javacream

String Performance

2011-06-26T16:13:00.001+03:00

Perhaps string concatenation is the most frequently used string operation in any language. In Java you can concat two string in this manner:

String concatdStr = "String1"+"String2"+" and String3";

But this is not a good idea. Behind the curtain , Java does string concatenation using StringBuffer class. So the above concat becomes :

String sBuf = new StringBuffer(32);
sBuf.append("String1").append("String2").append(" and String3");

Clearing WAR , EAR JAR files for .svn folders and library jars

2011-03-27T14:06:00.006+03:00

Big archieve sizes can be annoying. There can be some useless files within a war or jar file.

1. If you uploaded your library files to application server than you do not need to pack them everytime you create application archieve file.

2. There can be subversion folders which do not have a function in runtime.

I wrote a basic windows batch file. it uses 7-zip.

clearJavaArchieves.bat

cd %TEMP%
rem "C:\Program Files\7-Zip\7z.exe" d %1 *.jar -r -tzip
"C:\Program Files\7-Zip\7z.exe" d %1 .svn -r -tzip
rem "C:\Program Files\7-Zip\7z.exe" d %1 WEB-INF\lib -tzip
pause

paste this code to file.

first line is the for the temp files.

if you want to clear all jar files within your archieve , and you know what you are doing , delete rem part of line 2 , otherwise you can delete delete line 2.

3rd line deletes all .svn folders within your archieve file. if you donot want to do this , delete line 3.

if you want to delete web-inf/lib , delete rem part of 4th line. (warning be sure you uploaded all necessary jar files to your application server's lib folder) , otherwise you can delete delete line 4!!!

on windows , right-click to your arcive file -> open with -> choose default program -> (you can uncheck "always use the selected..." checkbox if you donot wanto to change your default program) -> browse.

once you open your archieve file with this script , windows adds it to open-with menu with the right-click.

use with caution and test before deploying to production.

Determine jar file of a class

2011-03-13T11:16:00.014+02:00

We sometimes need to determine from which jar file some particular class comes from. Here I wrote a small method to find out the name of the jar file we are looking for :

public static String getJarURL(Class mclass) {

URL clsUrl = mclass.getResource(mclass.getSimpleName() + ".class");

if (clsUrl != null) {

try {

URLConnection conn = clsUrl.openConnection();

if (conn instanceof JarURLConnection) {

JarURLConnection connection = (JarURLConnection) conn;

return connection.getJarFileURL().toString();

}

}

catch (IOException e) {

throw new RuntimeException(e);

}

}

return null;

} 

public static void main(String[] x){

System.out.println(getJarURL(java.net.HttpURLConnection.class));

}

which prints following output :

file:/C:/Program%20Files%20(x86)/Java/jre1.5.0_22/lib/rt.jar

Simple Java Exception Handling Tips

2011-03-06T14:49:00.013+02:00

Exceptions are errors that occur in runtime , that is , when the application is running.

A careful developer should estimate and care about possible runtime errors.

Let us give an example :

String s = args[0];
int i = Integer.parseInt(s);

this piece of code can cause some exceptions. For example args is an array of string. We want to read the first element of this array. What happens if args is null ? What happens if args is empty ?

so we need to ensure that the array is not null and not empty.

String s = null; // initialize s
// check the array is not null or empty
if (args != null && args.length > 0)
s = args[0];
int i = 0;
if (s != null) // if s is not null we can parse
i = Integer.parseInt(s);
else { // s is null , so show an error message and exit
System.err.println(
"Invalid argument , should be a valid integer value" );
System.exit(1);
}
System.out.println("square of "+i+" is ="+(i*i));

But there is still a possible cause of exception. We here assume that if s is not null than s contains an integer value that can be parsed without an error. But assumptions always cause exceptions. String s can be a "hello world" string and parseInt fails with this value.

so we need to rewrite our code :

String s = null; //initialize s
if ( args != null && args.length > 0 ) //check the argument array is null or empty
s = args[0];
int i = 0 ;
if ( s != null ) { //if s is not null we can try to parse
try {
i = Integer.parseInt(s);
} catch (NumberFormatException e) {
e.printStackTrace(); //dump the error
System.err.println("Invalid argument , should be a valid integer value");
System.exit(1);
}
}
else { // s is null , so show an error message and exit
System.err.println("Invalid argument , should be a valid integer value");
System.exit(1);
}
System.out.println("square of "+i+" is ="+(i*i));

as an alternative to try/catch blocks we can declare our methods as it throws exception , for example :

public int square(String s) throws NumberFormatException {

Schedule windows to close a program

2010-02-11T01:29:00.003+02:00

On windows If we need to close a program (maybe a java) at a certain time , use this command :

at 13:29 taskkill /F /IM 6976

6976 is the process id (you can find it on task manager - processes tab. If not seen , than on the top menu choose View-Select Coloumns and mark PID (processID) )

to cancel all scheduled commands

at /delete

you need to open cmd window as administrator to run at command. (startMenu-accessories right click on command window and choose "run as admin"

hop this helps.

