java 将本地文件转为外部能访问的的url

要将本地文件转为外部能访问的 URL，需要将该文件上传至一个能够提供文件访问服务的云存储平台，例如Amazon S3、Azure Blob Storage或者Google Cloud Storage等。以下是使用Amazon S3作为云存储平台的Java代码示例：

import com.amazonaws.AmazonClientException;
import com.amazonaws.AmazonServiceException;
import com.amazonaws.auth.profile.ProfileCredentialsProvider;
import com.amazonaws.services.s3.AmazonS3;
import com.amazonaws.services.s3.AmazonS3Client;
import com.amazonaws.services.s3.model.PutObjectRequest;

import java.io.File;

public class UploadToS3 {

    private static String bucketName = "your-bucket-name";
    private static String keyName = "your-file-name";
    private static String uploadFileName = "path/to/your/local/file";

    public static void main(String[] args) throws Exception {
        AmazonS3 s3client = new AmazonS3Client(new ProfileCredentialsProvider());

        try {
            System.out.println("Uploading a new object to S3 from a file\n");
            File file = new File(uploadFileName);
            s3client.putObject(new PutObjectRequest(
                    bucketName, keyName, file));

            String publicUrl = s3client.getUrl(bucketName, keyName).toString();
            System.out.println("Public URL of the uploaded file: " + publicUrl);

        } catch (AmazonServiceException ase) {
            System.out.println("Caught an AmazonServiceException, which means your request made it "
                    + "to Amazon S3, but was rejected with an error response for some reason.");
            System.out.println("Error Message:    " + ase.getMessage());
            System.out.println("HTTP Status Code: " + ase.getStatusCode());
            System.out.println("AWS Error Code:   " + ase.getErrorCode());
            System.out.println("Error Type:       " + ase.getErrorType());
            System.out.println("Request ID:       " + ase.getRequestId());
        } catch (AmazonClientException ace) {
            System.out.println("Caught an AmazonClientException, which means the client encountered "
                    + "a serious internal problem while trying to communicate with S3, "
                    + "such as not being able to access the network.");
            System.out.println("Error Message: " + ace.getMessage());
        }
    }
}

这段代码会将本地文件上传至Amazon S3，并返回该文件的公共访问URL。你可以将该URL分享给外部用户，让其访问该文件。请注意，为了使用Amazon S3服务，你需要在AWS Console中创建一个S3 Bucket，并且配置相应的访问权限。