Conditionally reset the transaction isolation level based on whether the user has altered it or not. Turns out it is expensive for some databases.

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

@@ -388,7 +388,7 @@ public final class HikariPool implements HikariPoolMBean
      * @param timeoutMs the timeout before we consider the test a failure
      * @return true if the connection is alive, false if it is not alive or we timed out
      */
-    private boolean isConnectionAlive(final Connection connection, long timeoutMs)
+    private boolean isConnectionAlive(final IHikariConnectionProxy connection, long timeoutMs)
     {
         if (timeoutMs < 1000)
         {
@@ -398,7 +398,10 @@ public final class HikariPool implements HikariPoolMBean
         try
         {
             connection.setAutoCommit(isAutoCommit);
-            connection.setTransactionIsolation(transactionIsolation);
+            if (connection.isTransactionIsolationDirty())
+            {
+                connection.setTransactionIsolation(transactionIsolation);
+            }
 
             try
             {
@@ -418,6 +421,8 @@ public final class HikariPool implements HikariPoolMBean
                 {
                     connection.commit();
                 }
+
+                connection.resetTransactionIsolationDirty();
             }
 
             return true;

src/main/java/com/zaxxer/hikari/proxy/ConnectionProxy.java

@@ -45,6 +45,7 @@ public abstract class ConnectionProxy implements IHikariConnectionProxy
 
     private boolean isClosed;
     private boolean forceClose;
+    private boolean isTransactionIsolationDirty;
     
     private final long creationTime;
     private volatile long lastAccess;
@@ -113,6 +114,16 @@ public abstract class ConnectionProxy implements IHikariConnectionProxy
         scheduler.schedule(leakTask, leakDetectionThreshold);
     }
 
+    public final boolean isTransactionIsolationDirty()
+    {
+        return isTransactionIsolationDirty;
+    }
+
+    public void resetTransactionIsolationDirty()
+    {
+        isTransactionIsolationDirty = false;
+    }
+
     public final boolean isBrokenConnection()
     {
         return forceClose;
@@ -399,6 +410,22 @@ public abstract class ConnectionProxy implements IHikariConnectionProxy
         }
     }
 
+    /** {@inheritDoc} */
+    public void setTransactionIsolation(int level) throws SQLException
+    {
+        checkClosed();
+        try
+        {
+            delegate.setTransactionIsolation(level);
+            isTransactionIsolationDirty = true;
+        }
+        catch (SQLException e)
+        {
+            checkException(e);
+            throw e;
+        }
+    }
+
     // **********************************************************************
     //                          Private Methods
     // **********************************************************************

src/main/java/com/zaxxer/hikari/proxy/IHikariConnectionProxy.java

@@ -36,6 +36,10 @@ public interface IHikariConnectionProxy extends Connection
 
     boolean isBrokenConnection();
 
+    boolean isTransactionIsolationDirty();
+
+    void resetTransactionIsolationDirty();
+
     long getCreationTime();
 
     long getLastAccess();