Author: Sarah Northway

Let’s face it: PayPal may be an unethical, bullying monopoly, but it’s a necessary evil when selling online. When we used PayPal to sell Fantastic Contraption, they suddenly froze our account because they were worried our digital goods (the full version of the game) might *poof* and disappear causing thousands of players to demand a refund.

PayPal has since then joined the modern age and added a new payment solution specifically for digital goods. It has lower rates for payments under $15 (5% + 5 cents, you just have to apply), and includes an optional login-free 2-click interface. You still need access to a browser window running Javascript, but it seems PayPal is making strides to support smaller, faster purchases.

I just added a PayPal micropayment system to Incredipede. I used to implement payment methods professionally, but for my own projects I prefer to keep things simple. The Java PayPal APIs I found were all too complicated, didn’t work with Google App Engine, or were out of date and incompatible with Digital Goods. So I rolled my own simple system which uses PayPal’s Express Checkout API. It has three public methods:

1. startPurchase – Begins the transaction and sends the user to PayPal
2. validateDetails – Verifies the payment when the user returns
3. finishPurchase – Completes the payment

I’ve simplified it down to one file for you with a bunch of inline constants and other bad practices. You may download PayPalManager.java (or copy & paste from below) and do as you will with it:

package com.incredipede.store;

import java.io.BufferedReader;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLDecoder;
import java.net.URLEncoder;
import java.util.HashMap;
import java.util.StringTokenizer;
import java.util.logging.Logger;

import javax.servlet.http.HttpServletResponse;

/**
 * Super simple implementation of Paypal for Digital Goods - Express Checkout API
 * Java NVP (name-value pairs) implementation (July 1012 version 69.0)
 * 
 * Using API and code samples from:
 * https://cms.paypal.com/cms_content/US/en_US/files/developer/PP_ExpressCheckout_IntegrationGuide_DG.pdf
 * https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_api_IntroducingExpressCheckoutDG
 * https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/howto_api_reference
 * https://www.paypal-labs.com/integrationwizard/ecpaypal/cart.php 
 *
 * Usage:
 *     When user presses a "Buy" button on your site:
 *         PayPalManager.startPurchase(2, 5, "Dapper Hat", A virtual item in the game Incredipede", "1.00", response);
 *     When user returns to RETURN_URL after authenitcating purchase on Paypal's website:
 *         String token = request.getParameter("token");
 *         int userId = Integer.parseInt(request.getParameter("userId"));
 *         int itemId = Integer.parseInt(request.getParameter("itemId"));
 *         // first check the status on paypal's system and make sure purchase is for reals
 *         String payerId = PayPalManager.validateDetails(token, userId, itemId);
 *         // next perform the purchase on your system to make sure it succeeds
 *         ...
 *         // finally take the user's money
 *         try {
 *             PayPalManager.finishPurchase(token, payerId, userId, itemId, "Dapper Hat", 
 *                 A virtual item in the game Incredipede", "1.00");
 *         } catch (Exception e) {
 *             // roll back the purchase on your system
 *             ...
 *         }
 * 
 * @author Sarah Northway
 */
public class PayPalManager
{
	// production creds
//	protected static String API_USERNAME = "xxxxxxxxxxxxxxxxxxxxxxxx";
//	protected static String API_PASSWORD = "XXXXXXXXXXXXX";
//	protected static String API_SIGNATURE = "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX";
//	protected static String API_URL = "https://api-3t.paypal.com/nvp";
//	protected static String REDIRECT_URL = "https://www.paypal.com/cgibin/webscr?cmd=_express-checkout";
	
	// sandbox creds
	protected static String API_USERNAME = "xxxxxxxxxxxxxxxxxxxxxxxx";
	protected static String API_PASSWORD = "XXXXXXXXXXXXX";
	protected static String API_SIGNATURE = "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX";
	protected static String API_URL = "https://api-3t.sandbox.paypal.com/nvp";
	protected static String REDIRECT_URL = "https://www.sandbox.paypal.com/webscr?cmd=_express-checkout";
	
	/** User will return to this page after the sale is successful */
	protected static String RETURN_URL = "http://127.0.0.1:8888/return;
	
