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Use three separate pieces rather than searching for one “Java factory pattern”: a stable service interface, Java’s ServiceLoader for runtime provider discovery, and an application-owned factory that selects and constructs the implementation. Use behavior-driven development (BDD) to agree on outcomes such as “the requested format is supported,” then automate those examples while keeping deployment mechanics in focused lower-level tests.
How the pieces fit together
A service is a well-known interface or class for which zero, one, or many providers can exist, as described by Oracle’s ServiceLoader API for Java SE 26. ServiceLoader answers “which providers are available?” It does not decide which provider is appropriate for a business request.
A factory answers a different question: “Which object should this application use, and how should it be constructed?” A service locator is a broader lookup abstraction. ServiceLoader can discover providers, and a discovered provider can itself be a factory, but the terms are not interchangeable.
A useful boundary is:
- Service contract: an interface or abstract class containing the operations and capability information needed by callers and selection policy.
- Provider registration: module metadata or class-path configuration that makes implementations discoverable.
- Factory or resolver: application code that applies deterministic selection rules and exposes a small method such as
createFor(request). - BDD examples: collaborative scenarios that describe externally visible results without coupling the business language to provider class names.
Define a service contract that can support a decision
Put behavior needed for a meaningful choice on the service contract. If a request asks for a particular format, region, protocol, or capability, the factory must be able to inspect that property before or during construction.
public interface DocumentRenderer {
Set<String> supportedFormats();
RenderedDocument render(Document document);
}
public record RenderRequest(String format) { }
Keep the contract stable and domain-oriented. A provider should expose capabilities in a way that allows selection without forcing callers to know implementation classes. If construction requires configuration, define that requirement explicitly in the provider contract or in a separate provider-factory abstraction.
Register providers for the deployment model
Named modules and class-path applications use different registration mechanisms. Choose the one that matches the project; these configurations are not interchangeable.
Named modules
The consuming module declares that it uses the service. A provider module declares which implementation supplies it:
// module-info.java in the application module
module com.example.app {
uses com.example.render.DocumentRenderer;
}
// module-info.java in a provider module
module com.example.pdf {
requires com.example.api;
provides com.example.render.DocumentRenderer
with com.example.pdf.PdfRenderer;
}
The Java API also permits a module provider to expose a public static no-argument provider() method. Otherwise, provider construction follows the documented public no-argument-constructor requirements for the Java version and deployment mode in use.
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Class path
For class-path deployment, create a UTF-8 file at META-INF/services/com.example.render.DocumentRenderer. List provider class names, one per line:
com.example.pdf.PdfRenderer
com.example.html.HtmlRenderer
Package that file in the provider artifact. A missing file, misspelled service name, or unavailable provider class prevents discovery even when the implementation itself compiles.
Build a factory around discovery and selection
Use ServiceLoader.load(ServiceType.class) to obtain a loader, then put application policy in a named factory. Use the provider stream when metadata can be inspected before creating instances; iterate when instances are required.
public final class RendererFactory {
private final ServiceLoader<DocumentRenderer> loader;
public RendererFactory(ClassLoader classLoader) {
this.loader = ServiceLoader.load(DocumentRenderer.class, classLoader);
}
public DocumentRenderer createFor(RenderRequest request) {
return loader.stream()
.filter(provider -> provider.type().getDeclaredConstructor() != null)
.map(ServiceLoader.Provider::get)
.filter(renderer -> renderer.supportedFormats()
.contains(request.format()))
.findFirst()
.orElseThrow(() -> new UnsupportedFormatException(request.format()));
}
}
The constructor check above is only illustrative; real selection should use provider metadata designed for that purpose rather than reflective assumptions. If obtaining a provider is expensive or can fail, handle that failure deliberately and preserve the provider and request context in logs or exceptions.
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Provider enumeration order is not an application policy. If several providers support a request, expose a priority or specificity value in the contract and select by an explicit rule. Make the winning provider observable in diagnostics so an installation change does not silently alter behavior.
Decide loader scope and refresh behavior
ServiceLoader loads providers lazily and caches providers it has loaded. Call reload() when the application intentionally needs to clear that cache. A ServiceLoader instance is not safe for concurrent use, so either confine it to one thread, synchronize access, or create loaders with a lifecycle that fits the application. Do not assume a single VM-wide cached loader is correct when context class loaders can differ between applications.
