6.15 Client Session State Tracker

Connector/J can receive information on client session state changes tracked by the server if the tracking has been enabled on the server. The reception of the information is enabled by setting the Connector/J connection property trackSessionState to true (default value is false for the property).

When the function is enabled, information on session state changes received from the server are stored inside the SessionStateChanges object, accessible through a ServerSessionStateController and its getSessionStateChanges() method:

ServerSessionStateChanges ssc =
  MysqlConnection.getServerSessionStateController().getSessionStateChanges();

In SessionStateChanges is a list of SessoinStateChange objects, accessible by the getSessionStateChangesList() method:

List<SessionStateChange> sscList = ssc.getSessionStateChangesList();

Each SessionStateChange has the fields type and values, accessible by the getType() and getValues() methods. The types and their corresponding values are described below:

Table 6.23 SessionStateChange Type and Values

Type	Number of Values in the value List	Values
SESSION_TRACK_SYSTEM_VARIABLES	2	The name of the changed system variable and its new value
SESSION_TRACK_SCHEMA	1	The new schema name
SESSION_TRACK_STATE_CHANGE	1	"1" or "0"
SESSION_TRACK_GTIDS	1	List of GTIDs as reported by server
SESSION_TRACK_TRANSACTION_CHARACTERISTICS	1	Transaction characteristics statement
SESSION_TRACK_TRANSACTION_STATE	1	Transaction state record

Connector/J receives changes only from the most recent OK packet sent by the server. With getSessionStateChanges(), some changes returned by the intermediate queries issued by Connector/J could be missed. However, the session state change information can also be received using a SessionStateChangesListener, which has to be registered with a ServerSessionStateController using the addSessionStateChangesListener() method. The following example implements SessionStateChangesListener in a class, which also provides a method to print the change information:

class SSCListener implements SessionStateChangesListener {
        ServerSessionStateChanges changes = null;

        public void handleSessionStateChanges(ServerSessionStateChanges ch) {
            this.changes = ch;
            for (SessionStateChange change : ch.getSessionStateChangesList()) {
                printChange(change);
            }
        }

        private void printChange(SessionStateChange change) {
            System.out.print(change.getType() + " == > ");
            int pos = 0;
            if (change.getType() == ServerSessionStateController.SESSION_TRACK_SYSTEM_VARIABLES) { 
                // There are two values with this change type, the system variable name and its new value
                System.out.print(change.getValues().get(pos++) + "=");
            }
            System.out.println(change.getValues().get(pos));
        }
    }
SessionStateChangesListener listener = new SSCListener();
MysqlConnection.getServerSessionStateController().addSessionStateChangesListener(listener);

With a registered SessionStateChangesListener, users have access to all intermediate results, though the listener might slow down the delivery of query results. That is because the listener is invoked immediately after the OK packet is consumed by Connector/J, before the ResultSet is constructed.