Agility demands good Software Design

From what i have seen, Agile Software Development is widely misinterpreted nowadays. Many people think to be agile, they just need a process like scrum to define a framework of sprints and iterations. They will sprint until they exhaust and change until they stagnate. But when you'd ask them what principles of objectoriented design are, they'd answer: "Aww yes, we use classes". And when you'd asked them if they did test driven development, they'd reply: "Ahh, this is the thing where you write tests first, isn't it?". Bottom line, people think to be agile without caring about good software design. But in the end, the most important thing is what nobody cares about. It is the agility of the software that they are producing. So if you want to be agile, a process and a mindset is not enough. You need to keep your software agile. By that i mean, you need to keep the design of the software as clean and flexible as possible, to be able to apply any future changes at minimal cost. Software design of course includes your source code. In the end, you are only as agile as your software is.

Why Agile?

A customer does not know what he wants in the first place. He might have a rough idea, but thats it. We cant even blame him for that, because the actual problem that we are supposed to solve is influenced by so many factors and opinions that it is almost impossible to define the best solution right away. Other than that, miscommunication will most likely make things worse. To solve these issues we start small, and deliver fast. Seeing the first result, the customer can verify if we are on track or not. He can check if we have understood, or if he himself made a bad call. He might recognize things that he hasnt thought of before. Delivering repeatedly and iteratively like that, our software will more likely be useful and accepted by the customer in the end. It will more likely lead our project to success. Of course we will have to deal with changes more often, which is why we have to make the software itself agile. In the end it is the software which needs to be changeable, not us.

What it takes

Not only does an agile development process put our software under frequent change, it also demands progress at a fast and steady pace. These facts can quickly cause our software to rot if we don't watch out. And when it does, we get slowed down even more and changes suddenly become impossible. So we'd better not forget about the most important part in agile software development: Keeping our software design as agile as possible, from high-level architecture to the very code itself. Sadly, this gets ignored way too often, and many agile projects fail because of it. They fail because they chose to go faster by neglecting quality, and they quickly rush into a deadend of code rot leading to stagnation. So please remember: It is the Software you are building that needs to be agile in order to adapt to change defined by the customer. It is the software that needs to be agile in order to react to change with minimal effort. Rushing is not gonna make your software agile. Rushing is not gonna make YOU agile. Agility comes hand in hand with quality. And high quality is not even slow, hell no. Good quality done right makes you go even faster.

So what is Agile Software Design?

Agile Software Design is what makes your software as clean and flexible as you need it to be. It demands deliberate adoption of discovered and well established design patterns and principles of objectoriented programming. This is not easy. In fact it's hard, it's very hard. Even if you are really good and experienced at it, you might fail at some point, having to rethink many of your previous design decisions. Agile Software Design is not something that you can rush. It needs time and deliberateness. But it will pay off in the long term by making you progress at a more constant pace, which will in fact be much faster than it would be without agile design. The graphic below should clarify how quick ignorant rushing can slow you down tremendously. The graphic is not based on statistics, but on experience.

Agile Software Design does not end at high-level architecture. It goes on with coding. Bad code can quickly make your project more clumsy than a bad high-level design could. So the devil is in the details.

What can you do to make your Software Design more Agile?

From a Managers perspective? Trust your Developers, don't push them. Invest in experience and expertise.

From a developers perspective? First of all, learn the principles and patterns of objectoriented programming - not just in theory. Being good at those needs lots of practice. There are plenty of books for this:
and many more.

Keep your code clean - don't underestimate this! When you are programming, you spend more time reading code, than writing code. Thus you want to minimize the time you have to read code by making it more readable. It's not a secret how clean code looks like. Take a look at Clean Code.
Other than that, use the right practices like

Test Driven Development  

Just do it already. And if you don't know how, go on and start learning. I have heard numerous opinions of TDD where people said
  • "Unittests are too expensive. The Customer wont pay for that."
  • "Unittests slow us down, we needed to write more code in that time."
  • "I don't know. I'm pretty good at programming, Test-First feels needlessly hard and awkward."
But these are just subjective opinions of people who are not experienced in TDD.
TDD should in fact boost your longterm development speed if you are doing it right. I have not seen a single project in my life that has been as small and simple as that it would not have taken a significant profit from a TDD approach. And i have in fact seen very simple ones. The thing is, you have to test it anyways. What else would speed up your testing more than to automate it? Also, TDD significantly reduces the amount of time you have to spend debugging which already eats up lots of your time - at least more than you'd think. Other than that those tests
  • become a reliable documentation of your code,
  • allow you to fearlessly clean up your code,
  • force you to think of the problem before thinking of the actual solution, which will improve your initial draft,
  • enforce decoupling of components, which will make your design more agile.

