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Java™ Platform Standard Ed. 6 |
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java.lang.Object java.nio.charset.CharsetDecoder
public abstract class 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 Invoke the Invoke the Invoke the 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 The default action for malformed-input and unmappable-character errors
is to 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 Instances of this class are not safe for use by multiple concurrent
threads.
Each invocation of the reset
method, unless it
has not been used before; decode
method zero or more times, as
long as additional input may be available, passing false for the
endOfInput argument and filling the input buffer and flushing the
output buffer between invocations; decode
method one final time, passing
true for the endOfInput argument; and then 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.
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.
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.
decodeLoop
method, which
encapsulates the basic decoding loop. A subclass that maintains internal
state should, additionally, override the implFlush
and
implReset
methods.
ByteBuffer
,
CharBuffer
,
Charset
,
CharsetEncoder
Constructor Summary | |
---|---|
protected |
CharsetDecoder(Charset cs,
float averageCharsPerByte,
float maxCharsPerByte)
Initializes a new decoder. |
Method Summary | |
---|---|
float |
averageCharsPerByte()
Returns the average number of characters that will be produced for each byte of input. |
Charset |
charset()
Returns the charset that created this decoder. |
CharBuffer |
decode(ByteBuffer in)
Convenience method that decodes the remaining content of a single input byte buffer into a newly-allocated character buffer. |
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. |
protected abstract CoderResult |
decodeLoop(ByteBuffer in,
CharBuffer out)
Decodes one or more bytes into one or more characters. |
Charset |
detectedCharset()
Retrieves the charset that was detected by this decoder (optional operation). |
CoderResult |
flush(CharBuffer out)
Flushes this decoder. |
protected CoderResult |
implFlush(CharBuffer out)
Flushes this decoder. |
protected void |
implOnMalformedInput(CodingErrorAction newAction)
Reports a change to this decoder's malformed-input action. |
protected void |
implOnUnmappableCharacter(CodingErrorAction newAction)
Reports a change to this decoder's unmappable-character action. |
protected void |
implReplaceWith(String newReplacement)
Reports a change to this decoder's replacement value. |
protected void |
implReset()
Resets this decoder, clearing any charset-specific internal state. |
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. |
float |
maxCharsPerByte()
Returns the maximum number of characters that will be produced for each byte of input. |
CharsetDecoder |
onMalformedInput(CodingErrorAction newAction)
Changes this decoder's action for malformed-input errors. |
CharsetDecoder |
onUnmappableCharacter(CodingErrorAction newAction)
Changes this decoder's action for unmappable-character errors. |
String |
replacement()
Returns this decoder's replacement value. |
CharsetDecoder |
replaceWith(String newReplacement)
Changes this decoder's replacement value. |
CharsetDecoder |
reset()
Resets this decoder, clearing any internal state. |
CodingErrorAction |
unmappableCharacterAction()
Returns this decoder's current action for unmappable-character errors. |
Methods inherited from class java.lang.Object |
---|
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait |
Constructor Detail |
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protected CharsetDecoder(Charset cs, float averageCharsPerByte, float maxCharsPerByte)
averageCharsPerByte
- A positive float value indicating the expected number of
characters that will be produced for each input bytemaxCharsPerByte
- A positive float value indicating the maximum number of
characters that will be produced for each input byte
IllegalArgumentException
- If the preconditions on the parameters do not holdMethod Detail |
---|
public final Charset charset()
public final String replacement()
public final CharsetDecoder replaceWith(String newReplacement)
This method invokes the implReplaceWith
method, passing the new replacement, after checking that the new
replacement is acceptable.
newReplacement
- The new replacement; must not be null
and must have non-zero length
IllegalArgumentException
- If the preconditions on the parameter do not holdprotected void implReplaceWith(String newReplacement)
The default implementation of this method does nothing. This method should be overridden by decoders that require notification of changes to the replacement.
newReplacement
- public CodingErrorAction malformedInputAction()
public final CharsetDecoder onMalformedInput(CodingErrorAction newAction)
This method invokes the implOnMalformedInput
method, passing the new action.
newAction
- The new action; must not be null
IllegalArgumentException
- If the precondition on the parameter does not holdprotected void implOnMalformedInput(CodingErrorAction newAction)
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.
public CodingErrorAction unmappableCharacterAction()
public final CharsetDecoder onUnmappableCharacter(CodingErrorAction newAction)
This method invokes the implOnUnmappableCharacter
method, passing the new action.
newAction
- The new action; must not be null
IllegalArgumentException
- If the precondition on the parameter does not holdprotected void implOnUnmappableCharacter(CodingErrorAction newAction)
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.
public final float averageCharsPerByte()
public final float maxCharsPerByte()
public final CoderResult decode(ByteBuffer in, CharBuffer out, boolean endOfInput)
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
is
CodingErrorAction.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 is
CodingErrorAction.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.
in
- The input byte bufferout
- The output character bufferendOfInput
- true if, and only if, the invoker can provide no
additional input bytes beyond those in the given buffer
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 exceptionpublic final CoderResult flush(CharBuffer out)
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.
out
- The output character buffer
CoderResult.UNDERFLOW
or
CoderResult.OVERFLOW
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
parameterprotected CoderResult implFlush(CharBuffer out)
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.
out
- The output character buffer
CoderResult.UNDERFLOW
or
CoderResult.OVERFLOW
public final CharsetDecoder reset()
This method resets charset-independent state and also invokes the
implReset
method in order to perform any
charset-specific reset actions.
protected void implReset()
The default implementation of this method does nothing. This method should be overridden by decoders that maintain internal state.
protected abstract CoderResult decodeLoop(ByteBuffer in, CharBuffer out)
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.
in
- The input byte bufferout
- The output character buffer
public final CharBuffer decode(ByteBuffer in) throws CharacterCodingException
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.
in
- The input byte buffer
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
CharacterCodingException
public boolean isAutoDetecting()
The default implementation of this method always returns false; it should be overridden by auto-detecting decoders to return true.
public boolean isCharsetDetected()
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.
UnsupportedOperationException
- If this decoder does not implement an auto-detecting charsetpublic Charset detectedCharset()
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.
IllegalStateException
- If insufficient bytes have been read to determine a charset
UnsupportedOperationException
- If this decoder does not implement an auto-detecting charset
|
Java™ Platform Standard Ed. 6 |
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