在Java的JDBC使用中设置事务回滚的保存点的方法

在Java的JDBC使用中设置事务回滚的保存点的方法

新的JDBC3.0保存点的接口提供了额外的事务控制。他们的环境中，如Oracle的PL/ SQL中的大多数现代的DBMS支持保存点。

当设置一个保存点在事务中定义一个逻辑回滚点。如果发生错误，过去一个保存点，则可以使用rollback方法来撤消要么所有的改变或仅保存点之后所做的更改。

Connection对象有两个新的方法，可帮助管理保存点：

setSavepoint(String savepointName): 定义了一个新的保存点。它也返回一个Savepoint 对象。

releaseSavepoint(Savepoint savepointName): 删除一个保存点。请注意，它需要一个Savepoint 对象作为参数。这个对象通常是由setSavepoint()方法生成一个保存点。

有一个rollback ( String savepointName ) 方法回滚工作到指定的保存点。

下面的例子演示如何使用Savepoint对象：

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);

}

在这种情况下没有上述INSERT语句会成功，一切都将被回滚。

下面是利用setSavepoint和事务教程描述回滚的例子。

基于对环境和数据库安装在前面的章节中做此示例代码已经解释。

复制下面的例子JDBCExample.java，编译并运行lttRFwy，如下所示：

//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

现在让我们来编译上面的例子如下：

C:>javac JDBCExample.java

当运行JDBCExample，它会产生以下结果：

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: KhanlttRFwy

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!

版权声明：本文内容由网络用户投稿，版权归原作者所有，本站不拥有其著作权，亦不承担相应法律责任。如果您发现本站中有涉嫌抄袭或描述失实的内容，请联系我们jiasou666@gmail.com 处理，核实后本网站将在24小时内删除侵权内容。