Dynamic Class Loading in the Java
Sheng Liang Gilad Bracha
Sun Microsystems Inc.
901 San Antonio Road, CUP02-302
Palo Alto, CA 94303
Class loaders are a powerful mechanism for dynamically
loading software components on the Java platform. They
are unusual in supporting all of the following features:
laziness, type-safe linkage, user-deﬁned extensibility, and multiple
We present the notion of class loaders and demonstrate
some of their interesting use s. In addition, we discuss how to
maintain type safety in the presence of user-deﬁned dynamic
In this paper, we investigate a n important feature of the
Java virtual machine: dynamic class loading. This is the
underlying mechanism that provides much of the power of
the Java platform: the ability to install software components
at runtime. An example of a component is an applet that is
downloaded into a web browser.
While many other sys tems   also support some
form of dynamic loading and linking, the Java platform is
the only system we know of that incorporates all of the
1. Lazy loading. Classes are loaded on demand. Class
loading is delayed as long as possible, reducing mem-
ory usage and improving system response time.
2. Type-safe linkage. Dynamic class loading must not
violate the type safety of the Java virtual machine.
Dynamic loading must not require additional run-time
check s in order to guarantee type safety. Additional
link-time checks are acceptable, because these checks
are performed only once.
3. User-deﬁna ble class loading policy. Class loaders are ﬁrst-
class obj e cts. Programmers have complete control of
dynamic cla ss loading. A user-deﬁned class loader can ,
for example, specify the remote location from which
To appear in the 13t h Annual ACM SIGPLAN Conference
on Object-Oriented Programming Systems, Languages,
and Applications (OOPSLA’98), Vancouver, BC, Canada,
October, 1998 .
the classes are loaded, or assign appropriate security
attributes to classes loaded from a particular source.
4. Multiple namespaces. Class loaders provide separate
namespaces for different software components. For
example, the Hotjava
browser loads applets from
different sources into separate class loaders. These
applets may contain c lasses of the same name, but the
classes are treated as distinct types by the Java virtual
In contrast, existing d ynamic linking mechan isms do
not support all of these features. Although most operating
systems support some form of dyna mic linked libraries, su ch
mechanis ms are targeted toward C/C++ code, and are not
type-safe. Dynamic languages such as Lisp , Smalltal k
, and Self  achieve type safety through additional
run-time checks, not link-time checks.
The main contribution of this paper is to provide the
ﬁrst in-depth description of class loaders, a novel c oncept
introduced by the Java platform. Class l oaders existed in
the ﬁrst version of the Java Development Kit (JDK 1 .0). T he
original purpose was to enable applet c lass loading in the
Hotjava browser. Since that time, the use of class loaders
has been extended to handle a wider range of software
components such as server-side components (servlets) ,
extensions  to the Java platform, and JavaBeans 
components. Despite the increasingly important role of class
loaders, the u nderlying mechanism has not been adequately
described in the literature.
A further contribution of this paper is to present a
solution to the long-standing type safety problem  with
class loaders. Early ver sions (1.0 and 1.1) of the JDK
contained a serious ﬂaw in class loader implementation.
Improperly written class loaders could defeat the type safety
guarantee of the Java virtual machine . Note that the type
safety problem did not impose any immediate security risks,
because untrusted code (such as a d ownloaded applet) was
not allowed to create c lass loaders. Nonetheless, application
programmers who had the need to write custom class loaders
could compromise type safety inad ver tently. Although the
issue had been known for some time, it remained an open
problem in the research community whether a satisfactory
solution exists. For example, earlier discussions centered
around whether the lack of type safety was a fundamental
limitation of user-deﬁnable class loaders, and whether we
would have to limit the power of class loaders, give up
lazy class loading, or introduce additional dynamic type-
check ing at runtime. The solution we present in this paper,
which has been implemented in JDK 1.2, solves the type
safety problem while preserving all of the other desira ble
features of class loaders.
