String vs StringBuffer vs StringBuilder in Java

Difference between String, Stringbuffer and StringBuilder

String is one of the most important classes in Java and anyone who starts with Java programming uses String to print something on console by using famous System.out.println() statements. Many Java beginners not aware that String is immutable and final in Java and every modification in String result creates a new String object. So How do you manipulate String in Java without creating String garbage? StringBuilder and StringBuffer is answer of this question. StringBuffer is old class but StringBuilder is newly added in Java 5 along with major improvements like Enum, Generics, varargs methods and Autoboxing in Java. No matter which kind of application you are working you will find heavy usage of Java String class but if you do profiling of your application you will find that String is the one class which creates lots of garbage because of many temporary String created in program. In this Java tutorial we will see What is String in Java, some important properties of String in Java, What is StringBuffer in Java , When to use StringBuffer in Java , StringBuilder in Java and how it can be used in place of StringBuffer,  What are differences between String and StringBuffer and StringBuilder in Java  which is a frequently asked core Java question and mostly String vs StringBuilder vs StringBuffer. Now let's start with String.

Differences between String, StringBuffer and StringBuilder in Java

String in Java

Before looking difference between String and StringBuffer or StringBuilder let’s see some fundamental properties of String Class in Java

1) String is immutable in Java:  String is by design immutable in Java you can check this post for reason. Immutability offers lot of benefit to the String class e.g. his hashcode value can be cached which makes it a faster hashmap key and one of the reason why String is a popular key in HashMap. Because String is final it can be safely shared between multiple threads  without any extra synchronization.

2)when we represent string in double quotes like "abcd" they are referred as String literal and String literals are created in String pools. When you compare two String literals using equality operator "==" it returns true because they are actually same instance of String. Anyway comparing object with equality operator is bad practice in Java and you should always use equals method to check equality.

3) "+" operator is overloaded for String and used to concatenated two string. Internally "+" operation is implemented using either StringBuffer or StringBuilder.

4) Strings are backed up by character Array and represented in UTF-16 format. By the way this behavior can cause memory leak in String because same character array is shared between source String and SubString which can prevent source String from being garbage collected. See How SubString works in Java for more details.

5) String class overrides equals() and hashcode() method and two Strings are considered to be equal if they contain exactly same character in same order and in same case. If you want ignore case comparison of two strings consider using equalsIgnoreCase() method. See  how to correctly override equals method in Java  to learn more about best practices on equals method. Another worth noting point is that equals method must be consistent with compareTo() method for String because SortedSet and SortedMap e.g. TreeMap uses compareTo method to compare String in Java.

7) toString() method provides String representation of any object and its declared in Object class and its recommended for other class to implement this and provide String representation.

8) String is represented using UTF-16 format in Java.

9) In Java you can create String from char array, byte array, another string, from StringBuffer or from StringBuilder. Java String class provides constructor for all of these.

Problem with String in Java

One of its biggest strength Immutability is also biggest problem of Java String if not used correctly. many a times we create a String and then perform a lot of operation on them e.g. converting string into uppercase, lowercase , getting substring out of it , concatenating with other string etc. Since String is an immutable class every time a new String is created and older one is discarded which creates lots of temporary garbage in heap. If String are created using String literal they remain in String pool. To resolve this problem Java provides us two Classes StringBuffer and StringBuilder. String Buffer is an older class but StringBuilder is relatively new and added in JDK 5.

Differences between String and StringBuffer in Java

Main difference between String and StringBuffer is String is immutable while StringBuffer is mutable means you can modify a StringBuffer object once you created it without creating any new object. This mutable property makes StringBuffer an ideal choice for dealing with Strings in Java. You can convert a StringBuffer into String by its toString() method. String vs StringBuffer or what is difference between StringBuffer and String is one of the popular Java interview questions for either phone interview or first round. Now days they also include StringBuilder and ask String vs StringBuffer vs StringBuilder. So be preparing for that. In the next section we will see difference between StringBuffer and StringBuilder in Java.

Difference between StringBuilder and StringBuffer in Java

StringBuffer is very good with mutable String but it has one disadvantage all its public methods are synchronized which makes it thread-safe but same time slow. In JDK 5 they provided similar class called StringBuilder in Java which is a copy of StringBuffer but without synchronization. Try to use StringBuilder whenever possible it performs better in most of cases than StringBuffer class. You can also use "+" for concatenating two string because "+" operation is internal implemented using either StringBuffer or StringBuilder in Java. If you see StringBuilder vs StringBuffer you will find that they are exactly similar and all API methods applicable to StringBuffer are also applicable to StringBuilder in Java. On the other hand String vs StringBuffer is completely different and there API is also completely different, same is true for StringBuilder vs String.

Summary

In summary here are list of difference between StringBuffer, String and StringBuilder in Java :

1) String is immutable while StringBuffer and StringBuilder is mutable object.

