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详解Java的JDBC API中事务的提交和回滚

2019-11-26 14:43:46
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如果JDBC连接是在自动提交模式下,它在默认情况下,那么每个SQL语句都是在其完成时提交到数据库。

这可能是对简单的应用程序,但有三个原因,你可能想关闭自动提交和管理自己的事务:

  1. 为了提高性能
  2. 为了保持业务流程的完整性
  3. 使用分布式事务

若要控制事务,以及何时更改应用到数据库。它把单个SQL语句或一组SQL语句作为一个逻辑单元,而且如果任何语句失败,整个事务失败。

若要启用,而不是JDBC驱动程序默认使用auto-commit模式手动事务支持,使用Connection对象的的setAutoCommit()方法。如果传递一个布尔值false到setAutoCommit(),关闭自动提交。可以传递一个布尔值true将其重新打开。

例如,如果有一个名为conn Connection对象,以下代码来关闭自动提交:

conn.setAutoCommit(false);

提交和回滚
一旦已经完成了变化,要提交更改,然后调用commit(在连接对象)方法,如下所示:

conn.commit( );

否则回滚更新对数据库所做的使用命名连接conn,使用下面的代码:

conn.rollback( );

下面的例子演示了如何使用一个提交和回滚对象:

try{  //Assume a valid connection object conn  conn.setAutoCommit(false);  Statement stmt = conn.createStatement();    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', 'Singh')";  stmt.executeUpdate(SQL);  // If there is no error.  conn.commit();}catch(SQLException se){  // If there is any error.  conn.rollback();}

在这种情况下没有上述INSERT语句会成功,一切都将被回滚。

事务提交和回滚示例
以下是使用事务提交和回滚描述的例子。

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学习过。

复制过去下面的例子中JDBCExample.java,编译并运行,如下所示:

//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 create statment with   // required arguments for RS example.   System.out.println("Creating statement...");   stmt = conn.createStatement(              ResultSet.TYPE_SCROLL_INSENSITIVE,              ResultSet.CONCUR_UPDATABLE);      //STEP 6: INSERT a row into Employees table   System.out.println("Inserting one row....");   String SQL = "INSERT INTO Employees " +          "VALUES (106, 20, 'Rita', 'Tez')";   stmt.executeUpdate(SQL);    //STEP 7: INSERT one more row into Employees table   SQL = "INSERT INTO Employees " +          "VALUES (107, 22, 'Sita', 'Singh')";   stmt.executeUpdate(SQL);   //STEP 8: Commit data here.   System.out.println("Commiting data here....");   conn.commit();    //STEP 9: 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 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...Inserting one row....Commiting data here....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: 28, First: Sumit, Last: MittalID: 106, Age: 20, First: Rita, Last: TezID: 107, Age: 22, First: Sita, Last: SinghGoodbye!
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