added synchronized to getObjectInstance()

src/main/java/com/zaxxer/hikari/HikariJNDIFactory.java

@@ -40,7 +40,7 @@ import com.zaxxer.hikari.util.PropertyElf;
 public class HikariJNDIFactory implements ObjectFactory
 {
    @Override
-   public Object getObjectInstance(Object obj, Name name, Context nameCtx, Hashtable<?, ?> environment) throws Exception
+   synchronized public Object getObjectInstance(Object obj, Name name, Context nameCtx, Hashtable<?, ?> environment) throws Exception
    {
       // We only know how to deal with <code>javax.naming.Reference</code> that specify a class name of "javax.sql.DataSource"
       if (!(obj instanceof Reference)) {