Category: Java

Learning about Class Literals after twenty years of Java

The story about a customer request that led to my discovery of a cool feature of Java: Class Literals (you’ve probably already heard about them).

I’ve programmed in Java nearly every day for twenty years now. At the beginning of my computer science studies, I was introduced to Java 1.0.x and have since accompanied every version of Java. Our professor made us buy the Java Language Specification on paper (it was quite a large book even back then) and I occassionally read it like you would read an encyclopedia – wading through a lot of already known facts just to discover something surprising and interesting, no matter how small.

With the end of my studies came the end of random research in old books – new books had to be read and understood. It was no longer efficient enough to randomly spend time with something, everything needed to have a clear goal, an outcome that improved my current position. This made me very efficient and quite effective, but I only uncover surprising facts and finds now if work forces me to go there.

An odd customer request

Recently, my work required me to re-visit an old acquaintance in the Java API that I’ve never grew fond of: The Runtime.exec() method. One of my customer had an recurring hardware problem that could only be solved by rebooting the machine. My software could already detect the symptoms of the problem and notify the operator, but the next logical step was to enable the software to perform the reboot on its own. The customer was very aware of the risks for such a functionality – I consider it a “sabotage feature”, but asked for it anyway. Because the software is written in Java, the reboot should be written in Java, too. And because the target machines are exclusively running on Windows, it was a viable option to implement the feature for that specific platform. Which brings me to Runtime.exec().

A simple solution for the reboot functionality in Java on Windows looks like this:


Runtime.exec("shutdown /r");

With this solution, the user is informed of the imminent reboot and has some time to make a decision. In my case, the reboot needed to be performed as fast as possible to minimize the loss of sensor data. So the reboot command needs to be expanded by a parameter:


Runtime.exec("shutdown /r /t 0");

And this is when the command stops working and politely tells you that you messed up the command line by printing the usage information. Which, of course, you can only see if you drain the output stream of the Process instance that performs the command in the background:


final Process process = Runtime.exec("shutdown /r /t 0");
try (final Scanner output = new Scanner(process.getInputStream())) {
    while (output.hasNextLine()) {
        System.out.println(output.nextLine());
    }
}

The output draining is good practice anyway, because the Process will just stop once the buffer is filled up. Which you will never see in development, but definitely in production – in the middle of the night on a weekend when you are on vacaction.

Modern thinking

In Java 5 and improved in Java 7, the Runtime.exec() method got less attractive by the introduction of the ProcessBuilder, a class that improves the experience of creating a correct command line and a lot of other things. So let’s switch to the ProcessBuilder:


final ProcessBuilder builder = new ProcessBuilder(
        "shutdown",
        "/r",
        "/t 0");
final Process process = builder.start();

Didn’t change a thing. The shutdown command still informs us that we don’t have our command line under control. And that’s true: The whole API is notorious of not telling me what is really going on in the background. The ProcessBuilder could be nice and offer a method that returns a String as it is issued to the operating system, but all we got is the ProcessBuilder.command() method that gives us the same command line parts we gave it. The mystery begins with our call of ProcessBuilder.start(), because it delegates to a class called ProcessImpl, and more specific to the static method ProcessImpl.start().

In this method, Java calls the private constructor of ProcessImpl, that performs a lot of black magic on our command line parts and ultimately disappears in a native method called create() with the actual command line (called cmdstr) as the first parameter. That’s the information I was looking for! In newer Java versions (starting with Java 7), the cmdstr is built in a private static method of ProcessImpl: ProcessImpl.createCommandLine(). If I could write a test program that calls this method directly, I would be able to see the actual command line by myself.

Disclaimer: I’m not an advocate of light-hearted use of the reflection API of Java. But for one-off programs, it’s a very powerful tool that gets the job done.

So let’s write the code to retrieve our actual command line directly from the ProcessImpl.createCommandLine() method:


public static void main(final String[] args) throws Exception {
    final String[] cmd = {
            "shutdown.exe",
            "/r",
            "/t 0",
    };
    final String executablePath = new File(cmd[0]).getPath();

    final Class<?> impl = ClassLoader.getSystemClassLoader().loadClass("java.lang.ProcessImpl");
    final Method myMethod = impl.getDeclaredMethod(
            "createCommandLine",
            new Class[] {
                    ????, // <-- Damn, I don't have any clue what should go here.
                    String.class,
                    String[].class
            });
    myMethod.setAccessible(true);

    final Object result = myMethod.invoke(
            null,
            2,
            executablePath,
            cmd);
    System.out.println(result);
}

The discovery

You probably noticed the “????” entry in the code above. That’s the discovery part I want to tell you about. This is when I met Class Literals in the Java Language Specification in chapter 15.8.2 (go and look it up!). The signature of the createCommandLine method is:


private static String createCommandLine(
        int verificationType,
        final String executablePath,
        final String cmd[])

Note: I didn’t remove the final keyword of verificationType, it isn’t there in the original code for unknown reasons.
When I wrote the reflection code above, it occurred to me that I had never attempted to lookup a method that contains a primitive parameter – the int in this case. I didn’t think much about it and went with Integer.class, but that didn’t work. And then, my discovery started:


final Method myMethod = impl.getDeclaredMethod(
        "createCommandLine",
        new Class[] {
                int.class, // <-- Look what I can do!
                String.class,
                String[].class
        });

As stated in the Java Language Specification, every primitive type of Java conceptionally “has” a public static field named “class” that contains the Class object for this primitive. We can even type void.class and gain access to the Class object of void. This is clearly written in the language specification and required knowledge for every earnest usage of Java’s reflection capabilities, but I somehow evaded it for twenty years.

I love when moments like this happen. I always feel dumb and enlightened at the same time and assume that everybody around me knew this fact for years, it is just me that didn’t get the memo.

The solution

Oh, and before I forget it, the solution to the reboot command not working is the odd way in which Java adds quote characters to the command line. The output above is:


shutdown /r "/t 0"

The extra quotes around /t 0 make the shutdown command reject all parameters and print the usage text instead. A working, if not necessarily intuitive solution is to separate the /t parameter and its value in order to never have spaces in the parameters – this is what provokes Java to try to help you by quoting the whole parameter (and is considered a feature rather than a bug):


final String[] cmd = {
        "shutdown",
        "/r",
        "/t",
        "0",
};

This results in the command line I wanted from the start:


shutdown /r /t 0

And reboots the computer instantaneous. Try it!

