The first method takes an object as argument and serializes it into a byte buffer (the object has to be serializable or externalizable). The byte array is then returned. This method is often used to serialize objects into the byte buffer of a message. The second method returns a reconstructed object from a buffer. Both methods throw an exception if the object cannot be serialized or unseriali

Prints the message given as argument in readable format. Returns a string.

Returns a strings containing a pretty-printed list of currently running threads.

Given a list of member addresses, pretty-prints the members and returns a string.

Interface Transport looks as follows:

                public interface Transport {
                    public void send(Message msg) throws Exception;
                    public Object receive(long timeout) throws Exception;
                }
            

It defines a very small subset of the functionality of a channel, essentially only the methods for sending and receiving messages. There are a number of classes that implement Transport , among others Channel . Many building blocks (see Chapter 4, Building Blocks ) require nothing else than a bare-bone facility to send and receive messages; therefore the Transport interface was created. It increases the genericness and portability of building blocks: being so simple, the Transport interface can easily be ported to a different toolkit, without requiring any modifications to building blocks.

Contrary to the pull-style of channels, some building blocks (e.g. PullPushAdapter ) provide an event-like push-style message delivery model. In this case, the entity to be notified of message reception needs to provide a callback to be invoked whenever a message has been received. The MessageListener interface below provides a method to do so:

                public interface MessageListener {
                    public void receive(Message msg);
                    byte[] getState();
                    void setState(byte[] state);
                }
            

Method receive() will be called when a message is received. The getState() and setState() methods are used to fetch and set the group state (e.g. when joining). Refer to Section 3.7.11, “Getting the group's state” for a discussion of state transfer.

JGroups release 2.3 introduces ExtendedMessageListener enabling partial state transfer (refer to Section 3.7.13, “Partial state transfer” ) while release 2.4 further expands ExtendedMessageListener with streaming state transfer callbacks:

                public interface ExtendedMessageListener extends MessageListener {
                    byte[] getState(String state_id);
                    void setState(String state_id, byte[] state);
                      
                    /*** since JGroups 2.4 *****/
                    void getState(OutputStream ostream);
                    void getState(String state_id, OutputStream ostream);
                    void setState(InputStream istream);
                    void setState(String state_id, InputStream istream);
                }
            

Method receive() will be called when a message is received. The getState() and setState() methods are used to fetch and set the group state (e.g. when joining). Refer to Section 3.7.11, “Getting the group's state” for a discussion of state transfer.

The MembershipListener interface is similar to the MessageListener interface above: every time a new view, a suspicion message, or a block event is received, the corresponding method of the class implementing MembershipListener will be called.

                public interface MembershipListener {
                    public void viewAccepted(View new_view);
                    public void suspect(Object suspected_mbr);
                    public void block();
                }
            

Oftentimes the only method containing any functionality will be viewAccepted() which notifies the receiver that a new member has joined the group or that an existing member has left or crashed. The suspect() callback is invoked by JGroups whenever a member if suspected of having crashed, but not yet excluded ^[1].

The block() method is called to notify the member that it will soon be blocked sending messages. This is done by the FLUSH protocol, for example to ensure that nobody is sending messages while a state transfer is in progress. When block() returns, any thread sending messages will be blocked, until FLUSH unblocks the thread again, e.g. after the state has been transferred successfully.

Therefore, block() can be used to send pending messages or complete some other work. However, sending of messages should be done on a different thread, e.g. the current thread blocks on a mutex, starts a different thread which notifies the mutex once the work has been done.

Note that block() should take a small amount of time to complete, otherwise the entire FLUSH protocol is blocked.

The ExtendedMembershipListener interface extends MembershipListener:

              public interface ExtendedMembershipListener extends MembershipListener {
                  public void unblock();
              }
          

The unblock() method is called to notify the member that the flush protocol has completed and the member can resume sending messages. If the member did not stop sending messages on block(), FLUSH simply blocked them and will resume, so no action is required from a member. Implementation of the unblock() callback is optional.

                public interface ChannelListener {
                    void channelConnected(Channel channel);
                    void channelDisconnected(Channel channel);
                    void channelClosed(Channel channel);
                    void channelShunned();
                    void channelReconnected(Address addr);
                }
            

A class implementing ChannelListener can use the Channel.setChannelListener() method to register with a channel to obtain information about state changes in a channel. Whenever a channel is closed, disconnected or opened a callback will be invoked.

