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lukaszlenart pushed a commit to branch WW-5539
in repository https://gitbox.apache.org/repos/asf/struts.git

commit 47ce7a8d3805054a1df64dab30feb28ffbc0aba5
Author: Lukasz Lenart <[email protected]>
AuthorDate: Tue Jul 21 16:57:05 2026 +0200

    WW-5539 Make StrutsTypeConverterHolder collections concurrent
    
    The holder is a container singleton whose HashMaps were read without any
    lock by XWorkConverter.lookup() while being written elsewhere, risking
    lost updates and torn reads during resize.
    
    Null TypeConverters are now ignored with a warning rather than stored,
    since ConcurrentHashMap forbids null values and a null converter left the
    holder in an inconsistent state.
---
 .../conversion/StrutsTypeConverterHolder.java      |  24 +++-
 .../conversion/StrutsTypeConverterHolderTest.java  | 133 +++++++++++++++++++++
 2 files changed, 151 insertions(+), 6 deletions(-)

diff --git 
a/core/src/main/java/org/apache/struts2/conversion/StrutsTypeConverterHolder.java
 
b/core/src/main/java/org/apache/struts2/conversion/StrutsTypeConverterHolder.java
index 3013c3bd4..09f5766bc 100644
--- 
a/core/src/main/java/org/apache/struts2/conversion/StrutsTypeConverterHolder.java
+++ 
b/core/src/main/java/org/apache/struts2/conversion/StrutsTypeConverterHolder.java
@@ -18,15 +18,20 @@
  */
 package org.apache.struts2.conversion;
 
-import java.util.HashMap;
-import java.util.HashSet;
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
+
 import java.util.Map;
+import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
 
 /**
  * Default implementation of {@link TypeConverterHolder}
  */
 public class StrutsTypeConverterHolder implements TypeConverterHolder {
 
+    private static final Logger LOG = 
LogManager.getLogger(StrutsTypeConverterHolder.class);
+
     /**
      * Record class and its type converter mapping.
      * <pre>
@@ -34,7 +39,7 @@ public class StrutsTypeConverterHolder implements 
TypeConverterHolder {
      * - TypeConverter - instance of TypeConverter
      * </pre>
      */
-    private final HashMap<String, TypeConverter> defaultMappings = new 
HashMap<>();  // non-action (eg. returned value)
+    private final Map<String, TypeConverter> defaultMappings = new 
ConcurrentHashMap<>();  // non-action (eg. returned value)
 
     /**
      * Target class conversion Mappings.
@@ -55,23 +60,30 @@ public class StrutsTypeConverterHolder implements 
TypeConverterHolder {
      *                    Element_property=foo.bar.MyObject
      * </pre>
      */
-    private final HashMap<Class, Map<String, Object>> mappings = new 
HashMap<>(); // action
+    private final Map<Class, Map<String, Object>> mappings = new 
ConcurrentHashMap<>(); // action
 
     /**
      * Unavailable target class conversion mappings, serves as a simple cache.
      */
-    private final HashSet<Class> noMapping = new HashSet<>(); // action
+    private final Set<Class> noMapping = ConcurrentHashMap.newKeySet(); // 
action
 
     /**
      * Record classes that doesn't have conversion mapping defined.
      * <pre>
      * - String -&gt; classname as String
      * </pre>
+     *
+     * @deprecated since 7.3.0, this field is an implementation detail and 
will be made private.
      */
-    protected HashSet<String> unknownMappings = new HashSet<>();     // 
non-action (eg. returned value)
+    @Deprecated
+    protected final Set<String> unknownMappings = 
ConcurrentHashMap.newKeySet();     // non-action (eg. returned value)
 