We assume the reader has basic knowledge of the Java
programming language . The remainder of this paper
is organized as follows: We ﬁrst give a more detailed
introduction to class loaders. Applications of class loaders
are dis cussed in section 3. Section 4 describes the type safety
problems that may arise due to the use of class loaders, a nd
their solutions. Section 5 relates our work to other research.
Finally, we present our conclusions in section 6.
The purpose of class loaders is to support dynamic loading
of software components on the Java platform. The unit of
software dis tribution is a class
. Classes are distributed us-
ing a machine-independent, standard, binar y representation
known as the class ﬁle format . The representation of an
individua l class is referred to as a class ﬁ le. Class ﬁles are
produced by Java compilers, a nd can be loaded into any Java
virtual machine. A class ﬁle does not have to be stored in an
actual ﬁle; it could be stored in a memory buffer, or obtained
from a network stream.
The Java virtual ma chine executes the byte code stored
in clas s ﬁles. Byte code sequences, however, are only part
of what the virtua l machine needs to execute a program. A
class ﬁle also contains s ymbolic references to ﬁelds, methods,
and names of other clas ses. Consider, for example, a class
declared as follows:
The cl ass ﬁle representing contains a symbolic reference
. Symbolic references are resolved at link time to
actual class types. Class types are reiﬁed ﬁrst-class objects in
the Java virtual machine. A class type is represented in user
code as an object of class
. In order to resolve a
symbolic reference to a class, the Java virtual machine must
load the cla ss ﬁle and create the class type.
The Java virtual machine uses class loaders to l oad class
ﬁles and create class objects. Class loaders are ordinary
objects that can be deﬁned in Java code. They are instances
of subclasses of the class , shown in Figure 1.
We have omitted the methods that are not directly relevant
Throughout this paper, we use the term class generically to denote both
classes and interfaces .
Figure 1: The class
Application class loader
System class loader
Applet class loaders
Figure 2: Class loaders in a web browser
to this presentation. The
takes a c lass name as argument, and returns a
that is the run-time representation of a class type. The
will be des cribed later.
In the a bove example, assume that
is loaded by the
class loader . i s referred to as ’s deﬁning loader. The Java
virtual machine will use
to load classes referenced by .
Before the virtual machine allocates an object of class
must resolve the reference to If has not yet been loaded,
the virtual machine will invoke the
method of ’s
Once has been loaded, the virtual machine can resolve
the reference and create an object of class .
A Java application may use several different kinds of class
loaders to manage various software components. For exam-
ple, Figure 2 shows how a web browser written in Java may
use class loaders.
This example illustrates the use of two types of class
loaders: user-deﬁned class loaders and the system class
loader supplied by the Java virtua l machin e . User-deﬁned
class loaders can be u sed to create classes that originate from
user-deﬁned sources. For example, the browser application
creates class loaders for downloaded applets. We use a
We use the notation to refer to an instance method deﬁned in
, although this is not legal syntax in the Java pro gramming language.
separate class loader for the web browser application itself.
All system c lasses (such as
) are loaded into
the system class loader. The sys tem class loader is s upported
directly by the Java virtual machine.
The arrows in the ﬁgure indicate the delegation relation-
ship between class loaders. A class loader
can ask another
to load a c lass on its behalf. In such a case,
delegates to . For example, applet and application class
loaders delegate all system classes to the system class loader.
As a result, all system c lasses are shared among the applets
and the application. This is desirabl e because type safety
would be violated if, for example, applet and system code
had a different notion of what the type
Delegating class loaders allow us to maintain namespace
separation while still sharing a common se t of classes. In
the Java virtual machine, a class type is uniquely determined
by the combination of the class na me and class loa der. Applet
and application class loaders delegate to the system class
loader. This guarantees that all system cla ss types, such
as , are unique. On the other hand, a class
loaded in applet 1 is considered a different type
from a cla ss named
in applet 2. Although these two
classes have the same name, they are deﬁned by different
class loader s. In fact, these two classes can be completely
unrelated. For example, they may have different methods or
Classes from one applet cannot interfere with classes in
another, because applets are loaded in separate class load-
ers. This is crucial in guaranteeing Java platform security.
