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<title>class Rack::Builder - rack-2.2.23 Documentation</title>

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</div>

<div id="class-metadata">
 
 <div id="parent-class-section" class="nav-section">
 <h3>Parent</h3>

Object
 
</div>

<div id="method-list-section" class="nav-section">
 <h3>Methods</h3>

<ul class="link-list" role="directory">
 
 <li ><a href="#method-c-app">::app</a>
 
 <li ><a href="#method-c-load_file">::load_file</a>
 
 <li ><a href="#method-c-new">::new</a>
 
 <li ><a href="#method-c-new_from_string">::new_from_string</a>
 
 <li ><a href="#method-c-parse_file">::parse_file</a>
 
 <li ><a href="#method-i-call">#call</a>
 
 <li ><a href="#method-i-freeze_app">#freeze_app</a>
 
 <li ><a href="#method-i-generate_map">#generate_map</a>
 
 <li ><a href="#method-i-map">#map</a>
 
 <li ><a href="#method-i-run">#run</a>
 
 <li ><a href="#method-i-to_app">#to_app</a>
 
 <li ><a href="#method-i-use">#use</a>
 
 <li ><a href="#method-i-warmup">#warmup</a>
 
 </ul>
</div>

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</nav>

<main role="main" aria-labelledby="class-Rack::Builder">
 <h1 id="class-Rack::Builder" class="class">
 class Rack::Builder
 </h1>

<section class="description">
 
<a href="Builder.html"><code>Rack::Builder</code></a> implements a small DSL to iteratively construct <a href="../Rack.html"><code>Rack</code></a> applications.

Example:

<pre class="ruby">require &#39;rack/lobster&#39;
app = Rack::Builder.new do
 use Rack::CommonLogger
 use Rack::ShowExceptions
 map &quot;/lobster&quot; do
 use Rack::Lint
 run Rack::Lobster.new
 end
end

run app
</pre>

Or

<pre class="ruby">app = Rack::Builder.app do
 use Rack::CommonLogger
 run lambda { |env| [200, {&#39;Content-Type&#39; =&gt; &#39;text/plain&#39;}, [&#39;OK&#39;]] }
end

run app
</pre>

<code>use</code> adds middleware to the stack, <code>run</code> dispatches to an application. You can use <code>map</code> to construct a <a href="URLMap.html"><code>Rack::URLMap</code></a> in a convenient way.

</section>

<section class="constants-list">
 <header>
 <h3>Constants</h3>
 </header>
 <dl>
 
 <dt id="UTF_8_BOM">UTF_8_BOM
 
 <dd><a href="https://stackoverflow.com/questions/2223882/whats-the-difference-between-utf-8-and-utf-8-without-bom">stackoverflow.com/questions/2223882/whats-the-difference-between-utf-8-and-utf-8-without-bom</a>
 
 
 </dl>
 </section>

<section id="public-class-5Buntitled-5D-method-details" class="method-section">
 <header>
 <h3>Public Class Methods</h3>
 </header>

<div id="method-c-app" class="method-detail ">
 
 <div class="method-heading">
 app(default_app = nil, &amp;block)
 
 click to toggle source
 
 </div>

<div class="method-description">
 
 Create a new <a href="Builder.html"><code>Rack::Builder</code></a> instance and return the <a href="../Rack.html"><code>Rack</code></a> application generated from it.

<div class="method-source-code" id="app-source">
 <pre># File lib/rack/builder.rb, line 130
def self.app(default_app = nil, &amp;block)
 self.new(default_app, &amp;block).to_app
end</pre>
 </div>
 
 </div>

</div>

<div id="method-c-load_file" class="method-detail ">
 
 <div class="method-heading">
 load_file(path, opts = Server::Options.new)
 
 click to toggle source
 
 </div>

<div class="method-description">
 
 Load the given file as a rackup file, treating the contents as if specified inside a <a href="Builder.html"><code>Rack::Builder</code></a> block.

