/*
 * Copyright (C) 2008 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.escape;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.base.Function;

/**
 * An object that converts literal text into a format safe for inclusion in a particular context
 * (such as an XML document). Typically (but not always), the inverse process of "unescaping" the
 * text is performed automatically by the relevant parser.
 *
 * 

For example, an XML escaper would convert the literal string {@code "Foo"} into {@code * "Foo<Bar>"} to prevent {@code ""} from being confused with an XML tag. When the * resulting XML document is parsed, the parser API will return this text as the original literal * string {@code "Foo"}. * *

An {@code Escaper} instance is required to be stateless, and safe when used concurrently by * multiple threads. * *

Because, in general, escaping operates on the code points of a string and not on its * individual {@code char} values, it is not safe to assume that {@code escape(s)} is equivalent to * {@code escape(s.substring(0, n)) + escape(s.substing(n))} for arbitrary {@code n}. This is * because of the possibility of splitting a surrogate pair. The only case in which it is safe to * escape strings and concatenate the results is if you can rule out this possibility, either by * splitting an existing long string into short strings adaptively around * {@linkplain Character#isHighSurrogate surrogate} {@linkplain Character#isLowSurrogate pairs}, or * by starting with short strings already known to be free of unpaired surrogates. * *

The two primary implementations of this interface are {@link CharEscaper} and * {@link UnicodeEscaper}. They are heavily optimized for performance and greatly simplify the task * of implementing new escapers. It is strongly recommended that when implementing a new escaper you * extend one of these classes. If you find that you are unable to achieve the desired behavior * using either of these classes, please contact the Java libraries team for advice. * *

Popular escapers are defined as constants in classes like * {@link com.google.common.html.HtmlEscapers} and {@link com.google.common.xml.XmlEscapers}. To * create your own escapers, use {@link CharEscaperBuilder}, or extend {@code CharEscaper} or * {@code UnicodeEscaper}. * * @author David Beaumont * @since 15.0 */ @Beta @GwtCompatible public abstract class Escaper { // TODO(user): evaluate custom implementations, considering package private constructor. /** Constructor for use by subclasses. */ protected Escaper() {} /** * Returns the escaped form of a given literal string. * *

Note that this method may treat input characters differently depending on the specific * escaper implementation. * *

    *
  • {@link UnicodeEscaper} handles UTF-16 * correctly, including surrogate character pairs. If the input is badly formed the escaper should * throw {@link IllegalArgumentException}. *
  • {@link CharEscaper} handles Java characters independently and does not verify the input for * well formed characters. A {@code CharEscaper} should not be used in situations where input is * not guaranteed to be restricted to the Basic Multilingual Plane (BMP). *
* * @param string the literal string to be escaped * @return the escaped form of {@code string} * @throws NullPointerException if {@code string} is null * @throws IllegalArgumentException if {@code string} contains badly formed UTF-16 or cannot be * escaped for any other reason */ public abstract String escape(String string); private final Function asFunction = new Function() { @Override public String apply(String from) { return escape(from); } }; /** * Returns a {@link Function} that invokes {@link #escape(String)} on this escaper. */ public final Function asFunction() { return asFunction; } }