Java Threads – an Introduction for all Skill Levels

Today we are talking about threads in Java. Threads are an essential means to implement concurrent algorithms and are therefore a key part of efficient everyday data processing. Imagine having a webserver without threads serving only one customer at a time: Every customer would need to pull a number for every single web request they make and our online shop would have a bad time.

So let’s head into it!

In modern days – Java threads with Lambda expressions

Since Java 8 we have been blessed with Lambda expression. Basically Lambda expressions serve the purpose of unnamed functions and are written as shown here:

(param1, param2, [...], paramN) -> {
    statement1; 
    statement2;
    [...]
    statementN;
}

If you just need one statement, it looks even better:

(param1, [...], paramN) -> statement

By harnessing their power we are able to write beautiful first level function style concurrent algorithms.

Let’s assume we want to sort a list with an even number of integers. We know that all ints in the second half of the list are greater than the greates number in the first half. Then we can do this:

import java.util.*;

public class ThreadExampleMain {

    public static void main(String[] args) throws InterruptedException {
        /* Elements of the 2nd half are larger than in the 1st half, otherwise unsorted. */
        List<Integer> test = Arrays.asList(3, 1, 5, 6, 21, 12, 18, 16);
        List<Integer> sortedFirstHalf = test.subList(0, 4);
        List<Integer> sortedSecondHalf = test.subList(4, 8);

        Thread t1 = new Thread(() -> {
            Collections.sort(sortedFirstHalf); // or any other more elaborate sorting algorithm
            System.out.println("First half sorted: " + sortedFirstHalf);
        });

        Thread t2 = new Thread(() -> {
            Collections.sort(sortedSecondHalf);
            System.out.println("Second half sorted: " + sortedSecondHalf);
        });

        t1.start();
        t2.start();
        t1.join();
        t2.join();

        // Merge the sorted list parts together from left to right...
        List<Integer> result = new ArrayList<>();
        result.addAll(sortedFirstHalf);
        result.addAll(sortedSecondHalf);

        // ...and show the result.
        System.out.println("Result: " + result);
    }

}

Try it out!

Notice that we feed the threads in our Java code with Lambdas to tell them what to do. Neat, isn’t it?

Legacy Java Threads with Runnable instances

And how has it been done in the past? When I started programming with Java around 5 and 6 we had to declare a full-blown Runnable-instance that we subsequently fed to the thread. Runnable is an interface type which means that during declaration we also had to implement it’s signature method run(). This method finally contains the functionality that the thread is supposed to run. Let me show what I mean:

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class ThreadExampleLegacyMain {

    public static void main(String[] args) throws InterruptedException {
        /* Elements of the 2nd half are larger than in the 1st half, otherwise unsorted. */
        List<Integer> test = Arrays.asList(3, 1, 5, 6, 21, 12, 18, 16);
        List<Integer> sortedFirstHalf = test.subList(0, 4);
        List<Integer> sortedSecondHalf = test.subList(4, 8);

        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                Collections.sort(sortedFirstHalf); // or any other more elaborate sorting algorithm
                System.out.println("First half sorted: " + sortedFirstHalf);
            }
        });

        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                Collections.sort(sortedSecondHalf);
                System.out.println("Second half sorted: " + sortedSecondHalf);
            }
        });

        t1.start();
        t2.start();
        t1.join();
        t2.join();

        // Merge the sorted list parts together from left to right...
        List<Integer> result = new ArrayList<>();
        result.addAll(sortedFirstHalf);
        result.addAll(sortedSecondHalf);

        // ...and show the result.
        System.out.println("Result: " + result);
    }

}

Try it out!

It was a bloated mess, but since it’s likely in use in modern day code, we got to list it here.

Conclusion

So long. I hope I could help you effectively processing data by leveraging the power of Java threads. Please note that the examples shown above are optimized for illustration of the topic only. Your way to sort a list of integers is certainly much better. If you’d like to read more about me doing Java, I have a post for you, where I talk about web test automation with Java. If you are already familiar with that and want to try something new, I recommend trying fuzz testing in Java. You will gain a fresh view on test automation and honestly it’s a blast to bomb an application with just random stuff.

Last but not least, if you want to dive in really deep into the world of Java threads, check out this article about Thread Pooling. It is quite advanced, but intuitive and powerful once you get the hang of it.

You have more questions regarding Java, me or anything else? As usual, feel free to drop me a line down below. I read every single one of your comments. Have a great day!