Pair Programming

It's not only that 4 eyes will see more defects than 2 eyes do, but also that the transfer of knowledge is taking place. You want all developers to learn from each other to become equally valuable. And you always want more than one person to know a specific module to improve the truck factor. Heck yeah, some people even do mobprogramming - Seriously.

Collective Code Ownership

Don't allow one guy to own that modules code. All code on the project must belong to everybody. Everyone is allowed and invited to view, question and improve everyones code. This might be hard for some people, and might provoke conflicts, but it is absolutely mandatory to make the code better.

Code Reviews

Constantly review code changes. It is the only way to stay up to date, and it will reveal code smell. You need to reveal code smell, because if you don't, you will get stuck.


Refactor often. Everytime you see something that could be refactored, you need to do it right on the spot. The earlier you do it, the more time you save. So if you do it right then, you save the most possible amount of time.

tl;dr: You are just as agile as your software design, which of course includes code. High speed does not demand low quality. It doesn't work like that. In fact high quality is needed to allow high speed. Agile design evolves high quality.

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spring-data-rest in Action

What is spring-data-rest?

spring-data-rest, a recent addition to the spring-data project, is a framework that helps you expose your entities directly as RESTful webservice endpoints. Unlike rails, grails or roo it does not generate any code achieving this goal. spring data-rest supports JPA, MongoDB, JSR-303 validation, HAL and many more. It is really innovative and lets you setup your RESTful webservice within minutes. In this example i'll give you a short overview of what spring-data-rest is capable of.

Initial Configuration 

I'm gonna use the new Servlet 3 Java Web Configuration instead of an ancient web.xml. Nothing really special here.

public class WebAppInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {

protected Class<?>[] getRootConfigClasses() {
return new Class<?>[]{AppConfiguration.class};

protected Class<?>[] getServletConfigClasses() {
return new Class[]{WebConfiguration.class};

protected String[] getServletMappings() {
return new String[]{"/"};
} on github

I am initializing hibernate as my database abstraction layer in the AppConfiguration class. I'm using an embedded database (hsql), since i want to keep this showcase simple stupid. Still, nothing special here.

public class AppConfiguration {

public DataSource dataSource() {
EmbeddedDatabaseBuilder builder = new EmbeddedDatabaseBuilder();
return builder.setType(EmbeddedDatabaseType.HSQL).build();

public LocalContainerEntityManagerFactoryBean entityManagerFactory() {
HibernateJpaVendorAdapter vendorAdapter = new HibernateJpaVendorAdapter();

LocalContainerEntityManagerFactoryBean factory = new LocalContainerEntityManagerFactoryBean();

return factory;

public PlatformTransactionManager transactionManager() {
return new JpaTransactionManager();
} on github

Now to the application servlet configuration: WebConfiguration

public class WebConfiguration extends RepositoryRestMvcConfiguration {
} on github

Oh, well thats a bit short isnt it? Not a single line of code required for a complete setup. This is a really nice application of the convention over configuration paradigm. We can now start creating spring-data-jpa repositories and they will be exposed as RESTful resources automatically. And we can still add custom configuration to the WebConfiguration class later if needed.

The Initialization was really short and easy. We didn't have to code anything special. The only thing we did was setting up a database connection and hibernate, which is obviously inevitable. Now, that we have setup our "REST Servlet" and persistence, lets move on to the application itself, starting with the model.

The Model

I'll keep it really simple creating only two related entities.
public class Book {

private String isbn;

private String title;

private String language;

private List<Author> authors;

} on github

public class Author {

@GeneratedValue(strategy = GenerationType.SEQUENCE)
private Integer id;

private String name;

@ManyToMany(mappedBy = "authors")
private List<Book> books;

} on github

To finally make the entities persistent and exposed as a RESTful webservice, we need spring-data repositories. A repository is basically a DAO. It offers CRUD functionality for our entities. spring-data takes away most of your programming effort creating such repositories. We just have to define an empty interface, spring-data does everything else out of the box. Still, it is easily customizable thanks to its design by convention over configuration!

