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

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

@@ -233,22 +233,29 @@ public final class PoolUtilities
     */
    public static boolean isJdbc40Compliant(final Connection connection) throws SQLException
    {
-      if (!jdbc40checked) {
-         try {
-            connection.isValid(5);  // This will throw AbstractMethodError or SQLException in the case of a non-JDBC 41 compliant driver
-            IS_JDBC40 = true;
-         }
-         catch (SQLFeatureNotSupportedException e) {
-            IS_JDBC40 = false;
-         }
-         catch (AbstractMethodError e) {
-            IS_JDBC40 = false;
-         }
-         catch (NoSuchMethodError e) {
-            IS_JDBC40 = false;
-         }
-         finally {
-            jdbc40checked = true;
+      // 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;
+               }
+               catch (SQLFeatureNotSupportedException e) {
+                  IS_JDBC40 = false;
+               }
+               catch (AbstractMethodError e) {
+                  IS_JDBC40 = false;
+               }
+               catch (NoSuchMethodError e) {
+                  IS_JDBC40 = false;
+               }
+               finally {
+                  jdbc40checked = checked = true;
+               }
+            }
          }
       }
       

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

@@ -215,16 +215,23 @@ public final class PoolUtilities
     */
    public static boolean isJdbc40Compliant(final Connection connection) throws SQLException
    {
-      if (!jdbc40checked) {
-         try {
-            connection.isValid(5);  // This will throw AbstractMethodError or SQLException in the case of a non-JDBC 41 compliant driver
-            IS_JDBC40 = true;
-         }
-         catch (NoSuchMethodError | AbstractMethodError | SQLFeatureNotSupportedException e) {
-            IS_JDBC40 = false;
-         }
-         finally {
-            jdbc40checked = true;
+      // 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;
+               }
+               catch (NoSuchMethodError | AbstractMethodError | SQLFeatureNotSupportedException e) {
+                  IS_JDBC40 = false;
+               }
+               finally {
+                  jdbc40checked = checked = true;
+               }
+            }
          }
       }