mailmessage.java

nachos操作系统框架

package nachos.network;

import nachos.machine.*;

/**
 * A mail message. Includes a packet header, a mail header, and the actual
 * payload.
 *
 * @see	nachos.machine.Packet
 */
public class MailMessage {
    /**
     * Allocate a new mail message to be sent, using the specified parameters.
     *
     * @param	dstLink		the destination link address.
     * @param	dstPort		the destination port.
     * @param	srcLink		the source link address.
     * @param	srcPort		the source port.
     * @param	contents	the contents of the packet.
     */
    public MailMessage(int dstLink, int dstPort, int srcLink, int srcPort,
		       byte[] contents) throws MalformedPacketException {
	// make sure the paramters are valid
	if (dstPort < 0 || dstPort >= portLimit ||
	    srcPort < 0 || srcPort >= portLimit ||
	    contents.length > maxContentsLength)
	    throw new MalformedPacketException();

	this.dstPort = (byte) dstPort;
	this.srcPort = (byte) srcPort;
	this.contents = contents;

	byte[] packetContents = new byte[headerLength + contents.length];

	packetContents[0] = (byte) dstPort;
	packetContents[1] = (byte) srcPort;

	System.arraycopy(contents, 0, packetContents, headerLength,
			 contents.length);

	packet = new Packet(dstLink, srcLink, packetContents);
    }
	
    /**
     * Allocate a new mail message using the specified packet from the network.
     *
     * @param	packet	the packet containg the mail message.
     */
    public MailMessage(Packet packet) throws MalformedPacketException {
	this.packet = packet;
	
	// make sure we have a valid header
	if (packet.contents.length < headerLength ||
	    packet.contents[0] < 0 || packet.contents[0] >= portLimit ||
	    packet.contents[1] < 0 || packet.contents[1] >= portLimit)
	    throw new MalformedPacketException();

	dstPort = packet.contents[0];
	srcPort = packet.contents[1];

	contents = new byte[packet.contents.length - headerLength];
	System.arraycopy(packet.contents, headerLength, contents, 0,
			 contents.length);
    }

    /**
     * Return a string representation of the message headers.
     */
    public String toString() {
	return "from (" + packet.srcLink + ":" + srcPort +
	    ") to (" + packet.dstLink + ":" + dstPort +
	    "), " + contents.length + " bytes";
    }
    
    /** This message, as a packet that can be sent through a network link. */
    public Packet packet;
    /** The port used by this message on the destination machine. */
    public int dstPort;
    /** The port used by this message on the source machine. */
    public int srcPort;
    /** The contents of this message, excluding the mail message header. */
    public byte[] contents;

    /**
     * The number of bytes in a mail header. The header is formatted as
     * follows:
     *
     * <table>
     * <tr><td>offset</td><td>size</td><td>value</td></tr>
     * <tr><td>0</td><td>1</td><td>destination port</td></tr>
     * <tr><td>1</td><td>1</td><td>source port</td></tr>
     * </table>
     */
    public static final int headerLength = 2;

    /** Maximum payload (real data) that can be included in a single mesage. */
    public static final int maxContentsLength =
	Packet.maxContentsLength - headerLength;

    /**
     * The upper limit on mail ports. All ports fall between <tt>0</tt> and
     * <tt>portLimit - 1</tt>.
     */    
    public static final int portLimit = 128;
}