Accessing an S3 Bucket in Java Spring Boot Using a Cognito User Token

You'd need to use the Cognito Identity Pool. An Amazon Cognito identity pool is a directory of federated identities that you can exchange for AWS credentials. Amazon Cognito identities are exchanged for credentials using web identity federation support in the AWS Security Token Service (AWS STS). The recommended way to obtain AWS credentials for your app users is to use AWS.CognitoIdentityCredentials. The identity in the credentials object is then exchanged for credentials using AWS STS.

See the blog post series: https://aws.amazon.com/blogs/mobile/understanding-amazon-cognito-authentication-part-3-roles-and-policies/