Go ahead and implement double-checked locking, this is a low traffic method.

hikaricp-java6/src/main/java/com/zaxxer/hikari/util/PoolUtilities.java

@@ -233,7 +233,12 @@ public final class PoolUtilities
     */
    public static boolean isJdbc40Compliant(final Connection connection) throws SQLException
    {
-      if (!jdbc40checked) {
+      // See http://en.wikipedia.org/wiki/Double-checked_locking#Usage_in_Java
+      boolean checked = jdbc40checked;
+      if (!checked) {
+         synchronized (PoolUtilities.class) {
+            checked = jdbc40checked;
+            if (!checked) {
                try {
                   connection.isValid(5);  // This will throw AbstractMethodError or SQLException in the case of a non-JDBC 41 compliant driver
                   IS_JDBC40 = true;
@@ -248,7 +253,9 @@ public final class PoolUtilities
                   IS_JDBC40 = false;
                }
                finally {
-            jdbc40checked = true;
+                  jdbc40checked = checked = true;
+               }
+            }
          }
       }
       

hikaricp/src/main/java/com/zaxxer/hikari/util/PoolUtilities.java

@@ -215,7 +215,12 @@ public final class PoolUtilities
     */
    public static boolean isJdbc40Compliant(final Connection connection) throws SQLException
    {
-      if (!jdbc40checked) {
+      // See http://en.wikipedia.org/wiki/Double-checked_locking#Usage_in_Java
+      boolean checked = jdbc40checked;
+      if (!checked) {
+         synchronized (PoolUtilities.class) {
+            checked = jdbc40checked;
+            if (!checked) {
                try {
                   connection.isValid(5);  // This will throw AbstractMethodError or SQLException in the case of a non-JDBC 41 compliant driver
                   IS_JDBC40 = true;
@@ -224,7 +229,9 @@ public final class PoolUtilities
                   IS_JDBC40 = false;
                }
                finally {
-            jdbc40checked = true;
+                  jdbc40checked = checked = true;
+               }
+            }
          }
       }