Introduction
This Tutorial will explain you, what struts
is, how it builds itself up and why it is advantage to use it for
your web application. First, I will provide you some informations
about Java Servlets, Java Server Pages and Java Beans, because they
are parts of struts.
Java Servlets
Servlets represents java programs that runs
on a web server. They allow the developer to produce dynamic web
sites with java.
A Servlet has the following tasks
-
It reads and processed data, which a
user typed in a HTML form on a web page.
-
If necessary other informations will be
processed. For Example what browser or system will be used.
-
It generate results with the existing
data. It calls the business logic directly in the servlet or another
class, which contain the logic or executes a database query.
-
The results will be formated. If the
browser. If the Browser expects an answer in the HTML format, then
the results must be formatted in accordance with the standard. It is
possible to return different formats of data with a servlet. (gif,
jpeg, doc, etc.).
-
Suitable answer parameters are set.
Befor the servlet return the data to the browser, it sends some
parameter. The parameter contains the format, that will returned by
the servlet, what time the browser use to cache the site and some
more.
-
Return the document, in the format that
it sends befor, to the browser.
Java Server Pages (JSP)
JavaServer Pages (JSP) are text documents,
which are similar to HTML Files. But you find also java code in the
JSP File. JavaServer Pages allow you to mix regulare, static HTML
with dynamic generated contents of servlets. The java code is
inserted in the HTML document on a JSP File, differently to a
servlet, where the HTML code is embedded in the java code.
Java Beans
Java Beans are
nothing else as classes, which keep a fixed naming convention,
defined by Sun, for their event processing and methods. The
attributes (variables) of the java beans are private. The access to
these attributes is managed by access methods. The java specification
specify these access methodes (getter and setter methods). If a java
bean contains a attribute name, you can set or get the value of the
attributes with the methode setName() or getName().
Business logic
The business logic is the core of the
application. The processes are implemented in the business logic to
manage the data. For Example: when someone borrows a book in a
library, that is a process in the business logic. The process change
the state of the data (is the book borrowsed or not) or reads the
state and then provide this information for a dialog.
This short outline should be sufficient, in
order to understand the structure of struts.
Why is struts helpful?
Sepration of dialogs and business logic
(functionality)
Some peoples develop web applications
with Perl or PHP and implement their SQL Querys and the business
logics directly in the HTML document.
The source code looks like the
following example:
<html><head><title>Important title</title></head>
<body>
<someScript>
dbConnection = openDBConnection(someDB)
resultSet = dbConnection.executeQuery('select bookName from books')
loop over resultSet{
print (resultSet.field('bookName') + '<br>')
}
</someScript>
</body></html>
You can develop in this way, when you
use java servlets or JSP. It is convenient in small projects. But
imagine you have 70 dialogs, many database querys in this dialogs and
you want to define a field status, setting if a book is deleted or
not.
Good luck
In order to alter functions and database
querys easily, we should seperate these from the dialogs.
Advantage of separation
-
Changes on the functionality, without
working in the dialogs.
-
Better overview, functionality not
mixed with the dialogs.
-
Easy maintain a application
-
Different dialogs, but the same
functionality
Central control
In order to control the interaction between
the dialogs and business processes, you need a central control unit.
This control unit manage all importants courses of the application,
when which business process and which dialog will be used.
You have some disadvantages, if you implement the
order of the processes directly to the business logic.
-
You can not reuse a part of the processes. In
the following picture the process "Give Money" will bring
you to the process "Select sort of icecream" every time.
-
If you want to change the order of the
processes or add more options, you have to change it directly in the
business logic.
Advantages of the central control
-
It is easy
to change the order of the business processes.
-
Better
overview, which dialogs will be used on which business processes.
-
A
central place to control the interaction between business processes
and dialogs.
Translation into the technical language
The name of this model is
Model-View-Controller (MVC)
Model
(business logic / business processes ? Java Beans)
View
(dialogs ? JavaServer Pages)
Controller
(central control unit - Java Servlets)
You will find
more about the MVC later in this tutorial.
