Responses to “The Closure Of No Return”

I knew as soon as I wrote about implementing a simple prime number algorithm using Groovy that someone would find a more elegant way of solving the problem. In this post, I want to highlight some of the responses I received.

In my previous post, The Closure Of No Return, I discussed implementing an isPrime method that worked for relatively small integers. The algorithm is to try to divide the given number by all integers from two up to the square root of the number, rounded up.

My initial attempt, and the actual motivation for the blog post, was to demonstrate that if you use a return keyword inside a closure, you only return from the closure. So my initial implementation didn’t work:

boolean isPrime1(int x) {
    if (x == 2) return true
    int limit = Math.sqrt(x) + 1
    (2..limit).each { n ->
        // nice try, but a return in a closure
        // returns only from the closure
        if (x % n == 0) return false
    return true

The each method takes a closure as an argument, which is like calling a completely separate method. The return statement inside the closure returns from that method, but not from the each loop itself.

In order to return the proper value, I introduced a local variable, called result, which I assigned inside the closure and then returned:

boolean isPrime2(int x) {
    if (x == 2) return true
    boolean result = true
    int limit = Math.sqrt(x) + 1
    (2..limit).each { n ->
        if (x % n == 0) {
            result = false
            // can't break out of the loop
    return result

Since I couldn’t use the break keyword inside the loop, this implementation was forced to check all the integers up to limit. I therefore switched at that point to Groovy’s for-in loop, which does allow the break:

boolean isPrime3(int x) {
    if (x == 2) return true
    boolean result = true
    int limit = Math.sqrt(x) + 1
    for (n in 2..limit) {
        if (x % n == 0) {
            result = false
    return result

That was my final implementation in the blog post. I posted that one the web and waited for the corrections to roll in.

One commenter, Eric Johansson, pointed out that once I switched to the for-in loop, I could go back to using return again and eliminate the local variable result. Of course that’s true, and I missed it because of the steps I went through to write the code. I wonder how often such hysteresis loops occur in practice.

Another person commenting on the post, identified at Tim but without an email address or a link, suggested using the any method:

boolean isPrime3(int x) {
    if (x == 2) return true
    int limit = Math.sqrt(x) + 1
    !(2..limit).any { n ->
        x % n == 0

The hope here is that the any method would stop at the first number that satisfied the closure. I wasn’t sure that was true, so I looked at the source code for Groovy, which can be found on GitHub. The any method is shown in the Groovy JDK as being part of the java.lang.Object class. One way methods are added to the Java standard library is through the metaprogramming methods in
org.codehaus.groovy.runtime.DefaultGroovyMethods. I looked in that class and found the any implementation, which adds the any method to the java.lang.Object class:

// from DefaultGroovyMethods
public static boolean any(Object self, Closure closure) {
    BooleanClosureWrapper bcw = new BooleanClosureWrapper(closure);
    for (Iterator iter = InvokerHelper.asIterator(self); iter.hasNext();) {
        if ( return true;
    return false;

If I’m interpreting this correctly, the iterator walks through each element one by one and returns true the first time an element satisfies the closure. In other words, yes, this approach would work and leave the loop at the proper time.

Soren (sorry, I don’t know how to put the line through the “o”) Berg Glasius ( suggested using the every method instead, which would also probably do the job.

Both great developer Jochen Theodorou and the indefatigable Tim Yates immediately identified the obvious alternative that I missed. Here’s Tim’s solution (which is a slight variation on Jochen’s):

boolean isPrime5(int x) {
    int limit = Math.sqrt(x) + 1
    x == 2 || !(2..limit).find { n -> x % n == 0 }

I remembered the findAll method, which I used in my tests, but I forgot all about the find method, which returns the first element that satisfies the closure. That’s pretty ironic, too, because (in Grails especially) I normally accidentally use find when I meant findAll.

(As an aside, as soon as I saw Tim’s name, I assumed his solution was going to be both elegant and have the take method in there somewhere. I was half right.)

The only problem I have with these solutions is that while they work, they invalidate the real reason for my blog post, which was to emphasize that a return inside a closure returns only from the closure. Still, that last one feels like a really good way to solve the actual problem, so I’m going to use that in the future.

The best part is that everyone who suggested an alternative was quite friendly about it. I may have felt a bit foolish for missing these alternatives, but that’s my own pressure on myself. One of the best features about the Groovy community is how helpful and easy to work with everyone is, which I really like. Hopefully by posting those solutions here, I’m paying some of that forward.

(Thanks to everyone who responded, whether I quoted them here or not. I really appreciate the feedback.)

The Closure Of No Return

Even in a language like Groovy that is normally so clean and intuitive, there are traps for the unwary. I fell into one again today (in front of a room full of students), and I think it’s high time I documented it, at least so I’ll remember it for next time.

I’m teaching a Groovy / Grails class this week, and one of the problems I posed to the students was to write an isPrime method that worked for integers less than, say, 1000. The easiest brute force way to solve the problem is to divide the given number by every integer from 2 up to the square root of the number, rounded up.

Here’s a first try:

boolean isPrime1(int x) {
    if (x == 2) return true
    int limit = Math.sqrt(x) + 1
    (2..limit).each { n ->
        if (x % n == 0) return false // Danger, Will Robinson!
    return true

assert (2..20).findAll { isPrime1(it) } == (2..20) // Wait, what??

In other words, the method returns true for every number, whether it’s prime or not.

The problem is the return statement inside the each loop. The argument to the each method is a closure, and returning from a closure returns only from the closure, not the whole method.

I should emphasize that, so maybe next time I’ll remember it:

A return inside a closure returns only from the closure, not from the method that called it.

It’s actually not that hard to understand, when I think it through. Calling the closure is like calling another method. When I use the return keyword inside the closure, it’s like returning from that method, so it only returns from there and not from the each method itself.

So, how do I fix this? One way is to define a boolean variable outside the closure and then set it inside, but that leads to a minor problem as well.

boolean isPrime2(int x) {
    if (x == 2) return true
    boolean result = true // local var to set in closure
    int limit = Math.sqrt(x) + 1
    (2..limit).each { n ->
        if (x % n == 0) {
            result = false
            // don't you wish you could break here?
    return result

assert (2..20).findAll { isPrime2(it) } == 
    [2, 3, 5, 7, 11, 13, 17, 19] // works, but no break

I defined a boolean variable called result and initialized it to true. Then, inside the closure, if a number is not prime, I set the variable to false, and returned the variable at the end.

