四步解决！OSS对象存储文件上传功能（服务端签名后直传，建议收藏）

<dependency>
    <groupId>com.alibaba.cloud</groupId>
    <artifactId>aliyun-oss-spring-boot-starter</artifactId>
</dependency>

alibaba:
  cloud:
    access-key: LTAI5tGDG4tqfdsfsfW2xQF             #账号 AccessKey
    secret-key: du12BXlruM8gfgfdfgpvxTekRxQSjw      #密码 secretKey
    oss:
      endpoint: oss-cn-beijing.aliyuncs.com         #给哪块进行上传 endpoint
      bucket: md-ossbucket                #自定义属性，bucket名称

package com.oss.controller;
import com.aliyun.oss.OSS;
import com.aliyun.oss.common.utils.BinaryUtil;
import com.aliyun.oss.model.MatchMode;
import com.aliyun.oss.model.PolicyConditions;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.web.bind.annotation.CrossOrigin;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.LinkedHashMap;
import java.util.Map;
@RestController
public class OssController {
  @Autowired
  OSS ossClient;
  @Value("${alibaba.cloud.oss.endpoint}")
  private String endpoint;    //从yml文件中读取
  @Value("${alibaba.cloud.oss.bucket}")
  private String bucket;      //从yml文件中读取
  @Value("${alibaba.cloud.access-key}")
  private String accessId;  //从yml文件中读取
  @GetMapping("/oss/policy")
  @CrossOrigin
  public Map<String, String> policy(){
    //https://md-ossbucket.oss-cn-beijing.aliyuncs.com/QQ%E6%88%AA%E5%9B%BE20210609114525.png  host的格式为 bucketname.endpoint
    String host = "https://" + bucket + "." + endpoint;
    String format = new SimpleDateFormat("yyyy-MM-dd").format(new Date()); //格式化一个当前的服务器时间
    String dir = format+"/"; // 用户上传文件时指定的前缀,我们希望以日期作为一个目录
    Map<String, String> respMap =null; //返回结果
    try {
      long expireTime = 30;
      long expireEndTime = System.currentTimeMillis() + expireTime * 1000;
      Date expiration = new Date(expireEndTime);
      // PostObject请求最大可支持的文件大小为5 GB，即CONTENT_LENGTH_RANGE为5*1024*1024*1024。
      PolicyConditions policyConds = new PolicyConditions();
      policyConds.addConditionItem(PolicyConditions.COND_CONTENT_LENGTH_RANGE, 0, 1048576000);
      policyConds.addConditionItem(MatchMode.StartWith, PolicyConditions.COND_KEY, dir);
      String postPolicy = ossClient.generatePostPolicy(expiration, policyConds);
      byte[] binaryData = postPolicy.getBytes("utf-8");
      String encodedPolicy = BinaryUtil.toBase64String(binaryData);
      String postSignature = ossClient.calculatePostSignature(postPolicy);
      respMap = new LinkedHashMap<String, String>();
      respMap.put("accessid", accessId);
      respMap.put("policy", encodedPolicy);
      respMap.put("signature", postSignature);
      respMap.put("dir", dir);
      respMap.put("host", host);
      respMap.put("expire", String.valueOf(expireEndTime / 1000));
      // respMap.put("expire", formatISO8601Date(expiration));
    } catch (Exception e) {
      // Assert.fail(e.getMessage());
      System.out.println(e.getMessage());
    } finally {
      ossClient.shutdown();
    }
    return respMap;
  }
}