Cooperation

Java's monitor supports two kinds of thread synchronization: mutual exclusion and cooperation . Mutual exclusion, which is supported in the Java virtual machine via object locks, enables multiple threads to independently work on shared data without interfering with each other. Cooperation, which is supported in the Java virtual machine via the wait and notify methods of class Object, enables threads to work together towards a common goal. 

Cooperation is important when one thread needs some data to be in a particular state and another thread is responsible for getting the data into that state. For example, one thread, a "read thread," may be reading data from a buffer that another thread, a "write thread," is filling. The read thread needs the buffer to be in a "not empty" state before it can read any data out of the buffer. If the read thread discovers that the buffer is empty, it must wait. The write thread is responsible for filling the buffer with data. Once the write thread has done some more writing, the read thread can do some more reading. 

Thread Synchronization by Bill Venner

package monitor;
import java.util.Vector;



class Producer extends Thread {
    static final int MAXQUEUE = 5;
    private Vector messages = new Vector(); 

    public void run() {

        try {
         while ( true ) {
                putMessage();
                sleep( 1000 );
            }
        }
        catch( InterruptedException e ) { }
    }

    private synchronized void putMessage() throws InterruptedException {
        while ( messages.size() == MAXQUEUE )
            wait();
        messages.addElement( new java.util.Date().toString() );
        notify();
    }

    // Called by Consumer
    public synchronized String getMessage() throws InterruptedException {
       notify();
       while ( messages.size() == 0 )
            wait();
        String message = (String)messages.firstElement();
        messages.removeElement( message );
        return message;
    }

}



class Consumer extends Thread {
    Producer producer;

    Consumer(Producer p) {
        producer = p;
    }

    public void run() {
        try {
            while ( true ) {
                String message = producer.getMessage();
                System.out.println("Got message: " + message);
                sleep( 2000 );
            }
        }
        catch( InterruptedException e ) { }
    } 

    public static void main(String args[]) {
        Producer producer = new Producer();
        producer.start();
        new Consumer( producer ).start();
    }

}
 

How to detect JVM Shutdown

Runtime.getRuntime().addShutdownHook(new ShutdownHook());

boolean jvmShuttingDown = false;


private class ShutdownHook extends Thread {
        public void run() {
               jvmShuttingDown = true;
        }
}

How to lock a table in Mysql

in a session
use following
--> lock tables mySchema.myTable write;
In another session try to get
-->select count(*) from mySchema.myTable;

It will be locked till u say

--> unlock tables

How to get OpenJpa Sql Traces

Put the following item in your persistence.xml

<properties>
            <property name="openjpa.Log" value="SQL=TRACE"/>
</properties>

Lets do some XStream test

Aim: Keep a finite state machine configuration in a xml file.

A sample synthetic scenario for this ;
    Get messageBoxes of the users. Put the messageBoxes into a blockingQueue , let some threads process the mailboxes and get new and old messeges from the boxes and seperate them to different queues .
    Then process new messages (exm:delete if it is keeped longer than expected) and put some data for notification sms to the owner of expired message.Some threads will process this queue and will send smses and they will put some data to the edr queue.
    If the message is old (read message) threads on the oldMessageQueue put data to the edr queue directly without sending sms.

Maybe we want to use this states and conditions in a state manager base implementation/code generator which is not written yet ;)

public class StateList {
    public StateList () {
    }

    public List getFsm () {
        return fsm;
    }

    public void setFsm (List fsm) {
        this.fsm = fsm;
    }

    private Listfsm = new ArrayList();
}

public class State {
    public State () {
    }

    private String queueName;
    private int threadCount;
    private int queueSize;
    private String executorClassName;
    private List transitions = new java.util.ArrayList();

    public String getExecutorClassName () {
        return executorClassName;
    }

    public String getQueueName () {
        return queueName;
    }

    public int getThreadCount () {
        return threadCount;
    }

    public int getQueueSize () {
        return queueSize;
    }

    public List getTransitions () {
        return transitions;
    }

    public void setExecutorClassName (String executorClassName) {
        this.executorClassName = executorClassName;
    }

    public void setQueueName (String queueName) {
        this.queueName = queueName;
    }

    public void setThreadCount (int threadCount) {
        this.threadCount = threadCount;
    }

    public void setQueueSize (int queueSize) {
        this.queueSize = queueSize;
    }

    public void setTransitions (List transitions) {
        this.transitions = transitions;
    }
}

public class Transition {
    public Transition () {
    }

    public String getCondition () {
        return condition;
    }

    public String getName () {
        return name;
    }

    public void setCondition (String condition) {
        this.condition = condition;
    }

    public void setName (String name) {
        this.name = name;
    }

    private String name ;
    private String condition;
}

package XstreamTester;

import java.util.*;
import com.thoughtworks.xstream.*;
import java.io.*;

public class XStreamTester {
    public XStreamTester () {
     /* messageboxGetter->//newMsgProcessor ->SmsSender->EdrWrite
                                       ->//oldMsgProcessor ->EdrWrite
     */
    }

    public static void main (String[] args) {
        XStreamTester xstreamtester = new XStreamTester ();

