com.google.common.escape.Escaper.java Source code

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Here is the source code for com.google.common.escape.Escaper.java

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/*
 * 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.
 *
 * <p>For example, an XML escaper would convert the literal string {@code "Foo<Bar>"} into {@code
 * "Foo&lt;Bar&gt;"} to prevent {@code "<Bar>"} 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<Bar>"}.
 *
 * <p>An {@code Escaper} instance is required to be stateless, and safe when used concurrently by
 * multiple threads.
 *
 * <p>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.
 *
 * <p>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.
 *
 * <p>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.
     *
     * <p>Note that this method may treat input characters differently depending on the specific
     * escaper implementation.
     *
     * <ul>
     * <li>{@link UnicodeEscaper} handles <a href="http://en.wikipedia.org/wiki/UTF-16">UTF-16</a>
     *     correctly, including surrogate character pairs. If the input is badly formed the escaper
     *     should throw {@link IllegalArgumentException}.
     * <li>{@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).
     * </ul>
     *
     * @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<String, String> asFunction = new Function<String, String>() {
        @Override
        public String apply(String from) {
            return escape(from);
        }
    };

    /**
     * Returns a {@link Function} that invokes {@link #escape(String)} on this escaper.
     */
    public final Function<String, String> asFunction() {
        return asFunction;
    }
}