As the comment shows, however, I can’t break out of the loop. You can only use break inside loops, and some reason (I don’t know why), each loops don’t qualify. I’m forced to follow the loop to the end.

There is a way to solve that problem, too. I just need to replace the each loop with Groovy’s for-in loop.

boolean isPrime3(int x) {
    if (x == 2) return true
    boolean result = true
    int limit = Math.sqrt(x) + 1
    for (n in 2..limit) {
        if (x % n == 0) {
            result = false
            break  // yay!
    return result

assert (2..20).findAll { isPrime3(it)} == 
    [2, 3, 5, 7, 11, 13, 17, 19] // works

Now it works. This is also one reason to sometimes favor the for-in loop over the each iterator, but usually they’re interchangeable.

As usual, I couldn’t leave well enough alone. A common idiom in the Groovy JDK is to use metaprogramming to replace a static method in one class with an instance method in another. For example, java.lang.Math has the static abs method, but the Groovy JDK adds abs as an instance method to java.lang.Number. It’s not hard to do the same thing here:

Number.metaClass.isPrime = { ->
    Integer x = delegate as Integer
    if (x == 2) return true
    boolean result = true
    int limit = Math.sqrt(x) + 1
    for (n in 2..limit) {
        if (x % n == 0) {
            result = false
    return result

assert (2..20).findAll { it.isPrime() } == [2, 3, 5, 7, 11, 13, 17, 19]

The delegate of the closure is the number the isPrime method was called on, so if I assign it to x I can copy and paste my implementation from above.

The truly fun part was that I assigned this problem to the students, and then decided to live code the implementation in front of them. Of course I went for the each implementation, falling right into the trap. Since I’ve seen this problem many times, I recognized it pretty quickly and therefore wrote the other two implementations after that. I’d like to believe that the students got some value out of seeing me mess it up and then fix it. If nothing else, they learned the value of test cases. Without those assert statements, I probably wouldn’t have noticed the error in the first place.

Baruch Sadogursky (@jbaruch on Twitter) is doing a presentation on Groovy and Grails Puzzlers at Gr8conf in Minneapolis at the end of July, and he’s always interested in issues like this, so I made sure to send it along to him. If you know of any others, I’m sure he’d appreciate them as well.

Start With A Story

Way back in the early 90s, I decided to revisit a notion I’d toyed with for years, which was to become a science fiction writer. As part of that effort, I subscribed to Asimov’s Science Fiction Magazine, which was (and is) the best short story market available.

Back then, Isaac Asimov himself was semi-retired (he occasionally lamented that the science fiction world had “passed him by”), but he always introduced each issue with an essay. Though I always enjoyed his writing and, like so many budding writers of my generation, grew up on the Foundation and I, Robot series, I wasn’t really interested in reading the intros. I wanted to move on to the actual stories, especially given how little time I had available to do non-professional reading.

Yet, his essays were irresistible. I never missed one, and remember some to this day.

It turns out that he was using an excellent writer’s trick that he explained in one of his many books on writing: he started each one with a brief story.

He’d talk about some event that took place in his life, involving travel, or children, or a conference, or pretty much anything. He wouldn’t spend too much time on it, and ultimately he’d tie it into what he really wanted to talk about that issue. But the effect was immediate — you were pulled into the story and wanted to know how it all turned out.

(I’d quote some of them here, but my copies of the issues from back then, if they even exist anymore, are buried in boxes deep in my basement bearing giant “Danger! Here Be Daemons!” labels. Actually, if they really were marked that way, they’d be easier to find.)

While I never became a real sf writer (a story for another day), I definitely took that advice to heart. Whenever the form allows it, I start anything I’m writing with a tiny story. Most of my blog posts work that way. I do the same thing in my presentations.

The structure can be remarkably loose, and doesn’t actually have to be about me. The key requirements are only that the story has to be interesting and that it has to be short. Some I’ve used recently:

  • “My wife has an old car with over 225,000 miles on it. My son and I have been bugging her about getting a new one. You know how here in Canada my phone doesn’t work? Well, I just got an email from my insurance company with my ‘new’ insurance card.”
  • “I’ve been reading about Bayes’ Rule and statistics recently. Did you know that Pierre Simon Laplace, one of the greatest mathematicians of all time, was disowned by his father when he refused to go into the clergy? I’ve been thinking about what jobs my son could go into that would cause me to disown him. My short list includes patent troll, spammer, TSA groper, and any I.T. job that has the word ‘evangelist’ in it.”
  • “I saw a great tweet yesterday. Do you know why George R. R. Martin can’t use Twitter? Because he’s already killed off all 140 characters.”
  • “You know how you know you’ve been traveling too much? I’m ashamed to admit I drove over to the site today and sat in the parking lot for ten solid minutes trying to remember what state I was in. That’s not good.”
  • “Last night I was looking on the Internet for a list of palindromes for some code we’re going to write today. Did you know that ‘Flee to me, remote elf?’ is a palindrome? How about ‘Go hang a salami; I’m a lasagna hog’? Or even, ‘Sex at noon taxes’?”
  • “Way back in the 90s, I tried to become a science fiction writer…” (I see what you did there)

Those are pretty generic. I try to keep an eye on slashdot or reddit or related sites to see if there are any relevant news items, too. Like during an Android class, I’ll show people the latest information about the mobile marketplace, or how there’s a huge debate going on right now about the best way to introduce apps into cars. Or if I’m teaching anything related to Java (including Groovy and/or Grails) I talk about the Java 8 release and ask about any plans for adoption. Or, as I did last January, I’ll remind those students I met in St. Cloud, MN (temperature -24F before windchill) that in this country we’re allowed to live in warmer places.

It’s simple, but it has the effect of getting everyone’s attention. Try it out on your next article, or tech talk, or lunch-and-learn, or whatever. Next week I expect to open with “Hey, did you notice that the UConn men’s AND women’s basketball teams just won the NCAA Championship? Well, nyah, nyah, nyah.”

