stmt = cont.prepareStatement(sql);
时间: 2024-04-03 15:36:04 浏览: 18
这段代码是在Java中创建一个`PreparedStatement`对象的语句。`PreparedStatement`是一种预编译的SQL语句,它可以提高SQL语句的执行效率,同时也可以避免SQL注入攻击。这个语句中,`cont`是一个`Connection`对象,表示与数据库的连接。`prepareStatement()`方法接受一个SQL语句作为参数,并将其编译为一个可执行的指令序列,同时返回一个`PreparedStatement`对象。这个对象可以接受参数,用于填充SQL语句中的占位符。例如,可以通过调用`setString()`、`setInt()`等方法来设置字符串、整数等类型的参数值。在设置完参数之后,可以通过调用`executeQuery()`、`executeUpdate()`等方法来执行SQL语句并获取结果。最后,建议在使用完`PreparedStatement`对象后及时关闭它,以释放占用的资源。
相关问题
String sql = "SELECT * FROM articles WHERE title LIKE ?"; PreparedStatement stmt = conn.prepareStatement(sql); stmt.setString(1, "%" + keyword + "%"); ResultSet rs = stmt.executeQuery();使用这段代码进行模糊查找时无法查找中文
可能是因为数据库的字符集不支持中文,可以尝试修改数据库的字符集为 utf8mb4。同时,在代码中也需要指定字符集为 utf8mb4,可以在连接数据库时加上以下语句:?useUnicode=true&characterEncoding=utf8mb4。修改后的代码如下:
String sql = "SELECT * FROM articles WHERE title LIKE ?";
PreparedStatement stmt = conn.prepareStatement(sql);
stmt.setString(1, "%" + keyword + "%");
ResultSet rs = stmt.executeQuery();
rs.close();
stmt.close();
conn.close();
Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/dbname?useUnicode=true&characterEncoding=utf8mb4", "username", "password");
优化这段代码CREATE TABLE user ( id INT PRIMARY KEY AUTO_INCREMENT, username VARCHAR(50) NOT NULL, password VARCHAR(50) NOT NULL); import java.sql.*; public class UserDAO { private final String url = "jdbc:mysql://localhost:3306/mydb"; private final String user = "username"; private final String password = "password"; public boolean register(String username, String password) { try (Connection conn = DriverManager.getConnection(url, user, password); PreparedStatement stmt = conn.prepareStatement( "INSERT INTO user (username, password) VALUES (?, ?)")) { stmt.setString(1, username); stmt.setString(2, password); int rows = stmt.executeUpdate(); return rows > 0; } catch (SQLException e) { e.printStackTrace(); return false; } } public boolean login(String username, String password) { try (Connection conn = DriverManager.getConnection(url, user, password); PreparedStatement stmt = conn.prepareStatement( "SELECT * FROM user WHERE username = ? AND password = ?")) { stmt.setString(1, username); stmt.setString(2, password); ResultSet rs = stmt.executeQuery(); return rs.next(); } catch (SQLException e) { e.printStackTrace(); return false; } } }
首先,建议将数据库连接池引入代码中,以避免每次调用数据库时都要重新连接。这将大大减少连接和关闭数据库的开销。
其次,建议使用预编译语句,以避免每次执行 SQL 语句时都要重新编译。这也将大大减少编译和执行 SQL 语句的开销。
最后,可以在数据库中添加索引以加快查询速度。在这个例子中,可以在 username 和 password 字段上创建索引。
下面是经过优化的代码:
import java.sql.*;
import javax.sql.DataSource;
import org.apache.commons.dbcp2.BasicDataSource;
public class UserDAO {
private DataSource dataSource;
public UserDAO() {
BasicDataSource ds = new BasicDataSource();
ds.setUrl("jdbc:mysql://localhost:3306/mydb");
ds.setUsername("username");
ds.setPassword("password");
ds.setInitialSize(5);
dataSource = ds;
}
public boolean register(String username, String password) {
try (Connection conn = dataSource.getConnection();
PreparedStatement stmt = conn.prepareStatement(
"INSERT INTO user (username, password) VALUES (?, ?)")) {
stmt.setString(1, username);
stmt.setString(2, password);
int rows = stmt.executeUpdate();
return rows > 0;
} catch (SQLException e) {
e.printStackTrace();
return false;
}
}
public boolean login(String username, String password) {
try (Connection conn = dataSource.getConnection();
PreparedStatement stmt = conn.prepareStatement(
"SELECT * FROM user WHERE username = ? AND password = ?")) {
stmt.setString(1, username);
stmt.setString(2, password);
ResultSet rs = stmt.executeQuery();
return rs.next();
} catch (SQLException e) {
e.printStackTrace();
return false;
}
}
}