谈谈基于SQL Server 的Exception Handling[中篇]
三、 TRY CATCH & Return 在上面一节中，我通过RAISERROR重写了创建User的Stored procedure，实际上上面的Stored procedure是有问题的。我之所以没有立即指出，是因为这是一个很容易犯的错误，尤其是习惯了.NET Exception Handling的人更容易犯这样的错误。我们知道在.NET Application中，如果出现一个未处理的Exception，程序将立即终止，后续的程序将不会执行，但是对于上面的SQL则不一样，虽然我们通过RAISERROR将Error抛出，但是SQL的指定并不会被终止，INSERT语句仍然会被执行的。我想很多人会说在RAISERROR后加一个Return就可以了嘛。不错这是一个常用的解决方案，但是我不倾向于使用这种方法。为了更清楚地说明这个问题，我们举另一个相关的例子，上面我们介绍了创建User的例子，我们现在来引入另一个例子：如何将一个User添加到一个Role里面。由于这个例子在后面还将使用，我先讲设计的Table的结构介绍一下：T_USERS和T_ROLES分别存放User和Role，User和Role不区分大小写并且唯一，两者通过T_USERS_IN_ROLES进行关联。现在我们来写将user添加到Role的Stored Procedure：首先验证User和Role是否存在，然后验证该User和Role是否已经存在，最后将Mapping关系添加到T_USERS_IN_ROLES中：
CREATE Procedure P_USERS_IN_ROLES_I
(
@user_name NVARCHAR(256),
@role_name NVARCHAR(256)
)
AS
DECLARE @user_id VARCHAR(50)
DECLARE @role_id VARCHAR(50)
SELECT @user_id = FROM dbo.T_USERS WHERE LOWERED_USER_NAME = LOWER(@user_name)
IF(@user_id IS NULL)
BEGIN
RAISERROR ('The user dose not exist',16,1)
RETURN
END
SELECT @role_id = FROM dbo.T_ROLES WHERE LOWERED_ROLE_NAME = LOWER(@role_name)
IF(@role_id IS NULL)
BEGIN
RAISERROR ('The role dose not exist',16,1)
RETURN
END
IF(EXISTS(SELECT * FROM T_USERS_IN_ROLES WHERE = @user_id AND ROLE_ID = @role_id))
BEGIN
RAISERROR ('The user is already in the role',16,1)
RETURN
END
INSERT INTO dbo.T_USERS_IN_ROLES(,ROLE_ID) VALUES(@user_id,@role_id)
虽然说在上面的Stored procedure中，我们在困难出现Exception的地方添加了RETURN，从而防止了后续的程序继续执行，但是对于一些我们无法预知的Exception呢？我们该如何添加这个RETURN呢？我想有人会说在每条语句执行之后都通过@@ERROR判断是否有Exception出现，我知道很多人喜欢这么做，而事实上，我现在真在维护的一些Stored procedure就是这么做的：全篇都是IF@@ERROR RETURN。其实我们完全可以通过其它的方式是我们的SQL看出来更加优雅一点。那就是使用我们很熟悉的TRY CATCH。在SQL Server中我们通过BEGIN TRY/END TRY和BEGIN CATCH/END CATCH这样的结构来进行Exception Handling。
通过TRY CATCH，上面的Stored procedure可以改成下面的样子：
CREATE Procedure P_USERS_IN_ROLES_I
(
@user_name NVARCHAR(256),
@role_name NVARCHAR(256)
)
AS
DECLARE @user_id VARCHAR(50)
DECLARE @role_id VARCHAR(50)
DECLARE @error_message NVARCHAR(256)
DECLARE @error_serverity INT
DECLARE @error_state INT
BEGIN TRY
SELECT @user_id = FROM dbo.T_USERS WHERE LOWERED_USER_NAME = LOWER(@user_name)
IF(@user_id IS NULL)
BEGIN
RAISERROR ('The user dose not exist',16,1)
END
SELECT @role_id = FROM dbo.T_ROLES WHERE LOWERED_ROLE_NAME = LOWER(@role_name)
IF(@role_id IS NULL)
BEGIN
RAISERROR ('The role dose not exist',16,1)
END
IF(EXISTS(SELECT * FROM T_USERS_IN_ROLES WHERE = @user_id AND ROLE_ID = @role_id))
BEGIN
RAISERROR ('The user is already in the role',16,1)
END
INSERT INTO dbo.T_USERS_IN_ROLES(,ROLE_ID) VALUES(@user_id,@role_id)
END TRY
BEGIN CATCH
SET @error_message = ERROR_MESSAGE)
SET @error_serverity = ERROR_SEVERITY()
SET @error_state = ERROR_STATE()
RAISERROR(@error_message,@error_serverity,@error_state)
END CATCh 当执行上面一个SQL的时候，碰到任何一个我们自己抛出的Exception和系统异常，都会跳到Catch Block中执行相应的操作。在CATCH中，我们把在TRY Block中遇到的Error从新抛出。
在这里有一些需要注意的是：并非所有的Error都会使用SQL的执行流入Catch Block，下面是两个主要的例外：
[*]
Severity20并且会马上中止Session
此外，相信大家也看见了在Catch中使用了一些Error作为前缀的Function，这些Function为系统定义的Function，用于返回当前Error的一些信息，这样的Function有：
[*]
ERROR_NUMBER()：返回Error Number，相当于@@ERROR。
[*]
ERROR_MESSAGE()：返回Error message.
