事务示例之转账

灰太狼 2022-09-30 04:58 209阅读 0赞

## 事务示例之转账 ##

### 1.创建数据库，填充数据： ###

数据库名称db\_account，表名tb\_account，插入几条数据。

![image][]

### 2.配置数据库信息 ###

在项目的WebContent/META-INF目录下创建一个context.xml文件。如图:  
![image][image 1]

在context.xml文件中配置：

<?xml version="1.0" encoding="UTF-8"?>
    <Context>      
        <Resource name="jdbc/account" auth="Container" type="javax.sql.DataSource" maxActive="100" maxIdle="30" maxWait="10000" username="root" password="root" driverClassName="com.mysql.jdbc.Driver" url="jdbc:mysql://localhost:3306/db_account"/>
    </Context>

这里采用的是Javaweb自带的DBCP配置，详细参考Javaweb配置常用的数据源配置

### 3.创建项目结构 ###

如图：

![image][image 2]

*  AccountDao
    
        public interface AccountDao { 
            /** * 更新账户 * @param account */
            public boolean updateAccount(Account account) throws SQLException;  
        
            /** * 查找账户 * @param account * @return */
            public Account findAccountByName(String account);
        }
 *  AccountDaoImpl
    
        public class AccountDaoImpl implements AccountDao{ 
        
            public AccountDaoImpl() {
        
            }
        
            @Override
            public boolean updateAccount(Account account) {
                Connection conn = ManagerThreadLocal.getConn();
                if(conn==null){
                    throw new NullPointerException("Connection is null");
                }
        
                PreparedStatement ps = null;
                try {
                     ps = conn.prepareStatement("update tb_account set money=? where name=?");
                     if(ps==null){
                         throw new NullPointerException("PreparedStatement is null");
                     }
                     ps.setDouble(1, account.getMoney());
                     ps.setString(2, account.getName());
                     int i = ps.executeUpdate();
                     return i>0;
        
                } catch (SQLException e) {
                    e.printStackTrace();
                }
        
                return false;
            }
        
            @Override
            public Account findAccountByName(String account) {
        
                Connection conn = ManagerThreadLocal.getConn();
        
                if(conn==null){
                    throw new NullPointerException("Connection is null");
                }
        
                Account a = null;
                PreparedStatement ps = null;
                ResultSet rs = null;
                try {
                     ps = conn.prepareStatement("select name,money from tb_account where name=?");
                     if(ps==null){
                         throw new NullPointerException("PreparedStatement is null");
                     }
                     ps.setString(1, account);
        
                     rs = ps.executeQuery();
                     if(rs==null){
                         throw new NullPointerException("ResultSet is null");
                     }
        
                     if(rs.next()){
                         a = new Account();
                         a.setName(rs.getString(1));
                         a.setMoney(rs.getDouble(2));
                         return a;
                     }
                } catch (SQLException e) {
                    e.printStackTrace();
                }
        
                return a;
            }
        }
 *  Account
    
        public class Account {
            private int id;
            private String name;
            private double money;
            public int getId() {
                return id;
            }
            public void setId(int id) {
                this.id = id;
            }
            public String getName() {
                return name;
            }
            public void setName(String name) {
                this.name = name;
            }
            public double getMoney() {
                return money;
            }
            public void setMoney(double money) {
                this.money = money;
            }
            @Override
            public String toString() {
                return "Account [id=" + id + ", name=" + name + ", money=" + money + "]";
            }   
           }
 *  AccountService
    
        public interface AccountService { 
                /** * 转账 * @param fromName * @param toName * @param money */
                public void transfer(String fromName,String toName,double money);
            }
 *  AccountServiceImpl
    
        public class AccountServiceImpl implements AccountService{ 
        
            @Override
            public void transfer(String fromName, String toName, double money) {
        
                AccountDao accountDao = new AccountDaoImpl();
                Account fromAccount = accountDao.findAccountByName(fromName);
                if(fromAccount==null){
                    throw new RuntimeException("你的账户不存在");
                }
                if(money > fromAccount.getMoney()){
                    throw new RuntimeException("你的账户的钱不够");
                }
                fromAccount.setMoney(fromAccount.getMoney() - money);
        
                Account toAccount = accountDao.findAccountByName(toName);
                if(toAccount==null){
                    throw new RuntimeException("对方的账户不存在");
                }
                toAccount.setMoney(toAccount.getMoney() + money);
        
                //开启事务
                ManagerThreadLocal.startTransaction();
        
                try {
                            accountDao.updateAccount(fromAccount);
                    //int i = 10/0; //测试事务代码
                    accountDao.updateAccount(toAccount);
                    //提交事务
                    ManagerThreadLocal.commit();
                } catch (SQLException e) {
                    //事务回滚
                    ManagerThreadLocal.rollback();
                    e.printStackTrace();
                }finally {
                    ManagerThreadLocal.close();
                }
            }
        }
 *  TransferServlet
    
        @WebServlet({ "/TransferServlet", "/transfer" })
        public class TransferServlet extends HttpServlet { 
            private static final long serialVersionUID = 1L;
        
                public TransferServlet() {
                    super();
        
                }
        
            protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        
                AccountService accountService = new AccountServiceImpl();
                accountService.transfer("10010", "10000", 100);
        
        
            }
        
            protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
                doGet(request, response);
            }
        
        }
 *  ManagerThreadLocal
    
        public class ManagerThreadLocal {
        
            private static ThreadLocal<Connection> tl = new ThreadLocal<>();
        
            private static Connection getConnection(){
                Connection conn  = null;
                try {
                    Context context = new InitialContext();
                    DataSource dataSource = (DataSource) context.lookup("java:/comp/env/jdbc/account");
                    conn = (Connection) dataSource.getConnection();
                } catch (SQLException e) {
                    e.printStackTrace();
                } catch (NamingException e) {
                    e.printStackTrace();
                }
                return conn;
            }
        
            public static Connection getConn(){
                Connection conn = null;
                conn = tl.get();
                if(conn==null){
                    conn = getConnection();
                    tl.set(conn);
                }
                return conn;
            }
        
            public static void startTransaction(){
                try {
                    getConn().setAutoCommit(false);
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
        
            public static void commit(){
                try {
                    getConn().commit();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
        
            public static void rollback(){
                try {
                    getConn().rollback();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
        
            public static void close(){
                try {
                    getConn().close();
                    tl.remove();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
        }

**ManagerThreadLocal的作用是通过ThreadLocal类（内部是由Map实现）缓存一个Connection对象，保证连接的conn对象是同一个，将Connection对象解耦出来，实现事务操作（保证了事务的conn对象是同一个）**

执行结果：注释AccountServiceImpl类中transfer方法中的如下代码，数据库数据同时改变，注释打开，数据保持不变。

//int i = 10/0; //测试事务代码

[image]: http://oe9ggtbcb.bkt.clouddn.com/$P%259DWA1EK8BU%7DLMUOA%28R12.png
[image 1]: http://oe9ggtbcb.bkt.clouddn.com/AWAJO%5DQM3$K%5BNR%7D8RMX$@HK.png
[image 2]: http://oe9ggtbcb.bkt.clouddn.com/S@D%28PEK0C%7D%6088%609_Y34G%5D%28B.png