Top 10 Spring Boot Interview Questions

25+ Spring Boots Interview Questions

In this article, we will discuss some top 10 interview questions in Spring Boot. These questions are a bit tricky and trending heavily, nowadays, in the job market.

1) What does the @SpringBootApplication annotation do internally?

As per the Spring Boot doc, the @SpringBootApplication annotation is equivalent to using @Configuration@EnableAutoConfiguration, and @ComponentScan with their default attributes. Spring Boot enables the developer to use a single annotation instead of using multiple. But, as we know, Spring provided loosely coupled features that we can use for each individual annotation as per our project needs.

2) How to exclude any package without using the basePackages filter?

There are different ways you can filter any package. But Spring Boot provides a trickier option for achieving this without touching the component scan. You can use the exclude attribute while using the annotation  @SpringBootApplication. See the following code snippet:

@SpringBootApplication(exclude= {Employee.class})
public class FooAppConfiguration {}

3) How to disable a specific auto-configuration class?

You can use the exclude attribute of@EnableAutoConfiguration, if you find any specific auto-configuration classes that you do not want are being applied.

//By using “exclude”
@EnableAutoConfiguration(exclude={DataSourceAutoConfiguration.class})

On the other foot, if the class is not on the classpath, you can use the excludeName attribute of the annotation and specify the fully qualified name instead.

//By using “excludeName”
@EnableAutoConfiguration(excludeName={Foo.class})

Also, Spring Boot provides the facility to control the list of auto-configuration classes to exclude by using the spring.autoconfigure.exclude property. You can add into the application.properties. And you can add multiple classes with comma separated.

//By using property file
spring.autoconfigure.exclude=org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration

4) What is Spring Actuator? What are its advantages?

This is one of the most common interview questions in Spring Boot. As per the Spring doc:

“An actuator is a manufacturing term that refers to a mechanical device for moving or controlling something. Actuators can generate a large amount of motion from a small change.”

As we know, Spring Boot provides lots of auto-configuration features that help developers quickly develop production components. But if you think about debugging and how to debug, if something goes wrong, we always need to analyze the logs and dig through the data flow of our application to check to see what’s going on. So, the Spring Actuator provides easy access to those kinds of features. It provides many features, i.e. what beans are created, the mapping in the controller, the CPU usage, etc. Automatically gathering and auditing health and metrics can then be applied to your application.

It provides a very easy way to access the few production-ready REST endpoints and fetch all kinds of information from the web. But by using these endpoints, you can do many things to see here the endpoint docs. There is no need to worry about security; if Spring Security is present, then these endpoints are secured by default using Spring Security’s content-negotiation strategy. Or else, we can configure custom security by the help of RequestMatcher.

5) How to enable/disable the Actuator?

Enabling/disabling the actuator is easy; the simplest way is to enable features to add the dependency (Maven/Gradle) to the spring-boot-starter-actuator, i.e. Starter. If you don’t want the actuator to be enabled, then don’t add the dependency.

Maven dependency:

<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-actuator</artifactId>
</dependency>
</dependencies>

6) What is the Spring Initializer?

This may not be a difficult question, but the interviewer always checks the subject knowledge of the candidate. It’s often that you can’t always expect questions that you have prepared. However, this is a very common question asked almost all of the time.

The Spring Initializer is a web application that generates a Spring Boot project with everything you need to start it quickly. As always, we need a good skeleton of the project; it helps you to create a project structure/skeleton properly. You can learn more about the Initializer here.

7) What is a shutdown in the actuator?

Shutdown is an endpoint that allows the application to be gracefully shutdown. This feature is not enabled by default. You can enable this by using management.endpoint.shutdown.enabled=true in your application.properties file. But be careful about this if you are using this.

8) Is this possible to change the port of Embedded Tomcat server in Spring boot?

Yes, it’s possible to change the port. You can use the application.properties file to change the port. But you need to mention “server.port” (i.e. server.port=8081). Make sure you have application.properties in your project classpath; REST Spring framework will take care of the rest. If you mention server.port=0 , then it will automatically assign any available port.

9) Can we override or replace the Embedded Tomcat server in Spring Boot?

Yes, we can replace the Embedded Tomcat with any other servers by using the Starter dependencies. You can use spring-boot-starter-jetty  or spring-boot-starter-undertow as a dependency for each project as you need.

10) Can we disable the default web server in the Spring Boot application?

The major strong point in Spring is to provide flexibility to build your application loosely coupled. Spring provides features to disable the web server in a quick configuration. Yes, we can use the application.properties to configure the web application type, i.e.  spring.main.web-application-type=none.

All the best!

For More : https://www.bipinwebacademy.com/2019/04/spring-boot-interview-questions.html

Using Cache in Spring Boot

Let’s imagine a web application, where for each request received, it must read some configuration data of a database. That data doesn’t change usually, but the application, in each request, must connect, execute the correct instructions to read the data, pick it up from the network, etc. Imagine also that the database is very busy or the connection is slow. What would happen? We would have a slow application because it is reading continuously data that hardly changes.

