详解Java的JDBC API中事务的提交和回滚

如果JDBC连接是在自动提交模式下，它在默认情况下，那么每个SQL语句都是在其完成时提交到数据库。

这可能是对简单的应用程序，但有三个原因，你可能想关闭自动提交和管理自己的事务：

1. 为了提高性能
2. 为了保持业务流程的完整性
3. 使用分布式事务

若要控制事务，以及何时更改应用到数据库。它把单个SQL语句或一组SQL语句作为一个逻辑单元，而且如果任何语句失败，整个事务失败。

若要启用，而不是JDBC驱动程序默认使用auto-commit模式手动事务支持，使用Connection对象的的setAutoCommit()方法。如果传递一个布尔值false到setAutoCommit()，关闭自动提交。可以传递一个布尔值true将其重新打开。

例如，如果有一个名为conn Connection对象，以下代码来关闭自动提交：

conn.setAutoCommit(false);

提交和回滚
一旦已经完成了变化，要提交更改，然后调用commit（在连接对象）方法，如下所示：

conn.commit( );

否则回滚更新对数据库所做的使用命名连接conn，使用下面的代码：

conn.rollback( );

下面的例子演示了如何使用一个提交和回滚对象：

try{
  //Assume a valid connection object conn
  conn.setAutoCommit(false);
  Statement stmt = conn.createStatement();
  
  String SQL = "INSERT INTO Employees " +
        "VALUES (106,20,'Rita','Tez')";
  stmt.executeUpdate(SQL); 
  //Submit a malformed SQL statement that breaks
  String SQL = "INSERTED IN Employees " +
        "VALUES (107,22,'Sita','Singh')";
  stmt.executeUpdate(SQL);
  // If there is no error.
  conn.commit();
}catch(SQLException se){
  // If there is any error.
  conn.rollback();
}

在这种情况下没有上述INSERT语句会成功，一切都将被回滚。

事务提交和回滚示例
以下是使用事务提交和回滚描述的例子。

基于对环境和数据库安装在前面的章节中做此示例代码已学习过。

复制过去下面的例子中JDBCExample.java，编译并运行，如下所示：

//STEP 1. Import required packages
import java.sql.*;

public class JDBCExample {
  // JDBC driver name and database URL
  static final String JDBC_DRIVER = "com.mysql.jdbc.Driver"; 
  static final String DB_URL = "jdbc:mysql://localhost/EMP";

  // Database credentials
  static final String USER = "username";
  static final String PASS = "password";
  
 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 database...");
   conn = DriverManager.getConnection(DB_URL,USER,PASS);

   //STEP 4: Set auto commit as false.
   conn.setAutoCommit(false);

   //STEP 5: Execute a query to create statment with
   // required arguments for RS example.
   System.out.println("Creating statement...");
   stmt = conn.createStatement(
              ResultSet.TYPE_SCROLL_INSENSITIVE,ResultSet.CONCUR_UPDATABLE);
   
   //STEP 6: INSERT a row into Employees table
   System.out.println("Inserting one row....");
   String SQL = "INSERT INTO Employees " +
          "VALUES (106,'Tez')";
   stmt.executeUpdate(SQL); 

   //STEP 7: INSERT one more row into Employees table
   SQL = "INSERT INTO Employees " +
          "VALUES (107,'Singh')";
   stmt.executeUpdate(SQL);

   //STEP 8: Commit data here.
   System.out.println("Commiting data here....");
   conn.commit();
  
  //STEP 9: Now list all the available records.
   String sql = "SELECT id,first,last,age FROM Employees";
   ResultSet rs = stmt.executeQuery(sql);
   System.out.println("List result set for reference....");
   printRs(rs);

   //STEP 10: Clean-up environment
   rs.close();
   stmt.close();
   conn.close();
  }catch(SQLException se){
   //Handle errors for JDBC
   se.printStackTrace();
   // If there is an error then rollback the changes.
   System.out.println("Rolling back data here....");
  try{
  if(conn!=null)
      conn.rollback();
   }catch(SQLException se2){
     se2.printStackTrace();
   }//end try

  }catch(Exception e){
   //Handle errors for Class.forName
   e.printStackTrace();
  }finally{
   //finally block used to close resources
   try{
     if(stmt!=null)
      stmt.close();
   }catch(SQLException se2){
   }// nothing we can do
   try{
     if(conn!=null)
      conn.close();
   }catch(SQLException se){
     se.printStackTrace();
   }//end finally try
  }//end try
  System.out.println("Goodbye!");
}//end main

  public static void printRs(ResultSet rs) throws SQLException{
   //Ensure we start with first row
   rs.beforeFirst();
   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);
   }
   System.out.println();
  }//end printRs()
}//end JDBCExample

现在让我们来编译上面的例子如下：

C:>javac JDBCExample.java

当运行JDBCExample，它会产生以下结果：

C:>java JDBCExample

Connecting to database...
Creating statement...
Inserting one row....
Commiting data here....
List result set for reference....
ID: 100,Age: 18,First: Zara,Last: Ali
ID: 101,Age: 25,First: Mahnaz,Last: Fatma
ID: 102,Age: 30,First: Zaid,Last: Khan
ID: 103,Age: 28,First: Sumit,Last: Mittal
ID: 106,Age: 20,First: Rita,Last: Tez
ID: 107,Age: 22,First: Sita,Last: Singh
Goodbye!