How to calculate MD5 as zero-padded hex string in Groovy?

import java.security.MessageDigest
 
def digest = MessageDigest.getInstance("MD5")
 
def bytes = data.getBytes()
def md5 = digest.digest(bytes)
 
// Convert to hex, left-padded with 0 to 32 chars
def hex = new BigInteger(1, md5).toString(16).padLeft(32, "0")

Gist: https://gist.github.com/sjtai/5733901

How to invoke JavaScript in JMeter?

A colleague complained that there is little documentation about how to invoke JavaScript in JMeter. Specifically, his problem was about how to use the session ID to compute a value to be passed to the server. After playing with JMeter for an hour, we managed to figure it out. This is how it works:

Since the session ID (JSESSIONID) is set as a cookie, we only need to figure how to retrieve the cookie in JMeter and pass it to the JavaScript. The first step is to enable cookies in JMeter so that they are accessible by using variables. In jmeter.properties, set CookieManager.save.cookie=true.

Next, make sure you add a HTTP Cookie Manager to your Thread Group. Add a BSF PostProcessor and write your JavaScript in the text area provided. You can also externalize your script to a file and specify the file name in the text field above the text area. You can access the JSESSIONID cookie this way:

var sid = "${COOKIE_JSESSIONID}";

To invoke a JavaScript function and set the return value as a variable:

var val = compute(sid);
vars.put("someKey", val);

You can access the value later as ${someKey}.

How to extract a range of lines from a large file?

It is not advisable to use vi to view a large file as on most system vi will create a swap file of similar size. It is better to extract the lines of interest to another file. Say you want to extract lines 20001 to 30000 from a log file. There are many ways to accomplish that and I am going to show you 2 of them here.

sed (my preferred way)
sed -n -e '20001,30000p' log.txt > /tmp/log-small.txt
-n means don't print anything unless the p command is used. -e means execute the following expression (those inside '....'). 20001,30000p means for lines 20001 to 30000 inclusive, execute the command p, which prints them to the console.

the head & tail way

head -30000 log.txt | tail -10000 > /tmp/log-small.txt

The head command will output lines 1 to 30000 and pipe to tail which will print the last 10000 lines, i.e. lines 20001 to 30000.

As you can see, the head & tail way requires some mental arithmetics and so it is not my preferred way.

How to find out Solaris version number?

Depending on what information you want, the following commands will provide different information about a Solaris server:

• cat /etc/release
• showrev
• uname -a

How to Get IP Address from MAC

One way to get the IP address of a remote host in the same subnet given a MAC address is by using the "arp -an" command. However, this will only work if the we have previously connected to the remote host.

With the help of nmap, we can find out the IP address. For example, the command below will scan the 10.1.1.0/24 subnet with nmap. After the scan we search through the output of arp. If the remote host that we are searching for is up, its MAC address will be displayed together with its IP address.

$ nmap -sP 192.168.0.0/24 > /dev/null
$ arp -an | grep -i "12:34:56:78:90:ab"
? (192.168.0.105) at 12:34:56:78:90:ab [ether] on wlan0

The amazing date command

I use the Linux date command regularly. Normally I use it to format current date, e.g.

date +"%Y-%m-%d"

Yesterday, while writing a backup script, I found out that the command also understands simple English. E.g.

date +"%Y-%m-%d %H:%M:%S" --date "yesterday"
date +"%Y-%m-%d %H:%M:%S" --date "-7days"
date +"%Y-%m-%d %H:%M:%S" --date "-7days 00:00:00"
date +"%Y-%m-%d %H:%M:%S" --date "2 weeks ago"

This helps when I want a date that represents, say, 3pm yesterday.

log4j Configuration Tips

Here are some of the tips/mistakes/solutions that I stumbled upon while configuring log4j. I am making a note for myself in case I need them in the future.

