R – Excel with Reporting Services via WCF

You can use a library called ExcelLibrary. It's a free, open source library posted on Google Code:

ExcelLibrary

This looks to be a port of the PHP ExcelWriter that you mentioned above. It will not write to the new .xlsx format yet, but they are working on adding that functionality in.

It's very simple, small and easy to use. Plus it has a DataSetHelper that lets you use DataSets and DataTables to easily work with Excel data.

ExcelLibrary seems to still only work for the older Excel format (.xls files), but may be adding support in the future for newer 2007/2010 formats.

You can also use EPPlus, which works only for Excel 2007/2010 format files (.xlsx files). There's also NPOI which works with both.

There are a few known bugs with each library as noted in the comments. In all, EPPlus seems to be the best choice as time goes on. It seems to be more actively updated and documented as well.

Also, as noted by @АртёмЦарионов below, EPPlus has support for Pivot Tables and ExcelLibrary may have some support (Pivot table issue in ExcelLibrary)

Here are a couple links for quick reference:
ExcelLibrary - GNU Lesser GPL
EPPlus - GNU (LGPL) - No longer maintained
EPPlus 5 - Polyform Noncommercial - Starting May 2020
NPOI - Apache License

Here some example code for ExcelLibrary:

Here is an example taking data from a database and creating a workbook from it. Note that the ExcelLibrary code is the single line at the bottom:

//Create the data set and table
DataSet ds = new DataSet("New_DataSet");
DataTable dt = new DataTable("New_DataTable");

//Set the locale for each
ds.Locale = System.Threading.Thread.CurrentThread.CurrentCulture;
dt.Locale = System.Threading.Thread.CurrentThread.CurrentCulture;

//Open a DB connection (in this example with OleDB)
OleDbConnection con = new OleDbConnection(dbConnectionString);
con.Open();

//Create a query and fill the data table with the data from the DB
string sql = "SELECT Whatever FROM MyDBTable;";
OleDbCommand cmd = new OleDbCommand(sql, con);
OleDbDataAdapter adptr = new OleDbDataAdapter();

adptr.SelectCommand = cmd;
adptr.Fill(dt);
con.Close();

//Add the table to the data set
ds.Tables.Add(dt);

//Here's the easy part. Create the Excel worksheet from the data set
ExcelLibrary.DataSetHelper.CreateWorkbook("MyExcelFile.xls", ds);

Creating the Excel file is as easy as that. You can also manually create Excel files, but the above functionality is what really impressed me.

To get started with WCF, it might be easiest to just use the default SOAP format and HTTP POST (rather than GET) for the web-service bindings. The easiest HTTP binding to get working is "basicHttpBinding". Here is an example of what the ServiceContract/OperationContract might look like for your login service:

[ServiceContract(Namespace="http://mycompany.com/LoginService")]
public interface ILoginService
{
    [OperationContract]
    string Login(string username, string password);
}

The implementation of the service could look like this:

public class LoginService : ILoginService
{
    public string Login(string username, string password)
    {
        // Do something with username, password to get/create sessionId
        // string sessionId = "12345678";
        string sessionId = OperationContext.Current.SessionId;

        return sessionId;
    }
}

You can host this as a windows service using a ServiceHost, or you can host it in IIS like a normal ASP.NET web (service) application. There are a lot of tutorials out there for both of these.

The WCF service config might look like this:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>


    <system.serviceModel>
        <behaviors>
            <serviceBehaviors>
                <behavior name="LoginServiceBehavior">
                    <serviceMetadata />
                </behavior>
            </serviceBehaviors>
        </behaviors>

        <services>
            <service name="WcfTest.LoginService"
                     behaviorConfiguration="LoginServiceBehavior" >
                <host>
                    <baseAddresses>
                        <add baseAddress="http://somesite.com:55555/LoginService/" />
                    </baseAddresses>
                </host>
                <endpoint name="LoginService"
                          address=""
                          binding="basicHttpBinding"
                          contract="WcfTest.ILoginService" />

                <endpoint name="LoginServiceMex"
                          address="mex"
                          binding="mexHttpBinding"
                          contract="IMetadataExchange" />
            </service>
        </services>
    </system.serviceModel>
</configuration>

(The MEX stuff is optional for production, but is needed for testing with WcfTestClient.exe, and for exposing the service meta-data).

You'll have to modify your Java code to POST a SOAP message to the service. WCF can be a little picky when inter-operating with non-WCF clients, so you'll have to mess with the POST headers a little to get it to work. Once you get this running, you can then start to investigate security for the login (might need to use a different binding to get better security), or possibly using WCF REST to allow for logins with a GET rather than SOAP/POST.

Here is an example of what the HTTP POST should look like from the Java code. There is a tool called "Fiddler" that can be really useful for debugging web-services.

POST /LoginService HTTP/1.1
Content-Type: text/xml; charset=utf-8
SOAPAction: "http://mycompany.com/LoginService/ILoginService/Login"
Host: somesite.com:55555
Content-Length: 216
Expect: 100-continue
Connection: Keep-Alive

<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
<s:Body>
<Login xmlns="http://mycompany.com/LoginService">
<username>Blah</username>
<password>Blah2</password>
</Login>
</s:Body>
</s:Envelope>