MySQL Connector/J 6.0 Developer Guide

Pre-General Availability Draft: 2018-06-26

Chapter 14 Troubleshooting Connector/J Applications

This section explains the symptoms and resolutions for the most commonly encountered issues with applications using MySQL Connector/J.


Questions and Answers

14.1: When I try to connect to the database with MySQL Connector/J, I get the following exception:

SQLException: Server configuration denies access to data source
SQLState: 08001
VendorError: 0

What is going on? I can connect just fine with the MySQL command-line client.

MySQL Connector/J must use TCP/IP sockets to connect to MySQL, as Java does not support Unix Domain Sockets. Therefore, when MySQL Connector/J connects to MySQL, the security manager in MySQL server will use its grant tables to determine whether the connection is permitted.

You must add the necessary security credentials to the MySQL server for this to happen, using the GRANT statement to your MySQL Server. See GRANT Syntax, for more information.


Testing your connectivity with the mysql command-line client will not work unless you add the "host" flag, and use something other than localhost for the host. The mysql command-line client will use Unix domain sockets if you use the special host name localhost. If you are testing connectivity to localhost, use as the host name instead.


Changing privileges and permissions improperly in MySQL can potentially cause your server installation to not have optimal security properties.

14.2: My application throws an SQLException 'No Suitable Driver'. Why is this happening?

There are three possible causes for this error:

14.3: I'm trying to use MySQL Connector/J in an applet or application and I get an exception similar to:

SQLException: Cannot connect to MySQL server on host:3306.
Is there a MySQL server running on the machine/port you
are trying to connect to?

SQLState: 08S01
VendorError: 0

Either you're running an Applet, your MySQL server has been installed with the "skip-networking" option set, or your MySQL server has a firewall sitting in front of it.

Applets can only make network connections back to the machine that runs the web server that served the .class files for the applet. This means that MySQL must run on the same machine (or you must have some sort of port re-direction) for this to work. This also means that you will not be able to test applets from your local file system, you must always deploy them to a web server.

MySQL Connector/J can only communicate with MySQL using TCP/IP, as Java does not support Unix domain sockets. TCP/IP communication with MySQL might be affected if MySQL was started with the "skip-networking" flag, or if it is firewalled.

If MySQL has been started with the "skip-networking" option set (the Debian Linux package of MySQL server does this for example), you need to comment it out in the file /etc/mysql/my.cnf or /etc/my.cnf. Of course your my.cnf file might also exist in the data directory of your MySQL server, or anywhere else (depending on how MySQL was compiled for your system). Binaries created by us always look in /etc/my.cnf and datadir/my.cnf. If your MySQL server has been firewalled, you will need to have the firewall configured to allow TCP/IP connections from the host where your Java code is running to the MySQL server on the port that MySQL is listening to (by default, 3306).

14.4: I have a servlet/application that works fine for a day, and then stops working overnight

MySQL closes connections after 8 hours of inactivity. You either need to use a connection pool that handles stale connections or use the autoReconnect parameter (see Section 6.3, “Configuration Properties”).

Also, catch SQLExceptions in your application and deal with them, rather than propagating them all the way until your application exits. This is just good programming practice. MySQL Connector/J will set the SQLState (see java.sql.SQLException.getSQLState() in your API docs) to 08S01 when it encounters network-connectivity issues during the processing of a query. Attempt to reconnect to MySQL at this point.

The following (simplistic) example shows what code that can handle these exceptions might look like:

Example 14.1 Connector/J: Example of transaction with retry logic

public void doBusinessOp() throws SQLException {
    Connection conn = null;
    Statement stmt = null;
    ResultSet rs = null;

    // How many times do you want to retry the transaction
    // (or at least _getting_ a connection)?
    int retryCount = 5;

    boolean transactionCompleted = false;

    do {
        try {
            conn = getConnection(); // assume getting this from a
                                    // javax.sql.DataSource, or the
                                    // java.sql.DriverManager


            // Okay, at this point, the 'retry-ability' of the
            // transaction really depends on your application logic,
            // whether or not you're using autocommit (in this case
            // not), and whether you're using transactional storage
            // engines
            // For this example, we'll assume that it's _not_ safe
            // to retry the entire transaction, so we set retry
            // count to 0 at this point
            // If you were using exclusively transaction-safe tables,
            // or your application could recover from a connection going
            // bad in the middle of an operation, then you would not
            // touch 'retryCount' here, and just let the loop repeat
            // until retryCount == 0.
            retryCount = 0;

            stmt = conn.createStatement();

            String query = "SELECT foo FROM bar ORDER BY baz";

            rs = stmt.executeQuery(query);

            while ( {

            rs = null;

            stmt = null;

            conn = null;

            transactionCompleted = true;
        } catch (SQLException sqlEx) {

            // The two SQL states that are 'retry-able' are 08S01
            // for a communications error, and 40001 for deadlock.
            // Only retry if the error was due to a stale connection,
            // communications problem or deadlock

            String sqlState = sqlEx.getSQLState();

            if ("08S01".equals(sqlState) || "40001".equals(sqlState)) {
                retryCount -= 1;
            } else {
                retryCount = 0;
        } finally {
            if (rs != null) {
                try {
                } catch (SQLException sqlEx) {
                    // You'd probably want to log this...

            if (stmt != null) {
                try {
                } catch (SQLException sqlEx) {
                    // You'd probably want to log this as well...

            if (conn != null) {
                try {
                    // If we got here, and conn is not null, the
                    // transaction should be rolled back, as not
                    // all work has been done

                    try {
                    } finally {
                } catch (SQLException sqlEx) {
                    // If we got an exception here, something
                    // pretty serious is going on, so we better
                    // pass it up the stack, rather than just
                    // logging it...

                    throw sqlEx;
    } while (!transactionCompleted && (retryCount > 0));


Use of the autoReconnect option is not recommended because there is no safe method of reconnecting to the MySQL server without risking some corruption of the connection state or database state information. Instead, use a connection pool, which will enable your application to connect to the MySQL server using an available connection from the pool. The autoReconnect facility is deprecated, and may be removed in a future release.

