con = GetDBConnection.connectDB(DBName, userName, password); ResultSet rs; try { PreparedStatement ps = con.prepareStatement("select * from books;"); rs = ps.executeQuery(); while (rs.next()) { int bookid = rs.getInt("bookid"); String bookname = rs.getString("bookname"); String bookauthor = rs.getString("bookauthor"); System.out.printf("%d\t", bookid); System.out.printf("%s\t", bookname); System.out.printf("%s\n", bookauthor); } con.close(); } catch (SQLException e) { System.out.println(e.getMessage()); }
时间: 2024-04-02 10:31:28 浏览: 49
这段代码是 Java 语言中的数据库操作代码,其作用是从指定的数据库中读取名为 `books` 的数据表中的所有数据,然后输出到控制台上。
具体来说,`con = GetDBConnection.connectDB(DBName, userName, password);` 这行代码是通过调用 `GetDBConnection` 类中的 `connectDB` 方法连接到指定的数据库,其中 `DBName`、`userName` 和 `password` 分别是数据库的名称、用户名和密码。
接着,使用 `PreparedStatement` 对象执行 SQL 查询语句 `select * from books;`,并将查询结果存储在 `ResultSet` 对象 `rs` 中。然后使用 `while (rs.next())` 循环遍历查询结果集,将每一行数据中的 `bookid`、`bookname` 和 `bookauthor` 字段值分别存储在变量 `bookid`、`bookname` 和 `bookauthor` 中,然后使用 `System.out.printf()` 方法将它们输出到控制台上。
最后,在 `try` 块的末尾关闭数据库连接。
如果在执行过程中出现了 SQL 异常,比如语法错误或数据访问错误,将会在控制台上输出异常信息。
相关问题
public static void add() { con = GetDBConnection.connectDB(DBName, userName, password); }利用Java和这个框架写一个增加图书
假设你已经有一个 Book 类,包含属性如下:
```
public class Book {
private int id;
private String title;
private String author;
private String publisher;
private int year;
private double price;
// getters and setters
}
```
那么你可以按照如下的步骤来写一个增加图书的函数:
1. 在数据库中创建一个表,用于存储图书信息。假设表名为 `books`,包含如下字段:
```
id int(11) NOT NULL AUTO_INCREMENT,
title varchar(255) NOT NULL,
author varchar(255) NOT NULL,
publisher varchar(255) NOT NULL,
year int(4) NOT NULL,
price double NOT NULL,
PRIMARY KEY (id)
```
2. 在 GetDBConnection 类中编写一个静态方法来获取数据库连接,例如:
```
public static Connection connectDB(String dbName, String userName, String password) throws SQLException {
String url = "jdbc:mysql://localhost/" + dbName;
Connection con = DriverManager.getConnection(url, userName, password);
return con;
}
```
3. 编写一个静态方法来添加图书信息,例如:
```
public static void addBook(Book book) throws SQLException {
con = GetDBConnection.connectDB(DBName, userName, password);
String sql = "INSERT INTO books (title, author, publisher, year, price) VALUES (?, ?, ?, ?, ?)";
PreparedStatement ps = con.prepareStatement(sql);
ps.setString(1, book.getTitle());
ps.setString(2, book.getAuthor());
ps.setString(3, book.getPublisher());
ps.setInt(4, book.getYear());
ps.setDouble(5, book.getPrice());
ps.executeUpdate();
ps.close();
con.close();
}
```
在这个方法中,我们首先获取数据库连接,然后使用 PrepareStatement 对象来执行 SQL 语句。我们将 Book 对象的属性作为参数设置到 PreparedStatement 对象中,然后执行 `executeUpdate()` 方法来执行 SQL 语句,将图书信息添加到数据库中。最后,记得关闭 PrepareStatement 和 Connection 对象。
4. 在主程序中调用 addBook() 方法,例如:
```
public static void main(String[] args) {
Book book = new Book();
book.setTitle("Java Programming");
book.setAuthor("John Smith");
book.setPublisher("Wiley");
book.setYear(2021);
book.setPrice(59.99);
try {
GetDBConnection.addBook(book);
System.out.println("Book added successfully!");
} catch (SQLException e) {
e.printStackTrace();
}
}
```
这段代码创建一个 Book 对象,设置属性,然后调用 addBook() 方法将图书信息添加到数据库中。如果添加成功,程序会输出 "Book added successfully!"。如果添加失败,程序会打印出 SQLException 的堆栈信息。
c# code:public class APPServerContext : DbContext { public readonly string DBAias; public readonly string UserID; public APPServerContext(string dbaias, string userid); public bool IsSqlServer { get; } public DbSet<sysBatchCodeNO> SYS_BATCHCODENO { get; set; } public DbSet<sysListCodeNO> SYS_LISTCODENO { get; set; } public DbSet<SYS_ORGSTRUCTURE> SYS_ORGSTRUCTURE { get; set; } public DbSet<sysPrgWorkbench> SYS_PRGWORKBENCH { get; set; } public DbSet<sysBarCodeNO> SYS_BARCODENO { get; set; } public DbSet<SYS_LOGINUSER> SYS_LOGINUSERS { get; set; } public bool IsMySql { get; } public DbSet<SYS_ENANNOU> SYS_ENANNOUS { get; set; } public bool IsOracle { get; } public DbParameter CreateParameter(string Key, object Value); public override void Dispose(); public string GetDBParName(string parameterName); public Hashtable PBValue(string KeyList); protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder); } using APPServerContext context1 = new APPServerContext(LoginInfo.DBAias, LoginInfo.UserID);await context1.Database.BeginTransactionAsync().ConfigureAwait(false); context1.Database.SqlQuery(exeSqlList[i]); error : Execution Timeout Expired. The timeout period elapsed prior to completion of the operation or the server is not responding. question :APPServerContext does not contain CommandTimeOut. How do I handle this error
You can set the Command Timeout value in the DbContext's configuration options using the "CommandTimeout" property. You can do this either in the OnConfiguring method or when creating a new instance of DbContextOptionsBuilder. Here's an example of setting the Command Timeout value to 60 seconds:
```
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
optionsBuilder.UseSqlServer(connectionString)
.ConfigureWarnings(warnings => warnings.Throw(RelationalEventId.QueryClientEvaluationWarning))
.CommandTimeout(60);
}
```
Alternatively, you can also set the Command Timeout value when executing the SQL query using the "CommandTimeout" property of the DbCommand object. Here's an example:
```
using APPServerContext context1 = new APPServerContext(LoginInfo.DBAias, LoginInfo.UserID);
await context1.Database.BeginTransactionAsync().ConfigureAwait(false);
var command = context1.Database.GetDbConnection().CreateCommand();
command.CommandText = exeSqlList[i];
command.CommandTimeout = 60;
context1.Database.OpenConnection();
var result = await command.ExecuteNonQueryAsync().ConfigureAwait(false);
```
This will set the Command Timeout value to 60 seconds for the current SQL query execution.
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