The Actual Repositories

@RestResource(path = "books", rel = "books")
public interface BookRepository extends PagingAndSortingRepository<Book, Long> {
} on github

@RestResource(path = "authors", rel = "authors")
public interface AuthorRepository extends PagingAndSortingRepository<Author, Integer> {
} on github

Again, there is nearly no code needed. Even the @RestResource annotation could be left out. But if i did, the path and rel would be named after the entity, which i dont want. A REST resource that contains multiple children should be named plural though.

Accessing The Result

Our RESTful webservice is now ready for deployment. Once run, it lists all available resources on the root, so you can navigate from there.

GET http://localhost:8080/
"links" : [ {
"rel" : "books",
"href" : "http://localhost:8080/books"
}, {
"rel" : "authors",
"href" : "http://localhost:8080/authors"
} ],
"content" : [ ]

Fine! Now lets create an author and a book.

POST http://localhost:8080/authors
{"name":"Uncle Bob"}

201 Created
Location: http://localhost:8080/authors/1
PUT http://localhost:8080/books/0132350882
"title": "Clean Code",
"authors": [
"rel": "authors",
"href": "http://localhost:8080/authors/1"

201 Created
Noticed how i used PUT to create the book? This is because its id is the actual isbn. I have to tell the server which isbn to use since he cant guess it. I used POST for the author as his id is just an incremental number that is generated automatically. Also, i used a link to connect both, the book (/books/0132350882) and the author (/authors/1). This is basically what hypermedia is all about: Links are used for navigation and relations between entities.

Now, lets see if the book was created accordingly.

GET http://localhost:8080/books
"links" : [ ],
"content" : [ {
"links" : [ {
"rel" : "books.Book.authors",
"href" : "http://localhost:8080/books/0132350882/authors"
}, {
"rel" : "self",
"href" : "http://localhost:8080/books/0132350882"
} ],
"title" : "Clean Code"
} ],
"page" : {
"size" : 20,
"totalElements" : 1,
"totalPages" : 1,
"number" : 1


Here is an Integration Test, following these steps automatically. It is also available in the example on github.

public class BookApiIT {

private final RestTemplate restTemplate = new RestTemplate();

private final String authorsUrl = "http://localhost:8080/authors";
private final String booksUrl = "http://localhost:8080/books";

public void testCreateBookWithAuthor() throws Exception {
final URI authorUri = restTemplate.postForLocation(authorsUrl, sampleAuthor()); // create Author

final URI bookUri = new URI(booksUrl + "/" + sampleBookIsbn);
restTemplate.put(bookUri, sampleBook(authorUri.toString())); // create Book linked to Author

Resource<Book> book = getBook(bookUri);

final URI authorsOfBookUri = new URI(book.getLink("books.Book.authors").getHref());
Resource<List<Resource<Author>>> authors = getAuthors(authorsOfBookUri);
assertFalse(authors.getContent().isEmpty()); // check if /books/0132350882/authors contains an author

private String sampleAuthor() {
return "{\"name\":\"Robert C. Martin\"}";

private final String sampleBookIsbn = "0132350882";

private String sampleBook(String authorUrl) {
return "{\"title\":\"Clean Code\",\"authors\":[{\"rel\":\"authors\",\"href\":\"" + authorUrl + "\"}]}";

private Resource<Book> getBook(URI uri) {
return, HttpMethod.GET, null, new ParameterizedTypeReference<Resource<Book>>() {

private Resource<List<Resource<Author>>> getAuthors(URI uri) {
return, HttpMethod.GET, null, new ParameterizedTypeReference<Resource<List<Resource<Author>>>>() {
} on github


We have created a complete RESTful webservice without much coding effort. We just defined our entities and database connection. spring-data-rest stated that everything else is just boilerplate, and i agree.

To consume the webservices manually, consider the rest-shell. It is a command-shell making the navigation in your webservice as easy and fun as it could be. Here is a screenshot:

The complete example is available on my github

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Good Bye Redeployment. spring-loaded, an Opensource Classreloader

Redeem yourself from hot deployments and OutOfMemoryException. spring-loaded is an opensource classreloader and a promising alternative to JRebel. It does not offer as many features as JRebel does, and it does not support any framework just yet. Nevertheless, its a great tool that can save the time you are waiting for that servletcontainer to restart again.

