Java tutorial
/* Copyright (c) 2008 Google Inc. * * 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.base; import java.io.*; import static com.google.common.base.Preconditions.*; /** * An {@link Escaper} 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<Bar>"} 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><b>Note:</b> This class is similar to {@link CharEscaper} but with one * very important difference. A CharEscaper can only process Java * <a href="http://en.wikipedia.org/wiki/UTF-16">UTF16</a> characters in * isolation and may not cope when it encounters surrogate pairs. This class * facilitates the correct escaping of all Unicode characters. * * <p>As there are important reasons, including potential security issues, to * handle Unicode correctly if you are considering implementing a new escaper * you should favor using UnicodeEscaper wherever possible. * * <p>A {@code UnicodeEscaper} instance is required to be stateless, and safe * when used concurrently by multiple threads. * * <p>Several popular escapers are defined as constants in the class {@link * CharEscapers}. To create your own escapers extend this class and implement * the {@link #escape(int)} method. * * */ public abstract class UnicodeEscaper implements Escaper { /** The amount of padding (chars) to use when growing the escape buffer. */ private static final int DEST_PAD = 32; /** * Returns the escaped form of the given Unicode code point, or {@code null} * if this code point does not need to be escaped. When called as part of an * escaping operation, the given code point is guaranteed to be in the range * {@code 0 <= cp <= Character#MAX_CODE_POINT}. * * <p>If an empty array is returned, this effectively strips the input * character from the resulting text. * * <p>If the character does not need to be escaped, this method should return * {@code null}, rather than an array containing the character representation * of the code point. This enables the escaping algorithm to perform more * efficiently. * * <p>If the implementation of this method cannot correctly handle a * particular code point then it should either throw an appropriate runtime * exception or return a suitable replacement character. It must never * silently discard invalid input as this may constitute a security risk. * * @param cp the Unicode code point to escape if necessary * @return the replacement characters, or {@code null} if no escaping was * needed */ protected abstract char[] escape(int cp); /** * Scans a sub-sequence of characters from a given {@link CharSequence}, * returning the index of the next character that requires escaping. * * <p><b>Note:</b> When implementing an escaper, it is a good idea to override * this method for efficiency. The base class implementation determines * successive Unicode code points and invokes {@link #escape(int)} for each of * them. If the semantics of your escaper are such that code points in the * supplementary range are either all escaped or all unescaped, this method * can be implemented more efficiently using {@link CharSequence#charAt(int)}. * * <p>Note however that if your escaper does not escape characters in the * supplementary range, you should either continue to validate the correctness * of any surrogate characters encountered or provide a clear warning to users * that your escaper does not validate its input. * * <p>See {@link PercentEscaper} for an example. * * @param csq a sequence of characters * @param start the index of the first character to be scanned * @param end the index immediately after the last character to be scanned * @throws IllegalArgumentException if the scanned sub-sequence of {@code csq} * contains invalid surrogate pairs */ protected int nextEscapeIndex(CharSequence csq, int start, int end) { int index = start; while (index < end) { int cp = codePointAt(csq, index, end); if (cp < 0 || escape(cp) != null) { break; } index += Character.isSupplementaryCodePoint(cp) ? 2 : 1; } return index; } /** * Returns the escaped form of a given literal string. * * <p>If you are escaping input in arbitrary successive chunks, then it is not * generally safe to use this method. If an input string ends with an * unmatched high surrogate character, then this method will throw * {@link IllegalArgumentException}. You should either ensure your input is * valid <a href="http://en.wikipedia.org/wiki/UTF-16">UTF-16</a> before * calling this method or use an escaped {@link Appendable} (as returned by * {@link #escape(Appendable)}) which can cope with arbitrarily split input. * * <p><b>Note:</b> When implementing an escaper it is a good idea to override * this method for efficiency by inlining the implementation of * {@link #nextEscapeIndex(CharSequence, int, int)} directly. Doing this for * {@link PercentEscaper} more than doubled the performance for unescaped * strings (as measured by CharEscapersBenchmark). * * @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 invalid surrogate characters are * encountered */ public String escape(String string) { int end = string.length(); int index = nextEscapeIndex(string, 0, end); return index == end ? string : escapeSlow(string, index); } /** * Returns the escaped form of a given literal string, starting at the given * index. This