1/**
2 * Copyright (c) 2008, Google Inc.
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.android.mail.common.base;
18
19/**
20 * An object that converts literal text into a format safe for inclusion in a particular context
21 * (such as an XML document). Typically (but not always), the inverse process of "unescaping" the
22 * text is performed automatically by the relevant parser.
23 *
24 * <p>For example, an XML escaper would convert the literal string {@code "Foo<Bar>"} into {@code
25 * "Foo&lt;Bar&gt;"} to prevent {@code "<Bar>"} from being confused with an XML tag. When the
26 * resulting XML document is parsed, the parser API will return this text as the original literal
27 * string {@code "Foo<Bar>"}.
28 *
29 * <p>An {@code Escaper} instance is required to be stateless, and safe when used concurrently by
30 * multiple threads.
31 *
32 * <p>The two primary implementations of this interface are {@link CharEscaper} and {@link
33 * UnicodeEscaper}. They are heavily optimized for performance and greatly simplify the task of
34 * implementing new escapers. It is strongly recommended that when implementing a new escaper you
35 * extend one of these classes. If you find that you are unable to achieve the desired behavior
36 * using either of these classes, please contact the Java libraries team for advice.
37 *
38 * <p>Several popular escapers are defined as constants in the class {@link CharEscapers}. To create
39 * your own escapers, use {@link CharEscaperBuilder}, or extend {@link CharEscaper} or {@code
40 * UnicodeEscaper}.
41 *
42 * @author dbeaumont@google.com (David Beaumont)
43 */
44public abstract class Escaper {
45  /**
46   * Returns the escaped form of a given literal string.
47   *
48   * <p>Note that this method may treat input characters differently depending on the specific
49   * escaper implementation.
50   *
51   * <ul>
52   * <li>{@link UnicodeEscaper} handles <a href="http://en.wikipedia.org/wiki/UTF-16">UTF-16</a>
53   *     correctly, including surrogate character pairs. If the input is badly formed the escaper
54   *     should throw {@link IllegalArgumentException}.
55   * <li>{@link CharEscaper} handles Java characters independently and does not verify the input
56   *     for well formed characters. A CharEscaper should not be used in situations where input is
57   *     not guaranteed to be restricted to the Basic Multilingual Plane (BMP).
58   * </ul>
59   *
60   * @param string the literal string to be escaped
61   * @return the escaped form of {@code string}
62   * @throws NullPointerException if {@code string} is null
63   * @throws IllegalArgumentException if {@code string} contains badly formed UTF-16 or cannot be
64   *         escaped for any other reason
65   */
66  public abstract String escape(String string);
67
68  /**
69   * Returns an {@code Appendable} instance which automatically escapes all text appended to it
70   * before passing the resulting text to an underlying {@code Appendable}.
71   *
72   * <p>Note that the Appendable returned by this method may treat input characters differently
73   * depending on the specific escaper implementation.
74   *
75   * <ul>
76   * <li>{@link UnicodeEscaper} handles <a href="http://en.wikipedia.org/wiki/UTF-16">UTF-16</a>
77   *     correctly, including surrogate character pairs. If the input is badly formed the escaper
78   *     should throw {@link IllegalArgumentException}.
79   * <li>{@link CharEscaper} handles Java characters independently and does not verify the input
80   *     for well formed characters. A CharEscaper should not be used in situations where input is
81   *     not guaranteed to be restricted to the Basic Multilingual Plane (BMP).
82   * </ul>
83   *
84   * <p>In all implementations the escaped Appendable should throw {@code NullPointerException} if
85   * given a {@code null} {@link CharSequence}.
86   *
87   * @param out the underlying {@code Appendable} to append escaped output to
88   * @return an {@code Appendable} which passes text to {@code out} after escaping it
89   */
90  public abstract Appendable escape(Appendable out);
91
92  private final Function<String, String> asFunction =
93      new Function<String, String>() {
94        public String apply(String from) {
95          return escape(from);
96        }
97      };
98
99  /**
100   * Returns a {@link Function} that invokes {@link #escape(String)} on this escaper.
101   */
102  public Function<String, String> asFunction() {
103    return asFunction;
104  }
105}