com.google.gdata.util.common.base
Class UnicodeEscaper

java.lang.Object
  com.google.gdata.util.common.base.UnicodeEscaper

All Implemented Interfaces:

Escaper

Direct Known Subclasses:

PercentEscaper

public abstract class UnicodeEscaper
extends Object
implements Escaper

An 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.

For example, an XML escaper would convert the literal string "Foo<Bar>" into "Foo<Bar>" to prevent "<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 "Foo<Bar>".

Note: This class is similar to CharEscaper but with one very important difference. A CharEscaper can only process Java UTF16 characters in isolation and may not cope when it encounters surrogate pairs. This class facilitates the correct escaping of all Unicode characters.

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.

A UnicodeEscaper instance is required to be stateless, and safe when used concurrently by multiple threads.

Several popular escapers are defined as constants in the class CharEscapers. To create your own escapers extend this class and implement the escape(int) method.

Constructor Summary

UnicodeEscaper()

Method Summary

protected static int	codePointAt(CharSequence seq, int index, int end) Returns the Unicode code point of the character at the given index.
Appendable	escape(Appendable out) Returns an Appendable instance which automatically escapes all text appended to it before passing the resulting text to an underlying Appendable.
protected abstract char[]	escape(int cp) Returns the escaped form of the given Unicode code point, or null if this code point does not need to be escaped.
String	escape(String string) Returns the escaped form of a given literal string.
protected String	escapeSlow(String s, int index) Returns the escaped form of a given literal string, starting at the given index.
protected int	nextEscapeIndex(CharSequence csq, int start, int end) Scans a sub-sequence of characters from a given CharSequence, returning the index of the next character that requires escaping.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

UnicodeEscaper

public UnicodeEscaper()

Method Detail

escape

protected abstract char[] escape(int cp)

Returns the escaped form of the given Unicode code point, or 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 0 <= cp <= Character#MAX_CODE_POINT.

If an empty array is returned, this effectively strips the input character from the resulting text.

If the character does not need to be escaped, this method should return null, rather than an array containing the character representation of the code point. This enables the escaping algorithm to perform more efficiently.

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.

Parameters:

cp - the Unicode code point to escape if necessary

Returns:

the replacement characters, or null if no escaping was needed

nextEscapeIndex

protected int nextEscapeIndex(CharSequence csq,
                              int start,
                              int end)

Scans a sub-sequence of characters from a given CharSequence, returning the index of the next character that requires escaping.

Note: 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 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 CharSequence.charAt(int).

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.

See PercentEscaper for an example.

Parameters:

csq - a sequence of characters

start - the index of the first character to be scanned

end - the index immediately after the last character to be scanned

Throws:

IllegalArgumentException - if the scanned sub-sequence of csq contains invalid surrogate pairs

escape

public String escape(String string)

Returns the escaped form of a given literal string.

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 IllegalArgumentException. You should either ensure your input is valid UTF-16 before calling this method or use an escaped Appendable (as returned by escape(Appendable)) which can cope with arbitrarily split input.

Note: When implementing an escaper it is a good idea to override this method for efficiency by inlining the implementation of nextEscapeIndex(CharSequence, int, int) directly. Doing this for PercentEscaper more than doubled the performance for unescaped strings (as measured by CharEscapersBenchmark).

Specified by:

escape in interface Escaper

Parameters:

string - the literal string to be escaped

Returns:

the escaped form of string

Throws:

NullPointerException - if string is null

IllegalArgumentException - if invalid surrogate characters are encountered

escapeSlow

protected final String escapeSlow(String s,
                                  int index)

Returns the escaped form of a given literal string, starting at the given index. This method is called by the 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 CharEscaperBuilder for an example usage.

This method is not reentrant and may only be invoked by the top level escape(String) method.

Parameters:

s - the literal string to be escaped

index - the index to start escaping from

Returns:

the escaped form of string

Throws:

NullPointerException - if string is null

IllegalArgumentException - if invalid surrogate characters are encountered

escape

public Appendable escape(Appendable out)

Returns an Appendable instance which automatically escapes all text appended to it before passing the resulting text to an underlying Appendable.

Unlike 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 close() method, and it is impossible to determine when the last characters have been appended.

The methods of the returned object will propagate any exceptions thrown by the underlying Appendable.

For well formed UTF-16 the escaping behavior is identical to that of escape(String) and the following code is equivalent to (but much slower than) escaper.escape(string):

StringBuilder sb = new StringBuilder();
   escaper.escape(sb).append(string);
   return sb.toString();

Specified by:

escape in interface Escaper

Parameters:

out - the underlying Appendable to append escaped output to

Returns:

an Appendable which passes text to out after escaping it

Throws:

NullPointerException - if out is null

IllegalArgumentException - if invalid surrogate characters are encountered

codePointAt

protected static final int codePointAt(CharSequence seq,
                                       int index,
                                       int end)

Returns the Unicode code point of the character at the given index.

Unlike Character.codePointAt(CharSequence, int) or String.codePointAt(int) this method will never fail silently when encountering an invalid surrogate pair.

The behaviour of this method is as follows:

1. If index >= end, IndexOutOfBoundsException is thrown.
2. If the character at the specified index is not a surrogate, it is returned.
3. If the first character was a high surrogate value, then an attempt is made to read the next character.
   1. If the end of the sequence was reached, the negated value of the trailing high surrogate is returned.
   2. If the next character was a valid low surrogate, the code point value of the high/low surrogate pair is returned.
   3. If the next character was not a low surrogate value, then IllegalArgumentException is thrown.
4. If the first character was a low surrogate value, IllegalArgumentException is thrown.

Parameters:

seq - the sequence of characters from which to decode the code point

index - the index of the first character to decode

end - the index beyond the last valid character to decode

Returns:

the Unicode code point for the given index or the negated value of the trailing high surrogate character at the end of the sequence