<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
  android:shape="rectangle" >
  <gradient
    android:angle="270"
    android:endColor="#000000"
    android:startColor="#ffffff" />
  <corners
    android:bottomLeftRadius="5dip"
    android:bottomRightRadius="5dip"
    android:topLeftRadius="5dip"
    android:topRightRadius="5dip" />
</shape>

LinearGradient(float x0, float y0, float x1, float y1, int[] colors, float[] positions, Shader.TileMode tile)
LinearGradient(float x0, float y0, float x1, float y1, int color0, int color1, Shader.TileMode tile)

Paint p=new Paint();
LinearGradient lg=new LinearGradient(0,0,100,100,Color.RED,Color.BLUE,Shader.TileMode.MIRROR);

p.setShader(lg);
canvas.drawCicle(0,0,200,p); //参数3为画圆的半径，类型为float型。

RadialGradient(float x, float y, float radius, int[] colors, float[] positions, Shader.TileMode tile)
RadialGradient(float x, float y, float radius, int color0, int color1, Shader.TileMode tile)

SweepGradient(float cx, float cy, int[] colors, float[] positions)

SweepGradient(float cx, float cy, int color0, int color1)

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    requestWindowFeature(Window.FEATURE_NO_TITLE); //设置没标题
    getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN , //全屏
           WindowManager.LayoutParams.FLAG_FULLSCREEN);
    setContentView(R.layout.login);//登录界面
    GradientDrawable grad = new GradientDrawable(//渐变色
      Orientation.TOP_BOTTOM,
      new int[]{Color.BLACK, Color.WHITE}
    );
    getWindow().setBackgroundDrawable(grad);//设置渐变颜色
}