     @Override
     public void addDefaultMapping(String className, TypeConverter 
typeConverter) {
+        if (typeConverter == null) {
+            LOG.warn("Ignoring null TypeConverter registered for class [{}]", 
className);
+            return;
+        }
         defaultMappings.put(className, typeConverter);
         unknownMappings.remove(className);
     }
diff --git 
a/core/src/test/java/org/apache/struts2/conversion/StrutsTypeConverterHolderTest.java
 
b/core/src/test/java/org/apache/struts2/conversion/StrutsTypeConverterHolderTest.java
new file mode 100644
index 000000000..6fc2eac80
--- /dev/null
+++ 
b/core/src/test/java/org/apache/struts2/conversion/StrutsTypeConverterHolderTest.java
@@ -0,0 +1,133 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.struts2.conversion;
+
+import org.apache.struts2.XWorkTestCase;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+import java.util.concurrent.TimeUnit;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class StrutsTypeConverterHolderTest extends XWorkTestCase {
+
+    private static final int THREADS = 16;
+    private static final int PER_THREAD = 200;
+
+    private static TypeConverter stubConverter() {
+        return (context, target, member, propertyName, value, toType) -> null;
+    }
+
+    public void testConcurrentDefaultMappingRegistrationLosesNothing() throws 
Exception {
+        StrutsTypeConverterHolder holder = new StrutsTypeConverterHolder();
+        ExecutorService pool = Executors.newFixedThreadPool(THREADS);
+        CountDownLatch start = new CountDownLatch(1);
+        List<Future<?>> futures = new ArrayList<>();
+
+        for (int t = 0; t < THREADS; t++) {
+            final int threadId = t;
+            futures.add(pool.submit(() -> {
+                start.await();
+                for (int i = 0; i < PER_THREAD; i++) {
+                    String className = "stub.Class" + threadId + "_" + i;
+                    holder.addDefaultMapping(className, stubConverter());
+                    // interleave reads with writes to provoke the race
+                    holder.containsDefaultMapping("stub.Class0_0");
+                    holder.getDefaultMapping(className);
+                }
+                return null;
+            }));
+        }
+
+        start.countDown();
+        for (Future<?> future : futures) {
+            future.get(60, TimeUnit.SECONDS);
+        }
+        pool.shutdown();
+
+        for (int t = 0; t < THREADS; t++) {
+            for (int i = 0; i < PER_THREAD; i++) {
+                String className = "stub.Class" + t + "_" + i;
+                assertThat(holder.getDefaultMapping(className))
+                        .as("lost registration for %s", className)
+                        .isNotNull();
+            }
+        }
+    }
+
+    public void 
testConcurrentNoMappingAndUnknownMappingRegistrationLosesNothing() throws 
Exception {
+        StrutsTypeConverterHolder holder = new StrutsTypeConverterHolder();
+        ExecutorService pool = Executors.newFixedThreadPool(THREADS);
+        CountDownLatch start = new CountDownLatch(1);
+        List<Future<?>> futures = new ArrayList<>();
+
+        for (int t = 0; t < THREADS; t++) {
+            final int threadId = t;
+            futures.add(pool.submit(() -> {
+                start.await();
+                for (int i = 0; i < PER_THREAD; i++) {
+                    holder.addUnknownMapping("stub.Unknown" + threadId + "_" + 
i);
+                    holder.containsUnknownMapping("stub.Unknown0_0");
+                }
+                return null;
+            }));
+        }
+
+        start.countDown();
+        for (Future<?> future : futures) {
+            future.get(60, TimeUnit.SECONDS);
+        }
+        pool.shutdown();
+
+        for (int t = 0; t < THREADS; t++) {
+            for (int i = 0; i < PER_THREAD; i++) {
+                String className = "stub.Unknown" + t + "_" + i;
+                assertThat(holder.containsUnknownMapping(className))
+                        .as("lost unknown mapping for %s", className)
+                        .isTrue();
+            }
+        }
+    }
+
+    public void testAddDefaultMappingIgnoresNullConverter() {
+        StrutsTypeConverterHolder holder = new StrutsTypeConverterHolder();
+
+        holder.addDefaultMapping("stub.NullConverter", null);
+
+        
assertThat(holder.containsDefaultMapping("stub.NullConverter")).isFalse();
+        assertThat(holder.getDefaultMapping("stub.NullConverter")).isNull();
+    }
+
+    public void testAddDefaultMappingClearsUnknownMapping() {
+        StrutsTypeConverterHolder holder = new StrutsTypeConverterHolder();
+
+        holder.addUnknownMapping("stub.Later");
+        assertThat(holder.containsUnknownMapping("stub.Later")).isTrue();
+
+        holder.addDefaultMapping("stub.Later", stubConverter());
+
+        assertThat(holder.containsUnknownMapping("stub.Later")).isFalse();
+        assertThat(holder.getDefaultMapping("stub.Later")).isNotNull();
+    }
+}

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