Fix #47: MergedNsContext.getPrefixes() returned null

release-notes/VERSION

@@ -14,6 +14,8 @@ Project: Stax2 API
  (contributed by @winfriedgerlach)
 #45: Fix swapped arguments in `AttributeEventImpl.writeUsing()`
  (contributed by @winfriedgerlach)
+#47: `MergedNsContext.getPrefixes()` returns null instead of result
+ (contributed by @winfriedgerlach)
 - Branch "master" renamed as "main"
 
 4.3.0 (31-Mar-2026)

src/main/java/org/codehaus/stax2/ri/evt/MergedNsContext.java

@@ -6,6 +6,8 @@
 import javax.xml.namespace.NamespaceContext;
 import javax.xml.stream.events.Namespace;
 
+import org.codehaus.stax2.ri.EmptyIterator;
+
 /**
  * Helper class used to combine an enclosing namespace context with
  * a list of namespace declarations contained, to result in a single
@@ -129,8 +131,7 @@ public Iterator<String> getPrefixes(String nsURI)
 
         // Any local bindings?
         ArrayList<String> l = null;
-        for (int i = 0, len = _namespaces.size(); i < len; ++i) {
-            Namespace ns = _namespaces.get(i);
+        for (Namespace ns : _namespaces) {
             if (nsURI.equals(ns.getNamespaceURI())) {
                 l = addToList(l, ns.getPrefix());
             }
@@ -144,21 +145,22 @@ public Iterator<String> getPrefixes(String nsURI)
                 // But is it masked?
                 String uri2 = getNamespaceURI(p2);
                 if (uri2.equals(nsURI)) {
-                    // No masking, we are good:
-                    l = addToList(l, p2);
+                    // No masking, we are good (but may already have it, if re-bound locally)
+                    l = addIfMissing(l, p2);
                 }
             }
         }
 
         // Ok, but how about pre-defined ones (for xml, xmlns)?
+        // (parent context may have already reported these)
         if (nsURI.equals(XMLConstants.XML_NS_URI)) {
-            l = addToList(l, XMLConstants.XML_NS_PREFIX);
+            l = addIfMissing(l, XMLConstants.XML_NS_PREFIX);
         }
         if (nsURI.equals(XMLConstants.XMLNS_ATTRIBUTE_NS_URI)) {
-            l = addToList(l, XMLConstants.XMLNS_ATTRIBUTE);
+            l = addIfMissing(l, XMLConstants.XMLNS_ATTRIBUTE);
         }
 
-        return null;
+        return l == null ? EmptyIterator.getInstance() : l.iterator();
     }
 
     /*
@@ -175,4 +177,12 @@ protected <T> ArrayList<T> addToList(ArrayList<T> l, T value)
         l.add(value);
         return l;
     }
+
+    private <T> ArrayList<T> addIfMissing(ArrayList<T> l, T value)
+    {
+        if (l != null && l.contains(value)) {
+            return l;
+        }
+        return addToList(l, value);
+    }
 }

src/test/java/org/codehaus/stax2/ri/evt/MergedNsContextTest.java

@@ -0,0 +1,59 @@
+package org.codehaus.stax2.ri.evt;
+
+import java.util.*;
+
+import javax.xml.XMLConstants;
+import javax.xml.stream.events.Namespace;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+/**
+ * Tests for {@link MergedNsContext}, mostly regarding
+ * [stax2-api#47]: {@code getPrefixes()} returned null.
+ */
+public class MergedNsContextTest
+{
+    @Test
+    public void testGetPrefixes()
+    {
+        List<Namespace> ns = new ArrayList<>();
+        ns.add(NamespaceEventImpl.constructNamespace(null, "a", "urn:test"));
+        ns.add(NamespaceEventImpl.constructNamespace(null, "b", "urn:test"));
+        ns.add(NamespaceEventImpl.constructNamespace(null, "c", "urn:other"));
+        MergedNsContext ctxt = MergedNsContext.construct(null, ns);
+
+        assertEquals(Arrays.asList("a", "b"), toList(ctxt.getPrefixes("urn:test")));
+        assertEquals(Arrays.asList("c"), toList(ctxt.getPrefixes("urn:other")));
+        // No match: must get empty iterator, not null
+        Iterator<String> it = ctxt.getPrefixes("urn:missing");
+        assertNotNull(it);
+        assertFalse(it.hasNext());
+    }
+
+    @Test
+    public void testGetPrefixesNoDuplicates()
+    {
+        MergedNsContext parent = MergedNsContext.construct(null,
+                Arrays.<Namespace>asList(NamespaceEventImpl.constructNamespace(null, "p", "urn:test")));
+        // Same binding re-declared locally
+        MergedNsContext ctxt = MergedNsContext.construct(parent,
+                Arrays.<Namespace>asList(NamespaceEventImpl.constructNamespace(null, "p", "urn:test")));
+
+        assertEquals(Arrays.asList("p"), toList(ctxt.getPrefixes("urn:test")));
+        // Parent also reports pre-defined "xml" prefix
+        assertEquals(Arrays.asList("xml"),
+                toList(ctxt.getPrefixes(XMLConstants.XML_NS_URI)));
+    }
+
+    private static List<String> toList(Iterator<String> it)
+    {
+        assertNotNull(it);
+        List<String> result = new ArrayList<>();
+        while (it.hasNext()) {
+            result.add(it.next());
+        }
+        return result;
+    }
+}