Specify behavior with BDD examples
BDD is a collaborative workflow, not merely a test framework. Cucumber describes three iterative practices: discover examples with business and technical participants, formulate them as automatable documentation, and automate the examples while implementing the smallest change that makes them pass.
Start with a user-visible outcome. Keep module descriptors, META-INF/services paths, and Java class names out of the scenario unless deployment configuration is itself the behavior under discussion.
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Feature: Choose a document renderer
Scenario: choose a provider that supports the requested format
Given the application has a provider for the requested format
When a client requests a service for that format
Then the application returns a service that supports the format
Write scenarios in the vocabulary of the product. Add examples for outcomes that matter to users:
- No provider is available and the application supplies a defined fallback or a clear error.
- Providers exist, but none supports the requested capability.
- Two providers qualify and the documented priority rule selects one deterministically.
Connect Gherkin steps to Java step definitions. Cucumber can run on the JVM through Java test runners, build tools, IDE integrations, or its command-line interface. Cucumber does not provide an assertion library, so use the assertion and test integration appropriate to the project.
Test the seams that BDD does not replace
End-to-end scenarios should verify behavior, while focused tests protect configuration and failure boundaries:
- Verify each provider advertises the capabilities expected by the contract.
- Verify module or class-path registration in an integration test using the actual packaging layout.
- Exercise malformed registrations and provider construction errors; Java reports discovery, loading, and instantiation failures as
ServiceConfigurationError. - Test duplicate or competing providers against the explicit priority rule.
- Test cache refresh behavior when the application supports dynamic provider changes.
- Test the no-provider path separately from the unsupported-capability path if users receive different remedies.
Do not swallow configuration errors and return a generic “not supported” result when the deployment is broken. Preserve enough context to distinguish absence, invalid registration, and provider-construction failure.
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Best Value
Choose an approach deliberately
| Approach | Who registers implementations | When selection occurs | Deployment extensibility | Lifecycle owner | Typical failure visibility | Behavior testing |
|---|---|---|---|---|---|---|
| Manual factory | Application wiring | Compile time or startup | Requires consuming-code changes for new implementations | Application code | Missing or invalid wiring is usually immediate | Simple when dependencies can be injected |
ServiceLoader plus factory |
Module descriptors or META-INF/services |
Startup or request time, depending on factory use | Provider artifacts can be added without changing consuming code | Application factory and providers | No provider, invalid configuration, and instantiation errors are distinct cases | Keep business examples separate from registration integration tests |
| Dependency-injection container | Container configuration and registrations | Usually container startup or resolution time | Depends on the selected container and its modules | Container-defined scopes | Container configuration and resolution errors | Inject fakes and test behavior independently of deployment |
| Service locator | Registry or locator configuration | Lookup time | Depends on registry design | Locator or application code | Lookup and registration failures | Can require locator seams or test registries |
The last two rows are intentionally broad: the cited Java and Oracle pattern documentation does not establish a current, framework-by-framework comparison. Select them only when their lifecycle, scope, and registration model solve a concrete problem.
Failure and lifecycle checklist
- Define the behavior when zero providers are found.
- Distinguish “no provider” from “providers found, but none supports this request.”
- Make tie-breaking deterministic and observable.
- Catch or propagate
ServiceConfigurationErrorwith actionable context. - Choose a loader scope that respects context class-loader boundaries.
- Do not share one
ServiceLoaderinstance across threads without synchronization. - Use
reload()only as an intentional cache-management operation. - Keep the factory API small enough to substitute in unit and BDD tests.
Frequently Asked Questions
Is ServiceLoader itself a factory?
No. ServiceLoader discovers and lazily loads providers. A factory applies application-specific selection and construction policy; a provider discovered by ServiceLoader may itself implement a factory.
Should every Java service use a global service locator?
No. Prefer an explicit factory or resolver with injected dependencies. Use a broader locator only when its lookup abstraction and lifecycle solve a demonstrated requirement.
Where should exact provider class names appear in BDD scenarios?
Usually in implementation-level registration tests, not user-facing scenarios. Keep business scenarios focused on capabilities and outcomes unless the class name is itself part of the intended behavior.
The Bottom Line
Define the service contract first, register providers using the correct module or class-path mechanism, and put deterministic selection behind a small factory. Let BDD establish the observable behavior; use focused integration and unit tests for discovery, caching, concurrency, and configuration failures.
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