在Spring Boot中使用FreeMarker生成文件的步骤如下：

添加FreeMarker依赖：在pom.xml文件中添加FreeMarker的依赖：

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

创建FreeMarker配置类：新建一个配置类用于配置FreeMarker的模板路径和其他配置：

@Configurationpublic class FreeMarkerConfig {@Autowiredprivate Environment env;@Beanpublic FreeMarkerConfigurationFactoryBean freeMarkerConfiguration() {FreeMarkerConfigurationFactoryBean bean = new FreeMarkerConfigurationFactoryBean();bean.setTemplateLoaderPath("classpath:/templates");Properties settings = new Properties();settings.setProperty("template_update_delay", "0");settings.setProperty("default_encoding", "UTF-8");settings.setProperty("locale", "zh_CN");settings.setProperty("number_format", "0.##########");settings.setProperty("datetime_format", "yyyy-MM-dd HH:mm:ss");settings.setProperty("date_format", "yyyy-MM-dd");settings.setProperty("boolean_format", "true,false");settings.setProperty("time_format", "HH:mm:ss");settings.setProperty("whitespace_stripping", "true");settings.setProperty("auto_import", "ftl/common/common.ftl as common,ftl/common/functions.ftl as fun");settings.setProperty("url_escaping_charset", "UTF-8");settings.setProperty("output_format", "HTMLOutputFormat");settings.setProperty("tag_syntax", "auto_detect");settings.setProperty("template_exception_handler", "rethrow");settings.setProperty("classic_compatible", "true");bean.setFreemarkerSettings(settings);return bean;}@Beanpublic FreeMarkerConfigurer freeMarkerConfigurer() throws IOException {FreeMarkerConfigurer configurer = new FreeMarkerConfigurer();configurer.setConfiguration(freeMarkerConfiguration().getObject());return configurer;}@Beanpublic FreeMarkerViewResolver freeMarkerViewResolver() {FreeMarkerViewResolver resolver = new FreeMarkerViewResolver();resolver.setCache(false);resolver.setSuffix(".ftl");resolver.setContentType("text/html; charset=UTF-8");resolver.setRequestContextAttribute("request");resolver.setExposeSpringMacroHelpers(true);return resolver;}}

创建模板文件：在src/main/resources/templates目录下创建FreeMarker模板文件（例如example.ftl）。

创建Controller：创建一个Controller类用于接收请求，并在请求处理方法中使用FreeMarker生成文件，例如：

@Controllerpublic class FileController {@Autowiredprivate Configuration configuration;@GetMapping("/generate")public ResponseEntity<Resource> generateFile() throws Exception {// 获取模板Template template = configuration.getTemplate("example.ftl");// 创建数据模型Map<String, Object> data = new HashMap<>();data.put("name", "John Doe");data.put("age", 30);// 创建输出流StringWriter writer = new StringWriter();template.process(data, writer);// 生成文件byte[] bytes = writer.toString().getBytes();ByteArrayResource resource = new ByteArrayResource(bytes);return ResponseEntity.ok().contentLength(bytes.length).contentType(MediaType.APPLICATION_OCTET_STREAM).body(resource);}}

在上述代码中，使用Configuration类获取模板对象，并将数据模型填充到模板中，最后使用ByteArrayResource返回生成的文件。

访问生成文件：启动Spring Boot应用，访问http://localhost:8080/generate即可生成文件并下载。

通过以上步骤，您可以在Spring Boot中使用FreeMarker生成文件。