netty Getting Started--reference
reference from:http://docs.jboss.org/netty/3.1/guide/html/start.html
- 1.1. Before Getting Started
- 1.2. Writing a Discard Server
- 1.3. Looking into the Received Data
- 1.4. Writing an Echo Server
- 1.5. Writing a Time Server
- 1.6. Writing a Time Client
- 1.7. Dealing with a Stream-based Transport
- 1.8. Speaking in POJO instead of ChannelBuffer
- 1.9. Shutting Down Your Application
- 1.10. Summary
This chapter tours around the core constructs of Netty with simple examples to let you get started quickly. You will be able to write a client and a server on top of Netty right away when you are at the end of this chapter.
If you prefer top-down approach in learning something, you might want to start from Chapter 2, Architectural Overview and get back here.
1.1. Before Getting Started
The minimum requirements to run the examples which are introduced in this chapter are only two; the latest version of Netty and JDK 1.5 or above. The latest version of Netty is available in the project download page. To download the right version of JDK, please refer to your preferred JDK vendor's web site.
Is that all? To tell the truth, you should find these two are just enough to implement almost any type of protocols. Otherwise, please feel free to contact the Netty project community and let us know what's missing.
At last but not least, please refer to the API reference whenever you want to know more about the classes introduced here. All class names in this document are linked to the online API reference for your convenience. Also, please don't hesitate to contact the Netty project community and let us know if there's any incorrect information, errors in grammar and typo, and if you have a good idea to improve the documentation.
1.2. Writing a Discard Server
The most simplistic protocol in the world is not 'Hello, World!' but DISCARD. It's a protocol which discards any received data without any response.
To implement the DISCARD protocol, the only thing you need to do is to ignore all received data. Let us start straight from the handler implementation, which handles I/O events generated by Netty.
package org.jboss.netty.example.discard;
@ChannelPipelineCoverage
("all")
public class DiscardServerHandler extends SimpleChannelHandler
{
@Override
public void messageReceived(ChannelHandlerContext
ctx,MessageEvent
e) {
} @Override
public void exceptionCaught(ChannelHandlerContext
ctx,ExceptionEvent
e) {
e.getCause().printStackTrace();Channel
ch = e.getChannel();
ch.close();
}
}
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We override the |
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So far so good. We have implemented the first half of the DISCARD server. What's left now is to write the main
method which starts the server with the DiscardServerHandler
.
package org.jboss.netty.example.discard; import java.net.InetSocketAddress;
import java.util.concurrent.Executors; public class DiscardServer { public static void main(String[] args) throws Exception {
ChannelFactory
factory =
newNioServerSocketChannelFactory
(
Executors.newCachedThreadPool(),
Executors.newCachedThreadPool());ServerBootstrap
bootstrap = newServerBootstrap
(factory); DiscardServerHandler handler = new DiscardServerHandler();
ChannelPipeline
pipeline = bootstrap.getPipeline();
pipeline.addLast("handler", handler);
bootstrap.setOption("child.tcpNoDelay", true);
bootstrap.setOption("child.keepAlive", true); bootstrap.bind(new InetSocketAddress(8080));
}
}
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Here, we add the |
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You can also set the parameters which are specific to the bootstrap.setOption("reuseAddress", true); |
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We are ready to go now. What's left is to bind to the port and to start the server. Here, we bind to the port |
Congratulations! You've just finished your first server on top of Netty.
1.3. Looking into the Received Data
Now that we have written our first server, we need to test if it really works. The easiest way to test it is to use the telnet
command. For example, you could enter "telnet localhost 8080
" in the command line and type something.
However, can we say that the server is working fine? We cannot really know that because it is a discard server. You will not get any response at all. To prove it is really working, let us modify the server to print what it has received.
We already know that MessageEvent
is generated whenever data is received and the messageReceived
handler method will be invoked. Let us put some code into the messageReceived
method of the DiscardServerHandler
:
@Override
public void messageReceived(ChannelHandlerContext
ctx,MessageEvent
e) {
ChannelBuffer
buf = (ChannelBuffer) e.getMessage();
while(buf.readable()) {
System.out.println((char) buf.readByte());
}
}
It is safe to assume the message type in socket transports is always Although it resembles to NIO |
If you run the telnet
command again, you will see the server prints what has received.