A solution to that problem could be using a cache, but how do you implement it? In that article, I explain how to use a basic cache in Spring Boot.

A Little Theory

The cache is replicated over functions, where for the same entry value, we are waiting for the same return value. That’s why we always have at least one parameter for entry and exit.

A typical example will be this:

@Cacheable(cacheNames="headers")
public int cachedFunction(int value){
..... complicated and difficult calculations ....
  return N;
}

And now, let’s suppose we have the next code for calling that function:

int value=cachedFunction(1);
int otherValue=cachedFunction(2);
int thirdValue=cachedFunction(1);

When executing the program, in the first line, Spring will execute the function and save the result that returns. In the second line, if it doesn’t know the value it must return for the input “2.” Nevertheless, in the third line, Spring will detect that a function tagged as @Cacheable   with the name “headers” was already called with the value “1.” It won’t execute the function, it will only return the value that in the first call it saved.

The cache’s name is important because, among other things, it permits us to have different independent caches, which we could clean to instruct Spring Boot to execute the functions again.

So, the idea is that in each call to a function tagged as @Cacheable it will save the return values for each call in an internal table, in such a way that if it already has a return value for one entry, it doesn’t call to the function.

The Practice

And now, let’s get to the practice.

An example project can be found here.

First, we must include the following dependency in our project.

<dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-cache</artifactId>
</dependency>

Now, we can use the tags that will allow us to use Cache  in our application.

The first tag set is  @EnableCaching. With this label, we tell Spring that it must prepare the support to use Cache. If we do not put it, it will simply not use Cache, regardless of whether we then mark the functions with cache tags.

@SpringBootApplication
@EnableCaching
public class CacheExampleApplication {
    public static void main(String[] args) {
          SpringApplication.run(CacheExampleApplication.class, args);
    }
}

In this example, we read the data of a database using REST requests.

Data  in the CacheDataImpl.java class which is in the package com.profesorp.cacheexample.impl

The function that reads the data is the following:

@Cacheable(cacheNames="headers", condition="#id > 1")
public DtoResponse getDataCache(int id) {         
    try {
        Thread.sleep(500);
    } catch (InterruptedException e) {
    }                              
    DtoResponse requestResponse=new DtoResponse();                     
    Optional<Invoiceheader> invoice=invoiceHeaderRepository.findById(id);
    .....MORE CODE WITHOUT IMPORTANCE ...
}

As can be seen, we have the tag  @Cacheable(cacheNames="headers", condition="#id > 1") 

With this, we told Spring two things:

  1. We want to cache the result of this function.
  2. We put it as a condition that it must store the results in cache if the input is greater than one.

Later, in the function flushCache we put the tag @CacheEvict that cleans the indicated cache. Also, in this case, we tell it to delete all the entries that it has in cache.

@CacheEvict(cacheNames="headers", allEntries=true)
public void flushCache() { }

In the function update we update the database and with the label @CachePut, we inform Spring that it updates the data for the existing value in dtoRequest.id.

Of course, this function must return an object equal to the function labeled with the tag @Cacheable , and we must indicate the input value on which we want to update the data

Running

To understand the application better, we will execute it and give it a request .

The application at the beginning has four invoices in the invoiceHeader table. You can see how it fills the table in the data.sql file

Let’s run the get function of the PrincipalController class. For this we write this:

> curl -s http://localhost:8080/2

The application will return the following:

{"interval":507,"httpStatus":"OK","invoiceHeader":{"id":2,"active":"N","yearFiscal":2019,"numberInvoice":2,"customerId":2}}

The field interval is the time in milliseconds that has takes the application making the request. As can be seen, it has taken more than half a second, because in the getDataCachefunction of CacheDataImpl.java we have a sleep 500instruction.

Now, we execute the call again:

> curl -s http://localhost:8080/2
{"interval":1,"httpStatus":"OK","invoiceHeader":{"id":2,"activo":"N","yearFiscal":2019,"numberInvoice":2,"customerId":2}}

Now the time the call has taken is 1, because Spring hasn’t executed the code of the function, and it has simply returned the value that it had cached.

However, if we request the id as 1, we have indicated that you should not cache this value, always execute the function and therefore we will have a time exceeding 500 milliseconds:

>curl -s http://localhost:8080/1
{"interval":503,"httpStatus":"OK","invoiceHeader":{"id":1,"activo":"S","yearFiscal":2019,"numberInvoice":1,"customerId":1}}
>curl -s http://localhost:8080/1
{"interval":502,"httpStatus":"OK","invoiceHeader":{"id":1,"activo":"S","yearFiscal":2019,"numberInvoice":1,"customerId":1}}
>curl -s http://localhost:8080/1
{"interval":503,"httpStatus":"OK","invoiceHeader":{"id":1,"activo":"S","yearFiscal":2019,"numberInvoice":1,"customerId":1}}

If we call to the flushcache function, we’ll clean the cache and therefore, the next call to the function will execute the code in it.

