Backend Java Developer Interview: Questions and Where to Look for Answers. Part 2

A GitHub repository with a full cheat sheet is here , and Habr is still a cake.

3 levels of caching:

• First-level cache. Enabled by default.
• Second-level cache. Disabled by default.
• (Query cache). .

:

• Baeldung
• First Level Cache Second Level Cache

Hibernate Vlad Mihalcea:

• High-Performance Java Persistence: 2, 16 — Caching
• How does Hibernate Query Cache work
• How does Hibernate Collection Cache work
• How does Hibernate store second-level cache entries
• How does Hibernate NONSTRICT_READ_WRITE CacheConcurrencyStrategy work
• How does Hibernate TRANSACTIONAL CacheConcurrencyStrategy work
• How does Hibernate READ_ONLY CacheConcurrencyStrategy work
• How does Hibernate READ_WRITE CacheConcurrencyStrategy work

• Eager Loading — , . (@OneToOne, @ManyToOne, @OneToMany, @ManyToMany) fetch = FetchType.EAGER. @OneToOne @ManyToOne.
• Lazy Loading — , . , (@OneToOne, @ManyToOne, @OneToMany, @ManyToMany) fetch = FetchType.LAZY. @OneToMany, @ManyToMany. proxy-, . LAZY- Hibernate, LazyInitializationException.

"N+1" .

Vlad Mihalcea: , LazyInitializationException .

N+1 Hibernate, .

N+1 , N , . N . slow query log, , .

plain sql (jdbc, JOOQ), ( ) . , . " !?". - , - . ? JOIN . .

Hibernate. "N+1", . .

, FetchType.EAGER. N+1. @ManyToOne . JPQL JOIN FETCH . , FetchType.LAZY.

FetchType.LAZY, , N+1. , . — . JOIN FETCH .

JOIN FETCH , bonjour. @OneToMany FetchMode.SUBSELECT — 2 , . .. .

— . @BatchSize .

:

• JPA EntityGraph
• Hibernate FetchProfile

N+1, db-util Vlad Mihalcea. .

JOOQ N+1 , 17- (01:16:36) +. JOOQ.

:

• Vlad Mihalcea:
• Hibernate —
• DOU JPA Hibernate
• JPoint " Hibernate "

: — , . .

Hibernate, @Embeddable. , Serializable equals hashcode.

, , @EmbeddedId.

Vlad Mihalcea Hibernate.

4 JPA (Hibernate):

• MappedSuperclass — . . @MappedSuperClass, @Entity. , , @AttributeOverride . . .
• Single table — . , -. @Entity- @Inheritance(strategy = InheritanceType.SINGLE_TABLE) @DiscriminatorColumn(name = "YOUR_DISCRIMINATOR_COLUMN_NAME") ( DTYPE VARCHAR). @DiscriminatorValue("ThisChildName") , - . , @DiscriminatorFormula, CASE...WHEN — JPA, Hibernate. . . — NULL.
• Joined table — , . , ( ) . @Entity- @Inheritance(strategy = InheritanceType.JOINED). JOIN, CASE...WHEN, _clazz, (0 (), 1, 2 ..) Hibernate .
• Table per class — MappedSuperclass, . . JPA 2.2 ( 2.12) , Hibernate , . @Entity @Inheritance(strategy = InheritanceType.TABLE_PER_CLASS). (@Id) . @AttributeOverride — . UNION . , Hibernate _clazz , (1, 2 ..). NULL AS some_field. .

:

• : ,
• Vlad Mahalcea: MappedSuperclass

, JavaDoc :

• JPA
• Hibernate

JPA 2.2. pdf.

Postgresql. Postgres Pro:

WHERE HAVING , WHERE , ( , ), HAVING . , WHERE ; . HAVING, , . ( , HAVING, , . WHERE.)

:

• How does a relational database work
• How does a relational database execute SQL statements and prepared statements
• How Database Internally Works
• Let's Build a Simple Database
• Database internals

• A — (Atomicity). , .
• C — (Consistency). .
• I — (Isolation). .
• D — (Durability). (, - ).

