how to mock apache-camel EL

i have the following RouteBuilder.

 public class MyRoute extends RouteBuilder {    

        @Override
        public void configure() {
            errorHandler(deadLetterChannel(errorHandlerEndpoint)
                    .maximumRedeliveries(1)
                    .redeliveryDelay(2000)
                    .retryAttemptedLogLevel(LoggingLevel.WARN));

        from(fromEndpoint).routeId("fetchPreprocessingResultFromMarklogicRoute")
         .setHeader("principal").method(principalService, "getPrincipal").id("getPrincipalServiceMethod") // put principal to header for latter use
         .multicast()
            .filter(xpath("//files[@ml-generated='true']"))
            .choice()
                    .when(PredicateBuilder.isEqualTo(constant(EPrincipal.VETCENTER.principal()), simple("${header.principal.principal.principal}")))
                        .log("Sending VetCenter preprocessing report...")
                        .inOnly(toVetCenterEmailNotificationEndpoint)
                    .when(PredicateBuilder.isEqualTo(constant(EPrincipal.OSTEOTHEK.principal()), simple("${header.principal.principal.principal}")))
                        .log("Sending Osteothek preprocessing report...")
                        .inOnly(toOsteothekEmailNotificationEndpoint)
                    .otherwise()
                        .log("Principal unknown. Don't sending a preprocessing report...")
                        .inOnly("direct:trash") // @ToDo: Write test
                .end()
                .inOnly(toCleaningPhaseDecisionEndpoint)
                .inOnly(deleteImportDirsEndpoint)
            .end();

        }
}

This is the JUnit-Test:

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(locations = {"classpath:application-context-test.xml"}) 
public class FetchPreprocessingResultFromMarklogicRoute_Test extends CamelTestSupport{
    private RouteBuilder route;

    @Value("${fetchPreprocessingResultFromMarklogicRoute.fromEndpoint}")
    private String testFromEndpoint;

    @Value("${fetchPreprocessingResultFromMarklogicRoute.toCleaningPhaseDecisionEndpoint}")
    private String toCleaningPhaseDecisionEndpoint;
    @Value("${fetchPreprocessingResultFromMarklogicRoute.vetcenter.toEmailNotificationEndpoint}")
    private String toEmailNotificationVetCenterEndpoint;
    @Value("${fetchPreprocessingResultFromMarklogicRoute.osteothek.toEmailNotificationEndpoint}")
    private String toEmailNotificationOsteothekEndpoint;
    @Value("${fetchPreprocessingResultFromMarklogicRoute.deleteImportDirsEndpoint}")
    private String testDeleteImportDirsEndpoint;
    @Value("${fetchPreprocessingResultFromMarklogicRoute.errorHandlerEndpoint}")
    private String testErrorHandlerEndpoint;

    private MockEndpoint emailNotificationVetCenterMOCKEndpoint;
    @Mock
    private PrincipalService principalServiceMOCK; 

    @Override
    protected RouteBuilder createRouteBuilder() throws Exception {
        if(route == null){

            route = new MyRoute(
                    testFromEndpoint, 
                    toCleaningPhaseDecisionEndpoint,
                    toEmailNotificationVetCenterEndpoint,
                    toEmailNotificationOsteothekEndpoint,
                    testDeleteImportDirsEndpoint, 
                    testErrorHandlerEndpoint,
                    principalServiceMOCK);
        }
        return route;
    }

    @Override
    public void setUp() throws Exception {
        MockitoAnnotations.initMocks(this);

        Principal principal = new Principal();
        principal.setPrincipal(EPrincipal.VETCENTER);

        Mockito.when(principalServiceMOCK.getPrincipal("anyIdentificator")).thenReturn(principal);
        super.setUp();
    }


    @Test
    public void test() throws InterruptedException{
        InputStream resourceInputStream = Thread.currentThread().getContextClassLoader().getResourceAsStream("testdata/importreport/inc/import-log-Vetcenter.xml");
        String message = new Scanner(resourceInputStream).useDelimiter("\\A").next();
        emailNotificationVetCenterMOCKEndpoint = getMockEndpoint(toEmailNotificationVetCenterEndpoint);

        emailNotificationVetCenterMOCKEndpoint.expectedBodiesReceived(message);
        // Herausforderung ist, die Datei aus dem  Verzeichnis ablesen und camel Regeln zu prüfen
        template.sendBody(testFromEndpoint, message);

        emailNotificationVetCenterMOCKEndpoint.assertIsSatisfied();
    }
}

i don't know how to make a mock for this snippet:

.when(PredicateBuilder.isEqualTo(constant(EPrincipal.VETCENTER.principal()), simple("${header.principal.principal.principal}")))

more exact for this part:

simple("${header.principal.principal.principal}")

It is expression language and i have no idiea how to create a mock for it. Is there other solution to create a unit test for it?

very many thanks for help