Your story?

What’s your “damn, I must’ve missed the memo” moment in the programming language you know best?

The Great Rational Explosion

A Dream to good to be true

A few years back I was doing mostly computational geometry for a while. In that field, floating point errors are often of great concern. Some algorithms will simply crash or fail when it’s not taken into account. Back then, the idea of doing all the required math using rationals seemed very alluring.
For the uninitiated: a good rational type based on two integers, a numerator and a denominator allows you to perform the basic math operations of addition, subtraction, multiplication and division without any loss of precision. Doing all the math without any loss of precision, without fuzzy comparisons, without imperfection.
Alas, I didn’t have a good rational type available at the time, so the thought remained in the realm of ideas.

A Dream come true?

Fast forward a couple of years to just two months ago. We were starting a new project and set ourselves the requirement of not introducing floating point errors. Naturally, I immediately thought of using rationals. That project is written in java and already using jscience, which happens to have a nice Rational type. I expected the whole thing to be a bit slower than math using build-in types. But not like this.
It seemed like a part that was averaging about 2000 “count rate” rationals was extremely slow. It seemed to take about 13 seconds, which we thought was way too much. Curiously, the problem never appeared when the count rate was zero. Knowing a little about the internal workings of rational, I quickly suspected the summation to be the culprit. But the code was doing a few other things to, so naturally my colleagues demanded proof that that was indeed the problem. Hence I wrote a small demo application to benchmark the problem.

The code that I measured was this:

Rational sum = Rational.ZERO;
for (final Rational each : list) {
    sum = sum.plus(each);
}
return sum;

Of course I needed some test data, that I generated like this:

final List<Rational> list = new ArrayList<>();
for (int i=0; i<2000; ++i) {
    list.add(Rational.valueOf(i, 100));
}
return list;

This took about 10ms. Bad, but not 13s catastrophic.

Now from using rational numbers in school, we remember that summing up numbers with equal denominators is actually quite easy. You just leave the denominator as is and add the two numerators. But what if the denominators are different? We need to find a common multiple of the two denominators before we can add. Usually we want the smallest such number, which is called the lowest common multiple (lcm). This is so that the numbers don’t just explode, i.e. get larger and larger with each addition. The algorithm to find this is to just multiply the two numbers and divide by their greatest common divisor (gcd). Whenever I held the debugger during my performance problems, I’d see the thread in a function called gcd. The standard algorithm to determine the gcd is the Euclidean Algorithm. I’m not sure if jscience uses it, but I suspect it does. Either way, it successively reduces the problem via a division to a smaller instance.

What does this all mean?

This means that much of the complexity involved happens only when there’s variation in the denominator. Looking at my actual data, I saw that this was the case for our problem. The numbers were actually close to one, but with the numerator and the denominator each close to about 4 million. This happened because the counts that we based this data on where “normalized” by a time value that was close, but not equal to one. So let’s try another input sequence:

final Random randomGenerator = new Random();
final List<Rational> list = new ArrayList<>();
for (int i=0; i<2000; ++i) {
    list.add(Rational.valueOf(4000000, 4000000 + randomGenerator.nextInt(2000)));
}
return list;

That already takes 10 seconds. Wow. Here’s the rational number it produced:

10925412090387826443356493315570970692092187751160448231723307006165619476894539076955047153673774291312083131288405937659569868481689058438177131164590263653191192222748118270107450103646129518975243330010671567374030224485257527751326361244902048868408897125206116588469496776748796898821150386049548562755608855373511995533133184126260366718312701383461146530915166140229600694518624868861494033238710785848962470254060147575168902699542775933072824986578365798070786548027487694989976141979982648898126987958081197552914965070718213744773755869969060335112209538431594197330895961595759802183116845875558763156976148877035872955887060171489805289872602037240200456259054999832744341644730504414071499368916837445036074038038173507351044919790626190261603929855676606292397018669058312742095714513746321779160111694908027299104638873374887446030780299702571350702255334922413606738293755688345624599836921568658488773148103958376515598663301740183540590772327963247869780883754669368812549202207109506869618137936835948373483789482539362351437914427056800252076700923528652746231774096814984445889899312297224641143778818898785578577803614153163690077765243456672395185549445788345311588933624794815847867376081561699024148931189645066379838249345071569138582032485393376417849961802417752153599079098811674679320452369506913889063163196412025628880049939111987749980405089109506513898205693912239150357818383975619592689319025227977609104339564104111365559856023347326907967378614602690952506049069808017773270860885025279401943711778677651095917727518548067748519579391709794743138675921116461404265591335091759686389002112580445715713768865942326646771624461518371508718346301286775279265940739820780922411618115665915206028180761758701198283575402598963356532479352810604578392844754856057089349811569436655814012237637615544417676166890247526742765145909088354349593431829508073735508662766171346365854920116894738553593715805698326801840647472004571022201012455368883190600587502030947401749733901881425019359516340993849314997522931836068574283213181677667615770392454157899894789963788314779707393082602321025304730355204512687710695657016587562258289968709342507303760359107314805479150337790244385189611378805094282650120553138575380568150214510972734241803176908917697662914714188030879994734853772797322420241420911735874903926141598416992690859929943631826094723456317312589265104334870907579391696178556354299428366394819280011410287891113591176612795009226826412471238783334239148961082442565804292473501012401378940718084589859443350905260282342990350362981901637062679381912861429756544396701574099199222399937752826106312708211791773562169940745686837853342547182813438086856565980815543626740277913678365142830117575847966404149038892476111835346566933160119385992791677587359063277202990220629004309670865867774206252830200897207368966439730136012024728717701204793182480513620275549665094200202565592742030772102704751736850897665353297536494739059325582661212315355306787427752670613324951121097833683795311514392922347268374097451268196257308005629903372871471809591087849716533132440301432155867780938535327925645340832637372702171777123816397448399703780105396941226655424025197472384099218081468916864256472238808237005121132164363385877692234230678011184351921814453560033879491735351402997266882544304106997065987376103362395437737475217181551336569975031721614790499945872209261769951117223344186839969922893394319287462384028859822057955389124951467203432571737865201780344423642467187208636881135573636815083891626138564337634176587161231028307776960866522346008589607259041199676560090157817882260300414906572885890188984036234226505815367029839231023461597364977306399898603903392434756572392816540125771578189640871020070756539777101197151773304409519870643142190955018579914630314940373832858007535828153361236115553577694543503842444481599944319287815162136101362705211937257383677282014480487759786222801447548899760241829116959865698836386442016721709983097509675552750221989521551169512674725876581185837611167980363615880958917421251873901289922888492447507837290336628975165062036681599909052030653421736716061426079882106810502703095803882805916960831442634085856041781093664688754713907512226706324967656091109936101526173370212867073380662492009726657437921033740063367290862521594119329592938626114166263957511012256023777676569002181500977475083845756500926631153405264250959378833667206532373995888322137324027620266863005721216133252342921697663864807284554205674829658250755046340838031118227643145562001361542532622713886266813492926885236832665609571019479812713355021295737820773552735161701716018010606040731647943600206193923458996150345093898644748170519757957470535978378479854546255651200511536560142431948781377187548218601919108870420102025378751015728281345799655926856602543107729659372984539588835599345223921737022220676709028150797109091782506736145801340069563865839397272145141831011878720095142353543406658905222847479419799336972983678887227301846161770296173667855239714987183774931791306562351516152727523242208973614372214119191610954209164193665209038399568256186789865817560942946289864468805486432823029457193832616134050058472575/5464071367689021145920376789564097484075057036929154325361292811121525852598101325663975897893163034606703637232625465056851133763148192531586105963101428155647567490545647427502233159652310898001125892675829093187577533366895512701590739143174316498475076791171065480136546725008720643752948237443019242161669077663609144434306128221503414531394096823915215979623953337930493605186601571769495144894636986997575880131117237208510857818613219593393080298986414277944012186244301930294333213815805316754678940597177696108355782072853392533822105101007621252067159549391148948251225551745385134586065334994558336331772298542454065874623247283672363609435360051294428160464673413148011783297358303182389731822629550376618544475198010869325105825675331154332311829064320240772190873197445422806128724029723364642376469088090535867746031708415054086042362900835830071439066729248803080864979591431807735444694059982355674331452510436222691302473693502522151731546119758216043039918795122874474779117841250524168339597048231340441394931963429195108468364711206679388129543777587115734312812004999951805288552516345754609724336541283126925634054615621607387407977448299658305191245949844785795188192329587881827885708672142850022336110188979606923986247270824545988992712993364088907981140326036171066083995899655482025878782401213185589085533281898090985040136079388970812804900747496605542168328539571899508434943602485570225905907644120282314027462195035830426406664327228058856521800556952281791943493734276488880843046612364784975328644388569123725347247071532065625240062618476483177429385912626131806870421725932016173249912942914992853735756415323674217279068679145528740177762103447741638280753518805769821177202801618302946904761889981101239141113424330262556585926281757788049928339119700279137278143586150134536970549327847433827708642630282821305576285238510965599311560018270085653975154329160951814845203648945184348538132528313719965960517692615234724933839415863834082163365616979319651204331888160659534780756175312928937580725581177783203569550625979825708835192412739747467314045215372710237518236109496682610777837094944201522879675742349874447002866525687741559327659019423524782141050034563799707395461801978917300210616390761873065433809737256362778865972956861423012424858347791919074000867019988914246185988825680768363450694954357708832975385526022445613099402197649574980990392753660371969220370462447697883319180076033708570759874351259644698980354866068784429511482751711329624930863629304545443308117342612677534958727764219861540475648492451312505519760969766401341016292089564143516344584197883268917604454917829927706134205489288603918892893943815239131020894621190795863599245502858307692284626604530452680973144088318876770254624083735638792267234407991769552946582638360068875509975769002847311018204789400549401560376453753063628076240879327047662612561558831502017122298425734701863181656884664851026481979959368508273072349808416302081774935491159815938813047108294310775621375475681754627108394755080076400344634873409823122370888409229666662703837096634538759103222727447751640114800586463155863216558367921497862783396136480309101772689723377535772396361300263399246855706948606216922101060308941294672554277063185792769730719177469014803853068307966008447841184403910224016078072279882935920347299431580783664128991812654780457701135185591782941754395263461459180940761928488860853515685525983743168631933069310771834569879795160576216776499263300025539821575767674288253419696114910586812284230709422230558006531335540918025940397918186306383352682903150594714754889631368556569576312855831657674738769087018659290558967429709446878600756119596020561987088170551193295467295484037656684414413021143993266548616775075905905925334335921152629477697210074629622902141063078657746149232034056642352265941452947845339051610301151130368316362022636711499114826671695540416257477345744126416363795225938900727969092692270788643742709421442552812735921534532156766228688341300473140115855278302077136595142902548557943030527129378779996403263450119984758775332533036452077654603781557033976641416371186050927154435643492983848051087282942559503555223474754690693742997370592039106889187781827888290578576160100274561900208861136548215743616634615083974725330617020874519606610812580339985836584711397102753448160446984238925180319975660479580098047098930179253806342651446549086560273369490291496196263969077175524002077318170194252067818804991902015489300563029607689935067689448990111925784018672159126316019672778424331051001652240822592173809393561688768265941132515679297537471384753992855568256114701369550476003237008840508060572457453896791033992685192069703595482787771374051189250065726225137532026227150630791972777126392521002221370419505711150022179216266239656298115575018443700688208844387102582445812545858014862427316785193110884751319467750425511273629581416588296942433219322216784262821066075700266485645060935996743388485096744598169509920624971167075214788838073469621687619355816573640360338756385397428237445511332615921133308459043700086925442114337299109227541364012352140312577797531234832237279430947774637533319353713938562360646441591033255036