	/** User will return here if they hit the cancel button during purchase */
	protected static String CANCEL_URL = "http://127.0.0.1:8888/cancel;

	protected final static Logger log = Logger.getLogger(PayPalManager.class.getName());
	
	/**
	 * Step 1: SetExpressCheckout
	 * 
	 * The first step of Express Checkout for Digital Goods: send a SetExpressCheckout
	 * request to PayPal and receive a token in response. Redirect the user to Paypal,
	 * then wait for their return through either the returnUrl or cancelUrl.
	 * 
	 * As of version 69.0, digital payments must set L_PAYMENTREQUEST_0_ITEMCATEGORY0=Digital, 
	 * must specify NOSHIPPING=1 and REQCONFIRMSHIPPING=0, 
	 * must use both AMT and ITEMAMT, and must have exactly one 
	 * payment (PAYMENTREQUEST_0_[...]) and one item (L_PAYMENTREQUEST_0_[...]0).
	 * https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_api_nvp_r_SetExpressCheckout
	 *
	 * @param userId The user's unique id in our system
	 * @param itemId The unique id in our system for the thing being bought
	 * @param itemName Shown in paypal as the name of the thing being bought eg "Dapper hat"
	 * @param itemDescription Shown in paypal beneath the item name eg "A virtual item in the game Incredipede"
	 * @param itemPriceDollars String price in USD must include decimal and two digits after eg "10.00"
	 */
	public static void startPurchase (int userId, int itemId, String itemName, String itemDescription, 
		String itemPriceDollars, HttpServletResponse resp)
	{
		// include the userId and itemId in the return urls so we can access them later
		String returnUrl = encodeValue(RETURN_URL + "&userId=" + userId + "&itemId=" + itemId);
		String cancelUrl = encodeValue(CANCEL_URL + "&userId=" + userId + "&itemId=" + itemId);
		
		String data = 
			"METHOD=SetExpressCheckout" +
			getAuthenticationData() +
			"&REQCONFIRMSHIPPING = 0" +
			"&NOSHIPPING = 1" +
			"&ALLOWNOTE = 0" +
			"&PAYMENTREQUEST_0_PAYMENTACTION=Sale" +
			"&PAYMENTREQUEST_0_CURRENCYCODE=USD" + 
			getPurchaseData(userId, itemId, itemName, itemDescription, itemPriceDollars) + 
			"&RETURNURL=" + returnUrl + 
			"&CANCELURL=" + cancelUrl +
			"";

		// tell paypal we want to start a purchase
		HashMap results = doServerCall(data);
		
		// forward the user on to payapal's site with the token identifying this transaction
		try {
			String token = results.get("TOKEN");
			String redirectUrl = resp.encodeRedirectURL(REDIRECT_URL + "&token=" + token);
			log.info("Sending user to paypal: " + redirectUrl);
			resp.sendRedirect(redirectUrl);
		} catch (IOException e) {
			e.printStackTrace();
			throw new RuntimeException("Failed to open PayPal link: " + e.getMessage());
		}
	}
	
	/**
	 * Step 2: GetExpressCheckoutDetails
	 * 
	 * Second step, performed when the user returns from paypal to validate the transaction
	 * details. If we cared about shipping info, the user's name etc it would be fetched here.
	 * Throws an exception if userId or purchase details don't match paypal's values, or if
	 * there's a problem with the purchase itself.
	 * https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_api_nvp_r_GetExpressCheckoutDetails
	 * 
	 * @param token The token created and returned by Paypal in step 1 (from the return url)
	 * @param userId The user's unique id in our system (from the return url)
	 * @param itemId The unique id in our system for the thing being bought (from the return url)
	 * @return Returns the user's paypal PayerId for use in the last step
	 */
	public static String validateDetails(String token, int userId, int itemId)
	{
		String data = 
			"METHOD=GetExpressCheckoutDetails" +
			getAuthenticationData() +
			"&TOKEN=" + encodeValue(token) +
			"";
	
		HashMap results = doServerCall(data);

		int resultsUserId = Integer.parseInt(results.get("PAYMENTREQUEST_0_CUSTOM"));
		if (resultsUserId != userId) {
			throw new RuntimeException("UserId does not match.");
		}
		
		int resultsItemId = Integer.parseInt(results.get("PAYMENTREQUEST_0_INVNUM"));
		if (resultsItemId != itemId) {
			throw new RuntimeException("ItemId does not match.");
		}
		
