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 */
16package com.android.mail.lib.base;
17
18/**
19 * An object that converts literal text into a format safe for inclusion in a particular context
20 * (such as an XML document). Typically (but not always), the inverse process of "unescaping" the
21 * text is performed automatically by the relevant parser.
22 *
23 * <p>For example, an XML escaper would convert the literal string {@code "Foo<Bar>"} into {@code
24 * "Foo&lt;Bar&gt;"} to prevent {@code "<Bar>"} from being confused with an XML tag. When the
25 * resulting XML document is parsed, the parser API will return this text as the original literal
26 * string {@code "Foo<Bar>"}.
27 *
28 * <p>An {@code Escaper} instance is required to be stateless, and safe when used concurrently by
29 * multiple threads.
30 *
31 * <p>The two primary implementations of this interface are {@link CharEscaper} and {@link
32 * UnicodeEscaper}. They are heavily optimized for performance and greatly simplify the task of
33 * implementing new escapers. It is strongly recommended that when implementing a new escaper you
34 * extend one of these classes. If you find that you are unable to achieve the desired behavior
35 * using either of these classes, please contact the Java libraries team for advice.
36 *
37 * <p>Several popular escapers are defined as constants in the class {@link CharEscapers}. To create
38 * your own escapers, use {@link CharEscaperBuilder}, or extend {@link CharEscaper} or {@code
39 * UnicodeEscaper}.
40 *
41 * @author dbeaumont@google.com (David Beaumont)
42 */
43public abstract class Escaper {
44  /**
45   * Returns the escaped form of a given literal string.
46   *
47   * <p>Note that this method may treat input characters differently depending on the specific
48   * escaper implementation.
49   *
50   * <ul>
51   * <li>{@link UnicodeEscaper} handles <a href="http://en.wikipedia.org/wiki/UTF-16">UTF-16</a>
52   *     correctly, including surrogate character pairs. If the input is badly formed the escaper
53   *     should throw {@link IllegalArgumentException}.
54   * <li>{@link CharEscaper} handles Java characters independently and does not verify the input
55   *     for well formed characters. A CharEscaper should not be used in situations where input is
56   *     not guaranteed to be restricted to the Basic Multilingual Plane (BMP).
57   * </ul>
58   *
59   * @param string the literal string to be escaped
60   * @return the escaped form of {@code string}
61   * @throws NullPointerException if {@code string} is null
62   * @throws IllegalArgumentException if {@code string} contains badly formed UTF-16 or cannot be
63   *         escaped for any other reason
64   */
65  public abstract String escape(String string);
66
67  /**
68   * Returns an {@code Appendable} instance which automatically escapes all text appended to it
69   * before passing the resulting text to an underlying {@code Appendable}.
70   *
71   * <p>Note that the Appendable returned by this method may treat input characters differently
72   * depending on the specific escaper implementation.
73   *
74   * <ul>
75   * <li>{@link UnicodeEscaper} handles <a href="http://en.wikipedia.org/wiki/UTF-16">UTF-16</a>
76   *     correctly, including surrogate character pairs. If the input is badly formed the escaper
77   *     should throw {@link IllegalArgumentException}.
78   * <li>{@link CharEscaper} handles Java characters independently and does not verify the input
79   *     for well formed characters. A CharEscaper should not be used in situations where input is
80   *     not guaranteed to be restricted to the Basic Multilingual Plane (BMP).
81   * </ul>
82   *
83   * <p>In all implementations the escaped Appendable should throw {@code NullPointerException} if
84   * given a {@code null} {@link CharSequence}.
85   *
86   * @param out the underlying {@code Appendable} to append escaped output to
87   * @return an {@code Appendable} which passes text to {@code out} after escaping it
88   */
89  public abstract Appendable escape(Appendable out);
90
91  private final Function<String, String> asFunction =
92      new Function<String, String>() {
93        public String apply(String from) {
94          return escape(from);
95        }
96      };
97
98  /**
99   * Returns a {@link Function} that invokes {@link #escape(String)} on this escaper.
100   */
101  public Function<String, String> asFunction() {
102    return asFunction;
103  }
104}