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Character array is missing „e“ notation exponential mark

I observed that many people out there visiting my blog have great interest in a particular NumberFormatException. It goes by the message Character array is missing „e“ notation exponential mark and was mentioned in my post about fuzzing in Java.

Today we will take a more in depth look at it.

How does that NumberFormatException look like?

To visualize the error effectively, let’s look at the following extreme example drawn from my fuzzing post mentioned above.

Consider the following (sub-par implemented) function dollar2euro that takes any input (hopefully it’s a number though!) and tries to convert it from USD to EUR:

public String dollar2euro(Object input){
    BigDecimal inputParsed = new BigDecimal(input.toString());
    BigDecimal dollars = inputParsed.setScale(2, BigDecimal.ROUND_HALF_EVEN);

    BigDecimal multiply = dollars.multiply(BigDecimal.valueOf(0.92));
    BigDecimal euros = multiply.setScale(2, BigDecimal.ROUND_HALF_EVEN);
    return String.valueOf(euros);
}

Now what happens, if we play along the method signature and put in the following characters: „뤇皽“? If you know what they mean, feel free to drop me a comment down below. Please note that this is exactly what my fuzztest tried to do. And of course it drops a heavy NumberFormatException upon us:

Input: 뤇皽
java.lang.NumberFormatException: Character 뤇 is neither a decimal digit number, decimal point, nor "e" notation exponential mark.

And what does it mean?

The problem here is that „뤇皽“ is not a number. The first and second part Character 뤇 is neither a decimal digit number, decimal point are straight forward. But what does the third part nor „e“ notation exponential mark mean?

In maths, we have the option of expressing numbers in power form with base 10. This comes in handy when we want to express Googol (the number that inspired Google’s name) – a 1 with 100 zeroes. Accordingly we would write 10100 instead. In Java you can do something very similar, but there are better sources to check how to properly use the e-format. For our context let’s acknowledge that it is a different way of displaying numbers. Which likewise could not be found in our input.

But how do we fix that?

We have to make sure that we provide a String to the BigDecimal Constructor that looks like a valid number – if we want to provide a String at all. An actual number like a double or int would be even better. That would cost us no more than a change in the method’s signature and a small adaption to line 2. If we really want to provide a String, we still can do that, but then we have to make sure that it is properly formatted. A valid input example would be: „1337.012342„.

If you apply this simple rule, you should be spared from ‚Character array is missing „e“ notation exponential mark‘ errors in the future.

Conclusion

So long! I hope this little post gave you an idea about what the error message ‚Character array is missing „e“ notation exponential mark‘ means. Of course this case was kinda constructed and you probably have a less explicit case. If so, feel free to post it in the comments down below and we see what we can do. But for demonstration purposes it should have provided a clear image of what is going on in your program.

As my linked post about fuzzing in Java implies, I’m an avid test automation person. If you want more about automating tests with Java, check out my introduction tutorial about Selenium in Java. If you are more of the Python person, no problem. Here’s the same Selenium tutorial in Python. And if Python and Java are both too mainstream for you, I recommend my tutorial about Cucumber in Rust .

Happy test automating & have a nice day!

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Introduction to Web Test Automation – Java Edition

The internet is an incredible market place hosting millions and millions of software products to make our lives indefinitely easier. To keep up with each other, software grew more and more elaborate to provide us customers with lots of different features and services to finally make us purchase a thing or two. Now due to the resulting increase of complexity we must take special care not to break existing features while developing the next big version brimming full of features. Testing that the next code increment does not break existing stuff is what we testers call „regression testing“, and there is one especially economical way of doing that: test automation.

That’s what we’re going to do today. We gonna test and we gonna automate and we do all that in Java this time. Because everyone knows about Java, right? So let’s write once and run everywhere. Given we have a browser there.

Shopping list

To start our test automation journey – again -, we will work with the following tools. The versions are the ones I used at the time of writing. I might update them in the future.

After downloading and installing these tools from their respective web sites, we’re ready to start our system under test.

Deploying the system under test

To achieve this, we simply run the following command:

docker run --name spree --rm -d -p 3000:3000 spreecommerce/spree:3.6.4

If it went well, a quick glance at http://localhost:3000/ should show a catalog page full of cute Rails merch. Now we will install and prepare Selenium.

