CharEscaper.java
/*
* Copyright (C) 2006 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 static com.google.common.base.Preconditions.checkNotNull;
import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import javax.annotation.CheckForNull;
/**
* 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<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>A {@code CharEscaper} instance is required to be stateless, and safe when used concurrently by
* multiple threads.
*
* <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 extend this class and implement the {@link #escape(char)} method.
*
* @author Sven Mawson
* @since 15.0
*/
@Beta
@GwtCompatible
@ElementTypesAreNonnullByDefault
public abstract class CharEscaper extends Escaper {
/** Constructor for use by subclasses. */
protected CharEscaper() {}
/**
* Returns the escaped form of a given literal string.
*
* @param string the literal string to be escaped
* @return the escaped form of {@code string}
* @throws NullPointerException if {@code string} is null
*/
@Override
public String escape(String string) {
checkNotNull(string); // GWT specific check (do not optimize)
// Inlineable fast-path loop which hands off to escapeSlow() only if needed
int length = string.length();
for (int index = 0; index < length; index++) {
if (escape(string.charAt(index)) != null) {
return escapeSlow(string, index);
}
}
return string;
}
/**
* Returns the escaped form of the given character, or {@code null} if this character does not
* need to be escaped. 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 a one-character array containing the character itself. This enables the escaping algorithm
* to perform more efficiently.
*
* <p>An escaper is expected to be able to deal with any {@code char} value, so this method should
* not throw any exceptions.
*
* @param c the character to escape if necessary
* @return the replacement characters, or {@code null} if no escaping was needed
*/
@CheckForNull
protected abstract char[] escape(char c);
/**
* 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.
*
* @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
*/
protected final String escapeSlow(String s, int index) {
int slen = s.length();
// Get a destination buffer and setup some loop variables.
char[] dest = Platform.charBufferFromThreadLocal();
int destSize = dest.length;
int destIndex = 0;
int lastEscape = 0;
// Loop through the rest of the string, replacing when needed into the
// destination buffer, which gets grown as needed as well.
for (; index < slen; index++) {
// Get a replacement for the current character.
char[] r = escape(s.charAt(index));
// If no replacement is needed, just continue.
if (r == null) {
continue;
}
int rlen = r.length;
int charsSkipped = index - lastEscape;
// This is the size needed to add the replacement, not the full size
// needed by the string. We only regrow when we absolutely must, and
// when we do grow, grow enough to avoid excessive growing. Grow.
int sizeNeeded = destIndex + charsSkipped + rlen;
if (destSize < sizeNeeded) {
destSize = sizeNeeded + DEST_PAD_MULTIPLIER * (slen - index);
dest = growBuffer(dest, destIndex, destSize);
}
// If we have skipped any characters, we need to copy them now.
if (charsSkipped > 0) {
s.getChars(lastEscape, index, dest, destIndex);
destIndex += charsSkipped;
}
// Copy the replacement string into the dest buffer as needed.
if (rlen > 0) {
System.arraycopy(r, 0, dest, destIndex, rlen);
destIndex += rlen;
}
lastEscape = index + 1;
}
// Copy leftover characters if there are any.
int charsLeft = slen - lastEscape;
if (charsLeft > 0) {
int sizeNeeded = destIndex + charsLeft;
if (destSize < sizeNeeded) {
// Regrow and copy, expensive! No padding as this is the final copy.
dest = growBuffer(dest, destIndex, sizeNeeded);
}
s.getChars(lastEscape, slen, dest, destIndex);
destIndex = sizeNeeded;
}
return new String(dest, 0, destIndex);
}
/**
* 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) {
if (size < 0) { // overflow - should be OutOfMemoryError but GWT/j2cl don't support it
throw new AssertionError("Cannot increase internal buffer any further");
}
char[] copy = new char[size];
if (index > 0) {
System.arraycopy(dest, 0, copy, 0, index);
}
return copy;
}
/** The multiplier for padding to use when growing the escape buffer. */
private static final int DEST_PAD_MULTIPLIER = 2;
}