java.io.NotSerializableException: com.mysql.jdbc.DatabaseMetaData

Question

I am using JSF 1.2 and am trying to use <a4j:keepAlive beanName="reportController"> , but I keep on getting this error:

HTTP Status 500

Caused by: java.io.NotSerializableException: com.mysql.jdbc.DatabaseMetaData at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1183

I am trying to use <a4j:keepAlive beanName="reportController"> because when I search for a specific report and then try to sort the data in the dataTable, it seems that it loses all the data in the dataTable.

Answer 1

Caused by: java.io.NotSerializableException: com.mysql.jdbc.DatabaseMetaData

This will happen when you get hold of java.sql.Connection or even directly DatabaseMetaData as instance variable of a serializable class like below.

public class ReportController implements Serializable {

    private Connection connection; // BAD!!
    private DatabaseMetaData metadata; // BAD!!

    // ...
}

You're not supposed to declare and get hold of external resources such as java.sql.Connection , Statement and ResultSet nor its properties as instance variables of a class. You should acquire, use and close them as soon as possible, exclusively within the method local scope. Get rid of those instance variables from the ReportController bean, move them into method local scope and this problem shall disappear. Only having DataSource (the server-managed connection pool) as instance variable is OK.

public class ReportController implements Serializable {

    @Resource("jdbc/someDB")
    private DataSource dataSource;

    public void someMethod() {
        try (Connection connection = dataSource.getConnection()) { // OK.
            // ...
        }
    }

    // ...
}

The <a4j:keepAlive> isn't exactly the cause of this problem. It just remembers the bean instance in the HTTP session across HTTP postback requests on the same page. HTTP session attributes are inherently usually serialized. This serialization merely triggered and exposed your hidden design problem. Volatile one-time-use resources such as database connection, statement, metadata, inputstream, outputstream, etc are absolutely not supposed to be serializable and hence this exception.

See also:

• Is it safe to use a static java.sql.Connection instance in a multithreaded system?
• Returning a ResultSet