Convert Java Date to Mysql Date

2009-10-16T11:05:00.004+03:00

Normal 0 21 false false false MicrosoftInternetExplorer4

public static String toMysqlDateStr(Date date){

if (date==null)

return "NULL";

SimpleDateFormat sdf =new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

return '\''+sdf.format(date)+'\'';

}

How to measure distance on google maps ?

2009-03-18T00:02:00.000+02:00

Use the page here , you just click on points and the distance will appear either in miles or in kilometers. You can also mark multiple points and calculate a real length of a path.

Check if a String contains letter

2009-03-11T18:25:00.002+02:00

public static boolean containsLetter(String s) {
if ( s == null )
return false;
boolean letterFound = false;
for (int i = 0; !letterFound && i < s.length(); i++)
letterFound = letterFound
|| Character.isLetter(s.charAt(i));
return letterFound;
}

Get MimeType of A File

2009-02-27T17:52:00.002+02:00

Use activation.jar to access javax.activation API.

File file = new File("foo.jpg");
String mimeType = new MimetypesFileTypeMap().getContentType(file)); // image/jpeg

alternatively , you can use filename to detech the mime type :

String mimeType = new MimetypesFileTypeMap().getContentType("foo.jpg")); // image/jpeg

Redirect StdErr to Stdin in Batch and Shell Scripts

2009-01-22T01:09:00.002+02:00

On windows command prompt :

mybatch.bat 1> log.txt 2>&1

This redirects stdin to log.txt and stderr to stdin

On Bash Linux Shell:

grep * &> allout.txt

Get Remote IP Address In Axis SOAP Server

2008-10-31T13:58:00.004+02:00

Obtain remote IP from ServletRequest (Axis 1.0 only)

import javax.servlet.http.HttpServletRequest;

import org.apache.axis.MessageContext;

import org.apache.axis.transport.http.HTTPConstants;

public static String getSOAPRemoteIP() {

MessageContext context = MessageContext.getCurrentContext();

String address = null;

if(context!=null &&

context.containsProperty(HTTPConstants.MC_HTTP_SERVLETREQUEST))

{

HttpServletRequest servletReq = (HttpServletRequest)

context.getProperty(HTTPConstants.MC_HTTP_SERVLETREQUEST);

if (servletReq != null)

address = servletReq.getRemoteAddr();

}

return address;

}

Google Chrome Ready For Download

2008-09-02T17:00:00.004+03:00

Google has released a new browser named Google Chrome. This browser is expected to become most widespred computer program ever written. Google Chrome will set the standarts for Web2.0 and Web3.0 technologies.

Chrome is a ligtweight browser and does not consume your CPU and memory and it makes Chrome fast and reliable.

Download Here

Here is a gread comic book for chrome on googlebooks site

At first impression Google Chrome is not working properly with PHP-Admin !!!

Console Beep On Error Line of a Log File

2008-08-22T19:33:00.006+03:00

Ok folks , this is non-java :)

Today I had to watch a log file and alert some ppl when an error phrase seen on log file. This is a terrible jıb todo. First I run on my linux server:

#tail -f mylogfile | grep ERRORSTRING

this worked but put me in front of the screen. Why not my console beep when ERRORSTRING exists in the log file ? I need an application to beep when read a line from stdin. This is piece of cake in java but i did not want to load JVM for this purpose because of memory needs. So I warped to good old days and decided to write a C program to do this job.


#include <stdio.h>
#include <string.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
        char buf[1024];
        int n;
        while (fgets(buf, sizeof buf, stdin) != NULL) {
                n = strlen(buf);
                if (n > 0 && buf[n-1] == '\n')  {
                        printf("\a");
                        printf("%s\n", buf);
                }
        }
}

and I compiled :
#cc beep.c -o mybeep

and later :

#tail -f mylogfile | grep ERRORSTRING | /home/hakan/utils/beep

and that's it , it worked ! I watch olympic games and when console beeps I called administration ppl :) There should be a lot of log watching utility but it is easier for me to write code than search google for utility programs and install them. Maybe this code can crash the server but it works for now , thanks in advance for comments.

Using Java Properties File

2008-07-23T13:36:00.002+03:00

Many programmers use properties files for keeping configuration parameters (DB Server IP etc.)

Here is a sample class code for accessing configuration parameters from a properties file. You can pass JVM parameter like this :

java -DMyPropertiesFile=/usr/local/java/proj/myconfig.properties MyClass


package javacream;

import java.io.InputStream;
import java.util.Properties;

public class AppProperties {
 private static Properties prop;

 static {
  initiate();
 }

 protected static void initiate() {
  try {
   //Pass Java VM following arguement
   // -DMyPropertiesFile="[PATH]/[PROPERTIESFILENAME]"

   prop = new Properties();

   String propertiesFile = System.getProperty("MyPropertiesFile");

   if (propertiesFile == null) {
    System.out
      .println("Could not find content.properties property in System Properties.");
    System.out
      .println("This property file is needed for the application to operate.");
    System.out
      .println("Send this parameter as -DMyPropertiesFile=%FILENAME% to java vm.");

    propertiesFile = "myconfiguration.properties";

    System.out
      .println("Setting properties file name to default value "
        + propertiesFile);

