Java by examples

Database prepares execution plan(steps) for each query fired on it. Preparing execution plan for each query will increase execution cost. To reduce cost on creating execution plan for each query database provides Prepared Statements.

Prepared Statement
Prepared Statements are pre-cached queries stored in database. Prepared Statement used when same query used with different values. Preparing execution plan in advance for particular query will increase query performance.

When, Prepared Statement?
Multiple occurrence of same query with different values will lead to Prepared Statement. Lets understand following queries...

/* Database will prepare two execution plan for following queries */
SELECT * FROM user_master WHERE user_name = 'vicky.thakor'
SELECT * FROM user_master WHERE user_name = 'chirag.thakor'

/* Prepared Statement */
SELECT * FROM user_master WHERE user_name = :param1

- First two queries are simple SQL statement, both are same except its value. For this queries database will prepare two execution plan.
- Third query will be pre-cached query and can accept n number of values and database don't have to prepare execution plan multiple times. Third query template/Prepared Statement used for above two query.

Rule for execution plan
Database will consider space, comma, special symbol while creating execution plan either its Prepared Statement or Simple SQL Query. Database will prepare different execution plan for following queries which seems similar to us but not for database.

/* Execution Plan 1; Comment is part of query*/
/* Native Query */ SELECT * FROM user_master WHERE user_name = 'vicky.thakor'

/* Execution Plan 2; Comment is part of query, We just changed 'q' in comment */
/* Native query */ SELECT * FROM user_master WHERE user_name = 'vicky.thakor'

/* Execution Plan 3 */
SELECT * FROM user_master WHERE user_name = 'vicky.thakor'

/* Execution Plan 4 (Prepared Statement)*/
SELECT * FROM user_master WHERE user_name = :param1

/* Execution Plan 5 (Prepared Statement); Comment is part of query */
/* Prepared Statement */SELECT * FROM user_master WHERE user_name = :param1

So far you noticed how importance of query writing and Prepared Statement. We all are using Prepared Statement from different programming languages, Program just prepare one of the above Prepared Statement and maintain same query structure for all database execution.

Now we will see how you can use Prepared Statements while writing Stored Procedures, functions in Microsoft SQL Server.

Step 1: Clear all available execution plan/Prepared Statements from cache.

DBCC FREEPROCCACHE
DBCC DROPCLEANBUFFERS

Step 2: We will see the execution plan for simple SQL Query

SELECT * FROM user_master WHERE user_name = 'vicky.thakor'
SELECT * FROM user_master WHERE user_name = 'chirag.thakor'

Step 3: Check your execution plan in cache. You will find two execution plan in it.

SELECT usecounts, size_in_bytes, cacheobjtype, text 
FROM sys.dm_exec_cached_plans
CROSS APPLY sys.dm_exec_sql_text(plan_handle)
WHERE cacheobjtype != 'Parse Tree'

Step 4: Now again clear all available execution plan/Prepared Statements from cache.

DBCC FREEPROCCACHE
DBCC DROPCLEANBUFFERS

Step 5: Now we will Prepare Statement in Database. Execute following query two times with different values. Change your table name, column name and value.

EXEC sp_executesql
@statement = N'SELECT * FROM user_master WHERE user_name = @param1',
@parameters = N'@param1 varchar(100)',
@param1 = N'vicky.thakor'

Step 6: Check your execution plan in cache. You will notice that same execution plan used twice.

SELECT usecounts, size_in_bytes, cacheobjtype, text 
FROM sys.dm_exec_cached_plans
CROSS APPLY sys.dm_exec_sql_text(plan_handle)
WHERE cacheobjtype != 'Parse Tree'

XML
XML stands for Extensible Markup Language. Its goal is to provide generality and simplicity. XML is set of rules to format document that readable by humans and machines both. XMLs are commonly used file format to handle request and response over Internet.

In this example we are using following XML file. We will see how you can parse XML to get multiple nodes and xml node attribute values.

<?xml version="1.0" encoding="UTF-8"?>
<breakfast_menu>
 <food>
  <name>Belgian Waffles</name>
  <price currency="USD" offer="20% Discount">$5.95</price>
  <description>Two of our famous Belgian Waffles with plenty of real maple syrup</description>
  <calories>650</calories>
  <restaurant>New York</restaurant>
  <restaurant>Los Angeles</restaurant>
 </food>
 <food>
  <name>Strawberry Belgian Waffles</name>
  <price currency="USD" offer="20% Discount">$7.95</price>
  <description>Light Belgian waffles covered with strawberries and whipped cream</description>
  <calories>900</calories>
  <restaurant>Los Angeles</restaurant>
  <restaurant>Las Vegas</restaurant>
 </food>
 <food>
  <name>Berry-Berry Belgian Waffles</name>
  <price currency="USD" offer="10% Discount">$8.95</price>
  <description>Light Belgian waffles covered with an assortment of fresh berries and whipped cream</description>
  <calories>900</calories>
  <restaurant>Las Vegas</restaurant>
 </food>
 <food>
  <name>French Toast</name>
  <price currency="USD">$4.50</price>
  <description>Thick slices made from our homemade sourdough bread</description>
  <calories>600</calories>
  <restaurant>Las Vegas</restaurant>
 </food>
 <food>
  <name>Homestyle Breakfast</name>
  <price currency="USD">$6.95</price>
  <description>Two eggs, bacon or sausage, toast, and our ever-popular hash browns</description>
  <calories>950</calories>
  <restaurant>Las Vegas</restaurant>
  <restaurant>New Jersey</restaurant>
 </food>
</breakfast_menu>

Source Code
Un-comment doc = db.parse(new URL("xml_url").openStream()); to read XML from Internet.

