Clever Geek Handbook

Entretien avec les développeurs Java Backend: Questions et où chercher des réponses. Partie 1



J'avais l'habitude de faire une série d'entretiens pour un développeur Backend Java et de noter des questions pour moi-même pour l'avenir, afin que plus tard je puisse revenir et me rafraîchir la mémoire. Je pensais que cette collection serait probablement utile non seulement pour moi, alors j'ai dépoussiéré, noté les réponses et la partager avec la communauté. Je ne prétends pas être original et exclusif: des articles similaires ont déjà été sur Habré, et dans de nombreux autres endroits - à la fin (dans la deuxième partie) je donnerai une liste de liens pour que la feuille de triche soit la plus complète possible.



— . - - middle, . . , .



, . Java Spring, — .



TL;DR

GitHub- , .





Java



1. Equals, hashcode HashMap.

. HashMap. ? ? , , . — ? , , ? . . . .



equals hashcode:



  1. .
  2. equals, .
  3. , equals ().
  4. , .


, - .



HashMap ( , Java 8, - ). , IDE. :)



2. : , , , , .

ArrayList vs LinkedList. , , — - , ArrayList , LinkedList , « » LinkedList. . , , ArrayList System.arraycopy, , .



3. `Object`.

HashMap, Java.



- ( :D), JavaDoc:



  • clone
  • equals
  • finalize (Deprecated)
  • getClass
  • hashCode
  • toString
  • notify
  • notifyAll
  • wait


, Object .



4. `wait`, `notify`, `notifyAll` `synchronized`.

, Baeldung . , , wait, notify, notifyAll synchronized . Oracle Concurrency Java.

, — . Java Language Specification, 17.1 17.2.



5. JMM. volatile. .

, JMM — , , . , , — , .



, JMM.



, JMM. , . ". Java- ": 1 2.



Java Language Specification, 17.4.



itsobes.ru.

JVM- How ‘volatile’ works on JVM level? Medium.



6. . ? ? JVM? , , - ?

Java Stack Heap.



Stack — , LIFO. frame — . , , , Heap. , Escape Analysis Java 6, , , . Escape Analysis ( ) , .



Frame Stack . Frame , , , . Stack . JVM Specification.



Heap . , telegram- Senior's Blog. :



Heap :



  1. Young Generation

    1. Eden
    2. Survivor 0 Survivor 1


  2. Old Generation

    1. Tenured






Young : Eden, Survivor 0 Survivor 1. Eden . Survivor . Eden , Eden Survivor Survivor, Eden Survivor . . , Survivor, Tenured.



, Tenured, , . , . Tenured ( ), , , . Mark-Sweep-Compact ( , , ).



-, , Eden, Survivor’ . Tenured.



, , , . Survivor , Eden .

GC:





, .

Java ", !" alygin — , , .

74 Podlodka . .



:





gc JVM (, ):



  • medium
  • — OpenJDK
  • Java Garbage Collection Handbook reachability algorithm
  • Tracing garbage collection
  • Simone Bordet ZGC Shenandoah
  • JVM Anatomy Quarks — JVM. , , , -- .


7. Executor ExecutorService, Thread pool ?

— . N (Thread pool) . . .



Executor (void execute​(Runnable command) — ) ExecutorService ( , Callable ) — , . . Executors. - - , , .



:



  • C
  • Baeldung:
  • Oracle


8. Java ? ? heap-dump?

. . heap-dump, jmap, memory profiler ( VisualVM)



:





9. ? ?

parallel stream ForkJoinPool.commonPool Runtime.getRuntime().availableProcessors() — 1. Common pool ForkJoinPool System::exit ( shutdown() shutdownNow()). common pool, pool . Common pool . stream ForkJoinPool — stream Callable submit ForkJoinPool. fork() ForkJoinPool ( ).



ForkJoinPool ExecutorService, ForkJoinTask (RecursiveAction RecursiveTask). pool . ForkJoinPool work stealing — , , . .



:





10. ? ?

2 Java Stream:



  • (Intermediate) — filter, map, sorted, peek .. Stream.
  • (Terminal) — collect, forEach, count, reduce, findFirst, anyMatch .. .


