springboot thymeleaf不渲染页面返回字符串

错误信息：

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This application has no explicit mapping for /error, so you are seeing this as a fallback. Sat Mar 30 01:59:07 CST 2019 There was an unexpected error (type=Internal Server Error, status=500). Error resolving template [index], template might not exist or might not be accessible by any of the configured Template Resolvers org.thymeleaf.exceptions.TemplateInputException: Error resolving template [index], template might not exist or might not be accessible by any of the configured Template Resolvers

@RestController 与 @Controller