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Use wkhtmltopdf for PDF output and wkhtmltoimage for images. Neither tool is a Java library: each is a headless Qt WebKit command-line executable. In Java, you either start the installed executable yourself (for maximum control) or use a third-party wrapper that starts it for you. The executable must be installed, executable by the service account, and tested in the same environment as your application.
This guide shows a direct Java implementation for an HTML string, explains wrapper trade-offs, covers deployment and security, and then shows a browser-free alternative with ScreenshotNeo.
Choose the correct executable
| Goal | Executable | Typical output |
|---|---|---|
| Paginated document | wkhtmltopdf |
|
| Rendered snapshot | wkhtmltoimage |
PNG, JPEG and other image formats |
Do not pass an image-output command when you need a PDF: wkhtmltoimage does not create PDFs. Both programs render with Qt WebKit and run headlessly.
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The upstream project identifies 0.12.6 as its stable series, released June 11, 2020. Its GitHub repository was archived and made read-only on January 2, 2023. That makes this legacy software: verify binary availability, operating-system compatibility and vulnerability exposure before standardizing on it.
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Prepare a complete HTML string
Include a doctype, character encoding and all CSS needed for deterministic rendering. Relative URLs resolve against a base location; for a self-contained conversion, use absolute URLs or inline assets.
String html = """
<!doctype html>
<html>
<head>
<meta charset="UTF-8">
<style>
@page { size: A4; margin: 18mm; }
body { font-family: sans-serif; color: #222; }
h1 { page-break-after: avoid; }
</style>
</head>
<body>
<h1>Invoice 1042</h1>
<p>Generated from a Java string.</p>
</body>
</html>
""";Java text blocks require Java 15 or newer. On older Java versions, build the same value with ordinary string concatenation and escaped quotes.
Direct Java conversion with ProcessBuilder
The most transparent integration is to write the string to a temporary UTF-8 file, invoke wkhtmltopdf, and read the resulting PDF. This avoids guessing a wrapper’s artifact version and lets you capture exit status and stderr.
- Install a compatible
wkhtmltopdfpackage in the image or host. Confirmwkhtmltopdf --versionworks for the account running Java. - Use an absolute executable path in production, such as an application configuration value, rather than relying on
PATH. - Write the HTML with UTF-8 and create an output file in a directory writable by the service account.
- Start the process, drain stdout and stderr, enforce a timeout, and check the exit code and output size.
import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.*;
import java.util.List;
import java.util.concurrent.TimeUnit;
public final class WkhtmlPdf {
public static Path convert(String html, Path output, String executable)
throws IOException, InterruptedException {
Path input = Files.createTempFile("wkhtml-", ".html");
try {
Files.writeString(input, html, StandardCharsets.UTF_8,
StandardOpenOption.TRUNCATE_EXISTING);
Files.createDirectories(output.toAbsolutePath().getParent());
List<String> command = List.of(
executable,
"--encoding", "utf-8",
input.toAbsolutePath().toString(),
output.toAbsolutePath().toString());
Process process = new ProcessBuilder(command)
.redirectErrorStream(true)
.start();
String log = new String(process.getInputStream().readAllBytes(),
StandardCharsets.UTF_8);
if (!process.waitFor(90, TimeUnit.SECONDS)) {
process.destroyForcibly();
throw new IOException("wkhtmltopdf timed out");
}
if (process.exitValue() != 0 || !Files.isRegularFile(output)
|| Files.size(output) == 0) {
throw new IOException("wkhtmltopdf failed (exit "
+ process.exitValue() + "): " + log);
}
return output;
} finally {
Files.deleteIfExists(input);
}
}
public static void main(String[] args) throws Exception {
String html = """
<!doctype html><meta charset="UTF-8">
<h1>Hello PDF</h1><p>Café</p>
""";
convert(html, Path.of("build/report.pdf"),
System.getenv().getOrDefault("WKHTMLTOPDF", "wkhtmltopdf"));
}
}For image output, keep the same structure but invoke wkhtmltoimage and use an image filename:
List<String> command = List.of(
wkhtmltoimage, "--format", "png", input.toString(), "report.png");Option names vary by version and platform. Consult the executable’s own --help output and validate every option in your target image.
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Using a Java wrapper
A third-party Java wrapper can expose a page/input API, conversion options and a success response instead of requiring you to assemble ProcessBuilder calls. The wrapper documentation demonstrates supplying page content from a string, configuring options, and converting to a destination. It explicitly requires a separately installed, working wkhtmltopdf executable; adding the Java dependency does not install the renderer.
Because wrapper coordinates and behavior change, select the current published Maven or Gradle artifact from that project’s documentation, pin the version, and inspect its release notes. Confirm how it discovers the binary, how it reports stderr, and whether it supports your operating system. One documented wrapper caveat is that concurrent PDF generations are not supported; it also reports timeout issues with some options. Serialize calls or isolate workers unless your exact version documents safe concurrency.
Wrapper integration checklist
- Set the executable path explicitly when the wrapper permits it.
- Pass the HTML string through the wrapper’s page/input method, not as an unescaped shell command.
- Set encoding, paper size, margins, headers and footers through typed options where available.
- Check the conversion result and preserve diagnostic output.
- Run a version-pinned integration test inside the production container.
Rendering options that affect real documents
Page geometry and pagination
Use CSS @page rules and, where needed, command options for paper size, orientation and margins. Test tables, long code blocks, widows/orphans and page breaks with representative data. A successful process does not guarantee attractive pagination.
Fonts and assets
Install every required font in the runtime image. Remote fonts and images require network access and valid certificates; local files may require the executable’s local-file policy. Prefer absolute, allow-listed asset URLs or inline data for reproducibility.
