1 /*
2 * Licensed to the Apache Software Foundation (ASF) under one or more
3 * contributor license agreements. See the NOTICE file distributed with
4 * this work for additional information regarding copyright ownership.
5 * The ASF licenses this file to You under the Apache License, Version 2.0
6 * (the "License"); you may not use this file except in compliance with
7 * the License. You may obtain a copy of the License at
8 *
9 * https://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 */
17 package org.apache.commons.configuration2;
18
19 import java.lang.reflect.InvocationHandler;
20 import java.lang.reflect.InvocationTargetException;
21 import java.lang.reflect.Method;
22 import java.util.Iterator;
23 import java.util.Objects;
24
25 /**
26 * <p>
27 * A specialized {@code InvocationHandler} implementation for supporting immutable configurations.
28 * </p>
29 * <p>
30 * An instance of this class is constructed with a reference to a {@code Configuration} object. All method invocations
31 * (which stem from the {@code ImmutableConfiguration} interface) are delegated to this object. That way all
32 * functionality is actually backed by the underlying {@code Configuration} implementation, but because the associated
33 * proxy only implements the {@code ImmutableConfiguration} interface manipulations are not possible.
34 * </p>
35 * <p>
36 * There is one caveat however: Some methods of the {@code ImmutableConfiguration} interface return an {@code Iterator}
37 * object. Using the iterator's {@code remove()} method it may be possible to remove keys from the underlying
38 * {@code Configuration} object. Therefore, in these cases a specialized {@code Iterator} is returned which does not
39 * support the remove operation.
40 * </p>
41 *
42 * @since 2.0
43 */
44 final class ImmutableConfigurationInvocationHandler implements InvocationHandler {
45 /**
46 * A specialized {@code Iterator} implementation which delegates to an underlying iterator, but does not support the
47 * {@code remove()} method.
48 */
49 private static final class ImmutableIterator implements Iterator<Object> {
50 /** The underlying iterator. */
51 private final Iterator<?> wrappedIterator;
52
53 /**
54 * Creates a new instance of {@code ImmutableIterator} and sets the underlying iterator.
55 *
56 * @param it the underlying iterator
57 */
58 public ImmutableIterator(final Iterator<?> it) {
59 wrappedIterator = it;
60 }
61
62 /**
63 * {@inheritDoc} This implementation just delegates to the underlying iterator.
64 */
65 @Override
66 public boolean hasNext() {
67 return wrappedIterator.hasNext();
68 }
69
70 /**
71 * {@inheritDoc} This implementation just delegates to the underlying iterator.
72 */
73 @Override
74 public Object next() {
75 return wrappedIterator.next();
76 }
77
78 /**
79 * {@inheritDoc} This implementation just throws an exception: removing objects is not supported.
80 */
81 @Override
82 public void remove() {
83 throw new UnsupportedOperationException("remove() operation not supported!");
84 }
85 }
86
87 /**
88 * Handles the result from the method invocation on the wrapped configuration. This implementation wraps result objects
89 * if necessary so that the underlying configuration cannot be manipulated.
90 *
91 * @param result the result object
92 * @return the processed result object
93 */
94 private static Object handleResult(final Object result) {
95 if (result instanceof Iterator) {
96 return new ImmutableIterator((Iterator<?>) result);
97 }
98 return result;
99 }
100
101 /** The underlying configuration object. */
102 private final Configuration wrappedConfiguration;
103
104 /**
105 * Creates a new instance of {@code ImmutableConfigurationInvocationHandler} and initializes it with the wrapped
106 * configuration object.
107 *
108 * @param configuration the wrapped {@code Configuration} (must not be <strong>null</strong>)
109 * @throws NullPointerException if the {@code Configuration} is <strong>null</strong>
110 */
111 public ImmutableConfigurationInvocationHandler(final Configuration configuration) {
112 wrappedConfiguration = Objects.requireNonNull(configuration, "configuration");
113 }
114
115 /**
116 * {@inheritDoc} This implementation delegates to the wrapped configuration object. Result objects are wrapped if
117 * necessary.
118 */
119 @Override
120 public Object invoke(final Object proxy, final Method method, final Object[] args) throws Throwable {
121 try {
122 return handleResult(method.invoke(wrappedConfiguration, args));
123 } catch (final InvocationTargetException e) {
124 // unwrap
125 throw e.getCause();
126 }
127 }
128 }