Month: May 2017

WebSocket with Spring Boot

Introduction

In this post I am going to talk briefly about developing a WebSocket based application using Spring Boot framework. WebSocket[1] is a full duplex protocol allows bi-directional communication. At the moment, widely using protocols such as HTTP is uni-directional and use long polling mechanisms to achieve bi-directional behavior. But using protocols such as WebSocket allows sending requests from server to client-side. However unlike HTTP, WebSocket does not form a strong application-level protocol. WebSocket sits as a thin layer on-top-of TCP protocol and allow application developers to come-up with a high-level messaging protocol design.

STOMP

Simple Text-Oriented Messaging Protocol (STOMP) is selected by the Spring framework for its WebSocket support. STOMP uses message brokers to provide bi-directional communication.

Server-side Code

Server-side code of a websocket server is as follows:


import org.springframework.messaging.handler.annotation.MessageMapping;
import org.springframework.messaging.handler.annotation.SendTo;
import org.springframework.stereotype.Controller;

@Controller
public class GreetingController {

@MessageMapping("/hello")
 @SendTo("/topic/greetings")
 public Greeting greeting(HelloMessage helloMessage) throws Exception {
 return new Greeting("Hello," + helloMessage.getName() + "!");
 }
}

In the above code @MessageMapping annotation is used to map the messages come with “/hello” in the path. @SendTo is used to specify the destination of the result. Here, HelloMessage and Greeting are bean classes which can be found in [2].

The server-side configuration is as follows:


@Configuration
@EnableWebSocketMessageBroker
public class WebsocketConfigurer extends AbstractWebSocketMessageBrokerConfigurer {

@Override
 public void configureMessageBroker(MessageBrokerRegistry registry) {
 registry.enableSimpleBroker("/topic");
 registry.setApplicationDestinationPrefixes("/app");
 }

@Override
 public void registerStompEndpoints(StompEndpointRegistry stompEndpointRegistry) {
 stompEndpointRegistry.addEndpoint("/gs-guide-websocket").withSockJS();
 }
}

WebsocketConfigurer class is annotated as the configuration class for the application. By configureMessageBroker method it enables message broker and add “/topic” as topic it holds. “/app” is used to identify messages which need to send to the controller.

In the second method, “/gs-guide-websocket” is used as a endpoint which clients can connect. So let’s move in to the client-side code. It contains 2 files, a static HTML and a JS file.

<html>
<head>
<title>Hello WebSocket</title>
	<link href="/webjars/bootstrap/css/bootstrap.min.css" rel="stylesheet">
<!--	<link href="/main.css" rel="stylesheet">-->
<script src="/webjars/jquery/jquery.min.js"></script>
    <script src="/webjars/sockjs-client/sockjs.min.js"></script>
<script src="/webjars/stomp-websocket/stomp.min.js"></script>
    <script src="/app.js"></script>
</head>
<body>
<noscript>
<h2 style="color: #ff0000">Seems your browser doesn't support Javascript! Websocket relies on Javascript being
enabled. Please enable
Javascript and reload this page!</h2>
</noscript>
<div id="main-content" class="container">
<div class="row">
<div class="col-md-6">
<form class="form-inline">
<div class="form-group">
<label for="connect">WebSocket connection:</label>
<button id="connect" class="btn btn-default" type="submit">Connect</button>
<button id="disconnect" class="btn btn-default" type="submit" disabled="disabled">Disconnect
</button>
</div>
</form>
</div>
<div class="col-md-6">
<form class="form-inline">
<div class="form-group">
<label for="name">What is your name?</label>
<input type="text" id="name" class="form-control" placeholder="Your name here...">
</div>
<button id="send" class="btn btn-default" type="submit">Send</button>
</form>
</div>
</div>
<div class="row">
<div class="col-md-12">
<table id="conversation" class="table table-striped">
<thead>
<tr>
<th>Greetings</th>
</tr>
</thead>
<tbody id="greetings"></tbody>
</table>
</div>
</div>
</div>
</body>
</html>

var stompClient = null;

function setConnected(connected) {
$("#connect").prop("disabled", connected);
$("#disconnect").prop("disabled", !connected);
if (connected) {
$("#conversation").show();
}
else {
$("#conversation").hide();
}
$("#greetings").html("");
}

function connect() {
var socket = new SockJS('/gs-guide-websocket');
stompClient = Stomp.over(socket);
stompClient.connect({}, function (frame) {
setConnected(true);
console.log('Connected: ' + frame);
stompClient.subscribe('/topic/greetings', function (greeting) {
showGreeting(JSON.parse(greeting.body).content);
});
});
}

function disconnect() {
if (stompClient != null) {
stompClient.disconnect();
}
setConnected(false);
console.log("Disconnected");
}

function sendName() {
stompClient.send("/app/hello", {}, JSON.stringify({'name': $("#name").val()}));
}

function showGreeting(message) {
$("#greetings").append("
<tr>
<td>" + message + "</td>
</tr>
");
}

