Sample standalone applications accessing Google Base data API feeds
This document assumes you know Java programming (including some basic knowledge of Http connections and SAX parsers) and that you are familiar with Google Base concepts. You need to understand the following concepts before you can take full advantage of the sample applications:
This tutorial consists of stepping through 5 examples. The first example shows how to connect to a Google Base data API feed and query data. The second example extends this by showing how to parse the result and extract data of interest from the feed. The third example introduces authentication and demonstrates how to display your own items, rather than querying snippets. The fourth example demonstrates how to insert your own item into Google Base, while the last example shows how to update a previously inserted item.
QueryExample1 is a simple Java application that runs from the
command line. It performs an unauthenticated query on the public snippets feed
(/feeds/snippets) and it dumps the query response to the console
(it won't look pretty!).
QueryExample1
javac sample.gbase/basic/QueryExample1.java
java sample.gbase/basic/QueryExample1
The output (conveniently formatted) will look like:
<feed>
<id>http://0.baseapitest.googlebase-api.jc.borg.google.com.:31911/base/feeds/snippets</id>
<updated>2006-08-22T14:14:11.984Z</updated>
<title type="text">Items matching query: cars [item type : products]</title>
<link rel="alternate" type="text/html" href="http://base.google.com"/>
<link rel="http://schemas.google.com/g/2005#feed" type="application/atom+xml" href="http://0.baseapitest.googlebase-api.jc.borg.google.com.:31911/base/feeds/snippets"/>
<link rel="self" type="application/atom+xml" href="http://0.baseapitest.googlebase-api.jc.borg.google.com.:31911/base/feeds/snippets?key=ABQ...9P2Y4A&bq=cars+%5Bitem+type+%3A+products%5D"/>
<link rel="next" type="application/atom+xml" href="http://0.baseapitest.googlebase-api.jc.borg.google.com.:31911/base/feeds/snippets?start-index=26&max-results=25&key=ABQ...9P2Y4A&bq=cars+%5Bitem+type+%3A+products%5D"/>
<generator version="1.0" uri="http://base.google.com">GoogleBase</generator>
<openSearch:totalResults>278120</openSearch:totalResults>
<openSearch:itemsPerPage>25</openSearch:itemsPerPage>
<entry>
<id>http://0.baseapitest.googlebase-api.jc.borg.google.com.:31911/base/feeds/snippets/10062394959501653657</id>
<published>2006-06-30T21:45:12.000Z</published>
<updated>2006-07-28T00:58:14.000Z</updated>
...
QueryExample1 codeAt the very beginning we define the url of the feed we are connecting to and the query that we are going to run:
private static final String SNIPPETS_FEED = "http://base.google.com/base/feeds/snippets"; private static final String QUERY = "cars [item type : products]";Feel free to change the query to a more relevant or interesting one. Take a look at the Google Base Query Language description if you need some inspiration.
After opening a connection on the snippets feed, we grab the connection's input stream, and dump its content to the output, character by character:
HttpURLConnection httpConnection = (HttpURLConnection)url.openConnection();
InputStream inputStream = httpConnection.getInputStream();
int ch;
while ((ch=inputStream.read()) > 0) {
System.out.print((char)ch);
}
In the main method, all we do is create a
QueryExample1 instance, and invoke its displayItems
method:
public static void main(String[] args) throws IOException, SAXException,
ParserConfigurationException {
new QueryExample1().displayItems();
}
One major problem with QueryExample1 is that it simply dumps
the items returned for the query to the console, and the result is barely
readable. In any real life application the query result would need to be
parsed, interpreted and relevant information should be extracted and
displayed to the user. In QueryExample2 we demonstrate a
possible way of doing this by using a SAX parser to extract each data item's
title from the result, and display it to the console. Some very basic
understanding of how SAX parsers work is necessary in order to fully
understand this example.
One might argue that for this task we don't even need a SAX parser. We
could just search for all <title> tags in the result and
display the characters that they enclose. Unfortunately, that wouldn't work,
as the Atom response also has a <title> tag, which is the
title of the feed:
<feed>
...
<title type="text">Items matching query: cars [item type : products]</title>
...
<entry>
...
<title type='text'>Great care for sale</title>
...
Atom does not mandate that the feed's <title> tag
should appear at a specific position inside the feed, so we need to make sure
we only display the <title> tags which are sub-elements of
an <entry> tag.QueryExample2
javac sample.gbase/basic/QueryExample2.java
java sample.gbase/basic/QueryExample2
The output will look like:
Item 1: Johnny Lightning MUSCLE CARS R8 1967 Chevelle SS
Item 2: Johnny Lightning MUSCLE CARS USA 2005 Ford GT
...
