Java persistence layer interview questions (2)

#1. Explain the role of namespace in MyBatis.

In large projects, there may be a large number of SQL statements. At this time, it is not easy to give each SQL statement a unique identification (ID). In order to solve this problem, in MyBatis, you can create a unique namespace for each mapping file, so that each SQL statement defined in this mapping file becomes an ID defined in this namespace.

As long as we can ensure that this ID is unique in each namespace, even if the statement IDs in different mapping files are the same, conflicts will no longer occur.

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2. What does dynamic SQL in MyBatis mean?

For some complex queries, we may specify multiple query conditions, but these conditions may or may not exist. If we do not use the persistence layer framework, we may need to assemble the SQL statements ourselves. , but MyBatis provides dynamic SQL functionality to solve this problem.

The elements used to implement dynamic SQL in MyBatis mainly include:

- if - choose / when / otherwise - trim - where - set - foreach

Usage examples:

   <select id="foo" parameterType="Blog" resultType="Blog">        
   select * from t_blog where 1 = 1
        <if test="title != null">            
           and title = #{title}
        </if>
        <if test="content != null">            
           and content = #{content}
        </if>
        <if test="owner != null">            
           and owner = #{owner}
        </if>
   </select>

3. What are the shortcomings of JDBC programming? How does MyBatis solve these problems?​​​​​

MyBatis: Configure the data link pool in SqlMapConfig.xml and use the connection pool to manage database links.

(2) JDBC: Sql statements are written in the code, making the code difficult to maintain. The actual application of sql may change greatly, and sql changes require changes to the java code.

MyBatis: Configure the Sql statement in the XXXXmapper.xml file and separate it from the java code.

(3) JDBC: It is troublesome to pass parameters to the sql statement, because the where condition of the sql statement is not necessarily certain, it may be more or less, and the placeholders need to correspond to the parameters one-to-one.

MyBatis: Mybatis automatically maps java objects to sql statements.

(4) JDBC: It is troublesome to parse the result set. SQL changes lead to changes in the parsing code, and it needs to be traversed before parsing. It would be more convenient to parse the database records into pojo objects.

MyBatis: Mybatis automatically maps sql execution results to java objects.

4. What are the differences between MyBatis and Hibernate?

(1) Mybatis is different from hibernate. It is not entirely an ORM framework because MyBatis requires programmers to write SQL statements themselves. However, mybatis can flexibly configure the SQL statements to be run through XML or annotations. , and map java objects and sql statements to generate the final executed sql, and finally map the results of sql execution to generate java objects.

(2) Mybatis has a low learning threshold and is easy to learn. Programmers can directly write original SQL, which can strictly control SQL execution performance. It is highly flexible and is very suitable for software development that does not have high requirements for relational data models, such as Internet software, enterprise operation software, etc., because the demand for this type of software changes frequently, once the demand changes, the results must be output quickly. However, the premise of flexibility is that mybatis cannot be database independent. If you need to implement software that supports multiple databases, you need to customize multiple sets of sql mapping files, which is a heavy workload.

(3) Hibernate has strong object/relational mapping capabilities and good database independence. For software with high requirements on relational models (such as customized software with fixed requirements), if you use hibernate to develop it, you can save a lot of code and improve efficiency. However, the disadvantage of Hibernate is that the threshold for learning is high, and the threshold for mastery is even higher. Moreover, how to design O/R mapping, how to weigh the performance and object model, and how to use Hibernate well requires strong experience and ability.​ ​ ​ ​

In short, as long as a software architecture with good maintainability and scalability can be made according to the needs of users in a limited resource environment, it is a good architecture, so the framework is only the best if it is suitable.

5. Briefly talk about the first-level cache and second-level cache of MyBatis?

Mybatis first queries the result set in the cache. If there is no result set, it queries the database. If there is, it retrieves the result set from the cache and returns it without going to the database. Mybatis internal storage cache uses a HashMap, and the key is hashCode sqlId Sql statement. value is the java object generated by mapping from the query.

Mybatis's second-level cache is the query cache. Its scope is the namespace of a mapper. That is, querying SQL in the same namespace can obtain data from the cache. The second-level cache can span SqlSession.

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