What it can do:
  • add/modify/delete methods/fields/constructors
  • modify annotations on types/methods/fields/constructors
  • add/remove/change values in enum types
What it cant:
  • support framework specific changes like Spring MVC @RequestMappings
  • change log configuration on the fly
It is not getting the attention that it deserves, so give it a try.

Just download from github already, and add the following to your JVM parameters:
java -javaagent:<pathTo>/springloaded-{VERSION}.jar -noverify

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Why REST is so important

This post is dedicated to REST, an architectural style of shaping webservices and one of the most misunderstood concept in the history of IT. This post is addressed to you who is designing webservice apis not being fully aware what REST actually means. I'm trying to give you the idea. This post is also addressed to you who think to know what REST means, when in reality you have no clue, just yet. Yes i have met such people in the past - plenty of them. It's not going into the details of the Richardson Maturity Model, and it's not gonna make you a REST expert. There are plenty of guides on the web for that: slides, youtube videos, blogposts, books and more. Rather than going into the details, i'm going to link some good resources at the end of this post.

So lets start with

The meaning of REST

Representational State Transfer. This sentence is not only what REST stands for, it is also the tiniest possible description of what REST actually means. Didn't get it? Read it again: Representational State Transfer. It is not a standard, rather a style describing the act of transfering a state of something by its representation.

Lets consider this:
Marcus is a farmer. He has a ranch with 4 pigs, 12 chickens and 3 cows. He is now simulating a REST api while i am the client. If i want to request the current state of his farm using REST i just ask him: "State?"
Marcus answers: "4 pigs, 12 chickens, 3 cows".
This is the most simple example of Representional State Transfer. Marcus transfered the state of his farm to me using a representation. The representation of the farm is the plain sentence: "4 pigs, 12 chickens, 3 cows".

So lets get to the next level. How would i tell Marcus to add 2 cows to his farm the REST way?
Maybe tell him: "Marcus, please add 2 cows to your farm".
Do you think this was REST? Are we transfering state by its representation here? NO! This was calling a remote procedure. The procedure of adding 2 cows to the farm.
Marcus sadly answers: "400, Bad Request. What do you mean?"
So lets try this again. How would we do this the REST way? What was the representation again? It was "4 pigs, 12 chickens, 3 cows". Ok. so lets try this again transfering the representation...
me: "Marcus, ... 4 pigs, 12 chickens, 5 cows ... please!".
Marcus: "Alright !".
me: "Marcus, ... what is your state now?".
Marcus: "4 pigs, 12 chickens, 5 cows".
me: "Ahh, great!"
See? It was really not that hard and it was REST.

Why RPC is a pain in the A**

So why would you favor REST over the remote procedure call (=RPC) from a logical perspective? Because it dramatically reduces the complexity of our communication by making the representation our only contract. We do not have to discuss what kinds of procedures we need (Add a cow?, Add an animal of a type? Double the chickens amount? Remove all pigs?). All we have to discuss is the representation, and use this representation to achieve anything we want. Easy, isn't it? The needless complexity of RPC is not helpful at all. It is rather increasing the risk of misunderstandings, which we don't want. We don't want our communication to fail because Marcus and I understood a procedure differently.
But this is just one of many problems RPC is creating. If you want to use RPC you need to design some kind of structure to embed your procedure into. This structure requires a place to store parameters, error codes, return values and so on. I have seen lots of developers and companies who really did this. They designed their own RPC-Structure arising huge problems in the implementation of clients and client-server interaction. Why would you do this? Why would you invent your own RPC-Structure? Do you think this is helpful? What if i wanted to make an application that uses many WebServices of multiple proprietary RPC-Formats? I would have to develop something like this:
image #1

If you really need RPC, at least choose a standard like SOAP. Don't make up your own stuff!

But SOAP is still bad

Still, even the standards of RPC are really painful. Well, i have to admit that with ACID Transactions, and a complete standardized Service Description Language, SOAP is not all that bad in some circumstances. Nevertheless, the overhead SOAP produces is massive and a huge performance killer. HTTP is a lightweight protocol. Its headers include anything you need. The only thing you want to put in the body is a representation - or not even that.