method is called by the {@link #escape(String)} method when it * discovers that escaping is required. It is protected to allow subclasses * to override the fastpath escaping function to inline their escaping test. * See {@link CharEscaperBuilder} for an example usage. * * <p>This method is not reentrant and may only be invoked by the top level * {@link #escape(String)} method. * * @param s the literal string to be escaped * @param index the index to start escaping from * @return the escaped form of {@code string} * @throws NullPointerException if {@code string} is null * @throws IllegalArgumentException if invalid surrogate characters are * encountered */ protected final String escapeSlow(String s, int index) { int end = s.length(); // Get a destination buffer and setup some loop variables. char[] dest = DEST_TL.get(); int destIndex = 0; int unescapedChunkStart = 0; while (index < end) { int cp = codePointAt(s, index, end); if (cp < 0) { throw new IllegalArgumentException("Trailing high surrogate at end of input"); } char[] escaped = escape(cp); if (escaped != null) { int charsSkipped = index - unescapedChunkStart; // This is the size needed to add the replacement, not the full // size needed by the string. We only regrow when we absolutely must. int sizeNeeded = destIndex + charsSkipped + escaped.length; if (dest.length < sizeNeeded) { int destLength = sizeNeeded + (end - index) + DEST_PAD; dest = growBuffer(dest, destIndex, destLength); } // If we have skipped any characters, we need to copy them now. if (charsSkipped > 0) { s.getChars(unescapedChunkStart, index, dest, destIndex); destIndex += charsSkipped; } if (escaped.length > 0) { System.arraycopy(escaped, 0, dest, destIndex, escaped.length); destIndex += escaped.length; } } unescapedChunkStart = index + (Character.isSupplementaryCodePoint(cp) ? 2 : 1); index = nextEscapeIndex(s, unescapedChunkStart, end); } // Process trailing unescaped characters - no need to account for escaped // length or padding the allocation. int charsSkipped = end - unescapedChunkStart; if (charsSkipped > 0) { int endIndex = destIndex + charsSkipped; if (dest.length < endIndex) { dest = growBuffer(dest, destIndex, endIndex); } s.getChars(unescapedChunkStart, end, dest, destIndex); destIndex = endIndex; } return new String(dest, 0, destIndex); } /** * Returns an {@code Appendable} instance which automatically escapes all * text appended to it before passing the resulting text to an underlying * {@code Appendable}. * * <p>Unlike {@link #escape(String)} it is permitted to append arbitrarily * split input to this Appendable, including input that is split over a * surrogate pair. In this case the pending high surrogate character will not * be processed until the corresponding low surrogate is appended. This means * that a trailing high surrogate character at the end of the input cannot be * detected and will be silently ignored. This is unavoidable since the * Appendable interface has no {@code close()} method, and it is impossible to * determine when the last characters have been appended. * * <p>The methods of the returned object will propagate any exceptions thrown * by the underlying {@code Appendable}. * * <p>For well formed <a href="http://en.wikipedia.org/wiki/UTF-16">UTF-16</a> * the escaping behavior is identical to that of {@link #escape(String)} and * the following code is equivalent to (but much slower than) * {@code escaper.escape(string)}: <pre>{@code * * StringBuilder sb = new StringBuilder(); * escaper.escape(sb).append(string); * return sb.toString();}</pre> * * @param out the underlying {@code Appendable} to append escaped output to * @return an {@code Appendable} which passes text to {@code out} after * escaping it * @throws NullPointerException if {@code out} is null * @throws IllegalArgumentException if invalid surrogate characters are * encountered * */ public Appendable escape(final Appendable out) { checkNotNull(out); return new Appendable() { int pendingHighSurrogate = -1; char[] decodedChars = new char[2]; public Appendable append(CharSequence csq) throws IOException { return append(csq, 0, csq.length()); } public Appendable append(CharSequence csq, int start, int end) throws IOException { int index = start; if (index < end) { // This is a little subtle: index must never reference the middle of a // surrogate pair but unescapedChunkStart can. The first time we enter // the loop below it is possible that index != unescapedChunkStart. int unescapedChunkStart = index; if (pendingHighSurrogate != -1) { // Our last append operation ended halfway through a surrogate pair // so we have to do some extra work first. char c = csq.charAt(index++); if (!Character.isLowSurrogate(c)) { throw new IllegalArgumentException("Expected low surrogate character but got " + c); } char[] escaped = escape(Character.toCodePoint((char) pendingHighSurrogate, c)); if (escaped != null) { // Emit the escaped character and adjust unescapedChunkStart to // skip the low surrogate we have consumed. outputChars(escaped, escaped.length); unescapedChunkStart += 1; } else { // Emit pending high surrogate (unescaped) but do not modify // unescapedChunkStart as we must still emit the low surrogate. out.append((char) pendingHighSurrogate); } pendingHighSurrogate = -1; } while (true) { // Find and append the next subsequence of unescaped characters. index = nextEscapeIndex(csq, index, end); if (index > unescapedChunkStart) { out.append(csq, unescapedChunkStart, index); } if (index == end) { break; } // If we are not finished, calculate the next code point. int cp = codePointAt(csq, index, end); if (cp < 0) { // Our sequence ended half way through a surrogate pair so just // record the state and exit. pendingHighSurrogate = -cp; break; } // Escape the code point and output the characters. char[] escaped = escape(cp); if (escaped != null) { outputChars(escaped, escaped.length); } else { // This shouldn't really happen if nextEscapeIndex is correct but // we should cope with false positives. int len = Character.toChars(cp, decodedChars, 0); outputChars(decodedChars, len); } // Update our index past the escaped character and continue. index += (Character.isSupplementaryCodePoint(cp) ? 2 : 1); unescapedChunkStart = index; } } return this; } public Appendable append(char c) throws IOException { if (pendingHighSurrogate != -1) { // Our last append operation ended halfway through a surrogate pair // so we have to do some extra work first. if (!Character.isLowSurrogate(c)) { throw new IllegalArgumentException( "Expected low surrogate character but got '" + c + "' with value " + (int) c); } char[] escaped = escape(Character.toCodePoint((char) pendingHighSurrogate, c)); if (escaped != null) { outputChars(escaped, escaped.length); } else { out.append((char) pendingHighSurrogate); out.append(c); } pendingHighSurrogate = -1; } else if (Character.isHighSurrogate(c)) { // This is the start of a (split) surrogate pair. pendingHighSurrogate = c; } else { if (Character.isLowSurrogate(c)) { throw new IllegalArgumentException( "Unexpected low surrogate character '" + c + "' with value " + (int) c); } // This is a normal (non surrogate) char. char[] escaped = escape(c); if (escaped != null) { outputChars(escaped, escaped.length); } else { out.append(c); } } return this; } private void outputChars(char[] chars, int len) throws IOException { for (int n = 0; n < len; n++) { out.append(chars[n]); } } }; } /** * Returns the Unicode code point of the character at the given index. * * <p>Unlike {@link Character#codePointAt(CharSequence, int)} or * {@link String#codePointAt(int)} this method will never fail silently when * encountering an invalid surrogate pair. * * <p>The behaviour of this method is as follows: * <ol> * <li>If {@code index >= end}, {@link IndexOutOfBoundsException} is thrown. * <li><b>If the character at the specified index is not a surrogate, it is * returned.</b> * <li>If the first character was a high surrogate value, then an attempt is * made to read the next character. * <ol> * <li><b>If the end of the sequence was reached, the negated value of * the trailing high surrogate is returned.</b> * <li><b>If the next character was a valid low surrogate, the code point * value of the high/low surrogate pair is returned.</b> * <li>If the next character was not a low surrogate value, then * {@link IllegalArgumentException} is thrown. * </ol> * <li>If the first character was a low surrogate value, * {@link IllegalArgumentException} is thrown. * </ol> * * @param seq the sequence of characters from which to decode the code point * @param index the index of the first character to decode * @param end the index beyond the last valid character to decode * @return the Unicode code point for the given index or the negated value of * the trailing high surrogate character at the end of the sequence */ protected static int codePointAt(CharSequence seq, int index, int end) { if (index < end) { char c1 = seq.charAt(index++); if (c1 < Character.MIN_HIGH_SURROGATE || c1 > Character.MAX_LOW_SURROGATE) { // Fast path (first test is probably all we need to do) return c1; } else if (c1 <= Character.MAX_HIGH_SURROGATE) { // If the high surrogate was the last character, return its inverse if (index == end) { return -c1; } // Otherwise look for the low surrogate following it char c2 = seq.charAt(index); if (Character.isLowSurrogate(c2)) { return Character.toCodePoint(c1, c2); } throw new IllegalArgumentException("Expected low surrogate but got char '" + c2 + "' with value " + (int) c2 + " at index " + index); } else { throw new IllegalArgumentException("Unexpected low surrogate character '" + c1 + "' with value " + (int) c1 + " at index " + (index - 1)); } } throw new IndexOutOfBoundsException("Index exceeds specified range"); } /** * Helper method to grow the character buffer as needed, this only happens * once in a while so it's ok if it's in a method call. If the index passed * in is 0 then no copying will be done. */ private static char[] growBuffer(char[] dest, int index, int size) { char[] copy = new char[size]; if (index > 0) { System.arraycopy(dest, 0, copy, 0, index); } return copy; } /** * A thread-local destination buffer to keep us from creating new buffers. * The starting size is 1024 characters. If we grow past this we don't * put it back in the threadlocal, we just keep going and grow as needed. */ private static final ThreadLocal<char[]> DEST_TL = new ThreadLocal<char[]>() { @Override protected char[] initialValue() { return new char[1024]; } }; }