:

• Read uncommitted ( ). (, ).
• Read committed ( ). . «» ( , , ).
• Repeatable read ( ). (, ).
• Serializable (). , , — (, ).

:

• MSSQL: , ,
• PostgreSQL. Postgres Professional, egorov PostgreSQL (, , ..). Postgres Professional .
• MySQL: ,

— , . . /. .

:

• B-Tree
• HASH-
• Bitmap
• Spatial grid
• Quadtree
• R-Tree
• .

. .

, :

• — , . B-.
• — , . .
  
  :
• — . ID , ; ID ; ; . .
• — , . , .

:

• Oracle, MySQL, PostgreSQL, MS SQL
• PostgreSQL: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
• «» MySQL (InnoDB)
• « » Postgres
• , , ?
• :
• : ,
• Database Indexes Explained
• Bitmap- B*tree-: ?
• MySQL
• PostgreSQL Java-
• 14 SQL Server,
• ,
• PostgreSQL
• Anatomy of an SQL Index
• SQL Indexing and Tuning e-Book
• Deep dive into Hash indexes for In-Memory OLTP tables
• Indexing based on Hashing
• SQL Server Indexes Interview Questions and Answers
• Top 25 SQL interview questions and answers about indexes
• An in-depth look at Database Indexin

(lock) — .

— , . , . . , , .

:

• shared (read) — , . , , .
• exclusive (write) — , . . , .

— , ( VERSION 0). , , . — (, ) , .

, , . , .

Vlad Mihalcea:

• A beginner’s guide to Java Persistence locking
• A beginner’s guide to database locking and the lost update phenomena
• How do LockModeType.PESSIMISTIC_READ and LockModeType.PESSIMISTIC_WRITE work in JPA and Hibernate
• How does LockModeType.PESSIMISTIC_FORCE_INCREMENT work in JPA and Hibernate
• How does LockModeType.OPTIMISTIC work in JPA and Hibernate
• How does LockModeType.OPTIMISTIC_FORCE_INCREMENT work in JPA and Hibernate
• How to prevent OptimisticLockException with Hibernate versionless optimistic locking
• How does database pessimistic locking interact with INSERT, UPDATE, and DELETE SQL statements
• How does MVCC (Multi-Version Concurrency Control) work
• How does the 2PL (Two-Phase Locking) algorithm work
• How to prevent lost updates in long conversations

:

• .
• Version JPA
• Hibernate – V.
• PostgreSQL: 1.
• PostgreSQL: 2.
• PostgreSQL: 3.
• PostgreSQL: 4.

:

• Pessimistic Locking in JPA
• Optimistic Locking in JPA
• Enabling Transaction Locks in Spring Data JPA
• Deadlock
• Enum LockModeType
• Postgresql
• PostgreSQL Concurrency with MVCC
• Locking Hibernate
• Optimistic locking JOOQ
• Optimistically Locking Your Spring Boot Web Services
• Optimistic and pessimistic locking with SQL

, . . , . , , , .

• S — (The Single Responsibility Principle). .
• O — / (The Open Closed Principle). , .
• L — (The Liskov Substitution Principle). . , .
• I — (The Interface Segregation Principle). , .
• D — (The Dependency Inversion Principle). , .

, , " " , . ? 3 " ". , , . , :)

, -. , . SOLID - . - SOLID .

, , , . Baeldung.

GoF. , , — refactoring.guru. , (, , , ), , . . - .

Coupling () — . .

Cohesion () — . .

.

. , :

• : . , .

coupling cohesion GRASP. : .

(scale cube, The Art of Scalability) : sharding, mirrorring microservices.

• Sharding (data partioning) — .
• Mirroring (horizontal duplication) — .
• Microservices — , -.

:

:

• .
• microservices.io.
• wiki pros cons.
• SCALING APPLICATIONS: THE SCALE CUBE.
• .
• .
• Microservice Patterns and Best Practices Microservices Patterns ( ).

-, . . - "".

— Microservice Patterns and Best Practices. Medium .

microservices.io : . — . .

