默认情况下,当我们创建一个数据库连接时,会运行在自动提交模式(Auto-commit)下。这意味着,任何时候我们执行一条SQL完成之后,事务都会自动提交。所以我们执行的每一条SQL都是一个事务,并且如果正在运行DML或者DDL语句,这些改变会在每一条SQL语句结束的时存入数据库。有时候我们想让一组SQL语句成为事务的一部分,那样我们就可以在所有语句运行成功的时候提交,并且如果出现任何异常,这些语句作为事务的一部分,我们可以选择将其全部回滚。
让我们通过一个简单的示例理解一下,这里使用JDBC的事务管理来支持数据的完整性。假设我们有一个名为UserDB的数据库,员工的信息分别存储在两张表中。比如我正在使用MySQL数据库,但是同样可以在Oracle和PostgreSQL等其他的关系型数据库上运行。
数据库表中存储员工信息和地址明细。两张表的DDL脚本如下:
CREATE TABLE `Employee` (
`empId` int(11) unsigned NOT NULL,
`name` varchar(20) DEFAULT NULL,
PRIMARY KEY (`empId`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
CREATE TABLE `Address` (
`empId` int(11) unsigned NOT NULL,
`address` varchar(20) DEFAULT NULL,
`city` varchar(5) DEFAULT NULL,
`country` varchar(20) DEFAULT NULL,
PRIMARY KEY (`empId`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
最终的工程如下图,我们将逐个查看这些类:
如图所示,在工程的build path中有一个 MySQL JDBC 的jar包,这样就可以连接到MySQL数据库。
DBConnection.java
package com.journaldev.jdbc.transaction;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
public class DBConnection {
public final static String DB_DRIVER_CLASS = "com.mysql.jdbc.Driver";
public final static String DB_URL = "jdbc:mysql://localhost:3306/UserDB";
public final static String DB_USERNAME = "pankaj";
public final static String DB_PASSWORD = "pankaj123";
public static Connection getConnection() throws ClassNotFoundException,
SQLException {
Connection con = null;
// load the Driver Class
Class.forName(DB_DRIVER_CLASS);
// create the connection now
con = DriverManager.getConnection(DB_URL, DB_USERNAME, DB_PASSWORD);
System.out.println("DB Connection created successfully");
return con;
}
}
在DBConnection类中,创建MySQL数据库连接供其他类使用。
EmployeeJDBCInsertExample.java
package com.journaldev.jdbc.transaction;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.SQLException;
public class EmployeeJDBCInsertExample {
public static final String INSERT_EMPLOYEE_QUERY = "insert into Employee (empId, name) values (?,?)";
public static final String INSERT_ADDRESS_QUERY = "insert into Address (empId, address, city, country) values (?,?,?,?)";
public static void main(String[] args) {
Connection con = null;
try {
con = DBConnection.getConnection();
insertEmployeeData(con, 1, "Pankaj");
insertAddressData(con, 1, "Albany Dr", "San Jose", "USA");
}
catch (SQLException | ClassNotFoundException e) {
e.printStackTrace();
}
finally {
try {
if (con != null)
con.close();
}
catch (SQLException e) {
e.printStackTrace();
}
}
}
public static void insertAddressData(Connection con,
int id,
String address,
String city,
String country) throws SQLException {
PreparedStatement stmt = con.prepareStatement(INSERT_ADDRESS_QUERY);
stmt.setInt(1, id);
stmt.setString(2, address);
stmt.setString(3, city);
stmt.setString(4, country);
stmt.executeUpdate();
System.out.println("Address Data inserted successfully for ID=" + id);
stmt.close();
}
public static void insertEmployeeData(Connection con, int id, String name)
throws SQLException {
PreparedStatement stmt = con.prepareStatement(INSERT_EMPLOYEE_QUERY);
stmt.setInt(1, id);
stmt.setString(2, name);
stmt.executeUpdate();
System.out.println("Employee Data inserted successfully for ID=" + id);
stmt.close();
}
}
这是一个简单的JDBC程序,向前面创建的Employee表和Address表中插入用户提供的数据。当我们将运行这个程序时,将得到以下输出:
Employee Data inserted successfully for ID=1
com.mysql.jdbc.MysqlDataTruncation: Data truncation: Data too long for column 'city' at row 1
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:2939)
at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1623)
at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:1715)
at com.mysql.jdbc.Connection.execSQL(Connection.java:3249)
at com.mysql.jdbc.PreparedStatement.executeInternal(PreparedStatement.java:1268)
at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:1541)