import java.io.File;
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.NamedNodeMap;
import org.w3c.dom.NodeList;

public class ReadXML {

    public static void main(String[] args) {
        try {
            File xmlFile = new File("D:\\Dropbox\\Workspace\\SampleXMLFile.xml");
            /* XML Parser base classes */
            DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
            DocumentBuilder db = dbf.newDocumentBuilder();
            Document doc = null;
            /* Check if file is exists or not */
            if (xmlFile.exists()) {
                /* Check you have permission to read file or not */
                if (xmlFile.canRead()) {
                    /* Parse XML file from local drive*/
                    doc = db.parse(xmlFile);
     
                    /* Read XML file from URL */
                    /* doc = db.parse(new URL("xml_url").openStream()); */
     
                    /* Get all "food" node from XML */
                    NodeList listRootNode = doc.getElementsByTagName("food");

                    /* Loop through all "food" node */
                    for (int i = 0; i < listRootNode.getLength(); i++) {
                        /* Get element from list */
                        Element nodeElement = (Element) listRootNode.item(i);
                        /* Get child node of "food" by its node name */
                        String Foodname = nodeElement.getElementsByTagName("name").item(0).getTextContent();
                        System.out.println("Name: "+Foodname);
                        
                        /* Get child node of "food" by its node name */
                        String FoodPrice = nodeElement.getElementsByTagName("price").item(0).getTextContent();
                        /* Get attribute of node */
                        NamedNodeMap nodeAttributes = nodeElement.getElementsByTagName("price").item(0).getAttributes();
                        System.out.print("Price: "+FoodPrice);
                        if(nodeAttributes != null){
                            if(nodeAttributes.getNamedItem("currency") != null){
                                /* Get attribute value */
                                System.out.print(" Currency: "+nodeAttributes.getNamedItem("currency").getNodeValue());                                 
                            }
                            
                            if(nodeAttributes.getNamedItem("offer") != null){
                                /* Get attribute value */
                                System.out.print(" Offer: "+nodeAttributes.getNamedItem("offer").getNodeValue());   
                            }
                        }
                        System.out.println("");
                        /* Get child node of "food" by its node name */
                        String FoodDescription = nodeElement.getElementsByTagName("description").item(0).getTextContent();
                        System.out.println("Description: "+FoodDescription);
                        
                        /* Get child node of "food" by its node name */
                        String FoodCalories = nodeElement.getElementsByTagName("calories").item(0).getTextContent();
                        System.out.println("Calories: "+FoodCalories);
                        
                        /* Get same name child node of "food" by its node name */
                        NodeList listRestaurant = nodeElement.getElementsByTagName("restaurant");
                        for (int j = 0; j < listRestaurant.getLength(); j++) {
                            String city = listRestaurant.item(j).getTextContent();
                            System.out.println("City: "+city);
                        }
                        System.out.println("*************************************************");
                    }
                } else {
                    /* Show message you don't have permission to read the file */
                    System.out.println("Don't have permission to read the file.");
                }
            } else {
                /* Show message file doesn't exsists the specified location. */
                System.out.println("Can't find file specified");
            }
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }
}

Output

Name: Belgian Waffles
Price: $5.95 Currency: USD Offer: 20% Discount
Description: Two of our famous Belgian Waffles with plenty of real maple syrup
Calories: 650
City: New York
City: Los Angeles
*************************************************
....
....

Other References:
How to read file in Java?
How to write file in Java?
How to append text to an existing file in Java?

Following is very common method to read any file in Java, However certain file type has their own formatting . It will not work in case of XML, PDF, Microsoft Excel, Microsoft Word, etc... There are bunch of APIs available on Internet that could help to read / parse this files.

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;

public class ReadFile {
    public static void main(String[] args) {
        /* Create an object of File and specify file path */
        File file = new File("D:\\Dropbox\\Workspace\\SampleTextFile.txt");
        /* String to hold new line value from file */
        String fileLine = "";
        /* Check if file is exists or not */
        if(file.exists()){
            /* Check you have permission to read file or not */
            if(file.canRead()){
                try {
                    /* BufferedReader to read file line by line */
                    BufferedReader br = new BufferedReader(new FileReader(file));
                    /* Loop until last line of file */
                    while((fileLine = br.readLine())!= null){
                        /* Print the line of file */
                        System.out.println(fileLine);
                    }
                } catch (Exception ex) {
                    /* Runtime exception */
                    ex.printStackTrace();
                }
            }else{
                /* Show message you don't have permission to read the file */
                System.out.println("Don't have permission to read the file.");
            }
        }else{
            /* Show message file doesn't exsists the specified location. */
            System.out.println("Can't find file specified");
        }
    }
}

Other References:
How to write file in Java?
How to append text to an existing file in Java?
How to read/parse XML file in Java?

SQL Server maintains cache for execution plan. This cache maintained to optimize query performance. Each query has its own execution plan stored in memory. It will lead to memory overhead for SQL Server. It will also be suppression to find exact execution. We need to cache only those queries which executed multiple times.

/* Get memory used for execution plan cache */
SELECT SUM(size_in_bytes)/1024.0/1024.0 AS [Total MB used]
FROM sys.dm_exec_cached_plans
CROSS APPLY sys.dm_exec_sql_text(plan_handle)

To understand the real time example lets turn off "optimize for ad hoc workloads" option and clear all execution plan from cache.

EXECUTE sp_configure 'optimize for ad hoc workloads', 0
RECONFIGURE
GO

DBCC FREEPROCCACHE
DBCC DROPCLEANBUFFERS
GO

Now execute simple select query on database and generate execution plan cache.

SELECT * FROM HumanResources.Employee

To check query execution plan cache execute below query.

SELECT usecounts, size_in_bytes, cacheobjtype, text
FROM sys.dm_exec_cached_plans
CROSS APPLY sys.dm_exec_sql_text(plan_handle)
WHERE cacheobjtype != 'Parse Tree'

Explanation
When you disable/turn off "optimize for ad hoc workloads" option, cache will stores whole execution plan regardless query will be used in future or not. This will create memory overhead. Lets see the counter part of this option.