I kid you not, that’s over 10000 digits! In the editor I’m writing this in, that’s roughly 3 pages. No wonder it took that long. Let’s use even more variation in the numbers:

final Random randomGenerator = new Random();
final List<Rational> list = new ArrayList<>();
for (int i=0; i<2000; ++i) {
    list.add(Rational.valueOf(4000000 + randomGenerator.nextInt(5000),
            4000000 + randomGenerator.nextInt(20000)));
}

Now that already takes 16 s, with about 14000 digits. Oh boy. Now the maximum number of values I expected to do this averaging for was about 4000, so let’s scale that up:

final Random randomGenerator = new Random();
final List<Rational> list = new ArrayList<>();
for (int i=0; i<4000; ++i) {
    list.add(Rational.valueOf(4000000 + randomGenerator.nextInt(5000),
            4000000 + randomGenerator.nextInt(20000)));
}
return list;

That took 77 seconds! More than 4 times as long as for half the amount of data. The resulting number has over 26000 digits. Obviously, this scales way worse than linear.

An Explanation

By now it was pretty clear what was happening: The ever so slightly not-1 values were causing an “explosion” in the denominator after all. When two denominators are coprime, i.e. their greatest common divisor is 1, the length of the denominators just adds up. The effect also happens when the gcd is very small, such as 2 or 3. This can happen quite a lot with huge numbers in a sufficiently large range. So when things go bad for your input data, the length of the denominator just keeps growing linearly with the number of input values, making each successive addition slower and slower. Your rationals just exploded.

Conclusion

After this, it became apparent that using rationals was not a great idea after all. You should be very careful when doing series of additions with them. Ironically, we were throwing away all the precision anyways before presenting the number to a user. There’s no way for anyone to grok a 26000 digit number anyways, especially if the result is basically 4000.xx. I learned my lesson and buried the dream of perfect arithmetic. I’m now using fixed point arithmetic instead.

The migration path for Java applets

Java applets have been a deprecated technology for a while. It has become increasingly difficult to run Java applets in modern browsers. Browser vendors are phasing out support for browser plugins based on the Netscape Plugin API (NPAPI) such as the Java plugin, which is required to run applets in the browser. Microsoft Edge and Google Chrome already have stopped supporting NPAPI plugins, Mozilla Firefox will stop supporting it at the end of 2016. This effectively means that Java applets will be dead by the end of the year.

However, it does not necessarily mean that Java applet based projects and code bases, which can’t be easily rewritten to use other technologies such as HTML5, have to become useless. Oracle recommends the migration of Java applets to Java Web Start applications. This is a viable option if the application is not required to be run embedded within a web page.

To convert an applet to a standalone Web Start application you put the applet’s content into a frame and call the code of the former applet’s init() and start() from the main() method of the main class.

Java Web Start applications are launched via the Java Network Launching Protocol (JNLP). This involves the deployment of an XML file on the web server with a “.jnlp” file extension and content type “application/x-java-jnlp-file”. This file can be linked on a web page and looks like this:

<?xml version="1.0" encoding="UTF-8"?>
<jnlp spec="1.0+" codebase="http://example.org/demo/" href="demoproject.jnlp">
 <information>
  <title>Webstart Demo Project</title>
  <vendor>Softwareschneiderei GmbH</vendor>
  <homepage>http://www.softwareschneiderei.de</homepage>
  <offline-allowed/>
 </information>
 <resources>
  <j2se version="1.7+" href="http://java.sun.com/products/autodl/j2se"/>
  <jar href="demo-project.jar" main="true"/>
  <jar href="additional.jar"/>
 </resources>
 <application-desc name="My Project" main-class="com.schneide.demo.WebStartMain">
 </application-desc>
 <security>
  <all-permissions/>
 </security>
 <update check="always"/>
</jnlp>

The JNLP file describes among other things the required dependencies such as the JAR files in the resources block, the main class and the security permissions.

The JAR files must be placed relative to the URL of the “codebase” attribute. If the application requires all permissions like in the example above, all JAR files listed in the resources block must be signed with a certificate.

deployJava.js

The JNLP file can be linked directly in a web page and the Web Start application will be launched if Java is installed on the client operating system. Additionally, Oracle provides a JavaScript API called deployJava.js, which can be used to detect whether the required version of Java is installed on the client system and redirect the user to Oracle’s Java download page if not. It can even create an all-in-one launch button with a custom icon:

<script src="https://www.java.com/js/deployJava.js"></script>
<script>
  deployJava.launchButtonPNG = 'http://example.org/demo/launch.png';
  deployJava.createWebStartLaunchButton('http://example.org/demo/demoproject.jnlp', '1.7.0');
</script>

Conclusion

The Java applet technology has now reached its “end of life”, but valuable applet-based projects can be saved with relatively little effort by converting them to Web Start applications.

Getting Shibboleth SSO attributes securely to your application

Accounts and user data are a matter of trust. Single sign-on (SSO) can improve the user experience (UX), convenience and security especially if you are offering several web applications often used by the same user. If you do not want to force your users to big vendors offering SSO like google or facebook or do not trust them you can implement SSO for your offerings with open-source software (OSS) like shibboleth. With shibboleth it may be even feasible to join an existing federation like SWITCH, DFN or InCommon thus enabling logins for thousands of users without creating new accounts and login data.

If you are implementing you SSO with shibboleth you usually have to enable your web applications to deal with shibboleth attributes. Shibboleth attributes are information about the authenticated user provided by the SSO infrastructure, e.g. the apache web server and mod_shib in conjunction with associated identity providers (IDP). In general there are two options for access of these attributes:

  1. HTTP request headers
  2. Request environment variables (not to confuse with system environment variables!)

Using request headers should be avoided as it is less secure and prone to spoofing. Access to the request environment depends on the framework your web application is using.

Shibboleth attributes in Java Servlet-based apps

In Java Servlet-based applications like Grails or Java EE access to the shibboleth attributes is really easy as they are provided as request attributes. So simply calling request.getAttribute("AJP_eppn") will provide you the value of the eppn (“EduPrincipalPersonName”) attribute set by shibboleth if a user is authenticated and the attribute is made available. There are 2 caveats though:

  1. Request attributes are prefixed by default with AJP_ if you are using mod_proxy_ajp to connect apache with your servlet container.
  2. Shibboleth attributes are not contained in request.getAttributeNames()! You have to directly access them knowing their name.