Likewise, because the b rowser resides in a separate class
loader, applets cannot access the classes used to implement
the browser. Applets are only allowed to acces s the standard
Java API exposed in the system classes.
The Java virtual mac hine starts up by creating the appli-
cation class loader and using it to load the i nitial browser
class. Application execution starts in the public class method
of the initial clas s. The invocation of this
method drives a ll fur ther execution. Execution of instruc-
tions may cause loading of a dditional class e s. In this
application, the browser also creates additional class loaders
for downloaded applets.
The garbage collector unloads applet cla sses that are no
longer referenc e d. E ach class object contains a reference to
its deﬁning loader; each class l oader refers to all the classes it
deﬁnes. This means that, from the garbage collector’s point
of view, classes are strongly connected with their deﬁning
loader. Classes are unloaded when their deﬁning loader is
We now walk through the implementation of a simple class
loader. As noted earlier, all user-deﬁned class loader classes
are subclasses of
. Subclasses of can
override the deﬁnition of , thus providing a user-
deﬁned loading pol icy. Here is a class loader that looks up
classes in a given directory:
The public constructor simply records
the directory name. In the deﬁnition of
, we use
the method to check whether the class has
already been loaded. (Section 4.1 will give a more precise de-
scription of the method.) If
returns , the class has not yet been loaded. We then dele-
gate to the system class loader by calling
the class we are trying to load is not a system class, we call
a helper method
to read in the class ﬁle.
After we have read in the class ﬁle, we pass it to the
method. The method constructs the
run-time representation of the class from the class ﬁle. Note
method syn chronizes on the class loader
object so tha t multiple threads may not load the same class
at the same time.
When one class loader delegates to another class loader, the
class loader that initiates the loading is not necessaril y the
same loader that completes the loading and deﬁnes the class.
Consider the following code segment:
Instances of the class delegate the load-
ing of to the system loader. Consequently,
is deﬁned by the system loader, even though
loading was i nitiated by
Deﬁnition 2.1 Let
be the result of . is the
deﬁning loader of or e quivalently, deﬁne s
Deﬁnition 2.2 Let be the result of . is an
initiating loader of
or e quivalently, initiates loading of
Figure 3: Class redirects to a new version of
In the Java virtual machine, every cl ass
associated with its deﬁning loader. It is
’s deﬁning loader
that initiates the loading of any class referenced by
In this section, we give a few examples that demonstrate the
power of cla ss loaders.
It is often desirable to upgrade software components in a
long-running application such as a ser ver. The upgrade must
not require the appl ication to shut down and restart.
On the Java platform, this ability translates to reloading
a subset of the classes already l oaded in a running virtual
machine. It corresponds to the schema evolution  problem,
which could be rather difﬁcult to solve in general. Here are
some of the difﬁculties:
There may b e live objects that are instances of a class
we want to reload. These objects must be migrated to
conform to the schema of the new class. For example,
if the new version of the class contains a different set
of instance ﬁelds, we must somehow map the existing
set of instance ﬁeld values to ﬁeld s in the new version
of the class.
Similarly, we may have to map the static ﬁeld values
to a different set of static ﬁelds in the reloaded version
of the class.
The application may be executing a method that be-
longs to a class we want to reload.
We do not address these problems in this paper. Instead,
we show how it is sometimes possible to bypass them using
class loaders. By organizing software components in separate
class loaders, programmers can often avoid dealing with
schema evolution. Instead, n e w classes are loaded by a
Figure 3 illustrates how a
class can dynamically
redirect the service requests to a new version of the
class. The key technique is to load the server class, old service
class, and new service class into separate class loaders. For
example, we can deﬁn e
using the class
introd uced in the last section.
