Download Excel file from GWT client - servlets

Question

I need to generate on my GWT application an xls file (generated on server side with apache-poi) from client.
I want that when the user click on a button, appears a file chooser the allow him to save the generated file.
To begin I've created my servlet:
public class DownloadServlet extends HttpServlet
{
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException
{
// What I have to insert here?!
}
public void getXlsFile()
{
XSSFWorkbook workbook = new XSSFWorkbook();
XSSFSheet sheet = workbook.createSheet("Datatypes in Java");
Object[][] datatypes = { { "Datatype", "Type", "Size(in bytes)" }, { "int", "Primitive", 2 }, { "float", "Primitive", 4 }, { "double", "Primitive", 8 }, { "char", "Primitive", 1 }, { "String", "Non-Primitive", "No fixed size" } };
int rowNum = 0;
for (Object[] datatype : datatypes) {
Row row = sheet.createRow(rowNum++);
int colNum = 0;
for (Object field : datatype) {
Cell cell = row.createCell(colNum++);
if( field instanceof String ) {
cell.setCellValue((String) field);
}
else if( field instanceof Integer ) {
cell.setCellValue((Integer) field);
}
}
}
try {
FileOutputStream outputStream = new FileOutputStream("MyFirstExcel.xlsx");
workbook.write(outputStream);
workbook.close();
}
catch (FileNotFoundException e) {
e.printStackTrace();
}
catch (IOException e) {
e.printStackTrace();
}
}
}
Now the questions are:
How do I have to fill doGet method?
And how I can call this servlet from client side?
Note that client side I don't know the xls file path because I want to create the xls file "on the fly" on server side.

Answer 1

You have created a servlet and have your xls file ready. All you have to do is to push the data into HttpServletResponse object.
First, you don't need to save the file. Change the getXlsFile() method to return XSSFWorkbook and remove the try / catch block at the end.
Now, doGet method:
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
HSSFWorkbook workbook = getXlsFile();
String fileName = "MyFirstExcel.xlsx";
resp.setStatus(HttpServletResponse.SC_OK);
resp.setHeader("Content-disposition", "attachment; filename=\"" + fileName + "\"");
resp.setContentType("application/vnd.ms-excel");
resp.getOutputStream().write(workbook.getBytes());
resp.getOutputStream().close();
resp.flushBuffer();
workbook.close();
}
How to call the server?
You need to add a servlet mapping in web.xml file:
<servlet>
<servlet-name>myServlet</servlet-name>
<servlet-class>fully.qualified.className</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>myServlet</servlet-name>
<url-pattern>/path/to/the/servlet</url-pattern>
</servlet-mapping>
In short: servlet mapping maps the url to the servlet class. So, when you open /path/to/the/servlet the fully.qualified.className servlet is called.