The full source code of the discard server is located in the org.jboss.netty.example.discard
package of the distribution.
1.4. Writing an Echo Server
So far, we have been consuming data without responding at all. A server, however, is usually supposed to respond to a request. Let us learn how to write a response message to a client by implementing the ECHOprotocol, where any received data is sent back.
The only difference from the discard server we have implemented in the previous sections is that it sends the received data back instead of printing the received data out to the console. Therefore, it is enough again to modify the messageReceived
method:
@Override
public void messageReceived(ChannelHandlerContext
ctx,MessageEvent
e) {
Channel
ch = e.getChannel();
ch.write(e.getMessage());
}
A |
If you run the telnet
command again, you will see the server sends back whatever you have sent to it.
The full source code of the echo server is located in the org.jboss.netty.example.echo
package of the distribution.
1.5. Writing a Time Server
The protocol to implement in this section is the TIME protocol. It is different from the previous examples in that it sends a message, which contains a 32-bit integer, without receiving any requests and loses the connection once the message is sent. In this example, you will learn how to construct and send a message, and to close the connection on completion.
Because we are going to ignore any received data but to send a message as soon as a connection is established, we cannot use the messageReceived
method this time. Instead, we should override thechannelConnected
method. The following is the implementation:
package org.jboss.netty.example.time; @ChannelPipelineCoverage
("all")
public class TimeServerHandler extendsSimpleChannelHandler
{ @Override
public void channelConnected(ChannelHandlerContext
ctx,ChannelStateEvent
e) {
Channel
ch = e.getChannel();ChannelBuffer
time =ChannelBuffers
.buffer(4);
time.writeInt(System.currentTimeMillis() / 1000);
ChannelFuture
f = ch.write(time);
f.addListener(new ChannelFutureListener
() {
public void operationComplete(ChannelFuture
future) {
Channel
ch = future.getChannel();
ch.close();
}
});
} @Override
public void exceptionCaught(ChannelHandlerContext
ctx,ExceptionEvent
e) {
e.getCause().printStackTrace();
e.getChannel().close();
}
}
As explained, |
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To send a new message, we need to allocate a new buffer which will contain the message. We are going to write a 32-bit integer, and therefore we need a On the other hand, it is a good idea to use static imports for import static org.jboss.netty.buffer. |
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As usual, we write the constructed message. But wait, where's the In contrast, NIO buffer does not provide a clean way to figure out where the message content starts and ends without calling the Another point to note is that the
Therefore, you need to call the |
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How do we get notified when the write request is finished then? This is as simple as adding a Alternatively, you could simplify the code using a pre-defined listener: f.addListener(
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1.6. Writing a Time Client
Unlike DISCARD and ECHO servers, we need a client for the TIME protocol because a human cannot translate a 32-bit binary data into a date on a calendar. In this section, we discuss how to make sure the server works correctly and learn how to write a client with Netty.
The biggest and only difference between a server and a client in Netty is that different Bootstrap
andChannelFactory
are required. Please take a look at the following code:
package org.jboss.netty.example.time; import java.net.InetSocketAddress;
import java.util.concurrent.Executors; public class TimeClient { public static void main(String[] args) throws Exception {
String host = args[0];
int port = Integer.parseInt(args[1]);ChannelFactory
factory =
newNioClientSocketChannelFactory
(
Executors.newCachedThreadPool(),
Executors.newCachedThreadPool());ClientBootstrap
bootstrap = newClientBootstrap
(factory); TimeClientHandler handler = new TimeClientHandler();
bootstrap.getPipeline().addLast("handler", handler); bootstrap.setOption("tcpNoDelay"
, true);
bootstrap.setOption("keepAlive", true); bootstrap.connect
(new InetSocketAddress(host, port));
}
}
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Please note that there's no |
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We should call the |
As you can see, it is not really different from the server side startup. What about the ChannelHandler
implementation? It should receive a 32-bit integer from the server, translate it into a human readable format, print the translated time, and close the connection:
package org.jboss.netty.example.time; import java.util.Date; @ChannelPipelineCoverage
("all")
public class TimeClientHandler extendsSimpleChannelHandler
{ @Override
public void messageReceived(ChannelHandlerContext
ctx,MessageEvent
e) {
ChannelBuffer
buf = (ChannelBuffer
) e.getMessage();
long currentTimeMillis = buf.readInt() * 1000L;
System.out.println(new Date(currentTimeMillis));
e.getChannel().close();
} @Override
public void exceptionCaught(ChannelHandlerContext
ctx,ExceptionEvent
e) {
e.getCause().printStackTrace();
e.getChannel().close();
}
}
It looks very simple and does not look any different from the server side example. However, this handler sometimes will refuse to work raising an IndexOutOfBoundsException
. We discuss why this happens in the next section.