Treats the first comment at the beginning of a line that starts with a backslash as options similar to options passed on a rackup command line.

Ignores content in the file after +__END__+, so that use of +__END__+ will not result in a syntax error.

Example config.ru file:

<pre>$ cat config.ru

#\ -p 9393

use Rack::ContentLength
require &#39;./app.rb&#39;
run App</pre>

<div class="method-source-code" id="load_file-source">
 <pre># File lib/rack/builder.rb, line 93
def self.load_file(path, opts = Server::Options.new)
 options = {}

cfgfile = ::File.read(path)
 cfgfile.slice!(/\A#{UTF_8_BOM}/) if cfgfile.encoding == Encoding::UTF_8

if cfgfile[/^#\\(.*)/] &amp;&amp; opts
 warn &quot;Parsing options from the first comment line is deprecated!&quot;
 options = opts.parse! $1.split(/\s+/)
 end

cfgfile.sub!(/^__END__\n.*\Z/m, &#39;&#39;)
 app = new_from_string cfgfile, path

return app, options
end</pre>
 </div>
 
 </div>

</div>

<div id="method-c-new" class="method-detail ">
 
 <div class="method-heading">
 new(default_app = nil, &amp;block)
 
 click to toggle source
 
 </div>

<div class="method-description">
 
 Initialize a new <a href="Builder.html"><code>Rack::Builder</code></a> instance. <code>default_app</code> specifies the default application if <code>run</code> is not called later. If a block is given, it is evaluted in the context of the instance.

<div class="method-source-code" id="new-source">
 <pre># File lib/rack/builder.rb, line 123
def initialize(default_app = nil, &amp;block)
 @use, @map, @run, @warmup, @freeze_app = [], nil, default_app, nil, false
 instance_eval(&amp;block) if block_given?
end</pre>
 </div>
 
 </div>

</div>

<div id="method-c-new_from_string" class="method-detail ">
 
 <div class="method-heading">
 new_from_string(builder_script, file = &quot;(rackup)&quot;)
 
 click to toggle source
 
 </div>

<div class="method-description">
 
 Evaluate the given <code>builder_script</code> string in the context of a <a href="Builder.html"><code>Rack::Builder</code></a> block, returning a <a href="../Rack.html"><code>Rack</code></a> application.

<div class="method-source-code" id="new_from_string-source">
 <pre># File lib/rack/builder.rb, line 112
def self.new_from_string(builder_script, file = &quot;(rackup)&quot;)
 # We want to build a variant of TOPLEVEL_BINDING with self as a Rack::Builder instance.
 # We cannot use instance_eval(String) as that would resolve constants differently.
 binding, builder = TOPLEVEL_BINDING.eval(&#39;Rack::Builder.new.instance_eval { [binding, self] }&#39;)
 eval builder_script, binding, file
 builder.to_app
end</pre>
 </div>
 
 </div>

</div>

<div id="method-c-parse_file" class="method-detail ">
 
 <div class="method-heading">
 parse_file(config, opts = Server::Options.new)
 
 click to toggle source
 
 </div>

<div class="method-description">
 
 Parse the given config file to get a <a href="../Rack.html"><code>Rack</code></a> application.

If the config file ends in <code>.ru</code>, it is treated as a rackup file and the contents will be treated as if specified inside a <a href="Builder.html"><code>Rack::Builder</code></a> block, using the given options.

If the config file does not end in <code>.ru</code>, it is required and <a href="../Rack.html"><code>Rack</code></a> will use the basename of the file to guess which constant will be the <a href="../Rack.html"><code>Rack</code></a> application to run. The options given will be ignored in this case.

