One of the many advantages of dynamic languages such as Ruby is the
ability to introspect---to examine aspects of the program from
within the program itself. Java, for one, calls this
feature reflection.
The word ``reflection''
conjures up an image of looking at oneself in the mirror---perhaps
investigating the relentless spread of that bald spot on the top of
one's head. That's a pretty apt analogy: we use reflection to examine
parts of our programs that aren't normally visible from where we stand.
In this deeply introspective mood, while we are contemplating our
navels and burning incense (being careful not to swap the two tasks),
what can we learn about our program? We might discover:
what objects it contains,
the current class hierarchy,
the contents and behaviors of objects, and
information on methods.
Armed with this information, we can look at particular objects
and decide which of their methods to call at runtime---even if the class of
the object didn't exist when we first wrote the code. We can also
start doing clever things, perhaps modifying the program as it's
running.
Sound scary? It needn't be. In fact, these reflection capabilities let
us do some very useful things. Later in this chapter we'll look at
distributed Ruby and marshaling, two reflection-based technologies
that let us send objects around the world and through time.
Have you ever craved the ability to traverse all the living objects
in your program? We have! Ruby lets you perform
this trick with ObjectSpace::each_object. We can use it to
do all sorts of neat tricks.
For example, to iterate over all objects of type Numeric, you'd
write the following.
a = 102.7
b = 95.1
ObjectSpace.each_object(Numeric) {|x| p x }
produces:
95.1
102.7
2.718281828
3.141592654
Hey, where did those last two numbers come from? We didn't define
them in our program. If you look on page 429, you'll
see that the Math module defines constants for e and PI; since we are
examining all living objects in the system, these turn up as
well.
However, there is a catch. Let's try the same example with different
numbers.
a = 102
b = 95
ObjectSpace.each_object(Numeric) {|x| p x }
produces:
2.718281828
3.141592654
Neither of the Fixnum objects we created showed up. That's because
ObjectSpace doesn't know about objects with immediate values:
Fixnum, true, false, and nil.
Once you've found an interesting object, you may be tempted to find
out just what it can do. Unlike static languages, where a variable's
type determines its class, and hence the methods it supports, Ruby
supports liberated objects. You really cannot tell exactly what an
object can do until you look under its hood.[Or under its bonnet, for
objects created to the east of the Atlantic.]
For instance, we can get a list of all the methods to which an object will
respond.
r = 1..10 # Create a Range object
list = r.methods
list.length
»
60
list[0..3]
»
["size", "end", "length", "exclude_end?"]
Or, we can check to see if an object supports a particular method.
r.respond_to?("frozen?")
»
true
r.respond_to?("hasKey")
»
false
"me".respond_to?("==")
»
true
We can determine our object's class and its unique object id, and test
its relationship to other classes.
Knowing about objects is one part of reflection, but to get the whole
picture, you also need to be able to look at classes---the methods
and constants that they contain.
Looking at the class hierarchy is easy. You can get the parent of any
particular class using Class#superclass. For
classes and modules, Module#ancestors lists both
superclasses and mixed-in modules.
klass = Fixnum
begin
print klass
klass = klass.superclass
print " < " if klass
end while klass
puts
p Fixnum.ancestors
If you want to build a complete class hierarchy, just run that code
for every class in the system. We can use ObjectSpace to iterate
over all Class objects:
ObjectSpace.each_object(Class) do |aClass|
# ...
end
We can find out a bit more about the methods and constants in a
particular object. Instead of just checking to see whether the object
responds to a given message, we can ask for methods by access level,
we can ask for just singleton methods, and we can have a look at
the object's constants.
class Demo
private
def privMethod
end
protected
def protMethod
end
public
def pubMethod
end
def Demo.classMethod
end
CONST = 1.23
end
Demo.private_instance_methods
»
["privMethod"]
Demo.protected_instance_methods
»
["protMethod"]
Demo.public_instance_methods
»
["pubMethod"]
Demo.singleton_methods
»
["classMethod"]
Demo.constants - Demo.superclass.constants
»
["CONST"]
Module.constants returns
all the constants available via a module, including
constants from the module's superclasses. We're not interested in
those just at the moment, so we'll subtract them from our list.
Given a list of method names, we might now be tempted to try calling them.
Fortunately, that's easy with Ruby.
