Defensive concurrent access to key set from java.util.Properties

Closes gh-23063

spring-core/src/main/java/org/springframework/core/env/PropertiesPropertySource.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2014 the original author or authors.
+ * Copyright 2002-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -35,7 +35,7 @@ import java.util.Properties;
  */
 public class PropertiesPropertySource extends MapPropertySource {
 
-	@SuppressWarnings({"unchecked", "rawtypes"})
+	@SuppressWarnings({"rawtypes", "unchecked"})
 	public PropertiesPropertySource(String name, Properties source) {
 		super(name, (Map) source);
 	}
@@ -44,4 +44,12 @@ public class PropertiesPropertySource extends MapPropertySource {
 		super(name, source);
 	}
 
+
+	@Override
+	public String[] getPropertyNames() {
+		synchronized (this.source) {
+			return super.getPropertyNames();
+		}
+	}
+
 }