, java.util.stream Collectors.



- , . , , - :





Java Doc, :





:





11. List<? extends Number>, List<? super Number>? , , ?

PECS — Producer extends, Consumer super (Joshua Bloch, Effective Java). — (, , ).



(covariance) — .

List<? extends T> , T , . List<? extends T> ( null) — , , . , T T, T.

, List<? extends Number> ArrayList<Number> ArrayList<Integer>, ArrayList<Object>. get Number, Integer Number.

.

, Java 5, .



List<?> List<? extends Object> .



(contravariance) — .

List<? super T> , T , . List<? super T> T , T . Object, .

, List<? super Number> ArrayList<Number>, ArrayList<Object>, Number(.. ArrayList<Integer>). Integer Double ( Number, ), — Object. get Object — .



— .

List<T> , T. T . T, .

, List<Number> ArrayList<Number>, ArrayList<Integer> ArrayList<Object>. Integer Double ( Number, ), — Object. get Number, Integer Number.



:





12. ConcurrentHashMap?

ConcurrentHashMap — (, , , ""), .

:



  • (Node<K,V>) val () next( ), (Node<K,V>[] table) volatile
  • CAS — , (insert, delete, replace)
  • volatile/atomic , CAS, intrinsics- (jdk.internal.misc.Unsafe)
  • Concurrent resizing
  • LongAdder


:



  • . , non-null , get(key) happens-before
  • ConcurrentHashMap — ConcurrentModificationException,
  • (size, isEmpty, containsValue),
  • null,
  • , ( ) , — forEach, search, reduce (bulk operations). , - ( forEach). parallelismThreshold — , parallelismThreshold. Long.MAX_VALUE . 1 ForkJoinPool.commonPool(),


— java 8 . Segment<K,V> , . concurrencyLevel initialCapacity - :



if (initialCapacity < concurrencyLevel)   // Use at least as many bins
    initialCapacity = concurrencyLevel;   // as estimated threads


ConcurrentMap. ConcurrentMap Baeldung.



13. Xmx Xms, Xss?

JVM Xms heap Xmx.

Xss .

:



java -Xmx<>< >


, (k), (m) (g).

:



java -jar my.jar -Xms256m -Xmx2048m


:



  • Mkyong
  • Tuning JVM Orcale
  • Factors Affecting Garbage Collection Performance Oracle
  • X CLI


14. ?

— , , .

— , , .. lock-free thread-safe . compare-and-swap (CAS) , . CAS.



. volatile value, compareAndSet(current, new), — current. CAS value , (.. current), compareAndSet(current, new). value , . , compareAndSet false. compareAndSet(current, new) current value.



:



  • compare-and-set — current CAS
  • set-and-get — current CAS ,


value VarHandle, Unsafe, . VarHandle — , , , . , /, volotile / compare-and-swap.



java.util.concurrent.atomic :





, :



public class NonReentrantSpinLock {

    private AtomicReference<Thread> owner = new AtomicReference<>();

    public void lock() {
      Thread currentThread = Thread.currentThread();

      while (!owner.compareAndSet(null, currentThread)) {}
    }

    public void unlock() {
      Thread currentThread = Thread.currentThread();
      owner.compareAndSet(currentThread, null);
    }
}


:





15. TreeSet/TreeMap? - ?

TreeMap — NavigableMap, - . Comparator, , . containsKey, get, put remove.

TreeSet — NavigableSet, TreeMap. Comparator, , . add, contains remove.



synchronized ConcurrentModificationException.



null, NullPointerException. null . 7- Java null TreeMap TreeSet .



- , , , . - :



  • ,
  • —


:





16. Java 8 Java <CURRENT_VERSION>?

Java . :



  • legacy- Java 8
  • Java 8,
  • Java 9+ ( 11 LTS, )


8 9 , , Java- . , , , , , , Java 8 , Java.



, :





API, , release notes API Java-.



, , Java:





GraalVM — JDK Java, , :



  • Java
  • Java
  • , , -
  • JIT AOT-
  • ..