14.5: I cannot connect to the MySQL server using Connector/J, and I'm sure the connection parameters are correct.

Make sure that the skip-networking option has not been enabled on your server. Connector/J must be able to communicate with your server over TCP/IP; named sockets are not supported. Also ensure that you are not filtering connections through a firewall or other network security system. For more information, see Can't connect to [local] MySQL server.

14.6: My application is deployed through JBoss and I am using transactions to handle the statements on the MySQL database. Under heavy loads, I am getting an error and stack trace, but these only occur after a fixed period of heavy activity.

This is a JBoss, not Connector/J, issue and is connected to the use of transactions. Under heavy loads the time taken for transactions to complete can increase, and the error is caused because you have exceeded the predefined timeout.

You can increase the timeout value by setting the TransactionTimeout attribute to the TransactionManagerService within the /conf/jboss-service.xml file (pre-4.0.3) or /deploy/jta-service.xml for JBoss 4.0.3 or later. See TransactionTimeout within the JBoss wiki for more information.

14.7: Updating a table that contains a primary key that is either FLOAT or compound primary key that uses FLOAT fails to update the table and raises an exception.

Connector/J adds conditions to the WHERE clause during an UPDATE to check the old values of the primary key. If there is no match, then Connector/J considers this a failure condition and raises an exception.

The problem is that rounding differences between supplied values and the values stored in the database may mean that the values never match, and hence the update fails. The issue will affect all queries, not just those from Connector/J.

To prevent this issue, use a primary key that does not use FLOAT. If you have to use a floating point column in your primary key, use DOUBLE or DECIMAL types in place of FLOAT.

14.8: I get an ER_NET_PACKET_TOO_LARGE exception, even though the binary blob size I want to insert using JDBC is safely below the max_allowed_packet size.

This is because the hexEscapeBlock() method in com.mysql.cj.jdbc.PreparedStatement.streamToBytes() may almost double the size of your data.

14.9: What should I do if I receive error messages similar to the following: Communications link failure – Last packet sent to the server was X ms ago?

Generally speaking, this error suggests that the network connection has been closed. There can be several root causes:

Although network connections can be volatile, the following can be helpful in avoiding problems:


Do not expect to be able to reuse a connection without problems if it has being lying idle for a period. If a connection is to be reused after being idle for any length of time, ensure that you explicitly test it before reusing it.

14.10: Why does Connector/J not reconnect to MySQL and re-issue the statement after a communication failure instead of throwing an Exception, even though I use the autoReconnect connection string option?

There are several reasons for this. The first is transactional integrity. The MySQL Reference Manual states that there is no safe method of reconnecting to the MySQL server without risking some corruption of the connection state or database state information. Consider the following series of statements for example:

  "UPDATE checking_account SET balance = balance - 1000.00 WHERE customer='Smith'");
  "UPDATE savings_account SET balance = balance + 1000.00 WHERE customer='Smith'");

Consider the case where the connection to the server fails after the UPDATE to checking_account. If no exception is thrown, and the application never learns about the problem, it will continue executing. However, the server did not commit the first transaction in this case, so that will get rolled back. But execution continues with the next transaction, and increases the savings_account balance by 1000. The application did not receive an exception, so it continued regardless, eventually committing the second transaction, as the commit only applies to the changes made in the new connection. Rather than a transfer taking place, a deposit was made in this example.

Note that running with autocommit enabled does not solve this problem. When Connector/J encounters a communication problem, there is no means to determine whether the server processed the currently executing statement or not. The following theoretical states are equally possible:

If you are running with autocommit enabled, it is not possible to guarantee the state of data on the server when a communication exception is encountered. The statement may have reached the server, or it may not. All you know is that communication failed at some point, before the client received confirmation (or data) from the server. This does not only affect autocommit statements though. If the communication problem occurred during Connection.commit(), the question arises of whether the transaction was committed on the server before the communication failed, or whether the server received the commit request at all.

The second reason for the generation of exceptions is that transaction-scoped contextual data may be vulnerable, for example:

These items are lost when a connection fails, and if the connection silently reconnects without generating an exception, this could be detrimental to the correct execution of your application.

In summary, communication errors generate conditions that may well be unsafe for Connector/J to simply ignore by silently reconnecting. It is necessary for the application to be notified. It is then for the application developer to decide how to proceed in the event of connection errors and failures.

14.11: How can I use 3-byte UTF8 with Connector/J?

To use 3-byte UTF8 with Connector/J set characterEncoding=utf8 and set useUnicode=true in the connection string.

14.12: How can I use 4-byte UTF8, utf8mb4 with Connector/J?

To use 4-byte UTF8 with Connector/J configure the MySQL server with character_set_server=utf8mb4. Connector/J will then use that setting as long as characterEncoding has not been set in the connection string. This is equivalent to autodetection of the character set.

14.13: Using useServerPrepStmts=false and certain character encodings can lead to corruption when inserting BLOBs. How can this be avoided?

When using certain character encodings, such as SJIS, CP932, and BIG5, it is possible that BLOB data contains characters that can be interpreted as control characters, for example, backslash, '\'. This can lead to corrupted data when inserting BLOBs into the database. There are two things that need to be done to avoid this:

  1. Set the connection string option useServerPrepStmts to true.