2.14 Serving Servlets

► |  | Web Server: HTTP Server

1 | Dispatching Server
2 | Dispatchers
3 | Launching Servers
4 | Web Servers
5 | Internal APIs
6 | Troubleshooting and Tips
| Index

▼ | 2 | Dispatchers

2.1 | General
2.2 | Mapping URLs to Paths
2.3 | Sequencing
2.4 | Timeouts
2.5 | Lifting Procedures
2.6 | Filtering Requests by URL
2.7 | Filtering Requests by Method
2.8 | Procedure Invocation upon Request
2.9 | Logging
2.10 | Basic Logging
2.11 | Password Protection
2.12 | Virtual Hosts
2.13 | Serving Files
2.14 | Serving Servlets
2.15 | Statistics
2.16 | Limiting Requests
2.17 | Wrapping Requests & Responses

► | 2.14 | Serving Servlets

2.14.1 | Setting Up Servlets
2.14.2 | Servlet Namespaces
2.14.3 | Internal Servlet Representation

On this page:

url->servlet/ c
make-cached-url->servlet
make
2.14.1 Setting Up Servlets
make-v1.servlet
make-v2.servlet
make-stateless.servlet
default-module-specs
path->servlet/ c
make-default-path->servlet
2.14.2 Servlet Namespaces
make-servlet-namespace/ c
make-make-servlet-namespace
2.14.2.1 Why this is useful
2.14.3 Internal Servlet Representation
servlet

Racket
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### 2.14 Serving Servlets🔗ℹ

(require web-server/dispatchers/dispatch-servlets)
| package: web-server-lib

The web-server/dispatchers/dispatch-servlets module defines a dispatcher constructor that runs servlets.

> value url->servlet/c : contract?

Equivalent to (-> url? servlet?)

> > procedure
> > (make-cached-url->servlet |  | url->path |
> > |  | path->serlvet) |
> | → |  |
> (->* () ((or/c false/c (listof url?)) (-> servlet? void?)) void?)
> url->servlet/c
> url->path : url->path/c
> path->serlvet : path->servlet/c

The first return value is a procedure that flushes the cache. If its first optional argument is #f (the default), all servlet caches are flushed. Otherwise, only those servlet caches to which url->path maps the given URLs are flushed. The second optional argument is a procedure which is invoked on each cached value before it is flushed, which can be used to finalize servlet resources. Beware that the default value void performs no finalization. In particular, it does not shut down the servlet’s custodian, instead allowing the servlet’s custodian-managed resources (such as threads) to persist.

The second return value is a procedure that uses url->path to resolve the URL to a path, then uses path->servlet to resolve that path to a servlet, caching the results in an internal table.

Changed in version 1.3 of package web-server-lib: Added optional argument to first return value for list of URLs.
Changed in version 1.3: Added optional argument to first return value for servlet finalizer procedure.

> > procedure
> > (make |  | url->servlet |
> > | [ | #:responders-servlet-loading responders-servlet-loading |
> > |  | #:responders-servlet responders-servlet]) |
> → dispatcher/c
> url->servlet : url->servlet/c
> | responders-servlet-loading |  | : |  | (url? exn? . -> . can-be-response?)
> |  |  | = |  | servlet-loading-responder
> | responders-servlet |  | : |  | (url? exn? . -> . can-be-response?)
> |  |  | = |  | servlet-error-responder

This dispatcher runs racket servlets, using url->servlet to resolve URLs to the underlying servlets. If servlets have errors loading, then responders-servlet-loading is used. Other errors are handled with responders-servlet. If a servlet raises calls next-dispatcher, then the signal is propagated by this dispatcher.

#### 2.14.1 Setting Up Servlets🔗ℹ

(require web-server/servlet/setup) | package: web-server-lib

This module is used internally to build and load servlets. It may be useful to those who are trying to extend the server.

