使用方法：

添加默认分割线：高度为2px，颜色为灰色

mRecyclerView.addItemDecoration(new RecycleViewDivider(mContext, LinearLayoutManager.VERTICAL));

添加自定义分割线：可自定义分割线drawable

mRecyclerView.addItemDecoration(new RecycleViewDivider(
mContext, LinearLayoutManager.VERTICAL, R.drawable.divider_mileage));

添加自定义分割线：可自定义分割线高度和颜色

mRecyclerView.addItemDecoration(new RecycleViewDivider(
mContext, LinearLayoutManager.VERTICAL, 10, getResources().getColor(R.color.divide_gray_color)));

万能分割线登场：

public class RecycleViewDivider extends RecyclerView.ItemDecoration {

private Paint mPaint;
private Drawable mDivider;
private int mDividerHeight = 2;//分割线高度，默认为1px
private int mOrientation;//列表的方向：LinearLayoutManager.VERTICAL或LinearLayoutManager.HORIZONTAL
private static final int[] ATTRS = new int[]{android.R.attr.listDivider};

/**
 * 默认分割线：高度为2px，颜色为灰色
 *
 * @param context
 * @param orientation 列表方向
 */
public RecycleViewDivider(Context context, int orientation) {
    if (orientation != LinearLayoutManager.VERTICAL && orientation != LinearLayoutManager.HORIZONTAL) {
        throw new IllegalArgumentException("请输入正确的参数！");
    }
    mOrientation = orientation;

    final TypedArray a = context.obtainStyledAttributes(ATTRS);
    mDivider = a.getDrawable(0);
    a.recycle();
}

/**
 * 自定义分割线
 *
 * @param context
 * @param orientation 列表方向
 * @param drawableId  分割线图片
 */
public RecycleViewDivider(Context context, int orientation, int drawableId) {
    this(context, orientation);
    mDivider = ContextCompat.getDrawable(context, drawableId);
    mDividerHeight = mDivider.getIntrinsicHeight();
}

/**
 * 自定义分割线
 *
 * @param context
 * @param orientation   列表方向
 * @param dividerHeight 分割线高度
 * @param dividerColor  分割线颜色
 */
public RecycleViewDivider(Context context, int orientation, int dividerHeight, int dividerColor) {
    this(context, orientation);
    mDividerHeight = dividerHeight;
    mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
    mPaint.setColor(dividerColor);
    mPaint.setStyle(Paint.Style.FILL);
}


//获取分割线尺寸
@Override
public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
    super.getItemOffsets(outRect, view, parent, state);
    outRect.set(0, 0, 0, mDividerHeight);
}

//绘制分割线
@Override
public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
    super.onDraw(c, parent, state);
    if (mOrientation == LinearLayoutManager.VERTICAL) {
        drawVertical(c, parent);
    } else {
        drawHorizontal(c, parent);
    }
}

//绘制横向 item 分割线
private void drawHorizontal(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 + mDividerHeight;
        if (mDivider != null) {
            mDivider.setBounds(left, top, right, bottom);
            mDivider.draw(canvas);
        }
        if (mPaint != null) {
            canvas.drawRect(left, top, right, bottom, mPaint);
        }
    }
}

//绘制纵向 item 分割线
private void drawVertical(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 + mDividerHeight;
        if (mDivider != null) {
            mDivider.setBounds(left, top, right, bottom);
            mDivider.draw(canvas);
        }
        if (mPaint != null) {
            canvas.drawRect(left, top, right, bottom, mPaint);
        }
    }
}
}

附：自定的drawable文件一份

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
android:shape="rectangle">
<size android:height="20dp" />
<solid android:color="#ff992900" />
</shape>

参考：

RecyclerView的万能分割线