在本教程将演示如何在JDBC应用程序中,查询数据库的一个表中数据记录。 在执行以下示例之前,请确保您已经准备好以下操作:
- 具有数据库管理员权限,以在给定模式中数据库表中查询数据记录。 要执行以下示例,需要用实际用户名和密码替换这里用户名(
username
)和密码(password
)。 - MySQL或数据库已启动并运行。
所需步骤
使用JDBC应用程序向表中查询数据记录需要以下步骤:
导入包:需要包含包含数据库编程所需的JDBC类的包。 大多数情况下,使用
import java.sql.*
就足够了。注册JDBC驱动程序:需要初始化驱动程序,以便可以程序中打开数据库的通信通道。
打开连接:需要使用
DriverManager.getConnection()
方法来创建一个Connection
对象,它表示与数据库服务器的物理连接。执行查询:需要使用类型为
Statement
的对象来构建和提交SQL语句,以在选择的数据库的表中查询数据记录。提取数据:执行SQL查询后,可以从表中获取记录。
清理环境:需要明确地关闭所有数据库资源,而不依赖于JVM的垃圾收集。
示例代码
复制以下示例代码保存到文件:SelectRecords.java中,然后编译并运行如下 -
//STEP 1. Import required packages
import java.sql.*;
public class SelectRecords {
// JDBC driver name and database URL
static final String JDBC_DRIVER = "com.mysql.jdbc.Driver";
static final String DB_URL = "jdbc:mysql://localhost/jdbc_db";
// Database credentials
static final String USER = "root";
static final String PASS = "123456";
public static void main(String[] args) {
Connection conn = null;
Statement stmt = null;
try{
//STEP 2: Register JDBC driver
Class.forName("com.mysql.jdbc.Driver");
//STEP 3: Open a connection
System.out.println("Connecting to a selected database...");
conn = DriverManager.getConnection(DB_URL, USER, PASS);
System.out.println("Connected database successfully...");
//STEP 4: Execute a query
System.out.println("Creating statement...");
stmt = conn.createStatement();
String sql = "SELECT id, first, last, age FROM student";
ResultSet rs = stmt.executeQuery(sql);
//STEP 5: Extract data from result set
while(rs.next()){
//Retrieve by column name
int id = rs.getInt("id");
int age = rs.getInt("age");
String first = rs.getString("first");
String last = rs.getString("last");
//Display values
System.out.print("ID: " + id);
System.out.print(", Age: " + age);
System.out.print(", First: " + first);
System.out.println(", Last: " + last);
}
rs.close();
}catch(SQLException se){
//Handle errors for JDBC
se.printStackTrace();
}catch(Exception e){
//Handle errors for Class.forName
e.printStackTrace();
}finally{
//finally block used to close resources
try{
if(stmt!=null)
conn.close();
}catch(SQLException se){
}// do nothing
try{
if(conn!=null)
conn.close();
}catch(SQLException se){
se.printStackTrace();
}//end finally try
}//end try
System.out.println("Goodbye!");
}//end main
}//end JDBCExample
编译上面代码,如下 -
F:\worksp\jdbc> javac -Djava.ext.dirs=F:\worksp\jdbc\libs SelectRecords.java
执行上面代码,如下 -
## 可先创建表或插入数据 -
## F:\worksp\jdbc> java -Djava.ext.dirs=F:\worksp\jdbc\libs InsertRecords
F:\worksp\jdbc>java -Djava.ext.dirs=F:\worksp\jdbc\libs SelectRecords
Connecting to a selected database...
Thu Jun 01 23:11:55 CST 2017 WARN: Establishing SSL connection without server's identity verification is not recommended. According to MySQL 5.5.45+, 5.6.26+ and 5.7.6+ requirements SSL connection must be established by default if explicit option isn't set. For compliance with existing applications not using SSL the verifyServerCertificate property is set to 'false'. You need either to explicitly disable SSL by setting useSSL=false, or set useSSL=true and provide truststore for server certificate verification.
Connected database successfully...
Creating statement...
ID: 100, Age: 18, First: C++, Last: Li
ID: 101, Age: 25, First: Python, Last: Py
ID: 102, Age: 30, First: Ruby, Last: Ru
ID: 103, Age: 28, First: Java, Last: Ja
Goodbye!
F:\worksp\jdbc>