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May 17, 2016

Groovy Goodness: Creating Files And Directories With Nice DSL Using FileTreeBuilder

Groovy has a lot of nice and useful gems. One of them is the FileTreeBuilder class. With this class we can create directories and files using a nice DSL with a builder syntax. The code already reflects the hierarchy of the directory structure, which makes it so more readable. We can use an explicit way of referring to methods in the FileTreeBuilder class, but there is also support for a more dynamic version, where Groovy's dynamic nature comes to play.

In the first example we use the explicit method names. We can use the method dir to create directories and the method file to create a file. We can specify a name for the file and also contents:

// Create new FileTreeBuilder. Default the current
// directory is the base directory for creating new
// files and directories.
// We can pas another directory in the constructor as
// the base directory.
final FileTreeBuilder treeBuilder = new FileTreeBuilder(new File('tree'))

// Add a file and set the contents using
// a closure. The delegate of the closure
// is the File object.
treeBuilder.file('README.adoc') {
    write '''\
        = Groovy rocks!

        Hidden features in Groovy are also cool.


// Append to file contents with a String argument.
treeBuilder.file('README.adoc', '== Extra heading')

final File sample = new File('sample')
sample.text = '''\
= Another sample

Testing the Groovy FileTreeBuilder.

// Or we use another File's contents to append to a file.
treeBuilder.file('README1.adoc', sample)

// Create a new directory.

// Create subdirectories and files
// using a closure. The delegate is
// is FileTreeBuilder again.
treeBuilder.dir('src') {
    dir('docs') {
        file('manuscript.adoc') {
            // Another way to write file contents.
            withWriter('UTF-8') { writer ->
                writer.write '= Building Apps With Grails 3'

assert new File('tree/README.adoc').exists()
assert new File('tree/src/docs/manuscript.adoc').exists()
assert new File('tree/src/docs/manuscript.adoc').text == '= Building Apps With Grails 3'

We can achieve the same result using a more compact DSL. The FileTreeBuilder will determine if a directory or file needs to be created. Notice that contents is always append to a file:

// We can even use a shorter syntax with the
// FileTreeBuilder where the node names are the name
// of a directory to be created (argument is a closure),
// or the name of a file and some contents.
// Notice that with the DSL all file contents is
// appended to existing file contents.
// We need to delete an existing file first if we
// don't want to append the contents.

final File newDir = new File('dsl')

// Remove existing dir, so file contents is
// only set by the FileTreeBuilder DSL,
// otherwise content is added to the existing files.
if (newDir.exists()) {

final FileTreeBuilder dir = new FileTreeBuilder(newDir)

dir {
    // Node name is the file name, followed by the contents.
        = Groovy rocks!

        Hidden features in Groovy are also cool.


    'README.adoc'('== Extra heading')

     // We cannot use a closure argument with this DSL,
     // like with the builder. The DSL assume that a node with a
     // closure is a directory.
     // But we can use the File object argument to set
     // the file contents.

// If name is follwed by closure than a directory
// name is assumed and created.
dir.out {}

// Created directory with subdirectories.
dir.src {
    // The name of the node is the directory name.
    docs {
        // And create file in the src/docs directory.
        'manuscript.adoc'('= Building Apps With Grails 3')

assert new File('dsl/README.adoc').exists()
assert new File('dsl/src/docs/manuscript.adoc').exists()
assert new File('dsl/src/docs/manuscript.adoc').text == '= Building Apps With Grails 3'

Written with Groovy 2.4.6.