   }

   try {
    InputStream is = new java.io.FileInputStream(propertiesFile);

    prop.load(is);
   } catch (Exception e) {
    e.printStackTrace();

    System.out.println("Could not read or find MyPropertiesFile");
    System.out
      .println("This property file is needed for the application to operate.");
    System.out
      .println("Send this parameter as -MyPropertiesFile=%FILENAME% to java vm.");
    return;
   }
  } catch (Throwable t) {
   t.printStackTrace();

   return;
  }

 }

 public static String get(String key) {
  String value = prop.getProperty(key);
  if ((value == null) && !prop.containsKey(key)) {
   System.out.println("Could not find the key " + key
     + " in Properties file.");

  }
  return (value);
 }

 public static String get(String key, String defaultValue) {
  String value = prop.getProperty(key);

  if ((value == null) && !prop.containsKey(key)) {
   System.out.println("Could not find the key " + key
     + " in Properties file, using default value");
  }

  return (prop.getProperty(key, defaultValue));

 }

 public static void main(String[] args) {
  System.out.println(AppProperties.get("UserName"));
 }

}

Java MD5 Hash Sample

2008-07-13T13:00:00.001+03:00

MD5 (Message Digest) is an encryption algorithm that creates a unique hash from a given data which is completely reverseble. This means you cannot recalculate original data using its hash , and hash is universally unique.

Here is a sample code that produces MD5 hash of a given string :

package javacream;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
public class MD5HashTest {
private static String getMD5Digest(String str) {
try {
byte[] buffer = str.getBytes();
byte[] result = null;
StringBuffer buf = null;
MessageDigest md5 = MessageDigest.getInstance("MD5");
//allocate room for the hash result = new byte[md5.getDigestLength()];
//calculate hash md5.reset();
md5.update(buffer);
result = md5.digest();
System.out.println(result);
//create hex string from the 16-byte hash buf = new StringBuffer(result.length * 2);
for (int i = 0;
i <
result.length;
i++) {
int intVal = result[i] & 0xff;
if (intVal <
0x10) {
buf.append("0");
}
buf.append(Integer.toHexString(intVal).toUpperCase());
}
return buf.toString();
}
catch (NoSuchAlgorithmException e) {
System.err.println("Exception caught: " + e);
e.printStackTrace();
}
return null;
}
public static void main(String[] args) {
String result = getMD5Digest("This is a text to hash");
System.out.println("result=" + result);
}
}

Java HTPPS SSL Client

2008-07-09T12:41:00.000+03:00

HTTPS secure HTTP connection , HTTP over secure socket layer (SSL).
To establish an HTTPS connection to a remote server , you should use the certificate obtained by the server. This certificate is validated using a local trust store. Default trust store is /lib/security/cacerts. You can define another trust store using keytool application in /bin. Certificates can be added to trust store with keytool application also.

You should give trust store parameter to JVM like this :

-Djavax.net.ssl.trustStore=/mypath/my.keystore
-Djavax.net.ssl.trustStorePassword=mypassword

Here is the sample code to create an HTTPS connection:


package javacream;

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.net.URLEncoder;
import java.security.Security;

import javax.net.ssl.SSLSocket;
import javax.net.ssl.SSLSocketFactory;

public class HTTPSClient {

 public static void main(String[] args) {
  String resp = sendReq("myserver.mydomain.com",
    "/myapp/findbookservlet", "bookID=2323");
  System.out.println(resp);
 }

 // Uses post to fetch data , return null if it fails.
 public static String sendReq(String host, String path, String data) {

  int port = 443; // default https port

  BufferedWriter wr = null;
  BufferedReader in = null;
  SSLSocket socket = null;

  try {
   Security.addProvider(new com.sun.net.ssl.internal.ssl.Provider());
   SSLSocketFactory factory = (SSLSocketFactory) SSLSocketFactory
     .getDefault();
   socket = (SSLSocket) factory.createSocket(host, port);
   String[] supported = socket.getSupportedCipherSuites();
   socket.setEnabledCipherSuites(supported);
   String encData = URLEncoder.encode(data, "UTF-8");

   wr = new BufferedWriter(new OutputStreamWriter(socket
     .getOutputStream(), "UTF8"));
   wr.write("POST " + path + " HTTP/1.0\r\n");
   wr.write("Content-Length: " + data.length() + "\r\n");
   wr.write("Content-Type: application/x-www-form-urlencoded\r\n");
   wr.write("\r\n");
   wr.write(data);
   wr.flush();
   // read response
   in = new BufferedReader(new InputStreamReader(socket
     .getInputStream(), "UTF-8"));
   int c;
   StringBuffer sb = new StringBuffer();

   while ((c = in.read()) != -1) {
    sb.append((char) c);
   }
   System.out.println("SSL Response:" + sb.toString());
   return sb.toString();
  } catch (Exception e) {
   e.printStackTrace();
  } finally {
   if (wr != null)
    try {
     wr.close();
    } catch (IOException e2) {
     e2.printStackTrace();
    }
   if (in != null)
    try {
     in.close();
    } catch (IOException e1) {
     e1.printStackTrace();
    }
   if (socket != null)
    try {
     socket.close();
    } catch (IOException e2) {
     e2.printStackTrace();
    }

