Welcome to the third lesson of our course on building an image generation service in Java! In our previous lessons, we created the PromptManager to format user inputs into detailed prompts and the ImageManager to handle storing and processing generated images. Now, we're ready to build the core component that brings everything together: the ImageGeneratorService.

The ImageGeneratorService is the central piece of our application that will:

1. Connect to Google's Gemini API to generate images
2. Use our PromptManager to format user inputs into effective prompts
3. Store generated images using our ImageManager
4. Provide access to all previously generated images

This service acts as the bridge between our application's components and the external AI service that actually creates the images. By encapsulating all the image generation logic in a dedicated service class, we maintain a clean separation of concerns in our application architecture.

In this lesson, we'll implement this service step by step, from setting up the HTTP client to handling responses and errors. By the end, you'll have a fully functional image generation service that you can later integrate into a Java web application.

Before we can generate images, we need to set up a way to communicate with Google's Gemini API. Instead of using a dedicated SDK, we'll use Java's HTTP client libraries to send requests directly to the API endpoint.

To do this, you'll need the following dependencies in your project:

These libraries allow us to send HTTP requests and parse JSON responses.

Next, let's create our ImageGeneratorService class and set up the HTTP client in the constructor. In Java, you would typically place this class in a package such as com.codesignal.services and save it in a file named ImageGeneratorService.java:

In this constructor, we're doing two important things:

1. Creating an instance of our ImageManager class to handle storing and retrieving images.
2. Reading the Gemini API key and base URL from environment variables.

In Java, environment variables can be accessed using System.getenv("VARIABLE_NAME"). This approach keeps sensitive information like API keys out of your source code.

Now that we have our HTTP client set up, let's implement the core method of our service: generateImage(). This method will take a user input string, format it into a detailed prompt using our PromptManager, send the request to the Gemini API using HTTP, and store the resulting image using our ImageManager.

Here is the implementation:

Generating images through an external API can fail for various reasons: network issues, API limits, invalid prompts, or server errors. To make our service robust, we've wrapped the API call in a try-catch block that catches any exceptions and throws a more informative RuntimeException.

This error handling is crucial for a production application, as it prevents crashes and provides meaningful error messages that can help with debugging and user feedback.

Now, let's add one more method to our service to retrieve all previously generated images:

This simple method delegates to our ImageManager's getImages() method, returning the complete list of stored images along with their associated prompts and IDs.

With these two methods, our ImageGeneratorService provides a complete interface for generating and retrieving images. The service integrates our previously built components (PromptManager and ImageManager) and connects them to the external Gemini API using HTTP.

Now that we've implemented our ImageGeneratorService, let's create a test class to verify that it works correctly. You can do this with a simple main method in a Java class, such as Main.java:

In this test code, we:

1. Import our ImageGeneratorService and PromptManager classes.
2. Define a sample user input for testing.
3. Format the prompt using our PromptManager (this is just for demonstration; the service does this internally).
4. Create an instance of our ImageGeneratorService.
5. Call the generateImage() method with our sample input.
6. Print the result (the base64-encoded image).

In this lesson, we've built the ImageGeneratorService, the core component of our image generation application. This service connects our previously built components (PromptManager and ImageManager) to Google's Gemini API using HTTP, allowing us to generate high-quality images from text prompts.

Let's review what we've learned:

1. We set up an HTTP client to communicate with Google's Gemini API in Java.
2. We implemented the generateImage() method to create images from user inputs.
3. We added robust error handling to deal with potential API issues.
4. We created a method to retrieve all previously generated images.
5. We tested our service with a sample prompt.

The ImageGeneratorService is a crucial piece of our application architecture. It encapsulates all the logic related to image generation, providing a clean interface for other components to use. In the next lesson, we'll build a controller that will use this service to handle HTTP requests in our Java web application.

In the upcoming practice exercises, you'll have the opportunity to work with the ImageGeneratorService, testing its functionality with different prompts and exploring how it integrates with the rest of our application. You'll also get to experiment with error handling and see how the service behaves in various scenarios.

Remember that to use this service in a real application, you'll need to:

1. Add the required HTTP and JSON libraries as dependencies in your Java project.
2. Obtain a valid API key from Google.
3. Set the GEMINI_API_KEY and GEMINI_BASE_URL environment variables before running your application.

With the in place, we're one step closer to having a complete image generation web application!