More helpfully capabilities
Internationalisation
If you do not
want to translate a dialog, you put the dialog texts directly into
the JSP File.
<html:form action="someAction">
Please give your name and the book title <br>
Name:
<html:text property="name"/> <br>
Title:
<html:text property="title"/> <br>
<html:submit/>
</html:form>
Struts support
resources files. In this files you can define a key for a text.
bookDialog.formIntro=Please give your name and the title of the book.
bookDialog.inputName=Name:
bookDialog.bookTitle=Title:
This key can be
used to diplay the text in the JSP file.
<html:form action="someAction">
<bean:message key="bookDialog.formIntro"/> <br>
<bean:message key="bookDialog.inputName"/>
<html:text property="name"/> <br>
<bean:message key="bookDialog.bookTitle"/>
<html:text property="title"/> <br>
<html:submit/>
</html:form>
Struts call the
locale specific resource file by using the browser settings. You may
overwrite this setting. It is quite easy to build an
internationalized application with struts.
Error handling
Struts saves
error messages in the business logic and can display this in the
dialogs. You can show all errors or bind an error to a field (ex.
Emailaddress incorrect) and show the error beside of it.
The error
messages will be added to the resource files, so the errors can be
internationalised too.
Validation of input fields
Whether an input
field of a form contains a valid date, an email, a number or
something else, in each application you have to check it. Struts
supports a complete solution to validate form fields. You only have
to define which form field will be checked and which error message
will be displayed.
Components of struts
JavaServer Pages
(JSP) take over the role of dialogs in struts,
Java Beans take
over the business logic and business processes and
Java Servlets
take over the central control unit
Thats why we talk about three great parts
of struts.
Model
(business logic / business processes ? Java Beans)
View
(dialogs ? JavaServer Pages)
Controller
(central control unit - Java Servlets)
The model represent the actual state
of the application. Two kinds of java beans are used. There are java
beans which contains the data of a form or data to display (ex. The
books of a library) and java beans which includes the functionality
of the application or call the business logic (when a user borrows a
book).
The view component is responsible for
the presentation of the data. The java server pages contain HTML, XML
and Java Script, like a normal HTML site. Futhermore you can use java
code. Struts provide tag libraries, a summary of functions, which can
be use to prepare the data for displaying.
The last component is the controller. The
controller manage the request of the web browser, which a user called
by an address (URL). But also forward to an action which are execute
and which dialogs will be used to display the informations.
The picture below illustrates the
interaction between these components.
If the user sends a query with a browser,
the controller(servlet) gets and processes this query. It decides
which action will be called or to which view component it must be
forward.
After the controller calls an action, the
action can read data from a database and provide this data to the
model component, java beans. The action (business logic) returns the
"next step" to the controller. The controller checks what
kind is the next step. (JSP View, next action, ...) and forwards to
it.
The view component (JSP) reads the updated
data from the model component and prepare this for the presentation.
Then it sends the answer as HTML site back to the browser. The user
sees the result of his query.
Struts configuration
Struts will be configured with various
configuration files. The following files are very important.
web.xml
struts-config.xml
Struts-Tag-Bibliotheken
Properties-Dateien
The illustration will show you, on which
places struts use the configuration files.
web.xml
With the web.xml you configure the web
server for the struts application. In this file you can set where the
web server find the struts-config.xml and some other global
properties.
struts-config.xml
The controller calls the business logic or a
view with a name. The allocation of the names to the action classes
or JSP Files(Views) will be set in the struts-config.xml. The
advantage is that you can change the definition of the Workflows
(Action Mapping), without using the long class names everytime. If
you change a class name, you only have to change the name in the
struts-config.xml. You don´t update any other part of your
application.
Struts-Tag-Libraries
The
Struts-Tag-Libraries, a summary of functions, extends the
functionality of JavaServer pages. They support the
internationalisation (multi-language) of your web application, and
the easy creation of form elements.
Properties files
This files will
be used to keep the internationalized textes of your web application.
You can create a seperated properties file for each language, which
contain all textes of the application.
轉(zhuǎn)自：http://www.laliluna.de