> curl -s http://localhost:8080/flushcache
Cache Flushed!
> curl -s http://localhost:8080/2
{"interval":508,"httpStatus":"OK","invoiceHeader":{"id":2,"activo":"N","yearFiscal":2019,"numberInvoice":2,"customerId":2}}
> curl -s http://localhost:8080/2
{"interval":0,"httpStatus":"OK","invoiceHeader":{"id":2,"activo":"N","yearFiscal":2019,"numberInvoice":2,"customerId":2}}

Finally, we will see as if we change the value of the field activo to N, since the function that makes the change is labeled with @CacheEvict, it will update the value of the cache, but the getDataCache function won’t execute in the next call.

> curl -X PUT   http://localhost:8080/   -H "Content-Type: application/json"   -d "{\"id\": 2, \"active\": \"N\"}"
>curl -s http://localhost:8080/2
{"interval":0,"httpStatus":"OK","invoiceHeader":{"id":2,"activo":"N","yearFiscal":2019,"numberInvoice":2,"customerId":2}}

Conclusions

Spring without any difficulty allows us to cache the results of the functions. However, you have to take into account that cache is very basic and it is realized in memory. Spring Boot permits us to use external libraries that will allow us to save the data in disc or database.

In the documentation, you can find the different implementations of cache that Spring Boot supports, one of which is EhCache with which you will can different kinds of backend for the data, as well as specify validity times for the data, and more.

Spring Framework Basics: What Is Inversion of Control?

Developers starting with the Spring Framework often get confused with the terminology, specifically dependencies, dependency injection, and Inversion of Control. In this article, we introduce you to the concept of Inversion of Control.

What You Will Learn

  • What is Inversion of Control?
  • What are some examples of Inversion of Control?
  • How does the Spring Framework implement Inversion of Control?
  • Why is Inversion of Control important and what are its advantages?

What Is Inversion of Control?

Approach-1

Have a look at the following implementation of ComplexAlgorithmImpl:

public class ComplexAlgorithmImpl {
BubbleSortAlgorithm bubbleSortAlgorithm = new BubbleSortAlgorithm();
//...
}

One of the numerous things that ComplexAlgorithmImpl does is sorting. It creates an instance of BubbleSortAlgorithm directly within its code.

Approach-2

Now, look at this implementation for a change:

public interface SortAlgorithm {
public int[] sort(int[] numbers);
}

@Component
public class ComplexAlgorithmImpl {
@AutoWired
private SortAlgorithm sortAlgorithm;
//...
}

ComplexAlgorithmImpl here makes use of the SortAlgorithm interface. It also provides a constructor or a setter method where you can set the SortAlgorithminstance into it. The user tells ComplexAlgorithmImpl, which sort algorithm to make use of.

Comparing Approach-1 and Approach-2

Approach-1

  • ComplexAlgorithmImpl can only use BubbleSortAlgorithm; it is tightly coupled.
  • If we need to change ComplexAlgorithmImpl to use quicksort, the relevant code needs to be changed entirely.
  • The control over the BubbleSortAlgorithm dependency is with the ComplexAlgorithmImpl class.

Approach-2

  • ComplexAlgorithmImpl is open to using any implementation of SortAlgorithm, it is loosely coupled.
  • We only need to change the parameter we pass to the constructor or setter of ComplexAlgorithmImpl.
  • The control over the SortAlgorithm dependency is with the user of ComplexAlgorithmImpl.

Inversion Of Control At Play!

In Approach-1, ComplexAlgorithmImpl is tied to a specific sort algorithm.

In Approach-2, it says: give me any sort algorithm and I will work with it.

This is Inversion of Control.

Instead of creating its own dependencies, a class declares its dependencies. The control now shifts from the class to the user of the class to provide the dependency.

Why Is Inversion of Control Important?

Once you write code with Inversion of Control, you can use frameworks like Spring to complete dependency injection and wire up beans and dependencies.

Advantages of Inversion Of Control

  • Inversion of Control makes your code loosely coupled
  • Inversion of Control also makes it easy for the programmer to write effective unit tests

Lastly, be sure to check out the video below on IoC:

image info

Summary

In this article, we talked about Inversion of Control. Instead of a class creating an instance of its own dependency, it leaves it to the user of the class to pass it in and makes code loosely coupled.

Hope you learned something! Let us know what you think in comments below.

50+ Java Interview Questions for Programmers

Hello, guys! Recently, I have been sharing a lot of Java Interview questions and discussion individually, and many of my readers requested to bring them together so that they can have them in the same page and prepare better and this post is the result of that.

This article contains more than 50 Java Interview questions covering all important topics like core Java fundamentals, Java Collection FrameworkJava Multithreading and ConcurrencyJava IOJDBCJVM InternalsCoding ProblemsObject-Oriented programming, etc.

