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A library for relational-database-based persistence in Java.
There is a newer version: 1.7.5
//
// Depot library - a Java relational persistence library
// https://github.com/threerings/depot/blob/master/LICENSE

package com.samskivert.depot.impl;

import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

import com.samskivert.depot.annotation.GeneratedValue;
import com.samskivert.depot.impl.jdbc.DatabaseLiaison;

import static com.samskivert.depot.Log.log;

/**
 * Defines the interface to our value generators.
 */
public abstract class ValueGenerator
{
    public ValueGenerator (GeneratedValue gv, DepotMarshaller dm, FieldMarshaller fm)
    {
        _allocationSize = gv.allocationSize();
        _initialValue = gv.initialValue();
        _migrateIfExists = gv.migrateIfExists();
        _dm = dm;
        _fm = fm;
    }

    /**
     * If true, this key generator will be run after the insert statement, if false, it will be run
     * before.
     */
    public abstract boolean isPostFactum ();

    /**
     * Ensures the generator is prepared for operation, creating it if necessary. Care is taken to
     * only run this the first time a column is created. However, if a column with a value
     * generator is renamed, we can't distinguish that from a newly created column and we call this
     * method again on the renamed column. If it's possible to not fail in that circumstance, try
     * to avoid doing so.
     */
    public abstract void create (Connection conn, DatabaseLiaison liaison)
        throws SQLException;

    /**
     * Fetch/generate the next primary key value.
     *
     * @param stmt if post-factum, the statement that was used to perform the insert which resulted
     * in the auto-generation of a value, otherwise null.
     */
    public abstract int nextGeneratedValue (Connection conn, DatabaseLiaison liaison, Statement stmt)
        throws SQLException;

    /**
     * Delete all database entities associated with this value generator.
     */
    public abstract void delete (Connection conn, DatabaseLiaison liaison)
        throws SQLException;

    /**
     * Scans the table associated with this {@link ValueGenerator} and returns either null, if
     * there are no rows, or an {@link Integer} containing the largest numerical value our
     * field attains.
     */
    protected Integer getFieldMaximum (Connection conn, DatabaseLiaison liaison)
        throws SQLException
    {
        String column = _fm.getColumnName();
        String table = _dm.getTableName();

        Statement stmt = conn.createStatement();
        ResultSet rs = stmt.executeQuery(
            " SELECT COUNT(*), MAX(" + liaison.columnSQL(column) + ") " +
            "   FROM " + liaison.tableSQL(table));
        if (!rs.next()) {
            log.warning("Query on count()/max() bizarrely returned no rows.");
            return null;
        }

        int cnt = rs.getInt(1);
        if (cnt > 0) {
            return Integer.valueOf(rs.getInt(2));
        }
        return null;
    }

    public DepotMarshaller getDepotMarshaller ()
    {
        return _dm;
    }

    public FieldMarshaller getFieldMarshaller ()
    {
        return _fm;
    }

    protected int _initialValue;
    protected int _allocationSize;
    protected boolean _migrateIfExists;

    protected DepotMarshaller _dm;
    protected FieldMarshaller _fm;
}