1 /*
2 * Copyright (C) 2008 The Guava Authors
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
5 * in compliance with the License. You may obtain a copy of the License at
6 *
7 * http://www.apache.org/licenses/LICENSE-2.0
8 *
9 * Unless required by applicable law or agreed to in writing, software distributed under the License
10 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
11 * or implied. See the License for the specific language governing permissions and limitations under
12 * the License.
13 */
14
15 package com.google.common.escape;
16
17 import com.google.common.annotations.GwtCompatible;
18 import com.google.common.base.Function;
19 import com.google.errorprone.annotations.DoNotMock;
20
21 /**
22 * An object that converts literal text into a format safe for inclusion in a particular context
23 * (such as an XML document). Typically (but not always), the inverse process of "unescaping" the
24 * text is performed automatically by the relevant parser.
25 *
26 * <p>For example, an XML escaper would convert the literal string {@code "Foo<Bar>"} into {@code
27 * "Foo<Bar>"} to prevent {@code "<Bar>"} from being confused with an XML tag. When the
28 * resulting XML document is parsed, the parser API will return this text as the original literal
29 * string {@code "Foo<Bar>"}.
30 *
31 * <p>An {@code Escaper} instance is required to be stateless, and safe when used concurrently by
32 * multiple threads.
33 *
34 * <p>Because, in general, escaping operates on the code points of a string and not on its
35 * individual {@code char} values, it is not safe to assume that {@code escape(s)} is equivalent to
36 * {@code escape(s.substring(0, n)) + escape(s.substring(n))} for arbitrary {@code n}. This is
37 * because of the possibility of splitting a surrogate pair. The only case in which it is safe to
38 * escape strings and concatenate the results is if you can rule out this possibility, either by
39 * splitting an existing long string into short strings adaptively around {@linkplain
40 * Character#isHighSurrogate surrogate} {@linkplain Character#isLowSurrogate pairs}, or by starting
41 * with short strings already known to be free of unpaired surrogates.
42 *
43 * <p>The two primary implementations of this interface are {@link CharEscaper} and {@link
44 * UnicodeEscaper}. They are heavily optimized for performance and greatly simplify the task of
45 * implementing new escapers. It is strongly recommended that when implementing a new escaper you
46 * extend one of these classes. If you find that you are unable to achieve the desired behavior
47 * using either of these classes, please contact the Java libraries team for advice.
48 *
49 * <p>Popular escapers are defined as constants in classes like {@link
50 * com.google.common.html.HtmlEscapers} and {@link com.google.common.xml.XmlEscapers}. To create
51 * your own escapers, use {@link CharEscaperBuilder}, or extend {@code CharEscaper} or {@code
52 * UnicodeEscaper}.
53 *
54 * @author David Beaumont
55 * @since 15.0
56 */
57 @DoNotMock("Use Escapers.nullEscaper() or another methods from the *Escapers classes")
58 @GwtCompatible
59 @ElementTypesAreNonnullByDefault
60 public abstract class Escaper {
61 // TODO(dbeaumont): evaluate custom implementations, considering package private constructor.
62 /** Constructor for use by subclasses. */
63 protected Escaper() {}
64
65 /**
66 * Returns the escaped form of a given literal string.
67 *
68 * <p>Note that this method may treat input characters differently depending on the specific
69 * escaper implementation.
70 *
71 * <ul>
72 * <li>{@link UnicodeEscaper} handles <a href="http://en.wikipedia.org/wiki/UTF-16">UTF-16</a>
73 * correctly, including surrogate character pairs. If the input is badly formed the escaper
74 * should throw {@link IllegalArgumentException}.
75 * <li>{@link CharEscaper} handles Java characters independently and does not verify the input
76 * for well formed characters. A {@code CharEscaper} should not be used in situations where
77 * input is not guaranteed to be restricted to the Basic Multilingual Plane (BMP).
78 * </ul>
79 *
80 * @param string the literal string to be escaped
81 * @return the escaped form of {@code string}
82 * @throws NullPointerException if {@code string} is null
83 * @throws IllegalArgumentException if {@code string} contains badly formed UTF-16 or cannot be
84 * escaped for any other reason
85 */
86 public abstract String escape(String string);
87
88 private final Function<String, String> asFunction =
89 new Function<String, String>() {
90 @Override
91 public String apply(String from) {
92 return escape(from);
93 }
94 };
95
96 /** Returns a {@link Function} that invokes {@link #escape(String)} on this escaper. */
97 public final Function<String, String> asFunction() {
98 return asFunction;
99 }
100 }