10 mybatis interview questions

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Here are MyBatis Interview questions , I believe everyone will feel familiar and strange 、 I've seen it. I may forget it immediately after a short interview .JavaPub Sort out these easily forgotten key knowledge and answer , Recommended collection , Review and consult often .

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MyBatis

This series 《 Minimum necessary interview questions 》

1. What is? MyBatis

This question is mainly about comparison JDBC Look at

  1. MyBatis It's a ORM( Object relation mapping ) frame , It encapsulates JDBC, You only need to pay attention to SQL Statement itself , It doesn't take a lot of effort to process the load driver , Create connection , establish statement And other complex processes . Developers do not need to write the original ecology sql, Can be strictly controlled sql Executive performance , High flexibility .

  2. MyBatis have access to xml Or annotations to configure mapping native information , take POJO Maps to records in the database , Almost all of them were avoided JDBC Code and manually set parameters and get the result set .

2. MyBatis The advantages of

  1. be based on SQL Statement programming , Relatively flexible ( be relative to hibernate), Support write dynamic sql Statement and can be used repeatedly .

  2. Reduce the amount of code , Eliminates redundant code .( Be similar to JDBC Encapsulation )

  3. And Spring Perfect integration .

  4. Provide mapping labels to support field relationship mapping .

3. #{} and ${} What's the difference ?

  1. #{} Precompiling 、 It's a placeholder ,${} It's string substitution 、 It's a tilde .

  2. Use #{} Can effectively prevent sql Inject , Improve the security of the system .

Mybatis Processing #{} Will be sql Medium #{} Replace with ? Number , call PreparedStatement To assign a value

/* SQL */
Such as :select * from user where name = #{userName}; set up userName=javapub
Looking at the log, we can see that the parsing will #{userName} replaced ?
select * from user where name = ?;
Then take it. javapub Put it in , Put single quotation marks outside

Mybatis Processing Of when Hou Just yes hold {} It's time to put Of when Hou Just yes hold {} Replace with the value of the variable , call Statement To assign a value

/* SQL */
Such as :select * from user where name = #{userName}; set up userName=javapub
Looking at the log, you can find that the values are spliced directly
select * from user where name = javapub;
This is very likely to happen sql Inject , Here is a simple example sql Injection cases

4. One Xml The mapping file , I can write a Dao The interface corresponds to it , This Dao How does the interface work ?

Dao Interface is what people often say Mapper Interface , The fully qualified name of the interface , It's the... In the mapping file namespace Value , The method name of the interface is in the mapping file MappedStatement Of id value , The parameters in the interface method are passed to sql Parameters of .

The methods in the interface are Cannot overload Of , the reason being that All names are limited + Method name The preservation and search strategy of .

Dao The working principle of the interface is JDK A dynamic proxy ,Mybatis Run time will Use JDK The dynamic proxy is Dao Interface generation agent proxy object , Proxy object proxy Blocking interface methods , Instead, execute the... Corresponding to the interface method MappedStatement Represented by the sql, And then sql The execution result returns .

MappedStatement:MappedStatement Maintained a <select|update|delete|insert> Encapsulation of nodes , Including the configuration of the input parameter mapping 、 Executive SQL sentence 、 Result mapping configuration and other information .

<select id="selectAuthorLinkedHashMap" resultType="java.util.LinkedHashMap">
select id, username from author where id = #{value}
</select>

5. How to get auto generated ( Lord ) Key value ?

usage :

stay <insert /> Add... To the tag useGeneratedKeys="true" Equal attribute

<insert id="insert" useGeneratedKeys="true" keyProperty="id" keyColumn="id" parameterType="person" >
INSERT INTO person(name, pswd)
VALUE (#{name}, #{pswd})
</insert>

When Mybatis analysis xml Node is , Read insert When configured , Will judge whether Have configuration useGeneratedKeys, If yes, it will be used Jdbc3KeyGenerator As sql The echo , Otherwise, it will take NoKeyGenerator Echo as primary key .

The bottom layer encapsulates JDBC Get auto increment primary key , That is, when you use prepareStatement perhaps Statement When , Can pass getGeneratedKeys obtain When the primary key of an insert statement is self incremented . Example

 Connection conn = DriverManager.getConnection(url, "root", "123456");
String[] columnNames = {
"id", "name"};
PreparedStatement stmt = conn.prepareStatement(sql, columnNames);
stmt.setString(1, "jack wang");
stmt.executeUpdate();
ResultSet rs = stmt.getGeneratedKeys();
int id = 0;
if (rs.next()) {

id = rs.getInt(1);
System.out.println("----------" + id);
}

6. Mybatis dynamic sql What's the usage? ? What's going on sql? Execution principle ?

Mybatis dynamic sql We can Xml In the mapping file , Write dynamic in the form of tags sql, Complete logical judgment and dynamic splicing sql The function of .

Mybatis Provides 9 Species dynamics sql label : trim | where | set | foreach | if | choose | when | otherwise | bind.

Its executive principle is , Use OGNL from sql Evaluate the value of the expression in the parameter object , Dynamically concatenate according to the value of the expression sql, To complete the dynamic sql The function of .