Variables. If you use PropertyConfigurator to configure logging, you can use the ${...} notation in the configuration file. E.g. in log4j.properties:

log4j.appender.A1.File=${user.home}/log/app.log

The catch is you can only use System variables defined in java.lang.System.

Layout pattern.

• To indicate EOL, use %n, not \n.
• %-5p is better than %6p as the log level will be left-aligned and you save 1 character per message.
• %5t doesn't do what you want it to do, just use %t.

NDC. Although the use of NDC is discouraged, I find that, when used properly, it helps in troubleshooting. E.g. in the MDB's onMessage(), you can push some unique identifier to the NDC by using NDC.push(s). For example, transaction number, account number, customer number, etc. The value will appear if you also add %x in the pattern. Remember to call NDC.remove() before your return from the MDB. Pattern:

NDC.push(id);
try {
    ...
}
finally {
    NDC.pop();
    if (NDC.getDepth() == 0) {
        NDC.remove();
    }
}

Logger syntax.

log4j.logger.some.package=INFO,appender


Additivity syntax.

log4j.additivity.some.package=false

Note that some web sites write log4j.some.package.additivity=false but PropertyConfigurator is looking for the prefix log4j.additivity.

log4j.properties & log4j.xml in the same folder. I noticed that when both files are found in the same folder, log4j.properties seems to have priority than the XML file unless your code specifically looks for log4j.xml.

Class name. Don't use %c because full class names are usually very long. Don't use %c{1} because you can't tell which package the class is in. I suggest you try %c{2}, which prints the last 2 components of the full class names, e.g. util.Date.

How to check available memory in Solaris?

There are many ways to check available memory in Solaris. One of the commands is:

prtconf | grep Memory

The Linux cut command

cut is one of the lesser-known commands in the same category of tools such as grep, awk and sed. Nevertheless, I find it more convenient than the other tools in some circumstances. Let's say I have the following lines with ":" as the field delimiter:

abc:12:DEF
ghi:3a4:JKLM
opqr:56b:STUV

To print out the values in the second column, I can use the command

cut -d: -f2

where -d: specifies that the field delimiter is ":", -f2 specifes that the field to print is 2.

If the delimiter is the space character, the command will become

cut -d\  -f2

Notice there are 2 space characters after the backslash. The backslash will escape the space character that follows it.

0.0 Is A BigDecimal In Groovy!

When you are so used to the convenience offered by Groovy and Grails, sometimes you are caught by simplicity itself.

I created a domain class recently:

class Foo {
    ...
    double price

    static constraints = {
        price(min: 0.0)
    }
}

Boom! The application failed to run with the error message:

Parameter for constraint [min] of property [value] of class [class Foo] 
must be the same type as property: [double]

The cause of the problem is that I declared price to be a double, but 0.0 is a BigDecimal in Groovy. To fix the problem, change the value to 0.0d. I found the answer from this thread.

When Class.forName() Can't Find The Domain Class

I was trying to use reflection to create a domain class:

try {
    def clazz = Class.forName(domainClassName)
    def obj = clazz.newInstance()
}
catch (ClassNotFoundException ex) {
    ...
}

To my surprise, I caught ClassNotFoundException at line 2. After searching for a while, I found an explanation in this thread. Burt Beckwith explained that classes loaded without a ClassLoader will not resolve dynamically added classes such as the domain classes. He suggested to use the other variant:

try {
    def clazz = Class.forName(domainClassName, true, Thread.currentThread().contextClassLoader)
    obj = clazz.newInstance()
}
catch (ClassNotFoundException ex) {
    ...
}

Thanks for the tips, Burt.

Self-printing Groovy Program

It always amazes me when I read about how people write a C program that print itself. The self-printing program, or more formally, quine, prints out its source code.