Shibboleth attributes in WSGI-based apps

If you are using a WSGI-compatible python web framework for your application you can get the shibboleth attributes from the wsgi.environ dictionary that is part of the request. In CherryPy for example you can use the following code to obtain the eppn:

eppn = cherrypy.request.wsgi_environ['eppn']

I did not find the name of the WSGI environment dictionary clearly documented in my efforts to make shibboleth work with my CherryPy application but after that everything was a bliss.

Conclusion

Accessing shibboleth attributes in a safe manner is straightforward in web environments like Java servlets and Python WSGI applications. Nevertheless, you have to know the above aspects regarding naming and visibility or you will be puzzled by the behaviour of the shibboleth service provider.

Every time you write a getter, a function dies

This blog post explores the difference between classic and Tell, don’t Ask-style code without any sourcecode examples.

Don’t be too alarmed by the title. Functions are immortal concepts and there’s nothing wrong with a getter method. Except when you write code under the rules of the Object Calisthenics (rule number 9 directly forbids getter and setter methods). Or when you try to adhere to the ideal of encapsulation, a cornerstone of object-oriented programming. Or when your code would really benefit from some other design choices. So, most of the time, basically. Nobody dies if you write a getter method, but you should make a concious decision for it, not just write it out of old habit.

One thing the Object Calisthenics can teach you is the immediate effect of different design choices. The rules are strict enough to place a lot of burden on your programming, so you’ll feel the pain of every trade-off. In most of your day-to-day programming, you also make the decisions, but don’t feel the consequences right away, so you get used to certain patterns (habits) that work well for the moment and might or might not work in the long run. You should have an alternative right at hands for every pattern you use. Otherwise, it’s not a pattern, it’s a trap.

Some alternatives

Here is an incomplete list of common alternatives to common patterns or structures that you might already be aware of:

  • if-else statement (explicit conditional): You can replace most explicit conditionals with implicit ones. In object-oriented programming, calling polymorphic methods is a common alternative. Instead of writing if and else, you call a method that is overwritten in two different fashions. A polymorphic method call can be seen as an implicit switch-case over the object type.
  • else statement: In the Object Calisthenics, rule 2 directly forbids the usage of else. A common alternative is an early return in the then-block. This might require you to extract the if-statement to its own method, but that’s probably a good idea anyway.
  • for-loop: One of the basic building blocks of every higher-level programming language are loops. These explicit iterations are so common that most programmers forget their implicit counterpart. Yeah, I’m talking about recursion here. You can replace every explicit loop by an implicit loop using recursion and vice versa. Your only limit is the size of your stack – if you are bound to one. Recursion is an early brain-teaser in every computer science curriculum, but not part of the average programmer’s toolbox. I’m not sure if that’s a bad thing, but its an alternative nonetheless.
  • setter method: The first and foremost alternative to a state-altering operation are immutable objects. You can’t alter the state of an immutable, so you have to create a series of edited copies. Syntactic sugar like fluent interfaces fit perfectly in this scenario. You can probably imagine that you’ll need to change the whole code dealing with the immutables, but you’ll be surprised how simple things can be once you let go of mutable state, bad conscience about “wasteful” heap usage and any premature thought about “performance”.

Keep in mind that most alternatives aren’t really “better”, they are just different. There is no silver bullet, every approach has its own advantages and drawbacks, both shortterm and in the long run. Your job as a competent programmer is to choose the right approach for each situation. You should make a deliberate choice and probably document your rationale somewhere (a project-related blog, wiki or issue tracker comes to mind). To be able to make that choice, you need to know about the pros and cons of as much alternatives as you can handle. The two lamest rationales are “I’ve always done it this way” and “I don’t know any other way”.

An alternative for get

In this blog post, you’ll learn one possible alternative to getter methods. It might not be the best or even fitting for your specific task, but it’s worth evaluating. The underlying principle is called “Tell, don’t Ask”. You convert the getter (aka asking the object about some value) to a method that applies a function on the value (aka telling the object to work with the value). But what does “applying” mean and what’s a function?

191px-Function_machine2.svgA function is defined as a conversion of some input into some output, preferably without any side-effects. We might also call it a mapping, because we map every possible input to a certain output. In programming, every method that takes a parameter (or several of them) and returns something (isn’t void) can be viewed as a function as long as the internal state of the method’s object isn’t modified. So you’ve probably programmed a lot of functions already, most of the time without realizing it.

In Java 8 or other modern object-oriented programming languages, the notion of functions are important parts of the toolbox. But you can work with functions in Java since the earliest days, just not as convenient. Let’s talk about an example. I won’t use any code you can look at, so you’ll have to use your imagination for this. So you have a collection of student objects (imagine a group of students standing around). We want to print a list of all these students onto the console. Each student object can say its name and matriculation number if asked by plain old getters. Damn! Somebody already made the design choice for us that these are our duties:

  • Iterate over all student objects in our collection. (If you don’t want to use a loop for this you know an alternative!)
  • Ask each student object about its name and matriculation number.
  • Carry the data over to the console object and tell the console to print both informations.

But because this is only in our imagination, we can go back in imagined time and eliminate the imagined choice for getters. We want to write our student objects without getters, so let’s get rid of them! Instead, each student object knows about their name and matriculation number, but cannot be asked directly. But you can tell the student object to supply these informations to the only (or a specific) method of an object that you give to it. Read the previous sentence again (if you’ve not already done it). That’s the whole trick. Our “function” is an object with only one method that happens to have exactly the parameters that can be provided by the student object. This method might return a formatted string that we can take to the console object or it might use the console itself (this would result in no return value and a side effect, but why not?).  We create this function object and tell each student object to use it. We don’t ask the student object for data, we tell it to do work (Tell, don’t Ask).

