...
Hibernate
...
Caching
...
Hibernate
...
offers
...
three
...
types
...
of
...
cache
...
to
...
reduce
...
the
...
number
...
of
...
database
...
hits
...
and
...
speed
...
up
...
performance.
...
They
...
are:
...
- Level
...
- 1
...
- cache
...
- Level
...
- 2
...
- cache
...
- Query
...
- cache
1.
...
Level
...
1
...
Cache
...
This
...
cache
...
is
...
associated
...
with
...
a
...
Hibernate
...
Session
...
and
...
is
...
mandatory.
...
You
...
don’t
...
have
...
to
...
do
...
anything
...
special
...
to
...
turn
...
on
...
level
...
1
...
caching
...
or
...
to
...
configure
...
it.
...
The
...
Hibernate
...
Session
...
exists
...
(usually)
...
for
...
the
...
duration
...
of
...
an
...
execution
...
thread,
...
and
...
in
...
the
...
web
...
app
...
world
...
this
...
typically
...
means
...
for
...
the
...
duration
...
of
...
a
...
single
...
HTTP
...
request.
...
Entities
...
loaded
...
from
...
the
...
database
...
are
...
stored
...
in
...
the
...
level
...
1
...
cache,
...
and
...
if
...
the
...
same
...
object
...
is
...
requested
...
subsequently
...
within
...
the
...
same
...
Hibernate
...
Session,
...
the
...
object
...
is
...
retrieved
...
from
...
the
...
cache
...
rather
...
than
...
the
...
database.
| Code Block |
|---|
{code} // First request - triggers a database SELECT of Student object with id = 500 Student s = (Student) session.load(Student.class, 500); // Second request in same session for Student 500: object retrieved from cache Student t = (Student) session.load(Student.class, 500); {code} h2. |
2.
...
Level
...
2
...
Cache
...
This
...
cache
...
is
...
associated
...
with
...
a
...
Hibernate
...
Session
...
Factory
...
and
...
is
...
optional.
...
In
...
a
...
web
...
application,
...
the
...
Session
...
Factory
...
is
...
typically
...
instantiated
...
when
...
the
...
web
...
app
...
starts
...
up
...
and
...
lives
...
until
...
the
...
web
...
app
...
is
...
closed
...
down.
...
The
...
factory
...
is
...
therefore
...
shared
...
by
...
all
...
users
...
of
...
the
...
web
...
app
...
-
...
and
...
so
...
the
...
level
...
2
...
cache
...
is
...
also
...
shared
...
by
...
all
...
users.
...
The
...
level
...
2
...
cache
...
must
...
be
...
explicitly
...
turned
...
on
...
in
...
the
...
application’s
...
Hibernate
...
configuration,
...
and
...
the
...
objects
...
to
...
be
...
handled
...
by
...
this
...
cache
...
are
...
specified
...
by
...
configuration.
...
The
...
cache
...
implementation
...
is
...
not
...
provided
...
by
...
Hibernate;
...
a
...
third-party
...
cache
...
must
...
be
...
plugged
...
in.
...
We
...
use
...
EHcache
...
as
...
our
...
implementation.
...
Unlike
...
the
...
level
...
1
...
cache,
...
the
...
entity
...
objects
...
themselves
...
are
...
not
...
cached
...
-
...
instead,
...
string
...
representations
...
of
...
the
...
objects
...
are
...
created
...
and
...
stored
...
in
...
the
...
cache.
...
The
...
lookup-key
...
for
...
an
...
entry
...
in
...
the
...
level
...
two
...
cache
...
is
...
the
...
object
...
type
...
(i.e.
...
class
...
name)
...
and
...
the
...
primary
...
key
...
value.
...
Now,
...
because
...
the
...
level
...
2
...
cache
...
is
...
shared
...
by
...
all
...
users,
...
we
...
would
...
run
...
into
...
concurrency
...
issues
...
in
...
the
...
cache
...
if
...
two
...
users
...
were
...
updating
...
the
...
same
...
object
...
at
...
the
...
same
...
time.
...
For
...
this
...
reason,
...
the
...
level
...
2
...
cache
...
should
...
be
...
used
...
only
...
for
...
“read-mostly”
...
data
...
-
...
data
...
which
...
changes
...
very
...
infrequently,
...
and
...
is
...
essentially
...
used
...
in
...
a
...
read-only
...
manner
...
by
...
the
...
application.
...
An
...
example
...
would
...
be
...
a
...
list
...
of
...
US
...
state
...
codes
...
and
...
names,
...
or
...
more
...
generally,
...
data
...
in
...
some
...
MITSIS
...
validation
...
tables.
...
3.
...
Query
...
Cache
...
This
...
is
...
a
...
cache
...
of
...
HQL
...
queries
...
and
...
a
...
representation
...
of
...
the
...
results
...
of
...
executing
...
the
...
query.
...
It
...
works
...
in
...
tandem
...
with
...
the
...
level
...
2
...
cache.
...
The
...
identifying
...
“key”
...
for
...
a
...
query
...
in
...
this
...
cache
...
is
...
the
...
HQL
...
string
...
itself
...
along
...
with
...
any
...
bind
...
variable
...
values.
...
The
...
cached
...
data
...
stored
...
for
...
this
...
key
...
is
...
a
...
list
...
of
...
primary
...
keys
...
of
...
all
...
the
...
objects
...
returned
...
by
...
the
...
query.
...
So
...
it
...
seems
...
that
...
the
...
query
...
cache
...
would
...
only
...
be
...
useful
...
if
...
a
...
query
...
is
...
done
...
frequently
...
using
...
the
...
same
...
bind
...
variables,
...
and
...
the
...
data
...
returned
...
by
...
the
...
query
...
is
...
not
...
expected
...
to
...
change.
...
It’s
...
hard
...
to
...
see
...
a
...
real-world
...
use
...
where
...
this
...
cache
...
would
...
provide
...
benefit.