这个 android 的东西很新，并得到一个我似乎无法弄清楚的 NullPointException。我正在尝试在我的 CameraActivity 中实现 onResume() 方法，并将 onCreate() 中的几乎所有原始代码移动到 onResume()，然后在 onCreate() 中调用 onResume()。当代码在 onCreate() 中时，活动运行良好，但在 onResume() 中时出现异常。是什么原因造成的？
package com.example.tensioncamapp_project;
import java.io.IOException;
import android.content.Context;
import android.hardware.Camera;
import android.util.Log;
import android.view.SurfaceHolder;
import android.view.SurfaceView;
/** A basic Camera preview class */
public class CameraPreview extends SurfaceView implements SurfaceHolder.Callback {
private static final String TAG = "PreviewAactivity";
private SurfaceHolder mHolder;
private Camera mCamera;
public CameraPreview(Context context, Camera camera) {
super(context);
this.mCamera = camera;
// Install a SurfaceHolder.Callback so we get notified when the
// underlying surface is created and destroyed.
this.mHolder = getHolder();
this.mHolder.addCallback(this);
}
/**Displays the picture on the camera */
public void surfaceCreated(SurfaceHolder holder) {
// The Surface has been created, now tell the camera where to draw the preview.
try {
this.mCamera.setPreviewDisplay(holder);
this.mCamera.startPreview();
} catch (IOException e) {
Log.d(TAG, "Error setting camera preview: " + e.getMessage());
}
}
public void surfaceDestroyed(SurfaceHolder holder) {
this.mCamera.release();
this.mCamera = null;
}
public void surfaceChanged(SurfaceHolder holder, int format, int w, int h) {
//add things here
}
}
和我的 CameraActivityClass
package com.example.tensioncamapp_project;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.sql.Date;
import java.text.SimpleDateFormat;
import android.content.Intent;
import android.app.Activity;
import android.graphics.Bitmap;
import android.hardware.Camera;
import android.hardware.Camera.Parameters;
import android.hardware.Camera.PictureCallback;
import android.os.Bundle;
import android.os.Environment;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.FrameLayout;
import android.widget.ImageButton;
import android.widget.ImageView;
public class CameraActivity extends Activity {
private ImageButton captureButton;
private Camera mCamera;
private CameraPreview mPreview;
private PictureCallback mPicture;
private ImageView imageView;
private static final int STD_DELAY = 400;
private static final int MEDIA_TYPE_IMAGE = 1;
protected static final String TAG = "CameraActivity";
/**Starts up the camera */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_camera);
onResume();
}
/**Connects the capture button on the view to a listener
* and redirects the client to a preview of the captures image*/
private void addListenerOnButton() {
this.captureButton = (ImageButton) findViewById(R.id.button_capture_symbol);
this.captureButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View capturebutton) {
mCamera.takePicture(null, null, mPicture);
delay();
Intent viewPic = new Intent(CameraActivity.this, ViewPicActivity.class);
startActivity(viewPic);
}
});
}
/** A safe way to get an instance of the Camera object. Code collected from elsewhere */
public static Camera getCameraInstance(){
Camera c = null;
try {
// attempt to get a Camera instance
c = Camera.open();
//getting current parameters
Camera.Parameters params = c.getParameters();
//setting new parameters with flash
params.setFlashMode(Parameters.FLASH_MODE_TORCH);
c.setParameters(params);
}
catch (Exception e){
// camera is not available (in use or does not exist)
}
// returns null if camera is unavailable
return c;
}
/**Generates a delay needed for application to save new pictures */
private void delay(){
try {
//Makes the program inactive for a specific amout of time
Thread.sleep(STD_DELAY);
} catch (Exception e) {
e.getStackTrace();
}
}
/**Method for releasing the camera immediately on pause event*/
@Override
protected void onPause() {
super.onPause();
//Shuts down the preview shown on the screen
mCamera.stopPreview();
//Calls an internal help method to restore the camera
releaseCamera();
}
/**Help method to release the camera */
private void releaseCamera(){
//Checks if there is a camera object active
if (this.mCamera != null){
//Releases the camera
this.mCamera.release();
//Restore the camera object to its initial state
this.mCamera = null;
}
}
/**Activates the camera and makes it appear on the screen */
protected void onResume() {
// TODO Auto-generated method stub
// deleting image from external storage
FileHandler.deleteFromExternalStorage();
// Create an instance of Camera.
this.mCamera = getCameraInstance();
// Create our Preview view and set it as the content of our activity.
