Send an email with Spring Boot and Thymeleaf

Using Spring Boot and Thymeleaf, it’s easy to send HTML emails to your users. This post will show the configuration and logic to send an email. We won’t go into details on how to setup the template, that’s for another time.

Project setup

Assuming you already use Spring Boot, we need to add two dependencies for this to work. The first is spring-boot-starter-mail, for sending the email. The second is spring-boot-starter-thymeleaf for handling the HTML template that we are going to use.

		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-mail</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-thymeleaf</artifactId>
		</dependency>

Email configuration

We need to configure two beans. To send the email, we’re going to use Spring’s JavaMailSender. To resolve the template. we’re going to use Thymeleaf’s TemplateResolver. Since both beans are specifically configured for sending emails, we configure them in the same class.

JavaMailSender configuration

We need to create a properties file containing the mail server configuration. Let’s put it in src/main/resources/email/emailconfig.properties

email.host=mail.server.address
email.username=username
email.password=password

Next we’ll create the EmailConfiguration class.

@Configuration
@PropertySource("classpath:/email/emailconfig.properties")
public class EmailConfiguration {

    @Value("${email.host}")
    private String host;

    @Value("${email.username}")
    private String username;

    @Value("${email.password}")
    private String password;
}

Using @PropertySource we reference the properties file we created earlier. The @Value annotation is used to reference the property in the properties file.

Let’s configure the JavaMailSender bean next.

    @Bean
    public JavaMailSender getJavaMailSender() {
        JavaMailSenderImpl mailSender = new JavaMailSenderImpl();
        mailSender.setHost(host);
        mailSender.setPort(25);

        mailSender.setUsername(username);
        mailSender.setPassword(password);

        Properties props = mailSender.getJavaMailProperties();
        props.put("mail.transport.protocol", "smtp");
        props.put("mail.smtp.auth", "true");
        props.put("mail.smtp.starttls.enable", "true");
        props.put("mail.debug", "true");

        return mailSender;
    }

This sets up a JavaMailSender configured to use the smtp protocol. It’s mostly self explanatory.

TemplateEngine configuration

To resolve and process the template, we need to configure a separate TemplateEngine. This TemplateEngine will only use one TemplateResolver. You could configure more if you need to.

    @Bean
    public TemplateEngine emailTemplateEngine(){
        final SpringTemplateEngine templateEngine = new SpringTemplateEngine();
        templateEngine.addTemplateResolver(templateResolver());
        return templateEngine;
    }

    private ITemplateResolver templateResolver() {
        final ClassLoaderTemplateResolver templateResolver = new ClassLoaderTemplateResolver();
        templateResolver.setOrder(Integer.valueOf(1));
        templateResolver.setResolvablePatterns(Collections.singleton("*"));
        templateResolver.setPrefix("/email/");
        templateResolver.setSuffix(".html");
        templateResolver.setTemplateMode(TemplateMode.HTML);
        templateResolver.setCharacterEncoding("UTF-8");
        templateResolver.setCacheable(false);
        return templateResolver;
    }
}

This TemplateResolver will look for templates in src/main/resources/email, with a name ending in .html. It will handle the file as an HTML file.

Email service

Now that the beans are configured, we’re going to create a service that actually sends the email.

@Log
@Service
public class EmailService {

    public static final String SENDER_ADDRESS = "from@email.com";
    public static final String SUBJECT = "Subject of the email";
    private final TemplateEngine templateEngine;

    private final JavaMailSender emailSender;

    @Autowired
    public EmailService(JavaMailSender emailSender, TemplateEngine emailTemplateEngine){
        this.templateEngine = emailTemplateEngine;
        this.emailSender = emailSender;
    }

    public void send(Context context, String template, String address, String senderAddress, String subject){
        try {
            final MimeMessage mimeMessage = this.emailSender.createMimeMessage();
            final MimeMessageHelper messageHelper = new MimeMessageHelper(mimeMessage, "UTF-8");
            messageHelper.setFrom(senderAddress);
            messageHelper.setTo(address);
            messageHelper.setSubject(subject);

            final String htmlContent = this.templateEngine.process(template, context);
            messageHelper.setText(htmlContent, true);

            emailSender.send(mimeMessage);
        } catch (MessagingException e){
            log.warning(e.getMessage());
        }
    }
}

One thing to note here is the Context, which contains the data Thymeleaf uses to fill out the template. The ‘template’ parameter contains the filename of the template to use, e.g. ’email.html’. This file needs to be placed where the TemplateResolver will look, in src/main/resources/email.

