I'm almost addicted to it. I can't sleep! Let a bug fuck me twice!

author ： Little brother Fu

Blog ：

https://bugstack.cn

precipitation 、 Share 、 grow up , Let yourself and others have something to gain ！

One 、 Preface ： One Bug

I didn't expect one Bug, I was fucked twice ！

I'm probably addicted , I can't sleep , Sit up and open Mac And external display , this Bug There is no reason , Silently looking at the screen with abnormal printing , One is mine , The other one is also mine .

Recently, it may be the volume source code , I'm addicted to cigarettes . First,

《 Handwriting Spring》

, Then the

《 Handwriting Mybatis》

, But I didn't expect a small problem to make me 2 Time ！

Today, this question is mainly reflected in the common Mybatis, When inserting data , We can return the returned value of the library table index through the input parameter object . But through my own handwriting Mybatis, Every time I come back 0, Instead of finally inserting the index value of the library table .

Because it's handwritten , Not directly Mybatis, So I will start from the file parsing 、 Object mapping 、SQL Query for 、 The packaging of the results has been checked , But the problem is not here ？！

• This is this.  selectKey  Configuration of , In the execution of the insert SQL after , Start to get the last index value .

• Usually as long as the configuration is OK , The return object also has a corresponding  id  Field , Then you can get the return value correctly .PS： Here's the problem , Written by little brother fu  Mybatis  It turns out that only one 0！

Two 、 analysis ： Abnormal diagnosis

Most of the R & D partners may not have read  Mybatis  Source code , So it's probably not clear what's going on here , Little brother Fu, draw a picture for you , Tell you what happened so that the result returned is 0 Of .

• Mybatis  The process of processing can be divided into two parts , One part is parsing , The other part is to use . When parsing, put  Mapper XML  Medium  insert  Label statements are parsed out , At the same time, analyze  selectKey  label . After the final parsing , Use the parsed statement information  MappedStatement  The mapping statement class is stored . Convenient for follow-up in  DefaultSqlSession  When performing an operation , It can be downloaded from  Configuration  It is obtained from the configuration item for use .

• So here is a very important point , Is to perform  insert  When inserting , It also contains a query operation . That means , We'll be at once  Insert  in , Contains two execution statements .
  a key
  ：bug It happened here , Why? ？ Because when these two statements are executed at the beginning , When getting links , Each one is to get a new link , So in other words ,insert xxx、select LAST_INSERT_ID()  In two  connection  During connection execution , It's not true , Unable to get the inserted index  ID, Only under a link or a transaction ( once  commit) Can have the characteristics of transaction , Get the auto increment after inserting data ID.

• And because this part was written at the beginning  JdbcTransaction  Realization  Transaction  When the interface gets the connection , Every time it's a new link , The code block is as follows ;
  

• Here's a link to get , At the beginning, there was no  if null  The judgment of the , Every time I get the link directly , So this is not a link under the two  SQL  operation , So we will not get the right result , It is equivalent to just executing alone  
  SELECT LAST_INSERT_ID()
    So the final query result is  0  When it's over ！
  You can test copying this statement to  SQL Query tool

3、 ... and 、 shock ： The same pit

  But in fact, there is such a link problem , Write in little brother Fu's handwriting Spring Also encountered in .

stay  Spring  There is a part about transaction processing , In fact, the operation of these transactions is also correct  JDBC  Packaging operations for , Rely on the links obtained from the data source to manage transactions . And we usually use  Spring  It is also combined with  Mybatis  Configure the data source , Then operate multiple transactions under one transaction  SQL  At the time of statement , How to get the same link .

Because from the above case , We know the characteristics of guaranteed transactions , Need to be under the same link , Even if there are multiple operations SQL

Due to the multiple SQL The operation of , It is equivalent to getting a new one every time  Session  There is a new link from the connection pool , But in order to achieve the characteristics of transactions , Therefore, multiple transactions are required  SQL  The transaction operation needs to be started before , Whether it's manual or annotation .

The transaction start action processing makes some restrictions on transaction propagation behavior and isolation level , In fact, the more important thing is to make multiple  SQL  The execution of get the link , The need is the same . So here we introduce  ThreadLocal  Based on the synchronization feature that it saves information under the same thread operation , In fact, the link obtained from the transaction here , In fact, it is saved to  TransactionSynchronizationManager#resources  Attribute .

Although it's just a small piece of content , But at the beginning of little brother Fu's handwriting Spring When , It was also left out . Until the time of the test , The link discovery transaction is always unsuccessful , At first, I thought that the whole facet logic was not cut in or my operation mode was wrong . Until the debugging code is checked step by step , It is found that there are many SQL The execution of is not the same link obtained , So there is no way to make the transaction effective .

Four 、 common ： Transaction failure

It may be such a small link problem , Sometimes it will cause a lot of exceptions , If we haven't learned the source code , Then you may not know how such a problem happened . Therefore, they often conduct in-depth research and exploration , So that you can explain a problem , It's simpler and more direct .

So you say , What are the reasons for transaction invalidation ？-  Share some common , If you encounter other , You can send it to the comment area to have a look .

• The database engine does not support transactions ： Here we use  MySQL  For example , Its  MyISAM  The engine does not support transaction operations ,InnoDB  It's the engine that supports transactions , Generally, support transactions will use  InnoDB.
  https://dev.mysql.com/doc/refman/8.0/en/storage-en...
    from  MySQL 5.5.5  The default storage engine to start with is ：InnoDB, Before the default is ：MyISAM, So it's worth noting , The underlying engine doesn't support transactions. No matter what happens, it's just a waste of time .

• The method is not  public  Of ： come from  Spring  Official documents 【When using proxies, you should apply the @Transactional annotation only to methods with public visibility. If you do annotate protected, private or package-visible methods with the @Transactional annotation, no error is raised, but the annotated method does not exhibit the configured transactional settings. Consider the use of AspectJ (see below) if you need to annotate non-public methods.】@Transactional  It can only be used for  public  On the way , Otherwise, the transaction will not fail , If it is to be used in non  public  On the way , Can be opened  AspectJ  The proxy pattern .

• Has not been  Spring  management ：
  // @Service -  It is commented out here  public class OrderServiceImpl implements OrderService { @Transactional public void placeOrder(Order order) { // ... } }

• The data source is not configured with transaction manager ： Generally, it comes from the self-developed database routing component  
  @Bean public PlatformTransactionManager transactionManager(DataSource dataSource) { return new DataSourceTransactionManager(dataSource); }

• Abnormally swallowed .catch  Then I ate it directly , Transaction exception cannot be rolled back . At the same time, configure the corresponding exception  
  @Transactional(rollbackFor = Exception.class)

5、 ... and 、 summary ： Learning experience

Many technical problems like this , It all comes from little brother Fu's study of the source code , The first thing is to look at the source code when you encounter problems , Although it is often difficult to smooth out the whole logic , However, a little bit of accumulation will indeed enable R & D personnel to have a more solid understanding of Technology .

So the reason why I write now Spring、 Handwriting Mybatis, I also hope to sort out all such knowledge , Learn the design scheme of complex logic from it 、 Design principles and how to use design patterns to solve the problems of complex scenes .PS： Usually our business code complexity is difficult to reach this level , So I have seen ” God “ after , In the future, it will be easy to design the business undertaken .

The other is to control all kinds of technical details , And accumulated in this experience to apply relevant technical design to some similar  SpringBoot Starter  Development, etc , Only such breadth 、 Height 、 depth , Can we really improve our R & D ability .PS： It is also to go further on the road of technology , Whether it's advanced development 、 Architects 、CTO！