com.google.cloud.dataflow.sdk.io

Interface Source.Reader<T>

All Superinterfaces:

java.lang.AutoCloseable

All Known Subinterfaces:

BoundedSource.BoundedReader<T>

All Known Implementing Classes:

BoundedSource.AbstractBoundedReader, ByteOffsetBasedSource.ByteOffsetBasedReader, DatastoreIO.DatastoreReader, FileBasedSource.FileBasedReader, Source.AbstractReader

Enclosing class:

Source<T>

public static interface Source.Reader<T>
extends java.lang.AutoCloseable

The interface that readers of custom input sources must implement.

This interface is deliberately distinct from Iterator because the current model tends to be easier to program and more efficient in practice for iterating over sources such as files, databases etc. (rather than pure collections).

To read a Reader:

 for (boolean available = reader.start(); available; available = reader.advance()) {
   T item = reader.getCurrent();
   ...
 }
 

Note: this interface is work-in-progress and may change.

Method Summary

Methods

Modifier and Type	Method and Description
boolean	advance() Advances the reader to the next valid record.
void	close() Closes the reader.
T	getCurrent() Returns the value of the data item that was read by the last start() or advance() call.
Source<T>	getCurrentSource() Returns a Source describing the same input that this Reader reads (including items already read).
Instant	getCurrentTimestamp() Returns the timestamp associated with the current data item.
boolean	start() Initializes the reader and advances the reader to the first record.

Method Detail

start

boolean start()
              throws java.io.IOException

Initializes the reader and advances the reader to the first record.

This method should be called exactly once. The invocation should occur prior to calling advance() or getCurrent(). This method may perform expensive operations that are needed to initialize the reader.

Returns:

true if a record was read, false if we're at the end of input.

Throws:

java.io.IOException

advance

boolean advance()
                throws java.io.IOException

Advances the reader to the next valid record. Invalidates the result of the previous getCurrent() call.

Returns:

true if a record was read, false if we're at the end of input.

Throws:

java.io.IOException

getCurrent

T getCurrent()
             throws java.util.NoSuchElementException

Returns the value of the data item that was read by the last start() or advance() call.

Throws:

java.util.NoSuchElementException - if the reader is at the beginning of the input and start() or advance() wasn't called, or if the last start() or advance() returned false.

getCurrentTimestamp

Instant getCurrentTimestamp()
                            throws java.util.NoSuchElementException

Returns the timestamp associated with the current data item.

If the source does not support timestamps, this should return BoundedWindow.TIMESTAMP_MIN_VALUE.

Throws:

java.util.NoSuchElementException

close

void close()
           throws java.io.IOException

Closes the reader. The reader cannot be used after this method is called.

Specified by:

close in interface java.lang.AutoCloseable

Throws:

java.io.IOException

getCurrentSource

Source<T> getCurrentSource()

Returns a Source describing the same input that this Reader reads (including items already read).

A reader created from the result of getCurrentSource, if consumed, MUST return the same data items as the current reader.