  }

  return null;
 }
}

Sample JSP Date Combo

2008-07-05T00:54:00.000+03:00

This method produces required html code for date time combos. Date parameter is selected in case of equality.


 public static String printDateCombos(String name , java.util.Date date) {
  
  Calendar today = Calendar.getInstance(new Locale("en-US")) ; 
  
  if (date == null) 
   today.setTimeInMillis( System.currentTimeMillis()  );
  else 
   today.setTime( date ) ;
  
  int year = today.get( Calendar.YEAR );
  int month = today.get( Calendar.MONTH );
  int day = today.get( Calendar.DAY_OF_MONTH );
  int hour = today.get( Calendar.HOUR_OF_DAY );
  int min = today.get( Calendar.MINUTE ); 
  StringBuffer out = new StringBuffer();
  
  out.append("<select name=\"");
  out.append(name);
  out.append("_day\">");

  for (int i=1;i<32;i++) {
   out.append("<option");
   if (i==day) out.append(" selected ");
   out.append(">");
   if (i<10) out.append('0');
   out.append(i);
   out.append("</option>");
  
  }
  out.append("</select>");

  // month
  out.append("/<select name=\"");
  out.append(name);
  out.append("_month\">");

  for (int i=0;i<12;i++) {
   out.append("<option");
   if (i==month) out.append(" selected");
   out.append(" value=");
   out.append(i+1);
   out.append(">");
   out.append( MONTHS[i] );
   out.append("</option>");
  
  }
  out.append("</select>");
  
  
  // year
  out.append("/<select name=\"");
  out.append(name);
  out.append("_year\">");

  for (int i=0;i<10;i++) {
   out.append("<option");
   if (i==0) out.append(" selected");
   out.append(">");
   out.append(year+i);
   out.append("</option>");
  
  }
  out.append("</select>");
  // hour
  out.append("-<select name=\"");
  out.append(name);
  out.append("_hour\">");

  for (int i=0;i<24;i++) {
   out.append("<option");
   if (i==hour) out.append(" selected");
   out.append(">");
   if (i<10) out.append('0');
   out.append(i);
   out.append("</option>");
  }
  out.append("</select>:");  

  // minute
  out.append("<select name=\"");
  out.append(name);
  out.append("_minute\">");

  for (int i=0;i<60;i++) {
   out.append("<option");
   if (i==min) out.append(" selected");
   out.append(">");
   if (i<10) out.append('0');
   out.append(i);
   out.append("</option>");
  }
  out.append("</select>");  

  return out.toString() ;  
 }

Convert Date object to Mysql Date Format

2008-06-18T11:31:00.003+03:00


 public static String toMysqlDateStr(Date date){
  if (date==null) return "NULL";
  SimpleDateFormat sdf =new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
  return sqlValueWithQuotas(sdf.format(date));
 }

 public static String sqlValueWithQuotas(Object obj){
  if ( obj == null ) return "NULL";
  
  String str = obj.toString();
  str.replaceAll("'", "\\'");
  str = '\''+str+'\'';
  
  return str;
  
 }

JSP Sessions Example

2008-06-14T19:55:00.001+03:00

Sessions are started automatically in JSP pages.
You can access current session by prebuilt session object.

Create sessions.jsp file on your Tomcat's webapps/ROOT directory :


<%@page import="java.util.Date"%>
<%
    out.print("Session Creation Time:" + new Date(session.getCreationTime()));
    out.print("<BR>");
    out.print("Last accessed Time:" + new Date(session.getLastAccessedTime()));
    out.print("<BR>");
    out.print("Session ID:"+session.getId());
%>

and launch it (for example http://localhost:8080/sessions.jsp)

it prints:

Session Creation Time:Sat Jun 14 19:28:06 EEST 2008
Last accessed Time:Sat Jun 14 19:33:34 EEST 2008
Session ID:365A3F239D1E54FD43EA0F7CBA1931EF

Now we are able to access the session. We can store objects in session objects.
This can help share objects between JSP pages.

Sessions are widely used for authentication purposes.

For example , let this be index.jsp as default page :


<%
String userName = request.getParameter("userName");
String password = request.getParameter("password");
if ("admin".equals(userName) && "adminpass".equals(password) )  { //Session start
    session.setAttribute("userName",userName);
    session.setAttribute("role","admin");
    response.sendRedirect("/main.jsp");
}
else out.print("Logon failed");
%>
<form action="index.jsp" method="post">
    Enter Username: <input name="userName"> <br>
    Enter Password: <input name="password"> <br>
    <input type="submit">
</form>

The sample code above get username and password from user. If username and password are ok , username and role objects are stored in session and redirects to main.jsp otherwise sends an http error.

And in main.jsp we authenticate user based on username and role :

<%
String userName = session.getAttribute("userName");
String role = session.getAttribute("role");
if ( userName == null || ! "admin".equals(role) ) {
response.sendError(403,"You are not authorized to view this page");
return;
}
%>

This authentication part can be a seperate jsp and included in every page that needs authentication. Ideally , username , password and role lookup should be from database or from a directory service like LDAP or Active Directory.

