1/*
2 * Copyright (C) 2009 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
17package com.google.common.xml;
18
19import com.google.common.annotations.Beta;
20import com.google.common.annotations.GwtCompatible;
21import com.google.common.escape.Escaper;
22import com.google.common.escape.Escapers;
23
24/**
25 * {@code Escaper} instances suitable for strings to be included in XML
26 * attribute values and elements' text contents. When possible, avoid manual
27 * escaping by using templating systems and high-level APIs that provide
28 * autoescaping. For example, consider <a href="http://www.xom.nu/">XOM</a> or
29 * <a href="http://www.jdom.org/">JDOM</a>.
30 *
31 * <p><b>Note:</b> Currently the escapers provided by this class do not escape
32 * any characters outside the ASCII character range. Unlike HTML escaping the
33 * XML escapers will not escape non-ASCII characters to their numeric entity
34 * replacements. These XML escapers provide the minimal level of escaping to
35 * ensure that the output can be safely included in a Unicode XML document.
36 *
37 *
38 * <p>For details on the behavior of the escapers in this class, see sections
39 * <a href="http://www.w3.org/TR/2008/REC-xml-20081126/#charsets">2.2</a> and
40 * <a href="http://www.w3.org/TR/2008/REC-xml-20081126/#syntax">2.4</a> of the
41 * XML specification.
42 *
43 * @author Alex Matevossian
44 * @author David Beaumont
45 * @since 15.0
46 */
47@Beta
48@GwtCompatible
49public class XmlEscapers {
50  private XmlEscapers() {}
51
52  private static final char MIN_ASCII_CONTROL_CHAR = 0x00;
53  private static final char MAX_ASCII_CONTROL_CHAR = 0x1F;
54
55  // For each xxxEscaper() method, please add links to external reference pages
56  // that are considered authoritative for the behavior of that escaper.
57
58  /**
59   * Returns an {@link Escaper} instance that escapes special characters in a
60   * string so it can safely be included in an XML document as element content.
61   * See section
62   * <a href="http://www.w3.org/TR/2008/REC-xml-20081126/#syntax">2.4</a> of the
63   * XML specification.
64   *
65   * <p><b>Note:</b> Double and single quotes are not escaped, so it is <b>not
66   * safe</b> to use this escaper to escape attribute values. Use
67   * {@link #xmlContentEscaper} if the output can appear in element content or
68   * {@link #xmlAttributeEscaper} in attribute values.
69   *
70   * <p>This escaper substitutes {@code 0xFFFD} for non-whitespace control
71   * characters and the character values {@code 0xFFFE} and {@code 0xFFFF} which
72   * are not permitted in XML. For more detail see section <a
73   * href="http://www.w3.org/TR/2008/REC-xml-20081126/#charsets">2.2</a> of the
74   * XML specification.
75   *
76   * <p>This escaper does not escape non-ASCII characters to their numeric
77   * character references (NCR). Any non-ASCII characters appearing in the input
78   * will be preserved in the output. Specifically "\r" (carriage return) is
79   * preserved in the output, which may result in it being silently converted to
80   * "\n" when the XML is parsed.
81   *
82   * <p>This escaper does not treat surrogate pairs specially and does not
83   * perform Unicode validation on its input.
84   */
85  public static Escaper xmlContentEscaper() {
86    return XML_CONTENT_ESCAPER;
87  }
88
89  /**
90   * Returns an {@link Escaper} instance that escapes special characters in a
91   * string so it can safely be included in XML document as an attribute value.
92   * See section
93   * <a href="http://www.w3.org/TR/2008/REC-xml-20081126/#AVNormalize">3.3.3</a>
94   * of the XML specification.
95   *
96   * <p>This escaper substitutes {@code 0xFFFD} for non-whitespace control
97   * characters and the character values {@code 0xFFFE} and {@code 0xFFFF} which
98   * are not permitted in XML. For more detail see section <a
99   * href="http://www.w3.org/TR/2008/REC-xml-20081126/#charsets">2.2</a> of the
100   * XML specification.
101   *
102   * <p>This escaper does not escape non-ASCII characters to their numeric
103   * character references (NCR). However, horizontal tab {@code '\t'}, line feed
104   * {@code '\n'} and carriage return {@code '\r'} are escaped to a
105   * corresponding NCR {@code "&#x9;"}, {@code "&#xA;"}, and {@code "&#xD;"}
106   * respectively. Any other non-ASCII characters appearing in the input will
107   * be preserved in the output.
108   *
109   * <p>This escaper does not treat surrogate pairs specially and does not
110   * perform Unicode validation on its input.
111   */
112  public static Escaper xmlAttributeEscaper() {
113    return XML_ATTRIBUTE_ESCAPER;
114  }
115
116  private static final Escaper XML_ESCAPER;
117  private static final Escaper XML_CONTENT_ESCAPER;
118  private static final Escaper XML_ATTRIBUTE_ESCAPER;
119  static {
120    Escapers.Builder builder = Escapers.builder();
121    // The char values \uFFFE and \uFFFF are explicitly not allowed in XML
122    // (Unicode code points above \uFFFF are represented via surrogate pairs
123    // which means they are treated as pairs of safe characters).
124    builder.setSafeRange(Character.MIN_VALUE, '\uFFFD');
125    // Unsafe characters are replaced with the Unicode replacement character.
126    builder.setUnsafeReplacement("\uFFFD");
127
128    /*
129     * Except for \n, \t, and \r, all ASCII control characters are replaced with
130     * the Unicode replacement character.
131     *
132     * Implementation note: An alternative to the following would be to make a
133     * map that simply replaces the allowed ASCII whitespace characters with
134     * themselves and to set the minimum safe character to 0x20. However this
135     * would slow down the escaping of simple strings that contain \t, \n, or
136     * \r.
137     */
138    for (char c = MIN_ASCII_CONTROL_CHAR; c <= MAX_ASCII_CONTROL_CHAR; c++) {
139      if (c != '\t' && c != '\n' && c != '\r') {
140        builder.addEscape(c, "\uFFFD");
141      }
142    }
143
144    // Build the content escaper first and then add quote escaping for the
145    // general escaper.
146    builder.addEscape('&', "&amp;");
147    builder.addEscape('<', "&lt;");
148    builder.addEscape('>', "&gt;");
149    XML_CONTENT_ESCAPER = builder.build();
150    builder.addEscape('\'', "&apos;");
151    builder.addEscape('"', "&quot;");
152    XML_ESCAPER = builder.build();
153    builder.addEscape('\t', "&#x9;");
154    builder.addEscape('\n', "&#xA;");
155    builder.addEscape('\r', "&#xD;");
156    XML_ATTRIBUTE_ESCAPER = builder.build();
157  }
158}
159