import java.sql.*;
/**類JDBCAdapter用于建立數(shù)據(jù)庫連接*/
class JDBCAdapter{
    /*聲明一個(gè)Connection對(duì)象,用于和數(shù)據(jù)庫建立連接*/
    Connection          connection;
    /*聲明一個(gè)Statement對(duì)象,用于對(duì)數(shù)據(jù)庫執(zhí)行SQL語句*/
    Statement           statement;

     /**下面是構(gòu)造方法,四個(gè)參數(shù)分別代表要連接的數(shù)據(jù)庫,驅(qū)動(dòng)程序,用戶名和密碼*/
    public JDBCAdapter(String url, String driverName,
                       String user, String passwd) {
        try {
        	/*返回driverName字符串對(duì)應(yīng)的類對(duì)象*/
            Class.forName(driverName);
            /*輸出"Opening db connection"提示信息*/
            System.out.println("Opening db connection");
			/*下面的語句進(jìn)行數(shù)據(jù)庫的連接*/
            connection = DriverManager.getConnection(url, user, passwd);
            /*創(chuàng)建SQL語句*/
            statement = connection.createStatement();
        }
        catch (ClassNotFoundException ex) {
            /*輸出沒有找到數(shù)據(jù)庫驅(qū)動(dòng)程序類的信息*/
            System.err.println("Cannot find the database driver classes.");
            System.err.println(ex);
        }
        catch (SQLException ex) {
        	/*輸出不能連接到數(shù)據(jù)庫的信息*/
            System.err.println("Cannot connect to database.");
            System.err.println(ex);
        }
     }
    /**方法executeUpdate用于對(duì)數(shù)據(jù)庫進(jìn)行更新，參數(shù)query指定更新操作的SQL語句*/
    public void executeUpdate(String query) {
    	/*if語句中的條件表示數(shù)據(jù)庫連接不成功或者沒有創(chuàng)建SQL語句*/
        if (connection == null || statement == null) {
            System.err.println("There is no database to execute the query.");
            return;
        }
        try {
        	/*執(zhí)行SQL語句,對(duì)數(shù)據(jù)庫進(jìn)行更新*/
            statement.executeUpdate(query);
        }
        catch (SQLException ex) {
            System.err.println(ex);
        }
    }
    /**方法executeQuery用于對(duì)數(shù)據(jù)庫進(jìn)行查詢，參數(shù)query指定查詢操作的SQL語句*/
    public void executeQuery(String query) {
    	/*if語句中的條件表示數(shù)據(jù)庫連接不成功或者沒有創(chuàng)建SQL語句*/
        if (connection == null || statement == null) {
            System.err.println("There is no database to execute the query.");
            return;
        }
        try {
        	/*執(zhí)行SQL語句,對(duì)數(shù)據(jù)庫進(jìn)行更新*/
            statement.executeQuery(query);
        }
        catch (SQLException ex) {
            System.err.println(ex);
        }
    }
    /**將前面生成的SQL語句對(duì)象和數(shù)據(jù)庫連接對(duì)象關(guān)閉*/
    public void close() throws SQLException {
        System.out.println("Closing db connection");
        statement.close();
        connection.close();
    }
    /*通過finalize()執(zhí)行上面的close()方法*/
    protected void finalize() throws Throwable {
        close();
        super.finalize();
    }
}