1.7. Dealing with a Stream-based Transport
1.7.1. One Small Caveat of Socket Buffer
In a stream-based transport such as TCP/IP, received data is stored into a socket receive buffer. Unfortunately, the buffer of a stream-based transport is not a queue of packets but a queue of bytes. It means, even if you sent two messages as two independent packets, an operating system will not treat them as two messages but as just a bunch of bytes. Therefore, there is no guarantee that what you read is exactly what your remote peer wrote. For example, let us assume that the TCP/IP stack of an operating system has received three packets:
+-----+-----+-----+
| ABC | DEF | GHI |
+-----+-----+-----+
Because of this general property of a stream-based protocol, there's high chance of reading them in the following fragmented form in your application:
+----+-------+---+---+
| AB | CDEFG | H | I |
+----+-------+---+---+
Therefore, a receiving part, regardless it is server-side or client-side, should defrag the received data into one or more meaningful frames that could be easily understood by the application logic. In case of the example above, the received data should be framed like the following:
+-----+-----+-----+
| ABC | DEF | GHI |
+-----+-----+-----+
1.7.2. The First Solution
Now let us get back to the TIME client example. We have the same problem here. A 32-bit integer is a very small amount of data, and it is not likely to be fragmented often. However, the problem is that it can be fragmented, and the possibility of fragmentation will increase as the traffic increases.
The simplistic solution is to create an internal cumulative buffer and wait until all 4 bytes are received into the internal buffer. The following is the modified TimeClientHandler
implementation that fixes the problem:
package org.jboss.netty.example.time; import static org.jboss.netty.buffer.ChannelBuffers
.*; import java.util.Date; @ChannelPipelineCoverage
("one")
public class TimeClientHandler extendsSimpleChannelHandler
{ private finalChannelBuffer
buf = dynamicBuffer();
@Override
public void messageReceived(ChannelHandlerContext
ctx,MessageEvent
e) {
ChannelBuffer
m = (ChannelBuffer
) e.getMessage();
buf.writeBytes(m);
if (buf.readableBytes() >= 4) {
long currentTimeMillis = buf.readInt() * 1000L;
System.out.println(new Date(currentTimeMillis));
e.getChannel().close();
}
} @Override
public void exceptionCaught(ChannelHandlerContext
ctx,ExceptionEvent
e) {
e.getCause().printStackTrace();
e.getChannel().close();
}
}
This time, |
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A dynamic buffer is a |
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First, all received data should be cumulated into |
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And then, the handler must check if |
There's another place that needs a fix. Do you remember that we added a TimeClientHandler
instance to thedefault ChannelPipeline
of the ClientBootstrap
? It means one same TimeClientHandler
instance is going to handle multiple Channel
s and consequently the data will be corrupted. To create a new TimeClientHandler
instance per Channel
, we have to implement a ChannelPipelineFactory
:
package org.jboss.netty.example.time; public class TimeClientPipelineFactory implementsChannelPipelineFactory
{ publicChannelPipeline
getPipeline() {
ChannelPipeline
pipeline =Channels
.pipeline();
pipeline.addLast("handler", new TimeClientHandler());
return pipeline;
}
}
Now let us replace the following lines of TimeClient
:
TimeClientHandler handler = new TimeClientHandler();
bootstrap.getPipeline().addLast("handler", handler);
with the following:
bootstrap.setPipelineFactory(new TimeClientPipelineFactory());
It might look somewhat complicated at the first glance, and it is true that we don't need to introduceTimeClientPipelineFactory
in this particular case because TimeClient
creates only one connection.
