布局文件：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"xmlns:tools="http://schemas.android.com/tools"android:layout_width="match_parent"android:layout_height="match_parent"android:orientation="vertical"><ImageView android:id="@+id/iv"android:layout_weight="1"android:layout_width="match_parent"android:layout_height="match_parent"/><Buttonandroid:id="@+id/btn"android:layout_weight="6"android:onClick="save"android:layout_width="match_parent"android:layout_height="match_parent"android:text="保存图片"/>
</LinearLayout>

MainActivity：

public class MainActivity extends Activity {private Button button;private ImageView p_w_picpathView;private Bitmap baseBitmap;private Canvas canvas;private Paint paint;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);p_w_picpathView=(ImageView) findViewById(R.id.iv);paint=new Paint();paint.setColor(Color.BLUE);//创建一个可修改的BitmapbaseBitmap=Bitmap.createBitmap(480, 320, Bitmap.Config.ARGB_8888);canvas=new Canvas(baseBitmap);canvas.drawColor(Color.WHITE);p_w_picpathView.setOnTouchListener(new OnTouchListener() {//定义手指坐标int startx;int starty;@Overridepublic boolean onTouch(View v, MotionEvent event) {switch (event.getAction()) {case MotionEvent.ACTION_DOWN:startx=(int) event.getX();starty=(int) event.getY();             break;case MotionEvent.ACTION_MOVE:int newx=(int) event.getX();int newy=(int) event.getY();canvas.drawLine(startx, starty, newx, newy, paint);//跟新画笔的开始位置startx=(int) event.getX();starty=(int) event.getY(); p_w_picpathView.setImageBitmap(baseBitmap);break;case MotionEvent.ACTION_UP:break;}return true;}});    }public void save(View v) {try {File file=new File(Environment.getExternalStorageDirectory(), System.currentTimeMillis()+".jpg");FileOutputStream fos=new FileOutputStream(file);baseBitmap.compress(CompressFormat.JPEG, 100, fos);//第一个参数为图片格式fos.close();Toast.makeText(this, "保存图片成功", 2000).show();//模拟消息：SD卡被重新挂载了Intent intent=new Intent();intent.setAction(intent.ACTION_MEDIA_MOUNTED);intent.setData(Uri.fromFile(Environment.getExternalStorageDirectory()));sendBroadcast(intent);} catch (FileNotFoundException e) {Toast.makeText(this, "保存图片失败", 2000).show();e.printStackTrace();} catch (IOException e) {e.printStackTrace();}              }
}

因为要存入SD中，所以还要加一个权限：

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

运行结果：