core/src/main/java/org/apache/struts2/components/MergeIterator.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *  http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
package org.apache.struts2.components;

import com.opensymphony.xwork2.util.ValueStack;
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import org.apache.struts2.components.Param.UnnamedParametric;
import org.apache.struts2.util.MakeIterator;
import org.apache.struts2.util.MergeIteratorFilter;
import org.apache.struts2.views.annotations.StrutsTag;
import org.apache.struts2.views.annotations.StrutsTagAttribute;

import java.io.Writer;
import java.util.ArrayList;
import java.util.List;

/**
 * <!-- START SNIPPET: javadoc -->
 * <p>Component for MergeIteratorTag, which job is to merge iterators and successive
 * call to the merged iterator will cause each merge iterator to have a chance to
 * expose its element, subsequently next call will allow the next iterator to expose
 * its element. Once the last iterator is done exposing its element, the first iterator
 * is allowed to do so again (unless it is exhausted of entries).</P>
 *
 * <p>Internally the task are delegated to MergeIteratorFilter</p>
 *
 * <p>Example if there are 3 lists being merged, each list have 3 entries, the following will
 * be the logic.</P>
 * <ol>
 *      <li>Display first element of the first list</li>
 *      <li>Display first element of the second list</li>
 *      <li>Display first element of the third list</li>
 *      <li>Display second element of the first list</li>
 *      <li>Display second element of the second list</li>
 *      <li>Display second element of the third list</li>
 *      <li>Display third element of the first list</li>
 *      <li>Display thrid element of the second list</li>
 *      <li>Display third element of the thrid list</li>
 * </ol>
 * <!-- END SNIPPET: javadoc -->
 *
 * <!-- START SNIPPET: params -->
 * <ul>
 *          <li>var (String) - the name where the resultant merged iterator will be stored in the stack's context</li>
 * </ul>
 * <!-- END SNIPPET: params -->
 *
 *
 * <!-- START SNIPPET: javacode -->
 * public class MergeIteratorTagAction extends ActionSupport {
 *
 *  private List myList1;
 *  private List myList2;
 *  private List myList3;
 *
 *  public List getMyList1() {
 *      return myList1;
 *  }
 *
 *  public List getMyList2() {
 *      return myList2;
 *  }
 *
 *  public List getMyList3() {
 *      return myList3;
 *  }
 *
 *
 *  public String execute() throws Exception {
 *
 *      myList1 = new ArrayList();
 *      myList1.add("1");
 *      myList1.add("2");
 *      myList1.add("3");
 *
 *      myList2 = new ArrayList();
 *      myList2.add("a");
 *      myList2.add("b");
 *      myList2.add("c");
 *
 *      myList3 = new ArrayList();
 *      myList3.add("A");
 *      myList3.add("B");
 *      myList3.add("C");
 *
 *      return "done";
 *  }
 * }
 * <!-- END SNIPPET: javacode -->
 *
 * <!-- START SNIPPET: example -->
 * &lt;s:merge var="myMergedIterator1"&gt;
 *      &lt;s:param value="%{myList1}" /&gt;
 *      &lt;s:param value="%{myList2}" /&gt;
 *      &lt;s:param value="%{myList3}" /&gt;
 * &lt;/s:merge&gt;
 * &lt;s:iterator value="%{#myMergedIterator1}"&gt;
 *      &lt;s:property /&gt;
 * &lt;/s:iterator&gt;
 * <!-- END SNIPPET: example -->
 *
 * <!-- START SNIPPET: description -->
 * This wil generate "1aA2bB3cC".
 * <!-- START SNIPPET: description -->
 *
 * @see org.apache.struts2.util.MergeIteratorFilter
 * @see org.apache.struts2.views.jsp.iterator.MergeIteratorTag
 *
 */
@StrutsTag(name="merge", tldTagClass="org.apache.struts2.views.jsp.iterator.MergeIteratorTag", description="Merge the values " +
                "of a list of iterators into one iterator")
public class MergeIterator extends ContextBean implements UnnamedParametric {

    private static final Logger LOG = LogManager.getLogger(MergeIterator.class);

    private MergeIteratorFilter mergeIteratorFilter = null;
    private List _parameters;

    public MergeIterator(ValueStack stack) {
        super(stack);
    }

    public boolean start(Writer writer) {

        mergeIteratorFilter = new MergeIteratorFilter();
        _parameters = new ArrayList();

        return super.start(writer);
    }

    public boolean end(Writer writer, String body) {

        for (Object iteratorEntryObj : _parameters) {
            if (! MakeIterator.isIterable(iteratorEntryObj)) {
                LOG.warn("param with value resolved as {} cannot be make as iterator, it will be ignored and hence will not appear in the merged iterator", iteratorEntryObj);
                continue;
            }
            mergeIteratorFilter.setSource(MakeIterator.convert(iteratorEntryObj));
        }

        mergeIteratorFilter.execute();

        // if id exists, we put it in the stack's context
        putInContext(mergeIteratorFilter);

        mergeIteratorFilter = null;

        return super.end(writer, body);
    }

    @StrutsTagAttribute(description="The name where the resultant merged iterator will be stored in the stack's context")
    public void setVar(String var) {
        super.setVar(var);
    }

    // == UnnamedParametric interface implementation ---------------------
    public void addParameter(Object value) {
        _parameters.add(value);
    }
}