2) StringBuffer is synchronized while StringBuilder is not which makes StringBuilder faster than StringBuffer.

3) Concatenation operator "+" is internal implemented using either StringBuffer or StringBuilder.

4) Use String if you require immutability, use Stringbuffer in java if you need mutable + thread-safety and use StringBuilder in Java if you require mutable + without thread-safety.

That's all on famous String vs StringBuffer or StringBuffer vs StringBuilder discussion. All these differences helps to avoid common coding mistake of using String in place of StringBuffer in many places. from Java 5 onwards either use + operator of StringBuilder for concatenating String in Java. 

Read more: http://javarevisited.blogspot.com/2011/07/string-vs-stringbuffer-vs-stringbuilder.html#ixzz426bqGNDk

Spring Interview Questions and Answers

Spring Framework is one of the most popular Java EE framework for web applications. Dependency Injection and Aspect Oriented Programming are at the heart of Spring framework.
If you are good in Spring Framework, chances of getting selected gets high. I have posted a lot of Spring Tutorials recently and this post will help you get through Spring interview Questions with Answers explaining the core concepts in detail.
You should bookmark this post because Spring Framework is a lot bigger and chances are that I will keep on adding more questions to this list and you don’t want to miss them out.

1. What is Spring Framework?
2. What are some of the important features and advantages of Spring Framework?
3. What do you understand by Dependency Injection?
4. How do we implement DI in Spring Framework?
5. What are the benefits of using Spring Tool Suite?
6. Name some of the important Spring Modules?
7. What do you understand by Aspect Oriented Programming?
8. What is Aspect, Advice, Pointcut, JointPoint and Advice Arguments in AOP?
9. What is the difference between Spring AOP and AspectJ AOP?
10. What is Spring IoC Container?
11. What is a Spring Bean?
12. What is the importance of Spring bean configuration file?
13. What are different ways to configure a class as Spring Bean?
14. What are different scopes of Spring Bean?
15. What is Spring Bean life cycle?
16. How to get ServletContext and ServletConfig object in a Spring Bean?
17. What is Bean wiring and @Autowired annotation?
18. What are different types of Spring Bean autowiring?
19. Does Spring Bean provide thread safety?
20. What is a Controller in Spring MVC?
21. What’s the difference between @Component, @Repository & @Service annotations in Spring?
22. What is DispatcherServlet and ContextLoaderListener?
23. What is ViewResolver in Spring?
24. What is a MultipartResolver and when its used?
25. How to handle exceptions in Spring MVC Framework?
26. How to create ApplicationContext in a Java Program?
27. Can we have multiple Spring configuration files?
28. What is ContextLoaderListener?
29. What are the minimum configurations needed to create Spring MVC application?
30. How would you relate Spring MVC Framework to MVC architecture?
31. How to achieve localization in Spring MVC applications?
32. How can we use Spring to create Restful Web Service returning JSON response?
33. What are some of the important Spring annotations you have used?
34. Can we send an Object as the response of Controller handler method?
35. How to upload file in Spring MVC Application?
36. How to validate form data in Spring Web MVC Framework?
37. What is Spring MVC Interceptor and how to use it?
38. What is Spring JdbcTemplate class and how to use it?
39. How to use Tomcat JNDI DataSource in Spring Web Application?
40. How would you achieve Transaction Management in Spring?
41. What is Spring DAO?
42. How to integrate Spring and Hibernate Frameworks?
43. What is Spring Security?
44. How to inject a java.util.Properties into a Spring Bean?
45. Name some of the design patterns used in Spring Framework?
46. What are some of the best practices for Spring Framework?

1. What is Spring Framework?

   Spring is one of the most widely used Java EE framework. Spring framework core concepts are “Dependency Injection” and “Aspect Oriented Programming”.
   Spring framework can be used in normal java applications also to achieve loose coupling between different components by implementing dependency injection and we can perform cross cutting tasks such as logging and authentication using spring support for aspect oriented programming.
   I like spring because it provides a lot of features and different modules for specific tasks such as Spring MVC and Spring JDBC. Since it’s an open source framework with a lot of online resources and active community members, working with Spring framework is easy and fun at same time.