C and Java programmers often find themselves writing some kind of
dispatch table: functions which are invoked based on a command. Think
of a typical C idiom where you have to translate a string to a
function pointer:
typedef struct {
char *name;
void (*fptr)();
} Tuple;
Tuple list[]= {
{ "play", fptr_play },
{ "stop", fptr_stop },
{ "record", fptr_record },
{ 0, 0 },
};
...
void dispatch(char *cmd) {
int i = 0;
for (; list[i].name; i++) {
if (strncmp(list[i].name,cmd,strlen(cmd)) == 0) {
list[i].fptr();
return;
}
}
/* not found */
}
In Ruby, you can do all this in one line. Stick all your command
functions into a class, create an instance of that class (we called it
commands), and ask that object to execute a method called the
same name as the command string.
commands.send(commandString)
Oh, and by the way, it does much more than the C version---it's
dynamic. The Ruby version will find new methods added at runtime just
as easily.
You don't have to write special command classes for send: it
works on any object.
"John Coltrane".send(:length)
»
13
"Miles Davis".send("sub", /iles/, '.')
»
"M. Davis"
Another way of invoking methods dynamically uses Method objects. A
Method object is like a Proc object: it represents a chunk of
code and a context in which it executes. In this case, the code is the
body of the method, and the context is the object that created the
method. Once we have our Method object, we can execute it sometime
later by sending it the message call.
trane = "John Coltrane".method(:length)
miles = "Miles Davis".method("sub")
trane.call
»
13
miles.call(/iles/, '.')
»
"M. Davis"
You can pass the Method object around as you would any other
object, and when you invoke Method#call, the method is run just
as if you had invoked it on the original object. It's like having a
C-style function pointer but in a fully object-oriented style.
You can also use Method objects with iterators.
def double(a)
2*a
end
mObj = method(:double)
[ 1, 3, 5, 7 ].collect(&mObj)
»
[2, 6, 10, 14]
As good things come in threes, here's yet another way to invoke
methods dynamically. The eval
method (and its variations such as
class_eval, module_eval, and instance_eval) will
parse and execute an arbitrary string of legal Ruby source code.
trane = %q{"John Coltrane".length}
miles = %q{"Miles Davis".sub(/iles/, '.')}
eval trane
»
13
eval miles
»
"M. Davis"
When using eval, it can be helpful to state explicitly the
context in which the expression should be evaluated, rather than using
the current context. You can obtain a context by calling
Kernel#binding at the desired point.
class CoinSlot
def initialize(amt=Cents.new(25))
@amt = amt
$here = binding
end
end
a = CoinSlot.new
eval "puts @amt", $here
eval "puts @amt"
produces:
$0.25USD
nil
The first eval evaluates @amt in the context of the instance
of class CoinSlot. The second eval evaluates @amt in
the context of Object, where the instance variable @amt is
not defined.
As we've seen in this section, there are several ways to invoke an
arbitrary method of some object: Object#send,
Method#call, and the various flavors of eval.
You may prefer to use any one of these techniques depending on your
needs, but be aware that eval is significantly slower than the
others (or, for optimistic readers, send and call are
significantly faster than eval).
require "benchmark" # from the Ruby Application Archive
include Benchmark
test = "Stormy Weather"
m = test.method(:length)
n = 100000
bm(12) {|x|
x.report("call") { n.times { m.call } }
x.report("send") { n.times { test.send(:length) } }
x.report("eval") { n.times { eval "test.length" } }
}
produces:
user system total real
call 0.220000 0.000000 0.220000 ( 0.214065)
send 0.210000 0.000000 0.210000 ( 0.217070)
eval 2.540000 0.000000 2.540000 ( 2.518311)
A hook is a technique that lets you trap some Ruby event, such
as object creation.
The simplest hook technique in Ruby is to intercept calls to methods
in system classes. Perhaps you want to log all the operating system
commands your program executes. Simply rename the method
Kernel::system[This Eiffel-inspired
idiom of renaming a
feature and redefining a new one is very useful, but be aware that
it can cause problems. If a subclass does the same thing, and
renames the methods using the same names, you'll end up with an
infinite loop. You can avoid this by aliasing your methods to a
unique symbol name or by using a consistent naming convention.]
and substitute it with one of your own that both logs the command and
calls the original Kernel method.
module Kernel
alias_method :old_system, :system
def system(*args)
result = old_system(*args)
puts "system(#{args.join(', ')}) returned #{result}"
result
end
end
system("date")
system("kangaroo", "-hop 10", "skippy")
produces:
Sun Jun 9 00:09:44 CDT 2002
system(date) returned true
system(kangaroo, -hop 10, skippy) returned false
A more powerful hook is catching objects as they are created.