The method redirects all incoming
requests to a
object stored in a private ﬁeld. It uses
the Java Core Reﬂection API  to invoke the “ ” method
object. In addition, the
method allows a new version of the class to b e
dynamically loaded, replacing the existing object.
supply the the location of the new
class ﬁles . Further requests will be redirected to the new
object referenced to by
To make reloading possi ble, the
class must not
directly refer to the
Once the class resolves the symbolic reference to
a class, it will contain a hard link to that clas s type.
An already-resolved reference cannot be changed. The type
conversion in the last line of the method
will fail for new versions of returned from the class
Reﬂection allows the
class to use the class
without a direct reference. Alternatively,
classes can share a common interface or superclass:
Dispatching through an interface i s typically more efﬁ-
cient than reﬂection. The interface type itself must not be
reloaded, because the
class can refer to only one
type. The method must return a
class that implements the same
After we call the method, all future requests
will be processed by the new
class. The old
class, however, may not have ﬁnished processing some of
the earlier requests. Thus two
classes may coexist
for a while, until all uses of the old cla ss are comple te, all
references to the old class are dropped, and the old cla ss is
A class loader can instrument the class ﬁle before makin g the
call. For example, in the example,
we can insert a call to change the contents of the class ﬁle:
An instrumented class ﬁle must be valid ac cording to
the Java virtual machine speciﬁcation . The virtual
machine will apply all the usual checks (such as run ning
the byte code veri ﬁer) to the instrumented class ﬁle. As
long as the class ﬁle format is obeyed, the programmer has
a great deal of freedom in modifying the class ﬁle. For
example, the instrumented class ﬁle may contain new byte
code instructions in e xisting methods, new ﬁelds, or ne w
methods. I t is also possible to delete existing methods, but
the resulting class ﬁle might not link with other cla sses.
The in strumented class ﬁle must deﬁne a class of the
same name as the original class ﬁle. The
should return a class object whose name matches the name
passed in as the argument. (Section 4.1 explains how this
rule is enforced by the vir tual machine.)
A class loader can only instrument the classes i t deﬁnes ,
not the classe s delegated to other loaders. All u ser-deﬁned
class loaders s hould ﬁrst delegate to the system class loader,
thus system classes cannot be instrumen ted through class
loaders. User-deﬁned class l oaders cannot bypass this re-
striction by trying to deﬁne system c lasses themselves. If,
for example, a cla ss l oader deﬁnes its own
cannot pass an object of that class to a Java API that expects
object. The virtu al machine will catch and
report these type errors (see section 4 for detail s).
Class ﬁle instrumentation is useful in many circum-
stances. For example, an instrumented class ﬁle may contain
proﬁlin g hooks that count how many times a certain method
is executed. Resource allocation may be monitored and
controlled by substituting references to certain classes with
references to resource-conscious versions of those classes
. A cla ss loader may be us e d to implement parameter-
ized classes, expanding and tailoring the code in a class ﬁ le
for each distinct invocation of a parametric type .
The examples presented so far have demonstrated the use-
fulness of multiple dele gating class loaders. As we will
see, however, ensuring type-safe linkage in the presence of
class loaders requires special care. The Java programming
langu age relies on name-based static typing. At compile
time, each static class type corresponds to a class name. At
runtime, class loaders introduce multiple namespaces. A
run-time class type is determined not by its name alone, but
by a pair: its class name and its deﬁning class loader. Hence,
namespaces introduced by user-deﬁned class loaders may
be inconsistent with the na mes pace managed by the Java
compiler, jeopardizing type safety.
The method may return differe nt class types for a
given name at different times. To maintain type safety, the
virtual machine must be able to consistently obtain the same
class type for a given class name and l oader. Consider, for
example, the two references to class
in the following c ode:
If ’s class loader were to map the two occurrences of
into different class types, the type safety of the method call
to inside would be compromised.
