public class DividerItemDecoration extends RecyclerView.ItemDecoration{
/*
* RecyclerView的布局方向，默认先赋值 为纵向布局
* RecyclerView 布局可横向，也可纵向
* 横向和纵向对应的分割线画法不一样
* */
private int mOrientation = LinearLayoutManager.VERTICAL;

private int mItemSize = 1;//item之间分割线的size，默认为1

private Paint mPaint;//绘制item分割线的画笔，和设置其属性

/**
 * 构造方法1 2 3个参数， 1 和2个参数的都是调用3个参数的构造方法，3个参数调用4个参数的构造方法
 *        构造方法4个参数直接写内容
 *
 * 构造方法1个参数(1.上下文)
 *        2个参数(1,上下文，2，绘制方向 )
 *        2个参数(1,上下文，2，绘制方向 3,分隔线颜色 颜色资源id)
 * @param context
 */

public DividerItemDecoration(Context context) {//构造方法非1个参数
    this(context,LinearLayoutManager.VERTICAL,R.color.colorAccent);
}

public DividerItemDecoration(Context context, int orientation) {//构造方法2个参数
    this(context,orientation, R.color.colorAccent);
}

public DividerItemDecoration(Context context, int orientation, int dividerColor){//构造方法3个参数
    this(context,orientation,dividerColor,20);//这里的最后一个参数填入1
}

/**
 * @param context
 * @param orientation 绘制方向
 * @param dividerColor 分割线颜色 颜色资源id
 * @param mItemSize 分割线宽度 传入dp值就行
 */
public DividerItemDecoration(Context context, int orientation, int dividerColor, int mItemSize){
    this.mOrientation = orientation;
    if(orientation != LinearLayoutManager.VERTICAL && orientation != LinearLayoutManager.HORIZONTAL){
        throw new IllegalArgumentException("请传入正确的参数") ;
    }
    //把dp值换算成px
    this.mItemSize = (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP,mItemSize,
            context.getResources().getDisplayMetrics());
    mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);

    mPaint.setColor(context.getResources().getColor(dividerColor));
}


@Override
public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
    if(mOrientation == LinearLayoutManager.VERTICAL){
        drawVertical(c,parent) ;//调用下方的绘制纵向的方法
    }else {
        drawHorizontal(c,parent) ;
    }
}

/**
 * 绘制纵向 item 分割线
 * @param canvas
 * @param parent
 */
private void drawVertical(Canvas canvas,RecyclerView parent){
    final int left = parent.getPaddingLeft() ;
    final int right = parent.getMeasuredWidth() - parent.getPaddingRight();
    final int childSize = parent.getChildCount();
    for(int i = 0 ; i < childSize ; i ++){
        final View child = parent.getChildAt( i ) ;
        RecyclerView.LayoutParams layoutParams = (RecyclerView.LayoutParams) child.getLayoutParams();
        final int top = child.getBottom() + layoutParams.bottomMargin ;
        final int bottom = top + mItemSize ;
        canvas.drawRect(left,top,right,bottom,mPaint);
    }
}

/**
 * 绘制横向 item 分割线
 * @param canvas
 * @param parent
 */
private void drawHorizontal(Canvas canvas,RecyclerView parent){
    final int top = parent.getPaddingTop() ;
    final int bottom = parent.getMeasuredHeight() - parent.getPaddingBottom() ;
    final int childSize = parent.getChildCount() ;
    for(int i = 0 ; i < childSize ; i ++){
        final View child = parent.getChildAt( i ) ;
        RecyclerView.LayoutParams layoutParams = (RecyclerView.LayoutParams) child.getLayoutParams();
        final int left = child.getRight() + layoutParams.rightMargin ;
        final int right = left + mItemSize ;
        canvas.drawRect(left,top,right,bottom,mPaint);
    }
}

/**
 * 设置item分割线的size
 * @param outRect
 * @param view
 * @param parent
 * @param state
 */
@Override
public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
    if(mOrientation == LinearLayoutManager.VERTICAL){
        outRect.set(0,0,0,mItemSize);//横向排列 底部偏移 （第4个参数）
    }else {
        outRect.set(0,0,mItemSize,0);//纵向排列 右边偏移 (在第三个参数)
    }
}

}

还需要在在RecyclerView 添加这么一行才行
recyclerView.addItemDecoration(new DividerItemDecoration(this));//添加分割线

运行效果 如下图所示

image.png