---

Now clear all execution plan cache and enable/turn on "optimize for ad hoc workloads" option.

EXECUTE sp_configure 'optimize for ad hoc workloads', 1
RECONFIGURE
GO

DBCC FREEPROCCACHE
DBCC DROPCLEANBUFFERS
GO

Execute same query and check the execution plan cache. Execute both the query separately.

SELECT * FROM HumanResources.Employee

SELECT usecounts, size_in_bytes, cacheobjtype, text
FROM sys.dm_exec_cached_plans
CROSS APPLY sys.dm_exec_sql_text(plan_handle)
WHERE cacheobjtype != 'Parse Tree'

Explanation
When you enable/turn on "optimize for ad hoc workloads" option, SQL server only store Compiled Plan Stub of query which executed once. If you fire same query again then SQL Server get to know that this query already executed once so it will remove Compiled Plan Stub from memory and replace it with Compiled Plan.

BlueCove
BlueCove is Java library for bluetooth. Its used to interact with Bluetooth devices using Java. BlueCove currently supports Mac OS X, WIDCOMM, BlueSoleil and Microsoft Bluetooth stack. Current version is 2.1.0. Read more on official website http://bluecove.org

Today I'm going to demonstrate "How to use BlueCove to search near by Bluetooth devices?" and "How to find Bluetooth device services using BlueCove".

RemoteDeviceDiscovery
RemoteDeviceDiscovery class is used to find paired and near by devices.

import java.util.Enumeration;
import java.util.Vector;
import javax.bluetooth.DeviceClass;
import javax.bluetooth.DiscoveryAgent;
import javax.bluetooth.DiscoveryListener;
import javax.bluetooth.LocalDevice;
import javax.bluetooth.RemoteDevice;
import javax.bluetooth.ServiceRecord;

public class RemoteDeviceDiscovery {

    public Vector getDevices() {
        /* Create Vector variable */
        final Vector devicesDiscovered = new Vector();
        try {
            final Object inquiryCompletedEvent = new Object();
            /* Clear Vector variable */
            devicesDiscovered.clear();

            /* Create an object of DiscoveryListener */
            DiscoveryListener listener = new DiscoveryListener() {

                public void deviceDiscovered(RemoteDevice btDevice, DeviceClass cod) {
                    /* Get devices paired with system or in range(Without Pair) */
                    devicesDiscovered.addElement(btDevice);
                }

                public void inquiryCompleted(int discType) {
                    /* Notify thread when inquiry completed */
                    synchronized (inquiryCompletedEvent) {
                        inquiryCompletedEvent.notifyAll();
                    }
                }

                /* To find service on bluetooth */
                public void serviceSearchCompleted(int transID, int respCode) {
                }

                /* To find service on bluetooth */
                public void servicesDiscovered(int transID, ServiceRecord[] servRecord) {
                }
            };

            synchronized (inquiryCompletedEvent) {
                /* Start device discovery */
                boolean started = LocalDevice.getLocalDevice().getDiscoveryAgent().startInquiry(DiscoveryAgent.GIAC, listener);
                if (started) {
                    System.out.println("wait for device inquiry to complete...");
                    inquiryCompletedEvent.wait();
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
        /* Return list of devices */
        return devicesDiscovered;
    }
}

ServicesSearch
ServicesSearch class used to find specific service on Bluetooth device. Each service identified using UUID (Universally unique identifier). All UUID listed on https://www.bluetooth.org/en-us/specification/assigned-numbers/service-discovery . Replace your required UUID in following code.

import java.util.ArrayList;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import javax.bluetooth.*;

public class ServicesSearch {

    /** 
     * UUID used to find specific service supported by bluetooth device
     * https://www.bluetooth.org/en-us/specification/assigned-numbers/service-discovery
     * Find UUIDs for all types of bluetooth services.
     */
    /* To find push object service */
    private UUID OBEX_OBJECT_PUSH_PROFILE = new UUID(0x1105);
    /* To find file transfer service */
    private UUID OBEX_FILE_TRANSFER_PROFILE = new UUID(0x1106);
    /* To find hands free service */
    private UUID HANDS_FREE = new UUID(0x111E);
    /* Get URL attribute from bluetooth service */
    private int URL_ATTRIBUTE = 0X0100;
    
    public Map<String, List<String>> getBluetoothDevices() {        
        /**
         * Find service on bluetooth device 
         * Note: In following line you can use one service at a time. I'm new to bluetooth programming it might me wrong perception.
         * UUID[] searchUuidSet = new UUID[]{OBEX_FILE_TRANSGER_PROFILE};
         * 
         * CORRECT: UUID[] searchUuidSet = new UUID[]{OBEX_FILE_TRANSGER_PROFILE};
         * WRONG: UUID[] searchUuidSet = new UUID[]{OBEX_FILE_TRANSGER_PROFILE, OBEX_OBJECT_PUSH_PROFILE};
         */
        /* Initialize UUID Array */
        UUID[] searchUuidSet = new UUID[]{HANDS_FREE};
        final Object serviceSearchCompletedEvent = new Object();
        int[] attrIDs = new int[]{URL_ATTRIBUTE};
        
        /* Create an object to get list of devices in range or paired */
        RemoteDeviceDiscovery remoteDeviceDiscovery = new RemoteDeviceDiscovery();
        /* Create map to return Bluetooth device address, name and URL */
        final Map<String, List<String>> mapReturnResult = new HashMap<String, List<String>>(); 

        try {
            /* Create an object of DiscoveryListener */
            DiscoveryListener listener = new DiscoveryListener() {

                /* To find bluetooth devices */
                public void deviceDiscovered(RemoteDevice btDevice, DeviceClass cod) {
                }

                /* To find bluetooth devices */
                public void inquiryCompleted(int discType) {
                }