Item 25: The Cars movie Hinged tool Box Toy Organize lunch RARE
QueryExample2 codeJust as in the previous example, we first send the query to the Google
Base data API server, and obtain an inputStream containing the
response:
URL url = new URL(SNIPPETS_FEED + "?bq=" +
URLEncoder.encode(QUERY, "UTF-8"));
HttpURLConnection httpConnection = (HttpURLConnection) url.openConnection();
InputStream inputStream = httpConnection.getInputStream();
We then use a standard SAX parser to parse the result. We obtain a SAXParser instance using a SAXParserFactory:
SAXParserFactory factory = SAXParserFactory.newInstance();
SAXParser parser = factory.newSAXParser();
We then parse the result inputStream using
DisplayTitlesHandler, our custom SAX event handler:
parser.parse(inputStream, new DisplayTitlesHandler());
DisplayTitlesHandler is derived from the no-op SAX parser,
org.xml.sax.helpers.DefaultHandler. The logic inside
DisplayTitlesHandler is pretty simple: we keep a stack of the
currently open XML tags and print all character data when we are inside a
<title> tag with a <entry> parent. The
insideEntryTitle flag is turned on each time we are inside a
<entry><title> pair:
public void startElement(String uri, String localName, String qName, Attributes attributes) throws SAXException {
if (qName.equals("title") && xmlTags.peek().equals("entry")) {
insideEntryTitle = true;
System.out.print("Item " + ++itemNo + ": ");
}
xmlTags.push(qName);
}
Since startElement is invoked each time the SAX parser
encounters an opening XML tag, we switch on insideEntryTitle only
if the currently parsed opening tag is <title> and the tag
on the top of the stack is <entry> . We are also printing
here the "Item no: " messages, rather than in the characters
method, as characters can be called multiple times for different
chunks of the title's character data.endElement method is invoked each time an XML tag closes.
All we need to do here is to remove the closing XML tag from the stack and,
in case the removed XML tag was an entry's title, flip
insideEntryTitle to false and go to a new line in the console,
in preparation for printing out the next title:
public void endElement(String uri, String localName, String qName) throws SAXException {
xmlTags.pop();
if (insideEntryTitle) {
insideEntryTitle = false;
System.out.println();
}
}
The characters method is invoked when the parser encounters
character data inside an XML element. If we are inside the
<title> tag, that is, if the insideEntryTitle
flag is on, we display the current characters: length characters
from the ch array, starting with start. As noted
earlier, we can't use println here to get to a new line, as
characters can be invoked multiple times for different chunks of the
same title:
public void characters(char[] ch, int start, int length) throws SAXException {
if (insideEntryTitle) {
System.out.print(new String(ch, start, length));
}
}
In the main method, all we do is create a
QueryExample2 instance, and invoke its displayItems
method:
public static void main(String[] args) throws IOException, SAXException, ParserConfigurationException {
new QueryExample2().displayItems();
}
The previous two examples demonstrated how to query the Google Base data API public feeds, also known as snippets. Snippets are accessible to anyone without authentication. The downside is that snippets are the "indexable" version of the items and can slightly differ from the original items. It can also take a while for a newly inserted or updated item to show up in the snippets feed. Therefore, if you need to change your own items, the Google Base data API server exposes the customer-specific "items" feed. This feed is very similar to the "snippets" feed, except that:
QueryExample3 connects to your "items" feed, and dumps your items to the
console, just as in QueryExample1. If you don't have any items
in Google Base yet, use InsertExample to insert one,
or go to the Google
Base Provider Frontenac and insert one.
QueryExample3EMAIL and PASSWORD static strings:
private static final String EMAIL = "";
private static final String PASSWORD = "";
javac sample.gbase/basic/QueryExample3.java
java sample.gbase/basic/QueryExample3
<?xml version='1.0' encoding='UTF-8'?>
<feed>
<id>http://base.google.com/base/feeds/items</id>
<updated>2006-08-22T12:00:00.000Z</updated>
<title type="text">Items matching query: [customer id(int):1870031]</title>
...
