package com.aem.demoproject.core.models;

import com.adobe.cq.export.json.ComponentExporter;
import com.adobe.cq.export.json.ExporterConstants;
import com.day.cq.wcm.api.Page;
import org.apache.sling.api.SlingHttpServletRequest;
import org.apache.sling.models.annotations.Exporter;
import org.apache.sling.models.annotations.Model;

import javax.annotation.Nonnull;
import javax.inject.Inject;

@Model(adaptables = SlingHttpServletRequest.class, adapters = {PageExporter.class,
        ComponentExporter.class}, resourceType = PageExporterImpl.RESOURCE_TYPE)
@Exporter(name = ExporterConstants.SLING_MODEL_EXPORTER_NAME, selector = "mobile", extensions = ExporterConstants.SLING_MODEL_EXTENSION)
public class PageExporterImpl implements PageExporter {

    protected static final String RESOURCE_TYPE = "demoproject/components/structure/page";

    @Inject
    private Page currentPage;

    @Nonnull
    @Override
    public String getExportedType() {
        return RESOURCE_TYPE;
    }

    @Override
    public String getLanguage() {
        return "en";
    }

    @Override
    public String getTitle() {
        return currentPage != null ? currentPage.getTitle() : "Default Title";
    }

    @Override
    public String getDescription() {
        return currentPage != null ? currentPage.getDescription() : "Default Desc";
    }

    public String getMessage(){
        return "Custom Page Exporter Impl";
    }
}

Thanks now its work. But now when I am using Sling model delegation pattern for retrieving page get exported items I am getting Null pointer exception.

@Model(adaptables = SlingHttpServletRequest.class, adapters = { PageExporter.class,
ComponentExporter.class, Page.class }, resourceType = PageExporterImpl.RESOURCE_TYPE, defaultInjectionStrategy=DefaultInjectionStrategy.OPTIONAL)
@3484101(name = ExporterConstants.SLING_MODEL_EXPORTER_NAME,selector = "mobile",extensions = ExporterConstants.SLING_MODEL_EXTENSION)
//@Model(adaptables = Resource.class, resourceType="/apps/contentservices/components/structure/page", defaultInjectionStrategy = DefaultInjectionStrategy.OPTIONAL)
//
//@Exporter(name = "jackson", selector="caas", extensions = "json", options = { @ExporterOption(name = "SerializationFeature.WRITE_DATES_AS_TIMESTAMPS", value = "true") })
public class PageExporterImpl implements PageExporter, Page{

protected static final String RESOURCE_TYPE = "contentservices/components/structure/page";

/** The logger. */
private final Logger logger = LoggerFactory.getLogger(PageExporterImpl.class);

/** The sling http servlet request. */
@1961677(injectionStrategy = InjectionStrategy.REQUIRED)
private SlingHttpServletRequest slingHttpServletRequest;

@ScriptVariable
protected com.day.cq.wcm.api.Page currentPage;

private Map<String, ComponentExporter> childModels = null;

protected String title;
protected String description;
@1961677 @2434638(type = ResourceSuperType.class)
private Page page;


@9944223
public String getTitle() {
return "title";
}
@9944223
public String getDescription() {
return description;
}

@9944223
public String getLanguage() {
return currentPage == null ? Locale.getDefault().toLanguageTag()
: currentPage.getLanguage(false).toLanguageTag();
}

@PostConstruct
protected void initModel() {
title = currentPage.getTitle();
description = currentPage.getDescription();

}


@9944223
public Map<String, ? extends ComponentExporter> getExportedItems() {
return page.getExportedItems();
}

@9944223
@JsonIgnore
public String getExportedType() {
// TODO Auto-generated method stub
return null;
}

}