hudson.remoting
Class Channel

java.lang.Object
  hudson.remoting.Channel

All Implemented Interfaces:

VirtualChannel

public class Channel
extends Object
implements VirtualChannel

Represents a communication channel to the remote peer.

A Channel is a mechanism for two JVMs to communicate over bi-directional InputStream/OutputStream pair. Channel represents an endpoint of the stream, and thus two Channels are always used in a pair.

Communication is established as soon as two Channel instances are created at the end fo the stream pair until the stream is terminated via close().

The basic unit of remoting is an executable Callable object. An application can create a Callable object, and execute it remotely by using the call(Callable) method or callAsync(Callable) method.

In this sense, Channel is a mechanism to delegate/offload computation to other JVMs and somewhat like an agent system. This is bit different from remoting technologies like CORBA or web services, where the server exposes a certain functionality that clients invoke.

Callable object, as well as the return value / exceptions, are transported by using Java serialization. All the necessary class files are also shipped over Channel on-demand, so there's no need to pre-deploy such classes on both JVMs.

Implementor's Note

Channel builds its features in a layered model. Its higher-layer features are built on top of its lower-layer features, and they are called layer-0, layer-1, etc.

• Layer 0: See Command for more details. This is for higher-level features, and not likely useful for applications directly.
• Layer 1: See Request for more details. This is for higher-level features, and not likely useful for applications directly.

Author:

Kohsuke Kawaguchi

Nested Class Summary

static class	Channel.Listener Callback "interface" for changes in the state of Channel.
static class	Channel.Mode Communication mode.

Field Summary

AtomicInteger	classLoadingCount Total counts of remote classloading activities.
AtomicLong	classLoadingTime Total number of nanoseconds spent for remote class loading.
static int	PIPE_WINDOW_SIZE
Capability	remoteCapability Capability of the remote Channel.
AtomicInteger	resourceLoadingCount Total count of remote resource loading.
AtomicLong	resourceLoadingTime Total number of nanoseconds spent for remote resource loading.

Constructor Summary

Channel(String name, ExecutorService exec, Channel.Mode mode, InputStream is, OutputStream os)

Channel(String name, ExecutorService exec, Channel.Mode mode, InputStream is, OutputStream os, OutputStream header)

Channel(String name, ExecutorService exec, Channel.Mode mode, InputStream is, OutputStream os, OutputStream header, boolean restricted)
Creates a new channel.

Channel(String name, ExecutorService exec, InputStream is, OutputStream os)

Channel(String name, ExecutorService exec, InputStream is, OutputStream os, OutputStream header)

Method Summary

void	addListener(Channel.Listener l) Registers a new Channel.Listener.
<V,T extends Throwable> V	call(Callable<V,T> callable) Makes a remote procedure call.
<V,T extends Throwable> Future<V>	callAsync(Callable<V,T> callable) Makes an asynchronous remote procedure call.
void	close() Performs an orderly shut down of this channel (and the remote peer.)
ListeningPort	createLocalToRemotePortForwarding(int recvPort, String forwardHost, int forwardPort) Starts a local to remote port forwarding (the equivalent of "ssh -L").
ListeningPort	createRemoteToLocalPortForwarding(int recvPort, String forwardHost, int forwardPort) Starts a remote to local port forwarding (the equivalent of "ssh -R").
static Channel	current() This method can be invoked during the serialization/deserialization of objects when they are transferred to the remote Channel, as well as during Callable.call() is invoked.
void	dumpExportTable(PrintWriter w) Dumps the list of exported objects and their allocation traces to the given output.
<T> T	export(Class<T> type, T instance) Exports an object for remoting to the other Channel by creating a remotable proxy.
long	getLastHeard()
<T> T	getProperty(ChannelProperty<T> key)
Object	getProperty(Object key) Gets the application specific property set by setProperty(Object, Object).
Object	getRemoteProperty(Object key)
void	join() Waits for this Channel to be closed down.
void	join(long timeout) Waits for this Channel to be closed down, but only up the given milliseconds.
boolean	preloadJar(Callable<?,?> classLoaderRef, Class... classesInJar) Preloads jar files on the remote side.
boolean	preloadJar(ClassLoader local, Class... classesInJar)
boolean	preloadJar(ClassLoader local, URL... jars)
boolean	removeListener(Channel.Listener l) Removes a listener.
void	resetPerformanceCounters() Resets all the performance counters.
Object	setProperty(Object key, Object value) Sets the property value on this side of the channel.
hudson.remoting.ExportTable.ExportList	startExportRecording()
void	syncIO() Blocks until all the I/O packets sent before this gets fully executed by the remote side, then return.
protected void	terminate(IOException e) Aborts the connection in response to an error.
String	toString()
Object	waitForProperty(Object key) Works like getProperty(Object) but wait until some value is set by someone.
Object	waitForRemoteProperty(Object key)

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Field Detail

classLoadingTime

public final AtomicLong classLoadingTime

Total number of nanoseconds spent for remote class loading.

Remote code execution often results in classloading activity (more precisely, when the remote peer requests some computation on this channel, this channel often has to load necessary classes from the remote peer.)

