1. jdbc connects to the database
1.1. Steps
-
Load the database driver
- mysql driver: com.mysql.jdbc.Driver
- oracle驱动：oracle.jdbc.driver.OracleDriver
Class.forName("com.mysql.jdbc.Driver") -
get connection
- mysql: jdbc: mysql://mysql host IP: port number (usually 3306) / database name? userUnicode=true&characterEncoding=utf8&useSSL=false
- oracle: jdbc:oracle:thin:@host IP:port number (usually 1521):database name
DriverManager.getConnection(url, user, password) -
execute sql
Use precompiled PreparedStatement to prevent sql injection problems
-
get result set
-
close connection
Don't forget to close the connection
1.2. Example code
package org.jdbc.util;
import java.io.IOException;
import java.io.InputStream;
import java.sql.*;
import java.util.Properties;
public class JdbcUtil {
private static String driver = null;
private static String url = null;
private static String username = null;
private static String password = null;
static {
try {
InputStream in = JdbcUtil.class.getClassLoader().getResourceAsStream("jdbc.properties");
Properties properties = new Properties();
properties.load(in);
driver = properties.getProperty("jdbc.driver");
url = properties.getProperty("jdbc.url");
username = properties.getProperty("jdbc.username");
password = properties.getProperty("jdbc.password");
// 加载驱动，只需要加载一次
Class.forName(driver);
} catch (Exception e) {
e.printStackTrace();
}
}
/**
* 获取连接
* @return
* @throws SQLException
*/
public static Connection getConnection() throws SQLException {
return DriverManager.getConnection(url, username, password);
}
/**
* 释放连接
* @param conn
* @param stat
* @param rs
*/
public static void release(Connection conn, Statement stat, ResultSet rs){
try {
if(rs!=null) {
rs.close();
}
} catch (Exception e) {
e.printStackTrace();
}
try {
if(stat != null) {
stat.close();
}
} catch (Exception e) {
e.printStackTrace();
}
try {
if(conn != null) {
conn.close();
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
jdbc.properties file
jdbc.driver = com.mysql.jdbc.Driver
jdbc.url = jdbc:mysql://localhost:3306/mine?userUnicode=true&characterEncoding=utf8&useSSL=false
jdbc.username = root
jdbc.password = root
2. Affairs
2.1. Overview
Either all succeed or both fail
ACID principles
-
atomicity
A transaction is an indivisible unit of work that either succeeds or both fail
-
consistency
total unchanged
-
isolation
Multiple processes do not interfere with each other, and the operations and data used within a transaction are isolated from other concurrent transactions
-
Persistence
Once a transaction is committed, itChanges to data in the database are permanentYes, subsequent other operations and database failures should not have any effect on it,
persisted to the database
Isolation question:
Dirty read: A transaction reads another uncommitted transaction
Non-repeatable read: In the same transaction, the data in the table is repeatedly read, and the table data has changed
False reading: In the same transaction, the data inserted by others is read, resulting in inconsistent reading results
2.2. Sample code:
@Test
public void testTransaction(){
Connection conn = null;
PreparedStatement ptst = null;
try {
// 获取连接
conn = JdbcUtil.getConnection();
conn.setAutoCommit(false); // 关闭自动提交，开启事务
String sql1 = "update account set money = money - 100 where name='A'";
// 预编译sql
ptst = conn.prepareStatement(sql1);
ptst.executeUpdate();
// 测试事务
int x = 1 / 0;
String sql2 = "update account set money = money + 100 where name='B'";
ptst = conn.prepareStatement(sql2);
ptst.executeUpdate();
// 业务完毕，提交事务
conn.commit();
System.out.println("成功");
} catch (SQLException e) {
// 出现异常，默认会自动调用rollback，回滚数据
e.printStackTrace();
} finally {
JdbcUtil.release(conn, ptst, null);
}
}