JGroups(3)

 

3 Building Blocks
    Building blocks位于org.jgroups.blocks包中，在逻辑上可以视为channels之上的一层，它提供了更复杂的接口。Building blocks并不必依赖于channels，部分building blocks只需要实现了Transport接口的类即可工作。以下简要介绍部分building blocks。

3．1 MessageDispatcher
    Channels 通常用于异步地发送和接收消息。然后有些情况下需要同步通信，例如发送者希望向集群发送消息并等待所有成员的应答，或者等待部分成员的应答。MessageDispatcher支持以同步或者异步的方式发送消息，它在构造时需要一个Channel型的参数。
    MessageDispatcher提供了Object handle(Message msg)方法，用于以push 方式的接收消息并返回应答（必须可以被序列化），该方法抛出的异常也会被传播到消息发送者。MessageDispatcher在内部使用了PullPushAdapter，PullPushAdapter也是org.jgroups.blocks包中的类，但是已经被标记为deprecated。这种方式被称为MessageDispatcher的server模式。
    MessageDispatcher的client模式是指通过调用castMessage或者sendMessage向集群发送消息并同步或者异步的等待应答。castMessage()方法向dests指定的地址发送消息，如果dest为null，那么向集群中所有成员发送消息。castMessage()方法的返回值是RspList，RspList 实现了Map<Address,Rsp> 接口。msg参数中的目的地址会被覆盖。mode参数（由org.jgroups.blocks.GroupRequest类定义）指定了消息是同步还是异步发送，其可选值如下：

• GET_FIRST 返回收到的第一个应答。
• GET_ALL 等待所有成员的应答（被怀疑崩溃的成员除外）。
• GET_MAJORITY 等待绝大多数成员（相对与成员的个数）的应答。
• GET_ABS_MAJORITY等待绝大多数成员（一个绝对的数值，只计算一次）的应答。
• GET_N 等待n个应答，如果n大于成员的个数，可能会一直阻塞下去。
• GET_NONE 不等待应答，直接返回，即异步方式。

    castMessage()方法的定义如下：

public RspList castMessage(Vector dests, Message msg, int mode, long timeout);

public RspList castMessage(Vector dests, Message msg, int mode, long timeout);

    sendMessage()方法允许向一个成员发送消息，msg参数的目的地址不能为null。如果mode参数是GET_NONE，那么消息的发送变成异步方式；否则mode参数会被忽略（缺省采用GET_FIRST）。sendMessage()方法的定义如下：

public Object sendMessage(Message msg, int mode, long timeout) throws TimeoutException;

public Object sendMessage(Message msg, int mode, long timeout) throws TimeoutException;

    以下是个使用MessageDispatcher的例子：

import java.io.BufferedReader;
import java.io.InputStreamReader;

import org.jgroups.Channel;
import org.jgroups.JChannel;
import org.jgroups.Message;
import org.jgroups.blocks.GroupRequest;
import org.jgroups.blocks.MessageDispatcher;
import org.jgroups.blocks.RequestHandler;
import org.jgroups.util.RspList;

public class MessageDispatcherTest {
    //
    private Channel channel;
    private MessageDispatcher dispatcher;
    private boolean propagateException = false;

    public void start() throws Exception {
        //
        channel = new JChannel();
        dispatcher = new MessageDispatcher(channel, null, null, new RequestHandler() {

            public Object handle(Message msg) {
                System.out.println("got a message: " + msg);
                if(propagateException) {
                    throw new RuntimeException("failed to handle message: " + msg.getObject());
                } else {
                    return new String("success");
                }
            }

        });
        channel.connect("MessageDispatcherTest");

        //
        sendMessage();