[*]
ERROR_SEVERITY()：返回Error严重级别.
[*]
ERROR_STATE() ：返回Error的状态.
[*]
ERROR_LINE() ：返回出现Error的行号.
ERROR_PROCEDURE() ：返回出现Error的Stored Procedure名称.
四、 Error message & sys.messages
从前面的部分我们可以主要介绍了一种基于RAISERROR和TRY/CATCH的异常处理机制，个人觉得这是一种值得推荐的做法。但是上面的处理有一种不太理想的做法是：在每个Stored procedure中为不同的Error定义了Message。其实在很多情况下，每个Stored procedure都需要处理一些共同的Error，而且对于.NET Application来说往往是通过Message来判断Exception的类型，所以保持各个Stored Procedure的Message的一致性和Stored procedure和Application的Message的一致性就显得尤为重要。所以我们希望的做法是一次定义，对此使用。在Oracle中，我们知道我们可以通过定义具有全局意义的常数来解决，而对于SQL Server，没有全局常数的概念（在我的印象中好像没有），我们需要寻求另一种解决方案：将Message 添加到sys.messages中。
在前面的部分我们说过，sys.messages是可以用于专门存放Error相关的信息：Error number, severity，state，message等。而且他不但可以用于系统与定义error的存储，也可以用于存放我们自定义的Error。更加可喜的是，SQL Server定义了一些built-in stored procedure来用于message的添加、删除和修改：
sp_addmessage [ @msgnum = ] msg_id , [ @severity = ] severity , [ @msgtext = ] 'msg'
[ , [ @lang = ] 'language' ]
[ , [ @with_log = ] 'with_log' ]
[ , [ @replace = ] 'replace' ]
sp_dropmessage [ @msgnum = ] message_number [ , [ @lang = ] 'language' ]
sp_altermessage [ @message_id = ] message_number , [ @parameter = ] 'write_to_log' , [ @parameter_value = ] 'value' 关于如何使用这些stored procedure，可以参阅SQL Server Books Online。在这里，我同下面的script添加我需要的Error。
sp_addmessage @msgnum = 50001,@severity = 16,@msgtext = N'This user is already existent' , @replace ='replace'
GO
sp_addmessage @msgnum = 50002,@severity = 16,@msgtext = N'This role is already existent', @replace ='replace'
Go
sp_addmessage @msgnum = 50003,@severity = 16,@msgtext = N'This user does not exist', @replace ='replace'
GO
sp_addmessage @msgnum = 50004,@severity = 16,@msgtext = N'This role does not exist', @replace ='replace'
GO
sp_addmessage @msgnum = 50005,@severity = 16,@msgtext = N'This user is already in the role', @replace ='replace'
GO
[注：直接操作sys.messages是不被允许的]
五、 ADO.NET Exception Handling
上面所有的都在介绍在Database层面如何进行Exception handling，下面我们同一个简单的Demo，简单介绍一个我么的.NET Application如何处理从Database Engine抛出的Exception。在这里我们使用一个简单的Cosole application模拟一个简单的Security方面的场景：创建用户、创建角色、添加用户到角色。大部分的功能都在上面提到了，在这里做一下总结：
1． 表结构：
2． Messages（通过上面一节末Script在sys.messages中创建）：
· 50001：This user is already existent
· 50002：This role is already existent
· 50003：This user does not exist
· 50004：This role does not exist
· 50005：This user is already in the role
3． Stored procedure
· Create User：P_USERS_I
CREATE Procedure P_USERS_I
(
@user_id varchar(50),
@user_name nvarchar(256)
)
AS
DECLARE @error_number INT
DECLARE @error_serverity INT
DECLARE @error_state INT
BEGIN TRY
IF(EXISTS(SELECT * FROMdbo.T_USERS WHERE LOWERED_USER_NAME = LOWER(@user_name) OR = @user_id))
BEGIN
RAISERROR (50001,16,1)
END
INSERT INTO dbo.T_USERS
(
,
,LOWERED_USER_NAME)
VALUES(@user_id, @user_name, LOWER(@user_name))
END TRY
BEGIN CATCH
SET @error_number = ERROR_NUMBER()
SET @error_serverity =ERROR_SEVERITY()
SET @error_state = ERROR_STATE()
RAISERROR(@error_number,@error_serverity,@error_state)
END CATCH
· Create Role：T_ROLES_I
CREATE Procedure T_ROLES_I
(
@role_id varchar(50),
@role_name nvarchar(256)
)
AS
DECLARE @error_number INT
DECLARE @error_serverity INT
DECLARE @error_state INT
BEGIN TRY
IF(EXISTS(SELECT * FROMdbo.T_ROLES WHERE LOWERED_ROLE_NAME = LOWER(@ROLE_name) OR = @role_id))