QueryExample3 codeAs opposed to the previous examples, here we need to obtain an
authorization token by authenticating with the Google Accounts server. We
then use this authorization token to invoke displayMyItems:
public static void main(String[] args) throws IOException {
QueryExample3 queryExample = new QueryExample3();
String token = queryExample.authenticate();
new QueryExample3().displayMyItems(token);
}
We authenticate using authentication
for installed applications. The authentication procedure is simple. We
make a POST request to AUTHENTICATION_URL:
private static final String AUTHENTICATION_URL = "https://www.google.com/accounts/ClientLogin";The POST request is constructed in
makeLoginRequest, and it
looks like this:
// POST /accounts/ClientLogin HTTP/1.0
// Content-type: application/x-www-form-urlencoded
// Email=johndoe@gmail.com&Passwd=north23AZ&service=gbase&source=Insert Example
makeLoginRequest sends the request to the Google Accounts
server and returns the response as a String. A successful response
will have the following structure:
// HTTP/1.0 200 OK
// Server: GFE/1.3
// Content-Type: text/plain
// SID=DQAAAGgA...7Zg8CTN
// LSID=DQAAAGsA...lk8BBbG
// Auth=DQAAAGgA...dk3fA5N
authenticate first obtains the authentication response from
makeLoginRequest and stores it in postOutput:
String postOutput = null;
try {
URL url = new URL(AUTHENTICATION_URL);
postOutput = makeLoginRequest(url);
} catch (IOException e) {
System.out.println("Authentication error: " + e.toString());
}
It then tokenizes postOutput, and returns the next token
after "Auth", or null if the token is not found:
StringTokenizer tokenizer = new StringTokenizer(postOutput, "=\n ");
String token = null;
while (tokenizer.hasMoreElements()) {
if (tokenizer.nextToken().equals("Auth")) {
if (tokenizer.hasMoreElements()) {
token = tokenizer.nextToken();
}
break;
}
}
The items are displayed in displayMyItems, which is very
similar to displayItems in QueryExample1, except that it injects
the authorization token in the Http header (as in this example):
connection.setRequestProperty("Authorization", "GoogleLogin auth=" + token);
The previous examples demonstrated how to query the Google Base data API server, both for snippets (unauthenticated feeds) and for items (authenticated feeds, containing a specific customer's items). Let's demonstrate now how to add to Google Base all the cool stuff you have, so that the world can see it. Again, you will need a Google Account email and password in order to run this example. If you don't have a Google Account, sign up for one here.
In this example we will connect to your "items" feed, and do an Http POST
operation (as opposed to GET, used for querying) to add a new item. The item
to be added is encoded in Atom format, and is defined a String
constant:
private static final String DATA_ITEM =
"<?xml version=\'1.0\'?>\n" +
"<entry xmlns=\'http://www.w3.org/2005/Atom\' xmlns:g=\'http://base.google.com/ns/1.0\'>\n" +
" <category scheme=\'http://base.google.com/categories/itemtypes\' term=\'Products\'/>\n" +
" <g:item_type type=\'text\'>Products</g:item_type>\n" +
" <title type=\'text\'>My cool car is for sale</title>" +
" <content type=\'xhtml\'>Light pink, yellow seats.</content>" +
"</entry>";
It's a very simple item, consisting only of an item type ("products", in
our case), a title and a content (description). Feel free to change these
fields to contain your personalized items or to add new attributes and labels
(use the responses dumped by QueryExample1 and
QueryExample3 as an inspiration for adding new attributes).
InsertExampleEMAIL and PASSWORD static strings:
private static final String EMAIL = "";
private static final String PASSWORD = "";
javac sample.gbase/basic/InsertExample.java
java sample.gbase/basic/InsertExample
Obtained authorization token: DQAAAGgA...dk3fA5N
<?xml version='1.0' encoding='UTF-8'?>
<entry>
<id>http://base.google.com/base/feeds/items/16024998325761524417</id>
<published>2006-08-23T15:18:55.184Z</published>
<updated>2006-08-23T15:18:55.184Z</updated>
<category scheme="http://base.google.com/categories/itemtypes" term="Products"/>
<title type="text">My cool car is for sale</title>
<content type="xhtml">Light pink, yellow seats.</content>
<link rel="self" type="application/atom+xml" href="http://base.google.com/base/feeds/items/16024998325761524417"/>
<link rel="edit" type="application/atom+xml" href="http://base.google.com/base/feeds/items/16024998325761524417"/>
<g:item_type type="text">Products</g:item_type>
</entry>
InsertExample codeWe use the same feed as in QueryExample3.java to insert the item:
private static final String ITEMS_FEED = "http://base.google.com/base/feeds/items";Authentication is also performed as in QueryExample3, using
makeLoginRequest to
request an authorization token and authenticate to parse the
authentication response. The insertion of the new data item is done in
postItem, which connects to the items feed:
HttpURLConnection connection = (HttpURLConnection)(new URL(ITEMS_FEED)).openConnection();
Once the connection is created, we need to set its properties: the Http
request method, the content type of the information that is being posted, the
authorization header:
connection.setRequestMethod("POST");
connection.setRequestProperty("Content-Type", "application/atom+xml");
connection.setRequestProperty("Authorization", "GoogleLogin auth=" + token);
We then obtain the output stream of the connection and dump
DATA_ITEM into it:
OutputStream outputStream = connection.getOutputStream();
outputStream.write(DATA_ITEM.getBytes());
outputStream.close();
The rest of the method is already familiar: we obtain the response code
and print out to the console the contents of the input stream corresponding to
the response code.The previous examples demonstrated how to query the Google Base data API server both for authenticated and unauthenticated feeds and how to insert a new item. We will combine this knowledge in order to demonstrate how to update an already existing item. Again, you will need a Google Account email and password in order to run this example. If you don't have a Google Account, sign up for one here. This example does not introduce important new concepts, but rather uses all the essentials that have been presented in the previous examples: we will authenticate, insert an item, and then update the inserted item by adding a new label to it.
