Android拍照没有takePictureIntent

Question

我写了这个测试程序来拍摄android中的照片

这是测试版，最终版本没有活动，因为程序将在后台运行

Camera camera;
Camera.ShutterCallback shutterCallback = new Camera.ShutterCallback(){
    @Override
    public void onShutter() {}
};
Camera.PictureCallback rawPictureCallback = new Camera.PictureCallback(){
    @Override
    public void onPictureTaken(byte[] arg0, Camera arg1) {}
};
Camera.PictureCallback jpegPictureCallback = new Camera.PictureCallback(){
    @Override
    public void onPictureTaken(byte[] arg0, Camera arg1) {
        Bitmap bitmapPicture = BitmapFactory.decodeByteArray(arg0, 0, arg0.length);
        Log.v("takePhoto", "photo here");
    }
};
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    Button btn=(Button)findViewById(R.id.button);

    camera=Camera.open();

    btn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            camera.takePicture(shutterCallback, rawPictureCallback, jpegPictureCallback);
        }
    });
}

我收到了运行时错误

Process: com.myapplication, PID: 14355
java.lang.RuntimeException: takePicture failed

我也试过这个

camera=Camera.open();
camera.startPreview();

但仍然有同样的错误

如何使这个程序有效？

更新

这是完整的错误信息

Process: com.myapplication, PID: 15122
java.lang.RuntimeException: takePicture failed
        at android.hardware.Camera.native_takePicture(Native Method)
        at android.hardware.Camera.takePicture(Camera.java:1290)
        at android.hardware.Camera.takePicture(Camera.java:1235)
        at com.myapplication.MainActivity$4.onClick(MainActivity.java:58)
        at android.view.View.performClick(View.java:4453)
        at android.view.View$PerformClick.run(View.java:18464)
        at android.os.Handler.handleCallback(Handler.java:733)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:136)
        at android.app.ActivityThread.main(ActivityThread.java:5064)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:797)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:613)
        at dalvik.system.NativeStart.main(Native Method)