JDBC - 设置保存点示例

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以下是示例，它使用了事务教程中描述的 setSavepoint 和 rollback 方法。

此示例代码是基于前面章节中完成的环境和数据库设置编写的。

使用 SavePoint 回滚到之前的稳定更改示例

在这个示例中，我们有六个静态字符串，包含数据库连接 URL、用户名、密码、SELECT 查询和两个 DELETE 查询。DELETE 查询用于删除 Employees 表中 ID 为 8 和 9 的两条记录。现在，使用 DriverManager.getConnection() 方法，我们准备了一个数据库连接。使用 setAutoCommit(false)，我们将自动提交设置为 false，默认为 true。连接准备就绪后，我们使用 connection.createStatement() 方法创建了一个 Statement 对象。在创建 Statement 对象时，我们使用了 ResultSet 类型 TYPE_SCROLL_INSENSITIVE 和 CONCUR_READ_ONLY。

使用 setSavepoint() 方法，我们设置了一个保存点，然后删除了 ID 为 8 的记录。下一步，使用回滚到之前的保存点来恢复更改。这将恢复删除行的更改。然后，我们删除了 ID 为 9 的记录，并使用 printResultSet() 方法打印记录，该方法打印结果集的所有记录。

将以下示例复制并粘贴到 JDBCExample.java 中，编译并运行如下：

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 JDBCExample {
   static final String DB_URL = "jdbc:mysql://127.0.0.1/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();
      } 
   }
}

输出

现在，让我们编译上面的示例，如下所示：

C:\>javac JDBCExample.java
C:\>

运行 JDBCExample 时，将产生以下结果：

C:\>java JDBCExample
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:\>

使用 SavePoint 回滚到之前的更改示例

在这个示例中，我们有六个静态字符串，包含数据库连接 URL、用户名、密码、SELECT 查询和两个 DELETE 查询。DELETE 查询用于删除 Employees 表中 ID 为 8 的记录。现在，使用 DriverManager.getConnection() 方法，我们准备了一个数据库连接。使用 setAutoCommit(false)，我们将自动提交设置为 false，默认为 true。连接准备就绪后，我们使用 connection.createStatement() 方法创建了一个 Statement 对象。在创建 Statement 对象时，我们使用了 ResultSet 类型 TYPE_SCROLL_INSENSITIVE 和 CONCUR_READ_ONLY。

使用 setSavepoint() 方法，我们设置了一个保存点，然后删除了 ID 为 8 的记录。下一步，使用回滚到之前的保存点来恢复更改。这将恢复删除行的更改。然后，使用 printResultSet() 方法打印记录，该方法打印结果集的所有记录。

将以下示例复制并粘贴到 JDBCExample.java 中，编译并运行如下：

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 JDBCExample {
   static final String DB_URL = "jdbc:mysql://127.0.0.1/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";

   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);

         // Create a savepoint object before executing the 
         // deleteQuery
         Savepoint beforeDeleteSavepoint = conn.setSavepoint();
         stmt.executeUpdate(DELETE_QUERY);  
		 
		 System.out.println("Table EMPLOYEES after executing deleteQuery");
		 ResultSet rs = stmt.executeQuery(QUERY);
		 printResultSet(rs);
         
		 // Rollback the changes after delete save point.
         conn.rollback(beforeDeleteSavepoint);
         
         System.out.println("Table Employees after rollback");
         rs = stmt.executeQuery(QUERY);
         printResultSet(rs);

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

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

输出

现在，让我们编译上面的示例，如下所示：

C:\>javac JDBCExample.java
C:\>

运行 JDBCExample 时，将产生以下结果：

C:\>java JDBCExample
Table EMPLOYEES after executing deleteQuery
ID: 1, Age: 18, First: Zara, Last: Ali
ID: 2, Age: 25, First: Mahnaz, Last: Fatma
ID: 3, Age: 30, First: Zaid, Last: Khan
ID: 4, Age: 28, First: Sumit, Last: Mittal
ID: 7, Age: 20, First: Rita, Last: Tez
Table Employees after rollback
ID: 1, Age: 18, First: Zara, Last: Ali
ID: 2, Age: 25, First: Mahnaz, Last: Fatma
ID: 3, Age: 30, First: Zaid, Last: Khan
ID: 4, Age: 28, First: Sumit, Last: Mittal
ID: 7, Age: 20, First: Rita, Last: Tez
ID: 8, Age: 20, First: Sita, Last: Singh

C:\>
jdbc-transactions.htm
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