This counter represents the total amount of time this channel had to spend loading classes from the remote peer. The time measurement doesn't include the time locally spent to actually define the class (as the local classloading would have incurred the same cost.)

classLoadingCount

public final AtomicInteger classLoadingCount

Total counts of remote classloading activities. Used in a pair with classLoadingTime.

resourceLoadingTime

public final AtomicLong resourceLoadingTime

Total number of nanoseconds spent for remote resource loading.

See Also:

classLoadingTime

resourceLoadingCount

public final AtomicInteger resourceLoadingCount

Total count of remote resource loading.

See Also:

classLoadingCount

remoteCapability

public final Capability remoteCapability

Capability of the remote Channel.

PIPE_WINDOW_SIZE

public static final int PIPE_WINDOW_SIZE

Constructor Detail

Channel

public Channel(String name,
               ExecutorService exec,
               InputStream is,
               OutputStream os)
        throws IOException

Throws:

IOException

Channel

public Channel(String name,
               ExecutorService exec,
               Channel.Mode mode,
               InputStream is,
               OutputStream os)
        throws IOException

Throws:

IOException

Channel

public Channel(String name,
               ExecutorService exec,
               InputStream is,
               OutputStream os,
               OutputStream header)
        throws IOException

Throws:

IOException

Channel

public Channel(String name,
               ExecutorService exec,
               Channel.Mode mode,
               InputStream is,
               OutputStream os,
               OutputStream header)
        throws IOException

Throws:

IOException

Channel

public Channel(String name,
               ExecutorService exec,
               Channel.Mode mode,
               InputStream is,
               OutputStream os,
               OutputStream header,
               boolean restricted)
        throws IOException

Creates a new channel.

Parameters:

name - Human readable name of this channel. Used for debug/logging. Can be anything.

exec - Commands sent from the remote peer will be executed by using this Executor.

mode - The encoding to be used over the stream.

is - Stream connected to the remote peer. It's the caller's responsibility to do buffering on this stream, if that's necessary.

os - Stream connected to the remote peer. It's the caller's responsibility to do buffering on this stream, if that's necessary.

header - If non-null, receive the portion of data in is before the data goes into the "binary mode". This is useful when the established communication channel might include some data that might be useful for debugging/trouble-shooting.

restricted - If true, this channel won't accept Commands that allow the remote end to execute arbitrary closures --- instead they can only call methods on objects that are exported by this channel. This also prevents the remote end from loading classes into JVM. Note that it still allows the remote end to deserialize arbitrary object graph (provided that all the classes are already available in this JVM), so exactly how safe the resulting behavior is is up to discussion.

Throws:

IOException

Method Detail

export

public <T> T export(Class<T> type,
                    T instance)

Exports an object for remoting to the other Channel by creating a remotable proxy.

All the parameters and return values must be serializable.

Specified by:

export in interface VirtualChannel

Parameters:

type - Interface to be remoted.

Returns:

the proxy object that implements T. This object can be transfered to the other Channel, and calling methods on it from the remote side will invoke the same method on the given local instance object.

preloadJar

public boolean preloadJar(Callable<?,?> classLoaderRef,
                          Class... classesInJar)
                   throws IOException,
                          InterruptedException

Preloads jar files on the remote side.

This is a performance improvement method that can be safely ignored if your goal is just to make things working.

Normally, classes are transferred over the network one at a time, on-demand. This design is mainly driven by how Java classloading works — we can't predict what classes will be necessarily upfront very easily.

Classes are loaded only once, so for long-running Channel, this is normally an acceptable overhead. But sometimes, for example when a channel is short-lived, or when you know that you'll need a majority of classes in certain jar files, then it is more efficient to send a whole jar file over the network upfront and thereby avoiding individual class transfer over the network.

That is what this method does. It ensures that a series of jar files are copied to the remote side (AKA "preloading.") Classloading will consult the preloaded jars before performing network transfer of class files.

Parameters:

classLoaderRef - This parameter is used to identify the remote classloader that will prefetch the specified jar files. That is, prefetching will ensure that prefetched jars will kick in when this Callable object is actually executed remote side.

RemoteClassLoaders are created wisely, one per local ClassLoader, so this parameter doesn't have to be exactly the same Callable to be executed later — it just has to be of the same class.

classesInJar - Class objects that identify jar files to be preloaded. Jar files that contain the specified classes will be preloaded into the remote peer. You just need to specify one class per one jar.

Returns:

true if the preloading actually happened. false if all the jars are already preloaded. This method is implemented in such a way that unnecessary jar file transfer will be avoided, and the return value will tell you if this optimization kicked in. Under normal circumstances your program shouldn't depend on this return value. It's just a hint.

Throws:

IOException - if the preloading fails.

InterruptedException

preloadJar

public boolean preloadJar(ClassLoader local,
                          Class... classesInJar)
                   throws IOException,
                          InterruptedException

Throws:

IOException

InterruptedException

preloadJar

public boolean preloadJar(ClassLoader local,
                          URL... jars)
                   throws IOException,
                          InterruptedException

Throws:

IOException

InterruptedException

call

public <V,T extends Throwable> V call(Callable<V,T> callable)
       throws IOException,
              T extends Throwable,
              InterruptedException

Makes a remote procedure call.

