http://groovy-lang.org/structure.html
3.2. Script class
A script is always compiled into a class. The Groovy compiler will compile the class for you, with the body of the script copied into a run method. The previous example is therefore compiled as if it was the following:
import org.codehaus.groovy.runtime.InvokerHelper
class Main extends Script {
def run() {
println ‘Groovy world!‘
}
static void main(String[] args) {
InvokerHelper.runScript(Main, args)
}
}The Main class extends the groovy.lang.Script class |
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groovy.lang.Script requires a run method returning a value |
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the script body goes into the run method |
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the main method is automatically generated |
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and delegates the execution of the script on the run method |
If the script is in a file, then the base name of the file is used to determine the name of the generated script class. In this example, if the name of the file is Main.groovy, then the script class is going to be Main.
3.3. Methods
It is possible to define methods into a script, as illustrated here:
int fib(int n) {
n < 2 ? 1 : fib(n-1) + fib(n-2)
}
assert fib(10)==89You can also mix methods and code. The generated script class will carry all methods into the script class, and assemble all script bodies into the run method:
println ‘Hello‘
int power(int n) { 2**n }
println "2^6==${power(6)}"| script begins | |
| a method is defined within the script body | |
| and script continues |
This code is internally converted into:
import org.codehaus.groovy.runtime.InvokerHelper
class Main extends Script {
int power(int n) { 2** n}
def run() {
println ‘Hello‘
println "2^6==${power(6)}"
}
static void main(String[] args) {
InvokerHelper.runScript(Main, args)
}
}the power method is copied as is into the generated script class |
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first statement is copied into the run method |
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second statement is copied into the run method |
| Even if Groovy creates a class from your script, it is totally transparent for the user. In particular, scripts are compiled to bytecode, and line numbers are preserved. This implies that if an exception is thrown in a script, the stack trace will show line numbers corresponding to the original script, not the generated code that we have shown. |
3.4. Variables
Variables in a script do not require a type definition. This means that this script:
int x = 1
int y = 2
assert x+y == 3will behave the same as:
x = 1
y = 2
assert x+y == 3However there is a semantic difference between the two:
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if the variable is declared as in the first example, it is a local variable. It will be declared in the
runmethod that the compiler will generate and will not be visible outside of the script main body. In particular, such a variable will not be visible in other methods of the script -
if the variable is undeclared, it goes into the script binding. The binding is visible from the methods, and is especially important if you use a script to interact with an application and need to share data between the script and the application. Readers might refer to the integration guide for more information.
If you want a variable to become a field of the class without going into the Binding, you can use the @Field annotation. |
