org.jcsp.net
Class Link

java.lang.Object
  org.jcsp.net.Link

All Implemented Interfaces:

CSProcess

public abstract class Link
extends Object
implements CSProcess

This class is an abstract class that all JCSP.NET protocol implementations must implement. Concrete implementations of the Link class must provide the mechanism for sending data to remote Nodes and receiving data back. When a concrete implementation of link is initiated by calling the run() method, it must inititate a handshaking procedure and call certain protected methods in this abstract class.

During handshaking, the runTests(ChannelInput, ChannelOutput, boolean) should be called. See the documentation for this method for a full explanation.

When a Link receives an object from over the network it should deliver the object to its destination by calling the deliverReceivedObject(Object) method.

Link implementations obtain objects to send to the remote Node by reading from the protected txChannel object.

Author:

Quickstone Technologies Limited

Field Summary

protected NodeID	remoteNodeID The remote NodeID.
protected boolean	sendNodeID True if the subclass must pass the ID of this node to the peer node.
protected FilteredAny2OneChannel	txChannel The channel used for TX over the link.

Constructor Summary

Link(ProtocolID protocolID, boolean client, boolean connected)
A constructor that must be called by sub-classes.

Method Summary

protected boolean	connect() Establishes a connection to the peer node.
protected boolean	createResources() Allocates the resources necessary for the actual connection.
protected void	deliverReceivedObject(Object obj) A protected method for concrete implementations of this class to call when they received a an object from the remote Node.
protected void	destroyResources() Deallocates any resources allocated by createResources.
boolean	equals(Object o) Compares another object with this object.
protected boolean	exchangeNodeIDs() Sends the ID of this node to the peer process and receives its ID.
long	getPingTime() A public accessor for obtaining the ping time between the local Node and this Link's remote Node.
protected ProtocolID	getProtocolID() A protected accessor for obtaining the identifier of the protocol implementing this Link object.
protected boolean[]	getReadSequence(boolean client) This is used by concrete Link implementations before calling the runTests method.
protected NodeID	getRemoteNodeID() Returns the other computer's ID.
protected ChannelOutput	getTxChannel() Returns channel to use for transmitting.
int	hashCode() Returns an int hash code for this Link.
NodeID	obtainNodeID() This returns the NodeID of the remote Node to which this link is connected.
boolean	performedPingTest() A public accessor for enquiring as to whether this Link object has performed a ping test.
long	ping() Performs a ping on the link.
protected boolean	readLinkDecision() Reads a boolean link decision as to whether this node can keep or discard the link.
protected Object	readTestObject() Reads a test object from the underlying connection.
void	run() Main process for the link, containing generic code that makes calls on the abstract methods that should be implemented by a concrete subclass.
protected void	runTxRxLoop() Performs send and receive actions for the link exchanging data with the peer node.
protected void	waitForReplies(int numAcknowledgements) Waits for numRepliesOutstanding instances of LinkLost to arrive from the txChannel.
protected void	writeLinkDecision(boolean use) Writes a boolean link decision as to whether the other node has the option to keep or discard the link.
protected void	writeTestObject(Object obj) Writes a test object to the underlying connection.

Methods inherited from class java.lang.Object

clone, finalize, getClass, notify, notifyAll, toString, wait, wait, wait

Field Detail

txChannel

protected FilteredAny2OneChannel txChannel

The channel used for TX over the link. In the future, this might be made private. It is recommended that implementations use the getTxChannel() to obtain the channel. Protocol implementations should NOT alter any of the filters on this channel.

remoteNodeID

protected NodeID remoteNodeID

The remote NodeID. The subclass must set this during exchangeNodeIDs.

sendNodeID

protected boolean sendNodeID

True if the subclass must pass the ID of this node to the peer node.

Constructor Detail

Link

public Link(ProtocolID protocolID,
            boolean client,
            boolean connected)

A constructor that must be called by sub-classes.

Parameters:

protocolID - A ProtocolID object for identifying the protocol that is implementing the Link.

client - Indicates whether this is the client (true) or server (false) end of the connection.

connected - true if a connection is already established; otherwise connect() will later be called

Method Detail

connect

protected boolean connect()
                   throws Exception

Establishes a connection to the peer node. Called when the process is started unless connected was true in the constructor. This is called internally from this class' run method - do not call it.

Returns:

true on success, false on failure

Throws:

Exception

createResources

protected boolean createResources()
                           throws Exception

Allocates the resources necessary for the actual connection. Called as the first part of the handshaking process. This is called internally from the class' handshake method - do not call it.

Returns:

true on success, false on failure

Throws:

Exception

destroyResources

protected void destroyResources()
                         throws Exception

Deallocates any resources allocated by createResources. Called in the event of an error during handshaking or when the link is dropped. An implementation of this method must cope with being called multiple times without an intervening call to createResources.

