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在Java的JDBC使用中设置事务回滚的保存点的方法

2015-12-29 08:53 1161 查看

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，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: 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!

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