Backend (Spring Boot)

Question 1

What is DBeaver?

Answer 1

A free and open source universal database tool for developers and database administrators.

Question 2

What is Maven (with command mvn)?

Answer 2

Maven is a build automation tool used primarily for Java projects. Unlike earlier tools like Apache Ant, it uses conventions for the build procedure, and only exceptions need to be written down. An XML file describes the software project being built, its dependencies on other external modules and components, the build order, directories, and required plug-ins.

Question 3

What is the JVM (Java Virtual Machine)?

Answer 3

It is a specification that provides runtime environment in which java bytecode can be executed. A JVM is platform dependent because configuration of each OS differs.

Question 4

What is the JRE (Java Runtime Environtment)?

Answer 4

It is the implementation of JVM. It physically exists. It contains a set of libraries and other files that JVM uses at runtime.

Question 5

What is the JDK (Java Development Kit)?

Answer 5

It is a software development environment used for developing Java applications and applets. It includes the JRE, an interpreter/loader (Java), a compiler (Javac), an archiver (Jar), a documentation generator (Javadoc) and other tools needed in Java development.

Question 6

What is Spring?

Answer 6

It is a framework that helps build web applications. It takes care of dependency injection, handles transactions, implements an MVC framework and provides foundation for the other Spring frameworks (including Spring Boot).

Question 7

What is Spring Boot?

Answer 7

While you can do everything in Spring without Spring Boot, Spring Boot helps you get things done faster:

• Simplifies Spring dependencies, no more version collisions.
• Can be run straight from a command line without an application container.
• Build more with less code: no need for XML, auto-configuration
• Useful tools for running in production, database initialization, environment specific config files, collecting metrics.

Simply put: Spring Boot = (Spring Framework) + (Embedded HTTP servers e.g. Tomcat, Jetty) - (XML <bean> configuration or @Configuration)</bean>

Question 8

What is a POM?

Answer 8

A Project Object Model or POM is the fundamental unit of work in Maven. It is an XML file that contains information about the project and configuration details used by Maven to build the project. It contains default values for most projects.

Question 9

What are the development requirements for Spring?

Answer 9

• Java JDK: $ java -version and $ javac -version
• Maven: $ mvn -v
• Gradle: $ gradle -v

Question 10

What is Spring Initializr?

Answer 10

Spring Initializr provides an extensible API to generate quickstart projects. It provides a simple web UI to configure the project to generate and endpoints that you can use via plain HTTP.

Question 11

What is the mvnw file which comes in the Spring Initializr zip file?

Answer 11

A maven wrapper used to run some commands without having maven installed on the system.

Question 12

$ mvn spring-boot:run

Answer 12

Compile and run an application using maven’s run goal.

Question 13

What is the Super POM?

Answer 13

The Super POM is Maven’s default POM. All POMs extend the Super POM unless explicitly set, meaning the configuration specified in the Super POM is inherited by the POMs you created for your projects.

Question 14

What is the use of parent POMs?

Answer 14

Maven also supports the notion of a parent POM. A parent POM enables you to define an inheritance style relationship between POMs. POM files at the bottom of the hierarchy declare that they inherit from a specific parent POM. The parent POM can then be used to share certain properties and details of configuration.

Question 15

What is JPA?

Answer 15

Java Persistence API, the official API and ORM (Object Relational Mapping) for working with relational data in Java. It is only a specification and not a concrete implementation.

Question 16

What are implementations of JPA?

Answer 16

• Hibernate with commercial support from Red Hat (72% marketshare)
• EclipseLink: reference implementation (13% marketshare)
• OpenJPA (2% marketshare)

Question 17

What is the use of JPA?

Answer 17

One API will support many relational databases so that developers get database independence.

Question 18

How to register a Java class to JPA persistence?

Answer 18

JPA classes are required to:

1. Be identified as being a JPA entity class with @Entity.
2. Have a default constructor.
3. At least have one property defined as the primary key.

Example:

import javax.persistence.*;

@Entity

public class Author {

    @Id

    @GeneratedValue(strategy = GenerationType.AUTO)

    private Long id;

    private String firstName;

    private String lastName;

    public Author(String firstName, String lastName) {

        this. firstName = firstName;
        this. lastName = lastName;

    }

}

Question 19

Define relationships with JPA.

Answer 19

Must be defined on two JPA classes:

• @OnetoOne
• @ManyToOne
• @OneToMany
• @ManyToMany

Example: many-to-many relationship between authors and books with a single join table.