The questions are also picked up from various interviews and they are, by no means, very difficult, and you might have seen them already in your telephonic or face-to-face round of interview.

The questions are also very good to revise important topics like multithreading and collections as I have also shared some useful resources for further learning and improvement like The Complete Java MasterClass to brush up and fill gaps in your Java skills.

So what are we waiting for here is the list of some of the frequently asked Java questions from interviews from both beginner and experienced Java developer of 2 to 5 years experience:

Java Interview Questions and Answers

1)  How Java achieves platform independence? (answer)
hint: bytecode and Java Virtual Machine

2)  What is ClassLoader in Java? (answer)
hint: part of JVM that loads bytecodes for classes. You can write your own.

3)  Write a Java program to check if a number is Even or Odd? (answer)
hint: you can use bitwise operator, e.g. bitwise AND &, remember, even the number has zero at the end in binary format and an odd number has 1 in the end.

4)  Difference between ArrayList and HashSet in Java? (answer)
hint: all differences between List and Set are applicable here, e.g. ordering, duplicates, random search, etc.

5)  What is double checked locking in Singleton? (answer)
hint: two-time check whether instances is initialized or not, first without locking and second with locking.

6)  How do you create thread-safe Singleton in Java? (answer)
hint: many ways, e.g. using Enum or by using double-checked locking pattern or using a nested static class.

7)  When to use volatile variable in Java? (answer)
hint: when you need to instruct the JVM that a variable can be modified by multiple threads and give hint to JVM that does not cache its value.

8)  When to use a transient variable in Java? (answer)
hint: when you want to make a variable non-serializable in a class, which implements the Serializable interface. In other words, you can use it for a variable whose value you don’t want to save. See The Complete Java MasterClass to learn about transient variables in Java.

9)  Difference between the transient and volatile variable in Java? (answer)
hint: totally different, one used in the context of serialization while the other is used in concurrency.

10) Difference between Serializable and Externalizable in Java? (answer)
hint: Externalizable gives you more control over the Serialization process.

11) Can we override the private method in Java? (answer)
hint: No, because it’s not visible in the subclass, a primary requirement for overriding a method in Java.

12) Difference between Hashtable and HashMap in Java? (answer)

hint: several but most important is Hashtable, which is synchronized, while HashMap is not. It’s also legacy and slow as compared to HashMap.

13) Difference between Listand Set in Java? (answer)
hint: List is ordered and allows duplicate. Set is unordered and doesn’t allow duplicate elements.

14) Difference between ArrayList and Vector in Java (answer)
hint: Many, but most important is that ArrayList is non-synchronized and fast while Vector is synchronized and slow. It’s also legacy class like Hashtable.

15) Difference between Hashtable and ConcurrentHashMap in Java? (answer)
hint: more scalable

16) How does ConcurrentHashMap achieve scalability? (answer)
hint: by dividing the map into segments and only locking during the write operation.

17) Which two methods you will override for an Object to be used as Key in HashMap? (answer)
hint: equals and hashcode

18) Difference between wait and sleep in Java? (answer)
hint: The wait() method releases the lock or monitor, while sleep doesn’t.

19) Difference between notify and notifyAll in Java? (answer)
hint: notify notifies one random thread is waiting for that lock while notifyAll inform to all threads waiting for a monitor. If you are certain that only one thread is waiting then use notify, or else notifyAll is better. See Threading Essentials Mini-Course by Java Champion Heinz Kabutz to learn more about threading basics.

20) Why you override hashcode, along with equals() in Java? (answer)
hint: to be compliant with equals and hashcode contract, which is required if you are planning to store your object into collection classes, e.g. HashMap or ArrayList.

21) What is the load factor of HashMap means? (answer)
hint: The threshold that triggers the re-sizing of HashMap is generally 0.75, which means HashMap resize itself if it’s 75 percent full.

22) Difference between ArrayList and LinkedList in Java? (answer)
hint: same as an array and linked list, one allows random search while other doesn’t. Insertion and deletion easy on the linked list but a search is easy on an array. See Java Fundamentals: Collections Richard Warburton course on Pluralsight to learn more about essential Collection data structure in Java.

23) Difference between CountDownLatch and CyclicBarrier in Java? (answer)
hint: You can reuse CyclicBarrier after the barrier is broken but you cannot reuse CountDownLatch   after the count reaches to zero.

24) When do you use Runnable vs Thread in Java? (answer)
hint: always

25) What is the meaning of Enum being type-safe in Java? (answer)
hint: It means you cannot assign an instance of different Enum type to an Enum variable. e.g. if you have a variable like DayOfWeek day then you cannot assign it value from DayOfMonth enum.

26) How does Autoboxing of Integer work in Java? (answer)
hint: using valueOf() method

27) Difference between PATH and Classpath in Java? (answer)
hint: PATH is used by the operating system while Classpath is used by JVM to locate Java binary, e.g. JAR files or Class files. See Java Fundamentals: The Core Platform to learn more about PATHClasspath, and other Java environment variable.