at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:1455)
at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:1440)
at com.journaldev.jdbc.transaction.EmployeeJDBCInsertExample.insertAddressData(EmployeeJDBCInsertExample.java:45)
at com.journaldev.jdbc.transaction.EmployeeJDBCInsertExample.main(EmployeeJDBCInsertExample.java:23)
从结果可以看到,在我们试图往Address表中插入数据时,由于输入的值超过了字段的大小,因此抛出了SQLException异常。
如果浏览Employee和Address表的内容,你会发现Employee表有数据,Address表却没有。这是一个严重的问题,因为只有部分数据正确地被插入。并且如果我们再次运行这个程序,它会再次试图向Employee表插入数据,并且引发下面的异常:
com.mysql.jdbc.exceptions.MySQLIntegrityConstraintViolationException: Duplicate entry '1' for key 'PRIMARY'
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:931)
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:2941)
at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1623)
at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:1715)
at com.mysql.jdbc.Connection.execSQL(Connection.java:3249)
at com.mysql.jdbc.PreparedStatement.executeInternal(PreparedStatement.java:1268)
at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:1541)
at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:1455)
at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:1440)
at com.journaldev.jdbc.transaction.EmployeeJDBCInsertExample.insertEmployeeData(EmployeeJDBCInsertExample.java:57)
at com.journaldev.jdbc.transaction.EmployeeJDBCInsertExample.main(EmployeeJDBCInsertExample.java:21)
所以,我们没有办法把Employee对应的Address数据保存到Address表中。这个程序造成了数据完整性的问题,这也是为什么需要用事务管理来确保两张表都得以成功插入,并且如果发生任何异常全部回滚。
JDBC事务管理
JDBC API提供了setAutoCommit()方法,通过它我们可以禁用自动提交数据库连接。自动提交应该被禁用,因为只有这样事务才不会自动提交,除非调用了连接的commit()方法。数据库服务器使用表锁来实现事务管理,并且它是一种紧张的资源。因此,在操作完成后应该尽快提交事务。让我们编写另外一个程序,这里我将使用JDBC事务管理特性来保证数据的完整性不被破坏。
EmployeeJDBCTransactionExample.java
package com.journaldev.jdbc.transaction;
import java.sql.Connection;
import java.sql.SQLException;
public class EmployeeJDBCTransactionExample {
public static void main(String[] args) {
Connection con = null;
try {
con = DBConnection.getConnection();
// set auto commit to false
con.setAutoCommit(false);
EmployeeJDBCInsertExample.insertEmployeeData(con, 1, "Pankaj");
EmployeeJDBCInsertExample.insertAddressData(con,
1,
"Albany Dr",
"San Jose",
"USA");
// now commit transaction
con.commit();
}
catch (SQLException e) {
e.printStackTrace();
try {
con.rollback();
System.out.println("JDBC Transaction rolled back successfully");
}
catch (SQLException e1) {
System.out.println("SQLException in rollback" + e.getMessage());
}
}
catch (ClassNotFoundException e) {
e.printStackTrace();
}
finally {
try {
if (con != null)
con.close();
}
catch (SQLException e) {
e.printStackTrace();
}
}
}
}
在运行程序之前,请确保你清楚地了解之前插入的数据。当你运行这个程序时,将得到下面的输出:
DB Connection created successfully
Employee Data inserted successfully for ID=1
com.mysql.jdbc.MysqlDataTruncation: Data truncation: Data too long for column 'city' at row 1
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:2939)
at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1623)
at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:1715)
at com.mysql.jdbc.Connection.execSQL(Connection.java:3249)
at com.mysql.jdbc.PreparedStatement.executeInternal(PreparedStatement.java:1268)
at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:1541)
at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:1455)
at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:1440)
at com.journaldev.jdbc.transaction.EmployeeJDBCInsertExample.insertAddressData(EmployeeJDBCInsertExample.java:45)
at com.journaldev.jdbc.transaction.EmployeeJDBCTransactionExample.main(EmployeeJDBCTransactionExample.java:19)
JDBC Transaction rolled back successfully