this.mPreview = new CameraPreview(this, this.mCamera);
FrameLayout preview = (FrameLayout) findViewById(R.id.camera_preview);
preview.addView(this.mPreview);
//add the capture button
addListenerOnButton();
// In order to receive data in JPEG format
this.mPicture = new PictureCallback() {
/**Creates a file when a image is taken, if the file doesn't already exists*/
@Override
public void onPictureTaken(byte[] data, Camera mCamera) {
File pictureFile = FileHandler.getOutputMediaFile(MEDIA_TYPE_IMAGE);
if (pictureFile == null){
Log.d(TAG, "Error creating media file, check storage permissions");
return;
}
try {
//Writes the image to the disc
FileOutputStream fos = new FileOutputStream(pictureFile);
fos.write(data);
fos.close();
} catch (FileNotFoundException e) {
Log.d(TAG, "File not found: " + e.getMessage());
} catch (IOException e) {
Log.d(TAG, "Error accessing file: " + e.getMessage());
}
}
};
super.onResume();
}
}
日志猫：
05-21 14:32:05.424: D/OpenGLRenderer(1030): Enabling debug mode 0
05-21 14:32:10.986: E/CameraActivity(1030): camera not availableFail to connect to camera service
05-21 14:32:11.033: I/Choreographer(1030): Skipped 66 frames! The application may be doing too much work on its main thread.
05-21 14:32:11.203: W/EGL_emulation(1030): eglSurfaceAttrib not implemented
05-21 14:32:13.013: D/AndroidRuntime(1030): Shutting down VM
05-21 14:32:13.013: W/dalvikvm(1030): threadid=1: thread exiting with uncaught exception (group=0x40a71930)
05-21 14:32:13.083: E/AndroidRuntime(1030): FATAL EXCEPTION: main
05-21 14:32:13.083: E/AndroidRuntime(1030): java.lang.NullPointerException
05-21 14:32:13.083: E/AndroidRuntime(1030): at com.example.tensioncamapp_project.CameraPreview.surfaceCreated(CameraPreview.java:33)
05-21 14:32:13.083: E/AndroidRuntime(1030): at android.view.SurfaceView.updateWindow(SurfaceView.java:569)
05-21 14:32:13.083: E/AndroidRuntime(1030): at android.view.SurfaceView.access$000(SurfaceView.java:86)
05-21 14:32:13.083: E/AndroidRuntime(1030): at android.view.SurfaceView$3.onPreDraw(SurfaceView.java:174)
05-21 14:32:13.083: E/AndroidRuntime(1030): at android.view.ViewTreeObserver.dispatchOnPreDraw(ViewTreeObserver.java:680)
05-21 14:32:13.083: E/AndroidRuntime(1030): at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1842)
05-21 14:32:13.083: E/AndroidRuntime(1030): at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:989)
05-21 14:32:13.083: E/AndroidRuntime(1030): at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:4351)
05-21 14:32:13.083: E/AndroidRuntime(1030): at android.view.Choreographer$CallbackRecord.run(Choreographer.java:749)
05-21 14:32:13.083: E/AndroidRuntime(1030): at android.view.Choreographer.doCallbacks(Choreographer.java:562)
05-21 14:32:13.083: E/AndroidRuntime(1030): at android.view.Choreographer.doFrame(Choreographer.java:532)
05-21 14:32:13.083: E/AndroidRuntime(1030): at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:735)
05-21 14:32:13.083: E/AndroidRuntime(1030): at android.os.Handler.handleCallback(Handler.java:725)
05-21 14:32:13.083: E/AndroidRuntime(1030): at android.os.Handler.dispatchMessage(Handler.java:92)
05-21 14:32:13.083: E/AndroidRuntime(1030): at android.os.Looper.loop(Looper.java:137)
05-21 14:32:13.083: E/AndroidRuntime(1030): at android.app.ActivityThread.main(ActivityThread.java:5041)
05-21 14:32:13.083: E/AndroidRuntime(1030): at java.lang.reflect.Method.invokeNative(Native Method)
05-21 14:32:13.083: E/AndroidRuntime(1030): at java.lang.reflect.Method.invoke(Method.java:511)
05-21 14:32:13.083: E/AndroidRuntime(1030): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
05-21 14:32:13.083: E/AndroidRuntime(1030): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
05-21 14:32:13.083: E/AndroidRuntime(1030): at dalvik.system.NativeStart.main(Native Method)