I’ll let you know how that goes over. :)

Groovy Groundhogs, revisited

For those people in the U.S. who are concerned that this morning Punxsutawney Phil, the Seer of Seers, Prognosticator of Prognosticators, emerged, reluctantly, but alertly, and stated in groundhog-ese, “I definitely see a shadow”, let me allay your fears using Groovy:

Calendar cal = Calendar.instance

cal.set(year:2014, month:Calendar.FEBRUARY, date:2)
def groundhogDay = cal.time

cal.set(year:2014, month:Calendar.MARCH, date:20)
def firstDayOfSpring = cal.time

int days = firstDayOfSpring - groundhogDay
println """There are $days days between Groundhog Day and the first Day of Spring.
That's ${(int) (days/7)} weeks and ${days % 7} days.

The result of that script is:

There are 46 days between Groundhog Day and the first Day of Spring.
That's 6 weeks and 4 days.

In other words, “six more weeks of winter” is a good thing, because otherwise we’d have to wait an extra four days.

A few notes are in order:

  • Here is the official Groundhog Day site.
  • According to the Wikipedia page, Phil has been predicting the weather since 1887. His accuracy rate is only about 39%, however.
  • Some kind of Groundhog Day celebration has been recorded since as far back as 1841. It bears similarities to the pagan festival of Imbolc, among others.
  • If you don’t like using the Calendar class (and who does?), the Groovy JDK also adds a parse method directly to java.util.Date:
    Date groundhogDay = Date.parse('MM/dd/yyyy', '02/02/2014')
    Date firstDayOfSpring = Date.parse('MM/dd/yyyy', '03/20/2014')

    and the rest is the same.

  • The Vernal Equinox this year occurs on March 20, 2014, at 1:57pm EDT.
  • I considered posting this blog entry over and over as some sort of weak tribute to the movie, but at least I spared you that. :)

There’s more, but I have to go get ready for our Super Bowl party, where the announcers have been talking about the weather all week and will no doubt be disappointed that the temperature will be in the 40s (Fahrenheit).

Groovy Weather: A New Groovy Example at

One of the main goals of Making Java Groovy is to show Java developers how much Groovy can make their lives easier. To that end, I just published a blog post (through Manning’s account) over a entitled, Groovy Weather: POGOs, Gson, and Open Weather. The blog post comes with a coupon code for 45% off the book. :)

(Spoiler: it’s “kousenjn“, but if you’re going to use it at least drop by the blog link.)

Another spoiler: it’s freakin’ cold outside. That’s partly what my blog post is about — consuming JSON data from Open Weather Map and displaying it at the console. That’s not terribly difficult, but the real value added comes from using Google’s Gson parser to convert the JSON objects into Groovy.

If you’re new to Groovy, the blog post shows a lot of detail. Regular readers of my blog, however, probably are at least comfortable with the language, so I thought I’d summarize the good parts here.

First, I need to show the JSON data, so I know what to map to.

(Er, “to what to map”? Ugh. Winston Churchill was once criticized for ending a sentence with a preposition. His reply was, “Madam, that is nonsense up with which I will not put.” That’s one of those possibly apocryphal stories that I don’t want to try to verify because I like it too much.)

Here’s the trivial Groovy expression to download the data:

import groovy.json.*

String url = ',ct'
String jsonTxt = url.toURL().text
println JsonOutput.prettyPrint(jsonTxt)

I’m using the static prettyPrint method of JsonOutput to make viewing the results easier. As a minor complaint, I have to mention that every time I use prettyPrint, I’m surprised it doesn’t actually print. It just formats the JSON data. I still need to print the result. That seems rather misleading to me.

Anyway, here’s the result:

    "coord": {
        "lon": -72.46,
        "lat": 41.63
    "sys": {
        "message": 0.193,
        "country": "United States of America",
        "sunrise": 1389096985,
        "sunset": 1389130583
    "weather": [
            "id": 800,
            "main": "Clear",
            "description": "Sky is Clear",
            "icon": "01d"
    "base": "gdps stations",
    "main": {
        "temp": 260.41,
        "humidity": 33,
        "pressure": 1025,
        "temp_min": 258.71,
        "temp_max": 262.15
    "wind": {
        "speed": 1.54,
        "deg": 0
    "clouds": {
        "all": 0
    "dt": 1389130569,
    "id": 4835003,
    "name": "Hartford",
    "cod": 200

The current temperature is buried inside the object, in the “temp” property of the “main” subobject. I could just parse this using a JsonSlurper, but instead I decided to map the whole thing using Gson.

Gson wants a class structure that maps to the JSON hierarchy. Here’s mine, which I just stuffed into a single Groovy class called Model.groovy.

class Model {
    Long dt
    Long id
    String name
    Integer cod

    Coordinates coord
    Main main
    System sys
    Wind wind
    Clouds clouds
    Weather[] weather

class Main {
    BigDecimal temp
    BigDecimal humidity
    BigDecimal pressure
    BigDecimal temp_min
    BigDecimal temp_max

class Coordinates {
    BigDecimal lat
    BigDecimal lon

    String toString() { "($lat, $lon)" }

class Weather {
    Integer id
    String main
    String description
    String icon

class System {
    String message
    String country
    Long sunrise
    Long sunset

class Wind {
    BigDecimal speed
    BigDecimal deg

class Clouds {
    BigDecimal all

I added the data types based on reading the docs, which are pretty thin, and following the nested structure of the JSON data. The names of the classes aren’t important. It’s the property names that have to match the keys in the JSON maps for deserialization to work.

Using Gson is almost trivial. All I need is the fromJson method in the Gson class:

import groovy.transform.*

class Model {

String url = ',ct'
String jsonTxt = url.toURL().text
Gson gson = new Gson()
println gson.fromJson(jsonTxt, Model)

Everything is converted to the Groovy class structure, just as expected.