28) Difference between method overloading and overriding in Java? (answer)
hint: Overriding happens at subclass while overloading happens in the same class. Also, overriding is a runtime activity while overloading is resolved at compile time.

29) How do you prevent a class from being sub-classed in Java? (answer)
hint: just make its constructor private

30) How do you restrict your class from being used by your client? (answer)
hint:  make the constructor private or throw an exception from the constructor

31) Difference between StringBuilder and StringBuffer in Java? (answer)
hint: StringBuilder is not synchronized while StringBuffer is synchronized.

32) Difference between Polymorphism and Inheritance in Java? (answer)
hint: Inheritance allows code reuse and builds the relationship between class, which is required by Polymorphism, which provides dynamic behavior. See Java Fundamentals: Object-Oriented Design to learn more about OOP features.

33) Can we override static method in Java? (answer)
hint: No, because overriding resolves at runtime while static method call is resolved at compile time.

34) Can we access the private method in Java? (answer)
hint: yes, in the same class but not outside the class

35) Difference between interface and abstract class in Java? (answer)
hint: from Java 8, the difference is blurred. However, a Java class can still implement multiple interfaces but can only extend one class.

36) Difference between DOM and SAX parser in Java? (answer)
hint: DOM loads whole XML File in memory while SAX doesn’t. It is an event-based parser and can be used to parse a large file, but DOM is fast and should be preferred for small files.

37) Difference between throw and throws keyword in Java? (answer)
hint: throws declare what exception a method can throw in case of error but throw keyword actually throws an exception. See Java Fundamentals: Exception Handling to learn more about Exception handling in Java.

38) Difference between fail-safe and fail-fast iterators in Java? (answer)
hint: fail-safe doesn’t throw ConcurrentModificationException while fail-fast does whenever they detect an outside change on the underlying collection while iterating over it.

39) Difference between Iterator and Enumeration in Java? (answer)
hint: Iterator also gives you the ability to remove an element while iterating while Enumeration doesn’t allow that.

40) What is IdentityHashMap in Java? (answer)
hint: A Map, which uses  the == equality operator to check equality instead of the equals() method.

41) What is String pool in Java? (answer)
hint: A pool of String literals. Remember it’s moved to heap from perm gen space in JDK 7.

42) Can a Serializable class contain a non-serializable field in Java? (answer)

hint: Yes, but you need to make it either static or transient.

43) Difference between this and super in Java? (answer)
hint: this refers to the current instance while super refers to an instance of the superclass.

44) Difference between Comparator and Comparable in Java? (answer)
hint: Comparator defines custom ordering while Comparable defines the natural order of objects, e.g. the alphabetic order for String. See The Complete Java MasterClass to learn more about sorting in Java.

45) Difference between java.util.Date and java.sql.Date in Java? (answer)
hint: former contains both date and time while later contains only date part.

46) Why wait and notify method are declared in Object class in Java? (answer)
hint: because they require lock which is only available to an object.

47) Why Java doesn’t support multiple inheritances? (answer)
hint: It doesn’t support because of bad experience with C++, but with Java 8, it does in some sense — only multiple inheritances of Type are not supported in Java now.

48) Difference between checked and unchecked Exception in Java? (answer)
hint: In case of checked, you must handle exception using catch block, while in case of unchecked, it’s up to you; compile will not bother you.

49) Difference between Error and Exception in Java? (answer)
hint: I am tired of typing please check the answer

50) Difference between race condition and deadlock in Java? (answer)
hint: both are errors that occur in a concurrent application, one occurs because of thread scheduling while others occur because of poor coding.

Additional Resources

Lifecycle of a Request-Response Process for a Spring REST API

Developing a REST API or microservice using the Spring Boot framework accelerates the development process, and allows API developers to only focus on writing the core business logic and not worry about all the underlying configurations and setup. This article describes the steps involved in the lifecycle of a request process and how the request is mapped to the appropriate controller method and how a response is returned to the client.

In order to create a REST API to serve a client with a list of users, the tasks involved are

  • Create a class with the @RestController annotation. Due to the annotation, this class will be auto-detected through classpath scanning and the methods annotated with @RequestMapping annotation will be exposed as HTTP endpoints. When an incoming request matches the requirements specified by the @RequestMapping annotation, the method will execute to serve the request.

For our example of a users API, the controller class will look like this:

@RestController
@RequestMapping("/users")
public class UserController {
                @Autowired
                UserService userService

                @RequestMapping(method = RequestMethod.GET)
                public List<UserDTO> findAllUsers() {
                                return userService.findAllUsers();
                }
}
  • Create a class for business logic.
  • Create a class to fetch data from the user table.

From a developer’s perspective, the flow to fetch the list of users from the database can be viewed as below:Image titleHowever, with Spring doing a lot of work for us behind the scenes, the lifecycle of the entire process for making an HTTP request to a resource to serving the response back to the client in either XML/JSON format involves many more steps.

This article describes the entire request to response lifecycle with steps which are managed by Spring.

