First Steps With EhCache

Thu, May 4, 2006

If you need to cache objects in your system, Ehcache is a simple cache written in Java, widely used and well tested. I will present here a short tutorial on how to use EhCache for people who don't want to look around the documentation at first, but just want to test if it works in their project and to see how easy it is to setup.

Installation

Download Ehcache from the Download link on http://ehcache.sourceforge.net. Current release is 1.2.

Unpack Ehcache with an unpacker that knows the tgz format. For unix users, it is trivial, for windows users, 7zip is a free (and open-source) unpacker. It is probably the most popular, but there are other ones like tugzip or izarc or winrar.

In your java project you need to have ehcache-1.2.jar, commons-collections-2.1.1.jar and commons-logging-1.0.4.jar (versions numbers may vary) in your classpath, those libraries are shipped with ehcache.

Cache Configuration

Write an ehcache.xml file where you describe what cache you want to use. There can be several files per project, several cache descriptions per file. I use here a persistent cache. Configuration file is well described at http://ehcache.sourceforge.net/documentation/configuration.html

</ehcache>

Code

static  {  
 //Create a CacheManager  using a specific  config file  
 cacheManager = CacheManager.create(TestClass.class.getResource( "/config/ehcache.xml"));
 cache = cacheManager.getCache("firstcache");
} 

/**
 * retrieves value  from cache if exists
 * if not  create it and add  it to cache */
public String doit(String key, String value) {
 //get an element  from cache by key
  Element e = cache.get(key);
 if (e != null)    {
   value  = (String)e.getValue();
   LOGGER.info("retrieved "+ value+" from cache ");
 }
  else   {
   value = "new value" ;
   cache.put(new Element(key, value));
 }
  return value;
}

/**
 * refresh value  for given key */
public void refresh(String key) { cache.remove(key); }

/**
 * to call eventually  when your application is  exiting  */
 public void shutdown() { cacheManager.shutdown(); }

Conclusion

Using EhCache is as simple a using a Java Map with an additional configuration file.

Null vs. Errors

Wed, Apr 26, 2006

I am not particularly a fan of JCS (Jakarta Cache System) as I find ehcache code very clean and simple. But I have to say the author has some good comments on the site:

Nulls vs. Errors

I started to support ObjectNotFoundExceptions for failed gets but the overhead and cumbersome coding needed to surround a simple get method is ridiculous. Instead the JCS return null.

For having seen too many times the ObjectNotFoundException "pattern", I can only agree!

Algorithms in Java (Third Edition) Book Review

Wed, Apr 26, 2006 The book Algorithms in Java is huge, but unlike the usual huge books, the content is very interesting. It can be used as reference material, or as toilet book (to learn things while you're wasting time in the toilets).

You will learn simple things, like what is the "raison d'être" of linked lists. The author gives very good examples to illustrate his propositions. He explains through the sieve of erathostene and through Josephus problem the advantages of arrays or linked list.

You will learn step by step everything that is to be known in algorithms. Recursion, divide and conquer, Tree knowledge will be useful for the later sorting and searching chapters.

The chapter on Hashing will make you understand very clearly why the source of String.hashCode() is

public int hashCode() {

  int h = hash;

  if (h == 0) {

     int off = offset;

     char val[] = value;

     int len = count;

     for (int i = 0; i <  len; i++) h = 31*h + val[off ++];

     hash = h;

  return h;

There might be too much info on different types of sorting algorithms and the book becomes there more a reference book than anything else. But overall, you will learn plenty with this book. It is very well written, complete, and will refresh one's memory. I find it useful to read back things I learnt after a few years as I then have a very different view of the subject, and I pay closer attention to some details I completely missed the first time (sometimes).
Tags: java book review

Caching HTTP Responses in Java

Wed, Apr 19, 2006

Caching HTTP response can dramatically improve performance of your app if what you generate is in reality not very dynamic. There are many free caching frameworks in Java. Most popular seem to be ehcache, oscache, jcs and JBoss Cache.

ehcache is quite simple to use and its code is clean. They have a CachingFilter that you can put in your webapp server to cache transparently HTTP responses. However as the framework only allows you to store Objects (which makes sense for most uses), I was wondering how they cached the HttpResponse which is a stream. I was a bit disappointed by the answer, they just create a copy ByteArrayOutputStream and call toBytes() to store it in the cache. While this is optimal for a memory cache store (the whole response will anyway be in the cache, although I am not sure if they check for particularly big responses to avoid caching those or try to cache those) I don’t think it is that good for a disk cache store.

Ideally one would like the response to be stored using a buffer, to avoid having the whole response in memory. This would enable a much higher concurrent use. I think it is doable by writing your own CachingFilter and by using the concurrent utils Queue to block writing when the buffer is full.

I googled for this kind of stuff without success. I only found solutions similar to ehcache one (for example sun CachingResponseWrapper and CachingFilter or oscache CacheFilter (a bit more careful, but still a toBytes())) I wonder why it is not already done and public.

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How To Use Java With Blogger: A Tutorial

Tue, Apr 18, 2006 Blogger has a REST API. I use it to retrieve particular posts or to post transformed data. There is no Java API that I know of, but you will see here it is not very difficult to interface with Blogger API in Java using plain old XML.