                public interface Receiver extends MessageListener, MembershipListener {
                }
            

A Receiver can be used to receive all relevant messages and view changes in push-style; rather than having to pull these events from a channel, they will be dispatched to the receiver as soon as they have been received. This saves one thread (application thread, pulling messages from a channel, or the PullPushAdapter thread

                public interface ExtendedReceiver extends ExtendedMessageListener, MembershipListener {
                }
            

This is a receiver who will be able to handle partial state transfer

The ViewId is used to uniquely number views. It consists of the address of the view creator and a sequence number. ViewIds can be compared for equality and put in a hashtable as they implement equals() and hashCode() methods.

Whenever a group splits into subgroups, e.g. due to a network partition, and later the subgroups merge back together, a MergeView instead of a View will be received by the application. The MergeView class is a subclass of View and contains as additional instance variable the list of views that were merged. As an example if the group denoted by view V1:(p,q,r,s,t) split into subgroups V2:(p,q,r) and V2:(s,t) , the merged view might be V3:(p,q,r,s,t) . In this case the MergeView would contains a list of 2 views: V2:(p,q,r) and V2:(s,t) .

A channel can be created in two ways: an instance of a subclass of Channel is created directly using its public constructor (e.g. new JChannel() ), or a channel factory is created, which -- upon request -- creates instances of channels. We will only look at the first method of creating channel: by direct instantiation. Note that instantiation may differ between the various channel implementations. As example we will look at JChannel .

The public constructor of JChannel looks as follows:

                public JChannel(Object properties) throws ChannelException {}
            

It creates an instance of JChannel . The properties argument defines the composition of the protocol stack (number and type of layers, parameters for each layer, and their order). For JChannel, this has to be a String. An example of a channel creation is:

                String props="UDP(mcast_addr=228.1.2.3;mcast_port=45566;ip_ttl=32):" +
                "PING(timeout=3000;num_initial_members=6):" +
                "FD(timeout=5000):" +
                "VERIFY_SUSPECT(timeout=1500):" +
                "pbcast.STABLE(desired_avg_gossip=10000):" +
                "pbcast.NAKACK(gc_lag=10;retransmit_timeout=3000):" +
                "UNICAST(timeout=5000;min_wait_time=2000):" +
                "FRAG:" +
                "pbcast.GMS(initial_mbrs_timeout=4000;join_timeout=5000;" +
                "shun=false;print_local_addr=false)";
          
                JChannel channel;
                try {
                channel=new JChannel(props);
                }
                catch(Exception ex) {
                // channel creation failed
                }
            

The argument is a colon-delimited string of protocols, specified from bottom to top (left to right). The example properties argument will be used to create a protocol stack that uses IP Multicast (UDP) as bottom protocol, the PING protocol to locate the initial members, FD for failure detection, VERIFY_SUSPECT for double-checking of suspected members, STABLE for garbage collection of messages received by all members, NAKACK for lossless delivery of multicast messages, UNICAST for lossless delivery of unicast messages and GMS for group membership (handling of join or leave requests).

If the properties argument is null, the default properties will be used. An exception will be thrown if the channel cannot be created. Possible causes include protocols that were specified in the property argument, but were not found, or wrong parameters to protocols.