		String payerId = results.get("PAYERID");
		if (payerId == null || payerId.trim().length() == 0) {
			throw new RuntimeException("Payment has not been initiated by the user.");
		}
		
		return payerId;
	}
	
	/**
	 * Step 3: DoExpressCheckoutPayment
	 * 
	 * Completes the payment that has already been started and authorized by the user
	 * via the paypal website. Requires passing in purchase information again.
	 * https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_api_nvp_r_DoExpressCheckoutPayment
	 * 
	 * @param userId The user's unique id in our system
	 * @param itemId The unique id in our system for the thing being bought
	 * @param itemName Shown in paypal as the name of the thing being bought eg "Dapper hat"
	 * @param itemDescription Shown in paypal beneath the item name eg "A virtual item in the game Incredipede"
	 * @param itemPriceDollars String price in USD must include decimal and two digits after eg "10.00"
	 */
	public static void finishPurchase(String token, String payerId, int userId, int itemId, String itemName, 
		String itemDescription, String itemPriceDollars)
	{
		try {
			String data = 
				"METHOD=DoExpressCheckoutPayment" +
				getAuthenticationData() +
				"&TOKEN=" + encodeValue(token) +
				"&PAYERID=" + encodeValue(payerId) +
				getPurchaseData(userId, itemId, itemName, itemDescription, itemPriceDollars) + 
				"";
		
			HashMap results = doServerCall(data);
			
			// warn if transaction type isn't completed or on the way to completed
			String status = results.get("PAYMENTINFO_0_PAYMENTSTATUS");
			if (status == null || !(status.equalsIgnoreCase("Completed") 
				|| status.equalsIgnoreCase("In-Progress")
				|| status.equalsIgnoreCase("Processed") 
				|| status.equalsIgnoreCase("Completed-Funds-Held"))) {
				ActionHandler.log.warning("Unexpected paypal purchase status: " + status 
					+ " for userId=" + userId + ", paypal payerId=" + payerId 
					+ ", transaction=" + results.get("PAYMENTINFO_0_TRANSACTIONID"));
			}
			
		// must rollback purchase if anything happens here, so make sure we catch them all
		} catch (RuntimeException e) {
			e.printStackTrace();
			throw new RuntimeException(e.getMessage());
		}
	}
	
	/**
	 * Return the name-value-pair parameters required for SetExpressCheckout and 
	 * DoExpressCheckoutPayment steps.
	 *
	 * @param userId The user's unique id in our system
	 * @param itemId The unique id in our system for the thing being bought
	 * @param itemName Shown in paypal as the name of the thing being bought eg "Dapper hat"
	 * @param itemDescription Shown in paypal beneath the item name eg "A virtual item in the game Incredipede"
	 * @param itemPriceDollars String price in USD must include decimal and two digits after eg "10.00"
	 */
	protected static String getPurchaseData(int userId, int itemId, String itemName, 
		String itemDescription, String itemPriceDollars)
	{
		return 
			"&PAYMENTREQUEST_0_AMT=" + itemPriceDollars + 
			"&PAYMENTREQUEST_0_ITEMAMT=" + itemPriceDollars + 
			"&PAYMENTREQUEST_0_DESC=" + itemDescription + 
			"&PAYMENTREQUEST_0_CUSTOM=" + userId + 
			"&PAYMENTREQUEST_0_INVNUM=" + itemId + 
			"&L_PAYMENTREQUEST_0_NAME0=" + itemName + 
			"&L_PAYMENTREQUEST_0_DESC0=" + itemDescription + 
			"&L_PAYMENTREQUEST_0_AMT0=" + itemPriceDollars + 
			"&L_PAYMENTREQUEST_0_QTY0=" + 1 + 
			"&L_PAYMENTREQUEST_0_ITEMCATEGORY0=Digital" +
			"";
	}

	/**
	 * Return the name-value-pair parameters required for all paypal api calls 
	 * to authenticate the seller account.
	 */
	protected static String getAuthenticationData()
	{
		return 
			"&VERSION=69.0" +
			"&USER=" + API_USERNAME +
			"&PWD=" + API_PASSWORD +
			"&SIGNATURE=" + API_SIGNATURE +
			"";
	}
	
