CharsetDecoder
public
abstract
class
CharsetDecoder
extends Object
java.lang.Object | |
↳ | java.nio.charset.CharsetDecoder |
An engine that can transform a sequence of bytes in a specific charset into a sequence of sixteen-bit Unicode characters.
The input byte sequence is provided in a byte buffer or a series of such buffers. The output character sequence is written to a character buffer or a series of such buffers. A decoder should always be used by making the following sequence of method invocations, hereinafter referred to as a decoding operation:
Reset the decoder via the
reset
method, unless it has not been used before;Invoke the
decode
method zero or more times, as long as additional input may be available, passingfalse
for theendOfInput
argument and filling the input buffer and flushing the output buffer between invocations;Invoke the
decode
method one final time, passingtrue
for theendOfInput
argument; and thenInvoke the
flush
method so that the decoder can flush any internal state to the output buffer.
decode
method will decode as many
bytes as possible from the input buffer, writing the resulting characters
to the output buffer. The decode
method returns when more
input is required, when there is not enough room in the output buffer, or
when a decoding error has occurred. In each case a CoderResult
object is returned to describe the reason for termination. An invoker can
examine this object and fill the input buffer, flush the output buffer, or
attempt to recover from a decoding error, as appropriate, and try again.
There are two general types of decoding errors. If the input byte sequence is not legal for this charset then the input is considered malformed. If the input byte sequence is legal but cannot be mapped to a valid Unicode character then an unmappable character has been encountered.
How a decoding error is handled depends upon the action requested for
that type of error, which is described by an instance of the CodingErrorAction
class. The possible error actions are to ignore the erroneous input, report the error to the invoker via
the returned CoderResult
object, or replace the erroneous input with the current value of the
replacement string. The replacement
has the initial value "\uFFFD"
;
its value may be changed via the replaceWith
method.
The default action for malformed-input and unmappable-character errors
is to report them. The
malformed-input error action may be changed via the onMalformedInput
method; the
unmappable-character action may be changed via the onUnmappableCharacter
method.
This class is designed to handle many of the details of the decoding
process, including the implementation of error actions. A decoder for a
specific charset, which is a concrete subclass of this class, need only
implement the abstract decodeLoop
method, which
encapsulates the basic decoding loop. A subclass that maintains internal
state should, additionally, override the implFlush
and
implReset
methods.
Instances of this class are not safe for use by multiple concurrent threads.
Summary
Protected constructors | |
---|---|
CharsetDecoder(Charset cs, float averageCharsPerByte, float maxCharsPerByte)
Initializes a new decoder. |
Public methods | |
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final
float
|
averageCharsPerByte()
Returns the average number of characters that will be produced for each byte of input. |
final
Charset
|
charset()
Returns the charset that created this decoder. |
final
CharBuffer
|
decode(ByteBuffer in)
Convenience method that decodes the remaining content of a single input byte buffer into a newly-allocated character buffer. |
final
CoderResult
|
decode(ByteBuffer in, CharBuffer out, boolean endOfInput)
Decodes as many bytes as possible from the given input buffer, writing the results to the given output buffer. |
Charset
|
detectedCharset()
Retrieves the charset that was detected by this decoder (optional operation). |
final
CoderResult
|
flush(CharBuffer out)
Flushes this decoder. |
boolean
|
isAutoDetecting()
Tells whether or not this decoder implements an auto-detecting charset. |
boolean
|
isCharsetDetected()
Tells whether or not this decoder has yet detected a charset (optional operation). |
CodingErrorAction
|
malformedInputAction()
Returns this decoder's current action for malformed-input errors. |
final
float
|
maxCharsPerByte()
Returns the maximum number of characters that will be produced for each byte of input. |
final
CharsetDecoder
|
onMalformedInput(CodingErrorAction newAction)
Changes this decoder's action for malformed-input errors. |
final
CharsetDecoder
|
onUnmappableCharacter(CodingErrorAction newAction)
Changes this decoder's action for unmappable-character errors. |
final
CharsetDecoder
|
replaceWith(String newReplacement)
Changes this decoder's replacement value. |
final
String
|
replacement()
Returns this decoder's replacement value. |
final
CharsetDecoder
|
reset()
Resets this decoder, clearing any internal state. |
CodingErrorAction
|
unmappableCharacterAction()
Returns this decoder's current action for unmappable-character errors. |
Protected methods | |
---|---|
abstract
CoderResult
|
decodeLoop(ByteBuffer in, CharBuffer out)
Decodes one or more bytes into one or more characters. |
CoderResult
|
implFlush(CharBuffer out)
Flushes this decoder. |
void
|
implOnMalformedInput(CodingErrorAction newAction)
Reports a change to this decoder's malformed-input action. |
void
|
implOnUnmappableCharacter(CodingErrorAction newAction)
Reports a change to this decoder's unmappable-character action. |
void
|
implReplaceWith(String newReplacement)
Reports a change to this decoder's replacement value. |
void
|
implReset()
Resets this decoder, clearing any charset-specific internal state. |
Inherited methods | |
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Protected constructors
CharsetDecoder
protected CharsetDecoder (Charset cs, float averageCharsPerByte, float maxCharsPerByte)
Initializes a new decoder. The new decoder will have the given
chars-per-byte values and its replacement will be the
string "\uFFFD"
.