Before printing the results, though, I need to do some data manipulation. If you looked at the current temperature value, you might have noticed it’s in Kelvin, of all things. As a US-based developer, I need to convert that to Fahrenheit.

def convertTemp(temp) {
    9 * (temp - 273.15) / 5 + 32

The time fields are based on “Unix time”, which measures seconds in the current epoch (beginning January 1, 1970 GMT). Java’s Date class has a constructor that takes a long representing milliseconds from the beginning of the epoch.

def convertTime(t) {
    new Date(t*1000)  // Unix time in sec, Java time in ms

Finally, the wind speed is in “meters per second” and I want “miles per hour”. When I was in high school (and dinosaurs roamed the Earth), I learned about the Factor Label method, which meant I could memorize a single length conversion and calculate any other.

def convertSpeed(mps) {
    // 1 m/sec * 60 sec/min * 60 min/hr * 100 cm/m * 1 in/2.54 cm * 1 ft/12 in * 1 mi/5280 ft
    mps * 60 * 60 * 100 / 2.54 / 12 / 5280

I added all of these to the Model class, and some getters to use them.

class Model {
  // ... as before ...

  def getTime() { convertTime dt }
  def getTemperature() { convertTemp main.temp }
  def getLow() { Math.floor(convertTemp(main.temp_min)) }
  def getHigh() { Math.ceil(convertTemp(main.temp_max)) }
  def getSunrise() { convertTime sys.sunrise }
  def getSunset() { convertTime sys.sunset }
  def getSpeed() { convertSpeed wind.speed }

Finally, here’s my nice, formatted toString method to print the results (also a part the Model class):

String toString() {
    Name         : $name
    Time         : $time
    Location     : $coord
    Weather      : ${weather[0].main} (${weather[0].description})
    Icon         :${weather[0].icon}.png
    Current Temp : $temperature F (high: $high F, low: $low F)
    Humidity     : ${main.humidity}%
    Sunrise      : $sunrise
    Sunset       : $sunset
    Wind         : $speed mph at ${wind.deg} deg
    Cloudiness   : ${clouds.all}%

I should mention that the Weather attribute of the Model class is a collection. Presumably that’s for when there are multiple weather stations associated with a given location. In the source code repository (linked below), I used Groovy’s each method to iterate over them all. Here I’m just using the first one.

The driver for the system is:


class OpenWeather {
    String base = ''
    Gson gson = new Gson()

    String getWeather(city='Marlborough', state='CT') {
        String jsonTxt = "$base$city,$state".toURL().text
        gson.fromJson(jsonTxt, Model).toString()

I like Groovy’s default arguments for methods. If I invoke the getWeather method without arguments (or as the weather property in the usual idiom), then the result is for “Marlborough, CT”. Otherwise I supply a city and a state and they’re used instead.

Clearly this needs to be tested, or at least the converters so. Here’s a Spock test for them:

import spock.lang.Specification

class ModelSpec extends Specification {
    Model model = new Model()

    def 'convertTemp converts from Kelvin to F'() {
        32 == model.convertTemp(273.15)
        212 == model.convertTemp(373.15)

    def 'convertSpeed converts from meters/sec to miles/hour'() {
        (2.23694 - model.convertSpeed(1)).abs() < 0.00001

    def 'convertTime converts from Unix time to java.util.Date'() {
        Calendar cal = Calendar.instance
        cal.set(1992, Calendar.MAY, 5)
        Date d = cal.time
        long time = d.time / 1000  // Java time in ms, Unix time in sec

        Date date = model.convertTime(time)

        d - date < 1

The mps to mph value I got from putting “1 meter per second in mph” into Google, which gave me the proper result.

Just to make sure the calls are working properly, here are a couple of tests for the Model class.

import spock.lang.Specification

class OpenWeatherSpec extends Specification {
    OpenWeather ow = new OpenWeather()

    def 'default city and state return weather string'() {
        String result =
        println result

        result  // not null is true in Groovy

    def "The weather is always great in Honolulu"() {
        String result = ow.getWeather('Honolulu', 'HI')
        println result


Here’s a script to run the whole system:

OpenWeather ow = new OpenWeather()
println  // called Marlborough, CT, but really Hartford

// Home of Paul King, co-author of _Groovy in Action_ and my personal hero
println ow.getWeather('Brisbane','Australia')

// Home of Guillaume Laforge, head of the Groovy project
// (also one of my heroes, along with Dierk Koenig, Graeme Rocher, Tom Brady, David Ortiz, ...)
println ow.getWeather('Paris','France')

// Have to check the weather in Java, right?
println ow.getWeather('Java','Indonesia')

// Any weather stations in Antarctica?
println ow.getWeather('', 'Antarctica')

Here are the results at the moment (Jan 7, 2014, at about 5:45pm EST):

Name         : Marlborough
Time         : Tue Jan 07 17:43:10 EST 2014
Location     : (41.63, -72.46)
Weather      : Clear (Sky is Clear)
Icon         :
Current Temp : 8.312 F (high: 11.0 F, low: 5.0 F)
Humidity     : 35%
Sunrise      : Tue Jan 07 07:16:25 EST 2014
Sunset       : Tue Jan 07 16:36:23 EST 2014
Wind         : 3.4448818898 mph at 258 deg
Cloudiness   : 0%

Name         : Brisbane
Time         : Tue Jan 07 17:36:03 EST 2014
Location     : (-27.47, 153.02)
Weather      : Clouds (broken clouds)
Icon         :
Current Temp : 78.566 F (high: 82.0 F, low: 77.0 F)
Humidity     : 46%
Sunrise      : Mon Jan 06 14:00:36 EST 2014
Sunset       : Tue Jan 07 03:47:48 EST 2014
Wind         : 10.51360057266 mph at 121 deg
Cloudiness   : 80%

Name         : Paris
Time         : Tue Jan 07 17:39:23 EST 2014
Location     : (48.85, 2.35)
Weather      : Clear (Sky is Clear)
Icon         :
Current Temp : 52.682 F (high: 54.0 F, low: 51.0 F)
Humidity     : 81%
Sunrise      : Tue Jan 07 02:42:36 EST 2014
Sunset       : Tue Jan 07 11:11:33 EST 2014
Wind         : 8.052970651396 mph at 220 deg
Cloudiness   : 0%