Having used Groovy for a while, I wonder if there is any self-printing Groovy program on the Internet. After some googling around, I found some Java programs on this page. After spending about 15 minutes, I came up with what I think is the first self-printing Groovy program to date. I used Bertram Felgenhauer's 2nd example as my starting point. Here is my version in Groovy (the code is written in one line):

class S{public static void main(a){def s="class S{public static void main(a){def s=;char c=34;println(s.substring(0,41)+c+s+c+s.substring(41));}}";char c=34;println(s.substring(0,41)+c+s+c+s.substring(41));}}

See if you can make it shorter.

Updates

Chrigel suggested the removal of "public". I have to adjust the parameter to substring() too. Here is a shorter version (195 bytes):

class S{static void main(a){def s="class S{static void main(a){def s=;char c=34;println(s.substring(0,34)+c+s+c+s.substring(34));}}";char c=34;println(s.substring(0,34)+c+s+c+s.substring(34));}}

paulk's version is 97 bytes:

def s="def s=;char c=34;println s[0..5]+c+s+c+s[6..-1]";char c=34;println s[0..5]+c+s+c+s[6..-1]

paulk's 2nd version is 46 bytes:

s="s=%c%s%c;printf s,34,s,34";printf s,34,s,34

paulk's 3rd version is 42 bytes. Keep going, paulk!

s='s=%c%s%1$c;printf s,39,s';printf s,39,s

Gavin Grover found a 38-char solution at http://golf.shinh.org/p.rb?Quine:

printf _='printf _=%c%s%1$c,39,_',39,_

Are we approaching the limit?

Grails Quick Reference

I have just finished version 1.1.0 of Grails Quick Reference. For your download convenience, here's the zip file containing the latest source of the quick reference: http://bitbucket.org/sjtai/grailsqr/get/tip.zip. The PDF can be downloaded from here.


Grails Quick Reference by Tai Siew Joon is licensed under a Creative Commons Attribution 2.5 Malaysia License.

The Y292M problem

In about 292 million years from now, we will face the Year 292 Million problem in all our Java/Groovy programs. Luckily it falls on a Sunday, so I think the impact on business will be minimum.

I use new Date(Long.MAX_VALUE) to calculate that date.

Groovy's groupBy and inject methods

groupBy

Aman Aggarwal blogged about a powerful method from the java.util.Collection class. The groupBy method converts a collection into a Map with the keys being returned by the closure passed to the method.

Example:

def map = invoices.groupBy { it.invoiceDate.format("MM/yyyy") }
map.each { k, v ->
  println "There are ${v.size()} invoices in ${k}"
}

Incidentally, the example above uses another cool function from java.util.Date: format. The method takes a date pattern (from the specification of java.text.SimpleDateFormat) and returns a String.

inject

While writing about groupBy, I thought I might as well talk about the inject() method too. It is a method that is best explained with an example. If I want to add all the numbers from 1 until 10, i.e. 1 + 2 + 3 + 4 + ... + 10, I can do it in one line:

def sum = (1..10).inject(0) { a, b -> a += b }

0 is passed as the initial value. In each iteration, the value from the last iteration is passed as a while the current value is passed as b. If we add a println statement, e.g.

def sum = (1..10).inject(0) { a, b ->
  println "${a} + ${b} = ${a + b}"
  a += b
}

The output will be:

0 + 1 = 1
1 + 2 = 3
3 + 3 = 6
6 + 4 = 10
10 + 5 = 15
15 + 6 = 21
21 + 7 = 28
28 + 8 = 36
36 + 9 = 45
45 + 10 = 55

Another typical example is to calculate the product of a range of numbers. For example, to calculate the product of 1 to 10 (the factorial of 10):

def product = (1..10).inject(1) { a, b -> a *= b }

The result is the expected 3628800 (= 10!).

New find and findAll methods for String in Groovy 1.6.1

In Ted Naleid's blog, he describes the patches to the String class that he contributed to the Groovy community recently. The patches have been released as Groovy 1.6.1 on April 7, 2009.