:





:





:





17. Java? String? String?

Java 9 UTF-16 (2 ) char.

Java 9 Compact String. Latin-1 ( ), 1 , char . char byte, Latin-1 . byte coder, Latin-1 UTF-16.



String hashcode.



, (final class). , . StringBuilder append. ! Java 9 JEP 280: Indify String Concatenation, . StringBuilder bytecode StringConcatFactory invokedynamic, +.



String pool — heap , . .



, [String.intern()](https://docs.oracle.com/en/java/javase/14/docs/api/java.base/java/lang/String.html#intern()) , . . — JVM Anatomy Quark #10: String.intern().



, equals ( indexOf) JIT .



: java.lang.String The Lord of the Strings: Two Scours.



:





18. ThreadLocal ?

ThreadLocal — , .



ThreadLocal- , Thread. Thread ThreadLocal.ThreadLocalMap threadLocals, ThreadLocal. ThreadLocal.ThreadLocalMap HashMap, WeakReference<ThreadLocal<?>>, ref field . ThreadLocal, — Object. null, (stale) .

, ThreadLocal , . , .

ThreadLocal- ( get), , threadLocals, , . ThreadLocal.

, ThreadLocal- , .



:





19. ? ?

:



  • byte — 1
  • short — 2
  • int — 4
  • long — 8
  • char — 2
  • float — 4
  • double — 8


boolean . — , . JVM. .



— JVM - . Java Objects Inside Out.



:





jvm ( - C++), openjdk. , , :





20. Java?

Java:





, get(). , null. null.



, PhantomReference , ReferenceQueue — , . SoftReference WeakReference , PhantomReference . .



:





Spring



21. scope Spring? ? singleton prototype? scope ? ' prototype singleton?'

Spring scope:



  • singleton ( )
  • prototype
  • request
  • session
  • application
  • websocket


scope , Bealdung. , , Spring- . 2.



prototype singleton :



  • @Lookup
  • prototype-
  • ProxyMod = ScopedProxyMode.TARGET_CLASS

    Bealdung Spring- . 2.


22. Spring. ? , ?

, — BeanCurrentlyInCreationException ( ).



:



  • , —
  • @Lazy
  • setter-,


Bealdung



23. , , , DI

Spring- 1 2. — .



.



24. @Transactional. ? ? @Transactional ? @Transactional @Transactional - ?

, Proxy , .



, MyServiceImpl 2 , @Transactional — method1 method2( Propagation.REQUIRES_NEW). method1 method2.



, Spring AOP, method1() . method1() MyServiceImpl. method1() method2(), , , , .

, .



? Spring- — 1 2. Proxy .



25. ( Boot) Spring- main-?

Spring- ( ), main-. war-. war- , , . Spring Boot war .



:





26. Spring Boot ?

-, spring-boot-starter-parent, spring-boot-dependencies, — , dependencyManagement pom. BOM.



Spring Boot — , properties-.



Spring Boot main- @SpringBootApplication run SpringApplication, ApplicationContext.



@SpringBootApplication
public class MyApplication {

    public static void main(String[] args) {
        SpringApplication.run(MyApplication.class, args);
    }
}


@SpringBootApplication @EnableAutoConfiguration, @ComponentScan @Configuration.



SpringBootApplication WebApplicationContext ( classpath Servlet ConfigurableWebApplicationContext), GenericApplicationContext.



META-INF/spring.factories — org.springframework.boot.autoconfigure.EnableAutoConfiguration - ( , @ComponentScan, @Import ..) .



@EnableAutoConfiguration AutoConfigurationImportSelector, . getCandidateConfigurations SpringFactoriesLoader loadFactoryNames, classpath META-INF/spring.factories - , .



Spring boot spring-boot-autoconfigure META-INF/spring.factories. - , @Conditional ( ) - .



Spring boot jar spring-boot-maven-plugin. jar META-INF/MANIFEST.MF Main-Class — org.springframework.boot.loader.JarLauncher, Start-Class main- . JarLauncher class path ( org.springframework.boot), BOOT-INF( lib class ), Start-Class.



:





:





27. http- Spring?