                /* Find service URL of bluetooth device */
                public void servicesDiscovered(int transID, ServiceRecord[] servRecord) {
                    for (int i = 0; i < servRecord.length; i++) {
                        /* Find URL of bluetooth device */
                        String url = servRecord[i].getConnectionURL(ServiceRecord.NOAUTHENTICATE_NOENCRYPT, false);
                        if (url == null) {
                            continue;
                        }
                        String temporaryString = "";
                        /* Get object of bluetooth device */
                        RemoteDevice rd = servRecord[i].getHostDevice();
                        /* Get attribute from ServiceRecord */
                        DataElement serviceName = servRecord[i].getAttributeValue(URL_ATTRIBUTE);
                        if (serviceName != null) {         
                            temporaryString = serviceName.getValue() + "\n" + url;
                            /* Put it in map */
                            mapReturnResult.get(rd.getBluetoothAddress()).add(temporaryString);
                        } else {
                            temporaryString = "Uknown service \n" + url;
                            /* Put it in map */
                            mapReturnResult.get(rd.getBluetoothAddress()).add(temporaryString);
                        }
                    }
                }

                public void serviceSearchCompleted(int transID, int respCode) {
                    /* Notify thread when search completed */
                    synchronized (serviceSearchCompletedEvent) {
                        serviceSearchCompletedEvent.notifyAll();
                    }
                }
            };

            /* Get list of bluetooth device from class RemoteDeviceDiscovery */
            for (Enumeration en = remoteDeviceDiscovery.getDevices().elements(); en.hasMoreElements();) {
                /* Get RemoteDevice object */
                RemoteDevice btDevice = (RemoteDevice) en.nextElement();
                /* Create list to return details */
                List<String> listDeviceDetails = new ArrayList<String>();
                
                try {
                    /* Add bluetooth device name and address in list */
                    listDeviceDetails.add(btDevice.getFriendlyName(false));
                    listDeviceDetails.add(btDevice.getBluetoothAddress());
                } catch (Exception e) {
                }
                
                /* Put bluetooth device details in map */
                mapReturnResult.put(btDevice.getBluetoothAddress(), listDeviceDetails);
                synchronized (serviceSearchCompletedEvent) {
                    LocalDevice.getLocalDevice().getDiscoveryAgent().searchServices(attrIDs, searchUuidSet, btDevice, listener);
                    serviceSearchCompletedEvent.wait();
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
        /* Return bluetooth devices detail */
        return mapReturnResult;
    }
}

BluetoothDevices
BluetoothDevices is simple JFrame used to give Uter Interface for program. Its created in NetBeans IDE, You may find some code unexplained which is generated by IDE.

import com.javaquery.bluetooth.ServicesSearch;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import javax.swing.DefaultListModel;
import javax.swing.JFrame;
import javax.swing.UIManager;

public class BluetoothDevices extends javax.swing.JFrame {

    /* DefaultListModel to attach it with JList */
    private DefaultListModel defaultModel;
    /* Map to get device details list */
    private Map<String, List<String>> mapReturnResult = new HashMap<String, List<String>>();
    /* Map to identify device on user click of JList */
    private Map<Integer, List<String>> mapDevicePosition = new HashMap<Integer, List<String>> ();

    public BluetoothDevices() {
        initComponents();
        defaultModel = new DefaultListModel();
    }

    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">//GEN-BEGIN:initComponents
    private void initComponents() {

        jScrollPane1 = new javax.swing.JScrollPane();
        JListBluetoothDevices = new javax.swing.JList();
        lblDeviceName = new javax.swing.JLabel();
        lblRuntimeDeviceName = new javax.swing.JLabel();
        lblDeviceAddress = new javax.swing.JLabel();
        lblRuntimeDeviceAddress = new javax.swing.JLabel();
        lblServiceDetails = new javax.swing.JLabel();
        jScrollPane2 = new javax.swing.JScrollPane();
        JTextAreaServiceDetails = new javax.swing.JTextArea();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
        setTitle("Bluecove Bluetooth Discovery");
        setResizable(false);
        addWindowListener(new java.awt.event.WindowAdapter() {
            public void windowOpened(java.awt.event.WindowEvent evt) {
                formWindowOpened(evt);
            }
        });

        JListBluetoothDevices.addMouseListener(new java.awt.event.MouseAdapter() {
            public void mouseClicked(java.awt.event.MouseEvent evt) {
                JListBluetoothDevicesMouseClicked(evt);
            }
        });
        jScrollPane1.setViewportView(JListBluetoothDevices);

        lblDeviceName.setText("Bluetooth Device Name");

        lblDeviceAddress.setText("Bluetooth Device Address");

        lblServiceDetails.setText("Service Details");

        JTextAreaServiceDetails.setColumns(20);
        JTextAreaServiceDetails.setRows(5);
        jScrollPane2.setViewportView(JTextAreaServiceDetails);

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(jScrollPane1, javax.swing.GroupLayout.PREFERRED_SIZE, 158, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addGroup(layout.createSequentialGroup()
                        .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                            .addComponent(lblDeviceName)
                            .addComponent(lblDeviceAddress))
                        .addGap(73, 73, 73)
                        .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                            .addComponent(lblRuntimeDeviceAddress)
                            .addComponent(lblRuntimeDeviceName, javax.swing.GroupLayout.PREFERRED_SIZE, 144, javax.swing.GroupLayout.PREFERRED_SIZE)))
                    .addComponent(lblServiceDetails)
                    .addComponent(jScrollPane2, javax.swing.GroupLayout.DEFAULT_SIZE, 475, Short.MAX_VALUE))
                .addContainerGap())
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addComponent(jScrollPane1, javax.swing.GroupLayout.DEFAULT_SIZE, 185, Short.MAX_VALUE)
                    .addGroup(layout.createSequentialGroup()
                        .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                            .addComponent(lblDeviceName)
                            .addComponent(lblRuntimeDeviceName))
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                        .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                            .addComponent(lblDeviceAddress)
                            .addComponent(lblRuntimeDeviceAddress))
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                        .addComponent(lblServiceDetails)
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                        .addComponent(jScrollPane2, 0, 0, Short.MAX_VALUE)))
                .addContainerGap())
        );

        pack();
    }// </editor-fold>//GEN-END:initComponents

 /* Search for bluetooth device when window opened */
    private void formWindowOpened(java.awt.event.WindowEvent evt) {//GEN-FIRST:event_formWindowOpened
        int intDevicePosition = 0;
        JListBluetoothDevices.setModel(defaultModel);        

