001/* 002 * Copyright (C) 2008 The Guava Authors 003 * 004 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except 005 * in compliance with the License. You may obtain a copy of the License at 006 * 007 * http://www.apache.org/licenses/LICENSE-2.0 008 * 009 * Unless required by applicable law or agreed to in writing, software distributed under the License 010 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express 011 * or implied. See the License for the specific language governing permissions and limitations under 012 * the License. 013 */ 014 015package com.google.common.escape; 016 017import com.google.common.annotations.GwtCompatible; 018import com.google.common.base.Function; 019 020/** 021 * An object that converts literal text into a format safe for inclusion in a particular context 022 * (such as an XML document). Typically (but not always), the inverse process of "unescaping" the 023 * text is performed automatically by the relevant parser. 024 * 025 * <p>For example, an XML escaper would convert the literal string {@code "Foo<Bar>"} into {@code 026 * "Foo<Bar>"} to prevent {@code "<Bar>"} from being confused with an XML tag. When the 027 * resulting XML document is parsed, the parser API will return this text as the original literal 028 * string {@code "Foo<Bar>"}. 029 * 030 * <p>An {@code Escaper} instance is required to be stateless, and safe when used concurrently by 031 * multiple threads. 032 * 033 * <p>Because, in general, escaping operates on the code points of a string and not on its 034 * individual {@code char} values, it is not safe to assume that {@code escape(s)} is equivalent to 035 * {@code escape(s.substring(0, n)) + escape(s.substring(n))} for arbitrary {@code n}. This is 036 * because of the possibility of splitting a surrogate pair. The only case in which it is safe to 037 * escape strings and concatenate the results is if you can rule out this possibility, either by 038 * splitting an existing long string into short strings adaptively around {@linkplain 039 * Character#isHighSurrogate surrogate} {@linkplain Character#isLowSurrogate pairs}, or by starting 040 * with short strings already known to be free of unpaired surrogates. 041 * 042 * <p>The two primary implementations of this interface are {@link CharEscaper} and {@link 043 * UnicodeEscaper}. They are heavily optimized for performance and greatly simplify the task of 044 * implementing new escapers. It is strongly recommended that when implementing a new escaper you 045 * extend one of these classes. If you find that you are unable to achieve the desired behavior 046 * using either of these classes, please contact the Java libraries team for advice. 047 * 048 * <p>Popular escapers are defined as constants in classes like {@link 049 * com.google.common.html.HtmlEscapers} and {@link com.google.common.xml.XmlEscapers}. To create 050 * your own escapers, use {@link CharEscaperBuilder}, or extend {@code CharEscaper} or {@code 051 * UnicodeEscaper}. 052 * 053 * @author David Beaumont 054 * @since 15.0 055 */ 056@GwtCompatible 057public abstract class Escaper { 058 // TODO(dbeaumont): evaluate custom implementations, considering package private constructor. 059 /** Constructor for use by subclasses. */ 060 protected Escaper() {} 061 062 /** 063 * Returns the escaped form of a given literal string. 064 * 065 * <p>Note that this method may treat input characters differently depending on the specific 066 * escaper implementation. 067 * 068 * <ul> 069 * <li>{@link UnicodeEscaper} handles <a href="http://en.wikipedia.org/wiki/UTF-16">UTF-16</a> 070 * correctly, including surrogate character pairs. If the input is badly formed the escaper 071 * should throw {@link IllegalArgumentException}. 072 * <li>{@link CharEscaper} handles Java characters independently and does not verify the input 073 * for well formed characters. A {@code CharEscaper} should not be used in situations where 074 * input is not guaranteed to be restricted to the Basic Multilingual Plane (BMP). 075 * </ul> 076 * 077 * @param string the literal string to be escaped 078 * @return the escaped form of {@code string} 079 * @throws NullPointerException if {@code string} is null 080 * @throws IllegalArgumentException if {@code string} contains badly formed UTF-16 or cannot be 081 * escaped for any other reason 082 */ 083 public abstract String escape(String string); 084 085 private final Function<String, String> asFunction = 086 new Function<String, String>() { 087 @Override 088 public String apply(String from) { 089 return escape(from); 090 } 091 }; 092 093 /** Returns a {@link Function} that invokes {@link #escape(String)} on this escaper. */ 094 public final Function<String, String> asFunction() { 095 return asFunction; 096 } 097}