Add WeakHashMapConverter.

xstream-distribution/src/content/changes.html

@@ -108,6 +108,13 @@ <h1 id="upcoming-1.4.x">Upcoming 1.4.x maintenance release</h1>
 
 	<p>Not yet released.</p>
 
+	<h2>Major changes</h2>
+
+	<ul>
+		<li>Add a converter for the WeakHashMap which does not write any elements of the map.  Avoids also access to the
+		ReentrantLock contained in the WeakHashMap since Java 19.</li>
+	</ul>
+
 	<h2>Minor changes</h2>
 
 	<ul>
@@ -125,6 +132,13 @@ <h2>API changes</h2>
 	<ul>
 		<li>Added constant c.t.x.io.xml.PrettyPrintWriter.XML_1_0_REPLACEMENT.</li>
 		<li>Added constant c.t.x.io.xml.PrettyPrintWriter.XML_1_1_REPLACEMENT.</li>
+		<li>Added c.t.x.converters.collections.WeakHashMapConverter.</li>
+	</ul>
+
+	<h2>Stream compatibility</h2>
+
+	<ul>
+		<li>The WeakHashMaps, that have been written with previous versions of XStream, can still be deserialized.</li>
 	</ul>
 
 	<h1 id="1.4.20">1.4.20</h1>

xstream-distribution/src/content/converters.html

@@ -301,6 +301,17 @@
                 If the Properties instance includes a set of default Properties, these are serialized in a nested <code>&lt;defaults&gt;</code> element.</td>
                 <td>normal</td>
             </tr>
+            <tr>
+                <td><a href="javadoc/com/thoughtworks/xstream/converters/collections/WeakHashMapConverter.html">WeakHashMapConverter</a></td>
+                <td>java.util.WeakHashMap</td>
+                <td class="example">
+                    &lt;weak-hash-map/&gt;
+                </td>
+                <td>A WeakHashMap is supposed to release its elements when they are no longer referenced. Therefore is at unmarshalling time no guarantee that
+                an entry is still available when it is referenced later in the stream.  As consequence the converter will marshal no elements at all, it
+                will create an empty WeakHashMap at unmarshalling time.</td>
+                <td>normal</td>
+            </tr>
             <tr>
                 <td><a href="javadoc/com/thoughtworks/xstream/converters/collections/SingletonCollectionConverter.html">SingletonCollectionConverter</a></td>
                 <td>java.util.Collections.singletonList().getClass()<br/>java.util.Collections.singleton().getClass()</td>

xstream/src/java/com/thoughtworks/xstream/XStream.java

@@ -1,6 +1,6 @@
 /*
  * Copyright (C) 2003, 2004, 2005, 2006 Joe Walnes.
- * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021, 2022 XStream Committers.
+ * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021, 2022, 2024 XStream Committers.
  * All rights reserved.
  *
  * The software in this package is published under the terms of the BSD
@@ -104,6 +104,7 @@
 import java.util.TreeSet;
 import java.util.UUID;
 import java.util.Vector;
+import java.util.WeakHashMap;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicInteger;
@@ -146,6 +147,7 @@
 import com.thoughtworks.xstream.converters.collections.SingletonMapConverter;
 import com.thoughtworks.xstream.converters.collections.TreeMapConverter;
 import com.thoughtworks.xstream.converters.collections.TreeSetConverter;
+import com.thoughtworks.xstream.converters.collections.WeakHashMapConverter;
 import com.thoughtworks.xstream.converters.enums.EnumConverter;
 import com.thoughtworks.xstream.converters.enums.EnumMapConverter;
 import com.thoughtworks.xstream.converters.enums.EnumSetConverter;
@@ -843,6 +845,7 @@ protected void setupAliases() {
         alias("hashtable", Hashtable.class);
         alias("linked-hash-map", LinkedHashMap.class);
         alias("linked-hash-set", LinkedHashSet.class);
+        alias("weak-hash-map", WeakHashMap.class);
         alias("concurrent-hash-map", ConcurrentHashMap.class);
         alias("atomic-boolean", AtomicBoolean.class);
         alias("atomic-int", AtomicInteger.class);
@@ -1001,6 +1004,7 @@ protected void setupConverters() {
         registerConverter(new EnumConverter(), PRIORITY_NORMAL);
         registerConverter(new EnumSetConverter(mapper), PRIORITY_NORMAL);
         registerConverter(new EnumMapConverter(mapper), PRIORITY_NORMAL);
+        registerConverter(new WeakHashMapConverter(), PRIORITY_NORMAL);
 
