In-memory HttpServer Handler

Christian Stein on November 6, 2023

Inspired by JEP’s 408 summary, which reads:

Summary Provide a command-line tool to start a minimal web server that serves static files in the current directory. This low-threshold utility will be useful for prototyping, ad-hoc coding, and testing purposes, particularly in educational contexts.

Here’s a http handler implementation that servers in-memory assets. This is useful for running http client tests that want to issue GET or HEAD requests against a well-known set of assets.

Asset

An asset record encapsulates a http return code, the bytes to send, and a type component that is used as the Content-Type response header.

public record Asset(int code, byte[] data, String type) {

  public static Asset of(byte... bytes) {
    return new Asset(200, bytes, "application/octet-stream");
  }

  public static Asset ofBase64(String base64, String type) {
    return new Asset(200, Base64.getDecoder().decode(base64), type);
  }

  public static Asset ofHtml(String html) {
    return Asset.ofText(200, html, "text/html");
  }
  
  public static Asset ofText(String text) {
    return Asset.ofText(200, text, "text/plain");
  }

  public static Asset ofText(int code, String text, String type) {
    return new Asset(code, text.getBytes(StandardCharsets.UTF_8), type);
  }

  // ... here be more convenient factory methods
}

All well-known assets are organized in a Map<String, Asset>, with absolute request paths used as keys.

Map<String, Asset> createAssets() {

    return Map.of(
        "/index.text", Asset.ofText("Index"),

        "/index.html", Asset.ofHtml(
        """
        <html>
          <body>
            <h1>Index</h1>
          </body>
        </html>
        """)
    
        // ... here be more path-asset mappings
    );
}

By default, all paths representing a directory (end with /) are mapped to an asset that generates a directory listing. All other unmapped paths (non-directory) are mapped to an asset that renders a 404 error page.

Command-line Demo

Launch an in-memory http server demo on the command-line via:

$ java InMemoryHttpServer.java
> InMemoryHttpServer
> http://127.0.0.1:59476

Open the URL in a browser:

Index