The virtual machine cannot trust any user-deﬁned
method to consistently return the same type for a given
name. In stead, it internall y maintains a loaded class cache. The
loaded cl ass c ache maps class names and initiating loaders
to class types. After the virtual machine obtains a cl ass from
the method, it performs the following operations:
The real name of the class is checked against the name
passed to the
method. An error is r aised
returns a c lass that does not have the
If the name matches, the resulting class is cached in the
loaded class cache. The virtual ma chine never invokes
method with the same name on the same
class loader more than once.
The method introduced in
section 2 performs a lookup in the loaded class cache.
We now describe the type safety problems that can arise with
delegating class loaders. The problem has been known for
some time. The ﬁrst published account was given by Vijay
Notatio n 4.1 We will represent a class type using the notation
, where denotes the n ame of the clas s, denotes the
class’s deﬁning loader, and denotes the loader that initiated
class loading. When we do not care about the deﬁning loader, we
use a simpliﬁed notation
to denote that is the initiating
. When we do not ca re about the initiating load er, we
use the simpliﬁed notation to de note that is deﬁned by
Note that if
delegates to , then = .
We will now give an e xample that demonstrates the type
safety problem. In order to make clear which class loaders
are involved, we use the above notation where class names
would ordinarily appear.
is deﬁned by . As a result, is used to initiate
the loading of the classes
deﬁnes Howe ver, delegates the
loading of to , which then deﬁnes
Because is deﬁned by , will use
to initiate the loading of As it happens, deﬁnes a
different type expects an instance of
to be returned by However, actually
returns an instance of
, which is a completely
different clas s.
This is an inconsistency between the namespaces of
and . If this inconsistency goes undetec ted, it allows one
type to be forged as another type using delegating c lass
loaders. To see a how this type safety problem can lead to
undesir able behaviors, suppose the two versions of
are deﬁned as follows:
Class is now able to reveal a private ﬁeld of an
and forge a pointer from an integer
We can access the private ﬁeld in a
instance because the ﬁeld is declared to be public in
. We are also able to forge an integer ﬁeld
instance as an integer array, and deref-
erence a pointer that is forged from the integer.
The underlying cause of the type-safety problem was the
virtual machine’s failure to take into acc ount that a class type
is determined by both the class name and the deﬁning loader.
Instead, the virtual machine relied on the Java programming
langu age notion of us ing class names alone as types during
type check ing. The problem has since been corrected, as
A straightforward solution to the type-safety problem is to
uniformly use both the class’s name and its deﬁning loader
to represent a class type in the Java virtual machine. The
only way to determine the deﬁning loader, however, is to
actually load the class through the initiating loader. In the
example in the previous section, before we can determine
’s call to is type-safe, we must ﬁrst
in both and , and see whether we obtain
the same deﬁning loader. The shortcoming of this approach
is that it sacri ﬁces lazy clas s loading.
Our solution pres e rves the type safety of the straightfor-
ward approach, but avoids eager class loading. The key idea
is to maintain a se t of loader constraints that are dynamically
updated as c lass loading takes place. In the above example,
instead of loading
in and , we simply record
a constraint that . If is later
or , we will need to verify that the existing
set of loader constraints will not be violated.
What if the constraint
is loaded by both and ? It is
too late to impose the constraint and undo previous class
We must therefore take both the loaded class cache and
loader constraint set into account at the same time. We need
to mai ntain the invariant: Each entry in the loaded class cache
satisﬁes all the loader constraints. The invariant is maintained
Every time a new entry is ab out to be added to the
loaded c lass cache, we verify that none of the existing
loader constraints will be violated. If the new entry
cannot be added to the loaded class cache without
violating one of the existing loader constraints, class
Every time a new loader constraint is added, we
verify that all loaded classes in the cache satisfy the
new constraint. If a new loader constraint cannot
be satisﬁed by all loaded classes, the operation that
triggered the addition of the new loader constraint
Let us see how these check s can be applied to the previous
example. The ﬁrst line of the
method causes the virtual
machine to generate the constraint
If and have already loaded the class when we
generate this constrain t, an exception will immediately be
raised in the program. Otherwise, the constraint will be suc-
cessfully recorded. Assuming
ﬁrst, an exception will be raised when tries to load
We now state the rules for generating constraints. These
correspond to situations when one class type may be referred
to by another class. When two such classes are deﬁned in
different loaders, there are opportunities for inconsistencies
across namespaces .