Geckodriver installation and management

Contrary to the Python version, we use WebDriverManager to free us from the installation hassle of Geckodriver, hence we are done with this step. The tip was sent to me by my good friend Sho. Thank you, mate. I owe you one! 🙂

Initializing our test automation project

Given a successful Gradle installation as linked in the shopping list we can now create our test project. Please open a terminal in a root directory of your choice and do:

mkdir jata_tutorial && cd jata_tutorial
gradle init

When prompted answer with 1 for a basic project and then 2 for Kotlin as our Gradle DSL.

Of course we will use the new APIs as well, since we love to live on the edge here. Please choose „yes“ here.

Finally, when asked, give your project the default name that is the project root directory’s name. Now your boilerplate project is set up and ready to go.

Now it’s time to define our dependencies. In build.gradle.kts please introduce the following lines:

plugins {
    java
}

group = "your.groupid"
version = "1.0-SNAPSHOT"

repositories {
    mavenCentral()
}

dependencies {
    implementation("org.slf4j:slf4j-api")
    implementation("ch.qos.logback:logback-classic:1.4.5")
    implementation("org.apache.commons:commons-lang3:3.12.0")

    testImplementation("org.seleniumhq.selenium:selenium-java:4.7.2")
    testImplementation("io.github.bonigarcia:webdrivermanager:5.3.1")
    testImplementation(platform("org.junit:junit-bom:5.9.1"))
    testImplementation("org.junit.jupiter:junit-jupiter:5.9.1")
}

tasks.getByName<Test>("test") {
    useJUnitPlatform()
}

This sets up our project with some basic metadata like our groupId and a (somewhat) meaningful version. We defined to use Java as our JVM language and JUnit as our test runner. Additionally we will have access to Maven’s central repository, where Gradle can download our project dependencies that we defined in the section of that exact same name.

With these dependencies, we are able to:

  • automagically download and install webdriver executables for Selenium (in our case: Geckodriver)
  • perform Selenium actions including opening and closing browser windows, enter URLs and clicking stuffs
  • generate random test input strings
  • log things to the console in a nice timestamped format (without being scared of Log4Shell)
  • and do all that in a modern JUnit5 – runner

Everything that we need to do very soon.

The boilerplate

Next up, we create the skeleton of our first test class. In src/test/java, please create a file named CrossTests.java containing the following code. Please note that .java source files should be named after the contained class. Here: CrossTests.

import org.junit.jupiter.api.AfterEach;
import org.junit.jupiter.api.BeforeAll;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;
import org.openqa.selenium.WebDriver;

class CrossTests {

    WebDriver driver;

    @BeforeAll
    static void setupClass() {
        
    }

    @BeforeEach
    void setup() {

    }

    @AfterEach
    void teardown() {

    }

    @Test
    void isOpen() {

    }
}

Great. That looks almost like a real test already. Finally we are going to open a browser window.

Starting and closing the browser

In setupClass we call the WebDriverManager and let it work its Geckodriver-initializing magic.

@BeforeAll
static void setupClass() {
    WebDriverManager.firefoxdriver().setup();
}

Then we ensure that a browser is opened before each of the individual tests marked with @Test:

@BeforeEach
void setup() {
    this.driver = new FirefoxDriver();
    this.driver.manage().timeouts().implicitlyWait(Duration.of(5, ChronoUnit.SECONDS));
}

This opens a Firefox window whenever we execute one of the class‘ test methods. This is a very pure and uncustomised instance; we could configure it further if we wanted. But for our purpose at the moment, it is perfectly fine.

Now before we perform any actions on the page, we must make sure that the browser window is closed appropriately after each test run. In teardown, we do:

    @AfterEach
    void teardown() {
        this.driver.quit();
    }

Caveat: confusion potential

Selenium’s WebDriver class has a close()-method, but please always use quit() instead. It performs a clean browser termination with regards to Selenium’s execution flow, whereas close() will just terminate the browser causing you lots of warnings in the log.

Alright! We have taken care of the browser handling. Time to do stuffs with it.

Opening the landing page

The most basic action ist to open a web page. Thankfully this task is handled by Selenium with a simple one-liner. Given that our sample web shop is up and running, we do:

this.driver.get("http://localhost:3000/");

We introduce the line to the test case method isOpen, which we introduced earlier in our boiler plate. But before we head into executing the test case, we have to be aware of the fact: We as the user can see the page being open, but the computer cannot. We have to programmatically verify that our expectation „landing page is open“ is met.