2. What are some of the important features and advantages of Spring Framework?

   Spring Framework is built on top of two design concepts – Dependency Injection and Aspect Oriented Programming.
   Some of the features of spring framework are:
   
   • Lightweight and very little overhead of using framework for our development.
   • Dependency Injection or Inversion of Control to write components that are independent of each other, spring container takes care of wiring them together to achieve our work.
   • Spring IoC container manages Spring Bean life cycle and project specific configurations such as JNDI lookup.
   • Spring MVC framework can be used to create web applications as well as restful web services capable of returning XML as well as JSON response.
   • Support for transaction management, JDBC operations, File uploading, Exception Handling etc with very little configurations, either by using annotations or by spring bean configuration file.
   Some of the advantages of using Spring Framework are:
   
   • Reducing direct dependencies between different components of the application, usually Spring IoC container is responsible for initializing resources or beans and inject them as dependencies.
   • Writing unit test cases are easy in Spring framework because our business logic doesn’t have direct dependencies with actual resource implementation classes. We can easily write a test configuration and inject our mock beans for testing purposes.
   • Reduces the amount of boiler-plate code, such as initializing objects, open/close resources. I like JdbcTemplate class a lot because it helps us in removing all the boiler-plate code that comes with JDBC programming.
   • Spring framework is divided into several modules, it helps us in keeping our application lightweight. For example, if we don’t need Spring transaction management features, we don’t need to add that dependency in our project.
   • Spring framework support most of the Java EE features and even much more. It’s always on top of the new technologies, for example there is a Spring project for Android to help us write better code for native android applications. This makes spring framework a complete package and we don’t need to look after different framework for different requirements.



3. What do you understand by Dependency Injection?

   Dependency Injection design pattern allows us to remove the hard-coded dependencies and make our application loosely coupled, extendable and maintainable. We can implement dependency injection pattern to move the dependency resolution from compile-time to runtime.
   Some of the benefits of using Dependency Injection are: Separation of Concerns, Boilerplate Code reduction, Configurable components and easy unit testing.
   Read more at Dependency Injection Tutorial. We can also use Google Guice for Dependency Injection to automate the process of dependency injection. But in most of the cases we are looking for more than just dependency injection and that’s why Spring is the top choice for this.
   



4. How do we implement DI in Spring Framework?

   We can use Spring XML based as well as Annotation based configuration to implement DI in spring applications. For better understanding, please read Spring Dependency Injection example where you can learn both the ways with JUnit test case. The post also contains sample project zip file, that you can download and play around to learn more.
   



5. What are the benefits of using Spring Tool Suite?

   We can install plugins into Eclipse to get all the features of Spring Tool Suite. However STS comes with Eclipse with some other important stuffs such as Maven support, Templates for creating different types of Spring projects and tc server for better performance with Spring applications.
   I like STS because it highlights the Spring components and if you are using AOP pointcuts and advices, then it clearly shows which methods will come under the specific pointcut. So rather than installing everything on our own, I prefer using STS when developing Spring based applications.



6. Name some of the important Spring Modules?

   Some of the important Spring Framework modules are:
   
   • Spring Context – for dependency injection.
   • Spring AOP – for aspect oriented programming.
   • Spring DAO – for database operations using DAO pattern
   • Spring JDBC – for JDBC and DataSource support.
   • Spring ORM – for ORM tools support such as Hibernate
   • Spring Web Module – for creating web applications.
   • Spring MVC – Model-View-Controller implementation for creating web applications, web services etc.



7. What do you understand by Aspect Oriented Programming?

   Enterprise applications have some common cross-cutting concerns that is applicable for different types of Objects and application modules, such as logging, transaction management, data validation, authentication etc. In Object Oriented Programming, modularity of application is achieved by Classes whereas in AOP application modularity is achieved by Aspects and they are configured to cut across different classes methods.
   AOP takes out the direct dependency of cross-cutting tasks from classes that is not possible in normal object oriented programming. For example, we can have a separate class for logging but again the classes will have to call these methods for logging the data. Read more about Spring AOP support at Spring AOP Example.
   



8. What is Aspect, Advice, Pointcut, JointPoint and Advice Arguments in AOP?

   Aspect: Aspect is a class that implements cross-cutting concerns, such as transaction management. Aspects can be a normal class configured and then configured in Spring Bean configuration file or we can use Spring AspectJ support to declare a class as Aspect using @Aspect annotation.
   Advice: Advice is the action taken for a particular join point. In terms of programming, they are methods that gets executed when a specific join point with matching pointcut is reached in the application. You can think of Advices as Spring interceptors or Servlet Filters.
   Pointcut: Pointcut are regular expressions that is matched with join points to determine whether advice needs to be executed or not. Pointcut uses different kinds of expressions that are matched with the join points. Spring framework uses the AspectJ pointcut expression language for determining the join points where advice methods will be applied.
   Join Point: A join point is the specific point in the application such as method execution, exception handling, changing object variable values etc. In Spring AOP a join points is always the execution of a method.
   Advice Arguments: We can pass arguments in the advice methods. We can use args() expression in the pointcut to be applied to any method that matches the argument pattern. If we use this, then we need to use the same name in the advice method from where argument type is determined.
   These concepts seems confusing at first, but if you go through Spring Aspect, Advice Example then you can easily relate to them.