Sends Callable to the remote system, executes it, and returns its result.

Specified by:

call in interface VirtualChannel

Throws:

IOException - If there's any error in the communication between Channels.

InterruptedException - If the current thread is interrupted while waiting for the completion.

T extends Throwable

callAsync

public <V,T extends Throwable> Future<V> callAsync(Callable<V,T> callable)
                    throws IOException

Makes an asynchronous remote procedure call.

Similar to VirtualChannel.call(Callable) but returns immediately. The result of the Callable can be obtained through the Future object.

Specified by:

callAsync in interface VirtualChannel

Returns:

The Future object that can be used to wait for the completion.

Throws:

IOException - If there's an error during the communication.

terminate

protected void terminate(IOException e)

Aborts the connection in response to an error.

Parameters:

e - The error that caused the connection to be aborted. Never null.

addListener

public void addListener(Channel.Listener l)

Registers a new Channel.Listener.

See Also:

removeListener(Listener)

removeListener

public boolean removeListener(Channel.Listener l)

Removes a listener.

Returns:

false if the given listener has not been registered to begin with.

join

public void join()
          throws InterruptedException

Waits for this Channel to be closed down. The close-down of a Channel might be initiated locally or remotely.

Specified by:

join in interface VirtualChannel

Throws:

InterruptedException - If the current thread is interrupted while waiting for the completion.

join

public void join(long timeout)
          throws InterruptedException

Waits for this Channel to be closed down, but only up the given milliseconds.

Specified by:

join in interface VirtualChannel

Throws:

InterruptedException - If the current thread is interrupted while waiting for the completion.

Since:

1.299

resetPerformanceCounters

public void resetPerformanceCounters()

Resets all the performance counters.

close

public void close()
           throws IOException

Performs an orderly shut down of this channel (and the remote peer.)

Specified by:

close in interface VirtualChannel

Throws:

IOException - if the orderly shut-down failed.

getProperty

public Object getProperty(Object key)

Gets the application specific property set by setProperty(Object, Object). These properties are also accessible from the remote channel via getRemoteProperty(Object).

This mechanism can be used for one side to discover contextual objects created by the other JVM (as opposed to executing Callable, which cannot have any reference to the context of the remote Channel.

getProperty

public <T> T getProperty(ChannelProperty<T> key)

waitForProperty

public Object waitForProperty(Object key)
                       throws InterruptedException

Works like getProperty(Object) but wait until some value is set by someone.

Throws:

InterruptedException

setProperty

public Object setProperty(Object key,
                          Object value)

Sets the property value on this side of the channel.

See Also:

getProperty(Object)

getRemoteProperty

public Object getRemoteProperty(Object key)

waitForRemoteProperty

public Object waitForRemoteProperty(Object key)
                             throws InterruptedException

Throws:

InterruptedException

createLocalToRemotePortForwarding

public ListeningPort createLocalToRemotePortForwarding(int recvPort,
                                                       String forwardHost,
                                                       int forwardPort)
                                                throws IOException,
                                                       InterruptedException

Starts a local to remote port forwarding (the equivalent of "ssh -L").

Parameters:

recvPort - The port on this local machine that we'll listen to. 0 to let OS pick a random available port. If you specify 0, use ListeningPort.getPort() to figure out the actual assigned port.

forwardHost - The remote host that the connection will be forwarded to. Connection to this host will be made from the other JVM that this Channel represents.

forwardPort - The remote port that the connection will be forwarded to.

Returns:

Throws:

IOException

InterruptedException

createRemoteToLocalPortForwarding

public ListeningPort createRemoteToLocalPortForwarding(int recvPort,
                                                       String forwardHost,
                                                       int forwardPort)
                                                throws IOException,
                                                       InterruptedException

Starts a remote to local port forwarding (the equivalent of "ssh -R").

Parameters:

recvPort - The port on the remote JVM (represented by this Channel) that we'll listen to. 0 to let OS pick a random available port. If you specify 0, use ListeningPort.getPort() to figure out the actual assigned port.

forwardHost - The remote host that the connection will be forwarded to. Connection to this host will be made from this JVM.

forwardPort - The remote port that the connection will be forwarded to.

Returns:

Throws:

IOException

InterruptedException

syncIO

public void syncIO()
            throws IOException,
                   InterruptedException

Blocks until all the I/O packets sent before this gets fully executed by the remote side, then return.

Throws:

IOException - If the remote doesn't support this operation, or if sync fails for other reasons.

InterruptedException

toString

public String toString()

Overrides:

toString in class Object

dumpExportTable

public void dumpExportTable(PrintWriter w)
                     throws IOException

Dumps the list of exported objects and their allocation traces to the given output.

Throws:

IOException

startExportRecording

public hudson.remoting.ExportTable.ExportList startExportRecording()

getLastHeard

public long getLastHeard()

See Also:

lastHeard

current

public static Channel current()

This method can be invoked during the serialization/deserialization of objects when they are transferred to the remote Channel, as well as during Callable.call() is invoked.

Returns:

null if the calling thread is not performing serialization.