$(function () {
$("form").on('submit', function (e) {
e.preventDefault();
});
$( "#connect" ).click(function() { connect(); });
$( "#disconnect" ).click(function() { disconnect(); });
$( "#send" ).click(function() { sendName(); });
});

Finally the pom file of the project is as follows:

<project xmlns="http://maven.apache.org/POM/4.0.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>1.4.1.RELEASE</version>
</parent>
<groupId>com.buddhima.websocket</groupId>
<artifactId>websocket-try</artifactId>
<version>1.0-SNAPSHOT</version>

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

<dependency>
<groupId>org.webjars</groupId>
<artifactId>webjars-locator</artifactId>
</dependency>
<dependency>
<groupId>org.webjars</groupId>
<artifactId>sockjs-client</artifactId>
<version>1.0.2</version>
</dependency>
<dependency>
<groupId>org.webjars</groupId>
<artifactId>stomp-websocket</artifactId>
<version>2.3.3</version>
</dependency>
<dependency>
<groupId>org.webjars</groupId>
<artifactId>bootstrap</artifactId>
<version>3.3.7</version>
</dependency>
<dependency>
<groupId>org.webjars</groupId>
<artifactId>jquery</artifactId>
<version>3.1.0</version>
</dependency>
</dependencies>

<properties>
<java.version>1.8</java.version>
</properties>

<build>
<plugins>
<plugin>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-maven-plugin</artifactId>
</plugin>
</plugins>
</build>
</project>

 

References

[1] WebSocket protocol RFC – https://tools.ietf.org/html/rfc6455

[2] Using WebSocket to build an interactive web application – https://spring.io/guides/gs/messaging-stomp-websocket/

[3] WebSocket Support – https://docs.spring.io/spring/docs/current/spring-framework-reference/html/websocket.html

Start Using Java 8 Lambda Expressions

Introduction

Java 8 comes with a bunch of new features for its developers. One such improvement is Lambda expressions. Lambda Expressions allows Java developers to use functions as values, and pass it as a value to a method. This might be familiar for people from functional programming background, but a bit difficult for people who tends to think in object-oriented mindset. In this article, I am going through series of examples to show how we can use lambda expressions in coding.

Prerequisites

Before you start, you need to install Java 8 in your machine and create a project using Java 8. Here I’m going describe things in more abstract-level. So you should possess knowledge on using your IDE for basic things such as creating classes, executing Java code etc.

Example Scenario

For this article I have selected a scenario where you need to go though set of books and select books based on different criteria. Those selection criteria are list all the books, list books which are novels, and list title of the books written in 20th century. So let’s start coding.

Creating entity class

First step of solving such problem is to create an entity class called Book which can represent a single instance of a book. So here it is.


public class Book {

private String name;
 private String author;
 private int year;
 private String language;
 private String category;

public Book(String name, String author, int year, String language, String category) {
 this.name = name;
 this.author = author;
 this.year = year;
 this.language = language;
 this.category = category;
 }

public String getName() {
 return name;
 }

public void setName(String name) {
 this.name = name;
 }

public String getAuthor() {
 return author;
 }

public void setAuthor(String author) {
 this.author = author;
 }

public int getYear() {
 return year;
 }

public void setYear(int year) {
 this.year = year;
 }

public String getLanguage() {
 return language;
 }

public void setLanguage(String language) {
 this.language = language;
 }

public String getCategory() {
 return category;
 }

public void setCategory(String category) {
 this.category = category;
 }

@Override
 public String toString() {
 return "Book{" +
 "name='" + name + '\'' +
 ", author='" + author + '\'' +
 ", year=" + year +
 ", language='" + language + '\'' +
 ", category='" + category + '\'' +
 '}';
 }
}

I think I no need to explain the above simple Java class as it should be familiar to you.