In this example, the result is the same. But our first approach centers the action around our “main” algorithm by gathering all the data and then acting on it. We don’t feel pain using this approach, but we were forced to use it by the absence of a function-accepting method and the presence of getters on the student objects. Our second approach prepares the action by creating the function object and then delegates the work to the objects holding the data. We were able to use it because of the presence of a function-accepting method on the student objects. The absence of getters in the second approach is a by-product, they simply aren’t necessary anymore. Why write getters that nobody uses?

We can observe the following characteristics: In a “traditional”, imperative style with getters, the data flows (gets asked) and the functionality stays in place. In a Tell, don’t Ask style with functions, the data tends to stay in place while the functionality gets passed around (“flows”).

Weighing the options

This is just one other alternative to the common “imperative getter” style. As stated, it isn’t “better”, just maybe better suited for a particular situation. In my opinion, the “functional operation” style is not straight-forward and doesn’t pay off immediately, but can be very rewarding in the long run. It opens the door to a whole paradigm of writing sourcecode that can reveal inherent or underlying concepts in your solution domain a lot clearer than the imperative style. By eliminating the getter methods, you force this paradigm on your readers and fellow developers. But maybe you don’t really need to get rid of the getters, just reduce their usage to the hard cases.

So the title of this blog post is a bit misleading. Every time you write a getter, you’ve probably considered all alternatives and made the informed decision that a getter method is the best way forward. Every time you want to change that decision afterwards, you can add the function-accepting method right alongside the getters. No need to be pure or exclusive, just use the best of two worlds. Just don’t let the functions die (or never be born) because you “didn’t know about them” or found the style “unfamiliar”. Those are mere temporary problems. And one of them is solved right now. Happy coding!

Recap of the Schneide Dev Brunch 2015-12-13

If you couldn’t attend the Schneide Dev Brunch at 13th of December 2015, here is a summary of the main topics.

brunch64-borderedTwo weeks ago, we held another Schneide Dev Brunch, a regular brunch on the second sunday of every other (even) month, only that all attendees want to talk about software development and various other topics. So if you bring a software-related topic along with your food, everyone has something to share. The brunch was small this time, but with enough stuff to talk about. As usual, a lot of topics and chatter were exchanged. This recapitulation tries to highlight the main topics of the brunch, but cannot reiterate everything that was spoken. If you were there, you probably find this list inconclusive:

Company strategies

Our first topic was about the changes that happen in company culture once a certain threshold is overstepped. The founders lose touch with their own groundwork and then with their own employees. Compliance frameworks are installed and then enforced, even if the rules make no sense in specific cases. A new hierarchy layer, the middle management, springs into existence and is populated by people that never worked on the topic but make all the decisions. The brightest engineers are promoted to a management position and find themselves helpless and overburdened. Adopting a new technology or tool takes forever now. The whole company stalls technologically.

Sounds familiar? We discussed several cases of this dramaturgy and some ways around it. One possible remedy is to never grow big enough. Stay small, stay fast and stay agile. That’s the Schneide way.

Code analysis with jDeodorant

We devoted a lot of time on getting to know jDeodorant, an eclipse-based code smell detection tool for Java. We grabbed a real project and analyzed it with the tool. Well, this step alone took its time, because the plugin cannot be operated in an intuitive manner. It presents itself as a collection of student thesis work without overarching narrative and a clear disregard of expectation conformity. If several experienced eclipse users cannot figure out how a tool works despite having used similar tools for years, something is afoul. We got past the bad user experience by viewing several screencasts, the most noteworthy being a plain feature demonstration.

Once you figure out the handling, the tool helps you to find code smells or refactoring opportunities. In our case, most of the findings were false alarms or overly picky. But in two cases, the tool provided a clear hint on how to make the code better (both being feature envies). If the project would really benefit from the proposed refactorings is subject for discussion. The tool acts like a very assiduous colleague in a code review when every improvement gets rewarded.

We really don’t know how to rate this tool. It’s hard to learn and provides little value on first sight, but might be useful on larger legacy code bases. We’ll keep it at the back of our minds.

Naming and syntax rules

During the discussion about jDeodorant, we talked about naming schemes and other syntax rules. We remembered horrific conventions like prefixed I and E or suffixed Exception. The last one got some curious looks, because it’s still a convention in the Java SDK and some names won’t make it without, like the beloved IOException. But what about the NullPointerException? Wouldn’t NullPointer describe the problem just as good? Kevlin Henney already talked about this and other ineffective coding habits (if you have audio degradation halfway through, try another video of the talk). It’s a good eye-opener to (some of) the habits we’ve adopted without questioning them. But challenging the status quo is a good thing if done in reasonable doses and with a constructive attitude.

Unit testing

When we played around with jDeodorant and surfed the code of the project that served as our testing ground, the Infinitest widget raised some questions. So we talked about Continuous Testing, unit tests and some pitfalls if your tests aren’t blazing fast. The eclipse plugin for MoreUnit was mentioned soon. Those two plugins really make a difference in working with tests. Especially the unannounced shortcut Ctrl+J is very helpful. I’ve even blogged about the topic back in 2011.

Epilogue

As usual, the Dev Brunch contained a lot more chatter and talk than listed here. The number of attendees makes for an unique experience every time. We are looking forward to the next Dev Brunch at the Softwareschneiderei. And as always, we are open for guests and future regulars. Just drop us a notice and we’ll invite you over next time.

What’s your time, database?

Time is a difficult subject. Especially time zones and daylight saving time. Adding layers makes things worse. Ask your database.

