FlowExecutionContinuationGroup


1   /*
2    * Copyright 2002-2006 the original author or authors.
3    *
4    * Licensed under the Apache License, Version 2.0 (the "License");
5    * you may not use this file except in compliance with the License.
6    * You may obtain a copy of the License at
7    *
8    *      http://www.apache.org/licenses/LICENSE-2.0
9    *
10   * Unless required by applicable law or agreed to in writing, software
11   * distributed under the License is distributed on an "AS IS" BASIS,
12   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13   * See the License for the specific language governing permissions and
14   * limitations under the License.
15   */
16  package org.springframework.webflow.execution.repository.continuation;
17  
18  import java.io.Serializable  ;
19  import java.util.HashMap  ;
20  import java.util.LinkedList  ;
21  import java.util.Map  ;
22  
23  /**
24   * A group of flow execution continuations. Simple typed data structure backed
25   * by a map and linked list. Supports expelling the oldest continuation once a
26   * maximum group size is met.
27   * 
28   * @author Keith Donald
29   */
30  class FlowExecutionContinuationGroup implements Serializable   {
31  
32      /**
33       * A map of continuations; the key is the continuation id, and the value is
34       * the {@link FlowExecutionContinuation} object.
35       */
36      private Map   continuations = new HashMap  ();
37  
38      /**
39       * An ordered list of continuation ids. Each continuation id represents an
40       * pointer to a continuation in the map. The first element is the oldest 
41       * continuation and the last is the youngest. 
42       */
43      private LinkedList   continuationIds = new LinkedList  ();
44  
45      /**
46       * The maximum number of continuations allowed in this group.
47       */
48      private int maxContinuations = -1;
49  
50      /**
51       * Creates a new flow execution continuation group.
52       * @param maxContinuations the maximum number of continuations that can be
53       * stored in this group, -1 for unlimited
54       */                                                                            
55      public FlowExecutionContinuationGroup(int maxContinuations) {
56          this.maxContinuations = maxContinuations;
57      }
58      
59      /**
60       * Returns the count of continuations in this group.
61       */
62      public int getContinuationCount() {
63          return continuationIds.size();
64      }
65  
66      /**
67       * Returns the continuation with the provided <code>id</code>, or
68       * <code>null</code> if no such continuation exists with that id.
69       * @param id the continuation id
70       * @return the continuation
71       * @throws ContinuationNotFoundException if the id does not match a
72       * continuation in this group
73       */
74      public FlowExecutionContinuation get(Serializable   id) throws ContinuationNotFoundException {
75          FlowExecutionContinuation continuation = (FlowExecutionContinuation)continuations.get(id);
76          if (continuation == null) {
77              throw new ContinuationNotFoundException(id);
78          }
79          return continuation;
80      }
81  
82      /**
83       * Add a flow execution continuation with given id to this group.
84       * @param continuationId the continuation id
85       * @param continuation the continuation
86       */
87      public void add(Serializable   continuationId, FlowExecutionContinuation continuation) {
88          continuations.put(continuationId, continuation);
89          continuationIds.add(continuationId);
90          // remove the oldest continuation if them maximium number of
91          // continuations has been exceeded
92          if (maxExceeded()) {
93              removeOldestContinuation();
94          }
95      }
96  
97      /**
98       * Has the maximum number of allowed continuations in this group been exceeded?
99       */
100     private boolean maxExceeded() {
101         return maxContinuations > 0 && continuationIds.size() > maxContinuations;
102     }
103 
104     /**
105      * Remove the olders continuation from this group.
106      */
107     private void removeOldestContinuation() {
108         continuations.remove(continuationIds.removeFirst());
109     }
110 }
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