         registerConverter(new FileConverter(), PRIORITY_NORMAL);
         if (JVM.isSQLAvailable()) {

xstream/src/java/com/thoughtworks/xstream/converters/collections/WeakHashMapConverter.java

@@ -0,0 +1,49 @@
+/*
+ * Copyright (C) 2024 XStream Committers.
+ * All rights reserved.
+ *
+ * The software in this package is published under the terms of the BSD
+ * style license a copy of which has been included with this distribution in
+ * the LICENSE.txt file.
+ *
+ * Created on 19. October 2024 by Joerg Schaible
+ */
+package com.thoughtworks.xstream.converters.collections;
+
+import java.util.WeakHashMap;
+
+import com.thoughtworks.xstream.converters.Converter;
+import com.thoughtworks.xstream.converters.MarshallingContext;
+import com.thoughtworks.xstream.converters.UnmarshallingContext;
+import com.thoughtworks.xstream.io.HierarchicalStreamReader;
+import com.thoughtworks.xstream.io.HierarchicalStreamWriter;
+
+
+/**
+ * Converts a WeakHashMap. A WeakHashMap is supposed to release its elements when they are no longer referenced.
+ * Therefore is at unmarshalling time no guarantee that an entry is still available when it is referenced later in the
+ * stream. As consequence the converter will marshal no elements at all, it will create an empty WeakHashMap at
+ * unmarshalling time.
+ *
+ * @author Joerg Schaible
+ * @since upcoming
+ */
+public class WeakHashMapConverter implements Converter {
+
+    @Override
+    public boolean canConvert(final Class<?> type) {
+        return WeakHashMap.class == type;
+    }
+
+    @Override
+    public void marshal(final Object source, final HierarchicalStreamWriter writer, final MarshallingContext context) {
+        // do nothing
+    }
+
+    @Override
+    public WeakHashMap<?, ?> unmarshal(final HierarchicalStreamReader reader, final UnmarshallingContext context) {
+        @SuppressWarnings("rawtypes")
+        final WeakHashMap<?, ?> weakHashMap = new WeakHashMap();
+        return weakHashMap;
+    }
+}

xstream/src/test/com/thoughtworks/acceptance/MapTest.java

@@ -1,6 +1,6 @@
 /*
  * Copyright (C) 2003, 2004, 2005 Joe Walnes.
- * Copyright (C) 2006, 2007, 2011, 2017, 2018, 2021 XStream Committers.
+ * Copyright (C) 2006, 2007, 2011, 2017, 2018, 2021, 2024 XStream Committers.
  * All rights reserved.
  *
  * The software in this package is published under the terms of the BSD
@@ -18,6 +18,7 @@
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.WeakHashMap;
 
 import com.thoughtworks.acceptance.objects.Hardware;
 import com.thoughtworks.acceptance.objects.Software;
@@ -109,6 +110,15 @@ public void testSupportsOldHashtables() {
         assertBothWays(hashtable, expected);
     }
 
+    public void testSupportsWeakHashMap() {
+        final WeakHashMap<String, String> hashtable = new WeakHashMap<>();
+        hashtable.put("hello", "world");
+
+        final String expected = "<weak-hash-map/>";
+
+        assertBothWays(hashtable, expected);
+    }
+
     static class ThingWithDifferentTypesOfMaps extends StandardObject {
         private static final long serialVersionUID = 200403L;
         final Map<?, ?> m1 = new HashMap<>();