9. What is the difference between Spring AOP and AspectJ AOP?

   AspectJ is the industry-standard implementation for Aspect Oriented Programming whereas Spring implements AOP for some cases. Main differences between Spring AOP and AspectJ are:
   
   • Spring AOP is simpler to use than AspectJ because we don’t need to worry about the weaving process.
   • Spring AOP supports AspectJ annotations, so if you are familiar with AspectJ then working with Spring AOP is easier.
   • Spring AOP supports only proxy-based AOP, so it can be applied only to method execution join points. AspectJ support all kinds of pointcuts.
   • One of the shortcoming of Spring AOP is that it can be applied only to the beans created through Spring Context.



10. What is Spring IoC Container?

    Inversion of Control (IoC) is the mechanism to achieve loose-coupling between Objects dependencies. To achieve loose coupling and dynamic binding of the objects at runtime, the objects define their dependencies that are being injected by other assembler objects. Spring IoC container is the program that injects dependencies into an object and make it ready for our use.
    Spring Framework IoC container classes are part of org.springframework.beans and org.springframework.context packages and provides us different ways to decouple the object dependencies.
    Some of the useful ApplicationContext implementations that we use are;
    
    • AnnotationConfigApplicationContext: For standalone java applications using annotations based configuration.
    • ClassPathXmlApplicationContext: For standalone java applications using XML based configuration.
    • FileSystemXmlApplicationContext: Similar to ClassPathXmlApplicationContext except that the xml configuration file can be loaded from anywhere in the file system.
    • AnnotationConfigWebApplicationContext and XmlWebApplicationContext for web applications.



11. What is a Spring Bean?

    Any normal java class that is initialized by Spring IoC container is called Spring Bean. We use Spring ApplicationContext to get the Spring Bean instance.
    Spring IoC container manages the life cycle of Spring Bean, bean scopes and injecting any required dependencies in the bean.
    



12. What is the importance of Spring bean configuration file?

    We use Spring Bean configuration file to define all the beans that will be initialized by Spring Context. When we create the instance of Spring ApplicationContext, it reads the spring bean xml file and initialize all of them. Once the context is initialized, we can use it to get different bean instances.
    Apart from Spring Bean configuration, this file also contains spring MVC interceptors, view resolvers and other elements to support annotations based configurations.
    



13. What are different ways to configure a class as Spring Bean?

    There are three different ways to configure Spring Bean.
    
    1. XML Configuration: This is the most popular configuration and we can use bean element in context file to configure a Spring Bean. For example:

       1	<bean name="myBean" class="com.journaldev.spring.beans.MyBean"></bean>

    2. Java Based Configuration: If you are using only annotations, you can configure a Spring bean using @Bean annotation. This annotation is used with @Configuration classes to configure a spring bean. Sample configuration is:

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       @Configuration @ComponentScan(value="com.journaldev.spring.main") public class MyConfiguration {       @Bean     public MyService getService(){         return new MyService();     } }

       To get this bean from spring context, we need to use following code snippet:
       

       1 2 3

       AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext(         MyConfiguration.class); MyService service = ctx.getBean(MyService.class);

    3. Annotation Based Configuration: We can also use @Component, @Service, @Repository and @Controller annotations with classes to configure them to be as spring bean. For these, we would need to provide base package location to scan for these classes. For example:

       1	<context:component-scan base-package="com.journaldev.spring" />


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14. What are different scopes of Spring Bean?

    There are five scopes defined for Spring Beans.
    
    1. singleton: Only one instance of the bean will be created for each container. This is the default scope for the spring beans. While using this scope, make sure spring bean doesn’t have shared instance variables otherwise it might lead to data inconsistency issues because it’s not thread-safe.
    2. prototype: A new instance will be created every time the bean is requested.
    3. request: This is same as prototype scope, however it’s meant to be used for web applications. A new instance of the bean will be created for each HTTP request.
    4. session: A new bean will be created for each HTTP session by the container.
    5. global-session: This is used to create global session beans for Portlet applications.
    Spring Framework is extendable and we can create our own scopes too, however most of the times we are good with the scopes provided by the framework.
    To set spring bean scopes we can use “scope” attribute in bean element or @Scope annotation for annotation based configurations.
    



15. What is Spring Bean life cycle?

    Spring Beans are initialized by Spring Container and all the dependencies are also injected. When context is destroyed, it also destroys all the initialized beans. This works well in most of the cases but sometimes we want to initialize other resources or do some validation before making our beans ready to use. Spring framework provides support for post-initialization and pre-destroy methods in spring beans.
    We can do this by two ways – by implementing InitializingBean and DisposableBean interfaces or using init-method and destroy-method attribute in spring bean configurations. For more details, please read Spring Bean Life Cycle Methods.
    



16. How to get ServletContext and ServletConfig object in a Spring Bean?

    There are two ways to get Container specific objects in the spring bean.
    
    1. Implementing Spring *Aware interfaces, for these ServletContextAware and ServletConfigAware interfaces, for complete example of these aware interfaces, please read Spring Aware Interfaces
    2. Using @Autowired annotation with bean variable of type ServletContext and ServletConfig. They will work only in servlet container specific environment only though.

