Closures

Summary

Groovy closures are true closures, with access to the variables in the context in which they are defined. They are:

• Contained in braces

• They are functions.

• Used in callbacks

• The default parameter (if you do not provide parameters) is it.

  [1,2,3].each {
     println it
  }

They can be used a lot like lambdas, passed as parameters to another function. (In other words, they are first-class objects.) But the Groovy documentation insists that they should not be thought of as lambdas.

Syntax

{ <type> param1, <type> param2 ... ->
   //Body
}

• Again, parameters are optional.
• If no parameters are listed, the default parameter is it.
• Parameters, if present, are listed (comma-separated) immediately after the first brace, on the first line of the closure, and followed by a arrow.
• The code body begins on the next line.
• The output of the last line is the return value, with or without the return statement.

Invoking Closures

Closures can be invoked like any other function:

closureName(<params...>)

Closures as Parameters

For a function/closure to take a Closure as a parameter, that parameter must be declared as the Closure datatype.1

Example

private defaultResponseHandler = { resp, reader ->
   assert resp.status == 200
   println "My response handler got response: ${resp.statusLine}"
   println "Response length: ${resp.headers.'Content-Length'}"
   System.out << reader // print response reader
}

void fetchContent(String path, Closure c = defaultResponseHandler){
    connect path, TEXT, c
}

In this case, from my HttpClient, I make the Closure parameter optional by giving it a default value. In either case, the closure works as a callback.

References

• http://groovy-lang.org/closures.html

Notes

1. Actually "must be declared" may be too strong. Perhaps def would work.