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在Java的JDBC使用中設置事務回滾的保存點的方法

2019-11-26 14:43:10
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新的JDBC3.0保存點的接口提供了額外的事務控制。他們的環境中,如Oracle的PL/ SQL中的大多數現代的DBMS支持保存點。

當設置一個保存點在事務中定義一個邏輯回滾點。如果發生錯誤,過去一個保存點,則可以使用rollback方法來撤消要么所有的改變或僅保存點之后所做的更改。

Connection對象有兩個新的方法,可幫助管理保存點:

setSavepoint(String savepointName): 定義了一個新的保存點。它也返回一個Savepoint 對象。

releaseSavepoint(Savepoint savepointName): 刪除一個保存點。請注意,它需要一個Savepoint 對象作為參數。這個對象通常是由setSavepoint()方法生成一個保存點。

有一個rollback ( String savepointName ) 方法回滾工作到指定的保存點。

下面的例子演示如何使用Savepoint對象:

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);}

在這種情況下沒有上述INSERT語句會成功,一切都將被回滾。
下面是利用setSavepoint和事務教程描述回滾的例子。

基于對環境和數據庫安裝在前面的章節中做此示例代碼已經解釋。

復制下面的例子JDBCExample.java,編譯并運行,如下所示:

//STEP 1. Import required packagesimport 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

現在讓我們來編譯上面的例子如下:

C:>javac JDBCExample.java

當運行JDBCExample,它會產生以下結果:

C:>java JDBCExample
Connecting to database...Creating statement...List result set for reference....ID: 95, Age: 20, First: Sima, Last: ChugID: 100, Age: 18, First: Zara, Last: AliID: 101, Age: 25, First: Mahnaz, Last: FatmaID: 102, Age: 30, First: Zaid, Last: KhanID: 103, Age: 30, First: Sumit, Last: MittalID: 110, Age: 20, First: Sima, Last: ChugDeleting row....Deleting row....List result set for reference....ID: 100, Age: 18, First: Zara, Last: AliID: 101, Age: 25, First: Mahnaz, Last: FatmaID: 102, Age: 30, First: Zaid, Last: KhanID: 103, Age: 30, First: Sumit, Last: MittalID: 110, Age: 20, First: Sima, Last: ChugGoodbye!
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