org.apache.pulsar.client.api

Interface Reader<T>

All Superinterfaces:

AutoCloseable, Closeable

@InterfaceAudience.Public
 @InterfaceStability.Stable
public interface Reader<T>
extends Closeable

A Reader can be used to scan through all the messages currently available in a topic.

Method Summary

Modifier and Type	Method and Description
CompletableFuture<Void>	closeAsync() Asynchronously close the reader and stop the broker to push more messages.
String	getTopic()
boolean	hasMessageAvailable() Check if there is any message available to read from the current position.
CompletableFuture<Boolean>	hasMessageAvailableAsync() Asynchronously check if there is any message available to read from the current position.
boolean	hasReachedEndOfTopic() Return true if the topic was terminated and this reader has reached the end of the topic.
boolean	isConnected()
Message<T>	readNext() Read the next message in the topic.
Message<T>	readNext(int timeout, TimeUnit unit) Read the next message in the topic waiting for a maximum time.
CompletableFuture<Message<T>>	readNextAsync() Read asynchronously the next message in the topic.
void	seek(long timestamp) Reset the subscription associated with this reader to a specific message publish time.
void	seek(MessageId messageId) Reset the subscription associated with this reader to a specific message id.
CompletableFuture<Void>	seekAsync(long timestamp) Reset the subscription associated with this reader to a specific message publish time.
CompletableFuture<Void>	seekAsync(MessageId messageId) Reset the subscription associated with this reader to a specific message id.

Methods inherited from interface java.io.Closeable

close

Method Detail

getTopic

String getTopic()

Returns:

the topic from which this reader is reading from

readNext

Message<T> readNext()
             throws PulsarClientException

Read the next message in the topic.

This method will block until a message is available.

Returns:

the next message

Throws:

PulsarClientException

readNext

Message<T> readNext(int timeout,
                    TimeUnit unit)
             throws PulsarClientException

Read the next message in the topic waiting for a maximum time.

Returns null if no message is received before the timeout.

Returns:

the next message(Could be null if none received in time)

Throws:

PulsarClientException

readNextAsync

CompletableFuture<Message<T>> readNextAsync()

Read asynchronously the next message in the topic.

readNextAsync() should be called subsequently once returned CompletableFuture gets complete with received message. Else it creates backlog of receive requests in the application.

The returned future can be cancelled before completion by calling .cancel(false) (CompletableFuture.cancel(boolean)) to remove it from the the backlog of receive requests. Another choice for ensuring a proper clean up of the returned future is to use the CompletableFuture.orTimeout method which is available on JDK9+. That would remove it from the backlog of receive requests if receiving exceeds the timeout.

Returns:

a future that will yield a message (when it's available) or PulsarClientException if the reader is already closed.

closeAsync

CompletableFuture<Void> closeAsync()

Asynchronously close the reader and stop the broker to push more messages.

Returns:

a future that can be used to track the completion of the operation

hasReachedEndOfTopic

boolean hasReachedEndOfTopic()

Return true if the topic was terminated and this reader has reached the end of the topic.

Note that this only applies to a "terminated" topic (where the topic is "sealed" and no more messages can be published) and not just that the reader is simply caught up with the publishers. Use hasMessageAvailable() to check for for that.

hasMessageAvailable

boolean hasMessageAvailable()
                     throws PulsarClientException

Check if there is any message available to read from the current position.

This check can be used by an application to scan through a topic and stop when the reader reaches the current last published message. For example:

 while (reader.hasMessageAvailable()) {
     Message<String> msg = reader.readNext();
     // Do something
 }

 // Done reading
 

Note that this call might be blocking (see hasMessageAvailableAsync() for async version) and that even if this call returns true, that will not guarantee that a subsequent call to readNext() will not block.

Returns:

true if the are messages available to be read, false otherwise

Throws:

PulsarClientException - if there was any error in the operation

hasMessageAvailableAsync

CompletableFuture<Boolean> hasMessageAvailableAsync()

Asynchronously check if there is any message available to read from the current position.

This check can be used by an application to scan through a topic and stop when the reader reaches the current last published message.

Returns:

a future that will yield true if the are messages available to be read, false otherwise, or a PulsarClientException if there was any error in the operation

isConnected

boolean isConnected()

Returns:

Whether the reader is connected to the broker

seek

void seek(MessageId messageId)
   throws PulsarClientException

Reset the subscription associated with this reader to a specific message id.

The message id can either be a specific message or represent the first or last messages in the topic.

• MessageId.earliest : Reset the reader on the earliest message available in the topic
• MessageId.latest : Reset the reader on the latest message in the topic

Note: this operation can only be done on non-partitioned topics. For these, one can rather perform the seek() on the individual partitions.

Parameters:

messageId - the message id where to reposition the reader

Throws:

PulsarClientException

seek

void seek(long timestamp)
   throws PulsarClientException

Reset the subscription associated with this reader to a specific message publish time.

Note: this operation can only be done on non-partitioned topics. For these, one can rather perform the seek() on the individual partitions.

Parameters:

timestamp - the message publish time where to reposition the reader

Throws:

PulsarClientException

seekAsync

CompletableFuture<Void> seekAsync(MessageId messageId)

Reset the subscription associated with this reader to a specific message id.

The message id can either be a specific message or represent the first or last messages in the topic.

• MessageId.earliest : Reset the reader on the earliest message available in the topic
• MessageId.latest : Reset the reader on the latest message in the topic

Note: this operation can only be done on non-partitioned topics. For these, one can rather perform the seek() on the individual partitions.

Parameters:

messageId - the message id where to position the reader

Returns:

a future to track the completion of the seek operation

seekAsync

CompletableFuture<Void> seekAsync(long timestamp)

Reset the subscription associated with this reader to a specific message publish time.

Note: this operation can only be done on non-partitioned topics. For these, one can rather perform the seek() on the individual partitions.

Parameters:

timestamp - the message publish time where to position the reader

Returns:

a future to track the completion of the seek operation