        //
        channel.close();
        dispatcher.stop();
    }

    private void sendMessage() throws Exception {
        boolean succeed = false;
        BufferedReader br = null;
        try {
            br = new BufferedReader(new InputStreamReader(System.in));
            while(true) {
                System.out.print("> ");
                System.out.flush();
                String line = br.readLine();
                if(line != null && line.equals("exit")) {
                    break;
                } else {
                    Message msg = new Message(null, null, line);
                    RspList rl = dispatcher.castMessage(null, msg, GroupRequest.GET_ALL, 0);
                    System.out.println("Responses:/n" + rl);
                }
            }
            succeed = true;
        } finally {
            if(br != null) {
                try {
                    br.close();
                } catch (Exception e) {
                    if(succeed) {
                        throw e;
                    }
                }
            }
        }
    }

    public static void main(String[] args) {
        try {
            new MessageDispatcherTest().start();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

import java.io.BufferedReader; import java.io.InputStreamReader; import org.jgroups.Channel; import org.jgroups.JChannel; import org.jgroups.Message; import org.jgroups.blocks.GroupRequest; import org.jgroups.blocks.MessageDispatcher; import org.jgroups.blocks.RequestHandler; import org.jgroups.util.RspList; public class MessageDispatcherTest { // private Channel channel; private MessageDispatcher dispatcher; private boolean propagateException = false; public void start() throws Exception { // channel = new JChannel(); dispatcher = new MessageDispatcher(channel, null, null, new RequestHandler() { public Object handle(Message msg) { System.out.println("got a message: " + msg); if(propagateException) { throw new RuntimeException("failed to handle message: " + msg.getObject()); } else { return new String("success"); } } }); channel.connect("MessageDispatcherTest"); // sendMessage(); // channel.close(); dispatcher.stop(); } private void sendMessage() throws Exception { boolean succeed = false; BufferedReader br = null; try { br = new BufferedReader(new InputStreamReader(System.in)); while(true) { System.out.print("> "); System.out.flush(); String line = br.readLine(); if(line != null && line.equals("exit")) { break; } else { Message msg = new Message(null, null, line); RspList rl = dispatcher.castMessage(null, msg, GroupRequest.GET_ALL, 0); System.out.println("Responses:/n" + rl); } } succeed = true; } finally { if(br != null) { try { br.close(); } catch (Exception e) { if(succeed) { throw e; } } } } } public static void main(String[] args) { try { new MessageDispatcherTest().start(); } catch (Exception e) { e.printStackTrace(); } } }

3．2 RpcDispatcher
    RpcDispatcher 继承自MessageDispatcher，它允许远程调用集群中其它成员上的方法，并可选地等待应答。跟MessageDispatcher相比，不需要为RpcDispatcher指定RequestHandler。RpcDispatcher的构造函数接受一个Object server_obj参数，它是远程调用的目标对象。RpcDispatcher的callRemoteMethods系列方法用于远程调用目标对象上的方法，该方法可以由MethodCall指定，也可以通过方法名、参数类型指定。跟MessageDispatcher的castMessage()方法和sendMessage()方法类似，callRemoteMethods系列方法也接受一个int mode参数，其含义也相同。以下是个简单的例子：

import java.io.BufferedReader;
import java.io.InputStreamReader;

import org.jgroups.Channel;
import org.jgroups.JChannel;
import org.jgroups.blocks.GroupRequest;
import org.jgroups.blocks.RpcDispatcher;
import org.jgroups.util.RspList;

public class RpcDispatcherTest {
    private Channel channel;
    private RpcDispatcher dispatcher;

    public int print(int number) throws Exception {
        return number * 2;
    }

    public void start() throws Exception {
        channel = new JChannel();
        dispatcher = new RpcDispatcher(channel, null, null, this);
        channel.connect("RpcDispatcherTest");

        //
        sendMessage();

