Shipping Java 11 to iOS: Benchmarking Gluon’s New Cloud Pipeline

So there I was at 2 AM on a Tuesday, staring at a massive Xcode stack trace on my M3 Max running Sonoma 14.4. I was trying to figure out why a perfectly good Java 11 enterprise library was refusing to compile to an iOS binary. The linking stage just kept hanging.

Then Gluon dropped their new OpenJDK Mobile Resources and automated build pipelines. But people outside of enterprise consulting probably think everyone migrated to Java 21 years ago. The reality? Massive chunks of backend business logic are still firmly stuck on Java 11. And right now, product managers are demanding that exact same logic run natively and offline on iPads for field workers. Rewriting hundreds of thousands of lines of tested Java into Swift isn’t happening. We have to compile it Ahead-Of-Time (AOT) and ship it.

Actually, let me back up — Gluon has been doing Java-on-iOS for a while, but configuring the local toolchains was always a headache. Their latest update moves the heavy lifting to their cloud runners and provides pre-packaged OpenJDK 11 mobile resources. I ripped out my broken local config and wired up their new pipeline to see if it actually worked.

The Code We’re Shipping

To test this properly, I didn’t want a “Hello World” app. I pulled a chunk of actual data synchronization logic from our Java 11 codebase. It uses the native HTTP Client, interfaces, and streams to process offline records before sending them to the server.

Here is the stripped-down version of what I fed into the Gluon pipeline:

import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;
import java.time.Duration;
import java.util.List;
import java.util.function.Predicate;
import java.util.stream.Collectors;

// 1. The Interface defining our contract
public interface DataSynchronizer {
    List<String> cleanOfflineRecords(List<String> rawData);
    boolean pushToServer(List<String> cleanedData);
}

// 2. The Implementation Class
public class IOSSyncService implements DataSynchronizer {
    
    // Using the Java 11 HttpClient
    private final HttpClient httpClient;
    
    public IOSSyncService() {
        this.httpClient = HttpClient.newBuilder()
            .version(HttpClient.Version.HTTP_2)
            .connectTimeout(Duration.ofSeconds(10))
            .build();
    }

    // 3. The Method handling data prep
    @Override
    public List<String> cleanOfflineRecords(List<String> rawData) {
        // 4. The Stream using Java 11 specific String methods
        return rawData.stream()
            // Predicate.not and String::isBlank were introduced in Java 11
            .filter(Predicate.not(String::isBlank))
            .map(String::strip) // Java 11 strip() handles Unicode whitespace better than trim()
            .map(record -> record.toUpperCase())
            .collect(Collectors.toList());
    }

    @Override
    public boolean pushToServer(List<String> cleanedData) {
        if (cleanedData.isEmpty()) return false;
        
        String payload = String.join(",", cleanedData);
        
        HttpRequest request = HttpRequest.newBuilder()
            .uri(URI.create("https://api.internal-system.com/v1/sync"))
            .header("Content-Type", "text/plain")
            .POST(HttpRequest.BodyPublishers.ofString(payload))
            .build();
            
        try {
            HttpResponse<String> response = httpClient.send(
                request, 
                HttpResponse.BodyHandlers.ofString()
            );
            return response.statusCode() == 200;
        } catch (Exception e) {
            System.err.println("Sync failed: " + e.getMessage());
            return false;
        }
    }
}

Pipeline Performance and The Inevitable Gotchas

Setting up the automated build was surprisingly straightforward. You feed their GitHub Action your Maven or Gradle project, point it at the new mobile resources target, and it spins up a runner to handle the GraalVM AOT compilation.

The speed difference caught me off guard. Local native-image compilation used to take me about 14m 20s and turn my laptop into a space heater. But the new pipeline knocked that down to 3m 15s on their infrastructure. And getting an iOS .ipa file spit out at the end of a standard CI run without managing local Apple provisioning profiles manually? That’s a massive relief.

But it wasn’t a completely smooth ride. I pushed the app to a physical iPhone 13 for testing. The UI loaded. The local data processing ran instantly. Yet the moment pushToServer() triggered, the app swallowed the network request and died. No logs. Just a silent failure.

I wasted three hours trying to debug network permissions in Xcode before I realized the issue was in the AOT configuration for the Java 11 HttpClient. When compiling Java for iOS via Substrate, the native image generator aggressively strips out anything it thinks you aren’t using. And by default, it doesn’t include the SSL certificate trust store required for HTTPS connections.

If you’re using this pipeline, you have to explicitly pass the HTTPS protocol flag in your build plugin configuration:

<!-- In your pom.xml under the gluonfx-maven-plugin -->
<configuration>
    <target>ios</target>
    <compilerArgs>
        <arg>-H:EnableURLProtocols=https</arg>
    </compilerArgs>
</configuration>

Once I added that single line, the Java 11 HTTP client happily negotiated the TLS handshake and synced the data from the iPhone to our backend.

I expect we’ll see a lot more enterprise teams quietly adopting this by Q1 2027. Rewriting complex, battle-tested Java validation rules into Swift or Kotlin Multiplatform just to get an iPad app out the door is an expensive risk. Being able to drop a legacy Java 11 JAR into a cloud pipeline and get a native iOS binary back changes the math entirely.

But just remember to configure your SSL flags before you spend half your night yelling at Xcode.