Spring Boot – Load users from database

This is the third article in a series on authentication with Spring Boot.
In the first article we authenticated using social networks, and allowed any user to access our application.
In the second article we used inMemoryAuthentication for users that used the login form. In essence, we hardcoded our users.
This article is about adding users to a database. We are not going to allow users to sign up, we’re just going to add the users manually.

Setup Postgres

For our user entity, we want to save the following fields:

  • username
  • password (optional)
  • role
  • email
  • name

You can add the clientIds for the social networks that you allow your users to connect with for extra security. But we won’t do that here.
But we do want to be the email to be unique. Every user must have his own email propperty.

CREATE TABLE public."user"
(
    user_name text NOT NULL,
    password text,
    role text NOT NULL,
    email text NOT NULL,
    name text,
	UNIQUE(email)
)

To test this our login later, we need to add a user. The password is BCrypt encoded for “password”.

insert into "user"
(user_name, password, role, email, name)
values
('user','$2a$10$bXetyuwpEai6LomSykjZAuQ5mxU8WqhMBXGuWYnxlveCySRlGxh2i', 'USER', 'test@example.com', 'Test User')

JPA Database access

Once we have the database in place, it would be nice to actually use it in our code. To do this, we need to configure Spring to connect to our database, create a representation of the database in our code, and create a Repository that glues it together.
First, the configuration. Since we created the database schema ourselves, we don’t want Hibernate to do that. You could set spring.jpa.hibernate.ddl-auto to ‘validate’ to make sure the schema matches your model. Also, we want to show the SQL it is executing, so it’s easier to see what’s wrong. You want to turn this off when you’re done, because it generates a lot of logging.

spring:
  jpa:
    hibernate:
      ddl-auto: none
    show-sql: true
  datasource:
    url: jdbc:postgresql://localhost:5432/database
    username: databaseuser
    password: password

For the model we’re going to use Lombok, so we don’t have to deal with the boilerplate code of getters and setters. We are using the Java Persistance API to handle the database mapping.

@Getter
@Setter
@Entity
@Table(name="user", schema = "public")
public class User {

    @Id
    private String email;

    @Column(name="user_name")
    private String userName;
    private String name;
    private String password;
    private String role;
}

We use a Repository to get the User objects from the database. Spring will do a lot of magic for us, so we only need to specify the JPA query and a method in an interface to retrieve the data.

@Repository
public interface UserRepository extends CrudRepository&lt;User, String&gt; {

    @Query("SELECT u FROM User u WHERE u.userName = :username")
    User getUserByUsername(String username);

    @Query("SELECT u FROM User u WHERE u.email = :email")
    User getUserByEmail(String email);
}

UserDetails and UserDetailsService

At this point we need to have UserDetails, and a service to get them from the database. Since we’re using bot FormLogin and OAuth2, I’ve decided to implement both UserDetails and OAuth2User in the same class. This makes things easier later on.

@Repository
public class MyUserDetails implements UserDetails, OAuth2User {

    private final User user;

    public MyUserDetails(User user){
        this.user = user;
    }

    @Override
    public Map&lt;String, Object&gt; getAttributes() {
        return Collections.emptyMap();
    }

    @Override
    public Collection&lt;? extends GrantedAuthority&gt; getAuthorities() {
        SimpleGrantedAuthority authority = new SimpleGrantedAuthority(user.getRole());
        return Collections.singletonList(authority);
    }

    @Override
    public String getPassword() {
        return user.getPassword();
    }

    @Override
    public String getUsername() {
        return user.getUserName();
    }

    @Override
    public boolean isAccountNonExpired() {
        return true;
    }

    @Override
    public boolean isAccountNonLocked() {
        return true;
    }

    @Override
    public boolean isCredentialsNonExpired() {
        return true;
    }

    @Override
    public boolean isEnabled() {
        return true;
    }

    @Override
    public String getName() {
        return user.getName();
    }
}

The service tries to load a user by username, and throws an exception when no user with that username could be found.