                    java org.javagroups.demos.Draw -props file:/home/bela/vsync.xml
                

                    java org.javagroups.demos.Draw -props http://www.jgroups.org/udp.xml
                

              <config>
                  <UDP
                       mcast_addr="${jgroups.udp.mcast_addr:228.10.10.10}"
                       mcast_port="${jgroups.udp.mcast_port:45588}"
                       discard_incompatible_packets="true"
                       max_bundle_size="60000"
                       max_bundle_timeout="30"
                       ip_ttl="${jgroups.udp.ip_ttl:2}"
                       enable_bundling="true"
                       use_concurrent_stack="true"
                       thread_pool.enabled="true"
                       thread_pool.min_threads="1"
                       thread_pool.max_threads="25"
                       thread_pool.keep_alive_time="5000"
                       thread_pool.queue_enabled="false"
                       thread_pool.queue_max_size="100"
                       thread_pool.rejection_policy="Run"
                       oob_thread_pool.enabled="true"
                       oob_thread_pool.min_threads="1"
                       oob_thread_pool.max_threads="8"
                       oob_thread_pool.keep_alive_time="5000"
                       oob_thread_pool.queue_enabled="false"
                       oob_thread_pool.queue_max_size="100"
                       oob_thread_pool.rejection_policy="Run"/>
                  <PING timeout="2000"
                          num_initial_members="3"/>
                  <MERGE2 max_interval="30000"
                          min_interval="10000"/>
                  <FD_SOCK/>
                  <FD timeout="10000" max_tries="5"   shun="true"/>
                  <VERIFY_SUSPECT timeout="1500"  />
                  <BARRIER />
                  <pbcast.NAKACK
                                 use_mcast_xmit="false" gc_lag="0"
                                 retransmit_timeout="300,600,1200,2400,4800"
                                 discard_delivered_msgs="true"/>
                  <UNICAST timeout="300,600,1200,2400,3600"/>
                  <pbcast.STABLE stability_delay="1000" desired_avg_gossip="50000"
                                 max_bytes="400000"/>
                  <VIEW_SYNC avg_send_interval="60000"   />
                  <pbcast.GMS print_local_addr="true" join_timeout="3000"
                              shun="false"
                              view_bundling="true"/>
                  <FC max_credits="20000000"
                                  min_threshold="0.10"/>
                  <FRAG2 frag_size="60000"  />
                  <pbcast.STATE_TRANSFER  />
              </config>
          

A number of options can be set in a channel. To do so, the following method is used:

              public void setOpt(int option, Object value);
          

Arguments are the options number and a value. The following options are currently recognized:

The equivalent method to get options is getOpt() :

                public Object getOpt(int option);
            

Given an option, the current value of the option is returned.

When a client wants to join a group, it connects to a channel giving the name of the group to be joined:

                public void connect(String clustername) throws ChannelClosed;
            

The cluster name is a string, naming the cluster to be joined. All channels that are connected to the same name form a cluster. Messages multicast on any channel in the cluster will be received by all members (including the one who sent it ^[2] ).

The method returns as soon as the group has been joined successfully. If the channel is in the closed state (see Figure 3.2, “Channel states” ), an exception will be thrown. If there are no other members, i.e. no other client has connected to a group with this name, then a new group is created and the member joined. The first member of a group becomes its coordinator . A coordinator is in charge of multicasting new views whenever the membership changes ^[3] .

Clients can also join a cluster group and fetch cluster state in one operation. The best way to conceptualize connect and fetch state connect method is to think of it as an invocation of regular connect and getstate methods executed in succession. However, there are several advantages of using connect and fetch state connect method over regular connect. First of all, underlying message exchange is heavily optimized, especially if the flush protocol is used in the stack. But more importantly, from clients perspective, connect and join operations become one atomic operation.

		      	public void connect(string cluster_name, address target, string state_id, long timeout) throws channelexception;
		      

Just as in regular connect method cluster name represents a cluster to be joined. Address parameter indicates a cluster member to fetch state from. Null address parameter indicates that state should be fetched from the cluster coordinator. If state should be fetched from a particular member other than coordinator clients can provide an address of that member. State id used for partial state transfer while timeout bounds entire join and fetch operation.

Method getLocalAddress() returns the local address of the channel. In the case of JChannel , the local address is generated by the bottom-most layer of the protocol stack when the stack is connected to. That means that -- depending on the channel implementation -- the local address may or may not be available when a channel is in the unconnected state.

                public Address getLocalAddress();
            

Method getClusterlName() returns the name of the cluster in which the channel is a member:

                public String getClusterName();
            

Again, the result is undefined if the channel is in the unconnected or closed state.

The following method can be used to get the current view of a channel:

                public View getView();
            

This method does not retrieve a new view (message) from the channel, but only returns the current view of the channel. The current view is updated every time a view message is received: when method receive() is called, and the return value is a view, before the view is returned, it will be installed in the channel, i.e. it will become the current view.

Calling this method on an unconnected or closed channel is implementation defined. A channel may return null, or it may return the last view it knew of.

Once the channel is connected, messages can be sent using the send() methods:

                public void send(Message msg) throws ChannelNotConnected, ChannelClosed;
                public void send(Address dst, Address src, Object obj) throws ChannelNotConnected, ChannelClosed;
            

The first send() method has only one argument, which is the message to be sent. The message's destination should either be the address of the receiver (unicast) or null (multicast). When it is null, the message will be sent to all members of the group (including itself). The source address may be null; if it is, it will be set to the channel's address (so that recipients may generate a response and send it back to the sender).

