diff -r 1612dbcec451 -r 639991d0808a java/cewolf-1.0/src/main/java/de/laures/cewolf/DatasetProducer.java
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/java/cewolf-1.0/src/main/java/de/laures/cewolf/DatasetProducer.java	Sat Feb 28 21:31:02 2009 +0100
@@ -0,0 +1,87 @@
+/* ================================================================
+ * Cewolf : Chart enabling Web Objects Framework
+ * ================================================================
+ *
+ * Project Info:  http://cewolf.sourceforge.net
+ * Project Lead:  Guido Laures (guido@laures.de);
+ *
+ * (C) Copyright 2002, by Guido Laures
+ *
+ * This library is free software; you can redistribute it and/or modify it under the terms
+ * of the GNU Lesser General Public License as published by the Free Software Foundation;
+ * either version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
+ * without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License along with this
+ * library; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330,
+ * Boston, MA 02111-1307, USA.
+ */
+
+package de.laures.cewolf;
+
+import java.io.Serializable;
+import java.util.Date;
+import java.util.Map;
+
+/**
+ * Produces a {@link org.jfree.data.Dataset} which will be rendered 
+ * as a chart 	afterwards.
+ * @see org.jfree.data.Dataset
+ * @author  Guido Laures
+ * @since 0.1
+ */
+public interface DatasetProducer extends Serializable {
+    
+    /**
+     * By default the the name of the JSP attribute 
+     * holding the producer instance is passed to the 
+     * produceDataset method as a prameter.
+     */
+    public static final String PRODUCER_ATTRIBUTE_NAME = "de.laures.cewolf.DatasetProducer.id";
+	
+    /**
+     * Produces a {@link org.jfree.data.Dataset} object.
+     * @param params additional params for the dataset production. All elements
+     * of this HashMap are of type <code>java.io.Serializable</code>. This is
+     * necessary to ensure the the serialization of the dataset producer into
+     * the http session. To provide a producer with additional production
+     * parameters the &lt;param&gt; tag is used (see tag library documentation).
+     * It is recommended to synchronize implementations of this method to avoid
+     * concurrency problems.
+     * @return an object of type <code>org.jfree.data.Dataset</code>.
+     * @throws DatasetProduceException if an error occured during production
+     * @since 0.2
+     */
+    Object produceDataset(Map params) throws DatasetProduceException;
+	
+	/**
+	 * This method is called by the Cewolf framework to check if a formerly
+	 * produced data can be reused. If the data which had already been used
+	 * for chart rendering is still valid this method should return <code>true</code>.
+	 * If possible the Cewolf framework will try to reuse the rendered chart
+	 * image. If this is not possible because of some circumstances (e.g. the chart
+	 * had been removed from the image cache) the produceDataset method is called afterwards.
+	 * Therefore there is no guarantee that the dataset production is always 
+	 * avoided if this method returns <code>true</true>.
+	 * @param params the production parameters of the already produced data
+	 * @param since the point in time when the already produced data had been produced
+	 * @return <code>true</code> if the data which had been produced with the 
+	 * passed in parameters has expired since its creation, <code>false</code> 
+	 * otherwise
+	 * @since 0.9
+	 */
+	boolean hasExpired(Map params, Date since);
+	
+	/**
+	 * Tis method returns a unique ID for a DatasetProducer from this class.
+	 * Producers with the same ID are supposed to produce the same data when
+	 * called with the same paramters.
+	 * @return the unique ID for instances of this poducer class
+	 * @since 0.9
+	 */
+	String getProducerId();
+
+}