01: /**
02: * Copyright 2006 Webmedia Group Ltd.
03: *
04: * Licensed under the Apache License, Version 2.0 (the "License");
05: * you may not use this file except in compliance with the License.
06: * You may obtain a copy of the License at
07: *
08: * http://www.apache.org/licenses/LICENSE-2.0
09: *
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: **/package org.araneaframework.backend.list.memorybased.expression;
16:
17: import org.araneaframework.backend.list.memorybased.Expression;
18: import org.araneaframework.backend.list.memorybased.ExpressionEvaluationException;
19: import org.araneaframework.backend.list.memorybased.ExpressionFactory;
20:
21: /**
22: * Expression that is initialized lazily.
23: *
24: * @author Rein Raudjärv
25: *
26: * @since 1.1
27: */
28: public class LazyExpression implements Expression {
29:
30: private static final long serialVersionUID = 1L;
31:
32: private final ExpressionFactory factory;
33: private Expression expression;
34:
35: public LazyExpression(ExpressionFactory factory) {
36: if (factory == null) {
37: throw new IllegalArgumentException(
38: "ExpressionFactory must be provided");
39: }
40: this .factory = factory;
41: }
42:
43: public ExpressionFactory getFactory() {
44: return factory;
45: }
46:
47: public Expression getExpression() {
48: if (expression == null) {
49: expression = factory.createExpression();
50: if (expression == null) {
51: throw new AssertionError(
52: "ExpressionFactory must not create a null object");
53: }
54: }
55: return expression;
56: }
57:
58: public Object evaluate(VariableResolver resolver)
59: throws ExpressionEvaluationException {
60: return getExpression().evaluate(resolver);
61: }
62: }
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