. — Enterprise Integration Patterns .

CAP — , , :

• Consistency () — — . — - , , .
• Availability () — , .
• Partition tolerance ( ) — , ( ) - ( ).

MongoDB, (), — CP, … , . .

.

:

• : , , , ,
• Visual Guide to NoSQL Systems CAP
• The CAP FAQ
• bigdataschool
• MongoDB stackoverflow
• CAP Theorem IBM
• CAP Theorem and Distributed Database Management Systems
• What is the CAP Theorem? medium

- (SOA) — , , . SOA — , . SOA - .

:

• - ( , -, , ..);
• ;
• " " .
  
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  (MSA) — SOA, , , (). MSA , , .
  
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• (Unix way — );
• ;
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• " " .

, MSA , SOA — . . — , . MSA SOA . , .

:

• SOA
• , SOA API ibm SOA MSA
• Monolith vs SOA vs MSA medium
• Microservices vs SOA DZone
• MSA SOA
• SOA MSA

:

• () -> , , , , , ( )
• ->
• ,

:

• () — (/, ..)
• ,

:

• . : , , , , , 2018
• Wikipedia
• Microservices — Not A Free Lunch!
• —
• What are microservices?
• To go or not to go micro: the pros and cons of microservices
• Microservices in a Nutshell. Pros and Cons

:

• HeadHunter
• :
• ,

(composition) — " " (HAS-A Relationship), .

(aggregation) — " " (HAS-A Relationship), , .

:

• vs vs
• , ,

(Test Double) — ,

:

• Dummy — , , . : , null
• Fake — , , production-. : in-memory (fake database)
• Stub — (system under test — SUT)
• Spy — Stub, , , ..
• Mock — ,

Mock , — .

:

• Mock vs Stub
• Mocks Aren't Stubs
• Test Double
• Test Stub
• Test Spy
• Mock Object
• Fake Object
• Interaction Based Testing Spock Framework
• Mockito
• Mockito Mock vs. Spy in Spring Boot Tests
• Tag: Mockito Bealdung

:

“” , .

:

1. . :
   
   
   • — , , . (Generic).
   • (inclusive) — , . .
2. ( ad-hoc) — () . :
   
   
   • (overloading) , ().
   • — , , .

Java .

:

• Java Challengers #3:
• JVM
• Java
• Java Virtual Machine Specification: Invoking Methods, invokevirtual, Linking
• Java Language Specification: Inheritance, Overriding, and Hiding, Conversions and Contexts, Type Inference

REST – , web-.

REST, :

• URI
• — JSON, XML
• — HTTP. — , . .
• HTTP . GET: ; POST: ; PUT: ; DELETE: . — PUT vs PATCH vs POST.
• HTTP . HTTP- .
• HATEOAS , REST,

, , HTTP- URI, API RESTful. , . , , REST — , . — .

REST- :

• - (Client–server)
• (Stateless) —
• (Cacheable) —
• (Uniform interface) — , , "" , HATEOAS
• (Layered system) — , .
• ( ) (Code on demand (optional)) —

SOAP — .

SOAP:

• — XML (SOAP-XML — Envelope, Header, Body, Fault)
• SOAP- WSDL( XML)
• SOAP — (TCP, UDP, HTTP, SMTP, FTP ..)
• HTTP, GET POST

, web-, .

-:

• REST — , REST vs SOAP, Contract First, Code First, HATEOAS, Java Spring
• REST vs SOAP. 1. REST vs SOAP. 2. ?
• . ( !)
• REST API Best Practices
• RESTful
• RESTful API
• REST-like API
• web API
• RESTful API – ( 1) RESTful API – ( 2)
• : REST
• 7 REST API
• JAX-WS XSD Restrictions
• REST API Java
• Spring Boot: REST API Java
• RESTful Kotlin Spring boot
• SOAP- Java Apache CXF Spring
• SOAP Web- Spring-WS

-:

• REST — SOAP
• REST 200
• REST API
• RESTful API —
  
  :
• REST GraphQL
• REST? JSON-RPC
  
  :
• A Brief Introduction to REST
• REST CookBook
• RESTful Web APIs
• Introduction to Spring Data REST
• What is REST
• Building REST services with Spring
• SOAP Web-
• SOAP

, ! !

