Wednesday, January 11, 2006, 04:12 PM - Java
Symptom
While using java 1.5, the following is allowed:
Integer number1=1;
Integer number2=1;
if(number1==number2) System.out.println("number1 is number2");
else System.out.println("number1 is not number2");
This fails.
Solution
It is not allowed in the java standard to compare two objects. You are actually comparing the object identifiers instead of the object values. Use the intValue or the compareTo methods to do the correct comparisson.
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Wednesday, January 4, 2006, 10:23 AM - Linux
Sometimes there are multiple jobs to be killed. This bash script is handy for that purpose:
TOBEKILLED=your unique identifying name of the program
for PROCESS in `ps aux | grep $TOBEKILLED | grep -v grep | awk '{print $2}'`
do
kill -9 $PROCESS
done
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Saturday, December 31, 2005, 09:37 PM - Java
Symptom
There are several exceptions in the style of "key not found exception" while there are references to the message resource file in the struts-config.xml or web.xml from tomcat.
Solution
The advice is to use the struts-config file only setup and not to include the message resource as an init parameter in the web.xml file.
To include the message resource files put the following in the struts-config.xml:
<message-resources
null="false"
parameter="your.preferred.package.ApplicationResources" />
The file in this should be named: ApplicationResources.properties, or with the language extention to it: ApplicationResources_es.properties.
The resource file has to be included in your build by adjusting ant or any other tool you are using.
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Friday, December 30, 2005, 03:08 PM - General applicable solutions
The memory expansion of an asus notebook of the a2 series (a2500H) is not in the compartment on the bottom of the laptop which they mention in the manual. It is actually under the keyboard of the system. The compartment on the bottom contains a set of copper heatpipes and a passive CPU cooler. It is protected by a void warranty sticker on one of the screws.
So to avoid any problems with warranty:
To get to the correct compartment, take of the bezel above the keyboard by pushing it to the left, it it will not go easy! Break a nail or two, but it is possible. UPDATE: There is a small hole in the bezel, in which a pin can be pushed. By pushing in a pin, you unlock the bezel, and the removal will go easier!
Now you can lift the keyboard by shifting it up a few milimeter and than flip it over. In the middle is a compartment protected by a cover with two screws. Open the compartment, insert the memory and place the keyboard and bezel back at its location.
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Thursday, December 29, 2005, 04:30 PM - Java
Symptom
In the catalina.out logfile from tomcat there is an exception:
ERROR org.apache.commons.digester.Digester[main] - End event threw exception
java.lang.reflect.InvocationTargetException
etc....
For the rest everything works fine.
Solution
Copy the log4j jar file to the /WEB-INF/lib location and reboot tomcat. The error is gone.
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Tuesday, December 27, 2005, 08:39 PM - Java
Struts with nested iterators
There are lots of examples on the internet concerning struts with the nested tag library. The following examples concenr less used ways of displaying nested data by using 100% struts compatible methods.
The data
The data to be displayed consists out of sets of data from which the main data is a single list of names, and the second set is a list of telephone numbers per name:
Jan: 123456789 122345678 122233344
John: 155522234 123354556 132311244
This data is to be displayed in such a way that it is readable to a user, in html. In this case: The name, with underneath the telephonenumbers:
jan
123456789
122345678
122233344
To be able to do this, the data will be organized in its own data storage class per name, after which this is data storage class is stored in a Collection.
The data storage class
Bean code
JSP code
Extra: multiple lists to be accessed
Suppose the data with the telephone numbers also has an address associated to it:
jan
123456789 home
122345678 office
122233344 mobile
And you want to display this data.
To do this, the data is all stored in the storage class, and the jsp is altered in the following way:
The changes to the storage class:
Conclusion
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Tuesday, December 27, 2005, 12:31 PM - Java
Symptom
In jsp with struts you are trying to iterate over a Collection, but the servlet does not recognize it as such.
Solution
Use a collection instead of the type you are using now. logic:iterate works over:
- Hashtable or other maps without accessing the nested elements.
- Collections: AbstractCollection, AbstractList, AbstractSet, ArrayList, HashSet, LinkedHashSet, LinkedList, TreeSet, Vector with access to the nested elements, so a Vector with Vectors stored within etc.
Sample code:
JAVA:
public ActionForward execute(..)
{
Vector test=new Vector();
Vector test2=new Vector();
test.add("test1");
test.add("test2");
test.add("test3");
test.add("test4");
test2.add(test);
test2.add(test);
request.setAttribute("test",test2);
return mapping.findForward("success");
}
JSP:
<logic:present name="test".
<logic:iterate id="myTest" name="test">
Element value: <bean:write name="myTest" /><br/>
<nested:root name="myTest">
<nested:iterate id="myTestInner" name="myTest">
Inner element value: <bean:write name="myTestInner" /><br/>
</nested:iterate>
</nested:root>
</logic:iterate>
</logic:present>
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Saturday, December 24, 2005, 10:52 PM - SuSE
Symptom
SuSE 10 (or openSuSE 10) recognized the prism54 chipset card, or other prism based card without problems, but still will not work.
ifconfig ethX up gives a response in /var/log/messages:
prism54: request_firmware() failed
eth1: could not upload firmware
Solution
openSuSE does not provide the firmware from the CD set. Run the online update and add the firmware in the software selection overview. Now the firmware is present and the card can finish its initialisation.
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Thursday, December 22, 2005, 02:19 PM - SuSE
Symptom
The quote and the double quote do not show until you press <SPACE>
Solution
The keyboard settings of you SuSE system are in international. The pressing of the quote or double qoute followed by a character like the o will give you a certain internationally used character. Most users find this behaviour annoying. Solve it by using YaST2, hardware, keyboard settings and set the keyboard from intl to basic. Now the keyboard behaves normal again
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Thursday, December 22, 2005, 11:43 AM - SuSE
Symptom
Under openSuSE 10.0 the numeric keypad fails completely and the mouse fails on some widgets under KDE. The logfiles showed a missing modules.dep file and some other severe errors.
