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, first, last, 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, age: 30, first: sumit, last: mittal
id: 110, age: 20, first: sima, last: chug

deleting row....
deleting row....
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: 30, first: sumit, last: mittal
id: 110, age: 20, first: sima, last: chug

goodbye!