Lombok — , , , , . Lombok .

? ? , . — .

— : " - ?". . , -, .

, , Lombok — IDEA. , . ! 2020.3 Lombok- IDEA.

Lombok , . , Hibernate ( ) Lombok entity-. @EqualsAndHashCode, @ToString @Data, equals, hashCode toString , entities. entities .

, , "+" 7-8 — . asm0dey Lombok Lombok . : " , , @SneakyThrows ?" ( ), " @Data. equals ?", " @EqualsAndHashCode StackOverflowException ?".

Lombok — — , . — . Lombok — Lombok, . - , , .

P.S. Lombok onX — (@__ Java 7) . , Spring-.

:

• Project Lombok
• Who Needs Lombok Anyhow
• Be Careful With Lombok
• Don’t use Lombok
• Lombok & Hibernate: How to Avoid Common Pitfalls
• Error handling with @Transactional and @SneakyThrows
• Lombok, sources.jar
• Some Dangers of Using Lombok

, (), .

OpenNET — , .

MSDN.

— .

.

:

• Apache JMeter — , opensource, HTTP, HTTPS, FTP, LDAP, SOAP, TCP, shell-. , java-based , JDBC, Message-oriented middleware (MOM) JMS Java Objects.
• LoadRunner — . , Enterprise.
• LoadNinja — , - . , .
• WebLOAD — . , , , CI/CD.
• LoadUI Pro — SOAP UI, . - , API.
• K6 — opensource , . JavaScript. , , , Kafka, Datadog, InfluxDB, JSON StatsD. CI-.
• . — , — hit-based- HTTP- Phantom jMeter. , , , .
• Gatling — opensource Scala Netty Akka. Scala, Jenkins.

:

• -
• :
• Gatling
• Top 10
• 9
• 10 Apache JMeter
• jMeter —
• Rest API Load testing with Apache JMeter
• Load Testing with Gatling — The Complete Guide
• -

(Virtual Machine — VM) — / , . — , . .. , , . . VM , . backup' VM .

VM .

— / , , .

KVM QEMU, XEN, VMWare Hyper-V.

Docker — , , , .. .

, . , . linux namespaces. cgroups.

, Dockerfile , () , . , , . .

Docker- CI/CD.

Docker- — Docker-Hub

Docker, .

Docker:

• VM Docker?
• " Docker": 1, 2, 3, 4, 5, 6
• Docker.
• docker- ( docker run runc)
• Docker: AWS
• : Docker .
• docker
• Linux namespaces
• Docker
• Docker
• Docker Y
• Docker , ,
• Docker-
• Docker
• 10 Docker: 1, 2
• Docker:
• Docker:
• containerd Docker
• : containerd "" CNCF

:

• . ?
• . 1.1.
• Hyper-V:
• Hyper-V Windows 10
• QEMU-KVM
• KVM.
• : KVM vs. Xen
• Hyper-V KVM?

, :

• Containers vs. virtual machines
• Play with Docker
• Docker
• The Docker EcoSystem
• Hardware-assisted virtualization
• Full virtualization
• : KVM, Hyper-V VMware?
• KVM
• KVM
• Unix. 1: Kernel-based Virtual Machine (KVM)
• Linux/Cgroups
• : cgroups
• , VM Docker
• containerd
• awesome-compose
• Learn Docker & Containers using Interactive Browser-Based Scenarios
• What Is Docker & Docker Container? A Deep Dive Into Docker!
• Docker for beginners

— - Java- — , URL- URL ..

( -, ( Java EE)):

• — Apache Tomcat. : (TomEE)
• Jetty
• Undertow
  
  — , Java EE ( Jakarta EE).
  
  :
• Wildfly
• Apache TomEE
• IBM WebSphere
• Eclipse GlassFish

" , Tomcat" .