If you can be
present when every object is born, you can do all sorts of interesting
things: you can wrap them, add methods to them, remove methods from them, add them
to containers to implement persistence, you name it. We'll show a
simple example here: we'll add a timestamp to every object as it's
created.
One way to hook object creation is to do our method renaming trick on
Class#new, the method that's called to allocate space for a new
object. The technique isn't perfect---some built-in objects, such as
literal strings, are constructed without calling new---but
it'll work just fine for objects we write.
class Class
alias_method :old_new, :new
def new(*args)
result = old_new(*args)
result.timestamp = Time.now
return result
end
end
We'll also need to add a timestamp attribute to every object in the
system. We can do this by hacking class Object itself.
class Object
def timestamp
return @timestamp
end
def timestamp=(aTime)
@timestamp = aTime
end
end
Finally, we can run a test. We'll create a couple of objects a few
seconds apart and check their timestamps.
class Test
end
obj1 = Test.new
sleep 2
obj2 = Test.new
obj1.timestamp
»
Sun Jun 09 00:09:45 CDT 2002
obj2.timestamp
»
Sun Jun 09 00:09:47 CDT 2002
All this method renaming is fine, and it really does work. However,
there are other, more refined ways to get inside a running
program. Ruby provides several callback methods that let you trap
certain events in a controlled way.
These techniques are all illustrated in the library descriptions for
each callback method. At runtime, these methods will be called by the
system when the specified event occurs. By default, these methods do
nothing. If you want to be notified when one of these events happens,
just define the callback method, and you're in.
Keeping track of method creation and class and module usage lets you
build an accurate picture of the dynamic state of your program. This
can be important. For example, you may have written code that wraps
all the methods in a class, perhaps to add transactional support or
to implement some form of delegation. This is only half the job: the
dynamic nature of Ruby means that users of this class could add new
methods to it at any time. Using these callbacks, you can write
code that wraps these new methods as they are created.
While we're having fun reflecting on all the objects and classes in
our programs, let's not forget about the humble statements that make
our code actually do things. It turns out that Ruby lets us look at
these statements, too.
First, you can watch the interpreter as it executes code.
set_trace_func
executes a Proc with all sorts of juicy
debugging information whenever a new source line is executed,
methods are called, objects are created, and so on. There's a full description
on page 422, but here's a taste.
class Test
def test
a = 1
b = 2
end
end
set_trace_func proc { |event, file, line, id, binding, classname|
printf "%8s %s:%-2d %10s %8s\n", event, file, line, id, classname
}
t = Test.new
t.test
produces:
line prog.rb:11 false
c-call prog.rb:11 new Class
c-call prog.rb:11 initialize Object
c-return prog.rb:11 initialize Object
c-return prog.rb:11 new Class
line prog.rb:12 false
call prog.rb:2 test Test
line prog.rb:3 test Test
line prog.rb:4 test Test
return prog.rb:4 test Test
There's also a method trace_var (described
on page 427) that lets you add a hook to a global variable; whenever
an assignment is made to the global, your Proc object is invoked.
A fair question, and one we ask ourselves regularly. Mental lapses
aside, in Ruby at least you can find out exactly ``how you got there''
by using the method caller,
which returns an Array of
String objects representing the current call stack.
def catA
puts caller.join("\n")
end
def catB
catA
end
def catC
catB
end
catC
Java features the ability to serialize objects, letting you
store them somewhere and reconstitute them when needed. You might
use this facility, for instance, to save a tree of objects that
represent some portion of application state---a document, a CAD
drawing, a piece of music, and so on.
Ruby calls this kind of serialization
marshaling.[Think of railroad marshaling yards
where individual cars are assembled in sequence into a complete
train, which is then dispatched somewhere.] Saving an object
and some or all of its components is done using the method
Marshal::dump. Typically, you will dump an entire object tree
starting with some given object. Later on, you can reconstitute the
object using Marshal::load.