        //
        channel.close();
        dispatcher.stop();
    }

    private void sendMessage() throws Exception {
        boolean succeed = false;
        BufferedReader br = null;
        try {
            br = new BufferedReader(new InputStreamReader(System.in));
            while(true) {
                System.out.print("> please input an int value:");
                System.out.flush();
                String line = br.readLine();
                if(line != null && line.equals("exit")) {
                    break;
                } else {
                    int param = 0;
                    try {
                        param = Integer.parseInt(line);
                    } catch(Exception e) {
                        System.out.println("invalid input: " + line);
                        continue;
                    }
                    RspList rl = dispatcher.callRemoteMethods(null, "print", new Object[]{new Integer(param)}, new Class[]{int.class}, GroupRequest.GET_ALL, 0);
                    System.out.println("Responses: /n" + rl);
                }
            }
            succeed = true;
        } finally {
            if(br != null) {
                try {
                    br.close();
                } catch (Exception e) {
                    if(succeed) {
                        throw e;
                    }
                }
            }
        }
    }

    public static void main(String[] args) {
        try {
            new RpcDispatcherTest().start();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

import java.io.BufferedReader; import java.io.InputStreamReader; import org.jgroups.Channel; import org.jgroups.JChannel; import org.jgroups.blocks.GroupRequest; import org.jgroups.blocks.RpcDispatcher; import org.jgroups.util.RspList; public class RpcDispatcherTest { private Channel channel; private RpcDispatcher dispatcher; public int print(int number) throws Exception { return number * 2; } public void start() throws Exception { channel = new JChannel(); dispatcher = new RpcDispatcher(channel, null, null, this); channel.connect("RpcDispatcherTest"); // sendMessage(); // channel.close(); dispatcher.stop(); } private void sendMessage() throws Exception { boolean succeed = false; BufferedReader br = null; try { br = new BufferedReader(new InputStreamReader(System.in)); while(true) { System.out.print("> please input an int value:"); System.out.flush(); String line = br.readLine(); if(line != null && line.equals("exit")) { break; } else { int param = 0; try { param = Integer.parseInt(line); } catch(Exception e) { System.out.println("invalid input: " + line); continue; } RspList rl = dispatcher.callRemoteMethods(null, "print", new Object[]{new Integer(param)}, new Class[]{int.class}, GroupRequest.GET_ALL, 0); System.out.println("Responses: /n" + rl); } } succeed = true; } finally { if(br != null) { try { br.close(); } catch (Exception e) { if(succeed) { throw e; } } } } } public static void main(String[] args) { try { new RpcDispatcherTest().start(); } catch (Exception e) { e.printStackTrace(); } } }

3．3 ReplicatedHashMap
    ReplicatedHashMap 继承自ConcurrentHashMap，并在内部使用了RpcDispatcher。ReplicatedHashMap构造函数的clustername参数指定了集群的名字，集群中所有的实例会包含相同的状态。新加入的实例在开始工作前会从集群中获得当前的状态。对实例的修改（例如通过put，remove方法）会传播到集群的其它实例中，只读的请求（例如get方法）则是本地调用。需要注意的是，ReplicatedHashMap的以下划线开头的方法是用于RpcDispatcher的远程调用的。在ReplicatedHashMap上可以注册 Notification，以便在实例的状态改变时进行回调，所有的回调也是本地的。以下是个简单的例子：

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.Iterator;
import java.util.Map;
import java.util.Vector;

import org.jgroups.Address;
import org.jgroups.ChannelFactory;
import org.jgroups.JChannelFactory;
import org.jgroups.View;
import org.jgroups.blocks.ReplicatedHashMap;

public class ReplicatedHashMapTest implements ReplicatedHashMap.Notification<String, String> {
    //
    private ReplicatedHashMap<String, String> map;

    public void start() throws Exception {
        ChannelFactory factory = new JChannelFactory();
        map = new ReplicatedHashMap<String, String>("ReplicatedHashMapTest", factory, "udp.xml", false, 10000);
        map.addNotifier(this);

        sendMessage();
        map.stop();
    }

    public void entryRemoved(String key) {
        System.out.println("in entryRemoved(" + key + ")");
    }

    public void entrySet(String key, String value) {
        System.out.println("in entrySet(" + key + "," + value + ")");
    }

    public void contentsSet(Map<String, String> m) {
        System.out.println("in contentsSet(" + printMap(m) + ")");
    }