        /* Create an object of ServicesSearch */
        ServicesSearch ss = new ServicesSearch();
        /* Get bluetooth device details */
        mapReturnResult = ss.getBluetoothDevices();

        /* Add devices in JList */
        for (Map.Entry<String, List<String>> entry : mapReturnResult.entrySet()) {            
            defaultModel.addElement(entry.getValue().get(0));
            mapDevicePosition.put(intDevicePosition, entry.getValue());
            intDevicePosition++;
        }
    }//GEN-LAST:event_formWindowOpened

 /* On click of any item in List Box */
    private void JListBluetoothDevicesMouseClicked(java.awt.event.MouseEvent evt) {//GEN-FIRST:event_JListBluetoothDevicesMouseClicked
  /* Get bluetooth device details from temporary list */
        List<String> tmpDeviceDetails = mapDevicePosition.get(JListBluetoothDevices.getSelectedIndex());
  /* Set bluetooth device name */
        lblRuntimeDeviceName.setText(tmpDeviceDetails.get(0));
  /* Set bluetooth device Address */
        lblRuntimeDeviceAddress.setText(tmpDeviceDetails.get(1));       
        
        if(tmpDeviceDetails.size() > 2 && tmpDeviceDetails.get(2) != null){
   /* Set bluetooth device service name and URL */
            JTextAreaServiceDetails.setText(tmpDeviceDetails.get(2));
        }else{
            JTextAreaServiceDetails.setText("Service not found");
        }
    }//GEN-LAST:event_JListBluetoothDevicesMouseClicked

    /**
     * @param args the command line arguments
     */
    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {

            public void run() {
                 /* To set new look and feel */
                JFrame.setDefaultLookAndFeelDecorated(true);
                try {
                    /**
                     * Change look and feel of JFrame to Nimbus 
                     * For other look and feel check
      * http://www.javaquery.com/2013/06/how-to-applyset-up-swing-look-and-feel.html
                     */
                    UIManager.setLookAndFeel("com.sun.java.swing.plaf.nimbus.NimbusLookAndFeel");
                } catch (Exception ex) {
                   ex.printStackTrace();
                }
                /* Create an object of BluetoothDevices */
                BluetoothDevices bluetoothDevicesFrame = new BluetoothDevices();
                /* make BluetoothDevices visible */
                bluetoothDevicesFrame.setVisible(true);
            }
        });
    }
    // Variables declaration - do not modify//GEN-BEGIN:variables
    private javax.swing.JList JListBluetoothDevices;
    private javax.swing.JTextArea JTextAreaServiceDetails;
    private javax.swing.JScrollPane jScrollPane1;
    private javax.swing.JScrollPane jScrollPane2;
    private javax.swing.JLabel lblDeviceAddress;
    private javax.swing.JLabel lblDeviceName;
    private javax.swing.JLabel lblRuntimeDeviceAddress;
    private javax.swing.JLabel lblRuntimeDeviceName;
    private javax.swing.JLabel lblServiceDetails;
    // End of variables declaration//GEN-END:variables
}

Downloads
NetBeans Project: https://drive.google.com/open?id=0B0ioAuPvwwCCUFJMMEllTE5KQVk
Driver: https://drive.google.com/open?id=0B0ioAuPvwwCCRVpzNmIwOS1kMHM

Client Server ChatApplication

Wikipedia: A network socket is an endpoint of an inter-process communication flow across a computer network.

Difference between Server and Client
Server Socket is one that accept connection on specific port and can respond. Where client can send message to server on specified port and can get response from server.

We will send messages over network only. In this demo application I'm sending and receiving messages on localhost. To create full-fledged chat application you need to modify many things like...

• Every system must have Socket Server running on local system to accept messages from another system.
• Get list of all system which is running your application. 
• If you are planning to create rich chat application, provide advance settings like Save chat log, Play sound when receives message, Send encrypted messages, etc... can be lot more.

Note: There will be plenty of code which is not explained in all source code which was generated by NetBeans IDE. NetBeans project code attached at the end of article.

Socket Server
It will accept connection and listen for messages and pass it to desired frame/user chat window. I create Server in form JFrame just to give better look.

/*
 * mainFrame.java
 * This program demonstarte client server message application.
 * mainFrame.java will act as a server program aimed to receive messages and respond to client.
 */
package com.javaquery.frames;

import java.awt.Frame;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;
import javax.swing.JFrame;
import javax.swing.UIManager;
import org.json.JSONObject;

public class mainFrame extends javax.swing.JFrame {

    /* Inner class to create socket server */
    private class mainServer extends Thread {

        /* Create ServerSocket variable */
        private ServerSocket serverSocket;

        /* Constructor to initialize serverSocket */
        public mainServer() throws IOException {
            serverSocket = new ServerSocket(6666);
        }

        /* Implement run() for Thread */
        public void run() {
            /* Keep Thread running */
            while (true) {
                try {
                    /* Accept connection on server */
                    Socket server = serverSocket.accept();
                    /* DataInputStream to get message sent by client program */
                    DataInputStream in = new DataInputStream(server.getInputStream());
                    /* We are receiving message in JSON format from client. Parse String to JSONObject */
                    JSONObject clientMessage = new JSONObject(in.readUTF());
                    
