Viewing the maximum number of open files allowed

To check the maximum number of files allowed for a user on Linux run the following command:
$ ulimit -n

To view the maximum number of files allowed on the system as a whole:
$ cat /proc/sys/fs/file-max

Removing a file/directory that starts with a "-" in Linux/Unix

Removing a file/directory that starts with the character "-" can be tricky in Linux/Unix. On Linux/Unix you may get this error when attempting to remove a file/directory:
rm: invalid option -- '1'

Some commands may give you a hint e.g.
Try `rm ./-1' to remove the file `-1'.

This is exactly what is required i.e just append the "./" to the file/folder to resolve your problem.

Slow startup of Weblogic server on Linux

Had a strange problem on our Linux server recently after installing Weblogic 10. The server was very slow to startup. Seems the problem is this bug in Java 5: Bug 6202721

The fix is to edit the java.security file in the jre/lib/security folder of your java installation on the server. Alter the securerandom.source field to be file:/dev/./urandom as suggested in this article: How to improve Weblogic Servers Startup Time

NullPointerException with Hibernate 4

java.lang.NullPointerException at org.hibernate.engine.jdbc.internal.JdbcServicesImpl.configure(JdbcServicesImpl.java:207) at org.hibernate.service.internal.StandardServiceRegistryImpl.configureService(StandardServiceRegistryImpl.java:75) at org.hibernate.service.internal.AbstractServiceRegistryImpl.initializeService(AbstractServiceRegistryImpl.java:159)

The exception above can be resolved by adding this Hibernate property:
<property name="hibernate.temp.use_jdbc_metadata_defaults">false</property>

"No runnable methods" when using Maven for a class that shouldn't be Tested

I recently ran into a problem doing a Maven build where I had a utility class to be used for my Test cases. Adding the @Ignore tag didn't seem to help. It turns out the solution was simply to add this plugin entry to my pom.xml file as my file had the suffix Util.java:

<plugin>
   <groupId>org.apache.maven.plugins</groupId>
   <artifactId>maven-surefire-plugin</artifactId>
   <configuration>
      <excludes>
         <exclude>**/*Util.java</exclude>
      </excludes>
   </configuration>
</plugin>


ClassNotFoundException: org.hibernate.hql.ast.HqlToken

When using Hibernate to read from a Database using Weblogic 10 and Springframework 2.5.6, I encountered the following error:

ClassNotFoundException: org.hibernate.hql.ast.HqlToken

The reason for this is that it by default will use Weblogic's default classloader to load the antlr jar file and doesn't use the version included within your .war file.

To solve this you need to wrap your .war in an ear file and within the META-INF/weblogic-application.xml file within you .ear file include the following:
 <prefer-application-packages>
    <package-name>antlr.*</package>
 </prefer-application-packages>
 

This should resolve this issue.

SQL String functions

Some STRING functions in SQL:

select RIGHT(column, chars) from table;
select SUBSTRING_INDEX(column, char, which char (i.e, 1,2...)) from table;
select REVERSE(string);
select LTRIM(string);
select RTRIM(string);
select LENGTH(string);

Java Pitfalls Book

This book "Java™ Puzzlers: Traps, Pitfalls, and Corner Cases" gives insight into some Java "fails" that one needs to watch out for when developing.

It explains what is wrong with this piece of code (It doesn't print out AB as one would expect):

public class PrintingSomeLetters {
  public static void main(String[] args) {
    System.out.println('A' + 'B');
  }
}

Getting a list of installed services on a Linux server

$ chkconfig --list

Design Patterns Book

Java 5 autoboxing fail on equality check

  Double d1 = 25d;
  Double d2 = 25d;

  if (d1 == d2) {
            System.out.println("equal (==)");
  } else if (d1.equals(d2)) {
            System.out.println("equal (using equals() method)");
  }

The output is:
equal (using equals() method)

Reverse search for a previous command on Linux

Use CTRL-r to reverse search through a history of your previous commands executed in Linux e.g.

$(reverse-i-search)`':

Type in the text for a command you are searching for e.g. 'cat'

$(reverse-i-search)`cat': cat test.txt

In this case the last command executed with the substring 'cat' is 'cat test.txt'.

To exit the reverse search just use CTRL-g.

Using SED to Append or Prepend characters

You can use sed to Append/Prepend characters to a String e.g.

We have a file (test.txt) below and we would like to Prepend each capital letter with a space:

$ cat test.txt

Hello.ThisIsATestFile.

$ sed "s/[A-Z]/ &/g" test.txt

Hello. This Is A Test File.

sed makes use of a special character "&" which implies the pattern found.

Similarly we could Append a sentence after the "Hello." string:

$ sed "s/Hello./&HowAreYou?/g" test.txt 

Hello.HowAreYou?ThisIsATestFile.

