Since this is a BST, the inorder traversal of its nodes results in a sorted list of values. Thus, the minimum absolute difference must occur in any adjacently traversed nodes. I use the global variable "prev" to keep track of each node's inorder predecessor.

```
public class Solution {
int minDiff = Integer.MAX_VALUE;
TreeNode prev;
public int getMinimumDifference(TreeNode root) {
inorder(root);
return minDiff;
}
public void inorder(TreeNode root) {
if (root == null) return;
inorder(root.left);
if (prev != null) minDiff = Math.min(minDiff, root.val - prev.val);
prev = root;
inorder(root.right);
}
}
```