Parameters | |
---|---|
cs |
Charset : The charset that created this decoder |
averageCharsPerByte |
float : A positive float value indicating the expected number of
characters that will be produced for each input byte |
maxCharsPerByte |
float : A positive float value indicating the maximum number of
characters that will be produced for each input byte |
Throws | |
---|---|
IllegalArgumentException |
If the preconditions on the parameters do not hold |
Public methods
averageCharsPerByte
public final float averageCharsPerByte ()
Returns the average number of characters that will be produced for each byte of input. This heuristic value may be used to estimate the size of the output buffer required for a given input sequence.
Returns | |
---|---|
float |
The average number of characters produced per byte of input |
charset
public final Charset charset ()
Returns the charset that created this decoder.
Returns | |
---|---|
Charset |
This decoder's charset |
decode
public final CharBuffer decode (ByteBuffer in)
Convenience method that decodes the remaining content of a single input byte buffer into a newly-allocated character buffer.
This method implements an entire decoding operation; that is, it resets this decoder, then it decodes the bytes in the given byte buffer, and finally it flushes this decoder. This method should therefore not be invoked if a decoding operation is already in progress.
Parameters | |
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in |
ByteBuffer : The input byte buffer |
Returns | |
---|---|
CharBuffer |
A newly-allocated character buffer containing the result of the decoding operation. The buffer's position will be zero and its limit will follow the last character written. |
Throws | |
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IllegalStateException |
If a decoding operation is already in progress |
MalformedInputException |
If the byte sequence starting at the input buffer's current
position is not legal for this charset and the current malformed-input action
is CodingErrorAction#REPORT |
UnmappableCharacterException |
If the byte sequence starting at the input buffer's current
position cannot be mapped to an equivalent character sequence and
the current unmappable-character action is CodingErrorAction.REPORT |
CoderMalfunctionError |
If an invocation of the decodeLoop method threw an unexpected exception |
NullPointerException |
if input buffer is null |
CharacterCodingException |
decode
public final CoderResult decode (ByteBuffer in, CharBuffer out, boolean endOfInput)
Decodes as many bytes as possible from the given input buffer, writing the results to the given output buffer.
The buffers are read from, and written to, starting at their current
positions. At most in.remaining()
bytes
will be read and at most out.remaining()
characters will be written. The buffers' positions will be advanced to
reflect the bytes read and the characters written, but their marks and
limits will not be modified.
In addition to reading bytes from the input buffer and writing
characters to the output buffer, this method returns a CoderResult
object to describe its reason for termination:
CoderResult#UNDERFLOW
indicates that as much of the input buffer as possible has been decoded. If there is no further input then the invoker can proceed to the next step of the decoding operation. Otherwise this method should be invoked again with further input.CoderResult#OVERFLOW
indicates that there is insufficient space in the output buffer to decode any more bytes. This method should be invoked again with an output buffer that has more remaining characters. This is typically done by draining any decoded characters from the output buffer.A malformed-input result indicates that a malformed-input error has been detected. The malformed bytes begin at the input buffer's (possibly incremented) position; the number of malformed bytes may be determined by invoking the result object's
length
method. This case applies only if the malformed action of this decoder isCodingErrorAction#REPORT
; otherwise the malformed input will be ignored or replaced, as requested.An unmappable-character result indicates that an unmappable-character error has been detected. The bytes that decode the unmappable character begin at the input buffer's (possibly incremented) position; the number of such bytes may be determined by invoking the result object's
length
method. This case applies only if the unmappable action of this decoder isCodingErrorAction.REPORT
; otherwise the unmappable character will be ignored or replaced, as requested.
The endOfInput
parameter advises this method as to whether
the invoker can provide further input beyond that contained in the given
input buffer. If there is a possibility of providing additional input
then the invoker should pass false
for this parameter; if there
is no possibility of providing further input then the invoker should
pass true
. It is not erroneous, and in fact it is quite
common, to pass false
in one invocation and later discover that
no further input was actually available. It is critical, however, that
the final invocation of this method in a sequence of invocations always
pass true
so that any remaining undecoded input will be treated
as being malformed.
This method works by invoking the decodeLoop
method, interpreting its results, handling error conditions, and
reinvoking it as necessary.
