Merge pull request #2901 from uniquewings/knownmapper_race

Fix a reace condition caused by one thread calls getMapper method whi…

Author: hazendaz

src/main/java/org/apache/ibatis/binding/MapperRegistry.java

@@ -17,9 +17,9 @@
 
 import java.util.Collection;
 import java.util.Collections;
-import java.util.HashMap;
 import java.util.Map;
 import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
 
 import org.apache.ibatis.builder.annotation.MapperAnnotationBuilder;
 import org.apache.ibatis.io.ResolverUtil;
@@ -34,7 +34,7 @@
 public class MapperRegistry {
 
   private final Configuration config;
-  private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<>();
+  private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new ConcurrentHashMap<>();
 
   public MapperRegistry(Configuration config) {
     this.config = config;