項目中經常會有這樣的需求，用戶註冊成功，需要給用戶發送一封郵件。郵件需要有一定格式和樣式。本次例子中用freemarker做樣式，其他的模版引擎類似。

首先Spring Boot項目，項目結構如下

在pom.xml文件中添加依賴

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

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

配置發件人信息，發件人郵箱需要開通POP3/SMTP服務，如下圖（我是采用126郵箱）：

然後在application.properties中添加配置文件：

然後在service層中添加MailService類，代碼如下

import freemarker.core.ParseException;
import freemarker.template.MalformedTemplateNameException;
import freemarker.template.Template;
import freemarker.template.TemplateException;
import freemarker.template.TemplateNotFoundException;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import
 
 org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.mail.SimpleMailMessage;
import org.springframework.mail.javamail.JavaMailSender;
import org.springframework.mail.javamail.MimeMessageHelper;
import org.springframework.stereotype.Service;
import org.springframework.ui.freemarker.FreeMarkerTemplateUtils;
import org.springframework.util.ResourceUtils;
import org.springframework.web.servlet.view.freemarker.FreeMarkerConfig;

import javax.mail.MessagingException;
import javax.mail.internet.MimeMessage;
import java.io.File;
import java.io.IOException;
import java.util.HashMap;
import java.util.Map;


@Service
public class MailService {

    private final static Logger logger = LoggerFactory.getLogger(MailService.class);

    @Value("${spring.mail.username}")
    private String from;

    @Autowired
    private JavaMailSender mailSender;

    @Autowired
    private FreeMarkerConfig freeMarkerConfig;

    // send simple email
    public String sendSimple(String to, String title, String content) {
        SimpleMailMessage message = new SimpleMailMessage();

        message.setFrom(from);
        message.setTo(to);
        message.setSubject(title);
        message.setText(content);

        mailSender.send(message);

        logger.info("{} send email to {}", from, to);
        return "SUCESS";
    }

    // send template mail
    public String sendTemplateMail(String to, String title) {
        MimeMessage mimeMessage = mailSender.createMimeMessage();
        MimeMessageHelper helper;
        try {
            helper = new MimeMessageHelper(mimeMessage, true);

            helper.setFrom(from);
            helper.setTo(to);
            helper.setSubject(title);

            Map<String, Object> model = new HashMap<>();
            model.put("params", from);
            Template template = freeMarkerConfig.getConfiguration().getTemplate("message.ftl");
            String text = FreeMarkerTemplateUtils.processTemplateIntoString(template, model);

            helper.setText(text, true);

            mailSender.send(mimeMessage);

        } catch (MessagingException e) {
            e.printStackTrace();
        } catch (MalformedTemplateNameException e) {
            e.printStackTrace();
        } catch (ParseException e) {
            e.printStackTrace();
        } catch (TemplateNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (TemplateException e) {
            e.printStackTrace();
        }


        return "SUCEESS";
    }

    // send template mail with attachment
    public String sendAttactmentMail(String to, String title){
        MimeMessage mimeMessage = mailSender.createMimeMessage();
        MimeMessageHelper helper;
        try {
            helper = new MimeMessageHelper(mimeMessage, true);

            helper.setFrom(from);
            helper.setTo(to);
            helper.setSubject(title);

            // add template
            Map<String, Object> model = new HashMap<>();
            model.put("params", from);
            Template template = freeMarkerConfig.getConfiguration().getTemplate("message.ftl");
            String text = FreeMarkerTemplateUtils.processTemplateIntoString(template, model);
            helper.setText(text, true);

            // add attachment
            File file = ResourceUtils.getFile("classpath:static/flcl.jpg");
            helper.addAttachment(file.getName(), file);

            mailSender.send(mimeMessage);

        } catch (MessagingException e) {
            e.printStackTrace();
        } catch (MalformedTemplateNameException e) {
            e.printStackTrace();
        } catch (ParseException e) {
            e.printStackTrace();
        } catch (TemplateNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (TemplateException e) {
            e.printStackTrace();
        }
        return "SUCCESS";
    }
}

這個service中寫了三個方法，三個方法的作用分別對應於：發送簡單郵件、發送模版郵件、發送帶附件的模版郵件

核心方法是 mailSender.send(message) ，message對象根據發送的郵件類型不同而不同，主要有MimeMessage MimeMessageHelper

發送帶附件的模板郵件方式是三者中最復雜，首先創建郵件對象MimeMessage ，然後通過MimeMessage對象創建MimeMessageHelper（此對象的作用：向MimeMessage對象中填充數據的助手），通過向MimeMessageHelper對象中添加一些屬性，例如發件人、收件人、郵件主題、模版、附件。其中freemarker模板文件xxx.ftl放在resources文件夾下中template（通過FreemakrerConfig獲取xxx.ftl的Template對象，然後轉換為String），其中附件flcl.jpg放在resources文件夾中的static中（通過ResourceUtils獲取文件對象File）。屬性添加完成後，就可以發送了。

然後在controller或者其他service層中調用該方法就可以了。

補充：

message.ftl文件內容如下

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Hello</title>
</head>
<body>
<h1>Send email</h1>
<div>this email come from ${params}</div>
</body>
</html> 

發送成功後，可在收件箱中收到郵件

Spring Boot學習筆記：JavaMailSender發送郵件