RadialGradient gradient = new RadialGradient(j/2,k/2,j/2,new int[]{0xff5d5d5d,0xff5d5d5d,0x00ffffff},new float[]{0.f,0.8f,1.0f}, Shader.TileMode.CLAMP);
paint.setShader(gradient);
canvas.drawCircle(j/2,k/2,j/2,paint);

/**
 * 转换图片成圆形
 * @param bitmap 传入Bitmap对象
 * @return
 */
 public Bitmap toRoundBitmap(Bitmap bitmap)
 {
  int width = bitmap.getWidth();
  int height = bitmap.getHeight();
  float roundPx;
  float left,top,right,bottom,dst_left,dst_top,dst_right,dst_bottom;
  if (width <= height) {
   roundPx = width / 2 -5;
   top = 0;
   bottom = width;
   left = 0;
   right = width;
   height = width;
   dst_left = 0;
   dst_top = 0;
   dst_right = width;
   dst_bottom = width;
  } else {
   roundPx = height / 2 -5;
   float clip = (width - height) / 2;
   left = clip;
   right = width - clip;
   top = 0;
   bottom = height;
   width = height;
   dst_left = 0;
   dst_top = 0;
   dst_right = height;
   dst_bottom = height;
  }
 
  Bitmap output = Bitmap.createBitmap(width,
    height, Bitmap.Config.ARGB_8888);
  Canvas canvas = new Canvas(output);
 
  final int color = 0xff424242;
  final Paint paint = new Paint();
  final Rect src = new Rect((int)left, (int)top, (int)right, (int)bottom);
  final Rect dst = new Rect((int)dst_left, (int)dst_top, (int)dst_right, (int)dst_bottom);
  final RectF rectF = new RectF(dst_left+15, dst_top+15, dst_right-20, dst_bottom-20);
 
  paint.setAntiAlias(true);
 
  canvas.drawARGB(0, 0, 0, 0);
  paint.setColor(color);
 
  canvas.drawRoundRect(rectF, roundPx, roundPx, paint);
 
  paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC_IN));
  canvas.drawBitmap(bitmap, src, dst, paint);
  return output;
 }

Paint paint = new Paint();
   paint.setColor(0xffffffff);
   paint.setStrokeWidth(10);
   paint.setStyle(Paint.Style.STROKE);
 
   canvas.drawCircle(j/2,k/2,j/2-20,paint);