Conventional Method

This is what we usually do to solve this type of problems.

public class BookFinderExample1 {

    public static void main(String[] args) {
        List<Book> books = Arrays.asList(
                new Book("Moby-Dick", "Herman Melville", 1851, "EN", "Novel"),
                new Book("War and Peace", "Leo Tolstoy", 1869, "RU", "Novel"),
                new Book("The Three Musketeers", "Alexandre Dumas", 1844, "FR", "Novel"),
                new Book("Les Miserables", "Victor Hugo", 1862, "FR", "Fiction"),
                new Book("Journey to the West", "Wu Cheng'en", 1592, "ZH", "Fiction"),
                new Book("Wild Swans", "Jung Chang", 1991, "ZH", "Biography"),
                new Book("The Reader", "Bernhard Schlink", 1995, "DE", "Novel"),
                new Book("Perfume", "Patrick Suskind", 1985, "DE", "Fiction")
        );

        // 1. print all books
        System.out.println("Print all books");
        printAllBooks(books);

        // 2. print all novels
        System.out.println("Print all novels");
        printAllNovels(books);

        // 3. print all books in 20th century
        System.out.println("Print all books in 20th century");
        printAllIn20thCentury(books);
    }

    private static void printAllBooks(List<Book> books) {
        for (Book book : books) {
            System.out.println(book.toString());
        }
    }

    private static void printAllNovels(List<Book> books) {
        for (Book book : books) {
            if (book.getCategory().equals("Novel"))
                System.out.println(book.toString());
        }
    }

    private static void printAllIn20thCentury(List&lt;Book&gt; books) {
        for (Book book : books) {
            if (book.getYear() > 1900 && book.getYear() < 2001)
                System.out.println(book.getName());
        }
    }
}

First it created a list of books (I won’t going to repeat this step in next examples). Then we create 3 methods which serves our purpose. And we call those methods one by one. Though this fulfills out requirement, seems that’s not a scalable solution. Once we have a new requirement, we need to create a new method and call it.

Using Generic Solution

If we carefully look into the methods we have implemented, they all do a common thing. They iterate though a given list of books, checks a condition and perform an action (eg: print the book). So we can use interfaces for this and stick with just one method. Let’s see how.

public class BookFinderExample2 {

    public static void main(String[] args) {
        List<Book> books = Arrays.asList(
                .......
        );

        // 1. print all books
        System.out.println("Print all books");
        printBooks(books, new Checker() {
            public boolean check(Book book) {
                return true;
            }
        }, new Action() {
            public void perform(Book book) {
                System.out.println(book.toString());
            }
        });

        // 2. print all novels
        System.out.println("Print all novels");
        printBooks(books, new Checker() {
            public boolean check(Book book) {
                return book.getCategory().equals("Novel");
            }
        }, new Action() {
            public void perform(Book book) {
                System.out.println(book.toString());
            }
        });

        // 3. print all books in 20th century
        System.out.println("Print all books in 20th century");
        printBooks(books, new Checker() {
            public boolean check(Book book) {
                return (book.getYear() > 1900 && book.getYear() < 2001);
            }
        }, new Action() {
            public void perform(Book book) {
                System.out.println(book.getName());
            }
        });
    }

    private static void printBooks(List<Book> books, Checker checker, Action action) {
        for (Book book : books) {
            if (checker.check(book)) {
                action.perform(book);
            }
        }
    }

    interface Checker {
        boolean check(Book book);
    }

    interface Action {
        void perform(Book book);
    }
}

In the above solution I have introduced two interfaces and both of them contains a single method exposed. With the use of just one method printBooks and objects implementation of those two interfaces we have achieved the same results as before. Now we have taken out the action and inject in to the printBooks method. The instances of Checker and Action is created with anonymous inner classes.

The above seems to be fine for generalizing the solution but the syntax seems to be so tedious. So far we have not used anything new in Java 8. Let’s use Java 8 new features to ease our coding.

Using Lambda Expressions

Lets have a look at one of our anonymous inner class

new Action() {
    public void perform(Book book) {
        System.out.println(book.toString());
    }
}

The above is an anonymous inner class created for Action interface. Action interface has only a single method called perform. That method takes one argument and prints it. So such Anonymous Inner classes can be written as follows.