       1 2	@Autowired ServletContext servletContext;




17. What is Bean wiring and @Autowired annotation?

    The process of injection spring bean dependencies while initializing it called Spring Bean Wiring.
    Usually it’s best practice to do the explicit wiring of all the bean dependencies, but spring framework also supports autowiring. We can use @Autowired annotation with fields or methods for autowiring byType. For this annotation to work, we also need to enable annotation based configuration in spring bean configuration file. This can be done by context:annotation-config element.
    For more details about @Autowired annotation, please read Spring Autowire Example.
    



18. What are different types of Spring Bean autowiring?

    There are four types of autowiring in Spring framework.
    
    1. autowire byName
    2. autowire byType
    3. autowire by constructor
    4. autowiring by @Autowired and @Qualifier annotations
    Prior to Spring 3.1, autowire by autodetect was also supported that was similar to autowire by constructor or byType. For more details about these options, please read Spring Bean Autowiring.



19. Does Spring Bean provide thread safety?

    The default scope of Spring bean is singleton, so there will be only one instance per context. That means that all the having a class level variable that any thread can update will lead to inconsistent data. Hence in default mode spring beans are not thread-safe.
    However we can change spring bean scope to request, prototype or session to achieve thread-safety at the cost of performance. It’s a design decision and based on the project requirements.



20. What is a Controller in Spring MVC?

    Just like MVC design pattern, Controller is the class that takes care of all the client requests and send them to the configured resources to handle it. In Spring MVC, org.springframework.web.servlet.DispatcherServlet is the front controller class that initializes the context based on the spring beans configurations.
    A Controller class is responsible to handle different kind of client requests based on the request mappings. We can create a controller class by using @Controller annotation. Usually it’s used with @RequestMapping annotation to define handler methods for specific URI mapping.
    



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

    @Component is used to indicate that a class is a component. These classes are used for auto detection and configured as bean, when annotation based configurations are used.
    @Controller is a specific type of component, used in MVC applications and mostly used with RequestMapping annotation.
    @Repository annotation is used to indicate that a component is used as repository and a mechanism to store/retrieve/search data. We can apply this annotation with DAO pattern implementation classes.
    @Service is used to indicate that a class is a Service. Usually the business facade classes that provide some services are annotated with this.
    We can use any of the above annotations for a class for auto-detection but different types are provided so that you can easily distinguish the purpose of the annotated classes.
    



22. What is DispatcherServlet and ContextLoaderListener?

    DispatcherServlet is the front controller in the Spring MVC application and it loads the spring bean configuration file and initialize all the beans that are configured. If annotations are enabled, it also scans the packages and configure any bean annotated with @Component, @Controller, @Repository or @Service annotations.
    
    ContextLoaderListener is the listener to start up and shut down Spring’s root WebApplicationContext. It’s important functions are to tie up the lifecycle of ApplicationContext to the lifecycle of the ServletContext and to automate the creation of ApplicationContext. We can use it to define shared beans that can be used across different spring contexts.
    



23. What is ViewResolver in Spring?

    ViewResolver implementations are used to resolve the view pages by name. Usually we configure it in the spring bean configuration file. For example:
    

    1 2 3 4 5

    <!-- Resolves views selected for rendering by @Controllers to .jsp resources in the /WEB-INF/views directory --> <beans:bean class="org.springframework.web.servlet.view.InternalResourceViewResolver">     <beans:property name="prefix" value="/WEB-INF/views/" />     <beans:property name="suffix" value=".jsp" /> </beans:bean>

    InternalResourceViewResolver is one of the implementation of ViewResolver interface and we are providing the view pages directory and suffix location through the bean properties. So if a controller handler method returns “home”, view resolver will use view page located at /WEB-INF/views/home.jsp.
    



24. What is a MultipartResolver and when its used?

    MultipartResolver interface is used for uploading files – CommonsMultipartResolver and StandardServletMultipartResolver are two implementations provided by spring framework for file uploading. By default there are no multipart resolvers configured but to use them for uploading files, all we need to define a bean named “multipartResolver” with type as MultipartResolver in spring bean configurations.
    Once configured, any multipart request will be resolved by the configured MultipartResolver and pass on a wrapped HttpServletRequest. Then it’s used in the controller class to get the file and process it. For a complete example, please read Spring MVC File Upload Example.
    



25. How to handle exceptions in Spring MVC Framework?

    Spring MVC Framework provides following ways to help us achieving robust exception handling.
    
    1. Controller Based – We can define exception handler methods in our controller classes. All we need is to annotate these methods with @ExceptionHandler annotation.
    2. Global Exception Handler – Exception Handling is a cross-cutting concern and Spring provides @ControllerAdvice annotation that we can use with any class to define our global exception handler.
    3. HandlerExceptionResolver implementation – For generic exceptions, most of the times we serve static pages. Spring Framework provides HandlerExceptionResolver interface that we can implement to create global exception handler. The reason behind this additional way to define global exception handler is that Spring framework also provides default implementation classes that we can define in our spring bean configuration file to get spring framework exception handling benefits.
    For a complete example, please read Spring Exception Handling Example.



