public abstract static class FileBasedSource.FileBasedReader<T> extends OffsetBasedSource.OffsetBasedReader<T>
reader that implements code common to readers of
FileBasedSources.
This reader uses a ReadableByteChannel created for the file represented by the
corresponding source to efficiently move to the correct starting position defined in the
source. Subclasses of this reader should implement startReading(java.nio.channels.ReadableByteChannel) to get access to this
channel. If the source corresponding to the reader is for a subrange of a file the
ReadableByteChannel provided is guaranteed to be an instance of the type
SeekableByteChannel, which may be used by subclass to traverse back in the channel to
determine the correct starting position.
Sequential reading is implemented using readNextRecord().
Then FileBasedReader implements "reading a range [A, B)" in the following way.
OffsetBasedSource.OffsetBasedReader.start() opens the file
OffsetBasedSource.OffsetBasedReader.start() seeks the SeekableByteChannel to A (reading offset ranges for
non-seekable files is not supported) and calls startReading()
OffsetBasedSource.OffsetBasedReader.start() calls OffsetBasedSource.OffsetBasedReader.advance() once, which, via readNextRecord(),
locates the first record which is at a split point AND its offset is at or after A.
If this record is at or after B, OffsetBasedSource.OffsetBasedReader.advance() returns false and reading is finished.
true sequential reading starts and
advance() will be called repeatedly
advance() calls readNextRecord() on the subclass, and stops (returns false) if
the new record is at a split point AND the offset of the new record is at or after B.
Since this class implements Source.Reader it guarantees thread safety. Abstract
methods defined here will not be accessed by more than one thread concurrently.
SPLIT_POINTS_UNKNOWN| Constructor and Description |
|---|
FileBasedReader(FileBasedSource<T> source)
Subclasses should not perform IO operations at the constructor.
|
| Modifier and Type | Method and Description |
|---|---|
protected boolean |
advanceImpl()
Advances to the next record and returns
true, or returns false if there is no next
record. |
void |
close()
Closes any
ReadableByteChannel created for the current reader. |
FileBasedSource<T> |
getCurrentSource()
Returns a
Source describing the same input that this Reader currently reads
(including items already read). |
protected abstract boolean |
readNextRecord()
Reads the next record from the channel provided by
startReading(java.nio.channels.ReadableByteChannel). |
protected boolean |
startImpl()
Initializes the
OffsetBasedSource.OffsetBasedReader and advances to the first record,
returning true if there is a record available to be read. |
protected abstract void |
startReading(ReadableByteChannel channel)
Performs any initialization of the subclass of
FileBasedReader that involves IO
operations. |
advance, getCurrentOffset, getFractionConsumed, getSplitPointsConsumed, getSplitPointsRemaining, isAtSplitPoint, isDone, isStarted, splitAtFraction, startgetCurrentTimestampgetCurrentpublic FileBasedReader(FileBasedSource<T> source)
startReading(java.nio.channels.ReadableByteChannel) method is invoked.public FileBasedSource<T> getCurrentSource()
BoundedSource.BoundedReaderSource describing the same input that this Reader currently reads
(including items already read).
Reader subclasses can use this method for convenience to access unchanging properties of the source being read. Alternatively, they can cache these properties in the constructor.
The framework will call this method in the course of dynamic work rebalancing, e.g. after
a successful BoundedSource.BoundedReader.splitAtFraction(double) call.
Source objects must always be immutable. However, the return value of
this function may be affected by dynamic work rebalancing, happening asynchronously via
BoundedSource.BoundedReader.splitAtFraction(double), meaning it can return a different
Source object. However, the returned object itself will still itself be immutable.
Callers must take care not to rely on properties of the returned source that may be
asynchronously changed as a result of this process (e.g. do not cache an end offset when
reading a file).
Source possible.