When a user makes a request for a resource, for example:

Request: http://localhost:8080/users 

Accept: application/json 

This incoming request is handled by the DispatcherServlet, which is auto-configured by Spring Boot. While creating a project through the Spring Boot framework, and when we mention the Spring Boot Starter Web as a dependency in pom.xml, Spring Boot’s auto-configuration feature configures dispatcherServlet, a default error page, and other dependent jar files.

When a Spring boot application is run, the log will have a message like this:

[ost-startStop-1] o.s.b.w.servlet.ServletRegistrationBean  : Mapping servlet: ‘dispatcherServlet’ to [/]

DispatcherServlet is the front controller and all incoming request goes through this single servlet.

The process from a request to response is shown in the below flow chart:

Image title

The blocks in the green are the ones which are implemented by developers.

In our request for /users resources, the activities below are performed in each step:

  1. In Step 1, the dispatcher servlet will intercept the request for the resource /users.
  2. In Step 2, the servlet determines the handler for the request (a good link on this topic).
  3. In Step 3, Spring checks which controller method matches the incoming lookup path of the “/users” request. Spring maintains a list of all mapping registries fetched from the @RequestMapping of the controller class and iterates the list to look for the matching method in the controller class implemented by the developer.
  4. In Step 4, after determining the right method it executes the controller method.
  5. Step 5 returns an ArrayList of users.
  6. The response type accepted by the client can be either JSON or XML. Therefore, Step 6 does the job of marshaling the Java object to the response type requested by the client. Spring takes the ArrayList of users and uses the message converter method to marshal it to the type requested by the client. If the converted message is not available, then the client will get a 406 error. In the case of users, as the requested type is JSON, thus a JSON object for users is returned as a response.

Conclusion

Understanding the lifecycle of the request and response process and other classes involved helps one to understand the issues better and troubleshoot it more easily. To check the process lifecycle, open the Eclipse Open Type DispatcherServlet class and add a breakpoint at the doDispatch method.

@RestController vs @Controller : Spring Framework

Spring MVC Framework and REST

Spring’s annotation-based MVC framework simplifies the process of creating RESTful web services. The key difference between a traditional Spring MVC controller and the RESTful web service controller is the way the HTTP response body is created. While the traditional MVC controller relies on the View technology, the RESTful web service controller simply returns the object and the object data is written directly to the HTTP response as JSON/XML.  For a detailed description of creating RESTful web services using the Spring framework, click here.

Image title

Figure 1: Spring MVC traditional workflow

Spring MVC REST Workflow

The following steps describe a typical Spring MVC REST workflow:

  1. The client sends a request to a web service in URI form.
  2. The request is intercepted by the DispatcherServlet which looks for Handler Mappings and its type.
    • The Handler Mappings section defined in the application context file tells DispatcherServlet which strategy to use to find controllers based on the incoming request.
    • Spring MVC supports three different types of mapping request URIs to controllers: annotation, name conventions, and explicit mappings.
  3. Requests are processed by the Controller and the response is returned to the DispatcherServlet which then dispatches to the view.

In Figure 1, notice that in the traditional workflow the ModelAndView object is forwarded from the controller to the client. Spring lets you return data directly from the controller, without looking for a view, using the @ResponseBody annotation on a method. Beginning with Version 4.0, this process is simplified even further with the introduction of the @RestController annotation. Each approach is explained below.

Using the @ResponseBody Annotation

When you use the @ResponseBody annotation on a method, Spring converts the return value and writes it to the http response automatically. Each method in the Controller class must be annotated with @ResponseBody.

3.x-diagram

Figure 2: Spring 3.x MVC RESTful web services workflow

Behind the Scenes

Spring has a list of HttpMessageConverters registered in the background. The responsibility of the HTTPMessageConverter is to convert the request body to a specific class and back to the response body again, depending on a predefined mime type. Every time an issued request hits @ResponseBody, Spring loops through all registered HTTPMessageConverters seeking the first that fits the given mime type and class, and then uses it for the actual conversion.

Code Example

Let’s walk through @ResponseBody with a simple example.

Project Creation and Setup

  1. Create a Dynamic Web Project with Maven support in your Eclipse or MyEclipse IDE.
  2. Configure Spring support for the project.• If you are using Eclipse IDE, you need to download all Spring dependencies and configure your pom.xml to contain those dependencies.• In MyEclipse, you only need to install the Spring facet and the rest of the configuration happens automatically.
  3. Create the following Java class named Employee. This class is our POJO.
package com.example.spring.model;
import javax.xml.bind.annotation.XmlRootElement;
 @XmlRootElement(name = “Employee”)
 public class Employee {
     String name
      String email;
    public String getName() {
       return name;
    }
    public void setName(String name) {
       this.name = name;
    }
     public String getEmail() {
                 return email;
     }
     public void setEmail(String email) {
       this.email = email;
     }
     public Employee() {
     }
 }