Sessions are Evil

Since i wrote solely about sessions and why they are bad in another post, i removed this paragraph. Please continue reading here: Sessions. A Pitfall

Dont reinvent Hypermedia

Since hypermedia is getting quite popular now, i beg you: Don't invent your own style.
We do already have plenty. There is
And we are slowly getting to the situation in image #1 again.

Further Resources

In this post, i have only scratched the surface of the advantages of REST.
Here are some good resources to get you a deeper understanding.

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Spring MVC - @RequestBody and @ResponseBody demystified

In this post i want to dig into spring mvc a little, revealing what happens behind the scenes when a request is converted to your parameter object and vice versa. Before we start, i want to explain the purpose of these annotations.

What are @RequestBody and @ResponseBody for?

They are annotations of the spring mvc framework and can be used in a controller to implement smart object serialization and deserialization. They help you avoid boilerplate code by extracting the logic of messageconversion and making it an aspect. Other than that they help you support multiple formats for a single REST resource without duplication of code. If you annotate a method with @ResponseBody, spring will try to convert its return value and write it to the http response automatically. If you annotate a methods parameter with @RequestBody, spring will try to convert the content of the incoming request body to your parameter object on the fly.

Here is an example

@RequestMapping(value = "/bookcase")
public class BookCaseController {

private BookCase bookCase;

@RequestMapping(method = RequestMethod.GET)
public BookCase getBookCase() {
return this.bookCase;

@RequestMapping(method = RequestMethod.PUT)
public void setBookCase(@RequestBody BookCase bookCase) {
this.bookCase = bookCase;


So what is Spring doing behind the scenes when we are using those Annotations?

Depending on your configuration, spring has a list of HttpMessageConverters registered in the background. A HttpMessageConverters responsibility 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 is hitting a @RequestBody or @ResponseBody annotation spring loops through all registered HttpMessageConverters seeking for the first that fits the given mime type and class and then uses it for the actual conversion.

How can i add a custom HttpMessageConverter?

By adding @EnableWebMvc respectively <mvc:annotation-driven />, spring registers a bunch of predefined messageconverters for JSON/XML and so on. You can add a custom converter like the following

public class WebConfiguration extends WebMvcConfigurerAdapter {

public void configureMessageConverters(List<HttpMessageConverter<?>> httpMessageConverters) {
httpMessageConverters.add(new BookCaseMessageConverter(new MediaType("text", "csv")));

In this example i've written a converter that handles the conversion of a BookCase, which is basically a List of Books. The converter is able to convert csv content to a BookCase and vice versa. I used opencsv to parse the text.

Here is the model

public class Book {

private String isbn;

private String title;

public Book(String isbn, String title) {
this.isbn = isbn;
this.title = title;

// ...

public class BookCase extends ArrayList<Book> {

public BookCase() {

public BookCase(Collection<? extends Book> c) {

and the actual converter

public class BookCaseMessageConverter extends AbstractHttpMessageConverter<BookCase> {

public BookCaseMessageConverter() {

public BookCaseMessageConverter(MediaType supportedMediaType) {

public BookCaseMessageConverter(MediaType... supportedMediaTypes) {

protected boolean supports(Class<?> clazz) {
return BookCase.class.equals(clazz);

protected BookCase readInternal(Class<? extends BookCase> clazz, HttpInputMessage httpInputMessage) throws IOException, HttpMessageNotReadableException {
CSVReader reader = new CSVReader(new InputStreamReader(httpInputMessage.getBody()));
List<String[]> rows = reader.readAll();
BookCase bookCase = new BookCase();
for (String[] row : rows) {
bookCase.add(new Book(row[0], row[1]));
return bookCase;

protected void writeInternal(BookCase books, HttpOutputMessage httpOutputMessage) throws IOException, HttpMessageNotWritableException {
CSVWriter writer = new CSVWriter(new OutputStreamWriter(httpOutputMessage.getBody()));
for (Book book : books) {
writer.writeNext(new String[]{book.getIsbn(), book.getTitle()});

The Result 

We can now issue text/csv requests to our Resource along with application/json and xml which are basically supported out of the box.

PUT /bookcase
Content-Type: text/csv
"123","Spring in Action"
"456","Clean Code"

204 No Content
GET /bookcase
Accept: text/csv

200 OK
"123","Spring in Action"
"456","Clean Code"

Thanks to the design of spring mvc, which is following the single responsibility principle, our controller stays thin. We don't have to add a single line if we want to support new media types.

The complete example is available on my github

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