public abstract class CharsetDecoder extends Object
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
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.
Modifier | Constructor | Description |
---|---|---|
protected |
Initializes a new decoder. |
Modifier and Type | Method | Description |
---|---|---|
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 |
Convenience method that decodes the remaining content of a single input byte buffer into a newly-allocated character buffer. |
final CoderResult |
decode |
Decodes as many bytes as possible from the given input buffer, writing the results to the given output buffer. |
protected abstract CoderResult |
decodeLoop |
Decodes one or more bytes into one or more characters. |
Charset |
detectedCharset() |
Retrieves the charset that was detected by this decoder (optional operation). |
final CoderResult |
flush |
Flushes this decoder. |
protected CoderResult |
implFlush |
Flushes this decoder. |
protected void |
implOnMalformedInput |
Reports a change to this decoder's malformed-input action. |
protected void |
implOnUnmappableCharacter |
Reports a change to this decoder's unmappable-character action. |
protected void |
implReplaceWith |
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. |
final float |
maxCharsPerByte() |
Returns the maximum number of characters that will be produced for each byte of input. |
final CharsetDecoder |
onMalformedInput |
Changes this decoder's action for malformed-input errors. |
final CharsetDecoder |
onUnmappableCharacter |
Changes this decoder's action for unmappable-character errors. |
final String |
replacement() |
Returns this decoder's replacement value. |
final CharsetDecoder |
replaceWith |
Changes 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 CharsetDecoder(Charset cs, float averageCharsPerByte, float maxCharsPerByte)
"\uFFFD"
.cs
- The charset that created this decoderaverageCharsPerByte
- 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 byteIllegalArgumentException
- If the preconditions on the parameters do not holdpublic final Charset charset()
public final String replacement()
null
and is never emptypublic 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
, must have non-zero length, and must not be longer than the value returned by the maxCharsPerByte
methodIllegalArgumentException
- 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
- The replacement valuepublic CodingErrorAction malformedInputAction()
null
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.
newAction
- The new actionpublic CodingErrorAction unmappableCharacterAction()
null
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.
newAction
- The new actionpublic 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 bufferIllegalStateException
- 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 operationCoderMalfunctionError
- 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 bufferCoderResult.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 bufferCoderResult.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 bufferpublic 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 bufferIllegalStateException
- If a decoding operation is already in progressMalformedInputException
- 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
- 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
OutOfMemoryError
- If the output character buffer for the requested size of the input byte buffer cannot be allocatedpublic boolean isAutoDetecting()
The default implementation of this method always returns false
; it should be overridden by auto-detecting decoders to return true
.
true
if, and only if, this decoder implements an auto-detecting charsetpublic 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.
true
if, and only if, this decoder has detected a specific charsetUnsupportedOperationException
- 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.
null
if the charset has not yet been determinedIllegalStateException
- If insufficient bytes have been read to determine a charsetUnsupportedOperationException
- If this decoder does not implement an auto-detecting charset
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