However, as your application gets more and more complex, you will almost always end up with writing aChannelPipelineFactory
, which yields much more flexibility to the pipeline configuration.
1.7.3. The Second Solution
Although the first solution has resolved the problem with the TIME client, the modified handler does not look that clean. Imagine a more complicated protocol which is composed of multiple fields such as a variable length field. Your ChannelHandler
implementation will become unmaintainable very quickly.
As you may have noticed, you can add more than one ChannelHandler
to a ChannelPipeline
, and therefore, you can split one monolithic ChannelHandler
into multiple modular ones to reduce the complexity of your application. For example, you could split TimeClientHandler
into two handlers:
TimeDecoder
which deals with the fragmentation issue, andthe initial simple version of
TimeClientHandler
.
Fortunately, Netty provides an extensible class which helps you write the first one out of the box:
package org.jboss.netty.example.time;
public class TimeDecoder extendsFrameDecoder
{ @Override
protected Object decode(
ChannelHandlerContext
ctx,Channel
channel,ChannelBuffer
buffer)
{ if (buffer.readableBytes() < 4) {
return null;
} return buffer.readBytes(4);
}
}
There's no |
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If |
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If non- |
If you are an adventurous person, you might want to try the ReplayingDecoder
which simplifies the decoder even more. You will need to consult the API reference for more information though.
package org.jboss.netty.example.time; public class TimeDecoder extendsReplayingDecoder
<VoidEnum
> { @Override
protected Object decode(
ChannelHandlerContext
ctx,Channel
channel,
ChannelBuffer
buffer,VoidEnum
state) { return buffer.readBytes(4);
}
}
Additionally, Netty provides out-of-the-box decoders which enables you to implement most protocols very easily and helps you avoid from ending up with a monolithic unmaintainable handler implementation. Please refer to the following packages for more detailed examples:
org.jboss.netty.example.factorial
for a binary protocol, andorg.jboss.netty.example.telnet
for a text line-based protocol.
1.8. Speaking in POJO instead of ChannelBuffer
All the examples we have reviewed so far used a ChannelBuffer
as a primary data structure of a protocol message. In this section, we will improve the TIME protocol client and server example to use a POJO instead of a ChannelBuffer
.
The advantage of using a POJO in your ChannelHandler
is obvious; your handler becomes more maintainable and reusable by separating the code which extracts information from ChannelBuffer
out from the handler. In the TIME client and server examples, we read only one 32-bit integer and it is not a major issue to useChannelBuffer
directly. However, you will find it is necessary to make the separation as you implement a real world protocol.
First, let us define a new type called UnixTime
.
package org.jboss.netty.example.time; import java.util.Date; public class UnixTime {
private final int value; public UnixTime(int value) {
this.value = value;
} public int getValue() {
return value;
} @Override
public String toString() {
return new Date(value * 1000L).toString();
}
}
We can now revise the TimeDecoder
to return a UnixTime
instead of a ChannelBuffer
.
@Override
protected Object decode(
ChannelHandlerContext
ctx,Channel
channel,ChannelBuffer
buffer) {
if (buffer.readableBytes() < 4) {
return null;
} return new UnixTime(buffer.readInt());
}
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With the updated decoder, the TimeClientHandler
does not use ChannelBuffer
anymore:
@Override
public void messageReceived(ChannelHandlerContext
ctx,MessageEvent
e) {
UnixTime m = (UnixTime) e.getMessage();
System.out.println(m);
e.getChannel().close();
}
Much simpler and elegant, right? The same technique can be applied on the server side. Let us update theTimeServerHandler
first this time:
@Override
public void channelConnected(ChannelHandlerContext
ctx,ChannelStateEvent
e) {
UnixTime time = new UnixTime(System.currentTimeMillis() / 1000);
ChannelFuture
f = e.getChannel().write(time);
f.addListener(ChannelFutureListener
.CLOSE);
}
Now, the only missing piece is the ChannelHandler
which translates a UnixTime
back into a ChannelBuffer
. It's much simpler than writing a decoder because there's no need to deal with packet fragmentation and assembly when encoding a message.