The system used is a 1.6Ghz Pentium 4 Dell dimension 4300.
Solution
Since this only has been observed after a system has been booted after install and after some sig 11 errors, the failure is most likely hardware-software interaction related, and is probably caused by the incorrect loading of some modules. The way out is to reboot the system. After that the errors are gone, and are not reproducable.
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Monday, December 19, 2005, 04:00 PM - Java
Symptom
You pressed a key combination concerning <SHIFT>,<CTRL> and <-, and now the eclipse v3.2 editor acts weird. No inserts of characters possible, however deletes of characters are possible. Also using the arrows controls the scrollbar for the up/down arrow and jumps words for the left/right arrow.
What is wrong
You hit a key combination which put you in a different editing mode, or you are straight in a bug. I have not been able to reproduce it yet.
Solution
The correct solution is to reset the editing mode with the menu:
windows->Navigation->Activate Editor <F12>
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Thursday, December 8, 2005, 03:46 PM - Java, MySQL
Symptom
While using JDBC, you get the following exception:
java.sql.SQLException: Can not issue data manipulation statements with executeQuery()
The query you wrote has INSERT, UPDATE or DELETE in it.
Solution
The JDBC standard does not allow this. So alter your way of executing the PreparedStatement.
Execute this with execute() or executeUpdate() instead of the very general executeQuery.
ExecuteQuery is only for real queries returning only values, not doing any data manipulation at all.
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Wednesday, December 7, 2005, 11:08 PM - General applicable solutions, Linux
To create a filesystem in a file, first a file needs to be create at the size of the filesystem. This can be done with dd:
dd if=/dev/zero of=fileXYZ bs=1024 count=1024
The count can be made bigger to get a bigger file.
Use your favourite filesystem creator on this file, and you will have the filesystem in a file available for all to use.
Mounting the filesystem is done by:
mount fileXYZ /mnt -o loop
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Tuesday, December 6, 2005, 09:25 PM - Linux
From the help of the kernel config:
mount none /tmp/fromhost -t hostfs -o /tmp/umlshare
Do this from inside your running UML system.
Also see the related link for more information
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Tuesday, December 6, 2005, 05:18 PM - MySQL
Symptom
A connection to the mysql server with a certain user/password combination fails with the error:
Host is not allowed to connect to this MySQL serverConnection
Solution
Alter the host column in the mysql database user table to the host on which it fails or to %. Restart mysql or reload the grant tables and the login should be possible.
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Tuesday, December 6, 2005, 05:15 PM - Java
Symptom
In the logfiles of your tomcat you find the following error:
Parse error in application web.xml
java.lang.ClassNotFoundException: org.apache.catalina.Container
Solution
Apparently one of the jars you copied or linked to the tomcat/common/lib has caused a strange conflict. Remove the last added jar(s) and start tomcat again. Now tomcat starts again, except that your application still does not run.
The class/jar you copied should have been in tomcat/shared/lib. Copy or link the jar to that location, and the error will disappear, while the added jar should work.
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Tuesday, December 6, 2005, 03:55 PM - Java, MySQL
Symptom
Trying to start tomcat, but it will not load because of the following exception:
Exception opening database connection
java.sql.SQLException: com.mysql.jdbc.Driver
etc.
Solution
Download the mysql jdbc connector from www.mysql.com and put it in the tomcat/common/lib directory. The driver will now load.
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Tuesday, December 6, 2005, 03:14 PM - Java
Symptom
java.lang.NoClassDefFoundError: some class exception in the logfile of Tomcat.
Solution
Add the jar class to the tomcat/common/lib directory.
Restart Tomcat and the exception will be gone.
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Monday, December 5, 2005, 11:41 PM - Linux
Note: This just works, it is not a complete configure so there is some work left for the rest of the setup.
Download a kernel newer than 2.6.8 from ftp.kernel.org. Newer than 2.6.8 because those versions do not need to be patched to get UML working (saves a few steps)
Unpack the kernel in your favourite location, and type:
make menuconfig ARCH=um
This starts the menuconfig in uml mode, giving you different options to configure just this kernel.
Configure what you want, but make sure you enable the following:
[b}Block devices->Virtual block device->Always do synchronous IO for UBD
This option is not obvious, but is needed to be able to access your root device.
Type:
make linux modules ARCH=um
This will create a executable file called linux in linux source directory. Copy this to a usefull location (/usr/bin for example)
While the kernel is being build, do the following:
Download a root filesystem from sourceforge.net (files starting with root_fs) or create on yourself by using dd:
dd if=/dev/hdX# of=root_fs_filename
So essentially you could copy your own install into an image.
Alter the downloaded or created image by mounting it and adding the disk locations needed for ubd:
mount -o loop -t auto root_fs /mnt
cd /mnt/dev
mkdir ubd
cd ubd
for i in 0 1 2 3 4 5 6 7; do mknod $i b 98 $[ $i * 16 ]; done
cd ..
chmod 660 ubd
chown -R root:disk ubd
cd /
umount /mnt
Start the user mode linux kernel with:
linux ubd=/yourlocation/root_fs_filename
The kernel should boot giving a running linux system on your console.
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Monday, December 5, 2005, 04:02 PM - MySQL
Symptom
A MySQL CREATE TABLE returns with this error statement:
ERROR 1005 (HY000): Can't create table './dbname/tablename.frm' (errno: 150)
Solution
One of the tables being referenced in the FOREIGN KEY syntax does not exist. Correct this and the error will be gone.
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