Programming Jakarta Struts, 2nd Edition [Electronic resources] نسخه متنی

2.1 An Architecture Overview

This section presents a high-level
architectural view of a
Struts
application. Although this section shows an architecture for an
enterprise application, not all applications written using Struts
will be of this size and makeup. However, this type of application
does allow us to present many facets of how Struts applications may
be configured.

Many applicationsespecially J2EE applicationscan be
described in terms of their tiers. The
application's functionality is separated across
these tiers, or functional
layers, to provide separation of responsibility, reusability,
improved scalability, and many other benefits. The separation of
tiers may be a physical separation where each is located on a
separate hardware resource, or it may be purely logical. In the
latter case, one or more tiers are collocated (i.e., arranged or
grouped together) on the same hardware resource, and the separation
exists in terms of software components. Figure 2-1
illustrates the tiers that may be used by a typical Struts
application.

Figure 2-1. Functional application tiers

Not every Struts application will contain all of the tiers
illustrated in Figure 2-1. For many smaller
applications, the middle tier may consist primarily of a web
container that interacts directly with a database in the
enterprise
information system (EIS) tier.

2.1.1 The Client Tier

The client tier
provides a way for users to interact with the application. This
interaction may be through a web browser, or it may be programmatic,
through a web services interface. Regardless of the type of client,
the interaction includes submitting a request and receiving some type
of response from the middle tier.

In the case of the Struts framework, the most common type of client
is a web browser. However, it is also possible to have clients such
as wireless devices and Java applets.

2.1.2 The Web Tier

Figure 2-1 shows the middle tier as an aggregate of
the web tier plus some type of application server component (in this
case, an EJB container). These two tiers often are combined, and many
application servers include web tier functionality.

The web tier allows
the client tier to communicate and interact with application logic
that resides in other tiers. In more traditional web applications,
it's not uncommon for some or all of the application
logic to reside in this tier. In larger, enterprise-scale
applications, the web tier acts as a translator and maps HTTP
requests into service invocations on the middle tier.

The web tier also is responsible for managing screen flow based on
application and user state. The web tier communicates with either a
database or, in the case of an enterprise application, an application
server. The web tier is the glue that binds client applications to
the core backend business systems.

The components that reside in the web tier allow developers to extend
the basic functionality of a web service. In the case of Struts, it
does this through framework components that run in a servlet
container.

2.1.3 The Middle Tier

The middle tier is often referred to as the
"application tier" or
"server." This is due in part to
the fact that there is often an application server within this tier.
Not all Struts applications have an application tier. This is
especially true for small web applicationsmany small projects
forgo using a large application server and communicate directly with
a database or some other data store. When an application server is
present, the web tier communicates with it using some variation of
Remote Method Invocation (RMI). If an EJB server is present in the
application tier, the communication protocol is RMI over IIOP
(Internet Inter-ORB Protocol).

When included, the application tier might provide a more scalable,
fault-tolerant, and highly available architecture. One of the main
purposes of using an application tier is to separate the
responsibilities of presentation from those of the model and the
business rules for the application. Today, many web applications use
EJB servers for their application tiers. They may not use all
available aspects of the J2EE architecture, such as EJBs, but there
are other benefits that can be leveraged from a J2EE server such as
JavaMail, JDBC and JMS to name a few.

2.1.4 The Enterprise Information System Tier

The EIS tier
contains data and services that are used throughout the enterprise.
It provides access to enterprise resources such as databases,
mainframes, customer relationship management (CRM) applications, and
resource-planning systems.

The middle tier communicates with components in the EIS tier using
resource-specific protocols. For example, to communicate with a
relational database, the middle tier normally will use a JDBC driver.
For enterprise resource planning (ERP) systems, a proprietary adapter
b is used, although some ERP systems and other enterprise resources
are starting to support a more web service-like access approach.

2.1.5 Where Does Struts Fit In?

As illustrated in Figure 2-2, the Struts framework
resides in the web tier. Struts applications are hosted by a web
container and can make use of services provided by the container,
such as handling requests via the HTTP and HTTPS protocols. This
frees developers to focus on building applications that solve
business problems.

Figure 2-2. The Struts framework is used within the web tier