KNOX-3332: Add listeners on gateway config changes

[hanicz]

KNOX-3332 - Add listeners on gateway config changes for observers

What changes were proposed in this pull request?

• Adds an observer pattern so clients are notified if gateway config changes.
• Knox LDAP Server Manager reloads the LDAP configs if there are changes and restart the LDAP service.

How was this patch tested?

Unit tests

Manual test:
Started a local Knox and LDAP instance. Changed the knoxsso ldap port to 33390 and tried to login into the homepage which failed. Updated the gateway-reloadable.xml with the new port 33395 and I was able to log into the homepage.

2026-05-29 11:26:10,426  INFO  knox.gateway (GatewayServer.java:refreshGatewayConfig(275)) - Refreshed gateway config
2026-05-29 11:26:10,426  INFO  services.ldap (KnoxLDAPServerManager.java:onGatewayConfigChanged(109)) - Reloading LDAP configuration
2026-05-29 11:26:10,427  INFO  services.ldap (KnoxLDAPServerManager.java:stop(218)) - Stopping LDAP service on port 33,390
2026-05-29 11:26:10,445  INFO  services.ldap (KnoxLDAPServerManager.java:stop(236)) - LDAP service stopped successfully
2026-05-29 11:26:10,448  INFO  services.ldap (BackendFactory.java:createBackend(39)) - Loading backend: ldap (via ServiceLoader)
2026-05-29 11:26:10,448  INFO  services.ldap (LdapProxyBackend.java:initialize(140)) - Loading backend: ldap (via Proxying dc=hadoop,dc=apache,dc=org to ldap://localhost:33389 (dc=hadoop,dc=apache,dc=org) with uid attribute using group searches)
2026-05-29 11:26:10,449  INFO  services.ldap (LdapProxyBackend.java:initializeConnectionPool(180)) - Loading backend: ldap (via Initialized connection pool with maxActive=8)
2026-05-29 11:26:10,449  INFO  services.ldap (KnoxLDAPServerManager.java:start(131)) - Starting LDAP service on port 33,395 with base DN: dc=hadoop,dc=apache,dc=org
2026-05-29 11:26:10,540  INFO  services.ldap (KnoxLDAPServerManager.java:start(190)) - LDAP service started successfully on port 33,395

gateway-site.xml

    <!-- KnoxLDAP Service Configuration -->
    <property>
        <name>gateway.ldap.enabled</name>
        <value>true</value>
    </property>
    <property>
        <name>gateway.ldap.port</name>
        <value>33390</value>
    </property>
    <property>
        <name>gateway.ldap.base.dn</name>
        <value>dc=hadoop,dc=apache,dc=org</value>
    </property>
    <property>
        <name>gateway.ldap.backend.type</name>
        <value>ldap</value>
    </property>

    <!-- LDAP Backend specific configuration (proxying to demo ldap) -->
    <property>
        <name>gateway.ldap.backend.ldap.url</name>
        <value>ldap://localhost:33389</value>
    </property>
    <property>
        <name>gateway.ldap.backend.ldap.remoteBaseDn</name>
        <value>dc=hadoop,dc=apache,dc=org</value>
    </property>
    <property>
        <name>gateway.ldap.backend.ldap.systemUsername</name>
        <value>uid=guest,ou=people,dc=hadoop,dc=apache,dc=org</value>
    </property>
    <property>
        <name>gateway.ldap.backend.ldap.systemPassword</name>
        <value>guest-password</value>
    </property>
    <property>
        <name>gateway.ldap.backend.ldap.userSearchBase</name>
        <value>ou=people,dc=hadoop,dc=apache,dc=org</value>
    </property>
    <property>
        <name>gateway.ldap.backend.ldap.groupSearchBase</name>
        <value>ou=groups,dc=hadoop,dc=apache,dc=org</value>
    </property>
    <property>
        <name>gateway.ldap.backend.ldap.groupMemberAttribute</name>
        <value>member</value>
    </property>

knoxsso.xml:

<param>
  <name>main.ldapRealm.contextFactory.url</name>
  <value>ldap://localhost:33395</value>
</param>  

gateway-reloadable.xml

<configuration>
    <property>
        <name>gateway.ldap.port</name>
        <value>33395</value>
    </property>
</configuration>

Integration Tests

N/A

UI changes

N/A

[github-actions]

Test Results

21 tests   21 ✅  2s ⏱️
 1 suites   0 💤
 1 files     0 ❌

Results for commit e507d5c.