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README.md

clojure.tools.namespace

Tools for managing namespaces in Clojure. Parse ns declarations from source files, extract their dependencies, build a graph of namespace dependencies within a project, update that graph as files change, and reload files in the correct order.

This is only about namespace dependencies within a single project. It has nothing to do with Leiningen, Maven, JAR files, or repositories.

Releases and Dependency Information

Leiningen dependency information:

[org.clojure/tools.namespace "0.2.0"]

Maven dependency information:

<dependency>
  <groupId>org.clojure</groupId>
  <artifactId>tools.namespace</artifactId>
  <version>0.2.0-SNAPSHOT</version>
</dependency>

Development Snapshots

Usage

API Documentation

tools.namespace consists of several parts:

clojure.tools.namespace.parse: A parser for namespace declarations in Clojure source files. Given a stream of characters from a Clojure source file, it can find the ns declaration and parse the :require and :use clauses to find the names of other namespaces that file depends on. This is all syntactic analysis: it does not evaluate any code.

clojure.tools.namespace.find: Utilities to search for Clojure namespaces on the filesystem, in directories or JAR files. Combined with java.classpath, it can search for namespaces on the Java classpath. This namespace contains most of the functions in clojure.tools.namespace version 0.1.x.

clojure.tools.namespace.repl: Utilities to load and reload code based on the namespace dependency graph. This takes some explaining, see below. c.t.n.repl is built out of smaller parts:

  • c.t.n.dependency - generic dependency graph data structure
  • c.t.n.track - namespace dependency tracker
  • c.t.n.file - file-reader extension to tracker
  • c.t.n.dir - directory-scanner extension to tracker
  • c.t.n.reload - namespace-reloading extension to tracker

You can recombine these parts in other ways, but c.t.n.repl is the primary public entry-point to their functionality.

Reloading Code: Motivation

c.t.n.repl is a smarter way to reload code.

The traditional way to reload Clojure code without restarting the JVM is (require ... :reload) or :reload-all or an editor/IDE feature that does the same thing. This has several problems:

  • If you modify two namespaces which depend on each other, you must remember to reload them in the correct order to avoid compilation errors.

  • If you remove definitions from a source file and then reload it, those definitions are still available in memory. If other code depends on those definitions, it will continue to work but will break the next time you restart the JVM.

  • If the reloaded namespace contains defmulti, you must also reload all of the associated defmethod expressions.

  • If the reloaded namespace contains defprotocol, you must also reload any records or types implementing that protocol and create new instances.

  • If the reloaded namespace contains macros, you must also reload any namespaces which use those macros.

  • If the running program contains functions which close over values in the reloaded namespace, those closed-over values are not updated. (This is common in web applications which construct the "handler stack" as a composition of functions.)

Often the only surefire way to reload Clojure code is to restart the JVM. A large Clojure application can take 20 seconds or more just to compile. I wrote tools.namespace to help speed up this development cycle.

Reloading Code: Usage

There's only one important function, refresh:

user=> (use '[clojure.tools.namespace.repl :only (refresh)])
nil

user=> (refresh)
:reloading (com.example.util com.example.app com.example.app-test)
:ok

The refresh function will scan all the directories on the classpath for Clojure source files, read their ns declarations, build a graph of their dependencies, and load them in dependency order. (You can change the directories it scans with set-refresh-dirs.)

Later on, after you have changed and saved a few files in your editor, run it again:

user=> (refresh)
:reloading (com.example.app com.example.app-test)
:ok

Based on file modification timestamps and the graph of dependencies, the refresh function will reload only the namespaces that have changed, in dependency order. But first, it will unload the namespaces that changed to clear out any old definitions.

This is quite unlike (require ... :reload). Calling refresh will blow away your old code. Sometimes this is helpful: it can catch trivial mistakes like deleting a function that another piece of code depends on. But sometimes it hurts when you have built-up application state stored in a Var that got deleted by refresh.

This brings us to the next section:

Reloading Code: Preparing Your Application

Being able to safely destroy and reload namespaces without breaking your application requires some discipline and careful design. It won't "just work" on any Clojure project.

No Global State

The first rule for making your application reload-safe is no global state. That means you should avoid things like this:

(def state-of-world (ref {}))
(def object-handle (atom nil))

c.t.n.repl/refresh will destroy those Vars when it reloads the namespace (even if you used defonce).

Instead of storing your state in global Vars, store it locally in an object that represents the running state of your application. Then provide a constructor function to initialize that state:

(defn create-application []
  {:state-of-world (ref {})
   :object-handle (atom nil)})

You can choose what representation works best for your application: map, vector, record, or even just a single Ref by itself.

Typically you'll still need one global def somewhere, perhaps in the REPL itself, to hold the current application instance. See "Managing Reloads" below.

Managed Lifecycle

The second rule for making your application reload-safe is having a consistent way to start and stop the entire system. I like to do this with a protocol implemented by each major component in the system:

(defprotocol Lifecycle
  (start [component])
  (stop [component]))

Smaller applications can probably get along fine with just a pair of functions.