                    /* Flag to check chat window is opened for user that sent message */
                    boolean flagChatWindowOpened = false;
                    /* Reading Message and Username from JSONObject */
                    String userName = clientMessage.get("Username").toString();
                    String message = clientMessage.getString("Message").toString();
                    
                    /* Get list of Frame/Windows opened by mainFrame.java */
                    for(Frame frame : Frame.getFrames()){
                        /* Check Frame/Window is opened for user */
                        if(frame.getTitle().equals(userName)){
                            /* Frame/ Window is already opened */
                            flagChatWindowOpened = true;
                            /* Get instance of ChatWindow */
                            ChatWindow chatWindow = (ChatWindow) frame;
                            /* Get previous messages from TextArea */
                            String previousMessage = chatWindow.getjTextArea1().getText();
                            /* Set message to TextArea with new message */
                            chatWindow.getjTextArea1().setText(previousMessage+"\n"+message);
                        }
                    }
                    
                    /* ChatWindow is not open for user sent message to server */
                    if(!flagChatWindowOpened){
                        /* Create an Object of ChatWindow */
                        ChatWindow chatWindow = new ChatWindow();
                        /**
                         * We are setting title of window to identify user for next message we gonna receive
                         * You can set hidden value in ChatWindow.java file.
                         */
                        chatWindow.setTitle(userName);
                        /* Set message to TextArea */
                        chatWindow.getjTextArea1().setText(message);
                        /* Make ChatWindow visible */
                        chatWindow.setVisible(true);
                    }
                    
                    /* Get DataOutputStream of client to repond */
                    DataOutputStream out = new DataOutputStream(server.getOutputStream());
                    /* Send response message to client */
                    out.writeUTF("Received from "+clientMessage.get("Username").toString());
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }
    }

    /**
     * To start SocketServer when mainFrame.java loads
     */
    public void startServer() {
        try {
            /* Create thread of Inner class mainServer */
            Thread t = new mainServer();
            /* Start Thread */
            t.start();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    /** Creates new form mainFrame */
    public mainFrame() {
        initComponents();
    }

    /** This method is called from within the constructor to
     * initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is
     * always regenerated by the Form Editor.
     */
    @SuppressWarnings("unchecked")
    //                           
    private void initComponents() {

        lblInformation = new javax.swing.JLabel();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
        setTitle("Server");
        setAlwaysOnTop(true);

        lblInformation.setText("Client - Server Sample Application");

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(lblInformation)
                .addContainerGap(229, Short.MAX_VALUE))
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(lblInformation)
                .addContainerGap(275, Short.MAX_VALUE))
        );

        pack();
    }//                         

    /**
     * @param args the command line arguments
     */
    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {

            public void run() {
                /* To set new look and feel */
                JFrame.setDefaultLookAndFeelDecorated(true);
                try {
                    /**
                     * Change look and feel of JFrame to Nimbus 
                     * For other look and feel check
                     * 
                     */
                    UIManager.setLookAndFeel("com.sun.java.swing.plaf.nimbus.NimbusLookAndFeel");
                } catch (Exception ex) {
                   ex.printStackTrace();
                }
                /* Create an Object of mainFrame */
                mainFrame mFrame = new mainFrame();
                /* make mainFrame visible */
                mFrame.setVisible(true);
                /* Call startServer method  */
                mFrame.startServer();
            }
        });
    }
    // Variables declaration - do not modify                     
    private javax.swing.JLabel lblInformation;
    // End of variables declaration                   
}

Chat Window Source Code
ChatWindow.java is common class which creates Chat window for user. We are identify particular user's chat window by its title. You can have your logic.

package com.javaquery.frames;

public class ChatWindow extends javax.swing.JFrame {

    /** Creates new form ChatWindow */
    public ChatWindow() {
        initComponents();
    }

    /** This method is called from within the constructor to
     * initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is
     * always regenerated by the Form Editor.
     */
    @SuppressWarnings("unchecked")
    //                           
    private void initComponents() {

        lblInfo = new javax.swing.JLabel();
        scrollPane = new javax.swing.JScrollPane();
        chatArea = new javax.swing.JTextArea();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
        setTitle("Message Box");

        lblInfo.setText("Messages");

        chatArea.setColumns(20);
        chatArea.setRows(5);
        scrollPane.setViewportView(chatArea);

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addComponent(scrollPane, javax.swing.GroupLayout.DEFAULT_SIZE, 380, Short.MAX_VALUE)
                    .addComponent(lblInfo))
                .addContainerGap())
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(lblInfo)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                .addComponent(scrollPane, javax.swing.GroupLayout.DEFAULT_SIZE, 253, Short.MAX_VALUE)
                .addContainerGap())
        );

        pack();
    }//                         

    /**
     * @param args the command line arguments
     */
    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {

            public void run() {
                new ChatWindow().setVisible(true);
            }
        });
    }
    // Variables declaration - do not modify                     
    private javax.swing.JTextArea chatArea;
    private javax.swing.JLabel lblInfo;
    private javax.swing.JScrollPane scrollPane;
    // End of variables declaration                   

    /**
     * @return the chatArea
     */
    public javax.swing.JTextArea getjTextArea1() {
        return chatArea;
    }

    /**
     * @param chatArea the chatArea to set
     */
    public void setjTextArea1(javax.swing.JTextArea jTextArea1) {
        this.chatArea = jTextArea1;
    }
}

Client Source Code
This will send message to sever and get response. We are sending 3 messages to server. Two messages from same username.

