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DatabasePeer.javaAPI DocExample7351Fri Jul 18 21:05:32 BST 1997imaginary.persist

DatabasePeer

public abstract class DatabasePeer extends Object implements PersistentPeer
The DatabasePeer class implements the persistence methods from PersistentPeer and performs the actual database access for individual persistent objects. Of course, at this level it is impossible to know everything about the application in order to isolate database access completely. Applications requiring database persistence should extend this class and implement the abstract methods that allow an application to formulate the SQL.

Fields Summary
Constructors Summary
Methods Summary
protected abstract java.lang.StringgetInsertSql(Persistent p)
Subclasses must implement this method to build an INSERT statement for the specified persistent object.

param
p the Persistent that is being inserted into the database
return
the SQL string to be sent to the database

protected abstract java.lang.StringgetRemoveSql(Persistent p)
Subclasses must implement this method to build a DELETE
statement for the specified persistent object.
param
p the Persistent that is being deleted from the database
return
the SQL DELETE statement
protected abstract java.lang.StringgetRestoreSql(Persistent p)
Subclasses must implement this method to build a SELECT
statement that will restore only the named persistent object.
At the time it gets passed to this method, the persistent
object has only its ID set. All other values are awaiting
a restore operation.
param
p the Persistent that is being restored from the database
return
the SQL SELECT statement
protected abstract java.lang.StringgetUpdateSql(Persistent p)
Subclasses must implement this method to build an UPDATE
statement that will update the specified persistent object.
param
p the Persistent object to be updated
return
the SQL UPDATE statement
public voidinsert(Persistent p, Transaction t)
This method implements the insert persistence operation
as prescribed by the PersistentPeer interface. It asks
its subclasses for the proper SQL and then triggers
a save using that SQL statement.
param
p the persistent object to be inserted
param
t the DatabaseTransaction to use for that insert
exception
imaginary.persist.PersistenceException An error occurred
in saving the object to the database.
        save((DatabaseTransaction)t, getInsertSql(p));
    
public voidremove(Persistent p, Transaction t)
This method implements the remove persistence operation
as prescribed by the PersistentPeer interface. It asks
its subclasses for the proper SQL and then triggers
a save using that SQL statement.
param
p the persistent object to be removed
param
t the DatabaseTransaction to use for that removal
exception
imaginary.persist.PersistenceException An error occurred
in removing the object from the database.
        save((DatabaseTransaction)t, getRemoveSql(p));
    
public voidrestore(Persistent p, Transaction trans)
This method implements the restore persistence operation
as prescribed by the PersistentPeer interface. It asks
its subclasses for the proper SQL and then triggers
a save using that SQL statement.
param
p the persistent object to be restored
param
trans the DatabaseTransaction to use for restoring the object
exception
imaginary.persist.PersistenceException An error occurred
in restoring the object from the database.
         String sql = getRestoreSql(p); // get the restore SQL
         Connection connection = null;
         Statement statement = null;
         ResultSet results = null;
         ResultSetMetaData meta = null;
        
         try {
             Hashtable h = new Hashtable(); // store the values here

             // Get the connection from the DatabaseTransaction
             connection = ((DatabaseTransaction)trans).getConnection();
             // Create a JDBC statement from the connection
             statement = connection.createStatement();
             // Execute the SQL
             results = statement.executeQuery(sql);
             // Get the meta data
             meta = results.getMetaData();
             // Make sure we got a row!
             if( !results.next() ) {
                 throw new PersistenceException("No rows found!");
             }
             // For each column in the result set
             for(int i=1; i<=meta.getColumnCount(); i++) {
                 Object ob = results.getObject(i);
                 // Put the value in the Hashtable using the column name
                 // as a key
                 h.put(meta.getColumnLabel(i), ob);
             }
             // Call the restore from Hashtable method in the Persistent
             p.restore(trans, h);
         }
         catch( SQLException e ) {
             if( statement != null ) { // This is not required
                 try { statement.close(); }
                 catch( SQLException e2 ) { }
             }
             throw new PersistenceException(e);
         }
    
private voidsave(DatabaseTransaction trans, java.lang.String sql)
        Connection connection = null;
        Statement statement = null;

        try {
            connection = trans.getConnection(); // Get the JDBC connection
            statement = connection.createStatement(); // Create a statement
            statement.executeUpdate(sql); // Execute the SQL
            statement.close(); // Close the statement (not required)
        }
        catch( SQLException e ) {
            if( statement != null ) { // not required
                try { statement.close(); }
                catch( SQLException e2) { }
            }
            throw new PersistenceException(e);
        }
    
public voidupdate(Persistent p, Transaction t)
This method implements the update persistence operation
as prescribed by the PersistentPeer interface. It asks
its subclasses for the proper SQL and then triggers
a save using that SQL statement.
param
p the persistent object to be updated
param
t the DatabaseTransaction to use for updating the object
exception
imaginary.persist.PersistenceException An error occurred
in updating the object in the database.
        save((DatabaseTransaction)t, getUpdateSql(p));