如何在Java SDK2.0中构建用于文件上传的亚马逊S3预签名PostPolicy？

import software.amazon.awssdk.auth.credentials.DefaultCredentialsProvider;
import software.amazon.awssdk.regions.Region;
import software.amazon.awssdk.services.s3.S3Client;
import software.amazon.awssdk.services.s3.model.GeneratePresignedUrlRequest;
import software.amazon.awssdk.services.s3.model.PostPolicy;
import software.amazon.awssdk.services.s3.model.S3Exception;

import java.net.URL;
import java.time.Duration;
import java.time.Instant;

public class S3PostPolicyExample {
    public static void main(String[] args) {
        // 创建S3客户端对象
        S3Client s3Client = S3Client.builder()
                .region(Region.US_EAST_1)
                .credentialsProvider(DefaultCredentialsProvider.create())
                .build();

        // 设置过期时间
        Instant expirationTime = Instant.now().plus(Duration.ofMinutes(15));

        // 创建PostPolicy对象
        PostPolicy postPolicy = PostPolicy.builder()
                .expiration(expirationTime)
                .conditions(
                        Arrays.asList(
                                Arrays.asList("starts-with", "$key", ""),
                                Arrays.asList("starts-with", "$Content-Type", ""),
                                Arrays.asList("content-length-range", "0", "1048576000")
                        )
                )
                .build();

        try {
            // 生成预签名URL
            GeneratePresignedUrlRequest request = GeneratePresignedUrlRequest.builder()
                    .bucket("your-bucket-name")
                    .key("your-file-key")
                    .expiration(expirationTime)
                    .fields(postPolicy.getFormFields())
                    .build();

            URL presignedUrl = s3Client.generatePresignedUrl(request);

            System.out.println("预签名URL: " + presignedUrl);
        } catch (S3Exception e) {
            e.printStackTrace();
        }
    }
}