26. How to create ApplicationContext in a Java Program?

    There are following ways to create spring context in a standalone java program.
    
    1. AnnotationConfigApplicationContext: If we are using Spring in standalone java applications and using annotations for Configuration, then we can use this to initialize the container and get the bean objects.
    2. ClassPathXmlApplicationContext: If we have spring bean configuration xml file in standalone application, then we can use this class to load the file and get the container object.
    3. FileSystemXmlApplicationContext: This is similar to ClassPathXmlApplicationContext except that the xml configuration file can be loaded from anywhere in the file system.



27. Can we have multiple Spring configuration files?

    For Spring MVC applications, we can define multiple spring context configuration files through contextConfigLocation. This location string can consist of multiple locations separated by any number of commas and spaces. For example;
    

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    <servlet>     <servlet-name>appServlet</servlet-name>     <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>     <init-param>         <param-name>contextConfigLocation</param-name>         <param-value>/WEB-INF/spring/appServlet/servlet-context.xml,/WEB-INF/spring/appServlet/servlet-jdbc.xml</param-value>     </init-param>     <load-on-startup>1</load-on-startup> </servlet>

    We can also define multiple root level spring configurations and load it through context-param. For example;
    

    1 2 3 4

    <context-param>     <param-name>contextConfigLocation</param-name>     <param-value>/WEB-INF/spring/root-context.xml /WEB-INF/spring/root-security.xml</param-value> </context-param>

    Another option is to use import element in the context configuration file to import other configurations, for example:
    

    1	<beans:import resource="spring-jdbc.xml"/>




28. What is ContextLoaderListener?

    ContextLoaderListener is the listener class used to load root context and define spring bean configurations that will be visible to all other contexts. It’s configured in web.xml file as:
    

    1 2 3 4 5 6 7 8

    <context-param>     <param-name>contextConfigLocation</param-name>     <param-value>/WEB-INF/spring/root-context.xml</param-value> </context-param>   <listener>     <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class> </listener>




29. What are the minimum configurations needed to create Spring MVC application?

    For creating a simple Spring MVC application, we would need to do following tasks.
    
    • Add spring-context and spring-webmvc dependencies in the project.
    • Configure DispatcherServlet in the web.xml file to handle requests through spring container.
    • Spring bean configuration file to define beans, if using annotations then it has to be configured here. Also we need to configure view resolver for view pages.
    • Controller class with request mappings defined to handle the client requests.
Above steps should be enough to create a simple Spring MVC Hello World application.



30. How would you relate Spring MVC Framework to MVC architecture?

    As the name suggests Spring MVC is built on top of Model-View-Controller architecture. DispatcherServlet is the Front Controller in the Spring MVC application that takes care of all the incoming requests and delegate it to different controller handler methods.
    Model can be any Java Bean in the Spring Framework, just like any other MVC framework Spring provides automatic binding of form data to java beans. We can set model beans as attributes to be used in the view pages.
    View Pages can be JSP, static HTMLs etc. and view resolvers are responsible for finding the correct view page. Once the view page is identified, control is given back to the DispatcherServlet controller. DispatcherServlet is responsible for rendering the view and returning the final response to the client.



31. How to achieve localization in Spring MVC applications?

    Spring provides excellent support for localization or i18n through resource bundles. Basis steps needed to make our application localized are:
    
    1. Creating message resource bundles for different locales, such as messages_en.properties, messages_fr.properties etc.
    2. Defining messageSource bean in the spring bean configuration file of type ResourceBundleMessageSource or ReloadableResourceBundleMessageSource.
    3. For change of locale support, define localeResolver bean of type CookieLocaleResolver and configure LocaleChangeInterceptor interceptor. Example configuration can be like below:

       1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

       <beans:bean id="messageSource"     class="org.springframework.context.support.ReloadableResourceBundleMessageSource">     <beans:property name="basename" value="classpath:messages" />     <beans:property name="defaultEncoding" value="UTF-8" /> </beans:bean>   <beans:bean id="localeResolver"     class="org.springframework.web.servlet.i18n.CookieLocaleResolver">     <beans:property name="defaultLocale" value="en" />     <beans:property name="cookieName" value="myAppLocaleCookie"></beans:property>     <beans:property name="cookieMaxAge" value="3600"></beans:property> </beans:bean>   <interceptors>     <beans:bean         class="org.springframework.web.servlet.i18n.LocaleChangeInterceptor">         <beans:property name="paramName" value="locale" />     </beans:bean> </interceptors>

    4. Use spring:message element in the view pages with key names, DispatcherServlet picks the corresponding value and renders the page in corresponding locale and return as response.
    For a complete example, please read Spring Localization Example.