Technorati Profile

Simple JSP Tutorial

2008-06-09T11:16:00.007+03:00

This jsp accepts a number from an html form and prints squareroot of that number on the same page. Paste this code to file "first.jsp" and move it to tomcat's webapps/ROOT directory. Start tomcat and browse to localhost:8080/first.jsp. We put java code in <% %> block , other parts are standart html. <%=variable %> prints variable into html code just like <%out.print(variable);%> does.

request , response and out objects are standart JSP objects that can be used to access http request and response.


<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
   pageEncoding="ISO-8859-1"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Sample JSP</title>
</head>
<body>
<%
String strNumber= request.getParameter("number");
double mySqrt = 0.0;
if ( strNumber == null )
    strNumber = "";
else {
    double dNumber = Double.parseDouble(strNumber);
    mySqrt = Math.sqrt(dNumber);
}
%>
<form action="first.jsp" method="post">
Enter Number: <input type="text" name="number" value="<%=strNumber%>">
<input type="submit">
<br><br>
The Sqrt is <%out.print(mySqrt);%>
</body>
</html>

JDBC Transaction

2008-05-31T17:49:00.006+03:00

In a Database Server , a transaction is a sequence of operations. Those operations are considered to be a single unit of work which either finish all or nothing , that is a transaction is atomic either lives or never exists.

For example you when your customer make a payment , you insert that payment into the payment table and update customer's balance in the customer table. If your update failse after the insert, you have to delete the payment to retry the operation. To avoid this we use transctions:

begin;
insert into customer_payment values(100,now(),12.0);
update customer set balance = balance+12.0 where customer_id = 100";

commit;

or something got wrong

rollback;

These two sql statements above will take affect into tables when you commit, if you rollback there will be no effect no update and no insert.

By default , every database in JDBC commits automatically. If you want to make use of transactions , you have to turn autocommit off.

Be careful , if you forget commit or rollback you can crash your db server because of full transaction log.

Here is the example java jdbc transaction code.:


        try {
            Class.forName("com.mysql.jdbc.Driver");
        } catch (ClassNotFoundException e2) {
            e2.printStackTrace();
        }

        Connection con = null;
        java.sql.PreparedStatement stmtInsertPayment = null;
        java.sql.PreparedStatement stmtUpdateBalance = null;
        ResultSet rs = null;

        try {
            con = DriverManager.getConnection(
                    "jdbc:mysql://localhost:3306/javacream", "javacream",
                    "creampass");
            con.setAutoCommit(false);
            stmtInsertPayment = con
                    .prepareStatement("insert into customer_payment values(?,?,now())");
            stmtInsertPayment.setInt(1, 100);
            stmtInsertPayment.setDouble(2, 12.0);
            stmtInsertPayment.executeUpdate();

            stmtUpdateBalance = con
                    .prepareStatement("update customer set balance = balance + ? where customer_id = ?");

            stmtUpdateBalance.setDouble(1, 12.0);
            stmtUpdateBalance.setInt(2, 100);
            stmtUpdateBalance.executeUpdate();
            con.commit();
            con.setAutoCommit(true);
        } catch (SQLException e1) {
            e1.printStackTrace();
        } finally {
            if (rs != null)
                try {
                    rs.close();
                } catch (SQLException e) {
                    e.printStackTrace();

                }

            if (stmtInsertPayment != null)
                try {
                    stmtInsertPayment.close();
                } catch (SQLException e) {
                    e.printStackTrace();

                }

            if (stmtUpdateBalance != null)
                try {
                    stmtUpdateBalance.close();
                } catch (SQLException e) {
                    e.printStackTrace();

                }

            if (con != null)
                try {
                    con.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }

        }

What is NullPointerException ?

2008-05-20T15:38:00.006+03:00

NullPointerException is the most common exception of the Java world.

It occurs when you try to call method or modify and attiribute of a null object.

Here is an example :

String strObject = null;
int len = strObject.length();// throws a NullPointerException

The code tried to access a null string's length() method so it is illegal. There is no strObject so there is no length() of it.

To avoid NullPointerException you should check the of object if it is null before accessing it's members.

For example in the jsp code below , we want to get userName from HTTP Post parameters , and print an error message if the length is less than 6 :

String userName = request.getParameter("userName");

if ( userName.length()<6)

out.print("Invalid Username");

return;

}

This code fails when the user in logon page sends an empty string for userName.

The code will be better if we check the object is null before access its method :

String userName = request.getParameter("userName");

if ( userName==null || userName.length()<6)

out.print("Invalid Username");

return;

}

Note that if statements exists after the first "true" reached , since this is an "OR" statement and does not run other part of If statement , otherwise userName.length()would throw exception in the if statement.

Escape HTML and XML in Java

2008-05-17T15:28:00.004+03:00

HTML and XML escape format is widely used in web and XML transactions.

There is a great utility for this purpose in Java world : Lang package in apache commons project

Download commons-lang and put the commons-lang-xxx.jar file in your classpath or in your project.

The StringEscapeUtils class is your key class here.

Use static escapeXml() , escapeHtml() , unescapeXml() , unescapeHtml() methods.

String escaped = StringEscapeUtils.escapeXml("");

Great & Lifesaving , thanks apache again !

