Fixes #1425: prevent StrinIndexOutOfBoundsException from JsonPointer.head()

release-notes/VERSION-2.x

@@ -14,6 +14,12 @@ a pure JSON library.
 === Releases ===
 ------------------------------------------------------------------------
 
+2.19.1 (not yet relesed)
+
+#1425: `JsonPointer.head()` throws `StringIndexOutOfBoundsException` for
+  valid JSON Pointers
+ (reported by @sergeykad)
+
 2.19.0 (24-Apr-2025)
 
 #633: Allow skipping `RS` CTRL-CHAR to support JSON Text Sequences

src/main/java/com/fasterxml/jackson/core/JsonPointer.java

@@ -169,7 +169,15 @@ protected JsonPointer(String fullString, int fullStringOffset,
         _matchingElementIndex = matchIndex;
     }
 
-    // @since 2.19
+    /**
+     * Copy-constructor used for creating transformed instances with
+     * re-linking textual contents to new "next" pointer instance.
+     *
+     * @param src Original pointer to copy "full String" from
+     * @param next New "next" pointed to link to
+     *
+     * @since 2.19
+     */
     protected JsonPointer(JsonPointer src, JsonPointer next) {
         _asString = src._asString;
         _asStringOffset = src._asStringOffset;
@@ -178,7 +186,16 @@ protected JsonPointer(JsonPointer src, JsonPointer next) {
         _matchingElementIndex = src._matchingElementIndex;
     }
 
-    // @since 2.19
+    /**
+     * Copy-constructor used for creating transformed instances without
+     * "next" linkage
+     *
+     * @param src Original pointer to copy "matchingXxx" fields from
+     * @param newFullString Full String to use
+     * @param newFullStringOffset Offset for new full String to use
+     *
+     * @since 2.19
+     */
     protected JsonPointer(JsonPointer src, String newFullString, int newFullStringOffset) {
         _asString = newFullString;
         _asStringOffset = newFullStringOffset;
@@ -841,22 +858,22 @@ protected JsonPointer _constructHead()
         if (last == this) {
             return EMPTY;
         }
-        // and from that, length of suffix to drop
-        final int suffixLength = last.length();
 
         // Initialize a list to store intermediate JsonPointers in reverse
         ArrayList<JsonPointer> pointers = new ArrayList<>();
 
         JsonPointer current = this;
-        String fullString = toString();
+        String origFullString = toString();
         // Make sure to share the new full string for path segments
-        fullString = fullString.substring(0, fullString.length() - suffixLength);
-        int offset = 0;
+        String fullString = origFullString.substring(0, origFullString.length() - last.length());
+
+        // Also: if there was an offset, must compensate (new String starts at 0)
+        final int offsetDiff = -_asStringOffset;
 
         while (current != last) {
+            // NOTE: since we drop from the end we can simply reuse offset (w/ possible modification)
             JsonPointer nextSegment = new JsonPointer(current,
-                    fullString, offset);
-            offset += suffixLength;
+                    fullString, current._asStringOffset + offsetDiff);
             pointers.add(nextSegment);
             current = current._nextSegment;
         }

src/test/java/com/fasterxml/jackson/core/jsonptr/JsonPointer1425Test.java

@@ -0,0 +1,41 @@
+package com.fasterxml.jackson.core.jsonptr;
+
+import org.junit.jupiter.api.Test;
+
+import com.fasterxml.jackson.core.JUnit5TestBase;
+import com.fasterxml.jackson.core.JsonPointer;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class JsonPointer1425Test extends JUnit5TestBase
+{
+
+    // [core#1425]
+    @Test
+    public void test1425Basic() {
+        JsonPointer ptr = JsonPointer.compile("/a/b/0/qwerty");
+        JsonPointer head = ptr.head();
+        assertEquals("/a/b/0", head.toString());
+        head = head.head(); // Exception happens here
+        assertEquals("/a/b", head.toString());
+        head = head.head();
+        assertEquals("/a", head.toString());
+        head = head.head();
+        assertEquals("", head.toString());
+    }
+
+    // [core#1425]
+    @Test
+    public void test1425Variations() {
+        JsonPointer ptr = JsonPointer.compile("/a/b/0/qwerty");
+        JsonPointer tail = ptr.tail();
+
+        assertEquals("/b/0/qwerty", tail.toString());
+        JsonPointer head = tail.head();
+        assertEquals("/b/0", head.toString());
+        head = head.head();
+        assertEquals("/b", head.toString());
+        head = head.head();
+        assertEquals("", head.toString());
+    }
+}