Here's a short example. We have a class Chord that holds a
collection of musical notes. We'd like to save away a particularly
wonderful chord so our grandchildren can load it into Ruby Version
23.5 and savor it, too. Let's start off with the classes for Note
and Chord.
class Note
attr :value
def initialize(val)
@value = val
end
def to_s
@value.to_s
end
end
class Chord
def initialize(arr)
@arr = arr
end
def play
@arr.join('-')
end
end
Now we'll create our masterpiece, and use Marshal::dump to save
a serialized version of it to disk.
c = Chord.new( [ Note.new("G"), Note.new("Bb"),
Note.new("Db"), Note.new("E") ] )
File.open("posterity", "w+") do |f|
Marshal.dump(c, f)
end
Finally, our grandchildren read it in, and are transported by our
creation's beauty.
Not all objects can be dumped: bindings, procedure objects, instances
of class IO, and singleton objects cannot be saved outside of the
running Ruby environment (a TypeError will be raised if you try).
Even if your object doesn't contain one of these problematic objects,
you may want to take control of object serialization yourself.
Marshal provides the hooks you need. In the objects that require
custom serialization, simply implement two methods: an instance method
called _dump,
which writes the object out to a string, and a
class method called _load, which reads a string that you'd
previously created and converts it into a new object.
For instance, here is a sample class that defines its own
serialization.
For whatever reasons, Special doesn't want to save one of its
internal data members, ``@volatile''.
class Special
def initialize(valuable)
@valuable = valuable
@volatile = "Goodbye"
end
def _dump(depth)
@valuable.to_str
end
def Special._load(str)
result = Special.new(str);
end
def to_s
"#{@valuable} and #{@volatile}"
end
end
a = Special.new("Hello, World")
data = Marshal.dump(a)
obj = Marshal.load(data)
puts obj
produces:
Hello, World and Goodbye
For more details, see the reference section on Marshal
beginning on page 428.
Since we can serialize an object or a set of objects into a form
suitable for out-of-process storage, we can use this capability for
the transmission of objects from one process to another.
Couple this capability with the power of networking, and
voilą: you have a distributed object system. To save you
the trouble of having to write the code, we suggest downloading
Masatoshi Seki's Distributed Ruby library (drb) from the RAA.
Using drb, a Ruby process may act as a server, as a client, or as both. A
drb server acts as a source of objects, while a client is a user of
those objects. To the client, it appears that the objects are local,
but in reality the code is still being executed remotely.
A server starts a service by associating an object with a given port.
Threads are created internally to handle incoming requests on that
port, so remember to join the drb thread before exiting your program.
require 'drb'
class TestServer
def doit
"Hello, Distributed World"
end
end
aServerObject = TestServer.new
DRb.start_service('druby://localhost:9000', aServerObject)
DRb.thread.join # Don't exit just yet!
A simple drb client simply creates a local drb object and associates
it with the object on the remote server; the local object is a proxy.
require 'drb'
DRb.start_service()
obj = DRbObject.new(nil, 'druby://localhost:9000')
# Now use obj
p obj.doit
The client connects to the server and calls the method doit, which
returns a string that the client prints out:
"Hello, Distributed World"
The initial nil argument to DRbObject indicates that we want
to attach to a new distributed object. We could also use an
existing object.
Ho hum, you say. This sounds like Java's RMI, or CORBA, or whatever.
Yes, it is a functional distributed object mechanism---but it is
written in just 200 lines of Ruby code. No C, nothing fancy, just
plain old Ruby code. Of course, there's no naming service or trader
service, or anything like you'd see in CORBA, but it is simple and
reasonably fast. On the 233MHz test system, this sample code runs at
about 50 remote message calls per second.
And, if you like the look of Sun's JavaSpaces, the basis of their JINI
architecture, you'll be interested to know that drb is distributed with
a short module that does the same kind of thing. JavaSpaces is based
on a technology called Linda. To prove that its Japanese author has a
sense of humor, Ruby's version of Linda is known as ``rinda.''
The important thing to remember about Ruby is that there isn't a big
difference between ``compile time'' and ``runtime.'' It's all the
same. You can add code to a running process. You
can redefine methods on the fly, change their scope from public
to private, and so on. You can even alter basic types, such
as Class and Object.
Once you get used to this flexibility, it is hard to go back to a
static language such as C++, or even to a half-static language such as
Java.
But then, why would you want to?
