javaMail直接发送附件，无须生成临时文件

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以excel附件为例：

先引入依赖：

		<dependency>
			<groupId>org.apache.poi</groupId>
			<artifactId>poi</artifactId>
			<version>3.17</version>
		</dependency>

邮件发送服务：

@Service
public class MailService {
	private final Logger logger = LoggerFactory.getLogger(this.getClass());
	
	private final JavaMailSender sender;
	
	@Value("${spring.mail.username}")
	private String from;

	public MailService(JavaMailSender sender) {
		this.sender = sender;
	}

	/**
	 * 利用helper设置各种邮件发送相关的信息
	 */
	private MimeMessageHelper setInfoByHelper(String to, String subject, String content, MimeMessage message)
			throws MessagingException {
		//true表示需要创建一个multipart message
		MimeMessageHelper helper = new MimeMessageHelper(message, true);
		helper.setFrom(from);
		helper.setTo(to);
		helper.setSubject(subject);
		helper.setText(content, true);
		return helper;
	}
	

	/**
	 * 发送带附件的邮件
	 * @param to 收件人列表
	 * @param subject 邮件标题
	 * @param content 邮件内容
	 * @param inputStreamSource 附件streamSource，可以这样获得：new ByteArrayResource(ByteArrayOutputStream.toByteArray());
	 * @param fileName 附件的文件名
	 */
	public void sendAttachmentsMail(String to, String subject, String content, InputStreamSource inputStreamSource, String fileName){
		MimeMessage message = sender.createMimeMessage();

		try {
			MimeMessageHelper helper = setInfoByHelper(to, subject, content, message);

			helper.addAttachment(fileName, inputStreamSource);

			sender.send(message);
			logger.debug("带附件的邮件已经发送。");
		} catch (MessagingException e) {
			logger.error("发送带附件的邮件时发生异常！", e);
		}
	}
}

方法调用（测试用例）：

利用ByteArrayOutputStream把excel文件输出到bytes[]中，然后由ByteArrayResource包装起来传递给邮件服务。

	@Test
	public void sendMailWithExcel() throws IOException {
		String[] headers = {"col1","col2","col3"};
		// 声明一个工作薄
		HSSFWorkbook wb = new HSSFWorkbook();
		// 生成一个表格
		HSSFSheet sheet = wb.createSheet();
		HSSFRow row = sheet.createRow(0);
		for (int i = 0; i < headers.length; i++) {
			HSSFCell cell = row.createCell(i);
			cell.setCellValue(headers[i]);
		}
		int rowIndex = 1;

		for(int j=0; j<3; j++){
			row = sheet.createRow(rowIndex);
			rowIndex++;
			HSSFCell cell1 = row.createCell(0);
			cell1.setCellValue(j);
			cell1 = row.createCell(1);
			cell1.setCellValue(j+1);
			cell1 = row.createCell(2);
			cell1.setCellValue(j+2);
		}
		for (int i = 0; i < headers.length; i++) {
			sheet.autoSizeColumn(i);
		}

		ByteArrayOutputStream os = new ByteArrayOutputStream(1000);
		wb.write(os);
		wb.close();

		InputStreamSource iss = new ByteArrayResource(os.toByteArray());
		os.close();

		mailService.sendAttachmentsMail("[email protected]",
				"attachmentMail subject",
				"I have an attachment",
				iss, "abc1.xlsx");

	}
}