mysql空指针异常处理

try {
    Connection conn = DriverManager.getConnection(url, username, password);
    if (conn != null) {
        // 执行数据库操作
    }
} catch (SQLException e) {
    e.printStackTrace();
} finally {
    if (conn != null) {
        try {
            conn.close();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

try {
    Statement stmt = conn.createStatement();
    ResultSet rs = stmt.executeQuery("SELECT * FROM table");
    if (rs != null) {
        while (rs.next()) {
            // 处理结果集
        }
    }
} catch (SQLException e) {
    e.printStackTrace();
} finally {
    if (rs != null) {
        try {
            rs.close();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
    if (stmt != null) {
        try {
            stmt.close();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

CallableStatement cstmt = conn.prepareCall("{call procedure_name(?, ?)}");
if (param1 != null) {
    cstmt.setObject(1, param1);
}
if (param2 != null) {
    cstmt.setObject(2, param2);
}
ResultSet rs = cstmt.executeQuery();