并发容器ThreadLocal实现银行转账功能

迈不过友情╰ 2023-06-07 10:37 16阅读 0赞

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ThreadLocal 在面试中经常提到，关于ThreadLocal使用不当造成OOM以及在特殊场景下，通过ThreadLocal可以轻松实现一些看起来复杂的功能，都说明值得花时间研究其原理。

ThreadLocal 不是 Thread，是一个**线程内部的数据存储类**，通过它可以在指定的线程中存储数据，**对数据存储后，只有在线程中才可以获取到存储的数据，对于其他线程来说是无法获取到数据。**可能这才是Local的真正含义吧。

**依旧用的三层架构**

**下面开始贴代码：**

AccountDaoImpl.java

package bank.dao.impl;
    
    import bank.dao.AccountDao;
    import bank.util.Account;
    import bank.util.JDBCUtil;
    import org.apache.commons.dbutils.QueryRunner;
    import org.apache.commons.dbutils.handlers.BeanHandler;
    
    import java.sql.Connection;
    import java.sql.SQLException;
    
    import static bank.util.JDBCUtil.getConnection;
    
    public class AccountDaoImpl implements AccountDao {
        @Override
        public Account queryAccountByCard(int  cardId) throws SQLException {
    
            QueryRunner runner = new QueryRunner();
            Connection conn = getConnection();
         //手工提交 QueryRunner();无参  query加connection
            Object[] params={cardId};
          Account account =runner.query(conn,"select * from account where  cardid =?",new BeanHandler<Account>(Account.class),params);
    
             return  account;
    
        }
    
        @Override
        public void updateAccount(Account account) throws SQLException {
            QueryRunner runner = new QueryRunner();
            Connection conn = getConnection();
            runner.update(conn,"update account set balance = ? where cardid = ? ",new Object[]{account.getBalance(),account.getCardID()});
    
        }
    }

package bank.Service.impl;
    
    import bank.Service.AccountService;
    import bank.dao.AccountDao;
    import bank.dao.impl.AccountDaoImpl;
    import bank.util.Account;
    import bank.util.JDBCUtil;
    import jdk.nashorn.internal.scripts.JD;
    
    import java.sql.SQLException;
    
    public class AccountServicecImpl implements AccountService {
        @Override
    
        public void transfer(int fromCardId, int toCardID, int money) {
    
            AccountDaoImpl accountDao = new AccountDaoImpl();
            //开启事务
            try {
                JDBCUtil.beginTransaction();
    
             Account fromaccount = accountDao.queryAccountByCard(fromCardId);
             Account toaccount =   accountDao.queryAccountByCard(toCardID);
    
             //转账
               if(fromaccount.getBalance()> money){
                    int fromBalance = fromaccount.getBalance()-money;
    
                   fromaccount.setCardID(fromCardId);
                    fromaccount.setBalance(fromBalance);
                  accountDao.updateAccount(fromaccount);
    
                    int  toBalance = toaccount.getBalance()+ money;
                    toaccount.setCardID(toCardID);
                    toaccount.setBalance(toBalance);
                  accountDao.updateAccount(toaccount);
    
                    System.out.println("转账成功");
                   //正常提交事务
                    JDBCUtil.commitTransaction();
    
    
                }else
                {
                    System.out.println("余额不足");
                }
    
    
    
            } catch (SQLException e) {
                try {
                    JDBCUtil.rollbackTransaction();
                    System.out.println("转账失败 ， 回滚");
                } catch (SQLException e1) {
                    e1.printStackTrace();
                }
                e.printStackTrace();
    
            }finally {
                try {
                    JDBCUtil.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
    
    
    
            //结束
        }
    }

**Account类**

package bank.util;
    
    public class Account {
        private  int cardID;
        private String  name ;
        private int balance;
    
        public Account() {
    
        }
    
        public Account(int cardID, String name, int balance) {
            this.cardID = cardID;
            this.name = name;
            this.balance = balance;
        }
    
        public void setCardID(int cardID) {
            this.cardID = cardID;
        }
    
    
        public int getCardID() {
            return cardID;
        }
    
        public void setName(String name) {
            this.name = name;
        }
        public String getName() {
            return name;
        }
    
    
    
        public void setBalance(int balance) {
            this.balance = balance;
        }
    
        public int getBalance() {
            return balance;
        }
    }

**dbcpDemo 数据源**

package bank.util;
    
    import org.apache.commons.dbcp2.BasicDataSource;
    
    import javax.sql.DataSource;
    import java.sql.ResultSet;
    
    //连接池访问数据库
    public class dbcpDemo {
    
        public static DataSource getDataSourceWIthDBCP(){
    
    
            BasicDataSource dbcp = new BasicDataSource();
            dbcp.setDriverClassName("oracle.jdbc.driver.OracleDriver");
            dbcp.setUrl("jdbc:oracle:thin:@127.0.0.1:1521:ORCL");
            dbcp.setUsername("sys as sysdba");
            dbcp.setPassword("123456");
            dbcp.setInitialSize(20);
    
            return dbcp;
        }
        
        
         public static void main (String[] args) throws Exception
         {
             System.out.println(getDataSourceWIthDBCP().getConnection());
    
                     String sql="select * from  student";
             ResultSet rs= getDataSourceWIthDBCP().getConnection().createStatement().executeQuery(sql);
    
             while(rs.next()){
                                 //获取stuname这列数据
                      String name = rs.getString("sname");
                                  //获取stuid这列数
                       int age= rs.getInt("sage");
    
                                 //输出结果
                                System.out.println(name + "\t" + age);
    
                           }
         }
    
    
    }

JDBCUtil  ThreadLocal

确保线程安全

package bank.util;
    
    import java.sql.Connection;
    import java.sql.SQLException;
    
    public class JDBCUtil {
        //ThreadLocal 保证线程安全   （一个连接 对应 一个事务  一个事务 内 含多个dml操作）
        private  static ThreadLocal<Connection> threadLocal = new ThreadLocal<>();
        //开启事务
    
        //
        public static Connection getConnection() throws SQLException {
    
           Connection conn=  threadLocal.get();//自动创建变量，如果不够自动创建副本
           if(conn==null){ //如果没连接 创建连接
             conn =   dbcpDemo.getDataSourceWIthDBCP().getConnection();
               threadLocal.set(conn);
           }
           return conn;
    
        }
    
        public static  void beginTransaction() throws SQLException {
            //连接
      Connection conn =   getConnection();
      conn.setAutoCommit(false);//开启手动提交 /  开启事务
    
        }
    
        public static  void commitTransaction() throws SQLException {
        //提交
            Connection conn =   getConnection();//进入threadlocal 拿副本
            //对象。方法 排空
            if(conn!=null)
            conn.commit();
    
        }
    
        public static  void rollbackTransaction() throws SQLException {
            //回滚
            Connection conn =   getConnection();
            if(conn!=null)
            conn.rollback();
        }
    
        public static void close() throws SQLException {
            //关闭
          Connection conn=   getConnection();
            if(conn!=null)
                conn.close();
            threadLocal.remove();
            conn=null;
        }
    
    
    }

**测试 Test2**

package bank;
    
    import bank.Service.AccountService;
    import bank.Service.impl.AccountServicecImpl;
    import bank.dao.AccountDao;
    import bank.dao.impl.AccountDaoImpl;
    import bank.util.Account;
    
    public class Test2 {
    
        public static void main (String[] args)throws Exception
        {
            AccountDao accountDao = new AccountDaoImpl();
            AccountService accountService =  new AccountServicecImpl();
          accountService.transfer(111 ,1234 , 1000);
    
        }
    
    }

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