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diff --git a/java/com/google/gerrit/extensions/registration/DynamicMap.java b/java/com/google/gerrit/extensions/registration/DynamicMap.java
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+// Copyright (C) 2012 The Android Open Source Project
+//
+// Licensed 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 com.google.gerrit.extensions.registration;
+
+import com.google.inject.Binder;
+import com.google.inject.Key;
+import com.google.inject.Provider;
+import com.google.inject.ProvisionException;
+import com.google.inject.Scopes;
+import com.google.inject.TypeLiteral;
+import com.google.inject.util.Types;
+import java.util.Collections;
+import java.util.Iterator;
+import java.util.Map;
+import java.util.SortedMap;
+import java.util.SortedSet;
+import java.util.TreeMap;
+import java.util.TreeSet;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
+
+/**
+ * A map of members that can be modified as plugins reload.
+ *
+ * <p>Maps index their members by plugin name and export name.
+ *
+ * <p>DynamicMaps are always mapped as singletons in Guice. Maps store Providers internally, and
+ * resolve the provider to an instance on demand. This enables registrations to decide between
+ * singleton and non-singleton members.
+ */
+public abstract class DynamicMap<T> implements Iterable<Extension<T>> {
+  /**
+   * Declare a singleton {@code DynamicMap<T>} with a binder.
+   *
+   * <p>Maps must be defined in a Guice module before they can be bound:
+   *
+   * <pre>
+   * DynamicMap.mapOf(binder(), Interface.class);
+   * bind(Interface.class)
+   *   .annotatedWith(Exports.named(&quot;foo&quot;))
+   *   .to(Impl.class);
+   * </pre>
+   *
+   * @param binder a new binder created in the module.
+   * @param member type of value in the map.
+   */
+  public static <T> void mapOf(Binder binder, Class<T> member) {
+    mapOf(binder, TypeLiteral.get(member));
+  }
+
+  /**
+   * Declare a singleton {@code DynamicMap<T>} with a binder.
+   *
+   * <p>Maps must be defined in a Guice module before they can be bound:
+   *
+   * <pre>
+   * DynamicMap.mapOf(binder(), new TypeLiteral&lt;Thing&lt;Bar&gt;&gt;(){});
+   * bind(new TypeLiteral&lt;Thing&lt;Bar&gt;&gt;() {})
+   *   .annotatedWith(Exports.named(&quot;foo&quot;))
+   *   .to(Impl.class);
+   * </pre>
+   *
+   * @param binder a new binder created in the module.
+   * @param member type of value in the map.
+   */
+  public static <T> void mapOf(Binder binder, TypeLiteral<T> member) {
+    @SuppressWarnings("unchecked")
+    Key<DynamicMap<T>> key =
+        (Key<DynamicMap<T>>)
+            Key.get(Types.newParameterizedType(DynamicMap.class, member.getType()));
+    binder.bind(key).toProvider(new DynamicMapProvider<>(member)).in(Scopes.SINGLETON);
+  }
+
+  /** Returns an empty DynamicMap instance * */
+  public static <T> DynamicMap<T> emptyMap() {
+    return new PrivateInternals_DynamicMapImpl<>();
+  }
+
+  final ConcurrentMap<NamePair, Provider<T>> items;
+
+  DynamicMap() {
+    items =
+        new ConcurrentHashMap<>(
+            16 /* initial size */,
+            0.75f /* load factor */,
+            1 /* concurrency level of 1, load/unload is single threaded */);
+  }
+
+  /**
+   * Lookup an implementation by name.
+   *
+   * @param pluginName local name of the plugin providing the item.
+   * @param exportName name the plugin exports the item as.
+   * @return the implementation. Null if the plugin is not running, or if the plugin does not export
+   *     this name.
+   * @throws ProvisionException if the registered provider is unable to obtain an instance of the
+   *     requested implementation.
+   */
+  public T get(String pluginName, String exportName) throws ProvisionException {
+    Provider<T> p = items.get(new NamePair(pluginName, exportName));
+    return p != null ? p.get() : null;
+  }
+
+  /**
+   * Get the names of all running plugins supplying this type.
+   *
+   * @return sorted set of active plugins that supply at least one item.
+   */
+  public SortedSet<String> plugins() {
+    SortedSet<String> r = new TreeSet<>();
+    for (NamePair p : items.keySet()) {
+      r.add(p.pluginName);
+    }
+    return Collections.unmodifiableSortedSet(r);
+  }
+
+  /**
+   * Get the items exported by a single plugin.
+   *
+   * @param pluginName name of the plugin.
+   * @return items exported by a plugin, keyed by the export name.
+   */
+  public SortedMap<String, Provider<T>> byPlugin(String pluginName) {
+    SortedMap<String, Provider<T>> r = new TreeMap<>();
+    for (Map.Entry<NamePair, Provider<T>> e : items.entrySet()) {
+      if (e.getKey().pluginName.equals(pluginName)) {
+        r.put(e.getKey().exportName, e.getValue());
+      }
+    }
+    return Collections.unmodifiableSortedMap(r);
+  }
+
+  /** Iterate through all entries in an undefined order. */
+  @Override
+  public Iterator<Extension<T>> iterator() {
+    final Iterator<Map.Entry<NamePair, Provider<T>>> i = items.entrySet().iterator();
+    return new Iterator<Extension<T>>() {
+      @Override
+      public boolean hasNext() {
+        return i.hasNext();
+      }
+
+      @Override
+      public Extension<T> next() {
+        Map.Entry<NamePair, Provider<T>> e = i.next();
+        return new Extension<>(e.getKey().pluginName, e.getKey().exportName, e.getValue());
+      }
+
+      @Override
+      public void remove() {
+        throw new UnsupportedOperationException();
+      }
+    };
+  }
+
+  static class NamePair {
+    private final String pluginName;
+    private final String exportName;
+
+    NamePair(String pn, String en) {
+      pluginName = pn;
+      exportName = en;
+    }
+
+    @Override
+    public int hashCode() {
+      return pluginName.hashCode() * 31 + exportName.hashCode();
+    }
+
+    @Override
+    public boolean equals(Object other) {
+      if (other instanceof NamePair) {
+        NamePair np = (NamePair) other;
+        return pluginName.equals(np.pluginName) && exportName.equals(np.exportName);
+      }
+      return false;
+    }
+  }
+}