Then, create the following @Controller class:

 package com.example.spring.rest;
 import org.springframework.stereotype.Controller;
 import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.ResponseBody;
import com.example.spring.model.Employee;
@Controller
@RequestMapping("employees")
public class EmployeeController {
     Employee employee = new Employee();
     @RequestMapping(value = “/{name}”, method = RequestMethod.GET, produces = “application/json”)
     public @ResponseBody Employee getEmployeeInJSON(@PathVariable String name) {
        employee.setName(name);
        employee.setEmail(“employee1@genuitec.com”);
     return employee;
    }
    @RequestMapping(value = “/{name}.xml”, method = RequestMethod.GET, produces = “application/xml”)
     public @ResponseBody Employee getEmployeeInXML(@PathVariable String name) {
        employee.setName(name);
        employee.setEmail(“employee1@genuitec.com”);
        return employee;
     }
 }
 Notice the @ResponseBody added to each of the @RequestMapping methods in the return value. After that, it’s a two-step process:
  1. Add the <context:component-scan> and <mvc:annotation-driven /> tags to the Spring configuration file.
    • <context:component-scan> activates the annotations and scans the packages to find and register beans within the application context.
    • <mvc:annotation-driven/> adds support for reading and writing JSON/XML if the Jackson/JAXB libraries are on the classpath.
    • For JSON format, include the jackson-databind jar and for XML include the jaxb-api-osgi jar to the project classpath.
  2. Deploy and run the application on any server (e.g., Tomcat). If you are using MyEclipse, you can run the project on the embedded Tomcat server.JSON—Use the URL: http://localhost:8080/SpringRestControllerExample/rest/employees/Bob and the following output displays:output_json-cropXML — Use the
    URL: http://localhost:8080/SpringRestControllerExample/rest/employees/Bob.xml and the following output displays:output_xml

Using the @RestController Annotation

Spring 4.0 introduced @RestController, a specialized version of the controller which is a convenience annotation that does nothing more than add the @Controller and @ResponseBody annotations. By annotating the controller class with @RestController annotation, you no longer need to add @ResponseBody to all the request mapping methods. The @ResponseBody annotation is active by default. Click here to learn more.
4.x-diagram

To use @RestController in our example, all we need to do is modify the @Controller to @RestController and remove the @ResponseBody from each method. The resultant class should look like the following:

 package com.example.spring.rest;
 import org.springframework.web.bind.annotation.PathVariable;
 import org.springframework.web.bind.annotation.RequestMapping;
 import org.springframework.web.bind.annotation.RequestMethod;
 import org.springframework.web.bind.annotation.RestController;
 import com.example.spring.model.Employee;
@RestController
 @RequestMapping(“employees”)
 public class EmployeeController {
     Employee employee = new Employee();
     @RequestMapping(value = “/{name}”, method = RequestMethod.GET, produces = “application/json”)
     public Employee getEmployeeInJSON(@PathVariable String name) {
        employee.setName(name);
        employee.setEmail(“employee1@genuitec.com”);
        return employee;
    }
     @RequestMapping(value = “/{name}.xml”, method = RequestMethod.GET, produces = “application/xml”)
     public Employee getEmployeeInXML(@PathVariable String name) {
        employee.setName(name);
        employee.setEmail(“employee1@genuitec.com”);
     return employee;
     }
 }

Note that we no longer need to add the @ResponseBody to the request mapping methods. After making the changes, running the application on the server again results in same output as before.

Conclusion

As you can see, using @RestController is quite simple and is the preferred method for creating MVC RESTful web services starting from Spring v4.0. I would like to extend a big thank you to my co-author, Swapna Sagi, for all of her help in bringing you this information!

@RestController vs. @Controller : Spring Framework

Spring MVC Framework and REST

Spring’s annotation-based MVC framework simplifies the process of creating RESTful web services. The key difference between a traditional Spring MVC controller and the RESTful web service controller is the way the HTTP response body is created. While the traditional MVC controller relies on the View technology, the RESTful web service controller simply returns the object and the object data is written directly to the HTTP response as JSON/XML.  For a detailed description of creating RESTful web services using the Spring framework, click here.

Image title

Figure 1: Spring MVC traditional workflow

Spring MVC REST Workflow

The following steps describe a typical Spring MVC REST workflow:

  1. The client sends a request to a web service in URI form.
  2. The request is intercepted by the DispatcherServlet which looks for Handler Mappings and its type.
    • The Handler Mappings section defined in the application context file tells DispatcherServlet which strategy to use to find controllers based on the incoming request.
    • Spring MVC supports three different types of mapping request URIs to controllers: annotation, name conventions, and explicit mappings.
  3. Requests are processed by the Controller and the response is returned to the DispatcherServlet which then dispatches to the view.

In Figure 1, notice that in the traditional workflow the ModelAndView object is forwarded from the controller to the client. Spring lets you return data directly from the controller, without looking for a view, using the @ResponseBody annotation on a method. Beginning with Version 4.0, this process is simplified even further with the introduction of the @RestController annotation. Each approach is explained below.