32. How can we use Spring to create Restful Web Service returning JSON response?

    We can use Spring Framework to create Restful web services that returns JSON data. Spring provides integration with Jackson JSON API that we can use to send JSON response in restful web service.
    We would need to do following steps to configure our Spring MVC application to send JSON response:
    
    1. Adding Jackson JSON dependencies, if you are using Maven it can be done with following code:

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       <!-- Jackson --> <dependency>     <groupId>com.fasterxml.jackson.core</groupId>     <artifactId>jackson-databind</artifactId>     <version>${jackson.databind-version}</version> </dependency>

    2. Configure RequestMappingHandlerAdapter bean in the spring bean configuration file and set the messageConverters property to MappingJackson2HttpMessageConverter bean. Sample configuration will be:

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       <!-- Configure to plugin JSON as request and response in method handler --> <beans:bean class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter">     <beans:property name="messageConverters">         <beans:list>             <beans:ref bean="jsonMessageConverter"/>         </beans:list>     </beans:property> </beans:bean>    <!-- Configure bean to convert JSON to POJO and vice versa --> <beans:bean id="jsonMessageConverter" class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter"> </beans:bean>

    3. In the controller handler methods, return the Object as response using @ResponseBody annotation. Sample code:

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       @RequestMapping(value = EmpRestURIConstants.GET_EMP, method = RequestMethod.GET) public @ResponseBody Employee getEmployee(@PathVariable("id") int empId) {     logger.info("Start getEmployee. ID="+empId);            return empData.get(empId); }

    4. You can invoke the rest service through any API, but if you want to use Spring then we can easily do it using RestTemplate class.
    For a complete example, please read Spring Restful Webservice Example.



33. What are some of the important Spring annotations you have used?

    Some of the Spring annotations that I have used in my project are:
    
    • @Controller – for controller classes in Spring MVC project.
    • @RequestMapping – for configuring URI mapping in controller handler methods. This is a very important annotation, so you should go through Spring MVC RequestMapping Annotation Examples
    • @ResponseBody – for sending Object as response, usually for sending XML or JSON data as response.
    • @PathVariable – for mapping dynamic values from the URI to handler method arguments.
    • @Autowired – for autowiring dependencies in spring beans.
    • @Qualifier – with @Autowired annotation to avoid confusion when multiple instances of bean type is present.
    • @Service – for service classes.
    • @Scope – for configuring scope of the spring bean.
    • @Configuration, @ComponentScan and @Bean – for java based configurations.
    • AspectJ annotations for configuring aspects and advices, @Aspect, @Before, @After, @Around, @Pointcut etc.



34. Can we send an Object as the response of Controller handler method?

    Yes we can, using @ResponseBody annotation. This is how we send JSON or XML based response in restful web services.
    



35. How to upload file in Spring MVC Application?

    Spring provides built-in support for uploading files through MultipartResolver interface implementations. It’s very easy to use and requires only configuration changes to get it working. Obviously we would need to write controller handler method to handle the incoming file and process it. For a complete example, please refer Spring File Upload Example.
    



36. How to validate form data in Spring Web MVC Framework?

    Spring supports JSR-303 annotation based validations as well as provide Validator interface that we can implement to create our own custom validator. For using JSR-303 based validation, we need to annotate bean variables with the required validations.
    For custom validator implementation, we need to configure it in the controller class. For a complete example, please read Spring MVC Form Validation Example.
    



37. What is Spring MVC Interceptor and how to use it?

    Spring MVC Interceptors are like Servlet Filters and allow us to intercept client request and process it. We can intercept client request at three places – preHandle, postHandle and afterCompletion.
    We can create spring interceptor by implementing HandlerInterceptor interface or by extending abstract class HandlerInterceptorAdapter.
    We need to configure interceptors in the spring bean configuration file. We can define an interceptor to intercept all the client requests or we can configure it for specific URI mapping too. For a detailed example, please refer Spring MVC Interceptor Example.



38. What is Spring JdbcTemplate class and how to use it?

    Spring Framework provides excellent integration with JDBC API and provides JdbcTemplate utility class that we can use to avoid bolier-plate code from our database operations logic such as Opening/Closing Connection, ResultSet, PreparedStatement etc.
    For JdbcTemplate example, please refer Spring JDBC Example.
    



39. How to use Tomcat JNDI DataSource in Spring Web Application?

    For using servlet container configured JNDI DataSource, we need to configure it in the spring bean configuration file and then inject it to spring beans as dependencies. Then we can use it with JdbcTemplate to perform database operations.
    Sample configuration would be:
    

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    <beans:bean id="dbDataSource" class="org.springframework.jndi.JndiObjectFactoryBean">     <beans:property name="jndiName" value="java:comp/env/jdbc/MyLocalDB"/> </beans:bean>

    For complete example, please refer Spring Tomcat JNDI Example.



