Struts Java Interview Questions – Set 01

by

Explain the Struts1/Struts2/MVC application architecture?

Struts was adopted by the Java developer community as a default web framework for developing web applications

The MVC(Model–view–controller) an application that consist of three distinct parts. The problem domain is represented by the Model. The output to the user is represented by the View. And, the input from the user is represented by Controller.

What is the difference between ArrayList and LinkedList? (ArrayList vs LinkedList.

when the internal array fills up. The arrayList has to create a new array and copy all the elements there. The ArrayList has a growth algorithm of (n*3)/2+1, meaning that each time the buffer is too small it will create a new one of size (n*3)/2+1 where n is the number of elements of the current buffer. Hence if we can guess the number of elements that we are going to have, then it java.util.ArrayList and java.util.LinkedList are two Collections classes used for storing lists of object references Here are some key differences:

  • ArrayList uses primitive object array for storing objects whereas LinkedList is made up of a chain of nodes. Each node stores an element and the pointer to the next node. A singly linked list only has pointers to next. A doubly linked list has a pointer to the next and the previous element. This makes walking the list backward easier.
  • ArrayList implements the RandomAccess interface, and LinkedList does not. The commonly used ArrayList implementation uses primitive Object array for internal storage. Therefore an ArrayList is much faster than a LinkedList for random access, that is, when accessing arbitrary list elements using the get method. Note that the get method is implemented for LinkedLists, but it requires a sequential scan from the front or back of the list. This scan is very slow. For a LinkedList, there’s no fast way to access the Nth element of the list.
  • Adding and deleting at the start and middle of the ArrayList is slow, because all the later elements have to be copied forward or backward. (Using System.arrayCopy()) Whereas Linked lists are faster for inserts and deletes anywhere in the list, since all you do is update a few next and previous pointers of a node.
  • Each element of a linked list (especially a doubly linked list) uses a bit more memory than its equivalent in array list, due to the need for next and previous pointers.
  • ArrayList may also have a performance issue makes sense to create a arraylist with that capacity during object creation (using construtor new ArrayList(capacity)). Whereas LinkedLists should not have such capacity issues.

In your experience, what do you don’t like about Spring? Are there any pitfalls

Spring has become very huge and bulky. So, don’t over do it by using all its features because of the hype that Spring is good. Look at what parts of Spring really provides some benefits for your project and use those parts. In most cases, it is much better to use proven frameworks like Spring than create your own equivalent solution from a maintenance and applying the best practices perspective. For example, all spring templates (jdbc, rest, jpa etc.) have the following advantages — perform common setup routines for you, let you skip the boilerplate and concentrate on the logic you want.

Spring MVC is probably not the best Web framework. There are other alternatives like Struts 2, Wicket, and JSF.  Having said this, Spring integrates well with the other Web frameworks like Struts, JSF, etc.

The XML files can get bloated. This can be minimized by carefully considering other options like annotations, JavaConfig, and having separate XML configuration files.

Can you describe the architecture of a medium-to-large scale system that you actually designed or implemented? Can you white board the components of the system you recently worked on? How would you go about designing a JEE shopping cart application? Can you discuss some of the high level architectures you are experienced with

There are a number of high level conceptual architectures as discussed below. These individual architectures can be mixed and matched to produce hybrid architectures.

Model-View-Controller Architecture

Most web and stand-alone GUI applications follow this pattern. For example, Struts and Spring MVC frameworks and Swing GUI.

  • The model represents the core business logic and state.
  • The view renders the content of the model state by adding display logic.
  • The controller translates the interaction with the view into action to be performed by the model.

The actions performed by a model include executing the business logic  and changing the state of the model.Based on the user interactions, the controller selects an appropriate view to render.The controller decouples the model from the view.

Service Oriented Architecture (SOA)

The business logic and application state are exposed as reusable services.

An Enterprise Service Bus (ESB) is used as an orchestration and mediation layer to decouple the applications from the services.

The above architecture has 5 tiers. The application tier could be using a typical MVC architecture.

  • The service orchestration tier could be using ESB products like OracleService Bus, TIBCO, etc and BPM products like Lombardi BPM, Pega BPM, etc.
  • In the abovediagram, the ESB integrates with the BPM via messaging queues.
  • The service tier consists of individual services that can be accessed through SOAP or RESTful web services.
  • The SOA implementation requires change agents to drive adoption of new approaches.
  • The BPM,application integration, and real-time information all contribute to dynamically changing how business users do their jobs.

So, it needs full support from the business, requiring  restructuring and also it can take some time to realize the benefits of SOA. Cloud computing is at the leading edge of its hype and as a concept compliments SOA as an architectural style. Cloud computing is expected to provide a computing capability that can scale up (to massive proportions) or scale down dynamically based on demand. This implies a very large pool of computing resources either be within the enterprise intranet or on the Internet (i.e on the cloud).

User Interface (UI) Component Architecture

This architecture is driven by a user interface that is made up of a number of discrete components.Each component calls a service that encapsulates business logic and hides lower level details. Components can be combined to form new composite components allowing richer functionality. These components can also be shared across a number ofapplications.

For example, JavaScript widgets, Java Server Faces (JSF) components, etc.

RESTful data composition Architecture

The user interface can be built by calling a number of underlying services that are each responsible for building part of a page.

The user interface translates and combine the data in different formats like XML(translate to HTML using XSLT), JSON (Java Script Object Notation), ATOM (feed for mail messages and calendar applications), RSS (for generating RSS feeds), etc.

HTML composition Architecture

In this architecture, multiple applications output fragments of HTML that are combined to generate the final user interface.

For example, Java portlets used inside a portal application server to aggregate individual content..

Plug-in Architecture

In this architecture, a core application defines an interface, and the functionality will be implemented as a set of plug-ins that conform to that interface.