Using the @ResponseBody Annotation

When you use the @ResponseBody annotation on a method, Spring converts the return value and writes it to the http response automatically. Each method in the Controller class must be annotated with @ResponseBody.

3.x-diagram

Figure 2: Spring 3.x MVC RESTful web services workflow

Behind the Scenes

Spring has a list of HttpMessageConverters registered in the background. The responsibility of the HTTPMessageConverter is to convert the request body to a specific class and back to the response body again, depending on a predefined mime type. Every time an issued request hits @ResponseBody, Spring loops through all registered HTTPMessageConverters seeking the first that fits the given mime type and class, and then uses it for the actual conversion.

Code Example

Let’s walk through @ResponseBody with a simple example.

Project Creation and Setup

  1. Create a Dynamic Web Project with Maven support in your Eclipse or MyEclipse IDE.
  2. Configure Spring support for the project.• If you are using Eclipse IDE, you need to download all Spring dependencies and configure your pom.xml to contain those dependencies.• In MyEclipse, you only need to install the Spring facet and the rest of the configuration happens automatically.
  3. Create the following Java class named Employee. This class is our POJO.
package com.example.spring.model;
import javax.xml.bind.annotation.XmlRootElement;
@XmlRootElement(name = "Employee")
public class Employee {
 String name; 
 String email;
 public String getName() {
 return name;
 }
 public void setName(String name) {
 this.name = name;
 }
 public String getEmail() {
 return email;
 }
 public void setEmail(String email) {
 this.email = email;
 }
 public Employee() {
 } 
}
 Then, create the following @Controller class:
package com.example.spring.rest;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.ResponseBody;
import com.example.spring.model.Employee;
@Controller
@RequestMapping("employees")
public class EmployeeController {
 Employee employee = new Employee();
 @RequestMapping(value = "/{name}", method = RequestMethod.GET, produces = "application/json")
 public @ResponseBody Employee getEmployeeInJSON(@PathVariable String name) {
 employee.setName(name);
 employee.setEmail("employee1@genuitec.com");
 return employee; 
 }
 @RequestMapping(value = "/{name}.xml", method = RequestMethod.GET, produces = "application/xml")
 public @ResponseBody Employee getEmployeeInXML(@PathVariable String name) {
 employee.setName(name);
 employee.setEmail("employee1@genuitec.com");
 return employee; 
 }
}
 Notice the @ResponseBody added to each of the @RequestMapping methods in the return value. After that, it’s a two-step process:
  1. Add the <context:component-scan> and <mvc:annotation-driven /> tags to the Spring configuration file.
    • <context:component-scan> activates the annotations and scans the packages to find and register beans within the application context.
    • <mvc:annotation-driven/> adds support for reading and writing JSON/XML if the Jackson/JAXB libraries are on the classpath.
    • For JSON format, include the jackson-databind jar and for XML include the jaxb-api-osgi jar to the project classpath.
  2. Deploy and run the application on any server (e.g., Tomcat). If you are using MyEclipse, you can run the project on the embedded Tomcat server.JSON—Use the URL: http://localhost:8080/SpringRestControllerExample/rest/employees/Bob and the following output displays:output_json-crop

    XML — Use the
    URL: http://localhost:8080/SpringRestControllerExample/rest/employees/Bob.xml and the following output displays:output_xml

Using the @RestController Annotation

Spring 4.0 introduced @RestController, a specialized version of the controller which is a convenience annotation that does nothing more than add the @Controller and @ResponseBody annotations. By annotating the controller class with @RestController annotation, you no longer need to add @ResponseBody to all the request mapping methods. The @ResponseBody annotation is active by default. Click here to learn more.
4.x-diagram

To use @RestController in our example, all we need to do is modify the @Controller to @RestController and remove the @ResponseBody from each method. The resultant class should look like the following:

package com.example.spring.rest;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RestController;
import com.example.spring.model.Employee;
@RestController
@RequestMapping("employees")
public class EmployeeController {
 Employee employee = new Employee();
 @RequestMapping(value = "/{name}", method = RequestMethod.GET, produces = "application/json")
 public Employee getEmployeeInJSON(@PathVariable String name) {
 employee.setName(name);
 employee.setEmail("employee1@genuitec.com");
 return employee;
 }
 @RequestMapping(value = "/{name}.xml", method = RequestMethod.GET, produces = "application/xml")
 public Employee getEmployeeInXML(@PathVariable String name) {
 employee.setName(name);
 employee.setEmail("employee1@genuitec.com");
 return employee; 
 } 
}

Note that we no longer need to add the @ResponseBody to the request mapping methods. After making the changes, running the application on the server again results in same output as before.

Conclusion

As you can see, using @RestController is quite simple and is the preferred method for creating MVC RESTful web services starting from Spring v4.0. I would like to extend a big thank you to my co-author, Swapna Sagi, for all of her help in bringing you this information!