Spring -:



  • Spring MVC
  • Spring WebFlux


Spring MVC DispatcherServlet, Servlet' Front Controller: Http- . DispatcherServlet WebApplicationContext. DispatcherServlet " " :



  1. HTTP- DispatcherServlet ( ) HandlerMapping, , Controller . HandlerMapping, : BeanNameUrlHandlerMapping RequestMappingHandlerMapping ( RequestMappingInfo @RequestMapping @Controller). HandlerMapping HttpServletRequest — handler- (, HandlerMethod). HandlerMapping HandlerInterceptor — - . HandlerInterceptor' handler- HandlerExecutionChain, DispatcherServlet.
  2. HandlerAdapter . HttpRequestHandlerAdapter ( , HttpRequestHandler), SimpleControllerHandlerAdapter ( , Controller) RequestMappingHandlerAdapter ( @RequestMapping).
  3. applyPreHandle HandlerExecutionChain. true, HandlerInterceptor . false , HandlerInterceptor .
  4. HandlerAdapter HandlerExecutionChain handle , - .
  5. - Controller ( handle) DispatcherServlet ModelAndView. ViewResolver DispatcherServlet , View .

    REST-Controller RESTful- , ModelAndView DispatcherServlet Controller null , , ViewResolver — HttpServletResponse handle. RESTful-, @ResponseBody @Controller @RestController, RESTful.
  6. HandlerExecutionChain applyPostHandle HandlerInterceptor.
  7. , HandlerExceptionResolver. ExceptionHandlerExceptionResolver ( , @ExceptionHandler), ResponseStatusExceptionResolver ( @ResponseStatus HTTP-) DefaultHandlerExceptionResolver ( Spring MVC HTTP-).
  8. Controller , View , DispatcherServlet View, HttpServletResponse. REST-Controller , HttpServletResponse.


HTTP Accept, Spring MVC HttpMessageConverter , , POJO Accept. HttpMessageConverter : Java , Java HTTP .

, Spring Boot HttpMessageConverter, , HttpMessageConverter .



, , Spring MVC javax.servlet.Filter , . Sring MVC .



Spring Security, . .



:





:





Spring WebFlux — -, Spring Framework 5.0. Servlet API ( Servlet 3.1+containers, Netty ( Spring Boot) Undertow), , Reactive Streams Reactor.

Spring WebFlux Spring MVC (RestController, RequestMapping ) . , HandlerFunction.



Spring WebFlux :



  • HttpHandler — HTTP- I/O, Reactive Streams back pressure Reactor Netty, Undertow ..
  • WebHandler — , API HTTP- .


HttpHandler HTTP-, WebExceptionHandler, WebFilter WebHandler. WebHttpHandlerBuilder ApplicationContext.



Spring WebFlux DispatcherHandler, WebHandler Front Controller: Http- . DispatcherHandler — Spring bean, ApplicationContextAware , . DispatcherHandler - webHandler WebHttpHandlerBuilder WebHandler.



DispatcherHandler http- " ", , . :



    @Override
    public Mono<Void> handle(ServerWebExchange exchange) {
        if (this.handlerMappings == null) {
            return createNotFoundError();
        }
        return Flux.fromIterable(this.handlerMappings)
                .concatMap(mapping -> mapping.getHandler(exchange))
                .next()
                .switchIfEmpty(createNotFoundError())
                .flatMap(handler -> invokeHandler(exchange, handler))
                .flatMap(result -> handleResult(exchange, result));
    }


  1. HandlerMapping (- , - ). (handler). HandlerMapping :

  2. , ( invokeHandler) HandlerAdapter, handle . HandlerResult, DispatcherHandler. HandlerAdapter — - DispatcherHandler. HandlerAdapter :

  3. HandlerResult ( handleResult) HandlerResultHandler. . HandlerResultHandler:



:







Dans la deuxième partie, nous parlerons d'Hibernate, des bases de données, des modèles et pratiques de développement, d'une bibliothèque populaire, du support et de la maintenance de nos applications, et nous examinerons également les feuilles de triche alternatives et les résumerons.



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