SOLR-13998: Add thread safety annotations to classes (#1053)

solr/CHANGES.txt

@@ -223,6 +223,9 @@ Other Changes
 
 * SOLR-13992: Code refactored to have collection name, slice name in Replica, Slice (noble)
 
+* SOLR-13998: Add thread safety annotations to Solr. This only introduces the annotations and doesn't add these to
+  existing classes. (Anshum Gupta, Mark Miller)
+
 ==================  8.3.1 ==================
 
 Consult the LUCENE_CHANGES.txt file for additional, low level, changes in this release.

solr/solrj/src/java/org/apache/solr/common/annotation/SolrSingleThreaded.java

@@ -0,0 +1,34 @@
+/*
+ * 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.solr.common.annotation;
+
+import static java.lang.annotation.ElementType.TYPE;
+import static java.lang.annotation.RetentionPolicy.SOURCE;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.Retention;
+import java.lang.annotation.Target;
+
+/**
+ * Annotation for classes in Solr that are not thread safe. This provides a clear indication of the thread safety of the class.
+ */
+@Documented
+@Retention(SOURCE)
+@Target(TYPE)
+public @interface SolrSingleThreaded {
+
+}

solr/solrj/src/java/org/apache/solr/common/annotation/SolrThreadSafe.java

@@ -0,0 +1,35 @@
+/*
+ * 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.solr.common.annotation;
+
+import static java.lang.annotation.ElementType.TYPE;
+import static java.lang.annotation.RetentionPolicy.SOURCE;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.Retention;
+import java.lang.annotation.Target;
+
+/**
+ * This annotation is applied to a class that is thread safe. This means that the objects of this class will never be
+ * in an inconsistent state, irrespective of interleaving accesses to the object.
+ */
+@Documented
+@Retention(SOURCE)
+@Target(TYPE)
+public @interface SolrThreadSafe {
+
+}