32. Stream Support

In many cases, application data is obtained from a stream. It is not recommended to send a reference to a stream as a message payload to a consumer. Instead, messages are created from data that is read from an input stream, and message payloads are written to an output stream one by one.

You need to include this dependency into your project:

Maven. 

<dependency>
    <groupId>org.springframework.integration</groupId>
    <artifactId>spring-integration-stream</artifactId>
    <version>5.1.3.BUILD-SNAPSHOT</version>
</dependency>

Gradle. 

compile "org.springframework.integration:spring-integration-stream:5.1.3.BUILD-SNAPSHOT"

32.1 Reading from Streams

Spring Integration provides two adapters for streams. Both ByteStreamReadingMessageSource and CharacterStreamReadingMessageSource implement MessageSource. By configuring one of these within a channel-adapter element, the polling period can be configured and the message bus can automatically detect and schedule them. The byte stream version requires an InputStream, and the character stream version requires a Reader as the single constructor argument. The ByteStreamReadingMessageSource also accepts the bytesPerMessage property to determine how many bytes it tries to read into each Message. The default value is 1024. The following example creates an input stream that creates messages that each contain 2048 bytes:

<bean class="org.springframework.integration.stream.ByteStreamReadingMessageSource">
  <constructor-arg ref="someInputStream"/>
  <property name="bytesPerMessage" value="2048"/>
</bean>

<bean class="org.springframework.integration.stream.CharacterStreamReadingMessageSource">
  <constructor-arg ref="someReader"/>
</bean>

The CharacterStreamReadingMessageSource wraps the reader in a BufferedReader (if it is not one already). You can set the buffer size used by the buffered reader in the second constructor argument. Starting with version 5.0, a third constructor argument (blockToDetectEOF) controls the behavior of the CharacterStreamReadingMessageSource. When false (the default), the receive() method checks whether the reader is ready() and returns null if not. EOF (end of file) is not detected in this case. When true, the receive() method blocks until data is available or EOF is detected on the underlying stream. When EOF is detected, a StreamClosedEvent (application event) is published. You can consume this event with a bean that implements ApplicationListener<StreamClosedEvent>.

[Note]Note

To facilitate EOF detection, the poller thread blocks in the receive() method until either data arrives or EOF is detected.

[Important]Important

The poller continues to publish an event on each poll once EOF has been detected. The application listener can stop the adapter to prevent this. The event is published on the poller thread. Stopping the adapter causes the thread to be interrupted. If you intend to perform some interruptible task after stopping the adapter, you must either perform the stop() on a different thread or use a different thread for those downstream activities. Note that sending to a QueueChannel is interruptible, so, if you wish to send a message from the listener, do it before stopping the adapter.

This facilitates "piping" or redirecting data to stdin, as the following two examples shows:

cat myfile.txt | java -jar my.jar
java -jar my.jar < foo.txt

This approach lets the application terminate when the pipe is closed.

Four convenient factory methods are available:

public static final CharacterStreamReadingMessageSource stdin() { ... }

public static final CharacterStreamReadingMessageSource stdin(String charsetName) { ... }

public static final CharacterStreamReadingMessageSource stdinPipe() { ... }

public static final CharacterStreamReadingMessageSource stdinPipe(String charsetName) { ... }

32.2 Writing to Streams

For target streams, you can use either of two implementations: ByteStreamWritingMessageHandler or CharacterStreamWritingMessageHandler. Each requires a single constructor argument (OutputStream for byte streams or Writer for character streams), and each provides a second constructor that adds the optional bufferSize. Since both of these ultimately implement the MessageHandler interface, you can reference them from a channel-adapter configuration, as described in Section 6.3, “Channel Adapter”.

<bean class="org.springframework.integration.stream.ByteStreamWritingMessageHandler">
  <constructor-arg ref="someOutputStream"/>
  <constructor-arg value="1024"/>
</bean>

<bean class="org.springframework.integration.stream.CharacterStreamWritingMessageHandler">
  <constructor-arg ref="someWriter"/>
</bean>

32.3 Stream Namespace Support

Spring Integration defines a namespace to reduce the configuration needed for stream-related channel adapters. The following schema locations are needed to use it:

<?xml version="1.0" encoding="UTF-8"?>
<beans:beans xmlns:int-stream="http://www.springframework.org/schema/integration/stream"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:beans="http://www.springframework.org/schema/beans"
  xsi:schemaLocation="http://www.springframework.org/schema/beans
      http://www.springframework.org/schema/beans/spring-beans.xsd
      http://www.springframework.org/schema/integration/stream
      http://www.springframework.org/schema/integration/stream/spring-integration-stream.xsd">

The following code snippet shows the different configuration options that are supported to configure the inbound channel adapter:

<int-stream:stdin-channel-adapter id="adapterWithDefaultCharset"/>

<int-stream:stdin-channel-adapter id="adapterWithProvidedCharset" charset="UTF-8"/>

Starting with version 5.0, you can set the detect-eof attribute, which sets the blockToDetectEOF property. See Section 32.1, “Reading from Streams” for more information.

To configure the outbound channel adapter, you can use the namespace support as well. The following example shows the different configuration for an outbound channel adapters:

<int-stream:stdout-channel-adapter id="stdoutAdapterWithDefaultCharset"
    channel="testChannel"/>

<int-stream:stdout-channel-adapter id="stdoutAdapterWithProvidedCharset" charset="UTF-8"
    channel="testChannel"/>

<int-stream:stderr-channel-adapter id="stderrAdapter" channel="testChannel"/>

<int-stream:stdout-channel-adapter id="newlineAdapter" append-newline="true"
    channel="testChannel"/>