He listed the following new methods:

• eachMatch(Pattern pattern) { fullMatch, group1, ... -> ... }


• find(String regex)


• find(String regex) { fullMatch, group1, ... -> ... }


• find(Pattern pattern)


• find(Pattern pattern) { fullMatch, group1, ... -> ... }


• findAll(String regex)


• findAll(String regex) { fullMatch, group1, ... -> ... }


• findAll(Pattern pattern)


• findAll(Pattern pattern) { fullMatch, group1, ... -> ... }

In other words, the find/findAll methods now accepts a regular expression and the matching groups are passed to the closure.

Thanks for the patches, Ted.

Worst Usability Example

To create a Windows service from the command line, one would normally use the sc command. I wasted almost half an hour today while settings Subversion's svnserve service. So I thought I had better document my ordeal.

To start with, sc has the worst command line syntax that I have seen in my entire life as a software developer. The syntax to create a service for Subversion, type:

sc create svn binPath= "c:\subversion\bin\svnserve.exe -r c:\repo" start= auto DisplayName= "Subversion Server" depend= Tcpip

Notice there is whitespace after the "=" character? No, it's not "start=auto". If you type that, sc will print out the command line syntax and the service is not created. Add in the space, you will have svnserve running as a Windows service.

Can someone explain why the syntax is so weird?

A simple example of the power of polymorphism

I often see codes written by the less experienced programmers that contain a lot of if/else blocks where they do something differently with different types of objects. For example,

public class Coordinator {

  public void doSomethingWithList(List list) {
    for (Object o : list) {
      if (o instanceof A) {
        A a = (A) o;
        doSomethingWithA(a);
      }
      else if (o instanceof B) {
        B b = (B) o;
        doSomethingWithB(b);
      }
      // and so forth for class C, D, E, F, ...
    }
  }
}

In the above codes, the current method is making all the coordination work as it determines what to do with different types of objects. By introducing a superclass or an interface, we can refactor the code to something more extensible, and you don't have to change the loop each time a new class is introduced.

First, declare an interface (or a superclass that all the different types of objects will inherit from):

public interface Foo {
  public void workWith(Coordinator coordinator);
}

Change types A, B, C, D, ... to implement this interface:

public class A implements Foo {
  public void workWith(Coordinator coordinator) {
    // do something specific to the A class
  }
}

Change Coordinator.doSomethingWithList() to:

public void doSomethingWithList(List<Foo> list) {
  for (Foo o : list) {
    o.workWith(this);
  }
}

You'll never have to change the loop again when you introduce new types. The Coordinator will invoke the workWith() method polymorphically since all implementations of Foo must define that method.

Testing Controllers That Are Protected By JSecurity Plugin

It can be a challenge to write a test case to test an action that depends on the current user. For example, the list action only returns the records that is accessible by the current user.

If you use the JSecurity Plugin, chances are you will use SecurityUtils.getSubject() to refer to the current user. If the user logs in through the login form provided by the plugin, the user name is stored as SecurityUtils.getSubject().principal. What we want to do is to find a way to inject the "current" user name during the test.

This is how I did it. In FooController, I define getSubject and list like this:

class FooController {

  def getSubject = {
    return SecurityUtils.getSubject()
  }

  def list = {
    def subject = getSubject()

    // Restrict results in listing
    ...
    [fooInstanceList: results, fooInstanceListTotal: total]
  }
}

and the test:

void testList() {
  FooController.metaClass.getSubject = { return [principal: 'user1'] }
  def ctrl = new FooController()
  ctrl.list()
  ...
}

In line 1, just before the FooController instance is created, the getSubject closure is modified to return a map that contains principal as the key. When the list action is invoked, the getSubject() method will return the map. That way, the listing will be restricted to the records which are accessible by the user user1.

How to rollback a transaction in a service

The Grails Framework Reference Documentation only mentions that a method in a service class can be set as transactional. However, it fails to mention how one should rollback a transaction.

It is actually easier than I thought. If the method throws an exception, the transaction will rollback. Although you can perform multi-table updates in a controller, it is good practice to do it in a service method.