "CalledFromWrongThreadException"?

2 Answers2