ETag使用

关于ETag相关知识：

https://blog.csdn.net/kikikind/article/details/6266101 

https://blog.csdn.net/jkxiaoxing/article/details/51499894

具体代码：

String clientEtag = request.getHeader("If-None-Match");
        if (md5.equals(clientEtag)) {
            response.setHeader("If-None-Match", "false");
            response.setStatus(304);
            return this.buildResult(RequestCode.success,"图片下载成功");
        }
        String contextType = "image/" + contentType;
        response.setHeader("content-type", contextType);
        response.setContentType(contextType);
        response.setHeader("ETag", md5);
        response.setHeader("If-None-Match", "true");
        OutputStream outputStream = response.getOutputStream();
        this.readStream(outputStream, inputStream);

md5为文件摘要，第一次加载图片时，将md5放入header的ETag中，并将header的If-None-Match设置为true

浏览器会将当前资源的ETag保存，并于下次请求时，加入If-None-Match中。请求接口时，判断If-None-Match的值是否和md5相同，如果相同则代表已经加载过，且未曾变更资源，将If-None-Match设置为false且将status设置为304即可。