在Java的JDBC使用中设置事务回滚的保存点的方法

新的JDBC3.0保存点的接口提供了额外的事务控制。他们的环境中，如Oracle的PL/ SQL中的大多数现代的DBMS支持保存点。

当设置一个保存点在事务中定义一个逻辑回滚点。如果发生错误，过去一个保存点，则可以使用rollback方法来撤消要么所有的改变或仅保存点之后所做的更改。

Connection对象有两个新的方法，可帮助管理保存点：

setSavepoint(String savepointName): 定义了一个新的保存点。它也返回一个Savepoint 对象。

releaseSavepoint(Savepoint savepointName): 删除一个保存点。请注意，它需要一个Savepoint 对象作为参数。这个对象通常是由setSavepoint()方法生成一个保存点。

有一个rollback ( String savepointName ) 方法回滚工作到指定的保存点。

下面的例子演示如何使用Savepoint对象：

try{
  //Assume a valid connection object conn
  conn.setAutoCommit(false);
  Statement stmt = conn.createStatement();
  
  //set a Savepoint
  Savepoint savepoint1 = conn.setSavepoint("Savepoint1");
  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','Tez')";
  stmt.executeUpdate(SQL);
  // If there is no error,commit the changes.
  conn.commit();

}catch(SQLException se){
  // If there is any error.
  conn.rollback(savepoint1);
}

在这种情况下没有上述INSERT语句会成功，一切都将被回滚。
下面是利用setSavepoint和事务教程描述回滚的例子。

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经解释。

复制下面的例子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 delete statment with
   // required arguments for RS example.
   System.out.println("Creating statement...");
   stmt = conn.createStatement();

  //STEP 6: 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 7: delete rows having ID grater than 104
   // But save point before doing so.
   Savepoint savepoint1 = conn.setSavepoint("ROWS_DELETED_1");
   System.out.println("Deleting row....");
   String SQL = "DELETE FROM Employees " +
          "WHERE ID = 110";
   stmt.executeUpdate(SQL); 
   // oops... we deleted too wrong employees!
   //STEP 8: Rollback the changes afetr save point 2.
   conn.rollback(savepoint1);

  // STEP 9: delete rows having ID grater than 104
   // But save point before doing so.
   Savepoint savepoint2 = conn.setSavepoint("ROWS_DELETED_2");
   System.out.println("Deleting row....");
   SQL = "DELETE FROM Employees " +
          "WHERE ID = 95";
   stmt.executeUpdate(SQL); 

  //STEP 10: Now list all the available records.
   sql = "SELECT id,age FROM Employees";
   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...
List result set for reference....
ID: 95,Age: 20,First: Sima,Last: Chug
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,First: Sumit,Last: Mittal
ID: 110,Last: Chug

Deleting row....
Deleting row....
List result set for reference....
ID: 100,Last: Chug

Goodbye!

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