UpdateExampleEMAIL and PASSWORD static strings:
private static final String EMAIL = "";
private static final String PASSWORD = "";
javac sample.gbase/basic/UpdateExample.java
java sample.gbase/basic/UpdateExample
Obtained authorization token: DQAAAGgA...dk3fA5N
Posting item:
<?xml version='1.0'?>
<entry xmlns='http://www.w3.org/2005/Atom' xmlns:g='http://base.google.com/ns/1.0'>
<category scheme='http://base.google.com/categories/itemtypes' term='Products'/>
<g:item_type type='text'>Products</g:item_type>
<title type='text'>My cool car is for sale</title>
<content type='xhtml'>Light pink, yellow seats.</content>
</entry>
Updating item: http://base.google.com/base/feeds/items/18020038538902937385
<?xml version='1.0' encoding='UTF-8'?>
<entry>
<id>http://base.google.com/base/feeds/items/18020038538902937385</id>
<updated>2006-08-23T16:20:21.601Z</updated>
<category scheme="http://base.google.com/categories/itemtypes" term="Products"/>
<title type="text">My cool car is for sale</title>
<content type="xhtml">Light pink, yellow seats.</content>
<link rel="self" type="application/atom+xml" href="http://base.google.com/base/feeds/items/18020038538902937385"/>
<link rel="edit" type="application/atom+xml" href="http://base.google.com/base/feeds/items/18020038538902937385"/>
<g:item_type type="text">Products</g:item_type>
</entry>
UpdateExample codeThe main method of UpdateExample provides a good
outline of the example's structure:
public static void main(String[] args) throws MalformedURLException, IOException {
UpdateExample updateExample = new UpdateExample();
String token = updateExample.authenticate();
System.out.println("Obtained authorization token: " + token);
System.out.println("Posting item:\n" + DATA_ITEM);
String itemUrl = updateExample.extractItemUrlFromResponse(
updateExample.postItem(token));
System.out.println("Updating item: " + itemUrl);
String updateResponse = updateExample.updateItem(token, itemUrl);
System.out.println(updateResponse);
}
Just like in the previous examples, we start by authenticating and
obtaining an authorization token using the authenticate method:
String token = updateExample.authenticate();
We then insert DATA_ITEM using postItem:
String itemUrl = updateExample.extractItemUrlFromResponse(
updateExample.postItem(token));
In the line above, we pass the output of the post operation
toextractItemUrlFromResponse (rather than dumping it out to the
console), which extracts the inserted item's id:
<id>http://base.google.com/base/feeds/items/18020038538902937385</id>
We assume that the item's id is surrounded by the first
<id></id> tags; this is true for the Google Base data API servers,
but it's not enforced by the Atom protocol. See how the title gets parsed in
Query Example 2, for a superior approach on parsing
the item's title.
Once DATA_ITEM is successfully inserted, we replace it with
NEW_DATA_ITEM using updateItem. Updating an item is
very similar to inserting a new one - in fact so similar that both operations
can be performed by the same method: makeHttpRequest.
makeHttpRequest receives as parameters the authorization token,
the url to connect to, the item to be inserted or posted, the http method to
use (this will be POST for inserting, and PUT for deleting) and the response
code to expect in case of a successful operation (HTTP_CREATED in case of
insert, HTTP_OK in case of update). Thus, postItem will contain
a simple invocation to makeHttpRequest:
public String postItem(String token) throws IOException {
return makeHttpRequest(token, ITEMS_FEED, NEW_DATA_ITEM, "POST", HttpURLConnection.HTTP_CREATED);
}
Similarly, updateItem invokes makeHttpRequest
with slightly different parameters:
public String updateItem(String token, String itemUrl) throws MalformedURLException, IOException {
return makeHttpRequest(token, itemUrl, NEW_DATA_ITEM, "PUT", HttpURLConnection.HTTP_OK);
}