If references a ﬁeld:
declared in class , then we gen e rate the con-
If references a method:
declared in class , then we gen e rate the con-
If overrides a method:
declared in class , then we gen e rate the con-
The constraint set indicates
must be loaded as the same class type in and
, and in and . Even if, during the execution of the
program, is never loaded b y , distinct versions of
could not be loaded by and .
If the loader constraints are violated, a
exception will be thrown. Loader constraints are removed
from the constraint set when the corresponding class loader
is garbag e -collected.
Saraswat has suggested another approac h to maintaini ng
type safety in the presence of delegating class loaders. That
proposal differs from ours in that it suggests that method
overriding should also be b ased upon dynamic types rather
than static (name-based) types. Saraswat’s idea is appealing,
in that it uses the dynamic concept of type uniformly from
link time onwards.
The following c ode illustrates the differences between his
model and ours:
Assume that and deﬁne different versions of
Saraswat considers the methods in and
to have different type signatures: takes an argument
of type whereas takes an argument of
. As a consequence, is not considered
in this model.
In our model, if is loaded by a linka ge error
results at the point where
is called. The behavior in
Saraswat’s model is very similar: a
The difference in approach becomes apparent when
is loaded by In our model, when is loaded by
the call to would invoke A linkage error would be
raised when code2 attempted to acce ss any ﬁelds or methods
of In Saraswat’s model the call to executes
(that is, does not override ).
We believe it is better to fail in this case than to silently
run code that was not meant to be executed. A programmer’s
expectation when writing the classes and above
does override in accordance with
the semantics of the Java programming languag e . These
expectations are violated in Saraswat’s proposal.
Saraswat also suggests a modiﬁcation to the class loader
API tha t would allow the virtual machine to determine
the run-time type of a symbolic reference without actually
loading it. This is necessary in order to implement his
proposal without the penalty of excessive class loading. We
believe it would be worth exploring this i dea independently
of the other aspects of Sar aswat’s proposal.
Other proposals have also focused on changing the pro-
tocol of the
class, or subdividing its functionality
among several classes. Such changes typical ly reduc e the
expressive power of class loaders.
Class loaders can be thought of as a reﬂective hook into the
system’s loading mechanism. Reﬂective systems in other
object-oriented languages [6, 14] have provided users the
opportunity to modify various aspects of system behavior.
One could use such mechanisms to provide user-extensible
class loading; however, we are n ot aware of any such
Some Lis p dialects [17 ] and some functional languages
 have a notion of ﬁrst-class environments, which support
multiple namespaces similar to those discussed in this paper.
Dean   has discussed the problem of type safety in
class loaders from a theoretical perspective. He suggests a
deep link between cla ss loading and dynamic scoping.
Jensen et al.  recently proposed a formalization of
dynamic class loading in the Java virtual machine. Among
other ﬁndings, the formal approach conﬁrmed the type
safety problem with cla ss loader s.
Roskind  has put in place link-time checks to ensure
class loader type safety in Netscape’s Java virtu al machine
implementation. The checks he implemented are more eager
and strict than ours.
The Oberon/F system [1 6] (now renamed Component
Pasc al) allows dyna mic loading and type-safe linkage of
modules. However, the dynamic loading mechanism is not
under user control , nor does it provide multiple namespaces.
Dynamically linked libraries have been supported by
many operating systems. These mechanisms typically do
not provide type-safe lin kage.