JavaScript and dynamic pages
Qt WebKit is not a modern browser. Pages that depend on current JavaScript frameworks, complex CSS or delayed API calls may render incompletely. Use a documented delay or wait condition only after measuring the page’s actual readiness; a fixed delay increases latency without proving that data loaded.
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Headers, cookies and authentication
Do not put secrets in query strings or HTML. Supply controlled headers or cookies through supported options, redact command logs, and ensure temporary files have restrictive permissions.
Security and isolation
The project documentation gives this warning: “Do not use wkhtmltopdf with any untrusted HTML – be sure to sanitize any user-supplied HTML/JS, otherwise it can lead to complete takeover of the server it is running on!” Treat sanitization as necessary, not sufficient. Run conversion in a locked-down container or sandbox with a non-privileged user, restricted filesystem access, limited outbound network access and CPU/memory/time quotas. Never expose the converter as an unrestricted network service.
Review the licenses for the 0.12.6 binary, native libraries, wrapper and any fonts before distribution. The upstream project identifies its software as LGPLv3; obligations can differ across bundled components.
Deployment, reliability and cost considerations
- Binary management: bake a known binary into the image or install it during deployment, then verify its checksum and run a smoke conversion.
- Process limits: use a bounded queue, timeout and maximum HTML size. Clean temporary files on success, failure and cancellation.
- Observability: record version, elapsed time, exit code and sanitized stderr. Keep the input identifier, not sensitive HTML, in application logs.
- Concurrency: the cited wrapper does not support concurrent PDF generations. Even with direct invocation, benchmark your CPU and memory limits before increasing worker count.
- Licensing and maintenance: the archived upstream project receives no ordinary repository development. Revalidate security and platform support whenever your base image or Java runtime changes.
When another renderer is a better fit
The project’s status guidance points to Puppeteer for pages dependent on dynamic JavaScript and suggests considering WeasyPrint or commercial Prince for controlled report generation. These are scenario recommendations, not a current benchmark. Compare Java integration, HTML/CSS and JavaScript fidelity, print pagination, runtime size, isolation, maintenance and license terms.
For a Java-focused alternative, iText’s pdfHTML material shows a Java API that converts HTML input to PDF. That example alone does not establish current license terms, supported CSS, or suitability for your document, so verify those points before adoption.
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Or skip the browser setup
ScreenshotNeo provides a website screenshot API and MCP server. One GET request returns PNG, JPEG, WebP or PDF. It accepts cookie/consent banners before capture and removes more than 60 known consent platforms, newsletter popups and chat widgets; each step can be disabled. Bot checks, CAPTCHAs, blank pages, timeouts, failed loads and cache hits are not billed, and response headers identify the page verdict and billing result.
For a URL-based capture, use the documented endpoint and options at https://screenshotneo.com/docs/:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpThe same request from Java is:
import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;
import java.nio.file.Files;
import java.nio.file.Path;
var uri = URI.create("https://api.screenshotneo.com/v1/shot"
+ "?access_key=YOUR_API_KEY&url=https%3A%2F%2Fstripe.com");
var response = HttpClient.newHttpClient().send(
HttpRequest.newBuilder(uri).GET().build(),
HttpResponse.BodyHandlers.ofByteArray());
if (response.statusCode() / 100 != 2) throw new RuntimeException("HTTP " + response.statusCode());
Files.write(Path.of("shot.webp"), response.body());ScreenshotNeo also supports full-page captures with lazy images, CSS-selector element capture, dark mode, device presets, arbitrary viewports, retina scale, PDF paper settings and ranges, HTML/CSS-to-image, custom CSS and JavaScript, clicks, selector waits, delays, network-idle waits, request blocking, headers, cookies, user agents, Authorization, timezone, geolocation, transparent backgrounds, resizing, configurable caching, signed links, asynchronous jobs with signed webhooks, bulk capture for 100 URLs per call, a usage API and an OpenAPI specification. An MCP server exposes take_screenshot, get_page_info and capture_pdf to Claude, Cursor and other MCP clients.
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“Cannot run program” or exit 127
The executable is missing or not on the service account’s PATH. Install it, use an absolute path, and verify permissions as the same user.
Empty or partial PDF
Inspect stderr and HTML separately. Check encoding, blocked local files, missing fonts, certificate errors, remote asset access and JavaScript timing. Capture a minimal self-contained document to isolate the cause.
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- Enjoy an easy-to-use, intuitive interface with a Drag and Drop file option.
- Burn your converted or original files directly to disc.
Timeouts
Look for unreachable assets, never-ending scripts and waits that cannot be satisfied. Set a finite process timeout, abort the child, and cap page size and network scope.
Works locally but fails in a container
Compare installed fonts, shared libraries, executable permissions, sandbox restrictions, DNS, certificates and writable temporary directories. Reproduce with a smoke test during image build.
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Use a bounded worker pool and serialized conversion when using the cited wrapper, or switch to isolated worker processes after validating direct invocation under load.
Frequently Asked Questions
Can a Java dependency install wkhtmltopdf automatically?
No. The Java wrapper and the renderer are separate; install and verify the wkhtmltopdf executable yourself.
Can wkhtmltoimage produce a PDF?
No. Use wkhtmltopdf for PDF output; wkhtmltoimage produces image formats.
Is wkhtmltopdf suitable for untrusted user HTML?
Not without strong sanitization and isolation. The project warns that unsafe HTML or JavaScript can enable complete server takeover.
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Evaluate a browser-based renderer such as Puppeteer, or a controlled-report engine such as WeasyPrint or Prince, against your fidelity, deployment and licensing requirements.
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