Storing Java objects in MySQL blobs
It's easy to store any serializable object in a blob in a MySQL database, if you use a PreparedStatement:
String query="replace into mysession (personid,data) values (?,?)";
PreparedStatement statement=conn.prepareStatement(query);
statement.setInt(1, ses.getPersonid());
statement.setObject(2, (Object)ses);
statement.execute();
When you try to retrieve the object, it gets work. It's supposed to work like this, using getObject():
query="select data from mysession where personid=? and time_to_sec(timediff(now(), datum)) < 600";
statement=conn.prepareStatement(query);
statement.setInt(1, personid);
resultSet = statement.executeQuery();
if (resultSet.next()) {
Object x=resultSet.getObject(1);
ses = (MySession)x;
} else {
However, the responsibility for deserializing the blob so that getObject() succeeds, lies with the database driver, and MySQL doesn't do this automatically.
You do get an Object back, and everything looks fine, but the class name of the object (read with o.getClass().toString()) is just garbage, containing stuff like "[B", and when you try to cast it to your own class you get a classCastException.
To make the MySQL driver deserialize blobs, you are supposed to add a parameter, "autoDeserialize=true", to your JDBC URL.
Unfortunately, the MySQL driver appears to exist in its own context, oblivious to any user-defined classes. Making your own classes globally available is usually not a good idea, so instead you can use an ObjectInputStream:
query="select data from mysession where personid=? and time_to_sec(timediff(now(), datum)) < 600";
statement=conn.prepareStatement(query);
statement.setInt(1, personid);
resultSet = statement.executeQuery();
if (resultSet.next()) {
InputStream is = resultSet.getBlob("data").getBinaryStream();
ObjectInputStream ois = new ObjectInputStream(is);
Object x = ois.readObject();
ses = (MySession)x;
} else {