	/**
	 * Send off the given data to PayPal's API and return the result in key-value pairs.
	 * Validate the ACK return value from paypal and throw an exception if it isn't "Success".
	 */
	protected static HashMap doServerCall (String data)
	{
		log.info("Sending data to paypal: " + data);
	
		String response = "";
		try {
			URL postURL = new URL(API_URL);
			HttpURLConnection conn = (HttpURLConnection)postURL.openConnection();
			conn.setDoInput(true);
			conn.setDoOutput(true);
			conn.setConnectTimeout(3000);
			conn.setReadTimeout(7000);
			conn.setRequestMethod("POST");
			
			DataOutputStream output = new DataOutputStream(conn.getOutputStream());
			output.writeBytes(data);
			output.flush();
			output.close();

			// Read input from the input stream.
			int responseCode = conn.getResponseCode();
			if (responseCode != HttpURLConnection.HTTP_OK) {
				throw new RuntimeException("Error " + responseCode + ": " + conn.getResponseMessage());
			}
			
			BufferedReader reader = new BufferedReader(new InputStreamReader(conn.getInputStream()));
			String line = null;
			while(((line = reader.readLine()) !=null)) {
				response = response + line;
			}
			reader.close();
		} catch(IOException e) {
			e.printStackTrace();
			throw new RuntimeException(e.getMessage());
		}

		log.info("Got response from paypal: " + response);
		if(response.length() <= 0) {
			throw new RuntimeException("Received empty response");
		}
		HashMap results = parsePaypalResponse(response);
		
		// first check for the new version (PAYMENTINFO_0_ACK)
		String ackString = results.get("PAYMENTINFO_0_ACK");
		if (ackString == null || !(ackString.equalsIgnoreCase("Success") || ackString.equalsIgnoreCase("SuccessWithWarning"))) {
			String errorCode = results.get("PAYMENTINFO_0_ERRORCODE");
			String errorLongMsg = results.get("PAYMENTINFO_0_LONGMESSAGE");
			if (errorCode != null && errorCode.trim().length() > 0) {
				throw new RuntimeException("Purchase Failed (code " + errorCode + "): " + errorLongMsg);
			}
			
			// sometimes API returns old version ACK instead of PAYMENTINFO_0_ACK
			ackString = results.get("ACK");
			if (ackString == null || !(ackString.equalsIgnoreCase("Success") || ackString.equalsIgnoreCase("SuccessWithWarning"))) {
				errorCode = results.get("L_ERRORCODE0");
				errorLongMsg = results.get("L_LONGMESSAGE0");
				throw new RuntimeException("Purchase Failed (code " + errorCode + "): " + errorLongMsg);
			}
		}
		
		return results;
	}
	
	/**
	 * Parse results from PayPal to a map of name/value pairs. Their format looks like: 
	 * "TOKEN=EC%2d80X519901R8632201&TIMESTAMP=2012%2d07%2d13T09%3a57%3a44Z&ACK=Success"
	 */
	protected static HashMap parsePaypalResponse (String data)
	{
		HashMap results = new HashMap();
		StringTokenizer tokenizer = new StringTokenizer(data, "&");
		while (tokenizer.hasMoreTokens()) {
			StringTokenizer tokenizer2 = new StringTokenizer(tokenizer.nextToken(), "=");
			if (tokenizer2.countTokens() != 2) {
				continue;
			}
			String key = decodeValue(tokenizer2.nextToken());
			String value = decodeValue(tokenizer2.nextToken());
			results.put(key.toUpperCase(), value);
		}
		return results;
	}
	
	/**
	 * Prepare a given string for transmission via HTTP. Spaces become %20, etc.
	 */
	protected static String encodeValue(String value)
	{
		try {
			return URLEncoder.encode(value, "UTF-8");
		} catch (Exception e) {
			e.printStackTrace();
			return "";
		}
	}
	
	/**
	 * Undo encoding of string that was sent via HTTP. %20 becomes a space, etc.
	 */
	protected static String decodeValue(String value)
	{
		try {
			return URLDecoder.decode(value, "UTF-8");
		} catch (Exception e) {
			e.printStackTrace();
			return "";
		}
	}
}