40. How would you achieve Transaction Management in Spring?

    Spring framework provides transaction management support through Declarative Transaction Management as well as programmatic transaction management. Declarative transaction management is most widely used because it’s easy to use and works in most of the cases.
    We use annotate a method with @Transactional annotation for Declarative transaction management. We need to configure transaction manager for the DataSource in the spring bean configuration file.
    

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    <bean id="transactionManager"     class="org.springframework.jdbc.datasource.DataSourceTransactionManager">     <property name="dataSource" ref="dataSource" /> </bean>




41. What is Spring DAO?

    Spring DAO support is provided to work with data access technologies like JDBC, Hibernate in a consistent and easy way. For example we have JdbcDaoSupport, HibernateDaoSupport, JdoDaoSupport and JpaDaoSupport for respective technologies.
    Spring DAO also provides consistency in exception hierarchy and we don’t need to catch specific exceptions.
    



42. How to integrate Spring and Hibernate Frameworks?

    We can use Spring ORM module to integrate Spring and Hibernate frameworks, if you are using Hibernate 3+ where SessionFactory provides current session, then you should avoid using HibernateTemplate or HibernateDaoSupport classes and better to use DAO pattern with dependency injection for the integration.
    Also Spring ORM provides support for using Spring declarative transaction management, so you should utilize that rather than going for hibernate boiler-plate code for transaction management.
    For better understanding you should go through following tutorials:
    
    • Spring Hibernate Integration Example
    • Spring MVC Hibernate Integration Example



43. What is Spring Security?

    Spring security framework focuses on providing both authentication and authorization in java applications. It also takes care of most of the common security vulnerabilities such as CSRF attack.
    It’s very beneficial and easy to use Spring security in web applications, through the use of annotations such as @EnableWebSecurity. You should go through following posts to learn how to use Spring Security framework.
    
    • Spring Security in Servlet Web Application
    • Spring MVC and Spring Security Integration Example



44. How to inject a java.util.Properties into a Spring Bean?

    We need to define propertyConfigurer bean that will load the properties from the given property file. Then we can use Spring EL support to inject properties into other bean dependencies. For example;
    

    [code]<bean id="propertyConfigurer"   class="org.springframework.context.support.PropertySourcesPlaceholderConfigurer">     <property name="location" value="/WEB-INF/application.properties" /> </bean> <bean class="com.journaldev.spring.EmployeeDaoImpl">     <property name="maxReadResults" value="${results.read.max}"/> </bean>[/code]

    If you are using annotation to configure the spring bean, then you can inject property like below.
    

    [code]

    @Value("${maxReadResults}") private int maxReadResults;

    [/code]




45. Name some of the design patterns used in Spring Framework?

    Spring Framework is using a lot of design patterns, some of the common ones are:
    
    1. Singleton Pattern: Creating beans with default scope.
    2. Factory Pattern: Bean Factory classes
    3. Prototype Pattern: Bean scopes
    4. Adapter Pattern: Spring Web and Spring MVC
    5. Proxy Pattern: Spring Aspect Oriented Programming support
    6. Template Method Pattern: JdbcTemplate, HibernateTemplate etc
    7. Front Controller: Spring MVC DispatcherServlet
    8. Data Access Object: Spring DAO support
    9. Dependency Injection and Aspect Oriented Programming



46. What are some of the best practices for Spring Framework?

    Some of the best practices for Spring Framework are:
    
    1. Avoid version numbers in schema reference, to make sure we have the latest configs.
    2. Divide spring bean configurations based on their concerns such as spring-jdbc.xml, spring-security.xml.
    3. For spring beans that are used in multiple contexts in Spring MVC, create them in the root context and initialize with listener.
    4. Configure bean dependencies as much as possible, try to avoid autowiring as much as possible.
    5. For application level properties, best approach is to create a property file and read it in the spring bean configuration file.
    6. For smaller applications, annotations are useful but for larger applications annotations can become a pain. If we have all the configuration in xml files, maintaining it will be easier.
    7. Use correct annotations for components for understanding the purpose easily. For services use @Service and for DAO beans use @Repository.
    8. Spring framework has a lot of modules, use what you need. Remove all the extra dependencies that gets usually added when you create projects through Spring Tool Suite templates.
    9. If you are using Aspects, make sure to keep the join pint as narrow as possible to avoid advice on unwanted methods. Consider custom annotations that are easier to use and avoid any issues.
    10. Use dependency injection when there is actual benefit, just for the sake of loose-coupling don’t use it because it’s harder to maintain.

That’s all for Spring Framework interview questions. I hope these questions will help you in coming Java EE interview. I will keep on adding more questions to the list as soon as I found them. If you know some more questions that should be part of the list, make sure to add a comment for it and I will include it.