<s>
The	O
Java	B-Language
Class	I-Language
Loader	I-Language
is	O
a	O
part	O
of	O
the	O
Java	B-Language
Runtime	I-Language
Environment	O
that	O
dynamically	B-Operating_System
loads	I-Operating_System
Java	B-Device
classes	O
into	O
the	O
Java	B-Language
Virtual	I-Language
Machine	I-Language
.	O
</s>
<s>
The	O
Java	B-Device
run	O
time	O
system	O
does	O
not	O
need	O
to	O
know	O
about	O
files	O
and	O
file	O
systems	O
as	O
this	O
is	O
delegated	O
to	O
the	O
class	B-Language
loader	I-Language
.	O
</s>
<s>
A	O
software	B-Library
library	I-Library
is	O
a	O
collection	O
of	O
related	O
object	B-Language
code	I-Language
.	O
</s>
<s>
In	O
the	O
Java	B-Language
language	I-Language
,	O
libraries	O
are	O
typically	O
packaged	O
in	O
JAR	B-Language
files	I-Language
.	O
</s>
<s>
The	O
most	O
important	O
type	O
of	O
object	O
contained	O
in	O
a	O
Jar	B-Language
file	I-Language
is	O
a	O
Java	B-Language
class	I-Language
.	O
</s>
<s>
The	O
class	B-Language
loader	I-Language
is	O
responsible	O
for	O
locating	O
libraries	O
,	O
reading	O
their	O
contents	O
,	O
and	O
loading	O
the	O
classes	O
contained	O
within	O
the	O
libraries	O
.	O
</s>
<s>
A	O
class	O
with	O
a	O
given	O
name	O
can	O
only	O
be	O
loaded	O
once	O
by	O
a	O
given	O
class	B-Language
loader	I-Language
.	O
</s>
<s>
Each	O
Java	B-Language
class	I-Language
must	O
be	O
loaded	O
by	O
a	O
class	B-Language
loader	I-Language
.	O
</s>
<s>
Furthermore	O
,	O
Java	B-Device
programs	O
may	O
make	O
use	O
of	O
external	B-Library
libraries	I-Library
(	O
that	O
is	O
,	O
libraries	O
written	O
and	O
provided	O
by	O
someone	O
other	O
than	O
the	O
author	O
of	O
the	O
program	O
)	O
or	O
they	O
may	O
be	O
composed	O
,	O
at	O
least	O
in	O
part	O
,	O
of	O
a	O
number	O
of	O
libraries	O
.	O
</s>
<s>
When	O
the	O
JVM	B-Language
is	O
started	O
,	O
three	O
class	B-Language
loaders	I-Language
are	O
used	O
:	O
</s>
<s>
The	O
bootstrap	O
class	B-Language
loader	I-Language
loads	O
the	O
core	O
Java	B-Device
libraries	O
located	O
in	O
the	O
<JAVA_HOME>/jre/lib ( or <JAVA_HOME>/jmods>	O
for	O
Java	B-Device
9	O
and	O
above	O
)	O
directory	O
.	O
</s>
<s>
This	O
class	B-Language
loader	I-Language
,	O
which	O
is	O
part	O
of	O
the	O
core	O
JVM	B-Language
,	O
is	O
written	O
in	O
native	O
code	O
.	O
</s>
<s>
by	O
the	O
java.ext.dirs	O
system	O
property	O
)	O
.	O
</s>
<s>
The	O
system	O
class	B-Language
loader	I-Language
loads	O
code	O
found	O
on	O
java.class.path	O
,	O
which	O
maps	O
to	O
the	O
CLASSPATH	B-Language
environment	O
variable	O
.	O
</s>
<s>
The	O
Java	B-Language
class	I-Language
loader	I-Language
is	O
written	O
in	O
Java	B-Device
.	O
</s>
<s>
It	O
is	O
therefore	O
possible	O
to	O
create	O