We have presented the notion of class loaders in the Java
platform. Class loaders combine four desirable features:
lazy loading, type-safe linkage, multiple namespaces, and
user extensibility. Type safety, in particular, requires special
attention. We have shown how to pres e rve type safety
without restricting the power of class loaders.
Class loaders are a simple yet powerful mechanism that
has proven to be extremely valuable in managing software
The authors wish to than k Drew Dean, Jim Roskind, and
Vijay Saraswat for focusing our attention on the type safety
problem, and for many valuable exchanges.
We owe a debt to David Connelly, Li Gong, Benjamin
Renaud, Roland S chemers, Bill Shannon, and many of our
other colleagues at Sun Java Software for countless discus -
sions on security and class loaders. Arthur van Hoff ﬁrst
conceived of class loaders.
Bill Maddox, Marianne Mueller, Nicholas Sterling, David
Stoutamire, and the anonymous reviewers for OOPSLA’98
suggested numerous improvements to this paper.
Finally, we thank James Gosling for creating the Java
 Ole Ages e n, Stephen N. Freund, and John C. Mitchell.
Adding type parameterization to the Java language.
In Proc. of the ACM Conf. on Object-Oriented Program-
ming, Systems, Languages and Applications, pages 49–65,
October 19 97.
 Andrew W. Appe l and David B. MacQueen. Standard
ML of New Jersey. In J. Maluszy ´nski and M. Wirsing,
editors, Programming Language Implementation and Logic
Programming, pages 1–13. Springer-Verlag, August 1991.
Lecture Notes in Computer Science 528.
 Gilles Barbedette. Schema modiﬁcations in the LISP
persistent object-oriented language. In European
Conference on Object-Oriented P rogramming, pages 77–96,
 Drew Dean, 1997. Private communication.
 Drew Dean. The security of static typing with dynamic
linking. In Fou rth ACM Conference on Computer an d
Communications Security, pages 18–27, April 1997.
 A. Goldberg and D. Robson. Smalltalk-80: the Language
and Its Implementation. Addison-Wesley, 1983.
 James Gosling, Bill Joy, and Guy Steele. The Java
Langu age Speciﬁcation. Addison-Wesley, Reading, Mas-
 JavaSoft, Sun Microsystems, Inc. JavaBeans Components
API for Java, 1997. JDK 1.1 documentation, available at
 JavaSoft, Sun Microsystems, Inc. Reﬂection,
1997. JDK 1.1 documentation, available at
 JavaSoft, Sun Microsystems, Inc. The Java Extensions
Framework, 1998. JDK 1.2 documentation, available at
 JavaSoft, Sun Microsystems, Inc. Servlet,
1998. JDK 1.2 documentation, available at
 Thomas Jensen, Daniel Le Metayer, and Tommy Thorn.
Security and dynamic class loading in Java: A formali-
sation. In Proceedings of IEEE International Conference on
Computer Languages, Chicago, Illinois, pages 4–15, May
 Sonya E. Keene. Object-Oriented Programming in Common
Lisp. Addison-Wesley, 1989.
 Gregor Kiczales, Jim de s Rivieres, and Daniel G. Bobrow.
The Art of the Metaobject Protocol. MIT Press, Cambridge,
 Tim Lind holm and Frank Yellin. The Jav a Virtual Machine
Speciﬁcation. Addison-Wesley, Reading, Massachusetts,
 Oberon Microsystems, Inc. Component Pas-
cal Language Report, 1997. Available at
 Jonathan A. Rees, Norman I. Adams, and James R.
Meehan. The T Manual, Fourth Edition. Department of
Computer Science, Yale University, January 198 4.
 Jim Roskind, 1997. Private communic ation.
 Vijay Saraswat. Matrix design notes.
 Vijay Saraswat. Java is not type-safe. available at
˜ , 1997.
 David Ungar and Randall Smith. SELF: The power of
simplicity. In Proc. of the ACM Conf. o n Object-Oriented
Programming, Systems, Languages and Applications, Octo-