/*
 * client.java
 * This program demonstarte client server message application.
 * client.java will act as a client program aimed to send messages and get server response
 */

package com.javaquery.network;

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.Socket;
import org.json.JSONObject;

public class client {

    public static void main(String[] args) {
        client c = new client();
        /* Create JSON variable */
        JSONObject transmitJSON = new JSONObject();

        /**
         * This is sample program so we are sending three messages from same system with different username.
         * In your case you'll receive message from multiple systems.
         */
        /* Fill up JSON variable with message and username */
        transmitJSON.put("Message", "Hello JavaQuery!");
        transmitJSON.put("Username", "Vicky.Thakor");
        /* Send message to server */
        c.sendMessage("localhost", transmitJSON.toString());
        
        transmitJSON.put("Message", "Hello Apple!");
        transmitJSON.put("Username", "Steve.Jobs");
        /* Send message to server */
        c.sendMessage("localhost", transmitJSON.toString());
        
        transmitJSON.put("Message", "How are you?");
        transmitJSON.put("Username", "Steve.Jobs");
        /* Send message to server */
        c.sendMessage("localhost", transmitJSON.toString());
    }

    /**
     * @author javaQuery
     * @param host
     * @param message 
     */
    public void sendMessage(String host, String message) {
        try {
            /* Create new socket connection with server host using port 6666 (port can be anything) */
            Socket client = new Socket(host, 6666);
            /* Get server's OutputStream */
            OutputStream outToServer = client.getOutputStream();
            /* Get server's DataOutputStream to write/send message */
            DataOutputStream out = new DataOutputStream(outToServer);
            /* Write message to DataOutputStream */
            out.writeUTF(message);
            /* Get InputStream to get message from server */
            InputStream inFromServer = client.getInputStream();
            /* Get DataInputStream to read message of server */
            DataInputStream in = new DataInputStream(inFromServer);
            /* Print message received from server */
            System.out.println("Server says..." + in.readUTF());
            /* Close connection of client socket */
            client.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Download Project

Swing
Swing is built on base of Abstract Window Toolkit. Swing is used to create desktop application. Swing is using AWT directly or indirectly to provide you richer user interface. Swing is placed under javax.swing




Understanding the Diagram
As you can see I created boundary around Parent JFrame, Child JFrame 1 and Child JFrame 2. It means you can access list of JFrame initiated by ParentJFrame. You can't access Standalone JFrame in Parent JFrame.


Frame.getFrames()
This method will return list of all Frame Object created by its parent and parent it self.


Parent JFrame Source Code

import java.awt.Frame;

public class ParentJFrame extends javax.swing.JFrame {

    /* Creates new form ParentJFrame */
    public ParentJFrame() {
        initComponents();
    }

    @SuppressWarnings("unchecked")
    // //GEN-BEGIN:initComponents
    private void initComponents() {

        lblInfo = new javax.swing.JLabel();
        btnOpenChildJFrame = new javax.swing.JButton();
        btnListJFrames = new javax.swing.JButton();
        scrollPane = new javax.swing.JScrollPane();
        textArea = new javax.swing.JTextArea();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
        setTitle("Parent JFrame");

        lblInfo.setText("Parent JFrame");

        btnOpenChildJFrame.setText("Open Child JFrame");
        btnOpenChildJFrame.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                btnOpenChildJFrameActionPerformed(evt);
            }
        });

        btnListJFrames.setText("List JFrame");
        btnListJFrames.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                btnListJFramesActionPerformed(evt);
            }
        });

        textArea.setColumns(20);
        textArea.setRows(5);
        scrollPane.setViewportView(textArea);

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addComponent(scrollPane, javax.swing.GroupLayout.DEFAULT_SIZE, 228, Short.MAX_VALUE)
                    .addComponent(lblInfo)
                    .addGroup(layout.createSequentialGroup()
                        .addComponent(btnOpenChildJFrame)
                        .addGap(18, 18, 18)
                        .addComponent(btnListJFrames)))
                .addContainerGap())
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(lblInfo)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(btnOpenChildJFrame)
                    .addComponent(btnListJFrames))
                .addGap(18, 18, 18)
                .addComponent(scrollPane, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
        );

        pack();
    }// //GEN-END:initComponents

    /**
     * Open Child JFrame button click event
     */
    private void btnOpenChildJFrameActionPerformed(java.awt.event.ActionEvent evt) {//GEN-FIRST:event_btnOpenChildJFrameActionPerformed
        /* Create an object of childJFrame */
        childJFrame child = new childJFrame();
        /* Make childJFrame visible */
        child.setVisible(true);
    }//GEN-LAST:event_btnOpenChildJFrameActionPerformed

    /**
     * List JFrame button click event
     */
    private void btnListJFramesActionPerformed(java.awt.event.ActionEvent evt) {//GEN-FIRST:event_btnListJFramesActionPerformed
        /* Get previous text of JFrame */
        String previousText = textArea.getText();
        String strNewFrames = "";
        /* It will give you list of JFrame created/Initiated by ParentJFrame */
        for(Frame frame: Frame.getFrames()){
            strNewFrames += frame.getTitle() + "\n";
        }
        /* Set text of JTextArea */
        textArea.setText(previousText+strNewFrames);
    }//GEN-LAST:event_btnListJFramesActionPerformed

    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {

            public void run() {
                new ParentJFrame().setVisible(true);
            }
        });
    }

    private javax.swing.JButton btnListJFrames;
    private javax.swing.JButton btnOpenChildJFrame;
    private javax.swing.JLabel lblInfo;
    private javax.swing.JScrollPane scrollPane;
    private javax.swing.JTextArea textArea;
}

Child JFrame Source Code

public class childJFrame extends javax.swing.JFrame {

    /** Creates new form childJFrame */
    public childJFrame() {
        initComponents();
    }

    @SuppressWarnings("unchecked")
    // //GEN-BEGIN:initComponents
    private void initComponents() {

        lblInfo = new javax.swing.JLabel();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
        setTitle("Child JFrame");

        lblInfo.setText("Child JFrame");

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(lblInfo)
                .addContainerGap(188, Short.MAX_VALUE))
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(lblInfo)
                .addContainerGap(91, Short.MAX_VALUE))
        );

        pack();
    }// //GEN-END:initComponents

    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {

            public void run() {
                new childJFrame().setVisible(true);
            }
        });
    }
    private javax.swing.JLabel lblInfo;
}

I used NetBeans to create sample code. I explained code that relate the title of article. Major code done only in Parent JFrame. Child JFrame only supports example.