The second send() method is a helper method and uses the former method internally. It requires the address of receiver and sender and an object (which has to be serializable), constructs a Message and sends it.

If the channel is not connected, or was closed, an exception will be thrown upon attempting to send a message.

Here's an example of sending a (multicast) message to all members of a group:

                Hashtable data; // any serializable data
                try {
                    channel.send(null, null, data);
                }
                catch(Exception ex) {
                    // handle errors
                }
            

The null value as destination address means that the message will be sent to all members in the group. The sender's address will be filled in by the bottom-most protocol. The payload is a hashtable, which will be serialized into the message's buffer and unserialized at the receiver's end. Alternatively, any other means of generating a byte buffer and setting the message's buffer to it (e.g. using Message.setBuffer()) would also work.

Here's an example of sending a (unicast) message to the first member (coordinator) of a group:

                Address receiver;
                Message msg;
                Hashtable data;
                try {
                    receiver=channel.getView().getMembers().first();
                    channel.send(receiver, null, data);
                }
                catch(Exception ex) {
                    // handle errors
                }
            

It creates a Message with a specific address for the receiver (the first member of the group). Again, the sender's address can be left null as it will be filled in by the bottom-most protocol.

Method receive() is used to receive messages, views, suspicions and blocks:

                public Object receive(long timeout) throws ChannelNotConnected, ChannelClosed, Timeout;
            

A channel receives messages asynchronously from the network and stores them in a queue. When receive() is called, the next available message from the top of that queue is removed and returned. When there are no messages on the queue, the method will block. If timeout is greater than 0, it will wait the specified number of milliseconds for a message to be received, and throw a TimeoutException exception if none was received during that time. If the timeout is 0 or negative, the method will wait indefinitely for the next available message.

Depending on the channel options (see Section 3.7.2, “Setting options” ), the following types of objects may be received:

The caller has to check the type of the object returned. This can be done using the instanceof operator, as follows:

                Object obj;
                Message msg;
                View v;
                obj=channel.receive(0); // wait forever
                if(obj instanceof Message)
                    msg=(Message)obj;
                else if(obj instanceof View)
                    v=(View)obj;
                else
                    ; // don't handle suspicions or blocks
            

If for example views, suspicions and blocks are disabled, then the caller is guaranteed to only receive return values of type Message . In this case, the return value can be cast to a Message directly, without using the instanceof operator.

If the channel is not connected, or was closed, a corresponding exception will be thrown.

The example below shows how to retrieve the "Hello world" string from a message:

                Message msg; // received above
                String s;
                try {
                    s=(String)msg.getObject(); // error if object not Serializable
                    // alternative: s=new String(msg.getBuffer());
                }
                catch(Exception ex) {
                    // handle errors, e.g. casting error above)
                }
            

The Message.getObject() method retrieves the message's byte buffer, converts it into a (serializable) object and returns the object.

Instead of pulling messages from a channel in an application thread, a Receiver can be registered with a channel; all received messages, view changes and state transfer requests will invoke callbacks on the registered Receiver:

                JChannel ch=new JChannel();
                ch.setReceiver(new ExtendedReceiverAdapter() {
                    public void receive(Message msg) {
                        System.out.println("received message " + msg);
                    }
                    public void viewAccepted(View new_view) {
                        System.out.println("received view " + new_view);
                    }
                });
                ch.connect("bla");
            

The ExtendedReceiverAdapter class implements all callbacks of ExtendedReceiver with no-ops, in the example above we override receive() and viewAccepted().

The advantage of using a Receiver is that the application doesn't have to waste 1 thread for pulling messages out of a channel. In addition, the channel doesn't have to maintain an (unbounded) queue of messages/views, which can quickly get large if the receiver cannot process messages fast enough, and the sender keeps sending messages.

Instead of removing the next available message from the channel, peek() just returns a reference to the next message, but does not remove it. This is useful when one has to check the type of the next message, e.g. whether it is a regular message, or a view change. The signature of this method is not shown here, it is the same as for receive() .

A newly joined member may wish to retrieve the state of the group before starting work. This is done with getState(). This method returns the state of one member (in most cases, of the oldest member, the coordinator). It returns true or false, depending on whether a valid state could be retrieved. For example, if a member is a singleton, then calling this method would always return false ^[4] .