这段输出和前面的程序很像,但是如果你查看数据库表,就会发现数据没有被插入Employee表。现在我们可以修改城市(city)的值,这样它就可以符合字段要求,重新运行程序就能够把数据插到两张表中。注意:只有当两个插入操作都执行成功时,连接才会提交。如果其中任何一个抛出异常,整个事务会回滚。
JDBC Savepoint示例
有时候一个事务可能是一组复杂的语句,因此可能想要回滚到事务中某个特殊的点。JDBC Savepoint帮我们在事务中创建检查点(checkpoint),这样就可以回滚到指定点。当事务提交或者整个事务回滚后,为事务产生的任何保存点都会自动释放并变为无效。把事务回滚到一个保存点,会使其他所有保存点自动释放并变为无效。
假设我们有一张日志表Logs,用来记录员工信息保存成功的日志。但是因为它只用于日志记录,当插入日志表有任何异常时,我们不希望回滚整个事务。我们来看一下如何用JDBC Savepoint来实现。
CREATE TABLE `Logs` (
`id` int(3) unsigned NOT NULL AUTO_INCREMENT,
`message` varchar(10) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
EmployeeJDBCSavePointExample.java
package com.journaldev.jdbc.transaction;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.SQLException;
import java.sql.Savepoint;
public class EmployeeJDBCSavePointExample {
public static final String INSERT_LOGS_QUERY = "insert into Logs (message) values (?)";
public static void main(String[] args) {
Connection con = null;
Savepoint savepoint = null;
try {
con = DBConnection.getConnection();
// set auto commit to false
con.setAutoCommit(false);
EmployeeJDBCInsertExample.insertEmployeeData(con, 2, "Pankaj");
EmployeeJDBCInsertExample.insertAddressData(con,
2,
"Albany Dr",
"SFO",
"USA");
// if code reached here, means main work is done successfully
savepoint = con.setSavepoint("EmployeeSavePoint");
insertLogData(con, 2);
// now commit transaction
con.commit();
}
catch (SQLException e) {
e.printStackTrace();
try {
if (savepoint == null) {
// SQLException occurred in saving into Employee or Address
// tables
con.rollback();
System.out.println("JDBC Transaction rolled back successfully");
} else {
// exception occurred in inserting into Logs table
// we can ignore it by rollback to the savepoint
con.rollback(savepoint);
// lets commit now
con.commit();
}
}
catch (SQLException e1) {
System.out.println("SQLException in rollback" + e.getMessage());
}
}
catch (ClassNotFoundException e) {
e.printStackTrace();
}
finally {
try {
if (con != null)
con.close();
}
catch (SQLException e) {
e.printStackTrace();
}
}
}
private static void insertLogData(Connection con, int i)
throws SQLException {
PreparedStatement stmt = con.prepareStatement(INSERT_LOGS_QUERY);
// message is very long, will throw SQLException
stmt.setString(1, "Employee information saved successfully for ID" + i);
stmt.executeUpdate();
System.out.println("Logs Data inserted successfully for ID=" + i);
stmt.close();
}
}
这段程序非常容易理解。在数据成功插入Employee表和Address表后,创建了一个Savepoint。如果抛出SQLException,而Savepoint为空,意味着在执行插入Employee或者Address表时发生了异常,所以需要回滚整个事务。
如果Savepoint不为空,意味着SQLException由插入日志表Logs操作引发,所以只回滚事务到保存点,然后提交。
运行上面的程序,可以看到下面的输出信息:
DB Connection created successfully
Employee Data inserted successfully for ID=2
Address Data inserted successfully for ID=2
com.mysql.jdbc.MysqlDataTruncation: Data truncation: Data too long for column 'message' at row 1
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:2939)
at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1623)
at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:1715)
at com.mysql.jdbc.Connection.execSQL(Connection.java:3249)
at com.mysql.jdbc.PreparedStatement.executeInternal(PreparedStatement.java:1268)
at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:1541)
at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:1455)
at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:1440)
at com.journaldev.jdbc.transaction.EmployeeJDBCSavePointExample.insertLogData(EmployeeJDBCSavePointExample.java:73)
at com.journaldev.jdbc.transaction.EmployeeJDBCSavePointExample.main(EmployeeJDBCSavePointExample.java:30)
如果查看数据库表,可以看到数据成功地插入到了Employee表和Address表。需要注意的是,我们有更简单的实现方式。当数据成功插入Employee表和Address表时提交事务,使用另一个事务管理插入日志的操作。这只是为了展示Java程序中JDBC Savepoint的用法。
通过上面的链接下载并使用示例项目,可以尝试使用复杂的Savepoint和JDBC事务API以学习更多内容。
--转自