CachingDynamicContentImpl.java

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package org.glassfish.appclient.server.core.jws.servedcontent;

import java.util.LinkedList;
import java.util.Properties;
import org.glassfish.appclient.server.core.jws.Util;
import org.glassfish.appclient.server.core.jws.servedcontent.DynamicContent.InstanceAdapter;

/**
 *
 * @author tjquinn
 */
/**
 *Represents dynamic content in template form.
 *<p>
 *This class also keeps track of the most recent times the object's template
 *was used to generate content that was different from the result of the
 *previous generation using this template.  This
 *information is used in responding to HTTP HEAD requests.  Java Web Start uses
 *HEAD requests to find out if a document on the server is more recent than
 *the locally cached copy on the client.  If so, then Java Web Start will 
 *request the updated version with a routine GET request. 
 *<p>
 *To avoid incorrectly reporting obsolete cached documents as current, this
 *class keeps track of when the content generated by the template is different
 *from the previous generation.  
 *<p>
 *The generated content can depend on request-time information
 *(such as command line arguments passed in the query string of the HTTP request).
 *We save and track only a few individual response instances, because the assumption
 *is that requests that carry query strings (which are converted into command line
 *arguments passed to ACC and on through to the app client) are likely to change
 *frequently and not necessarily be reused often.  Keeping a few allows caching
 *the different content resulting from a small number of different argument
 *value settings, but avoids the problems of caching every single response
 *which could become a large memory drain if each request specified a 
 *different set of arguments (for instance, one of the arguments could be a
 *timestamp that would change every time).
 */
public class CachingDynamicContentImpl extends Content.Adapter implements DynamicContent {

    /** maximum number of instances of content to keep for each template */
    private static final int DEFAULT_MAX_INSTANCES = 4;
    
    /**
     *the template which will be used at runtime to create the actual response
     *to the HTTP request
     */
    private final String template;

    /** the MIME type of the data represented by this CachingDynamicContentImpl instance */
    protected final String mimeType;

    /** content instances resulting from previous HTTP GET requests */
    private final LinkedList<Instance> instances = new LinkedList<Instance>();
    
    /** max. number of instances to cache */
    private final int maxInstances;

    /**
     * Returns a new instance of CachingDynamicContentImpl.
     * @param origin the ContentOrigin for the new content instance
     * @param contentKey the content key used to store and retrieve the content
     * @param path the path relative to the subcategory in which this document is addressable
     * @param mimeType the MIME type of data represented by the content generated by this
     * object.
     * @return new CachingDynamicContentImpl object
     */
    public CachingDynamicContentImpl(final String template, final String mimeType) {
        this(template, mimeType, DEFAULT_MAX_INSTANCES);
    }

    public CachingDynamicContentImpl(final String template, final String mimeType,
            final int maxInstances) {

        this.template = template;
        this.mimeType = mimeType;
        this.maxInstances = maxInstances;
    }

    /**
     * Returns the CachingDynamicContentImpl.InstanceImpl for this template corresponding to
     * the specified substitution token values.
     * @param tokenValues the name/value pairs to be substituted in the template
     * @param createIfAbsent selects whether a new CachingDynamicContentImpl.InstanceImpl should
     * be created for the resulting text if the content text resulting from the
     * substitution is not already cached by this CachingDynamicContentImpl.
     * @return the instance corresponding to the content generated by the tokenValues;
     * null if no such instance already exists for this CachingDynamicContentImpl and createIfAbsent
     * was false.
     */
    public Instance getExistingInstance(final Properties tokenValues) {
        return getOrCreateInstance(tokenValues, false);
    }

    public Instance getOrCreateInstance(final Properties tokenValues) {
        return getOrCreateInstance(tokenValues, true);
    }

    @Override
    public boolean isMain() {
        return false;
    }

    
    private Instance getOrCreateInstance(final Properties tokenValues, final boolean createIfAbsent) {
        /*
         * I generally avoid passing in flags to control the inner flow of a
         * method, but in this case we want the search and the optional addition
         * to take place atomically inside the synchronized block.  This way
         * the synchronized clause is in one place.
         */
        final String textWithPlaceholdersReplaced =
                Util.replaceTokens(template, tokenValues);

        /*
         * Look for an instance with its text matching the just-computed
         * replacement text.
         */
        Instance result = null;
        synchronized (instances) {
            for (Instance i : instances) {
                if (i.getText().equals(textWithPlaceholdersReplaced)) {
                    return i;
                }
            }
            
            if (createIfAbsent) {
                result = new InstanceAdapter(textWithPlaceholdersReplaced);
                addInstance(result);
            }
        }
        return result;
    }

    
    /**
     * Adds a new content instance to this dynamic content.  If adding the instance
     * makes the cache too long, discards the oldest instance.
     * @param newInstance the new instance to be added to the cache
     */
    private void addInstance(final Instance newInstance) {
        synchronized (instances) {
            instances.addFirst(newInstance);
            if (instances.size() > maxInstances) {
                instances.removeLast();
            }
        }
    }
    

    /**
     * Returns the MIME type associated with this content.
     * @return the MIME type for this content
     */
    public String getMimeType() {
        return mimeType;
    }

    /**
     * Clears the cached instances.
     */
    protected void clearInstances() {
        instances.clear();
    }
    
    /**
     * Returns a string representation of the CachingDynamicContentImpl.
     */
    @Override
    public String toString() {
        return super.toString() + ", template=" + template + ", MIME type=" + mimeType;// + ", most recent change in generated content=" + timestamp;
    }

    @Override
    public boolean equals(Object obj) {
        if (obj == null) {
            return false;
        }
        if (getClass() != obj.getClass()) {
            return false;
        }
        final CachingDynamicContentImpl other = (CachingDynamicContentImpl) obj;
        if ((this.template == null) ? (other.template != null) : !this.template.equals(other.template)) {
            return false;
        }
        if ((this.mimeType == null) ? (other.mimeType != null) : !this.mimeType.equals(other.mimeType)) {
            return false;
        }
        if (this.maxInstances != other.maxInstances) {
            return false;
        }
        return true;
    }

    @Override
    public int hashCode() {
        int hash = 7;
        hash = 89 * hash + (this.template != null ? this.template.hashCode() : 0);
        hash = 89 * hash + (this.mimeType != null ? this.mimeType.hashCode() : 0);
        hash = 89 * hash + this.maxInstances;
        return hash;
    }

    
}