To add browser-based SAML single sign-on to Javalin with pac4j, choose compatible library versions, configure a long-lived SAML2Client with your service provider (SP) keys and identity provider (IdP) metadata, register the SP metadata with the IdP, and connect pac4j’s security, callback, and logout handlers to the appropriate Javalin routes. The callback must accept the IdP’s POSTed assertion, and the client’s replay-cache state must persist between authentications.
Choose compatible versions before writing the integration
The javalin-pac4j README maps version 8 to Javalin 7, pac4j 6, and Java 17. It maps version 7 to Javalin 5.6, pac4j 6, and Java 17. The official Javalin SAML guide shows Javalin 7.0.1, javalin-pac4j 8.0.0, and pac4j-saml 6.5.8. Those are the versions in that example, not a guarantee that they are the latest releases.
Resolve a compatible set of currently released artifacts before implementation. Avoid mixing a Javalin integration version with a Javalin or pac4j major version outside the combinations documented by the integration README.
| javalin-pac4j line | Javalin | pac4j | Java |
|---|---|---|---|
| 8 | 7 | 6 | 17 |
| 7 | 5.6 | 6 | 17 |
These are compatibility combinations documented by the README, not a full dependency lockfile. Check the integration documentation and artifact metadata when selecting exact patch versions.
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Prepare the SP keys and metadata
Create and protect a keystore
The SAML client needs SP key material for signing and encryption operations. The Javalin guide demonstrates generating a keystore with Java’s keytool. Treat any passwords shown in a tutorial as examples only: keep the keystore password and private-key password in deployment-managed secrets, not source code or a checked-in configuration file.
pac4j’s SAML 2.0 client reference also describes a writable-resource option for automatic keystore creation. For production, decide deliberately how keys are created, stored, backed up, rotated, and made available to application instances. The private key should not be exposed through a public metadata URL.
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Set the SP identity and metadata location
Configure the SAML client with the keystore and its credentials, the IdP metadata, the SP entity ID, and the location where SP metadata is produced. The entity ID is the SP’s identifier; it is not interchangeable with the callback URL. Keep both values consistent with the SP registration at the IdP.
Configure one SAML client and pac4j Config
Create a SAML2Configuration from the SP and IdP settings, then construct a SAML2Client and place it in pac4j’s Config. The same configured client must be used for the protected flow and callback handling. After successful authentication, pac4j supplies a SAML2Profile; application code can use that type or the common UserProfile abstraction, depending on which attributes it needs.
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Do not create a fresh SAML client per request without an explicit state strategy. pac4j documents that the client’s replay cache must retain state across authentications and recommends a single client instance. If the deployment cannot retain that instance in the required way, the SAML reference points to a custom ReplayCacheProvider so replay state can be managed appropriately for the deployment.
Register the SP with the identity provider
Exchange the SP metadata generated from your configuration with the IdP administrator or registration workflow, and configure the application with the IdP metadata. The IdP must recognize the SP entity ID and the assertion consumer service (ACS) endpoint where it will send authentication responses.
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- Use the same SP entity ID in the application and IdP registration.
- Make the ACS URL match the application’s callback route, including scheme, host, path, and any externally visible proxy configuration.
- Use metadata and endpoint settings for the organization’s actual IdP; a public test provider in a tutorial is only an example.
An IdP error such as “unknown service provider” commonly indicates that the SP is not registered or that the entity ID sent by the application differs from the registered value.
Connect protection, callback, and logout routes
Protect only the intended routes
Use a Javalin before handler with pac4j’s SecurityHandler on the routes that require authentication. Javalin route patterns are significant: /protected and /protected/* are distinct, so add coverage for both the base path and nested paths if both should be protected. Apply the handler to every intended route rather than assuming the base pattern includes subpaths.
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Accept the SAML response at the callback
Register pac4j’s callback handling for the indirect SAML flow at the ACS path configured with the IdP. The IdP posts its assertion, so the callback route must be reachable using HTTP POST and its externally registered URL must match the configured callback URL. Ensure the SAML client name is consistent wherever the pac4j callback configuration refers to it.
Choose the logout behavior
Add pac4j’s LogoutHandler and decide whether the application needs only to end its local session or should also request logout at the IdP. Local logout and global single logout are different behaviors; configure the latter only when the IdP supports the needed endpoints and bindings and the application requires that user experience. The integration README documents support for both patterns.
Verify the IdP’s bindings and test the complete flow
Do not assume every provider exposes the same SAML endpoints or bindings. Check the actual IdP metadata and test login and logout against the provider and environment that will be used in production.
One documented compatibility case concerns SimpleSAMLphp: pac4j requires HTTP-POST bindings for both SSO and SLO, while SimpleSAMLphp’s default can expose HTTP-Redirect only. In that case, enable the required POST binding and register the SP entity ID with SimpleSAMLphp. This is provider-specific guidance, not a universal configuration rule for all IdPs.
Quick Recap
- Start the application with the intended production-compatible configuration and confirm that the SP metadata identifies the expected entity ID and ACS endpoint.
- Register that SP metadata with the IdP and confirm the IdP’s metadata and binding configuration are available to the application.
- Open a protected route in a browser and verify that unauthenticated access begins the SAML flow.
- Complete sign-in at the IdP and confirm the POST reaches the configured callback and the application receives the expected profile.
- Exercise the selected logout behavior and inspect the IdP’s response and application session state.
Troubleshoot common integration failures
| Symptom | What to check |
|---|---|
| IdP reports an unknown SP | Compare the application’s SP entity ID with the registered SP metadata. Confirm the ACS URL is also registered and matches the externally reachable callback. |
| Anonymous access reaches a protected page | Check that the SecurityHandler is attached to both the base route and the nested route pattern where needed. |
| Callback fails or does not receive a response | Confirm the callback accepts POST, the ACS URL matches the configured callback, and the SAML client name is consistent in pac4j configuration. |
| Provider rejects an endpoint or binding | Inspect the IdP metadata and provider-specific binding requirements. For the documented SimpleSAMLphp case, configure HTTP-POST for SSO and SLO. |
| Replay or state failures occur intermittently | Retain one SAML client instance across authentications, or implement an appropriate custom replay-cache provider for shared state. |
Implementation checklist
- Choose versions from a documented compatible Javalin, javalin-pac4j, pac4j, and Java combination.
- Generate and protect SP keys; keep credentials out of source control.
- Configure IdP metadata, SP entity ID, and metadata output location in one SAML client configuration.
- Register the resulting SP metadata and matching ACS URL with the IdP.
- Attach security, POST callback, and logout handlers to the routes and behavior you intend.
- Preserve replay-cache state and test against the actual IdP, including its bindings.
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