@ManyToMany(mappedBy = "authors")
private Set books = new HashSet();
...
@ManyToMany
@JoinTable(name = "author_book", joinColumns = @JoinColumn(name = "book_id"), inverseJoinColumns = @JoinColumn(name = "author_id"))
private Set authors = new HashSet();

Question 20

What are the equals() and hashcode() methods from java.lang.Object?

Answer 20

For comparing objects:

• equals(Object obj): indicates whether some other object passed as an argument is “equal to” the current instance. The default implementation provided by the JDK is based on memory location — two objects are equal if and only if they are stored in the same memory address.
• hashcode(): returns an integer representation of the object memory address. By default, this method returns a random integer that is unique for each instance, even between several executions of a program.

Question 21

What is advised to Java developers as for the equals() and hashcode() methods for large projects?

Answer 21

The default implementation is not enough to satisfy business needs. As per the Java documentation, developers should override both methods in order to achieve a fully working equality mechanism regardless of the memory addresses — it’s not enough to just implement the equals() method.

Question 22

What is the relation between equals() and hashcode()?

Answer 22

If two objects are equal according to the equals(Object) method, then calling the hashcode()method on each of the two objects must produce the same integer result.

Question 23

What are Spring Data repositories?

Answer 23

A Spring Data repository has methods for retrieving domain objects. These methods delegate to a specialized Repository object such that alternative storage implementations may be interchanged.

Question 24

What is the purpose of Spring Data repositories?

Answer 24

Allow to easily subsitute the persistence layer.

Question 25

What is the difference between a directory and a package in a Java project?

Answer 25

If configured correctly, adding a folder inside src/ is the same as adding a package from File > New Package using an IDE.

Question 26

How is defined the name of a package?

Answer 26

When you create a package/directory under src/ the package name starts from the first subdirectory after src/. So, src/com/sub/test will be package com.sub.test.

Question 27

How to create a Spring Data repository?

Answer 27

In the app directory, create a repositories/ directory. To inject Spring Data into a particular model, create a public interface like the following:

(Note: IdType can be Long, Int…)

package ###.repositories
import ####.model.MyModel
import org.springframework.data.repository.CrudRepository

public interface MyModelRepository extends CrudRepository < MyModel, IdType > { }

Question 28

How to populate the database with some initial data to play with during the development phase?

Answer 28

Create a bootstrap directory in the app directory, then create a DevBootstrap.java file. Example with Authors and Books:

(Note: InitData() is a function where data is created and saved to the database).

@Component
public class DevBootstrap implements ApplicationListener {

    private AuthorRepository authorRepository;
    private BookRepository bookRepository;

public DevBootstrap(AuthorRepository authorRepository, BookRepository bookRepository) {
    this.authorRepository = authorRepository;
    this.bookRepository = bookRepository;
} ~~~

@Override
public void onApplicationEvent(ContextRefreshedEvent contextRefreshedEvent) {
    initData();
}

private void initData() { ... } } ~~~

Question 29

What is the Spring Web MVC framework designed around?

Answer 29

The Spring Web MVC is designed around a DispatcherServlet that handles all HTTP requests and responses.

Question 30

What is the sequence of events corresponding to an incoming HTTP request to DispatcherServlet?

Answer 30

1. After receiving an HTTP request, DispatcherServlet consults the HandlerMapping to call the appropriate Controller.
2. The Controller takes the request and calls the appropriate service methods based on used GET or POST method. The service method will set model data based on defined business logic and returns view name to the DispatcherServlet.
3. The DispatcherServlet will take help from ViewResolver to pickup the defined view for the request.
4. Once view is finalized, The DispatcherServlet passes the model data to the view which is finally rendered on the browser.

Question 31

How to configure a controller in Spring?

Answer 31

1. Annotate Controller class with @Controller.
2. Map methods and/or class to HTTP request paths using @RequestMapping.

Example:

@Controller
public class HelloController { 
   private BookRepository = bookRepository;

public BookController(BookRepository bookRepository) {
this.bookRepository = bookRepository;
}
~~~

@RequestMapping(value = “/books”, method = RequestMethod.GET)
public String getBooks(Model model) {
model.addAttribute(“books”, bookRepository.findAll());
return “books”;
}
}
~~~

Question 32

What does the @Table annotation means? If not used, what happen?