Name         : Batununggal
Time         : Tue Jan 07 17:43:35 EST 2014
Location     : (-6.96, 107.65)
Weather      : Clouds (scattered clouds)
Icon         :
Current Temp : 68.5904 F (high: 69.0 F, low: 68.0 F)
Humidity     : 91%
Sunrise      : Mon Jan 06 17:40:32 EST 2014
Sunset       : Tue Jan 07 06:10:58 EST 2014
Wind         : 2.9303865426 mph at 200 deg
Cloudiness   : 44%

Name         :
Time         : Tue Jan 07 17:43:35 EST 2014
Location     : (-78.33, 20.61)
Weather      : Clear (Sky is Clear)
Icon         :
Current Temp : -20.2936 F (high: -20.0 F, low: -21.0 F)
Humidity     : 38%
Sunrise      : Tue Jan 07 16:43:35 EST 2014
Sunset       : Wed Jan 08 04:43:35 EST 2014
Wind         : 15.770400858984 mph at 15.0058 deg
Cloudiness   : 0%

So, yeah, it’s cold outside. I also freely admit it’s a little weird seeing those non-US temperatures converted to Fahrenheit.

The Gradle build file for this system is almost trivial:

apply plugin:'groovy'

repositories {

dependencies {
    compile 'org.codehaus.groovy:groovy-all:2.2.1'
    compile ''
    testCompile 'org.spockframework:spock-core:0.7-groovy-2.0'

All of this code can be found in the book’s Github repo. I added this to Chapter 2: Groovy by Example. Chapter 2 now uses a multi-project Gradle build, so it doesn’t look exactly that the one shown here, but it works the same way.

This process is easy enough to replicate for any RESTful service that returns JSON data. The time consuming part, if any, is writing the POGO structure to capture the JSON tree, but at least with POGOs the result is pretty simple.


Now for a few personal notes. First, the latest Amazon review is simply beautiful. It’s from a person who knew Java and didn’t expect much from Groovy, but found out how much it could help. That’s exactly what I’m trying to achieve. In fact, I’ve been amazed that every single Amazon reviewer, no matter how much he or she did or didn’t like specific aspects of the book, definitely understood the goal and appreciated it.

Second, I have a few new projects in the works, but I think I can mention one of them. I’m currently generating a series of screencasts for the book, which I’m calling “Making Java Groovy: The Director’s Cut”. The goal is to discuss why the book was written as it was — what was included, what was left out, what has changed since its publication in the Fall of 2013, and anything else I think might be interesting or helpful. Everyone at Manning has been very supportive of the effort so far. I hope to make at least the first set of screencasts available soon.

Third, I’ll be giving a talk at the Boston Grails Users’ Group on Thursday, January 9, at 7pm. I hope to bring some books to add to the raffle, along with an admission to the NFJS event in the Boston area in March. If you’re in the area, please drop by and say hello.

Fourth, if you’ve read this far, you deserve a reward! If you use the coupon code “kousen37” at, you can get 37% off any book at Manning. :)

Oh, and in case you didn’t hear me celebrating, I’m now officially a 2013 JavaOne Rock Star. Thank you very much to everyone who recommended me.

Finally, those of you battling the so-called “polar vortex”, stay warm! It’s cold here, but at least we’re still above 0 (F, not C). Next week I’m planning to brave the winds of Caradhras, otherwise known as central Minnesota. Brr.

As the saying goes, follow the great advice found on the side of a mayonnaise jar: “Keep cool but don’t freeze”.

Serving jokes locally with Ratpack and MongoDB

In two previous posts, I discussed applications I created that were simple client-side front ends for the Internet Chuck Norris Database (ICNDB), located at This post gives the details of the local server I created, using Groovy, MongoDB, and the cool Ratpack project (note new URL). The earlier posts contained parts of that app, but since then I’ve updated it to the latest version of Ratpack, revised the gradle build file accordingly, added a couple of integration tests, and checked the whole thing into GitHub.

I often use ICNDB in my Groovy presentations, because it’s easy to access, returns a simple JSON object, and is rather amusing.

(This, by the way, is in direct contrast to Mr. Norris himself, whose politics are somewhat to the right of Attila the Hun. Still, I only care about the jokes themselves, which are pretty funny.)

Accessing the site is trivial:

import groovy.json.JsonSlurper

def url = ''
def json = new JsonSlurper().parseText(url.toURL().text)
def joke = json?.value?.joke
println joke

The Groovy JDK adds a toURL method to java.lang.String, which returns an instance of It also adds a getText method to that returns the entire response as a string. I use the parseText method in JsonSlurper to parse it and just dig through the properties to get the contained joke.

Of course, my demo relies on that site being available, and that’s far from a sure thing, especially if Carlos Ray’s lawyers ever get to it. It also has a tendency to go down at inconvenient intervals, like when I’m trying to present the above script.

(Quick aside: this blog is hosted on WordPress, which has a much better uptime record than my actual home page. My site is hosted on a very old Windows laptop in my office. You’ve heard of “five nines” uptime, where a site guarantees it’ll be available 99.999% of the time? My site only promises “nine fives” uptime. It’s online a bit more than half the time.)

It behooves me, therefore, to keep a local copy if possible. That turns out to be pretty easy. First, there aren’t a whole lot of jokes. The ICNDB API offers the link to return the total number of jokes. That request returns a JSON object:

    "type": "success",
    "value": 546

I can certainly handle a database of that size locally. In fact, in my app I load the whole thing into memory because why not?

Since the format of the jokes is JSON, I decided to use a MongoDB database to store them. The native format for MongoDB is BSON, or binary JSON, so all I have to do is grab the jokes and I can just append them to a local database. To make the code easier, I use the GMongo project, which is just a Groovy wrapper around Mongo’s Java client library.

The other useful method in the ICNDB API is, where num represents the number of jokes you want returned. I want all of them, so I replace num with the total.

