Databse Transaction

                                                            Databse Transaction

      在更新数据库时，默认情况下，更改是永久性的写入（或提交）到数据库。然而，这种默认的行为可以通过编写程序来关闭。 在自动交付关闭的情况下，如果在更新时发生问题，则对数据库的每个更改都能够取消（或者说回退到最初的值）。如果更新 成功，那么，之后就可以将这些更改永久性的提交给数据库。这就是数据库的事物管理。 典型地我们可以用try/catch/finally块来正确地应对事务管理。首先，我们记录自动提交前的状态。然后在try块中，调用 setAutoCommit(false)并执行一系列的查询或更新。如果发生故障，则在catch块中调用rollbackl如果事务成功，则在try块 的节尾调用commit.不管哪种情况都在finally块中重置自动提交的状态。

比如:
Connection connection= DriverManage.getConnection(url,username,password);
boolean autoCommit=connection.getAutoCommit();
Statement statement;
try
{
connection.setAutoCommit(false);
statement=connecion.createStatement();
statement.executeQuery(.....);
statement.execute(.....);
connecion.commit();
}
catch(SQLException e)
 {
 connection.rollback();
 }
 finally
 {
statement.close(); connecion.setAutoCommit(autoCommit);
 }
getConnection方法会抛出SQLException异常，这个异常要么被外围的方法抛出,要么在单独的try/catch块中进行捕获.看下面的两段代码:

package coreservlet;

import java.sql.*;
import coreservlet.beans.*;

      /** An example to demonstrate submission of a block of SQL statements as s single transaction
       *  Specifically, four new records are inserted into the music table.Performed as a transaction
       *  block so that if a problem occurs, a rolback is performed and no changes are committed to
       *  the database.
       */

public class Transactons {
     public static void main(String[] args)
     {
         if(args.length<5)
         {
             printUsage();
             return;
         }
         String vendor= args[4];
         //Change to DriverUtilites2.loadDrivers() to load vendor drivers form an XML file instead of loading
         // hard-coded vendor drivers in Drivertilities.
         DriverUtilities.isValidVendow(vendor);
         if(!DriverUtilities.isValidVendor(vendor))
         {
             printUsage();
             return;
         }
         String driver=DriverUtilities.getDriver(vendor);
         String host=args[0];
         String dbName=args[1];
         String url=
                 DriverUtilities.makeURL(host,dbName,vendor);
         String username=args[2];
         String password=args[3];
         doTransactions(driver,url,username,password);
     }
     private static void doTransaction(String driver,
                                            String url,
                                            String username,
                                            String password)
     {
         String[] transaction=
         {
             "inset into music values"+
                     "(9,'Chopin','No.2 in F minor',100,17.99)",
             "Insert into music values"+
                     "(10,'Tchaikovsky','No.1 in Bb minor',100,24,99)",
             "Insert into music values"+
                     "(11,'Ravel','No.2 in D major',100,14.99)",
             "Insert into music values"+
                     "(12,'Schumann','No.1 in A minor',100,14.99)"
         };
         TransactionBean bean=new TransactionBean();
         try
         {
             bean.setConnection(driver,url,username,password);
             bean.execute(transaction);
         }
         catch(SQLException sqle)
         {
             System.err.println("Transaction failutr"+sqle);
         }
         finally
         {
             bean.close();
         }
     }
     private static void printUsage()
     {
         System.out.println("Usage:Transaction host"+
                 "dbName username password"+
                 "vendor.");
     }
}


package coreservlet;

import java.io.*;
import java.sql.*;
import java.util.*;
import coreservlet.*;

/** Bean for performing JDBC taransactions. After specifying the connection, submit a block of SQL statement
 *  as a single transaction by calling execute. If an SQLException occurs,any prior statements are automatically
 *  rolled back.
 */

public class TransactionBean {
    private Connection connection;
    public void setConnecion(Connection connection)
    {
        this.connection=connection;
    }
    public void setConnecion(String driver,String url,String username,String password)
    {
        setConnection(ConnectionInfoBean.getConnection(driver,url,username,password));
    }
    public Connecion getConnection()
    {
        return(connection);
    }
    public void execute(List list) throws SQLException
    {
        execute((String[])list.toArray(new String[list.size()]));
    }
    public void execute(String transaction) throws SQLException
    {
        execute(new String[]{transaction});
    }
   
    /** Execute a block of SQL Statement as s single transaction. If an SLQException occurs, a rollback
     *  is attempted and the exception is thrown.
     */
   
    public void execute(String[] transaction) throws SQLException
    {
        if(connecion==null)
        {
            throw new SQLException("No connection available.");
        }
        boolean autoCommit=connection.getAutoCommit();
        try
        {
            connection.setAutoCommit(false);
            Statement statement=connection.createStatement();
            for(int i=0;i<transaction.length;i++)
            {
                statement.execute(transaction[i]);
            }
            statment.close();
        }
        catch(SQLException sqle)
        {
            connection.rollback();
            throw sqle;
        }
        finally
        {
            connection.commit();
            connection.setAutoCommit(autoCommit);
        }
    }
    public void close()
    {
        if(connection==null)
        {
            try
            {
                connection.close();
            }
            catch(SQLException sqle)
            {
                System.err.print("Failed to close connecion:"+sqle);
            }
            finally
            {
                connnecion=null;
            }
        }
    }
}