Examples:

<pre class="ruby">Rack::Builder.parse_file(&#39;config.ru&#39;)
# Rack application built using Rack::Builder.new

Rack::Builder.parse_file(&#39;app.rb&#39;)
# requires app.rb, which can be anywhere in Ruby&#39;s
# load path. After requiring, assumes App constant
# contains Rack application

Rack::Builder.parse_file(&#39;./my_app.rb&#39;)
# requires ./my_app.rb, which should be in the
# process&#39;s current directory. After requiring,
# assumes MyApp constant contains Rack application
</pre>

<div class="method-source-code" id="parse_file-source">
 <pre># File lib/rack/builder.rb, line 64
def self.parse_file(config, opts = Server::Options.new)
 if config.end_with?(&#39;.ru&#39;)
 return self.load_file(config, opts)
 else
 require config
 app = Object.const_get(::File.basename(config, &#39;.rb&#39;).split(&#39;_&#39;).map(&amp;:capitalize).join(&#39;&#39;))
 return app, {}
 end
end</pre>
 </div>
 
 </div>

</div>

</section>
 
 <section id="public-instance-5Buntitled-5D-method-details" class="method-section">
 <header>
 <h3>Public Instance Methods</h3>
 </header>

<div id="method-i-call" class="method-detail ">
 
 <div class="method-heading">
 call(env)
 
 click to toggle source
 
 </div>

<div class="method-description">
 
 Call the <a href="../Rack.html"><code>Rack</code></a> application generated by this builder instance. Note that this rebuilds the <a href="../Rack.html"><code>Rack</code></a> application and runs the warmup code (if any) every time it is called, so it should not be used if performance is important.

<div class="method-source-code" id="call-source">
 <pre># File lib/rack/builder.rb, line 243
def call(env)
 to_app.call(env)
end</pre>
 </div>
 
 </div>

</div>

<div id="method-i-freeze_app" class="method-detail ">
 
 <div class="method-heading">
 freeze_app()
 
 click to toggle source
 
 </div>

<div class="method-description">
 
 Freeze the app (set using run) and all middleware instances when building the application in to_app.

<div class="method-source-code" id="freeze_app-source">
 <pre># File lib/rack/builder.rb, line 226
def freeze_app
 @freeze_app = true
end</pre>
 </div>
 
 </div>

</div>

<div id="method-i-map" class="method-detail ">
 
 <div class="method-heading">
 map(path, &amp;block)
 
 click to toggle source
 
 </div>

<div class="method-description">
 
 Creates a route within the application. Routes under the mapped path will be sent to the <a href="../Rack.html"><code>Rack</code></a> application specified by run inside the block. Other requests will be sent to the default application specified by run outside the block.

<pre class="ruby">Rack::Builder.app do
 map &#39;/heartbeat&#39; do
 run Heartbeat
 end
 run App
end
</pre>

The <code>use</code> method can also be used inside the block to specify middleware to run under a specific path:

<pre class="ruby">Rack::Builder.app do
 map &#39;/heartbeat&#39; do
 use Middleware
 run Heartbeat
 end
 run App
end
</pre>

This example includes a piece of middleware which will run before <code>/heartbeat</code> requests hit <code>Heartbeat</code>.

Note that providing a <code>path</code> of <code>/</code> will ignore any default application given in a <code>run</code> statement outside the block.

<div class="method-source-code" id="map-source">
 <pre># File lib/rack/builder.rb, line 219
def map(path, &amp;block)
 @map ||= {}
 @map[path] = block
end</pre>
 </div>
 
 </div>

</div>

<div id="method-i-run" class="method-detail ">
 
 <div class="method-heading">
 run(app)
 
 click to toggle source
 
 </div>

<div class="method-description">
 
 Takes an argument that is an object that responds to <a href="Builder.html#method-i-call"><code>call</code></a> and returns a <a href="../Rack.html"><code>Rack</code></a> response. The simplest form of this is a lambda object:

<pre class="ruby">run lambda { |env| [200, { &quot;Content-Type&quot; =&gt; &quot;text/plain&quot; }, [&quot;OK&quot;]] }
</pre>

However this could also be a class:

<pre class="ruby">class Heartbeat
 def self.call(env)
 [200, { &quot;Content-Type&quot; =&gt; &quot;text/plain&quot; }, [&quot;OK&quot;]]
 end
end

run Heartbeat
</pre>

<div class="method-source-code" id="run-source">
 <pre># File lib/rack/builder.rb, line 176
def run(app)
 @run = app
end</pre>
 </div>
 
 </div>

</div>

<div id="method-i-to_app" class="method-detail ">
 
 <div class="method-heading">
 to_app()
 
 click to toggle source
 
 </div>

<div class="method-description">
 
 Return the <a href="../Rack.html"><code>Rack</code></a> application generated by this instance.

