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Hibernate / MyBatis Interview questions

What is the Mapper proxy pattern, and how does MyBatis use it internally?

MyBatis generates a working implementation of a Mapper interface at runtime using Java's dynamic proxy mechanism, rather than requiring a hand-written implementation class — when application code calls session.getMapper(UserMapper.class), what's actually returned is a proxy object implementing that interface, backed by an InvocationHandler that intercepts every method call.

flowchart TD A[session.getMapper UserMapper.class] --> B[MyBatis creates a java.lang.reflect.Proxy] B --> C[Proxy implements UserMapper interface] C --> D[Application calls mapper.selectUserById 1] D --> E[Proxy's InvocationHandler intercepts the call] E --> F[Looks up MappedStatement by interface+method name] F --> G[Delegates to SqlSession's underlying execute methods] G --> H[Result mapped and returned as if it were a real method call]

When a method is called on the proxy, the InvocationHandler (MyBatis's internal MapperProxy class) intercepts the call, looks up the MappedStatement matching that interface's fully-qualified name plus the called method's name, and delegates the actual work to the underlying SqlSession's selectOne, selectList, insert, update, or delete methods as appropriate for the statement type.

This is what lets MyBatis provide a fully statically-typed data access API — the compiler checks method signatures against the interface just like any other Java code — while the actual implementation is generated and wired up entirely at runtime, without a developer ever writing (or needing to keep in sync) a concrete class implementing the Mapper interface by hand.

What Java mechanism does MyBatis use to implement a Mapper interface at runtime?
What does the proxy's InvocationHandler use to find the right SQL statement?

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What is MyBatis? What is the purpose of MyBatis? What are the key features of MyBatis? What is the difference between MyBatis and Hibernate? What is a SqlSessionFactory in MyBatis? What is a SqlSession in MyBatis? What is a Mapper interface in MyBatis? What is the mybatis-config.xml file used for? What is a Mapper XML file? How do you write a basic SELECT statement in MyBatis? What is parameter binding in MyBatis? What is result mapping? What are the supported statement types in MyBatis? What is the difference between #{} and ${} in MyBatis? What is a resultMap? How do you configure a data source in MyBatis? What is MyBatis-Spring? How do you integrate MyBatis with Spring Boot? What is dynamic SQL in MyBatis? List the dynamic SQL elements available in MyBatis? What is the difference between MyBatis and JPA/Hibernate in terms of philosophy? Why is #{} preferred over ${} for parameter binding? How does MyBatis prevent SQL injection? What is the difference between resultType and resultMap? What is the @Param annotation used for in MyBatis? Explain how MyBatis handles one-to-many mappings? Explain how MyBatis handles many-to-one mappings? What is the difference between association and collection in resultMap? What is lazy loading in MyBatis, and how do you configure it? Explain the lifecycle of a SqlSession? What is the difference between SqlSessionFactory and SqlSessionFactoryBuilder? How does MyBatis's first-level cache work? How does MyBatis's second-level cache work? What is the difference between first-level and second-level cache? Explain how to implement a custom TypeHandler in MyBatis? What is the purpose of the if, choose, when, otherwise dynamic SQL elements? Explain the internal working of the foreach element for batch operations? How do you handle batch inserts/updates in MyBatis? What is the Mapper proxy pattern, and how does MyBatis use it internally? Explain the execution flow of a MyBatis query from mapper call to result? What is the difference between MyBatis annotations and XML mapping? How do you use MyBatis with multiple data sources? Explain the role of the Executor in MyBatis's internal architecture, including the SIMPLE, REUSE, and BATCH executor types? How does MyBatis handle transactions, and how does it integrate with Spring's transaction management? What is a plugin/interceptor in MyBatis, and how do you write one? Explain how pagination is typically implemented in MyBatis? What is the N+1 select problem, and how does it relate to MyBatis? How do you troubleshoot a MyBatis mapping exception? Explain the internal working of MyBatis's dynamic SQL parsing? Explain the execution flow of MyBatis-Spring-Boot-Starter's auto-configuration?
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