Re: Storing/retrieving shared objects in a database

On 01/07/2012 20:26, Peter wrote:
> Hi guys,
>
> Basically, as the title says, I would like to store a shared object in a MySQL database
> and retrieve it at a later time. So I wrote a simple shared class Test:
>
> public class Test implements Serializable {
> private int statusId;
> public Test() {
> }
> public Test(int statusId) {
> this.statusId = statusId;
> }
> }
>
> Now I have an rpc that stores an instance of the Test class into a single-column table
> (the MySQL type is BLOB). This is the rpc implementation:
>
> public void storeTest(Test test) {
> try {
> Class.forName("com.mysql.jdbc.Driver");
> Connection conn = DriverManager.getConnection(MySQLConnection.DB_URL,
> MySQLConnection.DB_USER, MySQLConnection.DB_PWD);
> PreparedStatement statement = conn.prepareStatement("insert into test_table
> (test_object) values (?)");
> statement.setObject(1, test);
> statement.executeUpdate();
> } catch (SQLException e) {
> } catch (ClassNotFoundException e) {
> }
> }
>
> For retrieval, I have another rpc as follows (server side code again):
>
> public Test getTest() {
> Test t = new Test();
> try {
> Class.forName("com.mysql.jdbc.Driver");
> Connection conn = DriverManager.getConnection(MySQLConnection.DB_URL,
> MySQLConnection.DB_USER, MySQLConnection.DB_PWD);
> PreparedStatement statement = conn.prepareStatement("select test_object from
> test_table");
> ResultSet resultSet = statement.executeQuery();
> resultSet.next();
> t = (Test) resultSet.getObject(1);
> } catch (SQLException e) {
> } catch (ClassNotFoundException e) {
> }
> return t;
> }
>
> The storeTest(Test test) method runs fine (at least I think it does) - no errors are
> returned and the record is inserted into the MySQL table. However, the method getTest()
> fails during the conversion *t = (Test) resultSet.getObject(1)*. Now I've done some
> digging and apparently the object returned by resultSet.getObject(1) is an instance of a
> class called *[B*, which seems to be a JNI byte array which can't be converted into Test.
>
> It is unclear to me why this happens. I would expect that the BLOB column contains all the
> necessary info to rebuild the Test object. I'm pretty sure I'm missing something
> fundamental here, but can't quite figure it out.

I have no experience with storing object data in a blob of a SQL base. Maybe it acts like
serialization (I see you made your class serializable).

The first Google hit I got uses a slightly different, more indirect method:
http://www.easywayserver.com/blog/how-to-serializable-object-in-java-2/

The second hit explains why your method doesn't work, and why ObjectInputStream is
necessary: http://hans.liss.pp.se/node/700

--
Philippe Lhoste
-- (near) Paris -- France
-- http://Phi.Lho.free.fr
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