Configuring maven to autogenerate Serializable JAXB classes

 First you need to create a bindings file:

<jxb:bindings version="1.0"
xmlns:jxb="http://java.sun.com/xml/ns/jaxb"
xmlns:xs="http://www.w3.org/2001/XMLSchema">

<jxb:globalBindings>
  <jxb:serializable uid="1"/>
</jxb:globalBindings>

</jxb:bindings>
Save this file under "src/main/resources/" of your maven module. You can name it whatever you want e.g. jaxb-bindings.xjb.

<plugin>
  <groupId>org.jvnet.jaxb2.maven2</groupId>
  <artifactId>maven-jaxb2-plugin</artifactId>
  <executions>
    <execution>
      <id>Request</id>
      <goals>
      <goal>generate</goal>
      </goals>
      <configuration>
      <schemaDirectory>src/main/resources/xsd</schemaDirectory>
      <schemaIncludes>
      <include>*/*.xsd</include>
      </schemaIncludes>
      <generatePackage>mygeneratedfiles</generatePackage>
      <generateDirectory>src/main/java</generateDirectory>
      <bindingDirectory>src/main/resources</bindingDirectory>
      <bindingIncludes>
      <bindingInclude>jaxb-bindings.xjb</bindingInclude>
      </bindingIncludes>
      </configuration>
    </execution>
  </executions>
</plugin>

Getting the first day of the month in Oracle

select TRUNC(sysdate, 'MM') from dual;   

Getting the last day of the month in Oracle

select trunc(last_day(sysdate)) from dual;

Using a Singleton

Below is an example of a Singleton used to decrement a counter variable:

package singleton;
public class Counter {
   private static Counter instance;
   private int count;


   private Counter() {
   }


   public static synchronized Counter getInstance() {
      if (instance == null) {
          instance = new Counter();
      }
      return instance;
   }


   public synchronized void decrement() {
       count--;
       if( count<0 ) {
          count=0;
       }
   }


   public synchronized int getCount() {
       return count;
   }


   public synchronized void setCount(int countValue) {
       count = countValue;
   }
}

Test class for the singleton above:

package singleton;

import static org.junit.Assert.*;
import org.junit.Before;
import org.junit.Test;

public class CounterTest {
    private Counter counter1 = null, counter2 = null;

    @Before
    public void setUp() throws Exception {
       counter1 = Counter.getInstance();
       counter2 = Counter.getInstance();
    }

    /**
     * Test case to ensure that the getInstance Method
     * returns a non null object and to ensure
     * that getting 2 instances of Counter results in
     * the same object being returned.
     */
    @Test
    public void testGetInstance() {
       counter1 = Counter.getInstance();
       assertNotNull(counter1);
       counter2 = Counter.getInstance();
       assertNotNull(counter2);
       assert (counter1 == counter2);
    }

    /**
     * Test case that tests decrementing the count variable and
     * also ensures that both counter1 and counter2
     * returns the same count value.
     */
    @Test
    public void testDecrement() {
       counter1.setCount(20); 
       counter1.decrement();
       assert (counter1.getCount() == 19);
       assert (counter2.getCount() == 19);
    }

    /**
     * Tests that both instances of the Count returns
     * the same count value.
     */
    @Test
    public void testGetCount() {
       int count1 = counter1.getCount();
       int count2 = counter2.getCount();
       assert (count1 == count2);
       counter2.setCount(100);
       count1 = counter1.getCount();
       assert (count1 == 100);
       count2 = counter2.getCount();
       assert (count1 == count2);
    }

   /**
    * Tests that we are allowed to set the count variable
    * and that another instance reading that variable will
    * result in the correct value being returned. */
    @Test
    public void testSetCount() {
       counter2.setCount(100);
       int count1 = counter1.getCount();
       assert (count1 == 100);
    }
}

java.rmi.MarshalException: Failed to serialize Error

This error occurs when trying to invoke a Remote object that is not Serializable. You can fix this by making the object in question (if possible) Serializable by implementing java.io.Serializable.

Converting from java.util.Date to XMLGregorianCalendar

If you need to convert from java.util.date to an XMLGregorianCalendar format you can use the following code:

import javax.xml.datatype.DatatypeFactory;
import javax.xml.datatype.XMLGregorianCalendar;
import javax.xml.datatype.DatatypeConfigurationException;
import java.util.GregorianCalendar;

public class DateConversion {
   public XMLGregorianCalendar dateToXMLGregorianCalendar(
   final java.util.Date date)
   throws DatatypeConfigurationException {
      DatatypeFactory df = DatatypeFactory.newInstance();
      if (date == null) {
         return null;
      } else {
         GregorianCalendar gc = new GregorianCalendar();
         gc.setTimeInMillis(date.getTime());
         return df.newXMLGregorianCalendar(gc);
      }
   }
}

Touch all files within the current directory

If you need to update all files within the current directory with the latest timestamp you can use the following command:
find . -type f -exec touch {} \;