详解Java的JDBC API中事务的提交和回滚
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:
- 为了提高性能
- 为了保持业务流程的完整性
- 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:
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:
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:
下面的例子演示了如何使用一个提交和回滚对象:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 | 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:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 | //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 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:
当运行JDBCExample,它会产生以下结果:
1 2 3 4 5 6 7 8 9 10 11 12 | Connecting to database... Creating statement... Inserting one row.... Commiting data here.... 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: 28, First: Sumit, Last: Mittal ID: 106, Age: 20, First: Rita, Last: Tez ID: 107, Age: 22, First: Sita, Last: Singh Goodbye! |

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