    public void contentsCleared() {
        System.out.println("in contentsCleared()");
    }

    public void viewChange(View view, Vector<Address> newMembers,
            Vector<Address> oldMembers) {
        System.out.println("in viewChange(" + view + ")");
    }

    private void sendMessage() throws Exception {
        boolean succeed = false;
        BufferedReader br = null;
        try {
            br = new BufferedReader(new InputStreamReader(System.in));
            while (true) {
                System.out.print("> ");
                System.out.flush();
                String line = br.readLine();
                if (line != null && line.equals("exit")) {
                    break;
                } else {
                    if (line.equals("show")) {
                        System.out.println(printMap(map));
                    } else if (line.equals("clear")) {
                        map.clear();
                    } else if (line.startsWith("remove ")) {
                        String key = line.substring(line.indexOf(" ") + 1, line.length()).trim();
                        map.remove(key);
                    } else if (line.startsWith("put ")) {
                        line = line.replace("put ", "");
                        int index = line.indexOf("=");
                        if (index <= 0 || index >= (line.length() - 1)) {
                            System.out.println("invalid input");
                            continue;
                        }
                        String key = line.substring(0, index).trim();
                        String value = line.substring(index + 1, line.length())
                                .trim();
                        map.put(key, value);
                    } else {
                        System.out.println("invalid input: " + line);
                        continue;
                    }
                }
            }
            succeed = true;
        } finally {
            if (br != null) {
                try {
                    br.close();
                } catch (Exception e) {
                    if (succeed) {
                        throw e;
                    }
                }
            }
        }
    }

    private String printMap(Map<String, String> m) {
        StringBuffer sb = new StringBuffer();
        sb.append("[");
        for (Iterator<String> iter = map.keySet().iterator(); iter.hasNext();) {
            String key = iter.next();
            String value = map.get(key);
            sb.append(key).append("=").append(value);
            if (iter.hasNext()) {
                sb.append(",");
            }
        }
        sb.append("]");
        return sb.toString();
    }

    public static void main(String args[]) {
        try {
            new ReplicatedHashMapTest().start();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

import java.io.BufferedReader; import java.io.InputStreamReader; import java.util.Iterator; import java.util.Map; import java.util.Vector; import org.jgroups.Address; import org.jgroups.ChannelFactory; import org.jgroups.JChannelFactory; import org.jgroups.View; import org.jgroups.blocks.ReplicatedHashMap; public class ReplicatedHashMapTest implements ReplicatedHashMap.Notification<String, String> { // private ReplicatedHashMap<String, String> map; public void start() throws Exception { ChannelFactory factory = new JChannelFactory(); map = new ReplicatedHashMap<String, String>("ReplicatedHashMapTest", factory, "udp.xml", false, 10000); map.addNotifier(this); sendMessage(); map.stop(); } public void entryRemoved(String key) { System.out.println("in entryRemoved(" + key + ")"); } public void entrySet(String key, String value) { System.out.println("in entrySet(" + key + "," + value + ")"); } public void contentsSet(Map<String, String> m) { System.out.println("in contentsSet(" + printMap(m) + ")"); } public void contentsCleared() { System.out.println("in contentsCleared()"); } public void viewChange(View view, Vector<Address> newMembers, Vector<Address> oldMembers) { System.out.println("in viewChange(" + view + ")"); } private void sendMessage() throws Exception { boolean succeed = false; BufferedReader br = null; try { br = new BufferedReader(new InputStreamReader(System.in)); while (true) { System.out.print("> "); System.out.flush(); String line = br.readLine(); if (line != null && line.equals("exit")) { break; } else { if (line.equals("show")) { System.out.println(printMap(map)); } else if (line.equals("clear")) { map.clear(); } else if (line.startsWith("remove ")) { String key = line.substring(line.indexOf(" ") + 1, line.length()).trim(); map.remove(key); } else if (line.startsWith("put ")) { line = line.replace("put ", ""); int index = line.indexOf("="); if (index <= 0 || index >= (line.length() - 1)) { System.out.println("invalid input"); continue; } String key = line.substring(0, index).trim(); String value = line.substring(index + 1, line.length()) .trim(); map.put(key, value); } else { System.out.println("invalid input: " + line); continue; } } } succeed = true; } finally { if (br != null) { try { br.close(); } catch (Exception e) { if (succeed) { throw e; } } } } } private String printMap(Map<String, String> m) { StringBuffer sb = new StringBuffer(); sb.append("["); for (Iterator<String> iter = map.keySet().iterator(); iter.hasNext();) { String key = iter.next(); String value = map.get(key); sb.append(key).append("=").append(value); if (iter.hasNext()) { sb.append(","); } } sb.append("]"); return sb.toString(); } public static void main(String args[]) { try { new ReplicatedHashMapTest().start(); } catch (Exception e) { e.printStackTrace(); } } }

