SimpleAliasRegistry fully synchronizes registerAlias and removeAlias

Issue: SPR-16577

spring-core/src/main/java/org/springframework/core/SimpleAliasRegistry.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -45,6 +45,7 @@ public class SimpleAliasRegistry implements AliasRegistry {
 	public void registerAlias(String name, String alias) {
 		Assert.hasText(name, "'name' must not be empty");
 		Assert.hasText(alias, "'alias' must not be empty");
+		synchronized (this.aliasMap) {
 			if (alias.equals(name)) {
 				this.aliasMap.remove(alias);
 			}
@@ -64,6 +65,7 @@ public class SimpleAliasRegistry implements AliasRegistry {
 				this.aliasMap.put(alias, name);
 			}
 		}
+	}
 
 	/**
 	 * Return whether alias overriding is allowed.
@@ -92,11 +94,13 @@ public class SimpleAliasRegistry implements AliasRegistry {
 
 	@Override
 	public void removeAlias(String alias) {
+		synchronized (this.aliasMap) {
 			String name = this.aliasMap.remove(alias);
 			if (name == null) {
 				throw new IllegalStateException("No alias '" + alias + "' registered");
 			}
 		}
+	}
 
 	@Override
 	public boolean isAlias(String name) {