@Component
public class MyUserDetailsService implements UserDetailsService {

    private final UserRepository userRepository;

    @Autowired
    public MyUserDetailsService(UserRepository userRepository) {
        this.userRepository = userRepository;
    }

    @Override
    public UserDetails loadUserByUsername(String username)
            throws UsernameNotFoundException {
        User user = userRepository.getUserByUsername(username);

        if (user == null) {
            throw new UsernameNotFoundException("Could not find user");
        }

        return new MyUserDetails(user);
    }
}

Update FormLogin configuration

Previously we configured in-memory authentication. Now that we have all pieces in place to retrieve our users from the database, we need to configure it.
We need to configure the UserDetailsService.

@Bean
public UserDetailsService userDetailsService(){
	return new MyUserDetailsService(userRepository);
}

Then we’ll configure a DaoAuthenticationProvider using the UserDetailService. The passwordEncoder was already configured in the last blogpost.

@Bean
public DaoAuthenticationProvider authenticationProvider() {
	DaoAuthenticationProvider authProvider = new DaoAuthenticationProvider();
	authProvider.setUserDetailsService(userDetailsService());
	authProvider.setPasswordEncoder(passwordEncoder());
	return authProvider;
}

And then to tie it toghether, we use this AuthenticationProvider as the source of the users for the LoginForm

@Override
protected void configure(AuthenticationManagerBuilder auth) throws Exception {
	auth.authenticationProvider(authenticationProvider());
}

Update OAuth configuration

Since we now use the database to store our users, we also need to update the OAuth configuration. We need to verify whether the user who’s trying to login using OAuth is actually known to us. The key information that we can use here is the email address. That’s why there is a method to get the user by email address in the repository, which we are going to use here. If there is no user with the email address that was found in the OAuth2 principle, we throw an exception. Otherwise, we return that user.

@Bean
public OAuth2UserService&lt;OAuth2UserRequest, OAuth2User&gt; oauth2UserService() {
	DefaultOAuth2UserService delegate = new DefaultOAuth2UserService();
	return request -> {
		OAuth2User auth2User = delegate.loadUser(request);
		String email = auth2User.getAttribute("email");
		User user = userRepository.getUserByEmail(email);
		if (user != null){
			return new MyUserDetails(user);
		}
		throw new InternalAuthenticationServiceException("User not registered");
	};
}

Update WebController and frontend

Now that we have all the pieces in place to use the database for user verification, we want to use this information on our site. Since there will be problems with some login attempts (maybe the user misspelled his username), we would like to be able to show an error message on the login page. So we update the /login endpoint like this:

@RequestMapping(value = "/login")
public String login(HttpServletRequest request, Model model){
	if (request.getSession().getAttribute("error.message")!= null) {
		String errorMessage = request.getSession().getAttribute("error.message").toString();
		log.info("Error message: "+errorMessage);
		model.addAttribute("errormessage", errorMessage);
	}
	return "login";
}

On the login page, we need to add the following to display this error message:

<div class="alert alert-danger" role="alert" th:if="${errormessage}">
    <span id="user" th:text="${errormessage}"></span>
</div>

In other places we would like to get the user’s name. To do this, we need to get the principal from the authentication token.

private Optional<MyUserDetails> extractMyUserDetails(Principal principal){
	if (principal instanceof UsernamePasswordAuthenticationToken) {
		return Optional.of((MyUserDetails) ((UsernamePasswordAuthenticationToken) principal).getPrincipal());
	} else if (principal instanceof OAuth2AuthenticationToken){
		return Optional.of((MyUserDetails) ((OAuth2AuthenticationToken) principal).getPrincipal());
	}
	log.severe("Unknown Authentication token type!");
	return Optional.empty();
}

And then we get the username from the MyUserDetails class

@RequestMapping(value = "/welcome")
public String welcome(Principal principal, Model model) {
	MyUserDetails userDetails = extractMyUserDetails(principal)
			.orElseThrow(IllegalStateException::new);
	model.addAttribute("name", userDetails.getName());
	return "welcome";
}

Spring Boot login with a form

Previously I wrote about securing your application with social login. But not everybody has a social account. In this article we’re going to add formlogin to the application. Formlogin simply means that your users can log in with a username and password. We’re going to keep it as simple as possible, with in-memory authentication.

Adding users

Since we’re going to use usernames and passwords to allow users to login, we need to define a password encoder. Spring will not compare the literal password that it receives, but encodes it and then compare the encoded passwords. When the passwords are stored in the database, you don’t want to see them as plain text.