> procedure (make-v1.servlet directory timeout start) → servlet?
> directory : path-string?
> timeout : integer?
> start : (request? . -> . can-be-response?)

Creates a version 1 servlet that uses directory as its current directory, a timeout manager with a timeout timeout, and start as the request handler.

> procedure (make-v2.servlet directory manager start) → servlet?
> directory : path-string?
> manager : manager?
> start : (request? . -> . can-be-response?)

Creates a version 2 servlet that uses directory as its current directory, a manager as the continuation manager, and start as the request handler.

> > procedure
> > (make-stateless.servlet |  | directory |  |  |  |
> > |  | stuffer |  |  |  |
> > |  | manager |  |  |  |
> > |  | start) |  | → |  | servlet?
> directory : path-string?
> stuffer : (stuffer/c serializable? bytes?)
> manager : manager?
> start : (request? . -> . can-be-response?)

Creates a stateless web-server servlet that uses directory as its current directory, stuffer as its stuffer, and manager as the continuation manager, and start as the request handler.

> value default-module-specs : (listof module-path?)

The modules that the Web Server needs to share with all servlets.

> value path->servlet/c : contract?

Equivalent to (-> path? servlet?).

> > procedure
> > (make-default-path->servlet |
> > | [ | #:make-servlet-namespace make-servlet-namespace |
> > |  | #:timeouts-default-servlet timeouts-default-servlet]) |
> → path->servlet/c
> | make-servlet-namespace |  | : |  | make-servlet-namespace/c
> |  |  | = |  | (make-make-servlet-namespace)
> timeouts-default-servlet : integer? = 30

Constructs a procedure that loads a servlet from the path in a namespace created with make-servlet-namespace, using a timeout manager with timeouts-default-servlet as the default timeout (if no manager is given.)

#### 2.14.2 Servlet Namespaces🔗ℹ

(require web-server/configuration/namespace)
| package: web-server-lib

This module provides a function to help create the make-servlet-namespace procedure needed by the make function of web-server/dispatchers/dispatch-servlets.

> value make-servlet-namespace/c : contract?

Equivalent to

> (->* ()
> (#:additional-specs (listof module-path?))
> namespace?)

> procedure (make-make-servlet-namespace #:to-be-copied-module-specs to-be-copied-module-specs)
> → make-servlet-namespace/c
> to-be-copied-module-specs : (listof module-path?)

This function creates a function that when called will construct a new namespace that has all the modules from to-be-copied-module-specs and additional-specs, as well as racket and mred, provided they are already attached to the (current-namespace) of the call-site.

Example:

> (make-make-servlet-namespace
> #:to-be-copied-module-specs `((lib "database.rkt" "my-module")))

##### 2.14.2.1 Why this is useful🔗ℹ

A different namespace is needed for each servlet, so that if servlet A and servlet B both use a stateful module C, they will be isolated from one another. We see the Web Server as an operating system for servlets, so we inherit the isolation requirement on operating systems.

However, there are some modules which must be shared. If they were not, then structures cannot be passed from the Web Server to the servlets, because Racket’s structures are generative.

Since, on occasion, a user will actually wanted servlets A and B to interact through module C. A custom make-servlet-namespace can be created, through this procedure, that attaches module C to all servlet namespaces. Through other means (see Dispatchers) different sets of servlets can share different sets of modules.

#### 2.14.3 Internal Servlet Representation🔗ℹ

(require web-server/private/servlet)
| package: web-server-lib

> > struct
> > (struct |  | servlet (custodian namespace manager directory handler)
> #:extra-constructor-name make-servlet
> #:mutable)
> custodian : custodian?
> namespace : namespace?
> manager : manager?
> directory : path-string?
> handler : (request? . -> . can-be-response?)

Instances of this structure hold the necessary parts of a servlet: the custodian responsible for the servlet’s resources, the namespace the servlet is executed within, the manager responsible for the servlet’s continuations, the current directory of the servlet, and the handler for all requests to the servlet.
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