Throws:

Exception

exchangeNodeIDs

protected boolean exchangeNodeIDs()
                           throws Exception

Sends the ID of this node to the peer process and receives its ID. It is only necessary to send this node's ID if sendNodeID is true. The remoteNodeID attribute should be set to the ID of the remote node if received. This is called internally during handshaking - do not call it.

Returns:

true on success, false on failure.

Throws:

Exception

runTxRxLoop

protected void runTxRxLoop()
                    throws Exception

Performs send and receive actions for the link exchanging data with the peer node. This is called from this class' run method - do not call it directly.

Throws:

Exception

waitForReplies

protected void waitForReplies(int numAcknowledgements)
                       throws Exception

Waits for numRepliesOutstanding instances of LinkLost to arrive from the txChannel. This is called internally from this class' run method - do not call it directly.

Parameters:

numRepliesOutstanding - LinkLost instances to wait for.

Throws:

Exception

writeTestObject

protected void writeTestObject(Object obj)
                        throws Exception

Writes a test object to the underlying connection. Called internally during handshaking - do not call it directly. The value written must be delivered by readTestObject at the remote node.

Parameters:

obj - object to be written

Throws:

Exception

readTestObject

protected Object readTestObject()
                         throws Exception

Reads a test object from the underlying connection. Called internally during handshaking - do not call it directly. It must deliver the value passed to writeTestObject at the peer node.

Returns:

the object received.

Throws:

Exception

writeLinkDecision

protected void writeLinkDecision(boolean use)
                          throws Exception

Writes a boolean link decision as to whether the other node has the option to keep or discard the link. Called internally during handshaking - do not call it directly. The value written must be delivered by readLinkDecision at the peer node.

Parameters:

use - decision result

Throws:

Exception

readLinkDecision

protected boolean readLinkDecision()
                            throws Exception

Reads a boolean link decision as to whether this node can keep or discard the link. Called internally during handshaking - do not call it directly. It must return the value passed to writeLinkDecision at the peer node.

Returns:

decision result

Throws:

Exception

getRemoteNodeID

protected NodeID getRemoteNodeID()

Returns the other computer's ID. This method is safe to call while the process is running, however it will return null if the other computer has not yet identified itself.

Returns:

ID of connected computer.

getProtocolID

protected ProtocolID getProtocolID()

A protected accessor for obtaining the identifier of the protocol implementing this Link object.

Returns:

the ProtocolID of the protocol implementing this Link.

getTxChannel

protected ChannelOutput getTxChannel()

Returns channel to use for transmitting. This method is safe to call while the process is running. May block for if handshaking is still in progress. When written to, if the object supplied is not Serializable then an IllegalArgumentException Runtime exception.

Returns:

Channel you should send data to if you want to transmit on this Connection.

deliverReceivedObject

protected void deliverReceivedObject(Object obj)

A protected method for concrete implementations of this class to call when they received a an object from the remote Node. This method delivers the message to its destination (a Channel, Connection etc.).

Parameters:

obj - the object to deliver.

getPingTime

public long getPingTime()

A public accessor for obtaining the ping time between the local Node and this Link's remote Node.

Returns:

the ping time as a long.

ping

public long ping()

Performs a ping on the link.

Returns:

the ping time in ms.

performedPingTest

public boolean performedPingTest()

A public accessor for enquiring as to whether this Link object has performed a ping test.

Returns:

true iff a ping test has been performed.

getReadSequence

protected boolean[] getReadSequence(boolean client)

This is used by concrete Link implementations before calling the runTests method. When tests are run, a series of messages are exchanged between both sides of the link. This allows the link to perform its tests using a single thread.

Parameters:

client - a boolean indicating whether this Link object is the client side of the link. If this object is the client then the parameter should be true.

Returns:

an array of boolean values indicating the sequence of when to read and when to transmit.

equals

public final boolean equals(Object o)

Compares another object with this object. This implementation returns true iff the supplied object is the same instance object as this object.

Overrides:

equals in class Object

Parameters:

o - an object to compare with this object.

Returns:

true iff the parameter equals this object.

hashCode

public final int hashCode()

Returns an int hash code for this Link. The current implementation just uses the instance from Object.

Overrides:

hashCode in class Object

Returns:

an int hash code.

run

public final void run()

Main process for the link, containing generic code that makes calls on the abstract methods that should be implemented by a concrete subclass. The subclass will be required to connect() if it hasn't done so, perform handshaking, runTxRxLoop() and then waitForReplies() to acknowledge link termination.

Specified by:

run in interface CSProcess

obtainNodeID

public final NodeID obtainNodeID()

This returns the NodeID of the remote Node to which this link is connected. If a connection has not already been established, this method may connect to the remote Node and request its NodeID and then drop the connection.

Returns:

the remote Node's NodeID object.