Using libraries commons-httpclient and DOM4J it would be quite easy to implement your own Java Blogger API as the following code will suggest.

Authenticate
All requests need to be authenticated and are done in HTTPS. I use common-httpclient to perform requests. Here is how to setup the client:

private  HttpClient initHttpClient()
{
  HttpClient client  = new HttpClient();
  List authPrefs =  new ArrayList(2);
  authPrefs.add(AuthPolicy.DIGEST );
  authPrefs.add(AuthPolicy.BASIC);
  client.getParams().setParameter (AuthPolicy.AUTH_SCHEME_PRIORITY, authPrefs);
  client.getParams().setAuthenticationPreemptive(true);
  Credentials defaultcreds = new UsernamePasswordCredentials(user, password); 
  client.getState().setCredentials(new AuthScope( "www.blogger.com", 443, AuthScope.ANY_REALM), defaultcreds);
  return client;          
}

Get Your Posts
To retrieve the posts, you just have to query the right url, and parse the XML response. I prefer to use DOM4J, because of its handy asXML() method to print a node as XML. For simplicity I use a Map to store an XML entry.

public Collection getPosts() throws  HttpException, IOException, ParserConfigurationException, SAXException, DocumentException 
{
  GetMethod get = new  GetMethod("https://www.blogger.com/atom" +"/"+blogId);
  int  statusCode = client.executeMethod(get);
  if   (statusCode !=  HttpStatus.SC_OK)
  {
    throw new RuntimeException(" Could not make HTTP  request properly: " +get.getStatusLine());
  }
  InputStream response  = get.getResponseBodyAsStream();
  SAXReader reader = new  SAXReader();
  Document doc = reader.read(response);
  Collection posts = new ArrayList();
  List entries  = doc.getRootElement().elements("entry");
  if (LOG.isDebugEnabled()) 
  {
    LOG.debug("found  "+entries.size()+"  entries");
  }
  for (int  i = 0; i <entries.size();i ++)
  {
    Element entry  = (Element) entries.get(i); 
    Map m = new  HashMap();
    for (Iterator it = entry.elementIterator();it. hasNext();)
    {
      Element detail =  (Element) it.next();
      String name = detail.getName();
      if (name.equals("link"))
      {
        m.put("link ",detail.attribute("href").getValue());
      }
      else if (name. equals("content"))
      {
        m.put("content",detail.asXML());
      }
      else
      { 
        m.put(name,detail.getTextTrim());
      } 
    }
    posts.add(m);                   
    if  (LOG.isDebugEnabled())
    {
      LOG.debug( "found="+m.get(" title")+", url= "+m.get("link")); 
    }
  }
  return  posts;
}

Create XML for a new Post
Nothing particular here, just XML production.

private String createXmlForCreatePost(String postTitle, String postContent) throws   IOException, DocumentException
{
  SAXReader xmlReader =  new SAXReader();
  xmlReader.setValidation(false );
  Document doc = DocumentHelper.createDocument();
  QName rootName  = DocumentHelper.createQName("entry", new  Namespace("", "http://purl.org/atom/ns# "));
  Element root = doc.addElement(rootName);
  Element title = root.addElement("title"); 
  title.addAttribute("mode"," escaped");
  title.addAttribute("type ","text/plain");
  title.setText (postTitle);
  Element generator = root.addElement("generator ");
  generator.addAttribute("url" , "http://31416.org");
  generator. setText("31416 Java Generator ");
  Element content = root.addElement("content ");
  content.addAttribute("type" , "application/xhtml+xml");
  //Element  div = content.addElement(DocumentHelper.createQName("div",new  Namespace("","http://www.w3.org/1999/xhtml")));
  //div.add(...); //YOUR XHTML HERE
  StringWriter result = new  StringWriter();
  XMLWriter writer = new XMLWriter (result);
  writer.write(doc);
  writer.close();
  return result.toString();
}

That's it

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Tue, Apr 18, 2006

Web Services No Better Than CORBA?

Mon, Apr 10, 2006

Richard Monson-Haefel (RMH), author of the famous Enterprise Java Beans (O'Reilly) and of a Web Services book is making an alarming claim about JAX-WS, the new Java web services standard:

JAX-WS still sucks!

In an effort to write a client to real world web services (for Google, Amazon, Ebay), he mostly failed. Errors seem to be largely related to WSDL to Java conversion. Jason Green, a JBoss developer, managed to get quickly a web service working for Ebay, but analysis by RMH suggest that hundreds of classes were generated for this simple one method web service.

Recently I had to try to do CORBA client classes by hand in Java since the rmic tool was not working that well to generate classes that could be run on a client JVM of different version than the server JVM. It proved to be a no-go in the end, because of the complexity involved not only in coding, but more on maintaining those modified generated classes. This would have required coding a tool dedicated for that purpose. We chose to pass simpler objects, differently, which looks just like a hack to avoid having CORBA issues. I am amazed that after that many years of CORBA, there are still simple cases where it does not work properly automatically.

Seeing that there is the same kind of problem with Web Services (IDL to Java translation) makes me wonder about Web Services "improvement" over CORBA. And WSDL is definitely less readable than CORBA IDL.

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