your	O
own	O
class	B-Language
loader	I-Language
without	O
understanding	O
the	O
finer	O
details	O
of	O
the	O
Java	B-Language
Virtual	I-Language
Machine	I-Language
.	O
</s>
<s>
Every	O
Java	B-Language
class	I-Language
loader	I-Language
has	O
a	O
parent	O
class	B-Language
loader	I-Language
,	O
defined	O
when	O
a	O
new	O
class	B-Language
loader	I-Language
is	O
instantiated	O
or	O
set	O
to	O
the	O
virtual	O
machine	O
's	O
system	O
default	O
class	B-Language
loader	I-Language
.	O
</s>
<s>
to	O
load	O
or	O
unload	O
classes	O
at	O
runtime	O
(	O
for	O
example	O
to	O
load	O
libraries	O
dynamically	O
at	O
runtime	O
,	O
even	O
from	O
an	O
HTTP	B-Protocol
resource	O
)	O
.	O
</s>
<s>
This	O
is	O
one	O
of	O
the	O
foundations	O
of	O
CORBA	B-Architecture
/	O
RMI	B-Language
protocols	O
for	O
example	O
.	O
</s>
<s>
to	O
change	O
the	O
way	O
the	O
bytecode	B-Language
is	O
loaded	O
(	O
for	O
example	O
,	O
it	O
is	O
possible	O
to	O
use	O
encrypted	O
Java	B-Language
class	I-Language
bytecode	B-Language
)	O
.	O
</s>
<s>
to	O
modify	O
the	O
loaded	O
bytecode	B-Language
(	O
for	O
example	O
,	O
for	O
load-time	B-Operating_System
weaving	B-Application
of	O
aspects	O
when	O
using	O
aspect-oriented	B-Architecture
programming	I-Architecture
)	O
.	O
</s>
<s>
Jakarta	B-Language
EE	I-Language
(	O
formerly	O
Java	B-Language
EE	I-Language
and	O
J2EE	B-Language
)	O
application	O
servers	O
typically	O
load	O
classes	O
from	O
a	O
deployed	O
WAR	B-Language
or	O
EAR	B-Language
archive	O
by	O
a	O
tree	O
of	O
class	B-Language
loaders	I-Language
,	O
isolating	O
the	O
application	O
from	O
other	O
applications	O
,	O
but	O
sharing	O
classes	O
between	O
deployed	O
modules	O
.	O
</s>
<s>
So-called	O
"	O
servlet	B-Language
containers	I-Language
"	O
are	O
typically	O
implemented	O
in	O
terms	O
of	O
multiple	O
class	B-Language
loaders	I-Language
.	O
</s>
<s>
JAR	B-Language
hell	O
is	O
a	O
term	O
similar	O
to	O
DLL	O
hell	O
used	O
to	O
describe	O
all	O
the	O
various	O
ways	O
in	O
which	O
the	O
classloading	O
process	O
can	O
end	O
up	O
not	O
working	O
.	O
</s>
<s>
Three	O
ways	O
JAR	B-Language
hell	O
can	O
occur	O
are	O
:	O
</s>
<s>
If	O
versions	O
of	O
library	O
foo	O
use	O
the	O
same	O
class	O
names	O
,	O
there	O
is	O
no	O
way	O
to	O
load	O
the	O
versions	O
of	O
library	O
foo	O
with	O
the	O
same	O
class	B-Language
loader	I-Language
.	O
</s>
<s>
The	O
most	O
complex	O
JAR	B-Language
hell	O
problems	O
arise	O
in	O
circumstances	O
that	O
take	O
advantage	O
of	O
the	O
full	O
complexity	O
of	O
the	O
classloading	O
system	O
.	O
</s>
<s>
A	O
Java	B-Device
program	O
is	O
not	O
required	O
to	O
use	O
only	O
a	O
single	O
"	O
flat	O
"	O
class	B-Language
