<s>
EAR	O
(	O
Enterprise	B-Application
Application	I-Application
aRchive	B-Application
)	O
is	O
a	O
file	O
format	O
used	O
by	O
Jakarta	B-Language
EE	I-Language
for	O
packaging	O
one	O
or	O
more	O
modules	O
into	O
a	O
single	O
archive	B-Application
so	O
that	O
the	O
deployment	O
of	O
the	O
various	O
modules	O
onto	O
an	O
application	B-Application
server	I-Application
happens	O
simultaneously	O
and	O
coherently	O
.	O
</s>
<s>
It	O
also	O
contains	O
XML	B-Protocol
files	O
called	O
deployment	B-Language
descriptors	I-Language
which	O
describe	O
how	O
to	O
deploy	O
the	O
modules	O
.	O
</s>
<s>
Ant	B-Application
,	O
Maven	B-Language
,	O
or	O
Gradle	B-Language
can	O
be	O
used	O
to	O
build	O
EAR	O
files	O
.	O
</s>
<s>
An	O
EAR	O
file	O
is	O
a	O
standard	O
JAR	B-Language
file	I-Language
(	O
and	O
therefore	O
a	O
Zip	B-General_Concept
file	I-General_Concept
)	O
with	O
an	O
.ear	B-Language
extension	O
,	O
with	O
one	O
or	O
more	O
entries	O
representing	O
the	O
modules	O
of	O
the	O
application	O
,	O
and	O
a	O
metadata	O
directory	O
called	O
META-INF	B-Language
which	O
contains	O
one	O
or	O
more	O
deployment	B-Language
descriptors	I-Language
.	O
</s>
<s>
Developers	O
can	O
embed	O
various	O
artifacts	O
within	O
an	O
EAR	O
file	O
for	O
deployment	O
by	O
application	B-Application
servers	I-Application
:	O
</s>
<s>
A	O
Web	O
module	B-Language
has	O
a	O
.war	B-Language
extension	O
.	O
</s>
<s>
It	O
is	O
a	O
deployable	O
unit	O
that	O
consists	O
of	O
one	O
or	O
more	O
web	O
components	O
,	O
other	O
resources	O
,	O
and	O
a	O
web	B-Application
application	I-Application
deployment	B-Language
descriptor	I-Language
.	O
</s>
<s>
The	O
web	O
module	B-Language
is	O
contained	O
in	O
a	O
hierarchy	O
of	O
directories	O
and	O
files	O
in	O
a	O
standard	O
web	B-Application
application	I-Application
format	O
.	O
</s>
<s>
POJO	B-General_Concept
Java	O
classes	O
may	O
be	O
deployed	O
in	O
.jar	B-Language
files	O
.	O
</s>
<s>
An	O
Enterprise	B-Language
Java	I-Language
Bean	I-Language
module	B-Language
has	O
a	O
.jar	B-Language
extension	O
,	O
and	O
contains	O
in	O
its	O
own	O
META-INF	B-Language
directory	O
descriptors	O
describing	O
the	O
persistent	O
classes	O
deployed	O
.	O
</s>
<s>
Message	O
Beans	O
and	O
Session	B-Language
Beans	I-Language
are	O
available	O
for	O
remote	O
access	O
.	O
</s>
<s>
A	O
Resource	B-Language
Adapter	I-Language
module	B-Language
has	O
a	O
.rar	B-Language
extension	O
.	O
</s>
<s>
Most	O
application	B-Application
servers	I-Application
load	O
classes	O
from	O
a	O
deployed	O
EAR	O
file	O
as	O
an	O
isolated	O
tree	O
of	O
Java	B-Language
classloaders	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>
For	O
example	O
,	O
a	O
deployed	O
WAR	O
file	O
would	O
be	O
able	O
to	O
create	O
instances	O
of	O
classes	O
defined	O
in	O
a	O
JAR	B-Language
file	I-Language
that	O
was	O
also	O
included	O
in	O
the	O
containing	O
EAR	O
file	O
,	O
but	O
not	O
necessarily	O
those	O
in	O
JAR	B-Language
files	I-Language
in	O
other	O
EAR	O
files	O
.	O
</s>
<s>
The	O
JBoss	B-Language
application	I-Language
servers	I-Language
before	O
Version	O
5	O
were	O
notable	O
in	O
that	O
it	O
does	O
not	O
isolate	O
deployed	O
components	O
.	O
</s>
<s>
A	O
web	B-Application
application	I-Application
deployed	O
in	O
one	O
EAR	O
file	O
would	O
have	O
access	O
to	O
classes	O
in	O
other	O
EAR	O
and	O
WAR	O
files	O
.	O
</s>
<s>
The	O
Unified	O
Classloader	B-Language
design	O
reduces	O
communications	O
overhead	O
between	O
running	O
applications	O
,	O
as	O
class	O
data	O
can	O
be	O
shared	O
by	O
reference	O
or	O
simple	O
copies	O
.	O
</s>
<s>
It	O
also	O
allows	O
developers	O
to	O
avoid	O
having	O
to	O
understand	O
the	O
problems	O
that	O
a	O
tree	O
of	O
classloaders	B-Language
can	O
create	O
.	O
</s>
<s>
JBoss	B-Language
4.0.2	O
switched	O
to	O
a	O
hierarchical	O
classloader	B-Language
,	O
but	O
in	O
version	O
4.0.3	O
it	O
reverted	O
to	O
a	O
Unified	O
Classloader	B-Language
for	O
backwards	O
compatibility	O
reasons	O
.	O
</s>
<s>
JBoss	B-Language
5.x	O
,	O
6.x	O
and	O
7.x	O
no	O
longer	O
use	O
Unified	O
Classloading	O
.	O
</s>
<s>
The	O
META-INF	B-Language
directory	O
contains	O
at	O
least	O
the	O
application.xml	O
deployment	B-Language
descriptor	I-Language
,	O
known	O
as	O
the	O
Java	B-Language
EE	I-Language
Deployment	B-Language
Descriptor	I-Language
.	O
</s>
<s>
It	O
contains	O
the	O
following	O
XML	B-Protocol
entities	O
:	O
</s>
<s>
Each	O
module	B-Language
element	O
contains	O
an	O
ejb	B-Language
,	O
web	O
or	O
java	O
element	O
which	O
describes	O
the	O
individual	O
modules	O
within	O
the	O
application	O
.	O
</s>
<s>
Web	O
modules	O
also	O
provide	O
a	O
context-root	O
which	O
identifies	O
the	O
web	O
module	B-Language
by	O
its	O
URL	O
.	O
</s>
<s>
Next	O
to	O
the	O
Jakarta	B-Language
EE	I-Language
deployment	B-Language
descriptor	I-Language
there	O
can	O
be	O
zero	O
or	O
more	O
runtime	O
deployment	B-Language
descriptors	I-Language
.	O
</s>
<s>
These	O
are	O
used	O
to	O
configure	O
implementation-specific	O
Jakarta	B-Language
EE	I-Language
parameters	O
.	O
</s>