Tomcat JDBC Connection Pooling

2008-05-16T01:01:00.008+03:00

Connection pooling is a cool mechanism because it allows to us reuse db connections. In a DB server a database connection is an expensive thing because it consumes system resources. Also connection creation and closing are time consuming operations. Connection Pooling helps us to reduce number of concurrent connections and to escape from connection open/close by keeping connections alive.

Apache Tomcat is a great and ligthweight Java Application server for both development and production. Tomcat has an JDBC Connection Pooling mechanism. This feaute is configured in /META-INF/context.xml. Here is a sample:

<?xml version="1.0" encoding="UTF-8"?>
<Context docBase="/MyApp" path="/MyApp" reloadable="true">
<Resource name="jdbc/mydbresource" auth="Container" type="javax.sql.DataSource" factory="org.apache.tomcat.dbcp.dbcp.BasicDataSourceFactory"
username="myusername" password="mypass" driverClassName="com.mysql.jdbc.Driver"
url="jdbc:mysql://localhost:3306/mydb?autoReconnect=true"
maxWait="1000"
removeAbandoned="true"
maxActive="30"
maxIdle="10"
removeAbandonedTimeout="10"
logAbandoned="true"/>
</Context>
docBase and path are your application.

Resource name is the datasource name to be called in connection setup at your application. (arbitrary but give a meaningful name)

autoReconnect: as the name implies , it causes to reopen the closed connections immediately. Mysql closes idle connections in 8 minutes. with this option connections reopened immediately when closed.

maxWait: timeout for your java code to gett a connection from pool. if timeout reaches , an exception will be thrown.

removeAbandoned: if set true , a connection is removed from the pool after removeAbandonedTimout idle seconds. Not for heavy load production environments. It is only useful for freshman developers who forget to close jdbc connections.

logAbandoned: logs abondoned connections or statements. It consumes system resources , so should be false in mature production environments.

At the java side , environment should be initialized first. This code can be run in static part :

javax.sql.DataSource.DataSource ds = null;
javax.naming.Context initContext;
try {
initContext = new javax.naming.InitialContex();
javax.naming.Context envContext = (javax.naming.Context) initContext.lookup("java:/comp/env");
ds = (javax.sql.DataSource.DataSource) envContext.lookup("jdbc/mydbresource");
} catch (Exception e) {
e.printStackTrace();

}

And when you need a connection , reuse ds and get a connection from ds.

Connection con = null;

try {
con = ds.getConnection();
} catch (SQLException e) {
e.printStackTrace();
}

Java Mysql JDBC Tutorial

2008-05-12T22:54:00.009+03:00

Here is a simple piece of code explaining Java JDBC with Mysql.

Our case is to send email to customers who have negative balance. You cannot send email to a customer more than once a day.

First we select customer list to send email , try to send their balance and update last_email_sent_date if mail sent.

Our mysql server is on localhost and port 3306 (mysql default port)
We have to grant access to required database or objects for creating a jdbc connection to server.

here is the sample Mysql Command:

grant all on customer to 'dbuser'@'localhost' identified by 'dbpass';

which gives full access on all objects on customer database. Be carefull about that !

-----------------------------------------


  try {
   Class.forName("com.mysql.jdbc.Driver");
  } catch (ClassNotFoundException e) {
   //driver not found
   e.printStackTrace();
   System.exit(-1);
  }

  Connection con = null;
  Statement stmt = null;
  ResultSet rs = null;

  try {
   con = DriverManager.getConnection(
     "jdbc:mysql://localhost:3306/customer", "dbuser", "dbpass");
   stmt = con.createStatement();
   //get customers which you did not sent any emails today
   rs = stmt
     .executeQuery("select id , name , email , balance , last_mail_sent_date from customer where last_mail_sent_date < curdate()");
   while (rs.next()) {
    int id = rs.getInt(1);//coloumns indexes starts from 1
    String name = rs.getString(2);
    //you can use column name to fecth the value , but using indexes is faster
    String email = rs.getString("balance");
    double balance = rs.getDouble(4);
    if (balance < 0.0) {
     int sendMailStatus = sendMail(email, "Dear " + name
       + " your balance is " + balance
       + ".Please send some money");
     if (sendMailStatus == 1) //mail is sent
      //update the customer in order to not to send email again today
      con.createStatement().executeUpdate(
        "update customer set last_mail_sent_date = mow() where id = "
          + id);

    }//if balance
   }//while

  } catch (SQLException e) {
   e.printStackTrace();
  } finally { //clean up db connections and objects
   if (rs != null)
    try {
     rs.close();
    } catch (SQLException e) {
     e.printStackTrace();

    }

   if (stmt != null)
    try {
     stmt.close();
    } catch (SQLException e) {
     e.printStackTrace();
    }

   if (con != null)
    try {
     con.close();
    } catch (SQLException e) {
     e.printStackTrace();
    }

  }


//check the proper JDBC Driver

Mysql curdate() function returns todays date i.e. 2008-05-12

now() function returns current date and time : 2008-05-12 23:10:28

Sort a HashMap based on Keys or Values

2008-05-01T11:45:00.006+03:00

Sort based on keys is simple , just dump the map into TreeMap (TreeMap is sorted by nature)