For example, the the Eclipse RCP framework, Maven build tool, etc use this architecture.

Event Driven Architecture (EDA)

The EDA pattern decouples the interactions between the event publishers and the event consumers. Many to many communications are achieved via a topic, where one specific event can be consumed by many subscribers. The EDA also supports asynchronous operations and acknowledgments through event messaging.

This architecture requires effective monitoring in place to track queue depth, exceptions, and other possible problems. The traceability, isolation, and debugging of an event can be difficult in some cases. This architecture is useful in scenarios where the business process is inherently asynchronous, multiple consumers are interested in an event(e.g. order status has changed to partially-filled ), no immediate acknowledgment is required (e.g. an email is sent with the booking details and itinerary), and real-time request/response is not required (e.g. a long running report can be generated asynchronously and made available later via online or via email).

Most conceptual architectures use a hybrid approach using a combination of different architectures based on the benefits of each approach and its pertinence to your situation. Here is a sample hybrid approach depicting an online trading system.

FIX is a Financial Information eXchange protocol. You could also notice a number of synchronous calls using XML/HTTP or SOAP/HTTP and asynchronous calls using JMS. The above diagram also depicts that an enterprise architecture can be complex with a number of moving parts. So, it is imperative that all these moving parts are properly monitored and tested for any potential performance issues. Most of these services will be running as a cluster or a load balanced service with either active/active or active.passive configuration for high availability and scalability.

Can you give some examples of thread racing conditions you had experienced

  • Declaring variables in JSP pages are not thread-safe. The declared variables in JSP pages end-up as instance variables in the converted Servlets. <%! Calendar c = Calendar.getInstance(); %>
  • Decalring instance variables in Servlets is not thread safe, as Servlets are inherently multi-threaded and gets accessed by multiple-threads. Same is true for the Actionclasses in the struts framework.
  • Some of the Java standard library classes likeSimpleDateFormat is not thread-safe. Always check the API to see if a particular class is thread-safe. If a particular class or library is not therad-safe, you could do one of three things.

Provide your own wrapper class that decorates the third-party library with proper synchronization.

This is a typical use of the decorator design pattern.

Use an alternative library, which is thread-safe if available.

For example, Joda Time Library.

Use it in a thread-safe manner.

For example, you could use the SimpleDateFormat class as shown below within a ThreadLocal class.

Each thread will have its own instance of the SimpleDateFormat object.

public class DateFormatTest

{

//          anonymous inner class. Each thread will have its own copy

private final static ThreadLocal<SimpleDateFormat> shortDateFormat

=  new ThreadLocal<SimpleDateFormat>()

{

protected SimpleDateFormat initialValue()

{

return new SimpleDateFormat(“dd/MM/yyyy”);

}

};

public Date convert(String strDate)throws ParseException

{

//          get the SimpleDateFormat instance for this thread and parse the date string

Date d = shortDateFormat.get().parse(strDate);

return d;

}

}

  • The one that is very popular with the interviewers is writing the singleton classes that are not thread-safe.

In your experience, why would you use Spring framework

Spring has a layered architecture with over 20 modules to choose from. This means, use what you need and leave what you don’t need now. Spring simplifies JEE through POJO programming. There is no behind the scene magic in Spring as in JEE programming. POJO programming enables continuous integration and testability.

Spring framework’s core functionality is dependency injection (DI). Dependency injection promotes easy unit testing and more maintainable and flexible code. DI code is much easier to test. The functionality expressed by the object can be tested in a black box by building ‘mock’ objects implementing the interfaces expected by yourapplication logic. DI code is much easier to reuse as the ‘depended’ functionality is extrapolated into well defined interfaces, allowing separate objects whose configuration is handled by a suitable application platform to be plugged into other objects at will. DI code is more flexible. It is innately loosely coupled code to an extreme. This allows the programmer to pick and choose how objects are connected based exclusively on their required interfaces on one end and their expressed interfaces on the other.

Spring supports Aspect Oriented Programming (AOP), which enables cohesive development by separatingapplication business logic from system services. Supporting functionalities like auditing, gathering performance and memory metrics, etc can be enabled through AOP.

Spring also provides a lot of templates which act as base classes to make using the JEE standard technologies a breeze to work with. For example, the JdbcTemplate works well with JDBC, the JpaTemplate does good things with JPA, JmsTemplate makes JMS pretty straightforward. The RestTemplate is simply awesome in it’s simplicity. Simplicity means more readable and maintainable code.When writing software these days, it is important to try and decouple as much middleware code from your business logic as possible. The best approach when using remoting is to use Spring Remoting which can then use any messaging or remoting technology under the covers. Apache Camel is a powerful open source integration framework based on known Enterprise Integration Patterns with powerful Bean Integration. Apache Camel is designed to work nicely with the Spring Framework in a number of ways.

It also provides declarative transactions, job scheduling, authentication, a fully-fledged MVC webframework, and integration to other frameworks likeHibernate,iBatis,JasperReports,JSF,Struts,Tapestry,Seam, Quartz job scheduler, etc.

Spring beans can be shared between different JVMs using Terracotta. This allows you to take existing beans and spread them across a cluster, turn Spring application context events into distributed events, export clustered beans via Spring JMX, and make your Spring applications highly available and clustered. Spring also integrate well with other clustering solutions like Oracle’s Coherance.Spring favors unchecked exceptions and eliminates unsightly try, catch, and finally (and some times try/catch within finally itself) blocks. The Spring templates like JpaTemplate takes care of closing or releasing a database connection. This prevents any potential resource leaks and promotes more readable code.It prevents the proliferation of factory and singleton pattern classes that need to be created to promote loose coupling if not for using a DI framework like Spring or Guice.