(Book book) -> System.out.println(book.toString())

Since the Action interface has only one method, we no need to say it’s name. Since it has one line of code, no need to use curly-brackets. Now with single argument, no need of using brackets, and specifying argument type.

book -> System.out.println(book.toString())

That’s a lambda expression. So let’s substitute lambda expressions.

public class BookFinderExample3 {

    public static void main(String[] args) {
        List<Book> books = Arrays.asList(
                ....
        );

        // 1. print all books
        System.out.println("Print all books");
        printBooks(books, book ->  true, book -> System.out.println(book.toString()));

        // 2. print all novels
        System.out.println("Print all novels");
        printBooks(books, book -> book.getCategory().equals("Novel"), book -> System.out.println(book.toString()));

        // 3. print all books in 20th century
        System.out.println("Print all books in 20th century");
        printBooks(books, book ->  (book.getYear() > 1900 && book.getYear() < 2001), book -> System.out.println(book.getName()));
    }

    private static void printBooks(List<Book> books, Checker checker, Action action) {
        for (Book book : books) {
            if (checker.check(book)) {
                action.perform(book);
            }
        }
    }

    @FunctionalInterface
    interface Checker {
        boolean check(Book book);
    }

    @FunctionalInterface
    interface Action {
        void perform(Book book);
    }
}

We have introduced 2 interfaces for our work. But is it necessary? No. Java SDK developers have identified this and provide us a set of already defined interfaces inside java.util.function package. All we have to do it, reuse them!

public class BookFinderExample4 {

    public static void main(String[] args) {
        List<Book> books = Arrays.asList(
                ...
        );

        // 1. print all books
        System.out.println("Print all books");
        printBooks(books, book ->  true, book -> System.out.println(book.toString()));

        // 2. print all novels
        System.out.println("Print all novels");
        printBooks(books, book -> book.getCategory().equals("Novel"), book -> System.out.println(book.toString()));

        // 3. print all books in 20th century
        System.out.println("Print all books in 20th century");
        printBooks(books, book ->  (book.getYear() > 1900 && book.getYear() < 2001), book -> System.out.println(book.getName()));
    }

    private static void printBooks(List<Book> books, Predicate<Book> checker, Consumer<Book> action) {
        for (Book book : books) {
            if (checker.test(book)) {
                action.accept(book);
            }
        }
    }
}

In the above example we have used two interfaces Predicate and Consumer. you can find more information about those in Oracle’s official website. You can find plenty of such interfaces which can help your coding.

Using streams

We are not going to stop our simplification from there. If we look in to the printBooks method, it iterates through the list of books and perform things mention in the interface implementations. This kind of iteration is called external iteration, because for-loop iterates the list. In Java 8, streams provide the functionality of internal iteration, which is implemented by converting the list in to a stream. The stream can be considered as a conveyor belt which brings list items one by one. We can use filters etc., to filter-out the required elements and so what every action you like. So when using streams our code would look like as follows.

public class BookFinderExample5 {

    public static void main(String[] args) {
        List<Book> books = Arrays.asList(
                ...
        );

        // 1. print all books
        System.out.println("Print all books");
        books.stream()
                .filter(book ->  true)
                .forEach(book -> System.out.println(book.toString()));

        // 2. print all novels
        System.out.println("Print all novels");
        books.stream()
                .filter(book -> book.getCategory().equals("Novel"))
                .forEach(book -> System.out.println(book.toString()));

        // 3. print all books in 20th century
        System.out.println("Print all books in 20th century");
        books.stream()
                .filter(book ->  (book.getYear() > 1900 && book.getYear() < 2001))
                .forEach(book -> System.out.println(book.getName()));
    }

}

Conclusion

We have started our example from conventional coding and step-by-step brings the Java 8 new features such as lambda expressions, streams and forEach. Similarly you can apply Java 8 concepts in your development as well. Resources in references section below will help you to find more information about Java 8 Lambda Expressions.

References

[1] Oracle’s Java 8 website : http://docs.oracle.com/javase/8/docs/

[2] Oracle’s website on util-functions : https://docs.oracle.com/javase/8/docs/api/java/util/function/package-summary.html

[3] Java Brains Tutorial on Lambda Expressions : https://www.youtube.com/watch?v=gpIUfj3KaOc&list=PLqq-6Pq4lTTa9YGfyhyW2CqdtW9RtY-I3

Microservices with Spring Boot

Introduction

Currently enterprise application development is more interested towards building them as microservices. This trend started about 2 years back and some organizations take this as an opportunity to do a complete re-write of their products. To help developing microservices, several organizations have done framework implementations. In here I am talking about using Spring Boot to create a very basic microservice.