        XStream stateConfiguration = new XStream ();
        stateConfiguration.alias ("state", State.class);
        stateConfiguration.alias ("transition", Transition.class);
        stateConfiguration.alias ("states", StateList.class);

        StateList stateConfig = new StateList();
        List stateList = new ArrayList();
        // messagebox state
        State mboxState = new State ();
        mboxState.setExecutorClassName ("MboxGetter");

        List mboxTransitions = new ArrayList();
        Transition newMessageTransition = new Transition ();
        newMessageTransition.setName ("newMessageQueue");
        newMessageTransition.setCondition ("new");
        mboxTransitions.add (newMessageTransition);
        Transition oldMessageTransition = new Transition ();
        oldMessageTransition.setName ("oldMessageQueue");
        oldMessageTransition.setCondition ("old");
        mboxTransitions.add (oldMessageTransition);
        mboxState.setTransitions (mboxTransitions);

        mboxState.setQueueName ("messageBoxQueue");
        mboxState.setThreadCount (10);
        mboxState.setQueueSize (1000);
        stateList.add (mboxState);


        // new message state
        State newMsgState = new State ();
        newMsgState.setExecutorClassName ("newMessageProcessor");

        List newMsgTransitionList = new ArrayList();
        Transition smsTransition = new Transition ();
        smsTransition.setName ("smsQueue");
        smsTransition.setCondition ("deleted");
        newMsgTransitionList.add (smsTransition);
        newMsgState.setTransitions(newMsgTransitionList);

        newMsgState.setQueueName ("newMessageQueue");
        newMsgState.setThreadCount (10);
        newMsgState.setQueueSize (1000);
        stateList.add (newMsgState);

        // old message state
        State oldMsgState = new State ();
        oldMsgState.setExecutorClassName ("oldMessageProcessor");

        List oldMsgTransitionList = new ArrayList();
        Transition edrTransition = new Transition ();
        edrTransition.setName ("edrQueue");
        edrTransition.setCondition ("deleted");
        oldMsgTransitionList.add (edrTransition);
        oldMsgState.setTransitions(oldMsgTransitionList);

        oldMsgState.setQueueName ("oldMessageQueue");
        oldMsgState.setThreadCount (10);
        oldMsgState.setQueueSize (1000);
        stateList.add (oldMsgState);


        //sms state
        State smsState = new State ();
        smsState.setExecutorClassName ("smsQueueProcessor");

        List smsTransitionList = new ArrayList();
        Transition sms2EdrTransition = new Transition ();
        sms2EdrTransition.setName ("edr");
        sms2EdrTransition.setCondition ("smsSent");
        smsTransitionList.add (sms2EdrTransition);

        smsState.setTransitions(smsTransitionList);
        smsState.setQueueName ("smsQueue");
        smsState.setThreadCount (10);
        smsState.setQueueSize (1000);
        stateList.add (smsState);


        //edr state
        State edrState = new State ();
        smsState.setExecutorClassName ("edrQueueProcessor");

        edrState.setQueueName ("edrQueue");
        edrState.setThreadCount (10);
        edrState.setQueueSize (1000);
        stateList.add (edrState);


        stateConfig.setFsm(stateList);
        String xml = stateConfiguration.toXML (stateConfig);
        try {
            BufferedWriter out = new BufferedWriter (new FileWriter (
                "/home/serkans/jbproject/UM_Test/xml/stateConfiguration.xml"));
            out.write (xml);
            out.close ();
        } catch (IOException e) {
            e.printStackTrace ();
        }
        System.exit (0);
    }
}

Generated xml will be something like this.
<states>
  <fsm>
    <state>
      <queueName>messageBoxQueue</queueName>
      <threadCount>10</threadCount>
      <queueSize>1000</queueSize>
      <executorClassName>MboxGetter</executorClassName>
      <transitions>
        <transition>
          <name>newMessage</name>
          <condition>new</condition>
        </transition>
        <transition>
          <name>oldMessage</name>
          <condition>old</condition>
        </transition>
      </transitions>
    </state>
    <state>
      <queueName>newMessageQueue</queueName>
      <threadCount>10</threadCount>
      <queueSize>1000</queueSize>
      <executorClassName>newMessageProcessor</executorClassName>
      <transitions>
        <transition>
          <name>sms</name>
          <condition>deleted</condition>
        </transition>
      </transitions>
    </state>
    <state>
      <queueName>oldMessageQueue</queueName>
      <threadCount>10</threadCount>
      <queueSize>1000</queueSize>
      <executorClassName>oldMessageProcessor</executorClassName>
      <transitions>
        <transition>
          <name>edr</name>
          <condition>deleted</condition>
        </transition>
      </transitions>
    </state>
    <state>
      <queueName>smsQueue</queueName>
      <threadCount>10</threadCount>
      <queueSize>1000</queueSize>
      <executorClassName>edrQueueProcessor</executorClassName>
      <transitions>
        <transition>
          <name>edr</name>
          <condition>smsSent</condition>
        </transition>
      </transitions>
    </state>
    <state>
      <queueName>edrQueue</queueName>
      <threadCount>10</threadCount>
      <queueSize>1000</queueSize>
      <transitions/>
    </state>
  </fsm>
</states>