Page History

...

The

Education

Systems

framework

for

developing

Java-based

web

applications

can

seem

bewildering

to

a

new

developer.

Indeed,

some

of

us

who

have

worked

with

the

framework

for

a

while

still

find

ourselves

scratching

our

heads

from

time

to

time.

So

I

hope

this

document

will

serve

as

a

place

for

newbies

to

come

and

learn

about

how

we

put

web

apps

together,

and

also

for

old

hands

to

remind

themselves

about

some

of

the

dusty,

forgotten

corners

of

the

framework.

First

things

first

-

what

is

the

architecture,

design

pattern,

or

structure

we

follow

for

our

web

apps?

There's

no

pithy

phrase

that

captures

it

all,

but

here

are

the

essentials:

-

Code

is

written

in

Java

(6)

-

JSPs

are

used

for

the

front-end

views

-

Oracle

database

houses

the

data

-

Spring

MVC

for

the

web

layer

-

Spring

framework

provides

the

app

"container"

-

Hibernate

talks

to

the

database

We

use

a

fairly

standard

layered

structure

for

the

Java

code

in

the

web

apps.

Conceptually

this

structure

is:

UI

Layer

->

Service

Layer

->

Data

Access

Layer

We

also

have

a

Domain

component

that

contains

data-holding

objects.

These

domain

objects

can

live

in

any

of

the

other

three

layers

and

are

typically

passed

up

or

down

between

the

layers.

This

diagram

illustrates

these

layers:

This

layer

consists

mostly

of

Spring

MVC

Controllers,

and

is

identified

by

the

"web"

package

name

in

most

of

our

web

apps.

Controllers

are

all

about

web

input

and

output.

They

handle

input

from

the

user's

browser

(requests

for

retrieval

of

data,

form

submissions,

etc),

coordinate

processing

of

the

input

requests,

and

send

back

information

to

the

user's

browser.

There

should

be

no

"business

logic"

or

"business

rules"

implemented

in

a

controller.

Controllers

will

typically

call

services

to

do

business

logic.

Controllers

should

not

use

DAOs

(see

Data

Access

Later

below)

directly.

We

use

Spring's

@Controller

annotation

to

configure

controllers

as

Spring

beans.

You

may

see

some

older

controller

classes

configured

as

beans

via

XML,

but

new

code

should

always

use

the

annotation.

Note

that

we

configure

controllers

with

Spring's

default

"singleton"

scope.

This

means

that

there

is

only

one

instance

of

the

class

in

the

web

application;

all

users

will

use

the

same

cached

instance

of

the

controller.

We

must

therefore

make

sure

that

controllers

are

coded

in

a

thread-safe

way

-

no

state

should

be

kept

in

instance

variables.

In

fact,

instance

variables

should

only

be

other

Spring

beans,

injected

by

the

Spring

container.

This

is

where

the

business

logic

is

implemented.

Any

code

that

implements

a

business

function

(e.g.

"drop

a

class",

"change

a

grade")

should

be

in

a

service

class.

Service

classes

should

be

configured

as

Spring

beans

using

the

@Service

annotation.

Service

classes

should

never

call

any

UI

Layer

(controller)

methods.

They

should

also

never

have

any

logic

that

is

related

to

the

UI.

If

you

find

yourself

thinking

of

writing

a

service

method

called

"convertGradeDescriptionToHTML",

a

(imaginary

!)

warning

bell

should

go

off

and

you

should

place

this

code

in

the

UI

layer.

One

way

to

help

think

about

this

is

to

imagine

the

service

being

used

in

an

app

with

a

second

non-web

(i.e.

non-HTML)

interface

-

a

REST

interface

perhaps,

or

a

Swing

interface.

If

there's

something

in

your

service

code

that

would

not

apply

to

the

second

interface

(our

convertGradeDescriptionToHTML

method

for

example),

then

this

is

a

sign

that

the

code

shouldn't

be

in

the

service

layer,

but

up

in

the

appropriate

UI

layer.

As

with

the

controller

classes,

services

are

configured

as

Spring

singleton

beans,

so

should

be

coded

to

be

thread-safe.

All

services

are

coded

as

an

interface

and

an

implementing

class

(e.g.

RegistrationService

for

the

interface,

RegistrationServiceImpl

for

the

class).

This

is

helpful

in

unit

testing

of

classes

that

use

our

service

class

(controllers,

other

services),

allowing

us

to

inject

mock

implementations

of

our

service

class

into

the

class

under

test.

This

is

where

the

Data

Access

Objects

(DAOs)

live.

A

DAO's

purpose

is

to

provide

a

relatively

fine-grained

set

of

basic

data

operations

(e.g.

get

data

from

the

database,

update

data,

etc.).

There

should

be

no

business

logic

or

business

rules

in

a

DAO

-

just

pure

data

access.

Thinking

about

alternate

DAO

implementations

may

be

helpful

here.

For

example,

if

a

DAO

contains

code

like

"If

the

student

is

a

freshman, load this data, otherwise load that data", it's on the wrong track. This is business-related logic and belongs in the service layer.

Once again, DAOs are singleton Spring beans, so thread-safety is important.

DAOs are coded as interface & class pairs. You will find the DAO interfaces in a "dao" package, and their Hibernate implementations in a sub-package "dao/hibernate".

Naming

conventions

are

(for

example)

RegistrationDAO

for

the

interface

and

HibernateRegistrationDAO

for

the

class.

Again,

the

use

of

the

interface

helps

with

unit

testing,

allowing

us

to

test

service

classes

by

using

mock

DAOs

that

do

not

need

to

connect

to

the

database.

One

very

important

consideration

for

DAOs.

You

will

see

many

DAOs

in

the

framework

that

extend

Spring's

HibernateDaoSupport

class

and

use

Spring's

related

HibernateTemplate.

DO

NOT

USE

THIS

METHOD

FOR

WRITING

NEW

DAOs

!

This

style

has

been

deprecated

by

Spring

for

at

least

5

years

in

favor

of

DAOs

as

plain

POJOs,

and

use

of

the

Hibernate

Session

API.

Now,

with

Spring

3.1

beginning

to

support

Hibernate

4,

these

helper

classes

have

been

dropped

from

the

Spring

framework.

When

we

upgrade

our

framework

to

Hibernate

4,

we

will

need

to

rewrite

code

that

uses

these

helper

classes.

So

from

now

on

we

should

stay

away

from

these

deprecated

classes.

In

a

nutshell,

DAOs

should

be

POJOs

and

should

contain

a

private

instance

variable

of

type

SessionFactory.

The

session

factory

should

be

injected

by

Spring,

and

DAOs

should

use

the

getCurrentSession()

method

on

the

factory

object

to

obtain

the

Hibernate

Session,

and

perform

data

access

using

the

Session

API.