Grails 3 is build on top of Spring Boot and this adds a lot of the Spring Boot features to our Grails application. For example in Spring Boot we can store configuration properties in an external file. A default Grails application already adds application.yml in the grails-app/conf directory. If we want to specify different values for a configuration property for each environment (like development, test and production) we can use environment section in application.yml. We know this from previous Grails versions with a Groovy configuration file Config.groovy. But we can also create different configuration files per environment and set the value for the configuration property in each file. We can use the following naming pattern for the file: application-{env}.yml or application-{env}.properties. These files need be in:

1. a config directory in the directory the application is running from
2. the root of the directory the application is running from
3. in a /config package on the classpath
4. in the root of the classpath

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Grails 3 introduces GrailsAutoConfiguration as the base class for a Grails application. We can extend this class, add a main method and we are ready to go. By default this class is the Application class in the grails-app/init directory. We can override several Grails lifecycle methods from the GrailsAutoConfiguration base class. One of them is doWithSpring. This method must return a closure that can be used by the Grails BeanBuilder class to add Spring beans to the application context. The closure has the same syntax as what we already know for the grails-app/conf/spring/resources.groovy file, we know from previous Grails versions.

We start with a simple class we want to use as a Spring bean:

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In a previous post we learned how to pass Java system properties from the command-line to a Java process defined in a Gradle build file. Because Grails 3 uses Gradle as the build tool we can apply the same mechanism in our Grails application. We need to reconfigure the run task. This task is of type JavaExec and we can use the method systemProperties to assign the system properties we define on the command-line when we invoke the run task.

We have a simple Grails 3 application with the following controller that tries to access the Java system property sample.message:

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A lot of bugs are in some way related to state. So that is what we will be talking about today. We will start off with a quote from Einstein: “Insanity: doing the same thing over and over again and expecting different results. ” Code should be consistent, calling the same function with the same input should return the same result during the whole life cycle of the object. Insanity and bugs will follow if this rule is violated. This sounds logical, but in practise it is quite easy to violate this principle. The main cause of this, is object state. This state is used in a lot of functions, and can thus affect the behaviour of our program at runtime. This principle can be even be seen in the most basic of examples (imagine the impact on a more complicated class...). Given the following class:

public class Person {
    private String name;
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
}

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Since Grails 3 is based on Spring Boot we can re-use many of the Spring Boot features in our Grails application. For example in a Spring Boot application we can use environment variables to give configuration properties a value. We simply need to follow some naming rules: the name of the configuration property must be in uppercase and dots are replaced with underscores. For example a configuration property feature.enabled is represented by the environment variable FEATURE_ENABLED.

We create the following controller in a Grails 3 application with a message property. The value is set with the @Value annotation of the underlying Spring framework. With this annotation we tell the application to look for an (external) configuration property sample.message and assign it's value to the message property. If it cannot be set via a configuration property the default value is "gr8".

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In Grails we can use the list-plugins command to get a list of all available plugins. The list returns only the plugins that are available for the Grails version we are using. So if we invoke this command in Grails 3 we get a different list than a Grails 2.x version. To get more detailed information, like website, source code URL and dependency definition we use the plugin-info command.

Let's run the plugin-list command for Grails 3:

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Creating and working with mocks and stubs in Spock is easy. If we want to interact with our mocks and stubs we have several options to return values. One of the options is the triple right shift operator >>>. With this operator we can define different return values for multiple invocations of the stubbed or mocked method. For example we can use the >>> operator followed by a list of return values ['abc', 'def', 'ghi']. On the first invocation abc is return, the second invocation returns def and the third (and following) invocation(s) return ghi.

In the following specification we have a class under test StringUtil. The class has a dependency on an implementation of the Calculator interface. We mock this interface in our specification. We expect the calculateSize method is invoked 5 times, but we only provide 3 values for the invocations. This means the first time 1 is used, the second time 3 is used and the remaining invocations get the value 4:

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A very useful feature in Groovy is the use of named arguments. Instead of a list of arguments for a method or constructor we can use a Map argument. If the argument is the first in the list of arguments then Groovy allows use to use named arguments when we invoke the method or constructor. This means all key/value arguments are gathered together and assigned to the Map argument. Inside our method or constructor we can then access the Map argument and get the values for the keys. This leads to better readable code and that is very useful. IntelliJ IDEA has a Groovy intention to turn method parameters into a Map parameter for named arguments with a few mouse clicks.

Suppose we have the following source code with a simple method definition, 2 arguments, and the invocation of the method:

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IntelliJ IDEA has very good Groovy support. It also provides some intentions especially for the Groovy language. For example we can turn a map definition into a Groovy class definition with a few simple clicks.

The following screenshot shows a simple map declaration with two keys: username and alias. If we use the shortcut for intentions (Alt+Enter on my computer) we can choose the Convert to Class option:

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In a previous post we learned that configuration property values can be passed via environment variables. With Spring Boot we can also pass the values using Java system properties. So if we have a property sample.message then we can use -Dsample.message=value to pass a value when we run the application. If we use the Spring Boot Gradle plugin we must reconfigure the bootRun task to pass Java system properties from the command-line.

Let's reuse our sample application from the previous blog post:

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