Answer 32

@Table(name=table_name) maps the entity with the table. If no @Table is defined, the default value is used: the class name of the entity.

Question 33

What does the @Id annotation means?

Answer 33

@Id declares the identifier property of the entity.

Question 34

What does the @JoinColumn means?

Answer 34

@JoinColumn indicates the entity is the owner of the relationship: the corresponding table has a column with a foreign key to the referenced table.

Question 35

What does the mappedBy attribute means in a relationship e.g. @ManyToMany()?

Answer 35

mappedBy indicates the entity is the inverse of the relationship.

Question 36

What are the three most common generation strategies for the @GeneratedValue field annotation? Describe them succinctly.

Answer 36

1. @GeneratedValue(strategy = GenerationType.AUTO): lets the persistence provider choose the generation strategy. E.g. Hibernate usually selects GenerationType.Sequence.
2. @GeneratedValue(strategy = GenerationType.IDENTITY): auto-incremented value, very efficient from a database POV but prevent Hibernate to use some optimization techniques like JDBC batching.
3. @GeneratedValue(strategy = GenerationType.SEQUENCE): get the next value from a database sequence. Requires few additional SQL select statements.

Question 37

How to add a unique constraint to a field?

Answer 37

@Entity
@Table(name = "users", 
       uniqueConstraints = {
           @UniqueConstraint(columnNames = {
               "username"
        })
})

Question 38

What does the @NotBlank field annotation does?

Answer 38

It validates that the annotated string is not null or empty.

Question 39

What is the difference between @NotBlank and @NotEmpty?

Answer 39

@NotEmpty ignores trailing whitespaces.

Question 40

What is the difference between the FetchType.LAZY and FetchType.EAGER for relationships?

Answer 40

Suppose you have two entities and there is a relationship between them.

Now when you load entity1 from the database, JPA loads its fields. But you have two options for entity2: to load it together with the rest of the fields (i.e. eagerly) or to load it on-demand (i.e. lazily) when you call the entity1’s getSomeField() method.

Question 41

In which cases is it preferable to use lazy loading over eager loading?

Answer 41

• When an entity has many other entities through @OneToMany or @ManyToMany, it is not efficient to load all the other entities when they are not needed.
• For @OneToOne, eager loading is generally used.

Question 42

What is the difference between the @Column length attribute, the @Size annotation and the @length annotation?

Answer 42

1. @Column is a JPA annotation and the length attribute is used by the schema generation tool to set the associated SQL column length.
2. @Size is a Bean Validation annotation that validates that the associated String has a value whose length is bounded by the minimum and maximum values. It is preferable to @Length as it is more generic.
3. @Length is a Hibernate specific annotation and has the same meaning as @Size.

Question 43

What is the Optional<t> type? What is its purpose?</t>

Answer 43

It is a container type for a value which may be absent. It helps deal with NullPointerException when a developer does not add a null check in the code.

Question 44

When is Optional most commonly used?

Answer 44

To check if an entity is present in a database: e.g. Optional<user> findUserById(String userId);</user>

Question 45

How to return a default value if Optional is empty?

Answer 45

1. Using orElse(): User finalUser = optionalUser.orElse(new User(“Unknown User”));
2. Using orElseget(): User finalUser = optionalUser.orElseGet(() -> { return new User(“0”, “Unknown User”); });

Question 46

How to throw an exception when Optional is empty?

Answer 46

Using orElseThrow():

optionalUser.orElseThrow( () -> new ResourceNotFoundException("User not found with userId " + userId); );

Question 47

What is the purpose of a JSON Web Token (JWT)?

Answer 47

Used to prove that the sent data was actually created by an authentic source.

Question 48

How is a JWT composed?

Answer 48

It is composed of a header, a payload and a signature. The data inside a JWT is encoded and signed, not encrypted. So JWT do not guarantee any security for sensitive data.

Question 49

How are JWT used?

Question 50

What does the @EnableWebSecurity annotation means?

Answer 50

Primary Spring Security annotation used to enable web security in a project.

Question 51

What does the @EnableGlobalMethodSecurity(securedEnabled = true, jsr250Enabled = true, prePostEnabled = true) annotation means?

Answer 51

Used to enable method level security based on annotations: @Secured, @RolesAllowed, @PreAuthorize and @PostAuthorize.

Question 52

What does the security annotation @Secured means and how to use it?

Answer 52

Used to protect controller/service methods like the following:

@Secured("ROLE_ADMIN")
Some method...