Map myMap = new HashMap();
// put keys and values........
Map sortedMap = new TreeMap(myMap);

If you want to sort your HashMap based on values :

HashMap map1 = new HashMap();
map1.put("cat",5);
map1.put("cow",4);
map1.put("dog",3);
map1.put("horse",2);

List mpKeys = new ArrayList(map1.keySet());
List mpValues = new ArrayList(map1.values());

HashMap map = new LinkedHashMap();
TreeSet sortedSet = new TreeSet(mapValues);
Object[] sortedArray = sortedSet.toArray();
int size = sortedArray.length;
// descending

for (int i=size-1; i>=0; i--)
System.out.println(mpKeys.get(mpValues.indexOf(sortedArray[i]))+" "+sortedArray[i]);

Any discussions will be appreciated.

Read and Write Files in Java

2008-04-27T15:15:00.002+03:00

Here is a sample code I just wrote. This class copies one file to another.
By understanding this code below , you will comphrend both reading from and writing to file concepts in java. I used FileInputStream and FileOutputStream in this code.

package javacream;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;

public class FileUtil {

public static void copyFile(String strInputFileName , String strOutputFileName) throws IOException {

//Prepare file objects

File fInputFile = new File(strInputFileName);

File fOutputFile = new File(strOutputFileName);

//Prepare read write streams from file objects

FileInputStream fis = new FileInputStream(fInputFile);

FileOutputStream fos = new FileOutputStream(fOutputFile);

int c;

// read from the input file and write to the output file

while ((c = fis.read()) != -1) {

fos.write(c);

}

//we are done , close the streams.

fis.close();

fos.close();

}

public static void main(String[] args) {

try {

// use \\ for windows filenames , otherwise use single /

copyFile("d:\\sample.txt", "d:\\copyof_sample.txt");

} catch (IOException e) {

e.printStackTrace();

}

Alternatively , you can use FileReader object and BufferedReader. The sample code below counts the number of lines of a given file.

public static int lineCount(String fileName) throws IOException{

// file reader to read the file

FileReader input = new FileReader(fileName);

// Filter FileReader through a BufferedReader to read line by line

BufferedReader bufRead = new BufferedReader(input);

String line; // String that holds current file line

int count = 0; // Line count

// Read first line

line = bufRead.readLine();

count++;

// Read through file one line at time.

while (line != null){

line = bufRead.readLine();

count++;

}

bufRead.close();

return count;

}

Java HTTP GET POST

2008-04-22T01:25:00.005+03:00

As you know HTTP (Hypertext Transfer Protocol) is the transport protocol of www. Java has a strong support for HTTP.

HTTP addresses uses addressing syntax called URL (for example http://www.java.com). HTTP is based on TCP.

HTTP GET

This is the most widespred type of HTTP request. If you write http://www.java.com/ on your browser you have already made an HTTP GET Request. Here is a sample :

public static String doGET(String strURL) throws IOException {

String response = null;

InputStream in = null;

try {

// setup url connection

URL url = new URL(strURL);

HttpURLConnection urlConn = (HttpURLConnection) url

.openConnection();

in = urlConn.getInputStream();

StringBuilder sb = new StringBuilder();

int i;

while ((i = in.read()) > -1)

sb.append((char) i);

response = sb.toString();

} finally {

if (in != null)

try {

in.close();

} catch (IOException e) {

e.printStackTrace();

}

return response;

}

To run:
String getResult = doGET("http://javacream.blogspot.com/2008/04/java-http-get-post.html");
System.out.println(getResult);

HTTP POST

When you fill a form on your browser and submit it , most probably you did an HTTP Post to remote server. HTML forms uses POST method alot. In POST method , instead of adding parameters at the and of the url , you post in your request body. So , URL and Parameters are seperate. Check out this sample code for HTPP Post :

public static String doPost(String _url, String line)

throws IOException {

String response = null; //for reading from remote host

InputStream in = null; //for posting parameters to remote host

DataOutputStream out = null;

try {

// setup url connection. use POST to send forms data

URL url = new URL(_url);

HttpURLConnection urlConn = (HttpURLConnection) url

.openConnection();

urlConn.setRequestMethod("POST");

urlConn.setDoInput(true);

urlConn.setDoOutput(true);

urlConn.setRequestProperty("Content-Type",

"application/x-www-form-urlencoded");

out = new DataOutputStream(urlConn.getOutputStream());

out.writeBytes(line);

out.flush();

in = urlConn.getInputStream();

StringBuilder sb = new StringBuilder();

int i;

while ((i = in.read()) > -1)

sb.append((char) i);

response = sb.toString();

} finally {

if (in != null)

try {

in.close();

} catch (IOException e) {

e.printStackTrace();

}

if (out != null)

try {

out.close();

} catch (IOException e) {

e.printStackTrace();

}

return response;

}

To run:
String result = doPost("http://www.someserver.com/cgi-bin/form", "userid=Test&Password=Test");

Download a file using HTTP GET

This is a very simple sample code for downloading a file :

public static void downloadFile(String _url, String fileName)

throws IOException {

FileOutputStream outFile = null;

outFile = new FileOutputStream(fileName);

InputStream in = null;

try {

// setup url connection.

