文档 AWS SDK 示例 GitHub 存储库中还有更多 [S AWS DK 示例](https://github.com/awsdocs/aws-doc-sdk-examples)。

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# 使用 AWS 软件开发工具包从 Amazon S3 存储桶中获取对象，指定 If-Modified-Since标头
<a name="s3_example_s3_GetObject_IfModifiedSince_section"></a>

下面的代码示例显示如何从 S3 存储桶中的对象读取数据，但前提是该桶自上次检索以来未被修改。

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#### [ Rust ]

**适用于 Rust 的 SDK**  
 还有更多相关信息 GitHub。在 [AWS 代码示例存储库](https://github.com/awsdocs/aws-doc-sdk-examples/tree/main/rustv1/examples/s3#code-examples)中查找完整示例，了解如何进行设置和运行。

```
use aws_sdk_s3::{
    error::SdkError,
    primitives::{ByteStream, DateTime, DateTimeFormat},
    Client,
};
use s3_code_examples::error::S3ExampleError;
use tracing::{error, warn};

const KEY: &str = "key";
const BODY: &str = "Hello, world!";

/// Demonstrate how `if-modified-since` reports that matching objects haven't
/// changed.
///
/// # Steps
/// - Create a bucket.
/// - Put an object in the bucket.
/// - Get the bucket headers.
/// - Get the bucket headers again but only if modified.
/// - Delete the bucket.
#[tokio::main]
async fn main() -> Result<(), S3ExampleError> {
    tracing_subscriber::fmt::init();

    // Get a new UUID to use when creating a unique bucket name.
    let uuid = uuid::Uuid::new_v4();

    // Load the AWS configuration from the environment.
    let client = Client::new(&aws_config::load_from_env().await);

    // Generate a unique bucket name using the previously generated UUID.
    // Then create a new bucket with that name.
    let bucket_name = format!("if-modified-since-{uuid}");
    client
        .create_bucket()
        .bucket(bucket_name.clone())
        .send()
        .await?;

    // Create a new object in the bucket whose name is `KEY` and whose
    // contents are `BODY`.
    let put_object_output = client
        .put_object()
        .bucket(bucket_name.as_str())
        .key(KEY)
        .body(ByteStream::from_static(BODY.as_bytes()))
        .send()
        .await;

    // If the `PutObject` succeeded, get the eTag string from it. Otherwise,
    // report an error and return an empty string.
    let e_tag_1 = match put_object_output {
        Ok(put_object) => put_object.e_tag.unwrap(),
        Err(err) => {
            error!("{err:?}");
            String::new()
        }
    };

    // Request the object's headers.
    let head_object_output = client
        .head_object()
        .bucket(bucket_name.as_str())
        .key(KEY)
        .send()
        .await;

    // If the `HeadObject` request succeeded, create a tuple containing the
    // values of the headers `last-modified` and `etag`. If the request
    // failed, return the error in a tuple instead.
    let (last_modified, e_tag_2) = match head_object_output {
        Ok(head_object) => (
            Ok(head_object.last_modified().cloned().unwrap()),
            head_object.e_tag.unwrap(),
        ),
        Err(err) => (Err(err), String::new()),
    };

    warn!("last modified: {last_modified:?}");
    assert_eq!(
        e_tag_1, e_tag_2,
        "PutObject and first GetObject had differing eTags"
    );

    println!("First value of last_modified: {last_modified:?}");
    println!("First tag: {}\n", e_tag_1);

    // Send a second `HeadObject` request. This time, the `if_modified_since`
    // option is specified, giving the `last_modified` value returned by the
    // first call to `HeadObject`.
    //
    // Since the object hasn't been changed, and there are no other objects in
    // the bucket, there should be no matching objects.

    let head_object_output = client
        .head_object()
        .bucket(bucket_name.as_str())
        .key(KEY)
        .if_modified_since(last_modified.unwrap())
        .send()
        .await;

    // If the `HeadObject` request succeeded, the result is a typle containing
    // the `last_modified` and `e_tag_1` properties. This is _not_ the expected
    // result.
    //
    // The _expected_ result of the second call to `HeadObject` is an
    // `SdkError::ServiceError` containing the HTTP error response. If that's
    // the case and the HTTP status is 304 (not modified), the output is a
    // tuple containing the values of the HTTP `last-modified` and `etag`
    // headers.
    //
    // If any other HTTP error occurred, the error is returned as an
    // `SdkError::ServiceError`.

    let (last_modified, e_tag_2) = match head_object_output {
        Ok(head_object) => (
            Ok(head_object.last_modified().cloned().unwrap()),
            head_object.e_tag.unwrap(),
        ),
        Err(err) => match err {
            SdkError::ServiceError(err) => {
                // Get the raw HTTP response. If its status is 304, the
                // object has not changed. This is the expected code path.
                let http = err.raw();
                match http.status().as_u16() {
                    // If the HTTP status is 304: Not Modified, return a
                    // tuple containing the values of the HTTP
                    // `last-modified` and `etag` headers.
                    304 => (
                        Ok(DateTime::from_str(
                            http.headers().get("last-modified").unwrap(),
                            DateTimeFormat::HttpDate,
                        )
                        .unwrap()),
                        http.headers().get("etag").map(|t| t.into()).unwrap(),
                    ),
                    // Any other HTTP status code is returned as an
                    // `SdkError::ServiceError`.
                    _ => (Err(SdkError::ServiceError(err)), String::new()),
                }
            }
            // Any other kind of error is returned in a tuple containing the
            // error and an empty string.
            _ => (Err(err), String::new()),
        },
    };

    warn!("last modified: {last_modified:?}");
    assert_eq!(
        e_tag_1, e_tag_2,
        "PutObject and second HeadObject had different eTags"
    );

    println!("Second value of last modified: {last_modified:?}");
    println!("Second tag: {}", e_tag_2);

    // Clean up by deleting the object and the bucket.
    client
        .delete_object()
        .bucket(bucket_name.as_str())
        .key(KEY)
        .send()
        .await?;

    client
        .delete_bucket()
        .bucket(bucket_name.as_str())
        .send()
        .await?;

    Ok(())
}
```
+  有关 API 的详细信息，请参阅适用[GetObject](https://docs.rs/aws-sdk-s3/latest/aws_sdk_s3/client/struct.Client.html#method.get_object)于 *Rust 的AWS SDK API 参考*。

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