@Secured({"ROLE_USER", "ROLE_ADMIN"})
Some method...

@Secured("IS_AUTHENTICATED_ANONYMOUSLY")
Some method...

Question 53

How to use the @RolesAllowed security annotation?

Answer 53

@RolesAllowed("ROLE_ADMIN")
Some method...

Question 54

What is the purpose of the @PreAuthorize and @PostAuthorize security annotations?

Answer 54

• @PreAuthorize(): decide whether a method can actually be invoked or not.
• @PostAuthorize(): less commonly used, perform access-control check after the method has been invoked.

Question 55

What is the WebSecurityConfigurerAdapter from Spring Security?

Answer 55

This class implements Spring Security’s WebSecurityConfigurer interface. It provides default security configurations and allows other classes to extend it and customize the security configurations by overriding its methods.

Question 56

What is the purpose of having a CustomUserDetailsService that implements UserDetailsService interface and provides the implementation for loadUserByUsername() method?

Answer 56

To authenticate a user or perform various role-based checks, Spring Security needs to load users details.

Question 57

What does the loadUserByUsername() method of the CustomUserDetailsService class should return?

Answer 57

A custom UserDetails class that implements the Spring Security UserDetails interface, extended by a UserPrincipal class used by Spring Security to perform authentication and authorization.

Question 58

What is the custom JwtAuthenticationEntryPoint class?

Answer 58

• This class is used to return a 401 unauthorized error to clients that try to access a protected resource without proper authentication.
• Its commence() method is called whenever an exception is thrown due to an unauthenticated user trying to access a resource that requires authentication.
• It implements Spring Security’s AuthenticationEntryPoint interface.

Question 59

What is the custom JwtAuthenticationFilter class?

Answer 59

Use to implement a filter that:

• reads JWT authentication token from the Authorization header of all the requests,
• validates the token,
• loads the user details associated with that token,
• sets the user details in Spring Security’s SecurityContext.

Question 60

What is the Spring Security’s SecurityContext used for?

Answer 60

• Spring Security uses the user details to perform authorization checks.
• We can also access the user details stored in the SecurityContext in our controllers to perform our business logic.

Question 61

What is the AuhtenticationManagerBuilder class?

Answer 61

AuthenticationManagerBuilder is used to create an AuthenticationManagerinstance which is the main Spring Security interface for authenticating a user.

You can use AuthenticationManagerBuilder to build in-memory authentication, LDAP authentication, JDBC authentication, or add your custom authentication provider such as customUserDetailsService and passwordEncoder.

Question 62

What is the HttpSecurity class from Spring Security?

Answer 62

The HttpSecurity configurations are used to configure security functionalities like csrf, sessionManagement, and add rules to protect resources based on various conditions.

For example, we can permit access to static resources, few other public APIs to everyone, authorize custom JWT security filter and restricting access to other APIs to authenticated users only.

Question 63

What does @Transactional means?

Answer 63

The transactional annotation itself defines the scope of a single database transaction. The database transaction happens inside the scope of a persistence context.

Question 64

What is the persistence context in JPA?

Answer 64

The persistence context is in JPA the EntityManager, implemented internally using an Hibernate Session (when using Hibernate as the persistence provider).

The persistence context is just a synchronizer object that tracks the state of a limited set of Java objects and makes sure that changes on those objects are eventually persisted back into the database.

Question 65

What is the JwtTokenProvider class?

Answer 65

A utility class used to generate a JWT after a user logs in successfully, and validating the JWT sent in the Authorization header of the requests.

The utility class reads the JWT secret and expiration time from application.properties.

Question 66

What is a JWT secret?

Answer 66

The secret is a symmetric key known by both the sender and the receiver.

The secret can an arbitrary string of bytes. It can be generated or purposely chosen.

It is negotiated and distributed out of band to the intended recipient of the token.

Question 67

What is the purpose of the @AuthenticationPrincipal annotation?

Answer 67

It is used to access the currently authenticated user in the controllers. The CurrentUser annotation is a wrapper around the @AuthenticatedPrincipal to reduce the dependency on SpringSecurity in case we remove it.

Question 68

What are @GetMapping and @PostMapping?

Answer 68

They are shortcuts for @RequestMapping(method = RequestMethod.GET) to handle HTTP GET requests and They are shortcuts for @RequestMapping(method = RequestMethod.POST) to handle HTTP POST requests.