客户端/服务器对：临时端口

Question

I'm coding a client server pair in java and I would like the ports to be assigned at runtime by the system. From the server's side this can be easily done through APIs but how does the client know which port the server is listening on?

Answer 1

The client has no knowledge which ports the server is listening on, unless you specify it to the client
You may consider using other architectures, such as OGSI, or technologies in which you have a "service repository" which is known to all clients, so they can query it and get information on the service available and their ports.
Another option is to let the system administrator define in your DNS organization SRV records for your service , and then the client code can invoke an SRV record request, and get information on the service, but this might be an overkill for your needs and requires involving your sys admin
If you're interested at the DNS option I mentioned you should check for the DnsSrvLocator class at oVirt open source.
At oVirt we use this in order to detect Ldap servers (when Active Directory is installed , the Sys Admin should configure entries for the LDAP server indicating the host name and the port it listens to, then , using this utility you can provide the information of the srv record (the domain and the protocol and get a list of servers available)
Here is its code -

/**
 *
 */
package org.ovirt.engine.core.dns;

import java.net.URI;
import java.net.URISyntaxException;
import java.util.Arrays;
import java.util.Hashtable;
import java.util.InputMismatchException;
import java.util.Random;
import java.util.Scanner;
import java.util.regex.Pattern;

import javax.naming.Context;
import javax.naming.NamingException;
import javax.naming.directory.Attribute;
import javax.naming.directory.Attributes;
import javax.naming.directory.DirContext;
import javax.naming.spi.NamingManager;

import org.ovirt.engine.core.utils.log.Log;
import org.ovirt.engine.core.utils.log.LogFactory;

/**
 * Utility class to query DNS SRV records, and return results according to the priority/weights algorithm as specified
 * in RFC 2782
 *
 */
public class DnsSRVLocator {

    private static final String DNS_QUERY_PREFIX = "dns:///";
    private static final String SRV_RECORD = "SRV";
    private static Pattern SPACE_PATTERN = Pattern.compile(" ");
    private static SrvRecord invalidRecord = new SrvRecord(false, false, 0, 0, 0, "");
    private int numberOfValidRecords = 0;
    private Random random = new Random(System.currentTimeMillis());

    public static final String TCP = "_tcp";
    public static final String UDP = "_udp";

    /**
     * Holds information on a retrieved SRV record
     *
     */
    public static class SrvRecord implements Comparable<SrvRecord> {
        private boolean valid;
        private int weight;
        private int priority;
        private int sum;
        private String address;
        private boolean used;

        public String toString() {
            StringBuilder sb = new StringBuilder();
            sb.append("valid: ").append(valid).append(" sum: ").append(sum).append("priority: ").append(priority)
                    .append(" weight: ").append(weight).append(" hostport: ").append(address);
            return sb.toString();
        }

        public SrvRecord(int priority, int weight, String hostPort) {
            this(true, false, 0, priority, weight, hostPort);
        }

        public SrvRecord(boolean valid, boolean used, int sum, int priority, int weight, String address) {
            this.valid = valid;
            this.used = used;
            this.sum = sum;
            this.priority = priority;
            this.weight = weight;
            this.address = address;
        }

        public boolean isValid() {
            return valid;
        }

        public void setValid(boolean isValid) {
            this.valid = isValid;
        }

        public int getWeight() {
            return weight;
        }

        public void setWeight(int weight) {
            this.weight = weight;
        }

        public int getPriority() {
            return priority;
        }

        public void setPriority(int priority) {
            this.priority = priority;
        }

        public int getSum() {
            return sum;
        }

        public void setSum(int sum) {
            this.sum = sum;
        }

        public String getAddress() {
            return address;
        }

        public void setAddress(String address) {
            this.address = address;
        }

