# Servlet support

version 4.0.19

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