Time is a difficult subject. Especially time zones and daylight saving time. Sounds easy? Well, take a look.
Adding layers in software development complicates the issue and every layer has its own view of time. Let’s start with an example: we write a simple application which stores time based data in a SQL database, e.g. Oracle. The table has a column named ‘at’. Since we don’t want to mess around with timezones, we use a column type without timezone information, in Oracle this would be ‘Date’ if we do not need milliseconds and ‘Timestamp’ if we need them. In Java with plain JDBC we can extract it with a call to ‘getTimestamp’:

Date timestamp = resultSet.getTimestamp("at");

The problem is now we have a timestamp in our local timezone. Where is it converted? Oracle itself has two timezone settings: for the database and for the session. We can query them with:

select DBTIMEZONE from dual;

and

select SESSIONTIMEZONE from dual;

First Oracle uses the time zone set in the session, then the database one. The results from those queries are interesting though: some return a named timezone like ‘Europe/Berlin’, the other return an offset ‘+01:00’. Here a first subtle detail is important: the named timezone uses the offset and the daylight saving time from the respective timezone, the offset setting only uses the offset and no daylight saving. So ‘+01:00’ would just add 1 hour to UTC regardless of the date.
In our example changing these two settings does not change our time conversion. The timezone settings are for another column type: timestamp with (local) timezone.
Going up one layer the JDBC API reveals an interesting tidbit:

Timestamp getTimestamp(int columnIndex)
throws SQLException

Retrieves the value of the designated column in the current row of this ResultSet object as a java.sql.Timestamp object in the Java programming language.

Sounds about right, but wait there’s another method:

Timestamp getTimestamp(int columnIndex,
Calendar cal)
throws SQLException


Retrieves the value of the designated column in the current row of this ResultSet object as a java.sql.Timestamp object in the Java programming language. This method uses the given calendar to construct an appropriate millisecond value for the timestamp if the underlying database does not store timezone information.

Just as in Oracle we can use a named timezone or an offset:

Date timestamp = resultSet.getTimestamp("at", Calendar.getInstance(TimeZone.getTimeZone("GMT+1:00")));

This way we have control over what and how the time is extracted from the database. The next time you work with time based information take a close look. And if you work with Java use Joda Time.

Declaration-site and use-site variance explained

A common question posed by programming novices who have their first encounters with parametrized types (“generics” in Java and C#) is “Why can’t I use a List<Apple> as a List<Fruit>?” (given that Apple is a subclass of Fruit) Their reasoning usually goes like this: “An apple is a fruit, so a basket of apples is a fruit basket, right?”

Here’s another, similar, example:

Milk is a dairy product, but is a bottle of milk a dairy product bottle? Try putting a Cheddar cheese wheel into the milk bottle (without melting or shredding the cheese!). It’s obviously not that simple.

Let’s assume for a moment that it was possible to use a List<Apple> as a List<Fruit>. Then the following code would be legal, given that Orange is a subclass of Fruit as well:

List<Apple> apples = new ArrayList<>();
List<Fruit> fruits = apples;
fruits.add(new Orange());

// what's an orange doing here?!
Apple apple = apples.get(0);

This short code example demonstrates why it doesn’t make sense to treat a List<Apple> as a List<Fruit>. That’s why generic types in Java and C# don’t allow this kind of assignment by default. This behaviour is called invariance.

Variance of generic types

There are, however, other cases of generic types where assignments like this actually could make sense. For example, using an Iterable<Apple> as an Iterable<Fruit> is a reasonable wish. The opposite direction within the inheritance hierarchy of the type parameter is thinkable as well, e.g. using a Comparable<Fruit> as a Comparable<Apple>.

So what’s the difference between these generic types: List<T>, Iterable<T>, Comparable<T>? The difference is the “flow” direction of objects of type T in their interface:

  1. If a generic interface has only methods that return objects of type T, but don’t consume objects of type T, then assignment from a variable of Type<B> to a variable of Type<A> can make sense. This is called covariance. Examples are: Iterable<T>, Iterator<T>, Supplier<T>inheritance
  2. If a generic interface has only methods that consume objects of type T, but don’t return objects of type T, then assignment from a variable of Type<A> to a variable of Type<B> can make sense. This is called contravariance. Examples are: Comparable<T>, Consumer<T>
  3. If a generic interface has both methods that return and methods that consume objects of type T then it should be invariant. Examples are: List<T>, Set<T>

As mentioned before, neither Java nor C# allow covariance or contravariance for generic types by default. They’re invariant by default. But there are ways and means in both languages to achieve co- and contravariance.

Declaration-site variance

In C# you can use the in and out keywords on a type parameter to indicate variance:

interface IProducer<out T> // Covariant
{
    T produce();
}

interface IConsumer<in T> // Contravariant
{
    void consume(T t);
}

IProducer<B> producerOfB = /*...*/;
IProducer<A> producerOfA = producerOfB;  // now legal
// producerOfB = producerOfA;  // still illegal

IConsumer<A> consumerOfA = /*...*/;
IConsumer<B> consumerOfB = consumerOfA;  // now legal
// consumerOfA = consumerOfB;  // still illegal

This annotation style is called declaration-site variance, because the type parameter is annotated where the generic type is declared.

Use-site variance

In Java you can express co- and contravariance with wildcards like <? extends A> and <? super B>.

Producer<B> producerOfB = /*...*/;
Producer<? extends A> producerOfA = producerOfB; // legal
A a = producerOfA.produce();
// producerOfB = producerOfA; // still illegal

Consumer<A> consumerOfA = /*...*/;
Consumer<? super B> consumerOfB = consumerOfA; // legal
consumerOfB.consume(new B());
// consumerOfA = consumerOfB; // still illegal

This is called use-site variance, because the annotation is not placed where the type is declared, but where the type is used.

Arrays

The variance behaviour of Java and C# arrays is different from the variance behaviour of their generics. Arrays are covariant, not invariant, even though a T[] has the same problem as a List<T> would have if it was covariant:

Apple[] apples = new Apple[10];
Fruit[] fruits = apples;
fruits[0] = new Orange();
Apple apple = apples[0];

Unfortunately, this code compiles in both languages. However, it throws an exception at runtime (ArrayStoreException or ArrayTypeMismatchException, respectively) in line 3.

C++ inheritance for Java developers

Both Java and C++ are modern programming languages with native support for object oriented programming (OOP). While similar in syntax and features there are a bunch of differences in implementation and (default) behaviour which can be surprising for Java developers learning C++ or vice versa. In my post I will depict the basics of inheritance and polymorphism in C++ and stress the points Java developers may find surprising.