BEGIN
RAISERROR (50002,16,1)
END
INSERT INTO dbo.T_ROLES
(
,
,LOWERED_ROLE_NAME)
VALUES(@ROLE_id, @ROLE_name, LOWER(@ROLE_name))
END TRY
BEGIN CATCH
SET @error_number = ERROR_NUMBER()
SET @error_serverity =ERROR_SEVERITY()
SET @error_state = ERROR_STATE()
RAISERROR(@error_number,@error_serverity,@error_state)
END CATCH
· Add User in Role：P_USERS_IN_ROLES_I
CREATE Procedure P_USERS_IN_ROLES_I
(
@user_name NVARCHAR(256),
@role_name NVARCHAR(256)
)
AS
DECLARE @user_id VARCHAR(50)
DECLARE @role_id VARCHAR(50)
DECLARE @error_number INT
DECLARE @error_serverity INT
DECLARE @error_state INT
BEGIN TRY
SELECT @user_id = FROM dbo.T_USERS WHERE LOWERED_USER_NAME = LOWER(@user_name)
IF(@user_id IS NULL)
BEGIN
RAISERROR (50003,16,1)
END
SELECT @role_id = FROM dbo.T_ROLES WHERE LOWERED_ROLE_NAME = LOWER(@role_name)
IF(@role_id IS NULL)
BEGIN
RAISERROR (50004,16,1)
END
IF(EXISTS(SELECT * FROM T_USERS_IN_ROLES WHERE = @user_id AND ROLE_ID = @role_id))
BEGIN
RAISERROR (50005,16,1)
END
INSERT INTO dbo.T_USERS_IN_ROLES(,ROLE_ID) VALUES(@user_id,@role_id)
END TRY
BEGIN CATCH
SET @error_number = ERROR_NUMBER()
SET @error_serverity =ERROR_SEVERITY()
SET @error_state = ERROR_STATE()
RAISERROR(@error_number,@error_serverity,@error_state)
END CATCH
· Delete Data: P_CLEAR_DATA
CREATE Procedure P_CLEAR_DATA
AS
DELETE FROM dbo.T_USERS_IN_ROLES
DELETE FROM dbo.T_USERS
DELETE FROM dbo.T_ROLES
GO
4． Common Function：Utility. ExecuteCommand
private const string connectionStringName = "TestDb";
public static bool ExecuteCommand(string procedureName, Dictionary parameters)
{
ConnectionStringSettings connectionStringSection = ConfigurationManager.ConnectionStrings;
DbProviderFactory dbProviderFactory = DbProviderFactories.GetFactory(connectionStringSection.ProviderName);
using (DbConnection connection = dbProviderFactory.CreateConnection())
{
connection.ConnectionString = connectionStringSection.ConnectionString;
connection.Open();
DbCommand command = connection.CreateCommand();
command.CommandText = procedureName;
command.CommandType = CommandType.StoredProcedure;
DbParameter parameter;
foreach (KeyValuePair param in parameters)
{
parameter = dbProviderFactory.CreateParameter();
parameter.ParameterName = param.Key;
parameter.DbType = DbType.String;
parameter.Value = param.Value;
command.Parameters.Add(parameter);
}
DbTransaction transation = connection.BeginTransaction();
command.Transaction = transation;
try
{
command.ExecuteNonQuery();
transation.Commit();
return true;
}
catch
{
transation.Rollback();
throw;
}
}
}
5． Create User, Create Role, Add User In Role, Delete All Data
private const string ErrorUserExists = "This user is already existent";
private const string ErrorRoleExists = "This role is already existent";
private const string ErrorUserNotExists = "This user does not exist";
private const string ErrorRoleNotExists = "This role does not exist";
private const string ErrorUserInRole = "This user is already in the role";
public staticbool CreateUser(string userName)
{
string procedureName = "P_USERS_I";
Dictionary parameters = new Dictionary();
parameters.Add("user_id", Guid.NewGuid().ToString());
parameters.Add("user_name", userName);
try
{
ExecuteCommand(procedureName, parameters);
