1 /*
2 * Copyright (C) 2008 The Guava 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
17 package com.google.common.escape;
18
19 import com.google.common.annotations.Beta;
20 import com.google.common.annotations.GwtCompatible;
21 import com.google.common.base.Function;
22
23 /**
24 * An object that converts literal text into a format safe for inclusion in a particular context
25 * (such as an XML document). Typically (but not always), the inverse process of "unescaping" the
26 * text is performed automatically by the relevant parser.
27 *
28 * <p>For example, an XML escaper would convert the literal string {@code "Foo<Bar>"} into {@code
29 * "Foo<Bar>"} to prevent {@code "<Bar>"} from being confused with an XML tag. When the
30 * resulting XML document is parsed, the parser API will return this text as the original literal
31 * string {@code "Foo<Bar>"}.
32 *
33 * <p>An {@code Escaper} instance is required to be stateless, and safe when used concurrently by
34 * multiple threads.
35 *
36 * <p>Because, in general, escaping operates on the code points of a string and not on its
37 * individual {@code char} values, it is not safe to assume that {@code escape(s)} is equivalent to
38 * {@code escape(s.substring(0, n)) + escape(s.substing(n))} for arbitrary {@code n}. This is
39 * because of the possibility of splitting a surrogate pair. The only case in which it is safe to
40 * escape strings and concatenate the results is if you can rule out this possibility, either by
41 * splitting an existing long string into short strings adaptively around {@linkplain
42 * Character#isHighSurrogate surrogate} {@linkplain Character#isLowSurrogate pairs}, or by starting
43 * with short strings already known to be free of unpaired surrogates.
44 *
45 * <p>The two primary implementations of this interface are {@link CharEscaper} and {@link
46 * UnicodeEscaper}. They are heavily optimized for performance and greatly simplify the task of
47 * implementing new escapers. It is strongly recommended that when implementing a new escaper you
48 * extend one of these classes. If you find that you are unable to achieve the desired behavior
49 * using either of these classes, please contact the Java libraries team for advice.
50 *
51 * <p>Several popular escapers are defined as constants in classes like {@link
52 * com.google.common.html.HtmlEscapers}, {@link com.google.common.xml.XmlEscapers}, and {@link
53 * SourceCodeEscapers}. To create your own escapers, use {@link CharEscaperBuilder}, or extend
54 * {@code CharEscaper} or {@code UnicodeEscaper}.
55 *
56 * @author David Beaumont
57 * @since 15.0
58 */
59 @Beta
60 @GwtCompatible
61 public abstract class Escaper {
62 // TODO(user): evaluate custom implementations, considering package private constructor.
63 /** Constructor for use by subclasses. */
64 protected Escaper() {}
65
66 /**
67 * Returns the escaped form of a given literal string.
68 *
69 * <p>Note that this method may treat input characters differently depending on the specific
70 * escaper implementation.
71 *
72 * <ul>
73 * <li>{@link UnicodeEscaper} handles <a href="http://en.wikipedia.org/wiki/UTF-16">UTF-16</a>
74 * correctly, including surrogate character pairs. If the input is badly formed the escaper
75 * should throw {@link IllegalArgumentException}.
76 * <li>{@link CharEscaper} handles Java characters independently and does not verify the input for
77 * well formed characters. A {@code CharEscaper} should not be used in situations where input
78 * is not guaranteed to be restricted to the Basic Multilingual Plane (BMP).
79 * </ul>
80 *
81 * @param string the literal string to be escaped
82 * @return the escaped form of {@code string}
83 * @throws NullPointerException if {@code string} is null
84 * @throws IllegalArgumentException if {@code string} contains badly formed UTF-16 or cannot be
85 * escaped for any other reason
86 */
87 public abstract String escape(String string);
88
89 private final Function<String, String> asFunction =
90 new Function<String, String>() {
91 @Override
92 public String apply(String from) {
93 return escape(from);
94 }
95 };
96
97 /**
98 * Returns a {@link Function} that invokes {@link #escape(String)} on this escaper.
99 */
100 public final Function<String, String> asFunction() {
101 return asFunction;
102 }
103 }