Parameters | |
---|---|
in |
ByteBuffer : The input byte buffer |
out |
CharBuffer : The output character buffer |
endOfInput |
boolean : true if, and only if, the invoker can provide no
additional input bytes beyond those in the given buffer |
Returns | |
---|---|
CoderResult |
A coder-result object describing the reason for termination |
Throws | |
---|---|
IllegalStateException |
If a decoding operation is already in progress and the previous
step was an invocation neither of the reset
method, nor of this method with a value of false for
the endOfInput parameter, nor of this method with a
value of true for the endOfInput parameter
but a return value indicating an incomplete decoding operation |
CoderMalfunctionError |
If an invocation of the decodeLoop method threw an unexpected exception |
NullPointerException |
if input or output buffer is null |
detectedCharset
public Charset detectedCharset ()
Retrieves the charset that was detected by this decoder (optional operation).
If this decoder implements an auto-detecting charset then this
method returns the actual charset once it has been detected. After that
point, this method returns the same value for the duration of the
current decoding operation. If not enough input bytes have yet been
read to determine the actual charset then this method throws an IllegalStateException
.
The default implementation of this method always throws an UnsupportedOperationException
; it should be overridden by
auto-detecting decoders to return the appropriate value.
Returns | |
---|---|
Charset |
The charset detected by this auto-detecting decoder,
or null if the charset has not yet been determined |
Throws | |
---|---|
IllegalStateException |
If insufficient bytes have been read to determine a charset |
UnsupportedOperationException |
If this decoder does not implement an auto-detecting charset |
flush
public final CoderResult flush (CharBuffer out)
Flushes this decoder.
Some decoders maintain internal state and may need to write some final characters to the output buffer once the overall input sequence has been read.
Any additional output is written to the output buffer beginning at
its current position. At most out.remaining()
characters will be written. The buffer's position will be advanced
appropriately, but its mark and limit will not be modified.
If this method completes successfully then it returns CoderResult.UNDERFLOW
. If there is insufficient room in the output
buffer then it returns CoderResult#OVERFLOW
. If this happens
then this method must be invoked again, with an output buffer that has
more room, in order to complete the current decoding
operation.
If this decoder has already been flushed then invoking this method has no effect.
This method invokes the implFlush
method to
perform the actual flushing operation.
Parameters | |
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out |
CharBuffer : The output character buffer |
Returns | |
---|---|
CoderResult |
A coder-result object, either CoderResult#UNDERFLOW or
CoderResult#OVERFLOW |
Throws | |
---|---|
IllegalStateException |
If the previous step of the current decoding operation was an
invocation neither of the flush method nor of
the three-argument decode method
with a value of true for the endOfInput
parameter |
isAutoDetecting
public boolean isAutoDetecting ()
Tells whether or not this decoder implements an auto-detecting charset.
The default implementation of this method always returns
false
; it should be overridden by auto-detecting decoders to
return true
.
Returns | |
---|---|
boolean |
true if, and only if, this decoder implements an
auto-detecting charset |
isCharsetDetected
public boolean isCharsetDetected ()
Tells whether or not this decoder has yet detected a charset (optional operation).
If this decoder implements an auto-detecting charset then at a
single point during a decoding operation this method may start returning
true
to indicate that a specific charset has been detected in
the input byte sequence. Once this occurs, the detectedCharset
method may be invoked to retrieve the detected charset.
That this method returns false
does not imply that no bytes
have yet been decoded. Some auto-detecting decoders are capable of
decoding some, or even all, of an input byte sequence without fixing on
a particular charset.
The default implementation of this method always throws an UnsupportedOperationException
; it should be overridden by
auto-detecting decoders to return true
once the input charset
has been determined.
Returns | |
---|---|
boolean |
true if, and only if, this decoder has detected a
specific charset |
Throws | |
---|---|
UnsupportedOperationException |
If this decoder does not implement an auto-detecting charset |
malformedInputAction
public CodingErrorAction malformedInputAction ()
Returns this decoder's current action for malformed-input errors.
Returns | |
---|---|
CodingErrorAction |
The current malformed-input action, which is never null |
maxCharsPerByte
public final float maxCharsPerByte ()
Returns the maximum number of characters that will be produced for each byte of input. This value may be used to compute the worst-case size of the output buffer required for a given input sequence. This value accounts for any necessary content-independent prefix or suffix characters.
Returns | |
---|---|
float |
The maximum number of characters that will be produced per byte of input |
onMalformedInput
public final CharsetDecoder onMalformedInput (CodingErrorAction newAction)
Changes this decoder's action for malformed-input errors.
This method invokes the implOnMalformedInput
method, passing the new action.
