# 检查对象是否存在
<a name="s3_example_s3_Scenario_DoesObjectExist_section"></a>

以下代码示例演示如何检查对象是否存在。

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#### [ Java ]

**适用于 Java 的 SDK 2.x**  
 查看 GitHub，了解更多信息。在 [Amazon 代码示例存储库](https://github.com/awsdocs/aws-doc-sdk-examples/tree/main/javav2/example_code/s3#code-examples)中查找完整示例，了解如何进行设置和运行。
可以使用以下 `doesObjectExist` 方法来替代适用于 Java 的 SDK V1 [AmazonS3Client\#doesObjectExist(String, String)](https://docs.amazonaws.cn/AWSJavaSDK/latest/javadoc/com/amazonaws/services/s3/AmazonS3Client.html#doesObjectExist-java.lang.String-java.lang.String-) 方法。  

```
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import software.amazon.awssdk.services.s3.S3Client;
import software.amazon.awssdk.services.s3.model.HeadObjectRequest;
import software.amazon.awssdk.services.s3.model.NoSuchKeyException;
import software.amazon.awssdk.utils.Validate;

public class DoesObjectExist {
    private static final Logger logger = LoggerFactory.getLogger(DoesObjectExist.class);

    public static void main(String[] args) {
        DoesObjectExist doesObjectExist = new DoesObjectExist();

        final S3Client s3SyncClient = S3Client.builder().build();
        final String bucketName = "amzn-s3-demo-bucket"; // Replace with your bucket name.
        final String key = "my-key"; // Replace with your object key.

        boolean exists = doesObjectExist.doesObjectExist(bucketName, key, s3SyncClient);
        logger.info("Object exists: {}", exists);
    }

    /**
     * Checks if the specified object exists in the specified bucket.
     * <p>
     * Internally this method uses the
     * <a href="https://sdk.amazonaws.com/java/api/latest/software/amazon/awssdk/services/s3/S3Client.html#headObject(java.util.function.Consumer)">S3Client.headObject</a>
     * operation to determine whether the object exists.
     * <p>
     * This method is equivalent to the AWS SDK for Java V1's
     * <a href="https://docs.aws.amazon.com/AWSJavaSDK/latest/javadoc/com/amazonaws/services/s3/AmazonS3Client.html#doesObjectExist-java.lang.String-java.lang.String-">AmazonS3Client#doesObjectExist(String, String)</a>.
     * <p>
     * <b>Note:</b> This method returns {@code false} only when S3 responds with 404 (NoSuchKey). If the caller
     * does not have {@code s3:ListBucket} permission on the bucket, S3 may return 403 instead of 404 for
     * non-existent objects, which will be thrown as an exception.
     *
     * @param bucketName   The name of the bucket containing the object.
     * @param key          The key of the object to check.
     * @param s3SyncClient An {@code S3Client} instance.
     * @return {@code true} if the object exists; {@code false} if it does not exist.
     */
    public boolean doesObjectExist(String bucketName, String key, S3Client s3SyncClient) {
        try {
            Validate.notEmpty(bucketName, "The bucket name must not be null or an empty string.", "");
            Validate.notEmpty(key, "The object key must not be null or an empty string.", "");
            s3SyncClient.headObject(HeadObjectRequest.builder()
                    .bucket(bucketName)
                    .key(key)
                    .build());
            return true;
        } catch (NoSuchKeyException e) {
            return false;
        }
    }
}
```
+  有关 API 详细，请参阅《Amazon SDK for Java 2.x API 参考》**中的 [HeadObject](https://docs.amazonaws.cn/goto/SdkForJavaV2/s3-2006-03-01/HeadObject)。

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有关 Amazon SDK 开发人员指南和代码示例的完整列表，请参阅 [使用 Amazon SDK 通过 Amazon S3 进行开发](sdk-general-information-section.md)。本主题还包括有关入门的信息以及有关先前的 SDK 版本的详细信息。