Domain-Specific Languages

Groovy scripts are always compiled to classes. For example, a script as simple as:

is compiled to a class extending the abstract groovy.lang.Script class. This class contains a single abstract method called run. When a script is compiled, then its body will become the run method, while the other methods found in the script are found in the implementing class. The Script class provides base support for integration with your application through the Binding object, as illustrated in this example:

This is a very practical way to share data between the caller and the script, however it may be insufficient or not practical in some cases. For that purpose, Groovy allows you to set your own base script class. A base script class has to extend groovy.lang.Script and be a single abstract method type:

Then the custom script base class can be declared in the compiler configuration, for example:

As an alternative, it is also possible to use the @BaseScript annotation directly into a script:

where @BaseScript should annotate a variable which type is the class of the base script. Alternatively, you can set the base script class as a member of the @BaseScript annotation itself:

We have seen that the base script class is a single abstract method type that needs to implement the run method. The run method is executed by the script engine automatically. In some circumstances it may be interesting to have a base class which implements the run method, but provides an alternative abstract method to be used for the script body. For example, the base script run method might perform some initialization before the run method is executed. This is possible by doing this:

If you execute this code:

Then you will see that the script is executed, but the result of the evaluation is 1 as returned by the run method of the base class. It is even clearer if you use parse instead of evaluate, because it would allow you to execute the run method several times on the same script instance:

@groovy.lang.DelegatesTo is a documentation and compile-time annotation aimed at:

The Groovy language is a platform of choice for building DSLs. Using closures, it’s quite easy to create custom control structures, as well as it is simple to create builders. Imagine that you have the following code:

One way of implementing this is using the builder strategy, which implies a method, named email which accepts a closure as an argument. The method may delegate subsequent calls to an object that implements the from, to, subject and body methods. Again, body is a method which accepts a closure as an argument and that uses the builder strategy.

Implementing such a builder is usually done the following way:

the EmailSpec class implements the from, to, … methods. By calling rehydrate, we’re creating a copy of the closure for which we set the delegate, owner and thisObject values. Setting the owner and the this object is not very important here since we will use the DELEGATE_ONLY strategy which says that the method calls will be resolved only against the delegate of the closure.

The EmailSpec class has itself a body method accepting a closure that is cloned and executed. This is what we call the builder pattern in Groovy.

One of the problems with the code that we’ve shown is that the user of the email method doesn’t have any information about the methods that he’s allowed to call inside the closure. The only possible information is from the method documentation. There are two issues with this: first of all, documentation is not always written, and if it is, it’s not always available (javadoc not downloaded, for example). Second, it doesn’t help IDEs. What would be really interesting, here, is for IDEs to help the developer by suggesting, once they are in the closure body, methods that exist on the email class.

Moreover, if the user calls a method in the closure which is not defined by the EmailSpec class, the IDE should at least issue a warning (because it’s very likely that it will break at runtime).

One more problem with the code above is that it is not compatible with static type checking. Type checking would let the user know if a method call is authorized at compile time instead of runtime, but if you try to perform type checking on this code:

Then the type checker will know that there’s an email method accepting a Closure, but it will complain for every method call inside the closure, because from, for example, is not a method which is defined in the class. Indeed, it’s defined in the EmailSpec class and it has absolutely no hint to help it knowing that the closure delegate will, at runtime, be of type EmailSpec:

will fail compilation with errors like this one:

For those reasons, Groovy 2.1 introduced a new annotation named @DelegatesTo. The goal of this annotation is to solve both the documentation issue, that will let your IDE know about the expected methods in the closure body, and it will also solve the type checking issue, by giving hints to the compiler about what are the potential receivers of method calls in the closure body.

The idea is to annotate the Closure parameter of the email method:

What we’ve done here is telling the compiler (or the IDE) that when the method will be called with a closure, the delegate of this closure will be set to an object of type email. But there is still a problem: the default delegation strategy is not the one which is used in our method. So we will give more information and tell the compiler (or the IDE) that the delegation strategy is also changed:

Now, both the IDE and the type checker (if you are using @TypeChecked) will be aware of the delegate and the delegation strategy. This is very nice because it will both allow the IDE to provide smart completion, but it will also remove errors at compile time that exist only because the behaviour of the program is normally only known at runtime!

The following code will now pass compilation:

@DelegatesTo supports multiple modes that we will describe with examples in this section.

Whether you are using groovyc to compile classes or a GroovyShell, for example, to execute scripts, under the hood, a compiler configuration is used. This configuration holds information like the source encoding or the classpath but it can also be used to perform more operations like adding imports by default, applying AST transformations transparently or disabling global AST transformations.

The goal of compilation customizers is to make those common tasks easy to implement. For that, the CompilerConfiguration class is the entry point. The general schema will always be based on the following code:

Compilation customizers must extend the org.codehaus.groovy.control.customizers.CompilationCustomizer class. A customizer works:

You can implement your own compilation customizer but Groovy includes some of the most common operations.

