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In this final section, we demonstrate the life cycle of a portlet that uses background threads of execution in a web crawler application.
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Handling Concurrent Requests
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Because the portlet container will handle concurrent requests from clients by invoking the methods on the portlet on separate threads of execution, your portlet must be able to handle any combination and number of simultaneous calls to render() and/or processAction(). You must therefore implement your portlet to handle these concurrent requests safely. In practice this is not usually too tricky all the information you need to process a request is provided in a thread-safe manner in the parameter list, so if your portlets don t use instance variables and they don t access other resources external to the portlet, your application will automatically be thread-safe.
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It is guaranteed that your init() method will be called only once at the beginning of the life cycle and that no other methods will be invoked by the container until init() completes successfully, so your init() method does not have to be thread safe. Your render() and processAction() methods will be invoked with request and response objects. These are guaranteed to be unique to that invocation of the method during the lifetime of the method. Containers are likely to recycle these objects once the method in question has completed, so retaining a reference to them outside the scope of the method to which they were passed may result in unexpected behavior.
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Our Thread-Safe Crawler
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Our crawler class is implemented to be thread safe. It implements Runnable so it can be created and started within a background thread. Once the crawler is running, it can be queried at any time. The get methods return unmodifiable sets so it is not possible for the client to externally alter their state. The crawler can be stopped by an external thread by calling the stopCrawler() method. This is essential so that our portlet can be unloaded safely. Our crawler implementation follows, with a running commentary. Although this illustrates the functionality that s needed in a web crawler, you should note that it is a demonstration application only. We make a lot of assumptions and take shortcuts that would not be acceptable in a commercial product.
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package com.portalbook.crawler; import java.io.*; import java.net.*; import java.util.*; public class Crawler implements Runnable {
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Our simplest constructor creates an instance to crawl a given path. This will search only within the host of the path specified:
public Crawler(String path) throws MalformedURLException { this(path,DEFAULT_LINK_DEPTH); }
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The Portlet Life Cycle
This more complex constructor creates an instance to crawl a given path. It will search within the host of the path specified, and up to the specified number of sites (depth) away. If depth is 2, the crawler will look within the host of the path specified and within the hosts of sites referenced directly from this site, but no further:
public Crawler(String path, int depth) throws MalformedURLException { this(new HashSet( Arrays.asList( new Object[] { new URL(path) })), new HashSet(), new HashSet(), new HashSet(), new HashSet(), depth); }
This internal constructor is used directly or indirectly by the public ones to create the instance to crawl a given set of paths. It will search within the hosts specified, and up to the specified number of sites away from those sites. It will not search forbidden hosts, failed hosts, or already visited hosts:
protected Crawler( Set links, Set visited, Set visitedHosts, Set forbidden, Set failed, int depth ) { this.links this.visited this.forbidden this.failed this.currentHost this.depth } = links; = visited; = forbidden; = failed; = = depth;
this.visitedHosts = visitedHosts;
(URL)links.iterator().next();
Our crawler is designed to run as a thread, so it implements the Runnable interface. It can therefore be passed in as a parameter to a new Thread object.
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