The point is that you need a convenient way to destroy all the built-up state of your application and then recreate it from scratch. The "stop" function should stop any running processes, release all external resources, and zero-out any internal state. The "start" function should create internal state, acquire resources, and start processes.

It might take a few tries to get it right, but once you have a working start/stop you can have a workflow like this:

Step 1. Start up a REPL.

Step 2. Load the app:

user=> (use 'clojure.tools.namespace.repl)
user=> (refresh)
user=> (def my-app (create-application))
user=> (start my-app)

Step 3. Test it out.

Step 4. Modify some source files.

Step 5. Restart:

user=> (stop my-app)
user=> (refresh)
user=> (def my-app (create-application))
user=> (start my-app)

(You could also combine all those steps in a single utility function.)

After that, you've got a squeaky-clean new instance of your app running, in a fraction of the time it takes to restart the JVM.

Handling Errors

If an exception is thrown while loading a namespace, refresh stops, prints the namespace that caused the exception, and returns the exception. You can print the rest of the stacktrace with clojure.repl/pst; the exception itself is bound to *e.

user=> (refresh)
:reloading (com.example.app com.example.app-test)
:error-while-loading com.example.app
#<IllegalArgumentException java.lang.IllegalArgumentException:
  Parameter declaration cond should be a vector>

user=> (clojure.repl/pst)
IllegalArgumentException Parameter declaration cond should be a vector
        clojure.core/assert-valid-fdecl (core.clj:6567)
        clojure.core/sigs (core.clj:220)
        clojure.core/defn (core.clj:294)
        clojure.lang.Var.invoke (Var.java:427)
        ...

Remember that any namespaces which depend on the namespace that caused the exception do not exist at this point: they have been removed but not yet reloaded.

After you fix the problem, call refresh again and it will resume reloading where it left off.

user=> (refresh)
:reloading (com.example.app com.example.app-test)
:ok

If your current REPL namespace is one of those that has not yet been reloaded, then you will need to call refresh by its fully-qualified name clojure.tools.namespace.repl/refresh.

Managing Reloads

Some projects have a "project REPL" or a "scratch" namespace where you want keep state during development. You can use the functions disable-unload! and disable-reload! in clojure.tools.namespace.repl to prevent refresh from automatically un/reloading those namespaces.

Use this feature sparingly: it exists as a development-time convenience, not a work-around for code that is not reload-safe. Also, see the warnings about aliases, below. Aliases to reloaded namespaces will break if the namespace containing the alias is not reloaded also.

Warnings

Be careful when reloading the namespace in which you run your REPL. Because namespaces are removed when reloading, all your past definitions are lost. Either keep your REPL in a namespace which has no file associated with it, such as user, or put all your REPL definitions in a "scratch" namespace that can be reloaded.

Be careful when using fully-qualified symbol names without namespace aliases (require with no :as). If the namespace happens to be loaded already, it will not necessarily cause an error if you forget to require it, but the dependency graph of namespaces will be incorrect.

Beware of code which has references to old definitions, especially references to things you created in the REPL.

If you create your own instance of the dependency tracker, do not store it in a namespace which gets reloaded.

Warnings for Aliases

Namespace aliases created at the REPL will still refer to the old namespace after refresh. For example:

user=> (require '[com.example.foo :as foo])

user=> foo/bar

user=> (refresh)
:reloading (com.example.foo)
:ok

user=> foo/bar   ; this is the *old* foo/bar

If you try to recreate the alias with the new namespace, you will get an error:

user=> (require '[com.example.foo :as foo])
IllegalStateException Alias foo already exists in
namespace user, aliasing com.example.foo
clojure.lang.Namespace.addAlias (Namespace.java:224)

The only way out is to remove the alias before recreating it:

user=> (ns-unalias *ns* 'foo)
nil
user=> (alias 'foo 'com.example.foo)

Warnings for Protocols

When reloading namespaces which contain protocols, be careful that you do not leave any old instances of records or types implementing those protocols.

For example, if you have a namespace like this:

(ns com.example.foo)

(defprotocol IFoo
  (foo [this]))

(defrecord FooRecord []
  IFoo (foo [this] nil))

And you do something like the following at the REPL:

user=> (def my-foo (->FooRecord))
user=> (clojure.tools.namespace.repl/refresh)
user=> (foo my-foo)

You will get a confusing error message like this:

IllegalArgumentException
No implementation of method: :foo
of protocol: #'com.example.foo/IFoo
found for class: com.example.foo.FooRecord
clojure.core/-cache-protocol-fn (core_deftype.clj:527)

That's because my-foo is an instance of the old version of FooRecord, implementing the old version of IFoo. As far as the JVM is concerned, the old IFoo and the new IFoo are completely different classes.

To avoid this problem, always create new instances of records after a refresh.

Copyright and License

Copyright © 2012 Stuart Sierra

Licensed under the Eclipse Public License Version 1.0.

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