Basic inheritance or user defined types

Every Java programmer knows that all classes have java.lang.Object at the root of their inheritance hierarchy. This is not true for C++ classes: They do not share some common base class and do not inherit more or less useful methods like toString() oder clone(). C++ classes do not have any member functions besides the constructor and destructor by default. In Java all instance methods are virtual by default, meaning that if a subclass overrides a method from a superclass only the overridden version will be visible and callable on all instances of the subclass. We will see the different behaviour in the following example:

#include <iostream>
#include <memory>

using namespace std;

class Parent
{
public:
  void myName() { cout << "Parent\n"; }
  virtual void morph() { cout << "Base class\n"; }
};

class Child : public Parent
{
public:
  void myName() { cout << "Child\n"; }
  virtual void morph() { cout << "Child class\n"; }
};

int main()
{
  // initialisations more or less equivalent to
  // Parent parent = new Parent(); etc. in Java
  unique_ptr parent(new Parent);
  unique_ptr parentPointerToChild(new Child);
  unique_ptr child(new Child);
  parent->myName(); // prints Parent as expected
  parent->morph(); // prints Base class as expected
  parentPointerToChild->myName(); // surprise: prints Parent!
  parentPointerToChild->morph(); // prints Child class as expected
  child->myName(); // prints Child as expected
  child->morph(); // prints Child class as expected
  return 0;
}

The difference to Java becomes visible in line 29 where we call to an instance of Child using the type Parent. Since myName() is not virtual the implementation in the parent class is not overridden but only shadowed by the subclass. Depending on the type of our variable on which we call the method either the parent method or the child method (line 31) is invoked.

Access modifiers in Java and C++ are almost identical as both are offering public, protected and private. As there are no packages in C++ protected restricts access to child classes only.

Different syntax and fewer keywords

There are no keywords for interface or abstract class in C++ but the concepts are supported by pure virtual functions and multiple inheritance. So a class that defines a virtual function without a body like:


class Interface
{
public:
  virtual void m() = 0;
};

becomes abstract and cannot be instanciated. Subclasses must provide implementations for all pure virtual functions or become abstract themselves. A class having exclusively pure virtual functions serves as the equivalent of an Java interface in C++. Since C++ supports inheritance from multiple classes you can easily use these features for interface segregation (see ISP). Most people advise against multiple implementation inheritance although C++ makes that possible, too.

Class interface control

One problem with inheritance in Java is that you always inherit all non-private methods of the base classes. This can lead to large and sometimes unfocused interfaces when using inheritance blindly. Delegation gives you more control about your classes interface and means less coupling but requires a bit more writing effort. While you can (and should most of the time) do the same in C++ it offers more control to your classes interface using private/protected inheritance where you can decide on an per-method-basis which functions to expose and what virtual functions you would like to override:

class ReuseMe
{
public:
    void somethingGreat();
    void somethingSpecial();
};

// does expose somethingGreat() but hides sometingSpecial()
class PrivateParent : private ReuseMe
{
public:
    using ReuseMe::somethingGreat;
};

Children also gain access to their parents protected members.

Using super class functions and delegating constructors

Calling the functions of the parent class or other constructors looks a bit different in C++, as there is no super keyword and this is only a pointer to the current instance (please excuse the contrived example…):

class CoordinateSet
{
public:
    string toString() {
        stringstream s;
        std::copy(coordinates.begin(), coordinates.end(), std::ostream_iterator<double>(s, ","));
        return s.str();
    }
protected:
    vector<double> coordinates;
};

class Point : public CoordinateSet
{
public:
    // delegating constructor
    Point() : Point(0, 0) {}
    Point(double x, double y)
    {
        coordinates.push_back(x);
        coordinates.push_back(y);
    }
    // call a function of the parent class
    void print() { cout << CoordinateSet::toString() << "\n"; }
};

I hope my little guide helps some Java people getting hold of C++ inheritance features and syntax.

How I find the source of bugs

You know the situation: a user calls or emails you to tell you your program has a problem. Here I illustrate how I find the source of the problem

You know the situation: a user calls or emails you to tell you your program has a problem. When you are lucky he lists some steps he believe he did to reproduce the behaviour. When you are really lucky those steps are the right ones.
In some cases you even got a stacktrace on the logs. High fives all around. You follow the steps the problem shows and you get the exact position in the code where things get wrong. Now is a great time to write a test which executes the steps and shows the bug. If the bug isn’t data dependent you normally can nail it with your test. If it is dependent on the data in the production system you have to find the minimal set of data constraints which causes the problem. The test fails, fixing it should make it green and fix the problem. Done.
But there are cases where the problem is caused not in the last action but sometime before. If the data does not reflect that the problem is buried in layers between like caches, in memory structures or the particular state the system is in.
Here knowledge of the frameworks used or the system in question helps you to trace back the flow of the state and data coming to the position of the stack trace.
Sometimes the steps do not reproduce the behaviour. But most of the time the steps are an indicator for how to reproduce the problem. The stack trace should give you enough information. If it doesn’t, take a look in your log. If this also does not show enough info to find the steps you should improve your log around the position of the strack trace. Wait for the next instance or try your own luck and then you should have enough information to find the real problem.
But what if you have no stack trace? No position to start your hunt? No steps to reproduce? Just a message like: after some days I got an empty transmission happening every minute. After some days. No stack trace. No user actions. Just a log message that says: starting transmission. No error. No further info.
You got nothing. Almost. You got a message with three bits of info: transmission, every minute and empty.
I start with transmission. Where does the system transmit data. Good for the architecture but bad for tracing the transmission is decoupled from the rest of the system by using a message bus. Next.
Every minute. How does the system normally start recurring processes? By quartz, a scheduler for Java. Looking at the configuration of the live system no process is nearly triggered every minute. There must be another place. Searching the code and the log another message indicates a running process: a watchdog. This watchdog puts a message on the bus if it detects a problem. Which is then send by the transmission process. Bingo. But why is it empty?
Now the knowledge about the facilities the system uses comes into play: UMTS. Sometimes the transmission rate is so low that the connection does not transfer any packages. The receiving side records a transmission but gets no data.
Most of the time the problem can be found in your own code but all code has bugs. If you assume after looking at your code that the frameworks you use have a bug. Search the bug database of the framework hopefully it is found there and already fixed.