Use-case

This system is about handling patient records. So it is more like an CRUD application. To persist data, I am using a Mongo DB (embedded version). First, let’s see what would be structure of this project.

proj-structure

Fist you need to create a project with the above structure. You may find maven arc-types which helps to do that. Next the pom file should be created properly. Here, I’m showing the important sections of the pom file.

 <parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>1.4.1.RELEASE</version>
</parent>
...
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-actuator</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-data-mongodb</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-jersey</artifactId>
</dependency>

<dependency>
<groupId>de.flapdoodle.embed</groupId>
<artifactId>de.flapdoodle.embed.mongo</artifactId>
</dependency>

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

</dependencies>

 

Application.java file contains the main method to start the microservice. So it should looks like as follow:

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class Application {

 public static void main(String[] args) {
     SpringApplication.run(Application.class, args);
 }
}

ApplicationConfig.java file is used to provide configurations to Spring framework. Here we provide the location of the service and REST-Template. So it should look like follows:

 

import org.glassfish.jersey.server.ResourceConfig;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.client.RestTemplate;

import javax.inject.Named;

@Configuration
public class ApplicationConfig {
    @Named
    static class JerseyConfig extends ResourceConfig {
        public JerseyConfig() {
            this.packages("com.project.capsule.rest");
        }
    }

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

Next we can extend MongoRepository and create PatientReportRepository. This is very interesting capability of Spring framework as it can convert method names in to SQL queries directly.

import com.project.capsule.bean.PatientReport;
import org.springframework.data.mongodb.repository.MongoRepository;
import java.util.List;

public interface PatientReportRepository extends MongoRepository&lt;PatientReport, String&gt; {

 public List<PatientReport> findByName(String name);

 public List<PatientReport> findByNameLike(String name);

 public List<PatientReport> findByTimeBetween(long from, long to);

}

Now let’s create the bean class, PatientReport

 

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
import org.springframework.data.annotation.Id;
import java.util.Map;

@JsonIgnoreProperties(ignoreUnknown = true)
public class PatientReport {

@Id
public String id;

public String name;
public int age;
public String sex;
public String doctorName;
public long time;
public String reportType;
public Map<String, Object> reportData;
}

Finally the service class, PatientReportService. You can define any number of methods and implement a custom logic.

import com.project.capsule.PatientReportRepository;
import com.project.capsule.bean.PatientReport;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestParam;

import javax.inject.Named;
import javax.ws.rs.*;
import javax.ws.rs.core.MediaType;
import javax.ws.rs.core.Response;
import java.util.*;

@Named
@Path("/report")
public class PatientReportService {

@Autowired
private PatientReportRepository repository;

@POST
@Path("")
@Consumes(MediaType.APPLICATION_JSON)
public Response storePatientReport(@RequestBody PatientReport patientReport) {
repository.save(patientReport);
return Response.status(201).build();
}

@GET
@Path("{id}")
@Produces(MediaType.APPLICATION_JSON)
public PatientReport retrievePatientReport(@PathParam("id") int id) {
PatientReport patientReport = repository.findOne(String.valueOf(id));
return patientReport;
}

@POST
@Path("find")
public List<PatientReport> findReports(@RequestParam Map<String, Object> map) {
List<PatientReport> patientReports = new ArrayList<PatientReport>();
Map<String, PatientReport> resultantMap = new HashMap<String, PatientReport>();
List<PatientReport> resultantReports;

if (map.containsKey("name") && map.get("name") != null) {
String patientName = (String) map.get("name");
if (!patientName.trim().equalsIgnoreCase("")) {
resultantReports = repository.findByNameLike(patientName);

for (PatientReport report : resultantReports)
resultantMap.put(report.id, report);

}
}
return patientReports;
}

}

Once you run the Application.java file, microservice will start from port 8080. You can change the post by giving argument “-Dserver.port=8090” etc. Thereafter you can use a REST client to send HTTP requests and see how it works!

References

[1] https://spring.io/blog/2015/07/14/microservices-with-spring

[2] http://blog.scottlogic.com/2016/11/22/spring-boot-and-mongodb.html

[3] https://dzone.com/articles/spring-boot-creating