In practice, the implementation of this method should nearly always be one of the following:
BoundedSource.BoundedReader.getCurrentSource(): delegate to base class. In this case, it is almost always
an error for the subclass to maintain its own copy of the source.
public FooReader(FooSource<T> source) {
super(source);
}
public FooSource<T> getCurrentSource() {
return (FooSource<T>)super.getCurrentSource();
}
private final FooSource<T> source;
public FooReader(FooSource<T> source) {
this.source = source;
}
public FooSource<T> getCurrentSource() {
return source;
}
BoundedSource.BoundedReader that explicitly supports dynamic work rebalancing:
maintain a variable pointing to an immutable source object, and protect it with
synchronization.
private FooSource<T> source;
public FooReader(FooSource<T> source) {
this.source = source;
}
public synchronized FooSource<T> getCurrentSource() {
return source;
}
public synchronized FooSource<T> splitAtFraction(double fraction) {
...
FooSource<T> primary = ...;
FooSource<T> residual = ...;
this.source = primary;
return residual;
}
getCurrentSource in class OffsetBasedSource.OffsetBasedReader<T>protected final boolean startImpl()
throws IOException
OffsetBasedSource.OffsetBasedReaderOffsetBasedSource.OffsetBasedReader and advances to the first record,
returning true if there is a record available to be read. This method will be
invoked exactly once and may perform expensive setup operations that are needed to
initialize the reader.
This function is the OffsetBasedReader implementation of
BoundedReader#start. The key difference is that the implementor can ignore the
possibility that it should no longer produce the first record, either because it has exceeded
the original endOffset assigned to the reader, or because a concurrent call to
OffsetBasedSource.OffsetBasedReader.splitAtFraction(double) has changed the source to shrink the offset range being read.
startImpl in class OffsetBasedSource.OffsetBasedReader<T>IOExceptionBoundedReader#startprotected final boolean advanceImpl()
throws IOException
OffsetBasedSource.OffsetBasedReadertrue, or returns false if there is no next
record.
This function is the OffsetBasedReader implementation of
BoundedReader#advance. The key difference is that the implementor can ignore the
possibility that it should no longer produce the next record, either because it has exceeded
the original endOffset assigned to the reader, or because a concurrent call to
OffsetBasedSource.OffsetBasedReader.splitAtFraction(double) has changed the source to shrink the offset range being read.
advanceImpl in class OffsetBasedSource.OffsetBasedReader<T>IOExceptionBoundedReader#advancepublic void close()
throws IOException
ReadableByteChannel created for the current reader. This implementation is
idempotent. Any close() method introduced by a subclass must be idempotent and must
call the close() method in the FileBasedReader.close in interface AutoCloseableclose in class Source.Reader<T>IOExceptionprotected abstract void startReading(ReadableByteChannel channel) throws IOException
FileBasedReader that involves IO
operations. Will only be invoked once and before that invocation the base class will seek the
channel to the source's starting offset.
Provided ReadableByteChannel is for the file represented by the source of this
reader. Subclass may use the channel to build a higher level IO abstraction, e.g., a
BufferedReader or an XML parser.
If the corresponding source is for a subrange of a file, channel is guaranteed to
be an instance of the type SeekableByteChannel.
After this method is invoked the base class will not be reading data from the channel or adjusting the position of the channel. But the base class is responsible for properly closing the channel.
channel - a byte channel representing the file backing the reader.IOExceptionprotected abstract boolean readNextRecord()
throws IOException
startReading(java.nio.channels.ReadableByteChannel). Methods
Source.Reader.getCurrent(), OffsetBasedSource.OffsetBasedReader.getCurrentOffset(), and OffsetBasedSource.OffsetBasedReader.isAtSplitPoint() should return
the corresponding information about the record read by the last invocation of this method.
Note that this method will be called the same way for reading the first record in the source (file or offset range in the file) and for reading subsequent records. It is up to the subclass to do anything special for locating and reading the first record, if necessary.
true if a record was successfully read, false if the end of the
channel was reached before successfully reading a new record.IOException