@Bean
public BCryptPasswordEncoder passwordEncoder() {
	return new BCryptPasswordEncoder();
}

The next step is to add the users. We’re going to use inMemoryAuthentication with a single user, to keep it as simple as possible.

public class WebsiteApplication extends WebSecurityConfigurerAdapter {

	// some more code

	protected void configure(final AuthenticationManagerBuilder auth) throws Exception {
		auth.inMemoryAuthentication()
				.withUser("user").password(passwordEncoder().encode("password")).roles("USER");
	}
}

Note that we’re using the passwordEncoder defined in the previous step to encode the password here. Also, we need to define a role, even though we’re not using it yet.

Allow formlogin

Now we need to tell Spring Security to allow the use of our login form.

public class WebsiteApplication extends WebSecurityConfigurerAdapter {

	@Override
	protected void configure(HttpSecurity http) throws Exception {
		SimpleUrlAuthenticationFailureHandler handler = new SimpleUrlAuthenticationFailureHandler("/");
		// @formatter:off
		http
			.authorizeRequests(
					a -&gt; a
				.antMatchers("/error", "/webjars/**","/oauth/**","/login").permitAll()
				.anyRequest().authenticated()
			)
			.formLogin(f -&gt; f
					.loginPage("/login").permitAll()
			)
			// some more code
			;
		// @formatter:on
	}
	// some more code
}

Add login form

Now that we have enabled the formlogin in the backend, it’s time to add a login form in the frontend.

<form name='loginForm' th:action="@{/login}" method='POST'>
	<table>
		<tr>
			<td>User:</td>
			<td><input type='text' name='username' value=''></td>
		</tr>
		<tr>
			<td>Password:</td>
			<td><input type='password' name='password' /></td>
		</tr>
        <tr>
            <td><input name="submit" type="submit" value="submit" /></td>
        </tr>
    </table>
</form>

Webcontroller changes

There were two methods that use the authentication information. The first, index(), doesn’t need to change at all. Sure, we get a different Authentication implementation, but it still works unaltered.

@RequestMapping(value = {"/","/index"})
public String index(@CurrentSecurityContext(expression = "authentication") Authentication authentication) {
	if (authentication.isAuthenticated()) {
		return "redirect:/welcome";
	}
	return "redirect:/login";
}

The second method, where we try to find the username of the authenticated user does change. Here we drop the @CurrentSecurityContext annotation, and request the Principal directly. However, based on the method of authentication, we need to do something else to get the username. If the user used the login form, we can get the username directly from the principal. When the user authenticated using OAuth2, we need to dig a little deeper.

@RequestMapping(value = "/welcome")
public String welcome(Principal principal, Model model) {
	if (principal instanceof UsernamePasswordAuthenticationToken) {
		model.addAttribute("name", principal.getName());
	} else if (principal instanceof OAuth2AuthenticationToken){
		model.addAttribute("name", ((OAuth2AuthenticationToken) principal).getPrincipal().getAttribute("name"));
	}
	return "welcome";
}

Conclusion

Adding a login form to a Spring Boot application is easy. We’ve hardcoded a user with in memory authentication. Since we’re going to use a password, we need to have a password encoder. We’ve configured Spring Security to allow the user to login using a form. Then we’ve actually added a basic HTML login form, and as a last step we’ve modified the backend to work with a different type of principal.

Spring Boot and Oauth2 with Thymeleaf

Spring has a good tutorial explaining how to authenticate with your application using one or more external authentication providers, like GitHub or Google. This tutorial uses a single page application with a Rest endpoint. For a personal project I didn’t want a single page application, I wanted to use Thymeleaf. During implementation I discovered a few things that I’d like to share. This post continues where the tutorial stopped, so you might want to read the tutorial first.

Configure custom OAuth2UserService

When the user has authenticated using an external service, you probably want to do something with that information. Most commonly you’d want to find the user in your own database. You need to have a hook where you can get access to the authenticated user details. To do this, you can create your own OAuth2UserService bean which will be executed when the user has been authenticated. The bean itself can be quite basic, the following example only returns the authenticated user:

 	@Bean
	public OAuth2UserService<OAuth2UserRequest, OAuth2User> oauth2UserService() {
		DefaultOAuth2UserService delegate = new DefaultOAuth2UserService();
		return request -> {
			OAuth2User user = delegate.loadUser(request);

            // custom code

			return user;
		};
	}

Now we need to tell our application when to call this bean. In the configure(HttpSecurity http) method, we’re going to add the following fragment:

			.oauth2Login(o -> o.failureHandler((request, response, exception) -> {
						request.getSession().setAttribute("error.message", exception.getMessage());
						handler.onAuthenticationFailure(request, response, exception);
					})
				.userInfoEndpoint()
				.userService(oauth2UserService())
			);

Now we’ve specifically told Spring Security to use our own OAuth2UserService.

