在Java的JDBC使用中设置事务回滚的保存点的方法

前端技术 2023/09/09 Java

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!

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