package org.jboss.netty.example.time; import static org.jboss.netty.buffer.ChannelBuffers
.*; @ChannelPipelineCoverage
("all")
public class TimeEncoder extendsSimpleChannelHandler
{ public void writeRequested(ChannelHandlerContext
ctx,MessageEvent
e) {
UnixTime time = (UnixTime) e.getMessage();ChannelBuffer
buf = buffer(4);
buf.writeInt(time.getValue());Channels
.write(ctx, e.getFuture(), buf);
}
}
The |
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An encoder overrides the |
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Once done with transforming a POJO into a On the other hand, it is a good idea to use static imports for import static org.jboss.netty.channel. |
The last task left is to insert a TimeEncoder
into the ChannelPipeline
on the server side, and it is left as a trivial exercise.
1.9. Shutting Down Your Application
If you ran the TimeClient
, you must have noticed that the application doesn't exit but just keep running doing nothing. Looking from the full stack trace, you will also find a couple I/O threads are running. To shut down the I/O threads and let the application exit gracefully, you need to release the resources allocated byChannelFactory
.
The shutdown process of a typical network application is composed of the following three steps:
Close all server sockets if there are any,
Close all non-server sockets (i.e. client sockets and accepted sockets) if there are any, and
Release all resources used by
ChannelFactory
.
To apply the three steps above to the TimeClient
, TimeClient.main()
could shut itself down gracefully by closing the only one client connection and releasing all resources used by ChannelFactory
:
package org.jboss.netty.example.time; public class TimeClient {
public static void main(String[] args) throws Exception {
...
ChannelFactory
factory = ...;
ClientBootstrap
bootstrap = ...;
...
ChannelFuture
future
= bootstrap.connect(...);
future.awaitUninterruptible();
if (!future.isSuccess()) {
future.getCause().printStackTrace();
}
future.getChannel().getCloseFuture().awaitUninterruptibly();
factory.releaseExternalResources();
}
}
The |
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Wait for the returned |
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If failed, we print the cause of the failure to know why it failed. the |
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Now that the connection attempt is over, we need to wait until the connection is closed by waiting for the Even if the connection attempt has failed the |
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All connections have been closed at this point. The only task left is to release the resources being used by |
Shutting down a client was pretty easy, but how about shutting down a server? You need to unbind from the port and close all open accepted connections. To do this, you need a data structure that keeps track of the list of active connections, and it's not a trivial task. Fortunately, there is a solution, ChannelGroup
.
ChannelGroup
is a special extension of Java collections API which represents a set of open Channel
s. If a Channel
is added to a ChannelGroup
and the added Channel
is closed, the closed Channel
is removed from its ChannelGroup
automatically. You can also perform an operation on all Channel
s in the same group. For instance, you can close all Channel
s in a ChannelGroup
when you shut down your server.
To keep track of open sockets, you need to modify the TimeServerHandler
to add a new open Channel
to the global ChannelGroup
, TimeServer.allChannels
:
@Override
public void channelOpen(ChannelHandlerContext
ctx,ChannelStateEvent
e) {
TimeServer.allChannels.add(e.getChannel());
}
Yes, |
Now that the list of all active Channel
s are maintained automatically, shutting down a server is as easy as shutting down a client:
package org.jboss.netty.example.time; public class TimeServer { static finalChannelGroup
allChannels = newDefaultChannelGroup
("time-server"
); public static void main(String[] args) throws Exception {
...
ChannelFactory
factory = ...;
ServerBootstrap
bootstrap = ...;
...
Channel
channel
= bootstrap.bind(...);
allChannels.add(channel);
waitForShutdownCommand();
ChannelGroupFuture
future = allChannels.close();
future.awaitUninterruptibly();
factory.releaseExternalResources();
}
}
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The |
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Any type of |
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You can perform the same operation on all channels in the same |
1.10. Summary
In this chapter, we had a quick tour of Netty with a demonstration on how to write a fully working network application on top of Netty. More questions you may have will be covered in the upcoming chapters and the revised version of this chapter. Please also note that the community is always waiting for your questions and ideas to help you and keep improving Netty based on your feed back.
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