loader	I-Language
,	O
but	O
instead	O
may	O
be	O
composed	O
of	O
several	O
(	O
potentially	O
very	O
many	O
)	O
nested	O
,	O
cooperating	O
class	B-Language
loaders	I-Language
.	O
</s>
<s>
Classes	O
loaded	O
by	O
different	O
class	B-Language
loaders	I-Language
may	O
interact	O
in	O
complex	O
ways	O
not	O
fully	O
comprehended	O
by	O
a	O
developer	O
,	O
leading	O
to	O
errors	O
or	O
bugs	O
that	O
are	O
difficult	O
to	O
analyze	O
,	O
explain	O
,	O
and	O
resolve	O
.	O
</s>
<s>
The	O
OSGi	B-Application
Alliance	I-Application
specified	O
(	O
starting	O
as	O
JSR	O
8	O
in	O
1998	O
)	O
a	O
modularity	O
framework	O
that	O
aims	O
to	O
solve	O
JAR	B-Language
hell	O
for	O
current	O
and	O
future	O
VMs	O
in	O
ME	O
,	O
SE	O
,	O
and	O
EE	O
that	O
is	O
widely	O
adopted	O
.	O
</s>
<s>
Using	O
metadata	O
in	O
the	O
JAR	B-Language
manifest	O
,	O
JAR	B-Language
files	I-Language
(	O
called	O
bundles	O
)	O
are	O
wired	O
on	O
a	O
per-package	O
basis	O
.	O
</s>
<s>
To	O
remedy	O
the	O
JAR	B-Language
hell	O
problems	O
,	O
a	O
Java	B-Language
Community	I-Language
Process	I-Language
—	O
JSR	B-Language
277	I-Language
was	O
initiated	O
in	O
2005	O
.	O
</s>
<s>
The	O
resolution	O
—	O
Java	B-Language
Platform	I-Language
Module	I-Language
System	I-Language
—	O
intended	O
to	O
introduce	O
a	O
new	O
distribution	O
format	O
,	O
a	O
modules	O
versioning	O
scheme	O
,	O
and	O
a	O
common	O
modules	O
repository	O
(	O
similar	O
in	O
purpose	O
to	O
Microsoft	O
.NET	O
'	O
s	O
Global	O
Assembly	O
Cache	O
)	O
.	O
</s>
<s>
In	O
December	O
2008	O
,	O
Sun	O
announced	O
that	O
JSR	B-Language
277	I-Language
was	O
put	O
on	O
hold	O
.	O
</s>
<s>
The	O
Java	B-Language
Module	I-Language
System	I-Language
was	O
later	O
rebooted	O
as	O
"	O
project	O
Jigsaw	O
"	O
which	O
was	O
included	O
in	O
Java	B-Device
9	O
.	O
</s>
<s>
Released	O
in	O
2017	O
,	O
it	O
includes	O
support	O
for	O
modular	O
software	O
,	O
termed	O
"	O
Java	B-Language
Platform	I-Language
Module	I-Language
System	I-Language
"	O
,	O
which	O
is	O
controlled	O
on	O
source	O
level	O
with	O
module-info.java	O
files	O
.	O
</s>
<s>
It	O
follows	O
a	O
different	O
philosophy	O
from	O
the	O
OSGi	B-Application
architecture	O
that	O
aims	O
at	O
providing	O
modularity	O
for	O
the	O
Java	B-Language
Runtime	I-Language
Environment	O
in	O
a	O
backwards-compatible	O
way	O
that	O
uses	O
the	O
default	O
mechanism	O
of	O
loading	O
classes	O
that	O
the	O
JRE	O
provides	O
.	O
</s>
<s>
However	O
,	O
since	O
it	O
does	O
not	O
offer	O
the	O
ability	O
for	O
controlled	O
co-existence	O
of	O
libraries	O
with	O
different	O
versions	O
,	O
it	O
is	O
not	O
suited	O
for	O
tackling	O
the	O
JAR	B-Language
Hell	O
problem	O
.	O
</s>