3．4 NotificationBus
    NotificationBus 提供了向集群发送通知的能力，通知可以是任何可以被序列化的对象。NotificationBus在内部使用Channel，其start()和stop()方法用于启动和停止。NotificationBus的setConsumer()方法用于注册Consumer接口，其定义如下：

public interface Consumer {
    void handleNotification(Serializable n);
    Serializable getCache();
    void memberJoined(Address mbr);
    void memberLeft(Address mbr);
}

public interface Consumer { void handleNotification(Serializable n); Serializable getCache(); void memberJoined(Address mbr); void memberLeft(Address mbr); }

    NotificationBus的getCacheFromCoordinator() 和getCacheFromMember()用于请求集群的状态。前者是从coordinator得到状态，后者从指定地址的成员处得到状态。NotificationBus上注册的Consumer需要实现getCache()方法以返回状态。以下是个简单的例子：

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.Serializable;
import java.util.Iterator;
import java.util.LinkedList;

import org.jgroups.Address;
import org.jgroups.blocks.NotificationBus;

public class NotificationBusTest implements NotificationBus.Consumer {
    //
    private NotificationBus bus;
    private LinkedList<Serializable> cache;

    public void handleNotification(Serializable n) {
        System.out.println("in handleNotification(" + n + ")");
        if (cache != null) {
            cache.add(n);
        }
    }

    public Serializable getCache() {
        return cache;
    }

    public void memberJoined(Address mbr) {
        System.out.println("in memberJoined(" + mbr + ")");
    }

    public void memberLeft(Address mbr) {
        System.out.println("in memberLeft(" + mbr + ")");
    }

    @SuppressWarnings("unchecked")
    public void start() throws Exception {
        //
        bus = new NotificationBus("NotificationBusTest", null);
        bus.setConsumer(this);
        bus.start();
        cache = (LinkedList<Serializable>) bus.getCacheFromCoordinator(3000, 1);
        if (cache == null) {
            cache = new LinkedList<Serializable>();
        }
        System.out.println(printCache(cache));

        //
        sendNotification();

        //
        bus.stop();
    }

    private void sendNotification() throws Exception {
        boolean succeed = false;
        BufferedReader br = null;
        try {
            br = new BufferedReader(new InputStreamReader(System.in));
            while (true) {
                System.out.print("> ");
                System.out.flush();
                String line = br.readLine();
                if (line != null && line.equals("exit")) {
                    break;
                } else {
                    bus.sendNotification(line);
                }
            }
            succeed = true;
        } finally {
            if (br != null) {
                try {
                    br.close();
                } catch (Exception e) {
                    if (succeed) {
                        throw e;
                    }
                }
            }
        }
    }

    private String printCache(LinkedList<Serializable> c) {
        StringBuffer sb = new StringBuffer();
        sb.append("[");
        for (Iterator<Serializable> iter = c.iterator(); iter.hasNext();) {
            sb.append(iter.next());
            if (iter.hasNext()) {
                sb.append(",");
            }
        }
        sb.append("]");
        return sb.toString();
    }

    public static void main(String[] args) {
        try {
            new NotificationBusTest().start();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}