This exception caused by several issues. Issue may be solved by ensuring following checklist.

- Connected to Internet / Local Area Network (LAN)
Make sure you are connected to internet or local area network.

- Behind Firewall
Firewall is common issue faced by many developer. We were facing same issue.

java.net.ConnectException: Connection timed out: connect
 at java.net.PlainSocketImpl.socketConnect(Native Method)
 at java.net.PlainSocketImpl.doConnect(PlainSocketImpl.java:351)
 at java.net.PlainSocketImpl.connectToAddress(PlainSocketImpl.java:213)
 at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:200)
 at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:366)
 at java.net.Socket.connect(Socket.java:529)
 at java.net.Socket.connect(Socket.java:478)
 at sun.net.NetworkClient.doConnect(NetworkClient.java:163)
 at sun.net.www.http.HttpClient.openServer(HttpClient.java:394)
 at sun.net.www.http.HttpClient.openServer(HttpClient.java:529)
 at sun.net.www.http.HttpClient.(HttpClient.java:233)
 at sun.net.www.http.HttpClient.New(HttpClient.java:306)
 at sun.net.www.http.HttpClient.New(HttpClient.java:323)
 at sun.net.www.protocol.http.HttpURLConnection.getNewHttpClient(HttpURLConnection.java:970)
 at sun.net.www.protocol.http.HttpURLConnection.plainConnect(HttpURLConnection.java:911)
 at sun.net.www.protocol.http.HttpURLConnection.connect(HttpURLConnection.java:836)
 at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1172)
 at ReadWebpage.main(ReadWebpage.java:24)

Scenario
We were writing an applet program in Java to read webpage from internet. We can't even make request to well know search engine www.google.com even if its allowed in firewall rule. We called upon our network admin he said you must pass your request through proxy server. We set proxy in our Java code for HttpURLConnection and all went good.

Code

Proxy proxy = new Proxy(Proxy.Type.HTTP, new InetSocketAddress("192.168.0.202", 8080));
HttpURLConnection con = (HttpURLConnection) data.openConnection(proxy);

Use of Proxy
You need to take care for proxy connection and it depends upon whether you are write program for applet or desktop application. If you are writing code for applet then in production applet, you don't need proxy connection as server will have all access and will work without proxy. If you are working on desktop application that run within the Local Area Network then you must set proxy in case of firewall.

Applet
An applet is Java program designed to be delivered over Internet/Network. The Java program executed automatically by Java compatible browser.

Code Signing Certificate
Like in real world we sign document, Code Signing Certificate does the same with code. Code signing certificate is used to preserve authenticity of code. You can buy code signing certificate through different vendor like verisign, Thawte, etc...

Note: I am using .jks (Java KeyStore) file to sign my applet. I haven't tried with other certificate as its not available for free. You can also contact your vendor for the same.


Step 1: Start command prompt.

/**
 * Change jarsigner.exe path 
 * Change certificate name and path
 * Change jar file name and path
 */
cmd> "C:\Program Files\Java\jdk1.6.0_25\bin\jarsigner.exe" -keystore javaQueryThawte.jks applet.jar server

Step 3: Verifying jar you just signed.

/**
 * Change jarsigner.exe path 
 * Change jar file name and path
 */
cmd> "C:\Program Files\Java\jdk1.6.0_25\bin\jarsigner.exe" -verify -verbose -certs applet.jar

Lets say you required to sign same applet again and again, we'll act smart. Create executable batch file for windows system.

/**
 * Open notepad
 * Change jarsigner.exe location
 * Change certificate name
 * Change jar file name
 * Save below code with file extension .bat i.e: signApplet.bat
 */
@echo off
echo Applet Code Signing Console
"C:\Program Files\Java\jdk1.6.0_25\bin\jarsigner.exe" -keystore NextendersIndiaPvtLtd.jks POCDemo.jar server
"C:\Program Files\Java\jdk1.6.0_25\bin\jarsigner.exe" -verify -verbose -certs POCDemo.jar
pause

Place signApplet.bat, certificate and jar file in same folder. Now execute signApplet.bat and provide password of certificate. You are done with it.

You can't specify target table 'user_master' for update in FROM clause error comes up when you try to perform an update or delete operation while selecting data from table it self. This is something like How to update/delete self table in MySQL? We will produce same error and then will give you solution for the same.

Problem

/* UPDATING RECORDS */
UPDATE user_master
SET first_name = 'javaQuery'
WHERE id IN (SELECT id FROM user_master WHERE last_name = 'Jobs');

/* DELETING RECORDS */
DELETE 
FROM user_master
WHERE id IN (SELECT id FROM user_master WHERE user_name = 'javaQuery');

We are fetching all users whose Lastname is Jobs and replacing its Firstname in 1st query and deleting records in second query. This won't allow you to update or delete records.


Solution

/* UPDATING RECORDS */
UPDATE user_master
SET first_name = 'javaQuery'
WHERE id IN (SELECT * FROM (SELECT id FROM user_master WHERE user_name = 'javaQuery') AS userIDs);

/* DELETING RECORDS */
DELETE
FROM user_master
WHERE id IN (SELECT * FROM (SELECT id FROM user_master WHERE user_name = 'javaQuery') AS userIDs);

What we did?

• We are selecting data from query SELECT id FROM user_master WHERE user_name = 'javaQuery' to temporary table called userIDs.
• Select data from temporary table and use it in WHERE condition.