Thanks for sharing @vikrams57194913 .

Have a backup of your model class separately and use the below (have used the PageExporter interface similar to yours), see if it works. 

getMessage is just added for testing if it is from this impl (Its not part of the interface)

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";
    }
}

Result :