java.lang.Objectjava.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
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, passing
false
for the
endOfInput
argument and filling the input buffer and flushing the output buffer between invocations;
Invoke the
decode
method one final time, passing
true
for the
endOfInput
argument; and then
Invoke the
flush
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
"\uFFFD"
; its value may be changed via the
replaceWith
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
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
Each invocation of the
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.
implReset
reset
Instances of this class are not safe for use by multiple concurrent threads.
Constructor Summary | |
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protected |
CharsetDecoder
(
Charset
Initializes a new decoder. |
Method Summary | |
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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
Convenience method that decodes the remaining content of a single input byte buffer into a newly-allocated character buffer. |
CoderResult |
decode
(
ByteBuffer
in,
CharBuffer
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
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
Flushes this decoder. |
protected CoderResult |
implFlush
(
CharBuffer
Flushes this decoder. |
protected void |
implOnMalformedInput
(
CodingErrorAction
Reports a change to this decoder's malformed-input action. |
protected void |
implOnUnmappableCharacter
(
CodingErrorAction
Reports a change to this decoder's unmappable-character action. |
protected void |
implReplaceWith
(
String
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
Changes this decoder's action for malformed-input errors. |
CharsetDecoder |
onUnmappableCharacter
(
CodingErrorAction
Changes this decoder's action for unmappable-character errors. |
String |
replacement
() Returns this decoder's replacement value. |
CharsetDecoder |
replaceWith
(
String
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
Method Detail |
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public final Charsetcharset ()
public final Stringreplacement ()
public final CharsetDecoderreplaceWith ( String
This method invokes the implReplaceWith
protected void implReplaceWith ( String
The default implementation of this method does nothing. This method should be overridden by decoders that require notification of changes to the replacement.
public CodingErrorActionmalformedInputAction ()
public final CharsetDecoderonMalformedInput ( CodingErrorAction
This method invokes the implOnMalformedInput
protected void implOnMalformedInput ( CodingErrorAction
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 CodingErrorActionunmappableCharacterAction ()
public final CharsetDecoderonUnmappableCharacter ( CodingErrorAction
This method invokes the implOnUnmappableCharacter
protected void implOnUnmappableCharacter ( CodingErrorAction
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 CoderResultdecode ( ByteBuffer in, CharBuffer
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()
In addition to reading bytes from the input buffer and writing characters to the output buffer, this method returns a CoderResult
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
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
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.
public 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.
protected 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.
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.
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.
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.
public 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.