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 * http://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 /** The underlying configuration object. */
46 private final Configuration wrappedConfiguration;
47
48 /**
49 * Creates a new instance of {@code ImmutableConfigurationInvocationHandler} and initializes it with the wrapped
50 * configuration object.
51 *
52 * @param configuration the wrapped {@code Configuration} (must not be <b>null</b>)
53 * @throws NullPointerException if the {@code Configuration} is <b>null</b>
54 */
55 public ImmutableConfigurationInvocationHandler(final Configuration configuration) {
56 wrappedConfiguration = Objects.requireNonNull(configuration, "configuration");
57 }
58
59 /**
60 * {@inheritDoc} This implementation delegates to the wrapped configuration object. Result objects are wrapped if
61 * necessary.
62 */
63 @Override
64 public Object invoke(final Object proxy, final Method method, final Object[] args) throws Throwable {
65 try {
66 return handleResult(method.invoke(wrappedConfiguration, args));
67 } catch (final InvocationTargetException e) {
68 // unwrap
69 throw e.getCause();
70 }
71 }
72
73 /**
74 * Handles the result from the method invocation on the wrapped configuration. This implementation wraps result objects
75 * if necessary so that the underlying configuration cannot be manipulated.
76 *
77 * @param result the result object
78 * @return the processed result object
79 */
80 private static Object handleResult(final Object result) {
81 if (result instanceof Iterator) {
82 return new ImmutableIterator((Iterator<?>) result);
83 }
84 return result;
85 }
86
87 /**
88 * A specialized {@code Iterator} implementation which delegates to an underlying iterator, but does not support the
89 * {@code remove()} method.
90 */
91 private static final class ImmutableIterator implements Iterator<Object> {
92 /** The underlying iterator. */
93 private final Iterator<?> wrappedIterator;
94
95 /**
96 * Creates a new instance of {@code ImmutableIterator} and sets the underlying iterator.
97 *
98 * @param it the underlying iterator
99 */
100 public ImmutableIterator(final Iterator<?> it) {
101 wrappedIterator = it;
102 }
103
104 /**
105 * {@inheritDoc} This implementation just delegates to the underlying iterator.
106 */
107 @Override
108 public boolean hasNext() {
109 return wrappedIterator.hasNext();
110 }
111
112 /**
113 * {@inheritDoc} This implementation just delegates to the underlying iterator.
114 */
115 @Override
116 public Object next() {
117 return wrappedIterator.next();
118 }
119
120 /**
121 * {@inheritDoc} This implementation just throws an exception: removing objects is not supported.
122 */
123 @Override
124 public void remove() {
125 throw new UnsupportedOperationException("remove() operation not supported!");
126 }
127 }
128 }