For example, if I access, I get something like:

    "type": "success",
    "value": [
            "id": 204,
            "joke": "Nagasaki never had a bomb dropped on it. Chuck Norris jumped out of a plane and punched the ground",
            "categories": []
            "id": 329,
            "joke": "There are only two things that can cut diamonds: other diamonds, and Chuck Norris.",
            "categories": []
            "id": 348,
            "joke": "There?s an order to the universe: space, time, Chuck Norris.... Just kidding, Chuck Norris is first.",
            "categories": []
            "id": 360,
            "joke": "Two wrongs don't make a right. Unless you're Chuck Norris. Then two wrongs make a roundhouse kick to the face.",
            "categories": []
            "id": 406,
            "joke": "Chuck Norris doesn't say &quot;who's your daddy&quot;, because he knows the answer.",
            "categories": []

The only difference from the original URL is that now the value property returns all of the contained jokes, but that’s not a problem. The overall script is therefore:

import groovy.json.JsonSlurper
import com.gmongo.GMongo

// Drop the current icndb database, if it exists
GMongo mongo = new GMongo()
def db = mongo.getDB('icndb')

// Get the total number of available jokes
JsonSlurper slurper = new JsonSlurper()
String jsonTxt = ''.toURL().text
def json = slurper.parseText(jsonTxt)
int total = json.value.toInteger()

// Grab all of them at once
jsonTxt = "${total}".toURL().text
json = slurper.parseText(jsonTxt)

// Save them all locally
def jokes = json.value
jokes.each {
    db.cnjokes << it
assert total == jokes*.id.size()
assert total == db.cnjokes.find().count()

How cool is it that all I have to do is grab the jokes and append them to the collection to save them in the database? Truly, we live in magical times. :)

I can browse the database in Eclipse if I use the MonjaDB plugin. Here’s a screenshot showing it:

Now that the database is populated, I can build the Ratpack app. I started off using the lazybones builder inside of gvm, the Groovy enVironment Manager, which I discussed in the earlier post. Ratpack keeps evolving, though, and lazybones hasn’t kept up, so the changes I’ve made to the resulting app are a bit more substantial than I originally intended.

Here’s the JokeServer class. In the constructor, I load all the jokes into a Groovy map, where the keys are the id’s and the values are the joke strings themselves.

class JokeServer {
    GMongo mongo = new GMongo()
    Map jokes = [:]
    List ids = []
    JokeServer() {
        DB db = mongo.getDB('icndb')
        def jokesInDB = db.cnjokes.find([categories: 'nerdy'])
        jokesInDB.each { j ->
            jokes[] = j.joke
        ids = jokes.keySet() as List

The other method in the class returns a random joke by shuffling the id’s and returning the associated joke. (The id’s themselves aren’t consecutive, partly because I’m only using the “nerdy” ones and partly because the original site skipped ID values.)

    String getJoke(String firstName = 'Chuck', 
                   String lastName = 'Norris') {
        String joke = jokes[ids[0]]
        if (!joke) println "Null joke at id=$id"
        if (firstName != 'Chuck')
            joke = joke.replaceAll(/Chuck/, firstName)
        if (lastName != 'Norris')
            joke = joke.replaceAll(/Norris/, lastName)
        return joke

I’m using Groovy’s nice optional arguments mechanism here, so if I invoke getJoke without arguments I get the original joke, but if I supply a first name or a last name they’re used in the joke itself.

Here’s my test case to make sure at least this much is working. It’s a regular JUnit test, implemented in Groovy.

import static org.junit.Assert.*;

import org.junit.Before;
import org.junit.Test;

class JokeServerTest {
    JokeServer server = JokeServer.instance  // @Singleton on server

    public void testGetJokeFirstNameLastName() {
        String joke = server.getJoke('Patton', 'Boggs')
        assert !joke.contains('Chuck')
        assert !joke.contains('Norris')
        assert joke.contains('Patton')
        assert joke.contains('Boggs')

    public void testGetJoke() {
        String joke = server.joke
        assert joke


Did you notice that I added @Singleton on the JokeServer, just for fun? That’s why the test grabs the server using the instance property. My first test then uses the strings “Patton” and “Boggs”, the name of the law firm that sent me the takedown notice. The second test accesses the joke property, which calls the getJoke method by the normal Groovy idiom.

Ratpack applications use a script called ratpack.groovy to set the various handlers. Since my script is so simple, I just added everything to that handler:

import static ratpack.groovy.Groovy.*

import com.kousenit.JokeServer

JokeServer server = JokeServer.instance

ratpack {
    handlers {
        get {
            String message
            if(request.queryParams.firstName ||
               request.queryParams.lastName) {
                message = server.getJoke(
            } else {
                message = server.joke
            response.headers.set 'Content-Type', 'application/json'
            response.send message
        assets "public"

The ratpack method takes a closure containing the various handlers. I only have a single handler, which is accessed using a get request. Then I get the relevant joke, set the Content-Type header to the MIME type for JSON, and return it. Actually, since my server returns the actual string, I probably shouldn’t set the header at all, but it hasn’t hurt anything so far.

The only difference between this script and the one I showed previously is the import statement at the top. Now the ratpack method is a static method in the ratpack.groovy.Groovy class, which Eclipse (actually, Groovy / Grails Tool Suite) can’t find even though it’s in the dependencies.

The next piece of the puzzle is the Gradle build script itself. Ratpack recently changed its package structure and moved its deployed versions to JFrog. Here’s the updated build script:

apply plugin: "ratpack-groovy"

buildscript {
  repositories {
    maven { url "" }
  dependencies {
    classpath 'io.ratpack:ratpack-gradle:0.9.0-SNAPSHOT'

repositories {
  maven { url "" }
  maven { url "" } // for springloaded

dependencies {
    compile 'com.gmongo:gmongo:1.0'
    testCompile "org.spockframework:spock-core:0.7-groovy-2.0", {
        exclude module: "groovy-all"

    // SpringLoaded enables runtime hot reloading.
    springloaded "org.springsource.springloaded:springloaded-core:1.1.4"

task wrapper(type: Wrapper) {
    gradleVersion = "1.8"

The buildscript block includes the information for downloading the ratpack-groovy plugin, which has changed the group id of the ratpack dependency to io.ratpack. I excluded the groovy-all module from the Spock dependency because it’s already part of the ratpack-groovy plugin, and I updated the Gradle version property in the wrapper to 1.8, but otherwise this is the same as the one generated by lazybones.

