基础知识请看hongyang大神的http://blog.csdn.net/lmj623565791/article/details/45059587
一分钟添加只需三个步骤

1. 添加RecycleViewDivider类
2. 添加divider.xml
3. 主程序写一行代码

RecycleViewDivider类

public class RecycleViewDivider extends RecyclerView.ItemDecoration {
    private Drawable mDivider;
    private int mDividerHeight = 1;//分割线高度，默认为1px
    private static final int[] ATTRS = new int[]{android.R.attr.listDivider};

    /**
     * 自定义分割线
     *
     * @param context
     * @param dividerHeight 分割线高度
     * @param drawableId    分割线颜色
     */
    public RecycleViewDivider(Context context, int dividerHeight, int drawableId) {
        final TypedArray a = context.obtainStyledAttributes(ATTRS);
        mDivider = a.getDrawable(0);
        a.recycle();
        mDivider = ContextCompat.getDrawable(context, drawableId);
        mDividerHeight = dividerHeight;
    }

    //获取分割线尺寸
    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
        outRect.set(0, 0, 0, mDividerHeight);
    }
    //绘制分割线
    @Override
    public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
        drawDecoration(c, parent);
    }
    //绘制横向 item 分割线
    private void drawDecoration(Canvas canvas, RecyclerView parent) {
        final int left = parent.getPaddingLeft();
        final int right = parent.getWidth() - parent.getPaddingRight();
        final int childCount = parent.getChildCount();
        for (int i = 0; i < childCount; i++) {
            final View child = parent.getChildAt(i);
            final RecyclerView.LayoutParams layoutParams = (RecyclerView.LayoutParams) child.getLayoutParams();
            final int top = child.getBottom() + layoutParams.bottomMargin;
            final int bottom = top + mDividerHeight;
            if (mDivider != null) {
                mDivider.setBounds(left, top, right, bottom);
                mDivider.draw(canvas);
            }
        }
    }
}

divider.xml

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle">
    <gradient
        android:centerColor="#ffE6E6E6"
        android:endColor="#ffBDBDBD"
        android:startColor="#ffBDBDBD"
        android:type="linear" />
</shape>

主程序那行代码

mRecyclerView.addItemDecoration(new RecycleViewDivider(
                getActivity(),2,R.drawable.divider));