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Java > Open Source Codes > org > apache > taglibs > standard > tag > el > fmt > ParamTag


1 /*
2  * Copyright 1999-2004 The Apache Software Foundation.
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
17 package org.apache.taglibs.standard.tag.el.fmt;
18
19 import javax.servlet.jsp.JspException JavaDoc;
20
21 import org.apache.taglibs.standard.lang.support.ExpressionEvaluatorManager;
22 import org.apache.taglibs.standard.tag.common.fmt.ParamSupport;
23
24 /**
25  * <p>A handler for &lt;param&gt; that accepts message arguments as
26  * Strings and evaluates them as expressions at runtime.</p>
27  *
28  * @author Jan Luehe
29  */
30
31 public class ParamTag extends ParamSupport {
32
33     //*********************************************************************
34 // 'Private' state (implementation details)
35
36     private String JavaDoc value_; // stores EL-based property
37
38
39     //*********************************************************************
40 // Constructor
41
42     /**
43      * Constructs a new ParamTag. As with TagSupport, subclasses
44      * should not provide other constructors and are expected to call
45      * the superclass constructor
46      */
47     public ParamTag() {
48         super();
49         init();
50     }
51
52
53     //*********************************************************************
54 // Tag logic
55
56     // evaluates expression and chains to parent
57 public int doStartTag() throws JspException JavaDoc {
58
59         // evaluate any expressions we were passed, once per invocation
60 evaluateExpressions();
61
62     // chain to the parent implementation
63 return super.doStartTag();
64     }
65
66     // Releases any resources we may have (or inherit)
67 public void release() {
68         super.release();
69         init();
70     }
71
72
73     //*********************************************************************
74 // Accessor methods
75
76     // for EL-based attribute
77 public void setValue(String JavaDoc value_) {
78         this.value_ = value_;
79     this.valueSpecified = true;
80     }
81
82
83     //*********************************************************************
84 // Private (utility) methods
85
86     // (re)initializes state (during release() or construction)
87 private void init() {
88         // null implies "no expression"
89 value_ = null;
90     }
91
92     // Evaluates expressions as necessary
93 private void evaluateExpressions() throws JspException JavaDoc {
94         /*
95          * Note: we don't check for type mismatches here; we assume
96          * the expression evaluator will return the expected type
97          * (by virtue of knowledge we give it about what that type is).
98          * A ClassCastException here is truly unexpected, so we let it
99          * propagate up.
100          */
101
102     if (value_ != null) {
103         value = ExpressionEvaluatorManager.evaluate(
104             "value", value_, Object JavaDoc.class, this, pageContext);
105     }
106     }
107 }
108
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