        public boolean isUsed() {
            return used;
        }

        public void setUsed(boolean isUsed) {
            this.used = isUsed;
        }

        @Override
        public int compareTo(SrvRecord other) {

            // Sort in ascending order where invalid (non parsable) records are
            // last
            // Records with lower priority value come first
            // For a group of records with same priority, records with weight 0
            // come first
            if (valid && !other.valid) {
                return -1;
            }
            if (!valid && other.valid) {
                return 1;
            }
            if (priority < other.priority) {
                return -1;
            }
            if (priority > other.priority) {
                return 1;
            }
            if (weight == 0 && other.weight != 0) {
                return -1;
            }
            if (weight != 0) {
                return 1;
            }
            return 0;
        }
    }

    public static class DnsSRVResult {
        private int numOfValidAddresses;
        private String[] addresses;

        public DnsSRVResult(int numOfValidAddresses, String[] addresses) {
            this.numOfValidAddresses = numOfValidAddresses;
            this.addresses = addresses;
        }

        public int getNumOfValidAddresses() {
            return numOfValidAddresses;
        }

        public String[] getAddresses() {
            return addresses;
        }
    }

    public DnsSRVResult getService(String service, String protocol, String domain) throws Exception {
        StringBuilder dnsQuery = new StringBuilder();
        dnsQuery.append(service).append(".").append(protocol).append(".").append(domain);

        try {
            return getService(dnsQuery.toString());
        } catch (Exception ex) {
            log.errorFormat("Error: could not find DNS SRV record name: {0}.{1}.{2}.\nException message is: {3}\n" +
                    "Possible causes: missing DNS entries in the DNS server or DNS resolving" +
                    " issues from engine-core machine.\nPlease Ensure correct DNS entries exist in the DNS server" +
                    " and ensure the DNS server is reachable from the engine-core machine.",
                    service,
                    protocol,
                    domain,
                    ex.getMessage());
            throw ex;
        }
    }

    private DnsSRVResult getService(String dnsUrl) throws NamingException {
        Context ctx = NamingManager.getURLContext("dns", new Hashtable(0));

        if (!(ctx instanceof DirContext)) {
            return null; // cannot create a DNS context
        }

        StringBuilder fullDnsURL = new StringBuilder(DNS_QUERY_PREFIX);
        fullDnsURL.append(dnsUrl);
        Attributes attrs = ((DirContext) ctx).getAttributes(fullDnsURL.toString(), new String[] { SRV_RECORD });

        if (attrs == null) {
            return null;
        }

        Attribute attr = attrs.get(SRV_RECORD);
        if (attr == null) {
            return null;
        }

        int numOfRecords = attr.size();
        String[] records = new String[numOfRecords];
        for (int counter = 0; counter < numOfRecords; counter++) {
            records[counter] = (String) attr.get(counter);
        }

        return getSRVResult(records);
    }

    public DnsSRVResult getSRVResult(String[] recordsList) {
        int numOfRecords = recordsList.length;
        if (recordsList == null || numOfRecords == 0) {
            return null;
        }
        // Read records as retrieved from DNS
        SrvRecord[] records = new SrvRecord[numOfRecords];
        int counter = 0;
        for (String recordStr : recordsList) {
            SrvRecord srvRecord = parseSrvRecord(recordStr);
            records[counter++] = srvRecord;
        }
        // Sort the records
        Arrays.sort(records);
        int priority = -1;
        int lastPriorityIndex = -1;
        int priorityIndex = -1;
        int startPriorityIndex = -1;
        String[] addresses = new String[numOfRecords];
        // Total number of service addresses
        int numOfAddreses = -1;
        // Iterates over the records, and calculates for each
        // priority the index of the first SRV record that contains
        // the index of last SRVV record that contains the priority
        // For each group of records with same priorities, gets a list of services
        for (SrvRecord record : records) {
            if (!record.isValid()) {
                break;
            }
            lastPriorityIndex = priorityIndex;
            priorityIndex++;
            int currentPriority = record.getPriority();
            if (currentPriority != priority) {
                if (lastPriorityIndex != -1) {
                    // This means that this is the end of a group of records
                    // with same
                    // priority - get their service addresses
                    numOfAddreses = fillServiceAddress(records, startPriorityIndex, lastPriorityIndex, addresses,
                            numOfAddreses);

