Interview Backend-Java-Developer: questions and where to look for answers. Part 1

GitHub- , .

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

equals hashcode:

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

, - .

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

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

HashMap, Java.

- ( :D), JavaDoc:

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

, Object .

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

, — . Java Language Specification, 17.1 17.2.

, JMM — , , . , , — , .

, JMM.

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

Java Language Specification, 17.4.

itsobes.ru.

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

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:

• Serial Garbage Collector
• Parallel Garbage Collector. Java 8.
• Concurrent Mark Sweep (CMS). Deprecated Java 9.
• Garbage-First (G1). Java 9. . G1 Oracle.
• Z Garbage Collector (ZGC)
• Shenandoah Garbage Collector. Java 12. , , — ,

, .

Java ", !" alygin — , , .

74 Podlodka . .

:

• Method Area — , . JVM.
• Program Counter (PC) Register — .
• Run-time Constant Pool — Method Area . , . .
• Native Method Stack — Stack .

gc JVM (, ):

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

— . N (Thread pool) . . .

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

:

• C
• Baeldung:
• Oracle

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

:

• Baeldung , .
• Java
• Java
• Eclipse MAT
• Java ?
• OutOfMemoryError
• Java VisualVM — Browsing a Heap Dump
• VisualVM: , Java-
• , - -
• Different Ways to Capture Java Heap Dumps
• Analyze memory snapshots IntelliJ IDEA
• Analyze objects in the JVM heap IntelliJ IDEA

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 — , , . .

:

• Stream API & ForkJoinPool Fork/Join Framework Java 7
• ForkJoinPool Java 8
• Guide to the Fork/Join Framework in Java Guide to Work Stealing in Java Baeldung
• JavaDoc ForkJoinPool
• Think Twice Before Using Java 8 Parallel Streams DZone
• Java Parallel Streams Are Bad for Your Health! JRebel
• — Java Parallel Stream
• How does the Fork/Join framework act under different configurations?
• ?

2 Java Stream:

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

, java.util.stream Collectors.

- , . , , - :

• Java8 Code Kata
• Experience-Java-8
• — Java. Functional programming

Java Doc, :

• Java 8 Stream API
• The Java 8 Stream API Tutorial
• Java 8 Stream API . , !
• Java 4. Java Stream API
• Java Stream API: ,
• Spliterator

:

• letsCode — Java Stream API: , , !
• CSCenter — 8. Stream API
• Joker 2016 — Stream API

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.

:

• : Generics, API,
• Generics FAQ
• generic ?
• ?
• StackOverflow: , ,
• ,
• Wikipedia
• Wildcards Oracle

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.

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

— , , .

— , , .. 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 :

• AtomicBoolean, AtomicInteger, AtomicLong, AtomicIntegerArray, AtomicLongArray — , , .
• AtomicReference — .
• AtomicMarkableReference — [reference, boolean].
• AtomicStampedReference — [reference, int].
• AtomicReferenceArray —
• AtomicIntegerFieldUpdater, AtomicLongFieldUpdater, AtomicReferenceFieldUpdater — reflection.
• DoubleAccumulator, LongAccumulator — , , - (BinaryOperator) . , , -. - . , , .
• DoubleAdder, LongAdder — , . , - , 0.

, :

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);
    }
}

:

• ?
• Compare and Swap
• java.util.concurrent.*
• "Java Concurrency " —
• JDK concurrent package
• Atomic operations
• Concurrency: 6 shared state
• The Art of Multiprocessor Programming
• The JSR-133 Cookbook for Compiler Writers
• AtomicReference: A (Sometimes Easier) Alternative to Synchronized Blocks
• An Introduction to Atomic Variables in Java Bealdung
• Use AtomicReference to implement Reentrant Lock
• A comprehensive understanding of Java atomic variable classes
• Faster Atomic*FieldUpdaters for Everyone
• — Unsafe, : VarHandles
• Introduction to nonblocking algorithms

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

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

synchronized ConcurrentModificationException.

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

- , , , . - :

• ,
• —

:

• Java .
• Java TreeMap vs HashMap
• 10 TreeMap Java Interview Questions TreeSet Interview Questions
• Internal Working of TreeMap in Java
• A Guide to TreeMap in Java A Guide to TreeSet in Java Bealdung
• - :
• - —
• -
• - . .

Java . :

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

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

, :

• 9: Project Jigsaw aka , HTTP/2 Client (Incubator), jshell, G1 GC , Compact Strings .
• 10: Local-Variable Type Inference (var), Parallel Full GC G1, Graal JIT- .
• 11 LTS: var , single-file java, String, Epsilon GC (Experimental), ZGC (Experimental) .
• 12: Switch Expressions (Preview), Shenandoah (Experimental), G1, JMH
• 13: Text Blocks (Preview)
• 14: Pattern Matching instanceof (Preview), Packaging Tool (Incubator), NullPointerExceptions, Records (Preview) .
• 15: Sealed Classes (Preview), Hidden Classes, Nashorn JavaScript Engine JDK .

API, , release notes API Java-.

, , Java:

• Amber — , Java. JEP: var (JDK 10), Switch Expressions, Sealed Types, Records, Text Blocks, Pattern Matching instanceof .
• Panama — JVM . .
• Loom — Java . : .
• Valhalla — VM. : Inline types, Generics over Primitive Types, Enhanced volatiles .
• Lanai — Java- MacOS Metal Apple platform API. C 14 2020 Early-Access .

GraalVM — JDK Java, , :

• Java
• Java
• , , -
• JIT AOT-
• ..