Parameters | |
---|---|
newAction |
CodingErrorAction : The new action; must not be null |
Returns | |
---|---|
CharsetDecoder |
This decoder |
Throws | |
---|---|
IllegalArgumentException |
If the precondition on the parameter does not hold |
onUnmappableCharacter
public final CharsetDecoder onUnmappableCharacter (CodingErrorAction newAction)
Changes this decoder's action for unmappable-character errors.
This method invokes the implOnUnmappableCharacter
method, passing the new action.
Parameters | |
---|---|
newAction |
CodingErrorAction : The new action; must not be null |
Returns | |
---|---|
CharsetDecoder |
This decoder |
Throws | |
---|---|
IllegalArgumentException |
If the precondition on the parameter does not hold |
replaceWith
public final CharsetDecoder replaceWith (String newReplacement)
Changes this decoder's replacement value.
This method invokes the implReplaceWith
method, passing the new replacement, after checking that the new
replacement is acceptable.
Parameters | |
---|---|
newReplacement |
String : The new replacement; must not be
null , must have non-zero length,
and must not be longer than the value returned by the
maxCharsPerByte method |
Returns | |
---|---|
CharsetDecoder |
This decoder |
Throws | |
---|---|
IllegalArgumentException |
If the preconditions on the parameter do not hold |
replacement
public final String replacement ()
Returns this decoder's replacement value.
Returns | |
---|---|
String |
This decoder's current replacement,
which is never null and is never empty |
reset
public final CharsetDecoder reset ()
Resets this decoder, clearing any internal state.
This method resets charset-independent state and also invokes the
implReset
method in order to perform any
charset-specific reset actions.
Returns | |
---|---|
CharsetDecoder |
This decoder |
unmappableCharacterAction
public CodingErrorAction unmappableCharacterAction ()
Returns this decoder's current action for unmappable-character errors.
Returns | |
---|---|
CodingErrorAction |
The current unmappable-character action, which is never
null |
Protected methods
decodeLoop
protected abstract CoderResult decodeLoop (ByteBuffer in, CharBuffer out)
Decodes one or more bytes into one or more characters.
This method encapsulates the basic decoding loop, decoding as many
bytes as possible until it either runs out of input, runs out of room
in the output buffer, or encounters a decoding error. This method is
invoked by the decode
method, which handles result
interpretation and error recovery.
The buffers are read from, and written to, starting at their current
positions. At most in.remaining()
bytes
will be read, and at most out.remaining()
characters will be written. The buffers' positions will be advanced to
reflect the bytes read and the characters written, but their marks and
limits will not be modified.
This method returns a CoderResult
object to describe its
reason for termination, in the same manner as the decode
method. Most implementations of this method will handle decoding errors
by returning an appropriate result object for interpretation by the
decode
method. An optimized implementation may instead
examine the relevant error action and implement that action itself.
An implementation of this method may perform arbitrary lookahead by
returning CoderResult#UNDERFLOW
until it receives sufficient
input.
Parameters | |
---|---|
in |
ByteBuffer : The input byte buffer |
out |
CharBuffer : The output character buffer |
Returns | |
---|---|
CoderResult |
A coder-result object describing the reason for termination |
implFlush
protected CoderResult implFlush (CharBuffer out)
Flushes this decoder.
The default implementation of this method does nothing, and always
returns CoderResult#UNDERFLOW
. This method should be overridden
by decoders that may need to write final characters to the output buffer
once the entire input sequence has been read.
Parameters | |
---|---|
out |
CharBuffer : The output character buffer |
Returns | |
---|---|
CoderResult |
A coder-result object, either CoderResult#UNDERFLOW or
CoderResult#OVERFLOW |
implOnMalformedInput
protected void implOnMalformedInput (CodingErrorAction newAction)
Reports a change to this decoder's malformed-input action.
The default implementation of this method does nothing. This method should be overridden by decoders that require notification of changes to the malformed-input action.
Parameters | |
---|---|
newAction |
CodingErrorAction : The new action |
implOnUnmappableCharacter
protected void implOnUnmappableCharacter (CodingErrorAction newAction)
Reports a change to this decoder's unmappable-character action.
The default implementation of this method does nothing. This method should be overridden by decoders that require notification of changes to the unmappable-character action.
Parameters | |
---|---|
newAction |
CodingErrorAction : The new action |
implReplaceWith
protected void implReplaceWith (String newReplacement)
Reports a change to this decoder's replacement value.
The default implementation of this method does nothing. This method should be overridden by decoders that require notification of changes to the replacement.
Parameters | |
---|---|
newReplacement |
String : The replacement value |
implReset
protected void implReset ()
Resets this decoder, clearing any charset-specific internal state.
The default implementation of this method does nothing. This method should be overridden by decoders that maintain internal state.