CodeByAkram

What’s the difference between @Component, @Controller, @Repository & @Service annotations in Spring?

Spring 2.5 introduces further stereotype annotations: @Component, @Service, and @Controller. @Component is a generic stereotype for any Spring-managed component. @Repository, @Service, and @Controller are specializations of @Component for more specific use cases, for example, in the persistence, service, and presentation layers, respectively.

You may also like What do you mean by Aspect, Join Point, Advice?

Therefore, you can annotate your component classes with @Component, but by annotating them with @Repository, @Service, or @Controller instead, your classes are more properly suited for processing by tools or associating with aspects. For example, these stereotype annotations make ideal targets for pointcuts.

Thus, if you are choosing between using @Component or @Service for your service layer, @Service is clearly the better choice. Similarly, as stated above, @Repository is already supported as a marker for automatic exception translation in your persistence layer.

Differentiate between constructor injection and setter injection

Akram Spring , SpringBoot

Partial Dependency

In Setter Injection, partial dependency is possible, means if we have 4 dependencies as mentioned below,

Differentiate between constructor injection and setter injection, codebyakram

Then it is not necessary to inject all values if we are using setter injection.

But in Constructor Injection, partial dependency is not possible because we are calling the constructor of that class.

Overriding

Constructor Injection can not override the setter injected properties but Setter injection can override the constructor injected properties.

Changes

Setter injection makes bean class object as mutable but constructor injection makes bean class object as immutable.

Number of dependencies

If we have dependencies for example 17 in our bean class then, in this case setter injection is not recommended as we need to write almost 17 setters right, bean length will increase.

In this case, Constructor injection is highly recommended, as we can inject all the dependencies with in 3 to 4 lines by calling one constructor.

Java 8 Lambda Comparator

Akram Interview Questions , Java 8

Lets se how we can sort the object by using java 8 lambda comparator. For this let take a Employee class.

public class Employee {
    private String name;
    private int salary;
 
    // standard constructors, getters/setters, equals and hashcode
}

1. Classic sort (without lambda)

Comparator< Employee > bySalary = new Comparator< Employee >() {
  @Override
  public int compare(Employee o1, Employee o2) {
   return o1.getSalary().compareTo(o2.getSalary());
  }
 };

2. Lambda expression equivalent.

 Comparator< Employee > bySalary = 
  (Employee o1, Employee o2)->o1.getSalary().compareTo(o2.getSalary());

You may also like to read What do you mean by Aspect, Join Point, Advice?

How to get keys and values from Map in Java?

Akram Interview Questions , Java

As we know, map is based on key-value pair associations, so interviewer can ask you this question if you are a beginner or having less than 3 years of experience.

So lets see how we can the key and values from a Map?

In Java we have and Map.Entry method that returns a collection-view of map.

Map< string string=""> map = new HashMap<>();

 // Get keys and values
 for (Map.Entry< string string=""> entry : map.entrySet()) {
  String k = entry.getKey();
  String v = entry.getValue();
  System.out.println("Key: " + k + ", Value: " + v);
 }

 // In Java 8 we can use for each
 map.forEach((k, v) -> {
  System.out.println("Key: " + k + ", Value: " + v);
 });

How to set connection timeout for RestTemplate in spring?

Akram Interview Questions , SpringBoot

We can set the timeout for RestTemplate by doing some custom configuration for RestTemplate.

First you need to create a class named as HttpClientConfig in this class we configure HttpClient because RestTemplate internally uses the HttpClient.

package com.codebyakram;

import org.apache.http.client.config.RequestConfig;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.slf4j.Logger;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class HttpClientConfig {

 public static final Logger LOGGER = org.slf4j.LoggerFactory.getLogger(HttpClientConfig.class.getName());
 // Determines the timeout in milliseconds until a connection is established.
 private static final int CONNECT_TIMEOUT = 30000;

 // The timeout when requesting a connection from the connection manager.
 
 private int REQUEST_TIMEOUT = 10000;
 
 @Bean
 public CloseableHttpClient httpClient() {
  RequestConfig requestConfig = RequestConfig.custom().setConnectionRequestTimeout(REQUEST_TIMEOUT).build();
  return HttpClients.custom().setDefaultRequestConfig(requestConfig).build();
 }
}

Here in this class we setting HttpClient in RestTemplate

package com.codebyakram;

import org.apache.http.impl.client.CloseableHttpClient;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.client.HttpComponentsClientHttpRequestFactory;
import org.springframework.scheduling.TaskScheduler;
import org.springframework.scheduling.concurrent.ThreadPoolTaskScheduler;
import org.springframework.web.client.RestTemplate;

@Configuration
public class RestTemplateConfig {

 @Autowired
 CloseableHttpClient httpClient;


 @Bean
 public RestTemplate restTemplate() {
  RestTemplate restTemplate = new RestTemplate(clientHttpRequestFactory());
  return restTemplate;
 }

 @Bean
 public HttpComponentsClientHttpRequestFactory clientHttpRequestFactory() {
  HttpComponentsClientHttpRequestFactory clientHttpRequestFactory = new HttpComponentsClientHttpRequestFactory();
  clientHttpRequestFactory.setHttpClient(httpClient);
  return clientHttpRequestFactory;
 }

 @Bean
 public TaskScheduler taskScheduler() {
  ThreadPoolTaskScheduler scheduler = new ThreadPoolTaskScheduler();
  scheduler.setThreadNamePrefix("poolScheduler");
  scheduler.setPoolSize(50);
  return scheduler;
 }
}

So now we are done with the cofiguration of request timeout for RestTemplate.

What’s the difference between @Component, @Controller, @Repository & @Service annotations in Spring?

Differentiate between constructor injection and setter injection

Partial Dependency

Overriding

Changes

Number of dependencies

Java 8 Lambda Comparator

How to get keys and values from Map in Java?

How to set connection timeout for RestTemplate in spring?

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