Verifying the outcome

To do that we verify these 3 conditions:

  • the window title contains the landing page’s HTML title
  • the most prominent logo exists on the web page and
  • it’s actually visible (i.e. it does not have its display property set to none).

We add this code directly below the line that opens the website:

assertTrue(this.driver.getTitle().contains("Spree Demo"),
           "'Spree Demo' is not in the browser's title.");

WebElement logoElement = this.driver.findElement(By.id("logo"));
assertTrue(logoElement.isDisplayed());

The first line peeks into the page title, which is another staple of Selenium, and then checks if it contains „Spree Demo“. Using JUnit’s assertTrue, we verify that this is the case, and if not, we print a custom error message. Condition 1 done.

The second line depicts the method you will probably use the most whilst moving forward in your test automation career: Webdriver#findElement goes through the page’s DOM and grabs the element that meets the given criteria. Here we search for an element with the HTML id „logo“. If it fails to find the element, it drops a NoSuchElementError. Therefore, we just implicitly verified condition 2.

The return value is a WebElement object that we can do clicks, inputs and various other actions on. We will use the object’s method isDisplayed() to check if the logo is visible therefore verifying our 3rd condition.

Straight-forward so far. At this point your code should look like that:

import io.github.bonigarcia.wdm.WebDriverManager;
import org.junit.jupiter.api.*;
import org.openqa.selenium.By;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.WebElement;
import org.openqa.selenium.firefox.FirefoxDriver;

import java.time.Duration;
import java.time.temporal.ChronoUnit;

import static org.junit.jupiter.api.Assertions.*;

class CrossTests {

    WebDriver driver;

    @BeforeAll
    static void setupClass() {
        WebDriverManager.firefoxdriver().setup();
    }

    @BeforeEach
    void setup() {
        this.driver = new FirefoxDriver();
        this.driver.manage().timeouts().implicitlyWait(Duration.of(5, ChronoUnit.SECONDS));
    }

    @AfterEach
    void teardown() {
        this.driver.quit();
    }

    @Test
    void isOpen() {
        this.driver.get("http://localhost:3000/");
        assertTrue(this.driver.getTitle().contains("Spree Demo"),
                "'Spree Demo' is not in the browser's title.");

        WebElement logoElement = this.driver.findElement(By.id("logo"));
        assertTrue(logoElement.isDisplayed());
    }

}

Let’s execute it. In a terminal window, please switch to the project root and do:

./gradlew test

If everything went well, your console should display a big OK:

BUILD SUCCESSFUL in 7s
4 actionable tasks: 2 executed, 2 up-to-date

Congratulations, you just executed your first Java-Selenium-test:
Our shop’s landing page can be opened and it is displayed correctly.

What is this driver thingy by the way?

Simply put, the driver – in our case Geckodriver – is the glue between our code and Firefox. The same would be valid for Chrome with Chromedriver respectively. This means it is the interface to our browser handling all the requests that we perform :

  • findElement
  • click
  • „are you displayed?“
  • perform key strokes
  • „grab that text of an element“
  • etc. pp.

Under the hood, drivers are http server applications that receive requests sent by Selenium everytime we perform an action similar to these mentioned above. The driver then interacts with the browser in the way we tasked it to and sends a JSON-based response that we can base our future work on. Everything is encapsulated by Selenium so that we can work with simple objects during automating our tests without having to deal with HTTP or JSON.

Performing and validating browser actions

Now that we know the basics of how to write automated tests with Java and Selenium, let’s develop a more complex test.

The best candidates for automation are cross tests. These are tests that cover representative cross sections of the product. One example could be: Put items into the cart, view the cart page and perform the checkout. It is a great practise to start test automation for new projects by coding these cross tests first.

For the sake of brevity here, we will write up the test until reaching the checkout page. The rest is homework. Don’t worry, I got you covered with the full source code in my bitbucket repo.

Ok, without further ado, here’s the code. Please add the following method to the test class.