URL url = new URL(_url);

HttpURLConnection urlConn = (HttpURLConnection) url

.openConnection();

urlConn.setUseCaches(false);

in = urlConn.getInputStream();

int i = -1;

while ((i = in.read()) > -1)

outFile.write(i);

} finally {

if (in != null)

try {

in.close();

} catch (IOException e) {

e.printStackTrace();

}

if (outFile != null)

try {

outFile.close();

} catch (IOException e) {

e.printStackTrace();

}

Using with proxies

You have to define proxy before you open connection to your URL.

Properties sysProps = System.getProperties();
sysProps.put("http.proxyHost","proxy.local.intranet") ;
sysProps.put("http.proxyPort", "8080") ;

URL url = new URL(_url);

HttpURLConnection urlConn = (HttpURLConnection)url.openConnection();

If the proxy requires Authentication than the following java example code should be added:

sun.misc.BASE64Encoder encoder = new sun.misc.BASE64Encoder();
String basicAuthenticationUserPass =
        encoder.encode("mydomain\\MYUSER:MYPASSWORD".getBytes());
urlConn.setRequestProperty
        ("Proxy-Authorization", "Basic " + basicAuthenticationUserPass );

Arrays in Java

2008-04-17T16:34:00.003+03:00

Arrays in java are objects that hold fixed numbers of values of single type. Length of the array must be given at initalization , arrays cannot grow or shrink. Each element has an index value starting from 0 , so if you want to reach element 3 , index is 2..

Simple array initializations :

// array of integer with the capacity of 2 integers
int[] intArray = new int[2];

or ,

// [] goes to intArray which makes intArray[]
int intArray[] = new int[2];

Element indexes start from 0(zero) , so the last element index of array above is 1,

intArray[0] = 1;
intArray[1] = 2;

there is no intArray[2] because there is no third element. Capacity is 2.

System.out.println("the length of intArray="+intArray.length());

gives : the length of intArray=2

You can initialize our sample intArray easily like that :

int intArray[] = new int[]{1,2};

The length of the array will be the same with number of elements you specified.

You can also use Array classes' fill() method like this

Array.fill(intArray,10);

which fills the array with the specified value.

Arrays can hold not only primitive types as stated above, but also objects.

Arrays can also be multidimensional :

int[][] twoDimArray = { {1,2,3}, {4,5,6}, {7,8,9} };

or with String objects

String[][] friends = {{"Mr. ", "Mrs. ", "Ms. "},
{"Brown", "Parker"}}; //3x2 Array of Strings
System.out.println(friends[0][0] + friends[1][0]); //Mr. Brown
System.out.println(friends[0][2] + friends[1][1]); //Ms. Parker

Note that if you don't put elements in an array of objects all elements are null.

String[] strArray = new String[2];
strArray[0] = "first string";
if ( strArray[1] == null )
System.out.println("Second string is null");

gives the output Second string is null when launched

To copy an array into another use System.arraycopy() method.

String[] strArray1 = new String[]{"banana","apple"};
String[] strArray2 = new String[2];

//copy array strtArray1 into strArray2
//starting from element 0 in strArray1 and start copy
//to strArray2[0] with 2 elements
System.arraycopy(strArray1, 0 , strArray2, 0, 2);
System.out.println(Arrays.toString(strArray2));

gives the output
[banana, apple]

You can also sort the array above using Arrays.sort() method

Arrays.sort(strArray1);
System.out.println(Arrays.toString(strArray1));

gives the output
[apple, banana]

Note that elements of the array should be either primitive or implement Comperable interface to use Arrays.sort()

You can assign one element to another. But note that primitive types and immutable objects are assigned to value , while mutable objects are assigned to reference (just in the case of method call). Confused ? Look at the following sample code. int and String gets the only value of other element but mutable java.util.Date objects gets the reference of the other element and completely becomes the other element. (be to spoon and bend yourself) :

int[] intArray = new int[]{1,2}; //array of primitive
intArray[0] = intArray[1];
intArray[1] = 1000;
System.out.println("intArray[0]="+intArray[0]); //prints 2 not 1000

String[] strArray = new String[]{"string1","string2"};//array of immutable objects
strArray[0] = strArray[1];
strArray[1] = "Mr. Hide";
//print string2 no mrhide!
System.out.println("strArray[0]="+strArray[0]);

java.util.Date[] dateArray = new java.util.Date[2];//array of mutable objects
long tenMinutesAgo = System.currentTimeMillis() - 10 * 60 * 1000 ;
dateArray[0] = new Date();
dateArray[1] = new Date(tenMinutesAgo);
//prints the current date
System.out.println("dateArray[0]"+dateArray[0].toString());
dateArray[0] = dateArray[1];
//prints 10 minutes ago
System.out.println("dateArray[0]"+dateArray[0].toString());
long twelfMinutesAgo = tenMinutesAgo - 10 * 60 * 1000 ;
dateArray[1].setTime(twelfMinutesAgo);
//prints 20 minutes ago date1 equals date2
System.out.println("dateArray[0]"+dateArray[0].toString());