Authenticating with Google

Using our own custom OAuth2UserService, I discovered that authenticating with Google didn’t work. Or rather, the custom OAuth2UserService wasn’t executed while it was executed when using GitHub or Facebook. It turned out that when you don’t specify which scope you’re interested in, Google returned all scopes. Included in the list was “openid”, which is specifically filtered out by Spring Security. So, if you want to use your own OAuth2UserService with Google, you need to configure it with the scopes you need. Like this:

spring:
  security:
    oauth2:
      client:
        registration:
          google:
            clientId: google-client-ID
            clientSecret: google-client-secret
            scope:
              - email
              - profile

@CurrentSecurityContext

To obtain the authenticated user principal, the tutorial uses the annotation @AuthenticationPrincipal, like this:

    @GetMapping("/user")
    public Map<String, Object> user(@AuthenticationPrincipal OAuth2User principal) {
        return Collections.singletonMap("name", principal.getAttribute("name"));
    }

Usually you’re only interested in the user. However, if you need more information, you can use @SecurityContext. This article provides more information.
Here are two examples of how to use @CurrentSecurityContext with Thymeleaf:

    @RequestMapping(value = {"/","/index"})
    public String index(@CurrentSecurityContext(expression = "authentication") Authentication authentication) {
        if (authentication.isAuthenticated()) {
            return "redirect:/welcome";
        }
        return "redirect:/login";
    }
    @RequestMapping(value = "/welcome")
    public String welcome(@CurrentSecurityContext(expression = "authentication.principal") OAuth2User user, Model model) {
            model.addAttribute("name", user.getAttribute("name"));
            return "welcome";
    }

Custom Access Denied Page

The Spring Boot tutorial throws an unauthorized exception when the user tries to access a resource that he’s not allowed to. I wanted the website to redirect to the login page, assuming the user wasn’t authenticated. Or, if he was, then he should be redirected to the welcome page. We can configure this in the configure(HttpSecurity http) method:

			.exceptionHandling(e -> e
					.accessDeniedPage("/")
			)

This redirection ends up in the index() method of the previous section. Let’s look at that method again:

    @RequestMapping(value = {"/","/index"})
    public String index(@CurrentSecurityContext(expression = "authentication") Authentication authentication) {
        if (authentication.isAuthenticated()) {
            return "redirect:/welcome";
        }
        return "redirect:/login";
    }

When the user is authenticated, the user is redirected to the welcome page. Otherwise, the user is redirected to the login page.

Conclusion

While the Spring tutorial is quite good, there’s a lot more to OAuth2 authentication than it covers. This article covers some subjects that were beyond the scope of Spring’s tutorial. We’ve seen how to add your own processing of the authenticated user. Next we discussed some quirks when authenticating with Google. Then we’ve seen an alternative and more flexible way to get access to the user details. And last we redirected the access denied page to either the login page or the welcome page, using Thymeleaf.

Linux: Spring Boot as a service on port 80

Well, that’s a mouth full. This blog shows how to run your Spring Boot application on port 80 when your linux server starts. We are going to use Ubuntu or Linux Mint for this, and we’re going to assume that Java is installed.

Setup Spring Boot

The first thing we need to do is tell Maven to make our Spring Boot application runnable. We’ll configure the spring-boot-maven-plugin to make the jar that’s going to be built executable.