Before I run, though, I still want some sort of integration test. Most of the available examples online don’t have any tests (sigh), and I spent far too many hours figuring out how to get one to work, so I’m including it here.

package com.kousenit

import ratpack.groovy.test.LocalScriptApplicationUnderTest
import ratpack.groovy.test.TestHttpClient
import spock.lang.Specification

class ServerIntegrationSpec extends Specification {

    def aut = new LocalScriptApplicationUnderTest()
    @Delegate TestHttpClient client = aut.httpClient()

    def setup() {

    def "regular get request returns Chuck Norris string"() {
        String result = get('/').asString()

        println result
        result.contains('Chuck Norris')

    def "firstName and lastName parameters work"() {
        def response = get('?firstName=Carlos&lastName=Ray')?.asString()
        println response
        response.contains('Carlos Ray')

As with most Spock tests, the tests themselves are pretty self-explanatory. The hard part was figuring out the proper URLs to invoke and knowing to use the asString method, which wasn’t obvious at all. I’m also not clear on the mechanism used to get the TestHttpClient instance, but I’m sure Luke Daley will explain it to me when I see him next. :)

One last quirk should be noted for anyone trying to duplicate this. To run the server, I type:

> gradle run

which starts up the server on port 5050. Access the server using http://localhost:5050 or http://localhost:5050/firstName=Carlos&lastName=Ray and you’re all set.

For some reason, the script that runs the app requires the ratpack.groovy script to be located in the src/ratpack folder. If I want to run the tests, however,

> gradle test

then I have to have the ratpack.groovy file in the root of the project. I have no idea why, nor do I know how to configure them so I only need one and not the other, so I just copied the file into both locations.

(I know — that’s ugly and awkward. I promise to fix it when find out why it’s necessary in the first place.)

So, at long last, if you want the code, it’s all stored in my GitHub repository at Feel free to clone it and wreak whatever havoc upon it you desire.


For those who might be interested, a few notes about my book, Making Java Groovy:

  • The book page at Amazon now has 24 reviews. All are four or five stars but one, which is extremely gratifying. More importantly, the text of each makes it clear that the book is finding its intended audience: Java developers who want to make their jobs easier by adding Groovy.
  • The lone three-star review doesn’t say anything bad about the book anyway, so at least I’ve got that going for me, which is nice.
  • If you get the book at Amazon rather than at Manning, you can still get the ebook versions (pdf, mobi, and epub). The mechanism to do so is included in an insert in the book.
  • The response to my silly marketing ideas in my last couple of posts has been silence punctuated by the occasional cricket, so I’m going to stop doing that unless something really good occurs to me. Oh well. They made me laugh during the writing process, and anything that keeps you writing is a Good Thing(TM).

Making Java Groovy: A Celebrity (Non-)Endorsement

Several of my book author friends on the No Fluff, Just Stuff tour told me that writing a book would open doors for me. That doesn’t explain, though, why I seem to insist on climbing through windows.

I mean, writing Making Java Groovy put me on the NFJS tour, helped me become a speaker at Gr8 conf, DevNexus, and JavaOne*, gave me lots of new professional contacts, and means my author page at Amazon has some actual content. Sweet. But I’m always wondering what else I can do with it that most authors are too experienced, too normal, or maybe just too sane to consider.

*My evals so far from my presentation at JavaOne have been really good, so I’m still hoping that I’m awarded JavaOne Rock Star status. I totally want to party like a JavaOne rock star, party like a JavaOne rock star, etc.

For example, there are certain people that I only know through Twitter or other online media who I would live to meet. As one of my Silly Marketing Ideas (abbreviated SMI, as opposed to my Serious Marketing Ideas, abbreviated SMI), I thought if I could get one or more of them to endorse my book, that would be seriously cool, not to mention whatever it did for sales.

Of course, the silly part is that most of the people I want to approach aren’t even developers, to say nothing of Java people. If you place all this in a larger context, I call this entire effort an “experiment in accidental celebrity”, meaning I want to achieve D-List Twitter Celebrity status, but only if I can do the whole thing as a gag and blog about it here.

To that end, a couple weeks ago I chose four of the people I would love to meet somehow and sent them individualized emails.

That’s a long shot, since I’m sure the people I contacted get flooded with emails on a regular basis. Still, I had to start somewhere, and it’s not like I have a budget for this or anything.

What I do have is a set of author copies for my book. I showed the set in my last blog post when I arranged them on the coffee table in our living room:

After giving away a few to friends and family, I decided that I could part with a few more if they acted as my introduction to unsuspecting victims I wanted to meet. If they ultimately decided they loved the book, or even just the joke itself, and wanted to endorse it, so much the better.

One of the people I contacted was a person I truly enjoy and admire, Wil Wheaton. If you lost track of him after Stand By Me and his classic role as the much-loathed-but-not-his-fault-he-was-stuck-with-those-lines Wesley Crusher from Star Trek: The Next Generation, then you’ve missed a lot (here’s his IMDB page). His blog is excellent. I read his Just a Geek book, which I really enjoyed, and his Memories of the Future, Volume 1 book is among the funniest I’ve ever read. My wife and I have also enjoyed his guest appearances on Eureka, Criminal Minds, Leverage, and, of course, The Big Bang Theory.

He’s also great on Twitter, under the handle @wilw. If you follow him, be sure to also follow his wife, @AnneWheaton. I often say that following Wil is fun and following Anne is fun, but following both together is wonderful.

As I said, I ultimately mustered up my courage (which meant suppressing the nauseous feeling that this was not only particularly silly, but actively embarrassing), dug up his email address, and sent him a message. To summarize the message, I:

  • explained who I was and that I had just published a book
  • mentioned I was a fan and personalized the evidence enough to prove I wasn’t kidding
  • explained that this was a Silly Marketing Idea which he was free to ignore but gave me a hopefully non-creepy opportunity to contact him directly
  • asked him for a mailing address so I could send him a signed copy of my book

I didn’t get anything back right away, but I didn’t really expect to. I imagine he is inundated with email, and I’m sure I was one of millions. I expected that I would have to resend it roughly once a week to see what would happen**.

**I got that idea from The Shawshank Redemption. Andy sent a request for money to the town council once a week for two years until they finally realized he wouldn’t go away and sent some money for his prison library. I’ve always wondered about that, by the way. Did he send the same letter, or did he rewrite it each time? Did he vary the wording or the basic content? How did he keep from letting his annoyance or anger at being ignored seep into the letter? Neither the movie nor the original Stephen King story get into details.

