问题描述

Android处理程序出现NullPointerException错误

我在Activity中创建了一个新的Thread。通过输入流，run()方法获取vector对象。现在我想在名称为direc的ListView中显示vector对象的内容。但是程序没有正常运行，抛出了异常。
相关的代码如下：

public class FileExplorerActivity extends Activity implements Runnable
{
    public static final String TAG="ricky";
    Vector<String> dirs = new Vector<String>();
    ListView direc;
    static ArrayAdapter<String> arrayAdapter;
    Handler handler;

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.fileexplorer);
        direc = (ListView) findViewById(R.id.directoriesss);
        handler = new Handler();
        Thread thread = new Thread(this);
        thread.start();
    }

    public FileExplorerActivity()
    {

    }

    public void run()
    {
        Log.d(TAG, "FileExplorer: Thread started");
        try
        {
            Log.d(TAG, "FileExplorer: reading data");
            dirs = (Vector<String>) ConnectionThread.din.readObject();
            Log.d(TAG, "FileExplorer: read data");
            arrayAdapter = new ArrayAdapter<String>(this,
                                     android.R.layout.activity_list_item,
                                     android.R.id.text2 , dirs);
            Log.d(TAG, "FileExplorer: ArrayAdapter created");

            handler.post( new Runnable() {
                public void run()
                {
                    Log.d(TAG, "FileExplorer: In handler Thread");
                    direc.setAdapter(arrayAdapter);
                    Log.d(TAG, "FileExplorer: Updated UI");
                }
            });

            Log.d(TAG, "FileExplorer: Directory set");
        }catch (Exception e) {
            Log.d(TAG, "FileExplorerActivity: Exception:"+e.getMessage());
        }
    }
}

Logcat的错误：

> 1. 12-20 20:14:46.601  7759  7834 D ricky   : FileExplorer: Thread started
> 2. 12-20 20:14:46.601  7759  7834 D ricky   : FileExplorer: reading data
> 3. 12-20 20:14:46.648  7759  7834 D ricky   : FileExplorer: read data
> 4. 12-20 20:14:46.648  7759  7834 D ricky   : FileExplorer: ArrayAdapter created
> 5. 12-20 20:14:46.648  7759  7834 D ricky   : FileExplorer: Directory set
> 6. 12-20 20:14:46.656  7759  7759 D ricky   : FileExplorer: In handler Thread
> 7. 12-20 20:14:46.656  7759  7759 D ricky   : FileExplorer: Updated UI
> 8. 12-20 20:14:46.664  7759  7759 D AndroidRuntime: Shutting down VM
> 9. 12-20 20:14:46.664  7759  7759 W dalvikvm: threadid=1: thread exiting with uncaught 10. exception (group=0x4001e560)
> 11. 12-20 20:14:46.687  7759  7759 E AndroidRuntime: FATAL EXCEPTION: main
> 12. 12-20 20:14:46.687  7759  7759 E AndroidRuntime: java.lang.NullPointerException
> 13. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at android.widget.ArrayAdapter.createViewFromResource(ArrayAdapter.java:353)
> 14. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at android.widget.ArrayAdapter.getView(ArrayAdapter.java:323)
> 15. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at android.widget.AbsListView.obtainView(AbsListView.java:1456)
> 16. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at android.widget.ListView.measureHeightOfChildren(ListView.java :1291)
> 17. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at android.widget.ListView.onMeasure(ListView.java:1202)
> 18. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at android.view.View.measure(View.java:8355)
> 19. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java :3279)
> 20. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1017)
> 21. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at android.widget.LinearLayout.measureHorizontal(LinearLayout.ja va:701)
> 22. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at android.widget.LinearLayout.onMeasure(LinearLayout.java:311)
> 23. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at android.view.View.measure(View.java:8355)
> 24. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java :3279)
> 25. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at android.widget.FrameLayout.onMeasure(FrameLayout.java:250)
> 26. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at android.view.View.measure(View.java:8355)
> 27. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at android.widget.LinearLayout.measureVertical(LinearLayout.java :531)
> 28. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at android.widget.LinearLayout.onMeasure(LinearLayout.java:309)
> 29. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at android.view.View.measure(View.java:8355)
> 30. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java :3279)
> 31. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at android.widget.FrameLayout.onMeasure(FrameLayout.java:250)
> 32. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at android.view.View.measure(View.java:8355)
> 33. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at android.view.ViewRoot.performTraversals(ViewRoot.java:882)
> 34. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at android.view.ViewRoot.handleMessage(ViewRoot.java:1917)
> 35. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at android.os.Handler.dispatchMessage(Handler.java:99)
> 36. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at android.os.Looper.loop(Looper.java:130)
> 37. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at android.app.ActivityThread.main(ActivityThread.java:3859)
> 38. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at java.lang.reflect.Method.invokeNative(Native Method)
> 39. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at java.lang.reflect.Method.invoke(Method.java:507)
> 40. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
> 41. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
> 42. 12-20 20:14:46.687  7759  7759 E AndroidRuntime:        at dalvik.system.NativeStart.main(Native Method)

解决方案

注意引起NullPointerException错误的这一行：

android.widget.ArrayAdapter.createViewFromResource(ArrayAdapter.java:353)

这是因为android.R.layout.activity_list_item不包含android.R.id.text2。
你可以使用android.R.id.text1，或者使用simple_list_item_2来代替行布局。

解决方案二：

arrayAdapter = new ArrayAdapter<String>(this, android.R.layout.activity_list_item,
        android.R.id.text2 , dirs);

把run()方法中的这段代码移除，就会在UI线程中运行。你不能在UI线程外创建ArrayAdapter。