abstract protected JRExporter createExporter()
Create a JasperReports exporter for a specific output format,
which will be used to render the report to the HTTP response.
The useWriter method determines whether the
output will be written as text or as binary content.

 protected boolean generatesDownloadContent() 
{
	   
		return !useWriter();
}

 protected Map mergeExporterParameters(Map model) 
{
		Map mergedParameters = new HashMap();
		Map convertedExporterParameters = getConvertedExporterParameters();
		if (!CollectionUtils.isEmpty(convertedExporterParameters)) {
			mergedParameters.putAll(convertedExporterParameters);
		}
		for (Iterator it = model.keySet().iterator(); it.hasNext();) {
			Object key = it.next();
			if (key instanceof JRExporterParameter) {
				Object value = model.get(key);
				Object convertedValue = convertParameterValue((JRExporterParameter) key, value);
				mergedParameters.put(key, convertedValue);
			}
		}
		return mergedParameters;
}

Merges the configured JRExporterParameters with any specified in the supplied model data. JRExporterParameters in the model override those specified in the configuration.

 protected  void renderReport(JasperPrint populatedReport,
    Map model,
    HttpServletResponse response) throws Exception 
{
		JRExporter exporter = createExporter();
		// Set exporter parameters - overriding with values from the Model.
		Map mergedExporterParameters = mergeExporterParameters(model);
		if (!CollectionUtils.isEmpty(mergedExporterParameters)) {
			exporter.setParameters(mergedExporterParameters);
		}
		if (useWriter()) {
			renderReportUsingWriter(exporter, populatedReport, response);
		}
		else {
			renderReportUsingOutputStream(exporter, populatedReport, response);
		}
}

Perform rendering for a single Jasper Reports exporter, that is, for a pre-defined output format.

 protected  void renderReportUsingOutputStream(JRExporter exporter,
    JasperPrint populatedReport,
    HttpServletResponse response) throws Exception 
{
		// Apply the content type as specified - we don't need an encoding here.
		response.setContentType(getContentType());
		// Render report into local OutputStream.
		// IE workaround: write into byte array first.
		ByteArrayOutputStream baos = new ByteArrayOutputStream(OUTPUT_BYTE_ARRAY_INITIAL_SIZE);
		JasperReportsUtils.render(exporter, populatedReport, baos);
		// Write content length (determined via byte array).
		response.setContentLength(baos.size());
		// Flush byte array to servlet output stream.
		ServletOutputStream out = response.getOutputStream();
		baos.writeTo(out);
		out.flush();
}

We need to write binary output to the response OutputStream.

 protected  void renderReportUsingWriter(JRExporter exporter,
    JasperPrint populatedReport,
    HttpServletResponse response) throws Exception 
{
		// Copy the encoding configured for the report into the response.
		String contentType = getContentType();
		String encoding = (String) exporter.getParameter(JRExporterParameter.CHARACTER_ENCODING);
		if (encoding != null) {
			// Only apply encoding if content type is specified but does not contain charset clause already.
			if (contentType != null && contentType.toLowerCase().indexOf(WebUtils.CONTENT_TYPE_CHARSET_PREFIX) == -1) {
				contentType = contentType + WebUtils.CONTENT_TYPE_CHARSET_PREFIX + encoding;
			}
		}
		response.setContentType(contentType);
		// Render report into HttpServletResponse's Writer.
		JasperReportsUtils.render(exporter, populatedReport, response.getWriter());
}

We need to write text to the response Writer.

 abstract protected boolean useWriter()
Return whether to use a java.io.Writer to write text content
to the HTTP response. Else, a java.io.OutputStream will be used,
to write binary content to the response.