How can I read an HTML template from a file to send a mail with it using the JavaMail API?

I've got an assigment to send HTML mail with the JavaMail API. Here is a small part of my code:

MimeMessage message = sender.createMimeMessage();
MimeMessageHelper helper = new MimeMessageHelper(message);

try {
    helper.setTo(recipients);
    helper.setSubject("Simple mail template");
    helper.setText("<html><body>Hi There</body><html>",html:true);
} catch (MessagingException e) {
        e.printStackTrace();
}

Now I've got an assigment to move the HTML in a separate file, and create a class to read that HTML template and send a mail with it. Any suggestions on how to do that?

Solution

Use a template engine

I've created a minimal example with Thymeleaf as a template engine. You first wrote that you are using Spring Boot in your project, so I assume you can use it. I also assume you are using either Maven or Gradle as a build tool.

Adding the Thymeleaf dependency

Add the spring-boot-starter-thymeleaf dependency to your project. Are you using Maven or Gradle?

Maven

Your pom.xml dependencies should include:

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

Gradle

Your build.gradle dependencies should include:

compile('org.springframework.boot:spring-boot-starter-thymeleaf')

Configuring Thymeleaf in Spring Boot

Add the required @Beans. Those are:

@Bean
public ITemplateResolver templateResolver()
{
    ClassLoaderTemplateResolver templateResolver = new ClassLoaderTemplateResolver();
    templateResolver.setPrefix("templates/");
    templateResolver.setSuffix(".html");
    templateResolver.setTemplateMode(TemplateMode.HTML);

    return templateResolver;
}

@Bean
public TemplateEngine templateEngine()
{
    TemplateEngine templateEngine = new TemplateEngine();
    templateEngine.setTemplateResolver(this.templateResolver());

    return templateEngine;
}

They can go into any class annotated with @Configuration (or @SpringBootApplication).

Examples

Now, you can access your TemplateEngine from any class whose fields are injected by Spring.

@Component
public class SomeClass
{
    @Autowired
    private TemplateEngine templateEngine;

    public String generateMailHtml(String text)
    {
        Map<String, Object> variables = new HashMap<>();
        variables.put("mailtext", text);

        final String templateFileName = "mail"; //Name of the template file without extension
        String output = this.templateEngine.process(templateFileName, new Context(Locale.getDefault(), variables));

        return output;
    }
}

The mail.html should be located in the classpath (resources/) under templates/.

And it should look like this:

<html>
    <body data-th-text="${mailtext}"></body>
</html>

Your code snippet you posted could now look like this (@Autowired SomeClass as someClass in the class outside of the method):

MimeMessage message = sender.createMimeMessage();
MimeMessageHelper helper = new MimeMessageHelper(message);

try {
    helper.setTo(recipients);
    helper.setSubject("Simple mail template");
    helper.setText(someClass.generateMailHtml("Hi There"), true);
} catch (MessagingException e) {
    e.printStackTrace();
}

Of course, change the examples according to your needs!

Edit

You mentioned you need to fill the template "with a list of some names". This would be achieved like that:

public String generateMailHtml(List<String> names)
{
    Map<String, Object> variables = new HashMap<>();
    variables.put("names", names);

    final String templateFileName = "mail"; //Name of the template file without extension
    String output = this.templateEngine.process(templateFileName, new Context(Locale.getDefault(), variables));

    return output;
}

mail.html

<html>
    <body>
        <ul>
            <li data-th-each="name : ${names}" data-th-text="${name}"></li>
        </ul>
    </body>
</html>

Read more about data-th-each / th:each here. Note: You can use data-th- and th: interchangably, though data-th- is more HTML5-friendly.