Typical Plugins

public class MyBuildNumber extends BuildServerAdapter {

   public MyBuildNumber(SBuildServer aBuildServer) {
       aBuildServer.addListener(this);
   }

   public void buildStarted(SRunningBuild build) {

       // Build Numbers set to "my" in build number pattern will be replaced with custom build number
       if ("my".equals(build.getBuildNumber())) {
          build.setBuildNumber(createBuildNumber(build));
       }
   }

   public String createBuildNumber(SRunningBuild build) {
     return "my custom#";
   }
}

public class CustomLoginModule implements javax.security.auth.spi.LoginModule {
  private Subject mySubject;
  private CallbackHandler myCallbackHandler;
  private Callback[] myCallbacks;
  private NameCallback myNameCallback;
  private PasswordCallback myPasswordCallback;

  public void initialize(Subject subject, CallbackHandler callbackHandler, Map<String, ?> sharedState, Map<String, ?> options) {
    // We should remember callback handler and create our own callbacks.
    // TeamCity authorization scheme supports two callbacks only: NameCallback and PasswordCallback.
    // From these callbacks you will receive username and password entered on the login page.
    myCallbackHandler = callbackHandler;
    myNameCallback = new NameCallback("login:");
    myPasswordCallback = new PasswordCallback("password:", false);
    // remember references to newly created callbacks
    myCallbacks = new Callback[] {myNameCallback, myPasswordCallback};

    // Subject is a place where authorized entity credentials are stored.
    // When user is successfully authorized, the jetbrains.buildServer.serverSide.auth.ServerPrincipal
    // instance should be added to the subject. Based on this information the principal server will know a real name of
    // the authorized entity and realm where this entity was authorized.
    mySubject = subject;
  }

  public boolean login() throws LoginException {
    // invoke callback handler so that username and password were added
    // to the name and password callbacks
    try {
      myCallbackHandler.handle(myCallbacks);
    }
    catch (Throwable t) {
      throw new LoginException(t.toString());
    }

    // retrieve login and password
    final String login = myNameCallback.getName();
    final String password = new String(myPasswordCallback.getPassword());

    // perform authentication
    if (checkPassword(login, password)) {
      // create ServerPrincipal and put it in the subject
      mySubject.getPrincipals().add(new ServerPrincipal(null, login));
      return true;
    }

    return false;
  }

  private boolean checkPassword(final String login, final String password) {
    return true;
  }

  public boolean commit() throws LoginException {
    // simply return true
    return true;
  }

  public boolean abort() throws LoginException {
    return true;
  }

  public boolean logout() throws LoginException {
    return true;
  }
}

public class CustomLoginModuleDescriptor implements jetbrains.buildServer.serverSide.auth.LoginModuleDescriptor {
  // Spring framework will provide reference to LoginConfiguration when
  // the CustomLoginModuleDescriptor is instantiated
  public CustomLoginModuleDescriptor(LoginConfiguration loginConfiguration) {
    // register this descriptor in the login configuration
    loginConfiguration.registerLoginModule(this);
  }

  public Class<? extends LoginModule> getLoginModuleClass() {
    // return our custom login module class
    return CustomLoginModule.class;
  }

  public Map<String, ?> getOptions() {
    // return options which can be passed to our custom login module
    // for example, we could store reference to SBuildServer instance here
    return null;
  }

  public String getTextForLoginPage() {
    // return text to show on the login page which could describe
    // what should be entered in the user name and password fields
    return null;
  }

  public String getDisplayName() {
    // presentable name of this plugin
    return "My custom authentication plugin";
  }
}

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN//EN" "http://www.springframework.org/dtd/spring-beans.dtd">
<beans default-autowire="constructor">
  <bean id="customLoginModuleDescriptor" class="some.package.CustomLoginModuleDescriptor"/>
</beans>

public class CustomRunTypeImpl {
...
  public CustomRunType(RunTypeRegistry runTypeRegistry) {
    runTypeRegistry.registerRunType(this);
  }
...
}