The actual state is returned as the return value of one of the subsequent receive() calls, in the form of a SetStateEvent object. If getState() returned true, then a valid state (non-null) will be returned, otherwise a null state will be returned. Alternatively if an application uses MembershipListener (see Section 3.2.4, “MembershipListener” ) instead of pulling messages from a channel, the getState() method will be invoked and a copy of the current state should be returned. By the same token, setting a state would be accomplished by JGroups calling the setState() method of the state fetcher.

The reason for not directly returning the state as a result of getState() is that the state has to be returned in the correct position relative to other messages. Returning it directly would violate the FIFO properties of a channel, and state transfer would not be correct.

The following code fragment shows how a group member participates in state transfers:

                channel=new JChannel();
                channel.connect("TestChannel");
                boolean rc=channel.getState(null, 5000);

                ...

                Object state, copy;
                Object ret=channel.receive(0);
                if(ret instanceof Message)
                    ;
                else if(ret instanceof GetStateEvent) {
                    copy=copyState(state); // make a copy so that other msgs don't change the state
                    channel.returnState(Util.objectToByteBuffer(copy));
                }
                else if(ret instanceof SetStateEvent) {
                    SetStateEvent e=(SetStateEvent)ret;
                    state=e.getArg();
                }
            

A JChannel has to be created whose stack includes the STATE_TRANSFER or pbcast.STATE_TRANSFER protocols (see Chapter 5, Advanced Concepts ). Method getState() subsequently asks the channel to return the current state. If there is a current state (there may not be any other members in the group !), then true is returned. In this case, one of the subsequent receive() method invocations on the channel will return a SetStateEvent object which contains the current state. In this case, the caller sets its state to the one received from the channel.

Method receive() might return a GetStateEvent object, requesting the state of the member to be returned. In this case, a copy of the current state should be made and returned using JChannel.returnState() . It is important to a) synchronize access to the state when returning it since other accesses may modify it while it is being returned and b) make a copy of the state since other accesses after returning the state may still be able to modify it ! This is possible because the state is not immediately returned, but travels down the stack (in the same address space), and a reference to it could still alter it.

As an alternative to handling the GetStateEvent and SetStateEvent events, and calling Channel.returnState(), a Receiver could be used. The example above would look like this:

                class MyReceiver extends ReceiverAdapter {
                    final Map m=new HashMap();
                    public byte[] getState() {
                        synchronized(m) { // so nobody else can modify the map while we serialize it
                            byte[] state=Util.objectToByteBuffer(m);
                            return state;
                        }
                    }

                    public void setState(byte[] state) {
                        synchronized(m) {
                            Map new_m=(Map)Util.objectFromByteBuffer(state);
                            m.clear();
                            m.addAll(new_m);
                        }
                    }
                }

                channel=new JChannel(); // use default properties (has to include pbcast.STATE_TRANSFER protocol)
                channel.setReceiver(new MyReceiver());
                channel.connect("TestChannel");
                boolean rc=channel.getState(null, 5000);
            

In a group consisting of A,B and C, with D joining the group and calling Channel.getState(), the following sequence of callbacks happens:

Partial state transfer means that instead of transferring the entire state, we may want to transfer only a substate. For example, with HTTP session replication, a new node in a cluster may want to transfer only the state of a specific session, not all HTTP sessions. This can be done with either the pull or push model. The method to call would be Channel.getState(), including the ID of the substate (a string). In the pull model, GetStateEvent and SetStateEvent have an additional member, state_id, and in the push model, there are 2 additional getState() and setState() callbacks. The example below shows partial state transfer for the push model:

                class MyReceiver extends ExtendedReceiverAdapter {
                    final Map m=new HashMap();

                    public byte[] getState() {
                        return getState(null);
                    }

                    public byte[] getState(String substate_id) {
                        synchronized(m) { // so nobody else can modify the map while we serialize it
                            byte[] state=null;
                            if(substate_id == null) {
                                state=Util.objectToByteBuffer(m);
                            }
                            else {
                                Object value=m.get(substate_id);
                                if(value != null) {
                                    return Util.objectToByteBuffer(value);
                                }
                            }
                            return state;
                        }
                    }

                    public void setState(byte[] state) {
                        setState(null, state);
                    }

                    public void setState(String substate_id, byte[] state) {
                        synchronized(m) {
                            if(substate_id != null) {
                                Object value=Util.objectFromByteBuffer(state);
                                m.put(substate_id, value);
                            }
                            else {
                                Map new_m=(Map)Util.objectFromByteBuffer(state);
                                m.clear();
                                m.addAll(new_m);
                            }
                        }
                    }
                }

                channel=new JChannel(); // use default properties (has to include pbcast.STATE_TRANSFER protocol)
                channel.setReceiver(new MyReceiver());
                channel.connect("TestChannel");
                boolean rc=channel.getState(null, "MyID", 5000);
            