	<build>
		<plugins>
			<plugin>
				<groupId>org.springframework.boot</groupId>
				<artifactId>spring-boot-maven-plugin</artifactId>
				<configuration>
					<executable>
						true
					</executable>
				</configuration>
			</plugin>
		</plugins>
	</build>

By default Spring Boot runs on port 8080. There’s no need to change that here, we’ll deal with that later. But if you want, you could start the application on a different port by configuring it in application.properties

server.port=8079

Run Application as a Service

Set permissions

The first thing we need to do is to copy the application to its final destination, and create a user to run the app. By creating a separate user for this application, we limit the damage that can be done in case there is a security breach. I’m going to call the application “website” and I’m going to place it under /var/website, to make things easy. Once the application is in its place, change directory to /var/website, create a new user and transfer ownership to the new user. We’re going to allow the newly created user to read and execute the file, but not to write to the file. This, again, limits the damage that can be done when there’s a security breach.

sudo useradd website
sudo passwd website
sudo chown website:website website.jar
sudo chmod 500 website.jar

Setup systemd service

To manage starting and stopping our application, we’re going to turn it into a service maintained by systemd. We need to tell systemd how to run our service by creating a configuration file.

create /etc/systemd/system/website.service

[Unit]
Description=My spring-boot application

Wants=network.target
After=syslog.target

[Service]
User=website
Type=simple
ExecStart=/var/website/website.jar
Restart=on-failure
RestartSec=60
KillMode=mixed

[Install]
WantedBy=multi-user.target

This is what the options mean:
Description: A text description of what the service is for.
Wants: The services that should, but are not required to, be started before this service.
After: The services that should be started (if they are not already running) after this service has been started.
User: The user that should be used to start this service.
Type: This indicates the way the service is started. We’ll set it to simple.
ExecStart: The command with parameters that needs to be executed to start this service.
Restart: When the service should be restarted.
RestartSec: If the service needs to be restarted, how many seconds systemd should wait.
KillMode: How the process will be stopped by systemd. We’ll set it to mixed, which sends a SIGTERM signal to our service which allows the service to shut down itself.
WantedBy: We want our service to be started when the system is up and running, and accepting connections from users.

For a list of options, look here.
KillMode is described here.
For more info on WantedBy, see this.

For the security reasons mentioned above, we’ll change the permissions of this file:

sudo chmod 640 /etc/systemd/system/website.service

Enable service to start on boot

The next thing we want to do is to indicate that this service should be started when the server is starting.

sudo systemctl enable website

Setup nginx

Now we have our application installed as a service, and it’s started when the server starts up. But the application is running on port 8079, which is not what we want. Linux only allows the root user to open TCP ports below 1000. To fix this, we’re going to use nginx to forward port 80 to our application.

create /etc/nginx/sites-enabled/website.conf with the following content:

server {
        listen  80;
        server_name     _;
        location /  {
            proxy_pass          http://localhost:8079/;
            proxy_redirect      off;

            proxy_set_header   Host             $host;
            proxy_set_header   X-Real-IP        $remote_addr;
            proxy_set_header   X-Forwarded-For  $proxy_add_x_forwarded_for;
        }
}

This tells nginx to listen to port 80, for incoming connections on any hostname, and forward it to our application.

Spring Boot, MongoDB and raw JSON

Sometimes you want to store and retrieve raw JSON in MongoDB. With Spring Boot storing the JSON isn’t very hard, but retrieving can be a bit more challenging.

Setting up

To start using MongoDB from Spring Boot, you add the dependency to spring-boot-starter-data-mongodb

	<dependency>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-data-mongodb</artifactId>
	</dependency>

And then you inject MongoTemplate into your class

@Autowired
private MongoTemplate mongoTemplate;

Inserting into MongoDB

Inserting JSON is just a matter of converting the JSON into a Document, and inserting that document into the right collection

String json = getJson();
Document doc = Document.parse(json);
mongoTemplate.insert(doc, "CollectionName");

Retrieving JSON

Retrieving JSON is a bit more complicated. First you need to get a cursor for the collection. This allows you to iterate over all the documents within that collection. Then you’ll retrieve each document from the collection, and cast it to a BasicDBObject. Once you have that, you can retrieve the raw JSON.

DBCursor cursor = mongoTemplate.getCollection("CollectionName").find();
Iterator iterator = cursor.iterator();
while (iterator.hasNext()){
   BasicDBObject next = (BasicDBObject) iterator.next();
   String json = next.toJson();
   // do stuff with json
}

Transforming raw JSON to Object

With Jackson you can transform the retrieved JSON to an object. However, your object might miss a few fields, since MongoDB adds some to keep track of the stored documents. To get around this problem, you need to configure the ObjectMapper to ignore those extra fields.

ObjectMapper mapper = new ObjectMapper().configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)
MyObject object = mapper.readValue(json, MyObject.class);