                }
                startPriorityIndex = priorityIndex;
                priority = currentPriority;

            }
            lastPriorityIndex = priorityIndex;
        }
        numOfAddreses = fillServiceAddress(records, startPriorityIndex, lastPriorityIndex, addresses, numOfAddreses);

        // numOfAddresses points to the last index of valid address in the
        // addresses array
        // Increase it by 1 in order to truely reflect the number of valid
        // addresses
        return new DnsSRVResult(numOfAddreses + 1, addresses);
    }

    private int fillServiceAddress(SrvRecord[] records,
            int startPriorityIndex,
            int lastPriorityIndex,
            String[] addresses,
            int numOfAddressess) {

        // Run the following algorithm for determining the order of service entries for a
        // group of SRV records with same
        // priority:
        // 1. For each SRV record calculate its sum based on the sum of weights
        // of its weight and
        // the weight of all preceding SRV records
        // 2. Select a random value between 0 and the sum (inclusive)
        // 3. Iterate over the group until a record with sum that is greater or
        // above
        // to the generated random value is encountered
        // 4. This will be the next select SRV record - make it not used for
        // next round of the algorithm
        int numOfRepetitions = (lastPriorityIndex - startPriorityIndex) + 1;
        int totalSum = 0;
        for (int counter = 0; counter < numOfRepetitions; counter++) {
            for (int index = startPriorityIndex; index <= lastPriorityIndex; index++) {
                if (!records[index].isUsed()) {
                    totalSum += records[index].getWeight();
                    records[index].setSum(totalSum);
                }
            }
            int randResult = random.nextInt(totalSum + 1);
            for (int index = startPriorityIndex; index <= lastPriorityIndex; index++) {
                boolean found = false;
                if (!found && !records[index].isUsed()) {
                    if (records[index].getSum() >= randResult) {
                        records[index].setUsed(true);
                        addresses[++numOfAddressess] = records[index].getAddress();
                        found = true;
                    }
                }
            }
        }
        return numOfAddressess;
    }

    private SrvRecord parseSrvRecord(String recordStr) {

        // SRV record looks like: PRIORITY WEIGHT PORT HOST
        Scanner s = new Scanner(recordStr).useDelimiter(SPACE_PATTERN);
        try {
            int priority = s.nextInt();
            int weight = s.nextInt();
            String port = s.next();
            String host = s.next();
            StringBuilder sb = new StringBuilder(host);
            sb.append(":").append(port);
            numberOfValidRecords++;
            return new SrvRecord(priority, weight, sb.toString());
        } catch (InputMismatchException ex) {
            log.errorFormat("the record {0} has invalid format", recordStr);

            // In case there is a parsing error, the invalid record constant is
            // returned
            return invalidRecord;
        }
    }

    public URI constructURI(String protocol, String address) throws URISyntaxException {
        String[] parts = address.split("\\.:");
        if (parts.length != 2) {
            throw new IllegalArgumentException("the address in SRV record should contain host and port");
        }

        StringBuilder uriSB = new StringBuilder(protocol);
        uriSB.append("://").append(parts[0]).append(":").append(parts[1]);
        return new URI(uriSB.toString());
    }

    private static Log log = LogFactory.getLog(DnsSRVLocator.class);

}

Answer 2

Generally speaking, you'll need to use some other directory or discovery service to let the client know what port the server is on. There is no widely available, general purpose mechanism to ask a server "what port is server process X listening on"

If you're just looking to have flexibility in the port assignment (for example, to be able to use different ports in different installations, or to have multiple instances of your server running on different ports), consider using a simple environment variable or command line override to feed a port to the server. Eg,

./start-server -p 4000
./start-server -p 4001

Of course, you'll still need to make the client configurable to find the server, but at least you'll have predictability (and won't be relying on dynamically system-assigned ports).