Using this compilation customizer, your code will have imports added transparently. This is in particular useful for scripts implementing a DSL where you want to avoid users from having to write imports. The import customizer will let you add all the variants of imports the Groovy language allows, that is:

A detailed description of all shortcuts can be found in org.codehaus.groovy.control.customizers.ImportCustomizer

The AST transformation customizer is meant to apply AST transformations transparently. Unlike global AST transformations that apply on every class beeing compiled as long as the transform is found on classpath (which has drawbacks like increasing the compilation time or side effects due to transformations applied where they should not), the customizer will allow you to selectively apply a transform only for specific scripts or classes.

As an example, let’s say you want to be able to use @Log in a script. The problem is that @Log is normally applied on a class node and a script, by definition, doesn’t require one. But implementation wise, scripts are classes, it’s just that you cannot annotate this implicit class node with @Log. Using the AST customizer, you have a workaround to do it:

That’s all! Internally, the @Log AST transformation is applied to every class node in the compilation unit. This means that it will be applied to the script, but also to classes defined within the script.

If the AST transformation that you are using accepts parameters, you can use parameters in the constructor too:

As the AST transformation customizers works with objects instead of AST nodes, not all values can be converted to AST transformation parameters. For example, primitive types are converted to ConstantExpression (that is LOGGER is converted to new ConstantExpression('LOGGER'), but if your AST transformation takes a closure as an argument, then you have to give it a ClosureExpression, like in the following example:

For a complete list of options, please refer to org.codehaus.groovy.control.customizers.ASTTransformationCustomizer

This customizer will allow the developer of a DSL to restrict the grammar of the language, to prevent users from using some constructs, for example. It is only ``secure'' in that sense only and it is very important to understand that it does not replace a security manager. The only reason for it to exist is to limit the expressiveness of the language. This customizer only works at the AST (abstract syntax tree) level, not at runtime! It can be strange at first glance, but it makes much more sense if you think of Groovy as a platform to build DSLs. You may not want a user to have a complete language at hand. In the example below, we will demonstrate it using an example of language that only allows arithmetic operations, but this customizer allows you to:

For all those features, the secure AST customizer works using either a whitelist (list of elements that are allowed) or a blacklist (list of elements that are disallowed). For each type of feature (imports, tokens, …) you have the choice to use either a whitelist or a blacklist, but you can mix whitelists and blacklists for distinct features. In general, you will choose whitelists (disallow all, allow selected).

If what the secure AST customizer provides out of the box isn’t enough for your needs, before creating your own compilation customizer, you might be interested in the expression and statement checkers that the AST customizer supports. Basically, it allows you to add custom checks on the AST tree, on expressions (expression checkers) or statements (statement checkers). For this, you must implement org.codehaus.groovy.control.customizers.SecureASTCustomizer.StatementChecker or org.codehaus.groovy.control.customizers.SecureASTCustomizer.ExpressionChecker.

Those interfaces define a single method called isAuthorized, returning a boolean, and taking a Statement (or Expression) as a parameter. It allows you to perform complex logic over expressions or statements to tell if a user is allowed to do it or not.

For example, there’s no predefined configuration flag in the customizer which will let you prevent people from using an attribute expression. Using a custom checker, it is trivial:

Then we can make sure that this works by evaluating a simple script:

Statements can be checked using org.codehaus.groovy.control.customizers.SecureASTCustomizer.StatementChecker Expressions can be checked using org.codehaus.groovy.control.customizers.SecureASTCustomizer.ExpressionChecker

This customizer may be used as a filter on other customizers. The filter, in that case, is the org.codehaus.groovy.control.SourceUnit. For this, the source aware customizer takes another customizer as a delegate, and it will apply customization of that delegate only and only if predicates on the source unit match.

SourceUnit gives you access to multiple things but in particular the file being compiled (if compiling from a file, of course). It gives you the potential to perform operation based on the file name, for example. Here is how you would create a source aware customizer:

Then you can use predicates on the source aware customizer:

If you are using compilation customizers in Groovy code (like the examples above) then you can use an alternative syntax to customize compilation. A builder (org.codehaus.groovy.control.customizers.builder.CompilerCustomizationBuilder) simplifies the creation of customizers using a hierarchical DSL.

The code sample above shows how to use the builder. A static method, withConfig, takes a closure corresponding to the builder code, and automatically registers compilation customizers to the configuration. Every compilation customizer available in the distribution can be configured this way:

So far, we have described how you can customize compilation using a CompilationConfiguration class, but this is only possible if you embed Groovy and that you create your own instances of CompilerConfiguration (then use it to create a GroovyShell, GroovyScriptEngine, …).

If you want it to be applied on the classes you compile with the normal Groovy compiler (that is to say with  groovyc, ant or gradle, for example), it is possible to use a compilation flag named configscript that takes a Groovy configuration script as argument.

This script gives you access to the CompilerConfiguration instance before the files are compiled (exposed into the configuration script as a variable named configuration), so that you can tweak it.

It also transparently integrates the compiler configuration builder above. As an example, let’s see how you would activate static compilation by default on all classes.

If:

Then AST transformations are the way to go. Unlike the techniques used so far, AST transformations are meant to change or generate code before it is compiled to bytecode. AST transformations are capable of adding new methods at compile time for example, or totally changing the body of a method based on your needs. They are a very powerful tool but also come at the price of not being easy to write. For more information about AST transformations, please take a look at the compile-time metaprogramming section of this manual.

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