Then, however, fate took hold. On 10/17, Anne Wheaton tweeted:

Anne Witchon ‏@AnneWheaton 17 Oct
Texas tweety buddies! 10/17 Austin:
10/18 Houston:
10/20 Dallas:”

I knew I was traveling to Dallas this week, but it suddenly occurred to me his show was on Sunday and I was flying in on the same day. I checked my flight arrangements and realized I could actually make the show. I quickly bought a ticket online (not many were left, but I only needed one), donned my Star Trek polo shirt (the blue science officer one I use when I’m making Groovy presentations about the Spock testing framework) and drove over to the Granada Theater.

I brought a book with me, of course, but I didn’t sign it because I wasn’t sure I actually was going to meet Wil. I’m not the sort of person who goes to conventions, or even events like this, so I don’t really know how they work. In fact, I almost didn’t go at all***, because I was very tired after the flights and didn’t particularly want to leave my hotel room after I arrived. I went anyway, of course, or this would be a much shorter blog post.

***I know my wife Ginger will be shocked, SHOCKED! to hear that.

The show was called Wil Wheaton vs Paul and Storm. Here’s the poster:


I have to admit that I didn’t know Paul and Storm at all, except from Wil or Anne’s tweets about them. I knew they were a musical / comedy duo who wrote their own funny songs, but that’s about it.

Here’s the marquee from the theater:

The show was a blast. Before the show, the theater used a Twitter site that showed all tweets directed to the theater. I don’t remember many, but among the funniest were things like:

- What does the FOX say? Sorry we canceled Firefly

- Told my blind date I was the guy with the beard wearing a funny T-shirt. Crap.

- [Paul of Paul and Storm] Hanging out with this girl who was supposed to meet some guy with a beard and a funny T-shirt. Playing it cool…

Wil introduced the show, saying that any and all recordings were allowed as long as they were released under the Creative Commons license (“and if you don’t know what that is, ask a nerd and he’ll explain it to you in far more detail than you ever wanted”), but not to use flash photography because that distracted the performers and not to do anything to interfere with other audience members.

“In short,” he said, using one of his signature catch phrases, “Don’t Be A Dick, and you’ll be fine”.

Paul and Storm did their set first, and it was fantastic. Intelligent people tend to be quite clever but not comedian-level funny****. Both Paul and Storm were both. I highly recommend that you check out their website, listen to their music, buy their stuff, and go see them if at all possible. They are masters of the “X is my Y cover band” style of joke. I’d mention some here, but I don’t want to include any spoilers and I can’t remember them anyway. I guess I’ll just have to go see them again.

****I often put myself in that category.

After the intermission, Wil came out and told personal stories in the form of a comedy monologue. There were great (and no spoilers here), but the biggest impression I get from him is I never realized, even reading his Twitter feed, just how filthy he is. Even in his 40s he still looks like this pleasant guy next door, and you don’t expect him to be quite so foul. It works, though. :)

Wil and Paul and Storm then came out together and did a few songs filled with random riffs that the audience loved. Here is a link to some pictures from the show.

Here’s a Vine video Paul posted.

Finally, here’s a screen capture from that video showing the audience, with a circle showing me:


During the entire show, I was holding my book, hoping that I’d get a chance to meet Wil and explain why I was foisting it on him. I also had idle daydreams of him letting me take a picture of him holding it and tweeting it to his 2.4 million followers (!), thinking if only 1% of them bought it…

After the show, a line quickly formed in the lobby. It turned out that the rules were that Wil and Paul and Storm would sign autographs for everybody who wanted one, but that if you wanted a posed picture you’d have to get in the back of the line again.

I didn’t have anything to sign, but the couple in front of me in line gave me a few blank cards from a game called The World of Munchkin, which now gives me a perfect opportunity to show the difference between Paul and Storm and Wil Wheaton. Here are my autographs from Paul and Storm:

They’re basically illegible, sure, but I’m not one to talk. My signature is appalling. Anyway, here’s what Wil drew:
Yup. That’s the Wil Wheaton Experience right there.

While he was signing, I gave him my book. I told him I had just published it (“Hey, congratulations!” “Thanks!”), showed him where I had already signed it for him and Anne (“Cool!”), told him that I had sent him email if he was curious about me, and told him what a fan I was (yeah, I talked fast).

The rest of the conversation went something like this:

Wil: Will this help me learn this stuff if I don’t know anything about Java?

Me: Uh, some. Maybe. I’ll be happy to answer questions about it online if you like. That’s what I do. I teach technical training classes. I’ll come back around for a picture, but I was wondering if maybe I could get a picture with the book in it…

Wil: No, I can’t do that. That would be like an endorsement.

Me: (sound of internal imaginary bubble popping) I completely understand.

And I do understand. As I say, this is for fun, and if something else comes of it, fine, but if not, maybe I made an impression on him that the average fan doesn’t.

I did come back around for a picture. As we were setting up, I said:

Me: I meant to tell you that the front matter is pretty funny.

Wil: Great!

Me: Also, do you mind if I blog about this?

Wil: Of course!

Here’s my picture with my new buddies Paul and Storm and Wil and Anne Wheaton:
From left to right, that’s Storm, Anne, Me, Wil, and Paul. Or maybe Paul, Anne, Me, Wil, and Storm. (Just kidding. It’s the former. I think. Hey, I bought the USB drive with all their music on it, so I can make one joke at their expense.)

I have to say, I’m still basking in the afterglow. They show was fun, meeting them was fun, and getting the picture was even better. I’m really glad I gave him the book, too, though I have to point out that this does not in any way constitute an endorsement of Making Java Groovy (which is getting really great reviews at Amazon).

Yet. Now if I can only get him to read it…

(Upon re-reading this, it’s starting to sound a bit too mercenary. Look, all I want is my book to last a while, and maybe to make enough money that I can keep doing what I’m doing. It took me 40 years to find my dream job, and now that I have it I don’t want to give it up. Also, it’s kind of fun coming up with silly ideas like this, just to see if I can bring myself to actually do some of them. My next post will get back to technical issues, I promise.)


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