Java & MySQL - 儲存點

新的 JDBC 3.0 Savepoint 介面提供了額外的交易控制。大多數現代資料庫管理系統 (DBMS) 都支援其環境中的儲存點，例如 Oracle 的 PL/SQL。

設定儲存點時，您在事務中定義了一個邏輯回滾點。如果在儲存點之後發生錯誤，您可以使用回滾方法撤消所有更改或僅撤消儲存點之後進行的更改。

Connection 物件有兩個新方法可以幫助您管理儲存點：

• setSavepoint(String savepointName) - 定義一個新的儲存點。它還返回一個 Savepoint 物件。

• releaseSavepoint(Savepoint savepointName) - 刪除一個儲存點。請注意，它需要一個 Savepoint 物件作為引數。此物件通常是由 setSavepoint() 方法生成的儲存點。

有一個 rollback (String savepointName) 方法，它將工作回滾到指定的儲存點。

以下示例說明了 Savepoint 物件的使用：

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，以上任何 INSERT 語句都不會成功，並且所有內容都將回滾。

以下示例使用 setSavepoint 和 rollback 進行說明。

此示例程式碼是基於前面章節中完成的環境和資料庫設定編寫的。

將以下示例複製並貼上到 TestApplication.java 中，編譯並執行如下：

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Savepoint;
import java.sql.Statement;

public class TestApplication {
   static final String DB_URL = "jdbc:mysql:///TUTORIALSPOINT";
   static final String USER = "guest";
   static final String PASS = "guest123";
   static final String QUERY = "SELECT id, first, last, age FROM Employees";
   static final String DELETE_QUERY = "DELETE FROM Employees WHERE ID = 8";
   static final String DELETE_QUERY_1 = "DELETE FROM Employees WHERE ID = 9";

   public static void printResultSet(ResultSet rs) throws SQLException{
      // Ensure we start with first row
      rs.beforeFirst();
      while(rs.next()){
         // Display values
         System.out.print("ID: " + rs.getInt("id"));
         System.out.print(", Age: " + rs.getInt("age"));
         System.out.print(", First: " + rs.getString("first"));
         System.out.println(", Last: " + rs.getString("last"));
      }
      System.out.println();
   }

   public static void main(String[] args) {
      // Open a connection
      try(Connection conn = DriverManager.getConnection(DB_URL, USER, PASS);
         Statement stmt = conn.createStatement(
            ResultSet.TYPE_SCROLL_INSENSITIVE,
            ResultSet.CONCUR_UPDATABLE);				
      ) {		

         conn.setAutoCommit(false);
         ResultSet rs = stmt.executeQuery(QUERY);
         System.out.println("List result set for reference....");
         printResultSet(rs);

         // delete row having ID = 8
         // But save point before doing so.
         Savepoint savepoint1 = conn.setSavepoint("ROWS_DELETED_1");
         System.out.println("Deleting row....");
         stmt.executeUpdate(DELETE_QUERY);  
         // Rollback the changes after save point 1.
         conn.rollback(savepoint1);

         // delete rows having ID = 9
         // But save point before doing so.
         conn.setSavepoint("ROWS_DELETED_2");
         System.out.println("Deleting row....");

         stmt.executeUpdate(DELETE_QUERY_1);  

         rs = stmt.executeQuery(QUERY);
         System.out.println("List result set for reference....");
         printResultSet(rs);

         // Clean-up environment
         rs.close();

      } catch (SQLException e) {
         e.printStackTrace();
      } 
   }
}

現在，讓我們按如下方式編譯以上示例：

C:\>javac TestApplication.java
C:\>

執行 TestApplication 時，會產生以下結果：

C:\>java TestApplication
List result set for reference....
ID: 1, Age: 23, First: Zara, Last: Ali
ID: 2, Age: 30, First: Mahnaz, Last: Fatma
ID: 3, Age: 35, First: Zaid, Last: Khan
ID: 4, Age: 33, First: Sumit, Last: Mittal
ID: 5, Age: 40, First: John, Last: Paul
ID: 7, Age: 20, First: Sita, Last: Singh
ID: 8, Age: 20, First: Rita, Last: Tez
ID: 9, Age: 20, First: Sita, Last: Singh

Deleting row....
Deleting row....
List result set for reference....
ID: 1, Age: 23, First: Zara, Last: Ali
ID: 2, Age: 30, First: Mahnaz, Last: Fatma
ID: 3, Age: 35, First: Zaid, Last: Khan
ID: 4, Age: 33, First: Sumit, Last: Mittal
ID: 5, Age: 40, First: John, Last: Paul
ID: 7, Age: 20, First: Sita, Last: Singh
ID: 8, Age: 20, First: Rita, Last: Tez
C:\>