@Test
void findSpreeBagAndCheckout()throws InterruptedException{
    this.driver.get("http://localhost:3000/");

    // Landing and catalog page
    this.driver.findElement(By.cssSelector("[href='/t/spree']")).click();
    this.driver.findElement(By.cssSelector("a[href*='spree-bag']")).click();

    // Product page

    this.driver.findElement(By.cssSelector("input#quantity")).clear();
    this.driver.findElement(By.cssSelector("input#quantity")).sendKeys("3");
    this.driver.findElement(By.id("add-to-cart-button")).click();

    // Cart page
    assertTrue(this.driver.getTitle().contains("Shopping Cart"),"'Shopping Cart' is not in the browser's title.");
    this.driver.findElement(By.cssSelector("img[src$='/spree_bag.jpeg']")).isDisplayed();
    String qtyValue=this.driver.findElement(By.cssSelector(".line_item_quantity")).getAttribute("value");
    assertEquals(qtyValue,"3","The item quantity is not the same we typed in!");

    String cartTotal=this.driver.findElement(By.cssSelector(".cart-total > .lead")).getText();
    assertEquals(cartTotal,"$68.97","The total cart value does not match our expectations. That's a blocker!");
}

What’s new here?

If we look back at our first landing page test, you will notice that we use a lot of new things. We will cover them one by one in the next section.

CSS selectors

One thing that catches the eye is that we looked for elements by using CSS expressions. You may have heard about them from your frontend peers, or maybe you have even worked with them yourself already. If not, don’t worry. We will cover them in a followup post. For now, I will give you a few translations:

[href='/t/spree']

„Give me a page element, whose attribute href is equal to ‚/t/spree'“. Quick reminder: href is used in link elements for defining the target URL that is opened, when the user clicks the link.

a[href*='spree-bag']

„Give me any link („anchor“) element, whose attribute href contains ’spree-bag‘.“

img[src$='/spree_bag.jpeg']

„Give me an image element, whose attribute src ends with ‚/spree_bag.jpg‘.“

input#quantity

„Give me an input element, whose HTML id is equal to ‚quantity‘.“ Yes, we could have used By.ID with „quantity“ here, but in this case I wanted to be explicit about the element type input.

.line_item_quantity

„Give me an element that has line_item_quantity in its class attribute.“ The leading dot indicates that we are looking for elements that has the specified class in its class list. It is roughly equivalent to [class*='line_item_quantity'].

.cart-total > .lead

„Give me any element that has the lead class and that’s preceded by any element that has the cart-total class.“

By using CSS selectors you can be very specific about what element you want to use in your test case. This makes CSS selectors incredibly powerful and versatile while maintaining a solid degree of simplicity, especially compared to XPath (ugh).

New properties and methods used in our test case

Aside from CSS selectors, we used several new methods and properties in our automated test.

click() issues a click on the target element. Straight-forward.

.getText() returns the element’s written text. In <p>Some text</p> for example, we would get access to „Some text“.

getAttribute("someHtmlAttribute") gives us the content of an element’s HTML attribute. In our test we fetch an element’s value attribute that is often used to set the content of an input element. Another example might be the link element we talked about earlier. Given we want to have its href attribute for some further verifications; on our page:

<a id="mylink" href="https://www.example.org">an example link</a>

Then in our test we could do:

targetHref = this.driver.findElement(By.id("mylink"))
              .getAttribute("href");
this.doSomeStuffsOn(targetHref);

.sendKeys("My desired input") imitates key strokes on an element. We use it to type any string we want into our target text input element. In our test for example that would be the „quantity“ input box.

And finally, with clear() we have a little helper in our toolbox that removes any content from an input element. In our test we use that to remove the preset value. Otherwise we would accidentally order 31 bags: „3“ from us and „1“ from the value preset by the page.

Continuing your test automation journey from here

Alright, let’s sum up what we accomplished:

We have seen how to open and close a browser window, do various actions on the page and verify their outcomes. Finally, we used that to write a fairly large cross test, all of that on a full fledged ecommerce web app. Great job! Now where should we go next? For the next post, as promised, I’d like to go back one step and give you a deeper introduction to CSS expressions, because they will accompany you for a long time during your test automation journey. Afterwards we have a big problem to solve that you probably already noticed:

Copious amounts of repetition.

To tackle that we will leverage cucumber-jvm, a Behavior Driven Development framework that makes it possible to write test cases in human-readable text form on top of a DRY Java code base. Afterwards we will apply the Page Object Pattern, a test automation – specific design pattern that reduces repetition even more.

Okay! To keep you busy while I am busy, how about finishing the checkout? As a reminder, you can find the full source code in my bitbucket repo at any time. Alternatively you can spoil yourself a little about Cucumber in Rust or try Python instead of Java.

Stay curious and see you in the next post!

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