在Java的JDBC應用中設置事務回滾的保留點的辦法

在Java的JDBC應用中設置事務回滾的保留點的辦法。本站提示廣大學習愛好者：（在Java的JDBC應用中設置事務回滾的保留點的辦法）文章只能為提供參考，不一定能成為您想要的結果。以下是在Java的JDBC應用中設置事務回滾的保留點的辦法正文

新的JDBC3.0保留點的接口供給了額定的事務掌握。他們的情況中，如Oracle的PL/ SQL中的年夜多半古代的DBMS支撐保留點。

當設置一個保留點在事務中界說一個邏輯回滾點。假如產生毛病，曩昔一個保留點，則可使用rollback辦法來取消要末一切的轉變或僅保留點以後所做的更改。

Connection對象有兩個新的辦法，可贊助治理保留點：

setSavepoint(String savepointName): 界說了一個新的保留點。它也前往一個Savepoint 對象。

releaseSavepoint(Savepoint savepointName): 刪除一個保留點。請留意，它須要一個Savepoint 對象作為參數。這個對象平日是由setSavepoint()辦法生成一個保留點。

有一個rollback ( String savepointName ) 辦法回滾任務到指定的保留點。

上面的例子演示若何應用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);
}

在這類情形下沒有上述INSERT語句會勝利，一切都將被回滾。
上面是應用setSavepoint和事務教程描寫回滾的例子。

基於對情況和數據庫裝置在後面的章節中做此示例代碼曾經說明。

復制上面的例子JDBCExample.java，編譯並運轉，以下所示：

//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

如今讓我們來編譯下面的例子以下：

C:>javac JDBCExample.java

當運轉JDBCExample，它會發生以下成果：

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!