You can write (Java) Servlets in Groovy (called Groovlets).

There is also a GroovyServlet.

This feature will automatically compile your .groovy source files, turn them into bytecode, load the Class and cache it until you change the source file.

Here’s a simple example to show you the kind of things you can do from a Groovlet.

Notice the use of implicit variables to access the session, output and request. Also notice that this is more like a script as it does not have a class wrapper.

if (!session) {
  session = request.getSession(true)
}

if (!session.counter) {
  session.counter = 1
}

println """
<html>
    <head>
        <title>Groovy Servlet</title>
    </head>
    <body>
        <p>
Hello, ${request.remoteHost}: ${session.counter}! ${new Date()}
        </p>
    </body>
</html>
"""
session.counter = session.counter + 1

Or, do the same thing using MarkupBuilder:

if (!session) {
    session = request.getSession(true)
}

if (!session.counter) {
    session.counter = 1
}

html.html { // html is implicitly bound to new MarkupBuilder(out)
  head {
      title('Groovy Servlet')
  }
  body {
    p("Hello, ${request.remoteHost}: ${session.counter}! ${new Date()}")
  }
}
session.counter = session.counter + 1

1. Implicit variables

The following variables are ready for use in Groovlets:

variable name bound to note

request

ServletRequest

-

response

ServletResponse

-

context

ServletContext

-

application

ServletContext

-

session

getSession(false)

can be null! see <1>

params

a Map object

headers

a Map object

out

response.getWriter()

see <2>

sout

response.getOutputStream()

see <2>

html

new MarkupBuilder(out)

see <2>

json

new StreamingJsonBuilder(out)

see <2>

  1. The session variable is only set, if there was already a session object. See the if (session == null) checks in the examples above.

  2. These variables cannot be re-assigned inside a Groovlet. They are bound on first access, allowing to e.g. calling methods on the response object before using out.

2. Setting up groovlets

Add the following to your web.xml:

<servlet>
    <servlet-name>Groovy</servlet-name>
    <servlet-class>groovy.servlet.GroovyServlet</servlet-class>
</servlet>

<servlet-mapping>
    <servlet-name>Groovy</servlet-name>
    <url-pattern>*.groovy</url-pattern>
</servlet-mapping>

Then put the required groovy jar files into WEB-INF/lib.

Now put the .groovy files in, say, the root directory (i.e. where you would put your html files). The GroovyServlet takes care of compiling the .groovy files.

So for example using tomcat you could edit tomcat/conf/server.xml like this:

<Context path="/groovy" docBase="c:/groovy-servlet"/>

Then access it with http://localhost:8080/groovy/hello.groovy