### Instruction:
Create a Java application that connects to AWS Lambda and triggers a serverless function when a user uploads a file. The function should compress the uploaded file and return the compressed version to the user.


### Response:

To create a Java application that connects to AWS Lambda and triggers a serverless function when a user uploads a file, you'll need to follow these steps:

1. Create an AWS Lambda function with the required code for compressing files.
2. Set up an Amazon S3 bucket to store the uploaded files.
3. Create a Java application using the AWS SDK for Java to interact with AWS services.

Here's a step-by-step guide on how to do this:

Step 1: Create an AWS Lambda function

1. Sign in to your AWS Management Console and navigate to the Lambda service.
2. Click "Create function" and choose "Author from scratch."
3. Enter a name for your function (e.g., "CompressFileFunction") and select a runtime (e.g., Node.js or Python).
4. Add the necessary code to compress the input file. For example, if you're using Node.js, you can use the following code snippet:

```javascript
const zlib = require('zlib');
const fs = require('fs');

exports.handler = async (event) => {
    const inputFile = event.inputFile;
    const outputFile = `${inputFile}.gz`;

    await new Promise((resolve, reject) => {
        const gzip = zlib.createGzip();
        const inputStream = fs.createReadStream(inputFile);
        const outputStream = fs.createWriteStream(outputFile);

        inputStream.pipe(gzip).pipe(outputStream).on('finish', resolve).on('error', reject);
    });

    return { compressedFile: outputFile };
};
```

5. Save and deploy your Lambda function.

Step 2: Set up an Amazon S3 bucket

1. Navigate to the Amazon S3 service in the AWS Management Console.
2. Click "Create bucket" and enter a unique name for your bucket.
3. Follow the prompts to configure your bucket as needed and click "Create."

Step 3: Create a Java application

1. Install the AWS SDK for Java in your project using Maven or Gradle.
2. Configure your AWS credentials by following the instructions in the [official documentation](https://docs.aws.amazon.com/sdk-for-java/v1/developer-guide/setup-credentials.html).
3. Create a Java class with a method to upload a file to Amazon S3 and trigger the Lambda function:

```java
import com.amazonaws.services.lambda.AWSLambda;
import com.amazonaws.services.lambda.AWSLambdaClientBuilder;
import com.amazonaws.services.lambda.model.InvokeRequest;
import com.amazonaws.services.lambda.model.InvokeResult;
import com.amazonaws.services.s3.AmazonS3;
import com.amazonaws.services.s3.AmazonS3ClientBuilder;
import com.amazonaws.services.s3.model.PutObjectRequest;

import java.io.File;
import java.nio.ByteBuffer;
import java.nio.charset.Charset;

public class FileCompressor {

    private static final String BUCKET_NAME = "your-bucket-name";
    private static final String LAMBDA_FUNCTION_NAME = "CompressFileFunction";

    public void compressAndUploadFile(File inputFile) {
        // Upload the input file to Amazon S3
        AmazonS3 s3Client = AmazonS3ClientBuilder.defaultClient();
        s3Client.putObject(new PutObjectRequest(BUCKET_NAME, inputFile.getName(), inputFile));

        // Invoke the Lambda function
        AWSLambda lambdaClient = AWSLambdaClientBuilder.defaultClient();
        InvokeRequest invokeRequest = new InvokeRequest()
                .withFunctionName(LAMBDA_FUNCTION_NAME)
                .withPayload(String.format("{\"inputFile\": \"%s\"}", inputFile.getName()));
        InvokeResult invokeResult = lambdaClient.invoke(invokeRequest);

        // Get the compressed file name from the Lambda response
        String compressedFileName = parseCompressedFileNameFromResponse(invokeResult.getPayload());

        // Download the compressed file from Amazon S3
        File compressedFile = new File(compressedFileName);
        s3Client.getObject(BUCKET_NAME, compressedFileName, compressedFile);

        System.out.println("Compressed file saved as: " + compressedFile.getAbsolutePath());
    }

    private String parseCompressedFileNameFromResponse(ByteBuffer payload) {
        String responseJson = Charset.forName("UTF-8").decode(payload).toString();
        // Assuming the Lambda function returns a JSON object with a single property "compressedFile"
        return responseJson.substring(responseJson.indexOf(":") + 1, responseJson.length() - 1).replace("\"", "");
    }
}
```

4. Use the `FileCompressor` class in your Java application to compress and upload files:

```java
public class Main {

    public static void main(String[] args) {
        File inputFile = new File("path/to/your/input/file.txt");
        FileCompressor fileCompressor = new FileCompressor();
        fileCompressor.compressAndUploadFile(inputFile);
    }
}
```

Now, when you run your Java application, it will upload the input file to Amazon S3, trigger the AWS Lambda function to compress the file, and download the compressed version of the file back to your local machine.