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：
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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：
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//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：
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C:>javac JDBCExample.java |
當運行JDBCExample，它會產生以下結果：
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C:>java JDBCExample |
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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! |