:

• Javaswag:
• 172 Java ,

:

• Java 8-14
• API, - Java 8. 1
• JAVA 9. ?
• Java 9
• Java 9
• Java 9
• Java 10 General Availability
• Java 10
• Java 10
• Java 10
• " Java 10": Java 11
• 90 ( API) JDK 11
• Java 11: String
• Java 11 / JDK 11: General Availability
• 39 , Java 12
• Java 12! JEP-
• Java 12: The Teeing Collector
• Java 13
• Java 13 " "
• Introducing Java 13: Let's dive Into JDK's New Features
• Java 14
• Java 14 is coming
• Java 14: Record, instanceof, jpackage, switch-
• Java 14
• instanceof Java 14
• sealed Java 15
• Sealed classes. Semantics vs performance
• Sealed Java
• Java 15?
• Java 15
• Project Panama: Java " "?
• : Java. Project Loom
• Project Loom: Java
• , GraalVM
• Graal — JIT- JVM Java
• Graal: JIT- JVM
• GraalVM
• : OpenJDK-11 + GraalVM
• JavaScript, Java, ?
• GraalVM?
  
  :
• Java-
• State of Loom: 1 2
• GraalVM

:

• Cay Horstmann — Feature evolution in Java 13 and beyond
• — Java 9-14:
• — Java 9 . OSGi?
• Cay Horstmann — Java 9: the good parts (not modules)
• — Project Panama: Java “ ”?
• — GraalVM
• — Graal, Value Types, Loom
• — Java ahead of time GraalVM
• — , partial evaluation, GraalVM
• Project Loom JDK 14 letsCode
• GOTO 2019 • Life After Java 8 • Trisha Gee
• Dalia Abo Sheasha — Migrating beyond Java 8
• Project Loom: Helping Write Concurrent Applications on the Java Platform by Ron Pressler

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.

:

• String javadoc
• ?
• String?
• JDK 9/JEP 280:
• Java 9
• Guide to Java String Pool Baeldung
• Compact Strings in Java 9
• — invokedynamic
• Charles Nutter — Let's Talk About Invokedynamic
• JEP-303 invokedynamic

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- , .

:

• thread local ?
• ThreadLocal
• ThreadLocal
• An Introduction to ThreadLocal in Java
• ThreadLocal
• 5 ,

:

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

boolean . — , . JVM. .

— JVM - . Java Objects Inside Out.

:

• The Java Virtual Machine Specification
• ?
• ?
• ?
• Java
• Java Objects Inside Out
• jol
• JVM ?
• Java
• Measuring Object Sizes in the JVM Bealdung

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

• instanceOop.hpp
• klass.hpp
• instanceKlass.hpp
• objArrayKlass.hpp
• objArrayOop.hpp
• oopFactory.hpp

Java:

• Strong reference — Java. GC , ( ).
• — , . :
  
  
  • Soft reference — , SoftReference. GC OutOfMemoryError. , OutOfMemoryError .
  • Weak reference — , WeakReference. GC . . WeakHashMap.
  • Phantom reference — , PhantomReference. GC , . , ( , finalize(), , deprecated).

, get(). , null. null.

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

:

• PhantomReferences Java

Spring scope:

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

scope , Bealdung. , , Spring- . 2.

prototype singleton :

• @Lookup
• prototype-
• ProxyMod = ScopedProxyMode.TARGET_CLASS
  
  Bealdung Spring- . 2.

, — BeanCurrentlyInCreationException ( ).

:

• , —
• @Lazy
• setter-,

Bealdung

Spring- 1 2. — .

.

, Proxy , .

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

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

, .

? Spring- — 1 2. Proxy .

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

:

• WAR
• jar war?
• Spring Boot JAR WAR RUS ENG

-, 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.

:

• Boot yourself, Spring is coming: 1, 2. : 1, 2.
• Spring Boot Starter — ?
• — Spring Boot Starter

:

• : Spring Boot Auto-Configuration, spring-boot-starter Conditional Spring
• H Baeldung: A Comparison Between Spring and Spring Boot, Create a Custom Auto-Configuration with Spring Boot, Intro to Spring Boot Starters, Spring Boot: Configuring a Main Class
• What is Spring Boot? Autoconfigurations In-Depth
• Spring Boot for beginners
• Spring Boot Documentation

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 MVC —
• Spring Security
• An Intro to the Spring DispatcherServlet Bealdung
• HandlerAdapters in Spring MVC Bealdung
• Quick Guide to Spring Controllers Bealdung
• Spring RequestMapping Bealdung
• Http Message Converters with the Spring Framework Bealdung
• How to Define a Spring Boot Filter? Bealdung
• Spring Professional Study Notes
• Spring Security Architecture

:

• Spring Web MVC
• Spring MVC Auto-configuration Spring Boot

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 :
   
   
   • RequestMappingHandlerMapping -, @RequestMapping
   • RouterFunctionMapping
   • SimpleUrlHandlerMapping URL- -
2. , ( invokeHandler) HandlerAdapter, handle . HandlerResult, DispatcherHandler. HandlerAdapter — - DispatcherHandler. HandlerAdapter :
   
   
   • RequestMappingHandlerAdapter — , @RequestMapping
   • HandlerFunctionAdapter — HandlerFunctions
3. HandlerResult ( handleResult) HandlerResultHandler. . HandlerResultHandler:
   
   
   • ResponseEntityResultHandler — ResponseEntity, @Controller
   • ServerResponseResultHandler — ServerResponse,
   • ResponseBodyResultHandler — , @ResponseBody, @RestController
   • ViewResolutionResultHandler — View Resolution

:

• Spring WebFlux
• Spring WebFlux Auto-configuration Spring Boot