The example shows that the Channel.getState() method specifies the ID of the substate, in this case "MyID". The getState(String substate_id) method checks whether the substate ID is not null, and returns the substate pertaining to the ID, or the entire state if the substate_id is null. The same goes for setting the substate: if setState(String substate_id, byte[] state) has a non-null substate_id, only that part of the current state will be overwritten, otherwise (if null) the entire state will be overwritten.

Streaming state transfer allows transfer of application (partial) state without having to load entire state into memory prior to sending it to a joining member. Streaming state transfer is especially useful if the state is very large (>1Gb), and use of regular state transfer would likely result in OutOfMemoryException. Streaming state transfer was introduced in JGroups 2.4. JGroups channel has to be configured with either regular or streaming state transfer. The JChannel API that invokes state transfer (i.e. JChannel.getState(long timeout, Address member)) remains the same.

Streaming state transfer, just as regular byte based state transfer, can be used in both pull and push mode. Similarly to the current getState and setState methods of org.jgroups.MessageListener, the application interested in streaming state transfer in a push mode would implement streaming getState method(s) by sending/writing state through a provided OutputStream reference and setState method(s) by receiving/reading state through a provided InputStream reference. In order to use streaming state transfer in a push mode, existing ExtendedMessageListener has been expanded to include additional four methods:

public interface ExtendedMessageListener { 

      /*non-streaming callback methods ommitted for clarity*/
 
       void getState(OutputStream ostream);
       void getState(String state_id, OutputStream ostream);
       void setState(InputStream istream);
       void setState(String state_id, InputStream istream);

}            

For a pull mode (when application uses channel.receive() to fetch events) two new event classes will be introduced:

These two events/classes are very similar to existing GetStateEvent and SetStateEvent but introduce a new field; StreamingGetStateEvent has an OutputStream and StreamingSetStateEvent has an InputStream.

The following code snippet demonstrates how to pull events from a channel, processing StreamingGetStateEvent and sending hypothetical state through a provided OutputStream reference. Handling of StreamingSetStateEvent is analogous to this example:

   ...
   Object obj=channel.receive(0);
   if(obj instanceof StreamingGetStateEvent) {
     StreamingGetStateEvent evt=(StreamingGetStateEvent)obj;
     OutputStream oos = null;
     try {
         oos = new ObjectOutputStream(evt.getArg());
         oos.writeObject(state);
         oos.flush();
     }
     catch (Exception e) {}
     finally {
         try {
             oos.close();
         }
         catch (IOException e) {
             System.err.println(e);
         }
    }
  }                
  ...

JGroups has a great flexibility with state transfer methodology by allowing application developers to implement both byte based and streaming based state transfers. Application can, for example, implement streaming and byte based state transfer callbacks and then interchange state transfer protocol in channel configuration to use either streaming or byte based state transfer. However, one cannot configure a channel with both state transfers at the same time and then in runtime choose which particular state transfer type to use.

Disconnecting from a channel is done using the following method:

                public void disconnect();
            

It will have no effect if the channel is already in the disconnected or closed state. If connected, it will remove itself from the group membership. This is done (transparently for a channel user) by sending a leave request to the current coordinator. The latter will subsequently remove the channel's address from its local view and send the new view to all remaining members.

After a successful disconnect, the channel will be in the unconnected state, and may subsequently be re-connected to.

To destroy a channel instance (destroy the associated protocol stack, and release all resources), method close() is used:

                public void close();
            

It moves the channel to the closed state, in which no further operations are allowed (most throw an exception when invoked on a closed channel). In this state, a channel instance is not considered used any longer by an application and -- when the reference to the instance is reset -- the channel essentially only lingers around until it is garbage collected by the Java runtime system.