return true;
}
catch (Exception ex)
{
if (ex.Message == ErrorUserExists)
{
Console.WriteLine("The user \"{0}\" you specify is already existent!",userName);
return false; ;
}
Console.WriteLine("A unhandled exception is thrown for some unknown reason!");
return false;
}
}
public static bool CreateRole(string roleName)
{
string procedureName = "P_ROLES_I";
Dictionary parameters = new Dictionary();
parameters.Add("role_id", Guid.NewGuid().ToString());
parameters.Add("role_name", roleName);
try
{
ExecuteCommand(procedureName, parameters);
return true;
}
catch (Exception ex)
{
if (ex.Message==ErrorRoleExists)
{
Console.WriteLine("The role \"{0}\" you specify is already existent!",roleName);
return false; ;
}
Console.WriteLine("A unhandled exception is thrown for some unknown reason!");
return false;
}
}
public static bool AddUserInRole(string userName, string roleName)
{
string procedureName = "P_USERS_IN_ROLES_I";
Dictionary parameters = new Dictionary();
parameters.Add("user_name", userName);
parameters.Add("role_name", roleName);
try
{
ExecuteCommand(procedureName, parameters);
return true;
}
catch (Exception ex)
{
if (ex.Message==ErrorUserNotExists)
{
Console.WriteLine("The user \"{0}\" you specify is not existent!", userName);
return false;
}
if (ex.Message==ErrorRoleNotExists)
{
Console.WriteLine("The role \"{0}\" you specify is not existent!",roleName);
return false;
}
if (ex.Message == ErrorUserInRole)
{
Console.WriteLine("The user \"{0}\" is in the role \"{1}\"!",userName,roleName);
return false;
}
Console.WriteLine("A unhandled exception is thrown for some unknown reason!");
return false;
}
}
public static void Clear()
{
ExecuteCommand("P_CLEAR_DATA", new Dictionary());
}
6． Programs
class Program
{
static void Main(string[] args)
{
Utility.Clear();
if (Utility.CreateUser("Artech"))
{
Console.WriteLine("The user \"{0}\" has been sucessully created!", "Artech");
}
if (Utility.CreateUser("Artech"))
{
Console.WriteLine("The user \"{0}\" has been sucessully created!", "Artech");
}
if (Utility.CreateRole("Administrator"))
{
Console.WriteLine("\nThe role \"{0}\" has been sucessully created!", "Administrator");
}
if (Utility.CreateRole("Administrator"))
{
Console.WriteLine("The role \"{0}\" has been sucessully created!", "Administrator");
}
if (Utility.AddUserInRole("Artech", "Administrator"))
{
Console.WriteLine("\nThe user \"{0}\" has been successfully added in the role \"{1}\"", "Artech", "Administrator");
}
if (Utility.AddUserInRole("Dave Crane", "Administrator"))
{
Console.WriteLine("The user \"{0}\" has been successfully added in the role \"{1}\"", "Dave Crane", "Administrator");
}
if (Utility.AddUserInRole("Artech", "Super Administrator"))
{
Console.WriteLine("The user \"{0}\" has been successfully added in the role \"{1}\"", "Artech", "Super Administrator");
}
if (Utility.AddUserInRole("Artech", "Administrator"))
{
Console.WriteLine("The user \"{0}\" has been successfully added in the role \"{1}\"", "Artech", "Administrator");
}
}
7． 最终执行结果
[原创]谈谈基于SQL Server的Exception Handling - PART I
[原创]谈谈基于SQL Server 的Exception Handling - PART II
[原创]谈谈基于SQL Server 的Exception Handling - PART III
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