<div class="method-source-code" id="to_app-source">
 <pre># File lib/rack/builder.rb, line 231
def to_app
 app = @map ? generate_map(@run, @map) : @run
 fail &quot;missing run or map statement&quot; unless app
 app.freeze if @freeze_app
 app = @use.reverse.inject(app) { |a, e| e[a].tap { |x| x.freeze if @freeze_app } }
 @warmup.call(app) if @warmup
 app
end</pre>
 </div>
 
 </div>

</div>

<div id="method-i-use" class="method-detail ">
 
 <div class="method-heading">
 use(middleware, *args, &amp;block)
 
 click to toggle source
 
 </div>

<div class="method-description">
 
 Specifies middleware to use in a stack.

<pre class="ruby">class Middleware
 def initialize(app)
 @app = app
 end

def call(env)
 env[&quot;rack.some_header&quot;] = &quot;setting an example&quot;
 @app.call(env)
 end
end

use Middleware
run lambda { |env| [200, { &quot;Content-Type&quot; =&gt; &quot;text/plain&quot; }, [&quot;OK&quot;]] }
</pre>

All requests through to this application will first be processed by the middleware class. The <code>call</code> method in this example sets an additional environment key which then can be referenced in the application if required.

<div class="method-source-code" id="use-source">
 <pre># File lib/rack/builder.rb, line 153
def use(middleware, *args, &amp;block)
 if @map
 mapping, @map = @map, nil
 @use &lt;&lt; proc { |app| generate_map(app, mapping) }
 end
 @use &lt;&lt; proc { |app| middleware.new(app, *args, &amp;block) }
end</pre>
 </div>
 
 </div>

</div>

<div id="method-i-warmup" class="method-detail ">
 
 <div class="method-heading">
 warmup(prc = nil, &amp;block)
 
 click to toggle source
 
 </div>

<div class="method-description">
 
 Takes a lambda or block that is used to warm-up the application. This block is called before the <a href="../Rack.html"><code>Rack</code></a> application is returned by to_app.

<pre class="ruby">warmup do |app|
 client = Rack::MockRequest.new(app)
 client.get(&#39;/&#39;)
end

use SomeMiddleware
run MyApp
</pre>

<div class="method-source-code" id="warmup-source">
 <pre># File lib/rack/builder.rb, line 190
def warmup(prc = nil, &amp;block)
 @warmup = prc || block
end</pre>
 </div>
 
 </div>

</div>

</section>
 
 <section id="private-instance-5Buntitled-5D-method-details" class="method-section">
 <header>
 <h3>Private Instance Methods</h3>
 </header>

<div id="method-i-generate_map" class="method-detail ">
 
 <div class="method-heading">
 generate_map(default_app, mapping)
 
 click to toggle source
 
 </div>

<div class="method-description">
 
 Generate a <a href="URLMap.html"><code>URLMap</code></a> instance by generating new <a href="../Rack.html"><code>Rack</code></a> applications for each map block in this instance.

<div class="method-source-code" id="generate_map-source">
 <pre># File lib/rack/builder.rb, line 251
def generate_map(default_app, mapping)
 mapped = default_app ? { &#39;/&#39; =&gt; default_app } : {}
 mapping.each { |r, b| mapped[r] = self.class.new(default_app, &amp;b).to_app }
 URLMap.new(mapped)
end</pre>
 </div>
 
 </div>

</div>

</section>
 
 </section>

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