Is it a little confused , Read on :

The popular science :

OGNL yes Object-Graph Navigation Language Abbreviation , object - Graph line navigation language . for example #{} grammar .

OGNL The function is to do data interaction between objects and views , You can access the object's properties and call the object's methods , The structure diagram of the whole object can be iterated through the expression .

Refer to a very vivid example .

There is a student object student, The attributes are id = 10,name = ‘ Xiao Ming ’ and Course objects course, among course There are properties in the object : fraction score = 88, ranking rank = 5.

The object relationship diagram is as follows :

student
id:10
name: Xiao Ming
course:
score:88
rank:5

When context ( Environmental Science ) The object in is student When , That is to say Mybatis The parameter object passed in during query in is student When :

adopt OGNL Expression gets the property value of the object in the context directly , such as :

#{id} —> 10, Relative to the current context object .getId(), namely student.getId() .

#{name} —> Xiao Ming .

#{course.score} —> 88, amount to student.getCourse().getScore().

therefore , adopt OGNL expression , The structure diagram of the whole object can be iterated .

Release 《 Minimum necessary interview questions 》

7. What is? Mybatis The first level of 、 Second level cache ?

First level cache : be based on PerpetualCache Of HashMap Local cache , Its storage scope is Session, When Session flush or close after , The Session All in Cache Will be emptied , The default L1 cache is on .

When Mybaits And Spring When integrating , No Spring Within the method of the transaction , Every time a database is requested , Will create a new SqlSession, At this time, the L1 cache is not used . Except for business problems , And I called Sqlsession Modification of 、 add to 、 Delete 、commit()、close() Other methods , The L1 cache will also be emptied .

Second level cache It has the same mechanism as the first level cache , Default is also used PerpetualCache,HashMap Storage , The difference is that the storage scope is Mapper(Namespace). Even if the second level cache is turned on , Different sqlsession The cache data between users can't be accessed if they want to , We have to wait until sqlsession After it's closed , The data in the first level cache will be written to the second level cache . The second level cache is not turned on by default .

Most applications now support distributed , Generally, middleware is used as the cache layer , such as redis. Turn on MyBatis The L2 cache of will also be serialized and deserialized one more step , Impact on service performance .

8. MyBatis How it works

A picture is worth a thousand words

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  1. Read MyBatis The configuration file :mybatis-config.xml by MyBatis Global configuration file for , Configured with MyBatis Operation environment and other information , For example, database connection information .

  2. Load mapping file . The mapping file is SQL The mapping file , This file is configured to operate the database SQL sentence , Need to be in MyBatis The configuration file mybatis-config.xml Load in .mybatis-config.xml File can load multiple mapping files , Each file corresponds to a table in the database .

  3. Construct a conversation factory : adopt MyBatis Build session factory with configuration information such as environment SqlSessionFactory.

  4. Create session object : Created by session factory SqlSession object , This object contains the execution SQL All methods of statement .

  5. Executor actuator :MyBatis The bottom defines a Executor Interface to operate the database , It will be based on SqlSession The passed parameters dynamically generate what needs to be executed SQL sentence , Also responsible for query cache maintenance .

  6. MappedStatement object : stay Executor There is one of the execution methods of the interface MappedStatement Parameters of type , This parameter encapsulates the mapping information , Used to store the SQL Of the statement id、 Parameter information .

  7. Input parameter mapping : The input parameter type can be Map、List And so on , It can also be basic data types and POJO type . The input parameter mapping process is similar to JDBC Yes preparedStatement The process of setting parameters for objects .

  8. Output result mapping : The output result type can be Map、 List And so on , It can also be basic data types and POJO type . The output mapping process is similar to JDBC The process of parsing the result set .

9. What is? MyBatis Interface binding of ? What are the ways to do it ?

Interface binding , Is in the MyBatis Any interface defined in , Then the method in the interface and SQL Statement binding , We can call the interface method directly , It's much better than before SqlSession We can have more flexible choices and settings .

Interface binding can be implemented in two ways :

  • adopt Annotation binding , It is to add... To the interface method @Select、@Update Etc , It contains Sql Statement to bind ;

  • adopt xml It says in it SQL To bind , under these circumstances , To specify xml In the mapping file namespace Must be the full pathname of the interface . When Sql When the sentences are simple , Bind... With annotations , When SQL When the sentences are complicated , use xml binding , It's usually used xml There are a lot of binding .

10. Mybatis Paging principle of

Mybatis Use RowBounds Object to paginate , It's about ResultSet Memory paging performed by result set , Not physical paging , Therefore, it is generally not used . Can be in sql Write parameters with physical paging directly to complete the physical paging function , You can also use paging plug-ins to do physical paging .

The principle of the paging plug-in is to use MyBatis Provided plug-in interface , Implement custom plug-ins , In the plug-in's interception method , Intercept what is to be executed SQL, And then according to the settings dialect( dialect ), And set paging parameters , rewrite SQL , Generate... With paging statements SQL, Execute the rewritten SQL, This enables paging .

give an example :select * from student, Intercept sql Then rewrite it to :select t.* from (select * from student)t limit 0,10.

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