If you are using Java servlets directly, you will have to handle HTTP input and HTML output in Java classes. You need rich Java programming experience to build complex applications. The addition of JSP allows you to distinguish HTML expression logic from the complex business logic implanted in servlets. This means that expression layer code can be written by experienced scriptwriters, while advanced Java developers can focus on solving more complex problems in servlets and beans.
Whether you have Java programming knowledge or not, you can use JSP. JSP contains some server-side tags, so that dynamic data can be displayed without writing a line of Java code. You can directly access the bean to complete the operation, and then use the JSP tag to display the results as dynamic content. You can also use servlets to generate beans, and the calculation results of the servlets operation are stored in it, and then use JSP tags to display the results. There is no need to write Java code in the JSP page.
There are three ways to add Java code to your web page:
1. Use declarations (declarations), you can define global variables or Java methods that can be accessed anywhere in the page. The declaration is included in the tag <%!...%>.
2. Using scriptlets (script fragments), you can write any logic required for processing within the page, which is included in the <%...%> tag.
3. Expressions, included in <%=...%>. It provides an easy way to display the results of Java expressions. The attached expression will be calculated and displayed on the page as if you have explicitly written the value of the operation result in the code.
In your own code, you can use some implicit variables - predefined Java objects provided by JSP. In addition, by using JSP directives, non-Java code modules can also be included, such as HTML text from other files.
Let's take a closer look at these script elements, which will be used frequently when writing your own JSP scripts.
Directives
JSP defines three in-page instructions for setting JSP parameters or augmenting code. They are page, include and taglib and must be written on the first line of the JSP page. The syntax is as follows:
<%@ directive attribute="value" ... %>
The page directive allows you to set some basic parameters for the web page, including setting parameters of the script language used (default is Java), Java classes introduced in your script fragments, setting output buffers, etc. For the complete page directive parameter table, see Chapter 2.8.1 of "JSP Specification Version 1.0" ("JSP Specification 1.0").
Use the include directive to include the contents of other files, such as HTML headers and footers stored in separate files.
The taglib directive is used to extend the standard JSP tag set, which is beyond the scope of this article. However, it is beneficial to understand that JSP defines a way to expand its tag set, which is especially important when you are a software vendor who wants to expand the original functionality of JSP without breaking its compatibility.
Declarations (Declaration)
Using declarations, you can define methods or variables in a JSP page that can be accessed by other code in the same page. In most cases, you might define methods in your own bean. However, sometimes it may be easier to define methods within web pages, especially when the code is only used on a single page. Regardless of defining methods or variables, declarations are included in the <%! %> tag.
Note that the declaration does not produce any output within the JSP page. They are used only for definitions and do not generate output results. To generate output results, you should use JSP expressions or script fragments.
Expressions
Expressions is a very simple JSP tag that is used to convert the value of a JSP expression defined in <%= %> into a string and send this value as dynamic text. Expression is indeed a shortcut to generating text. With it, you don't have to call the print() method every time you want to display a piece of dynamic text. A typical application is that you can use expressions to display the return value of a simple variable value or method in a bean.
For example, the following code will generate the return value of the getName() method:
<H2>Welcome, <%= () %></H2>
In fact, before generating dynamic output, JSP must convert the return value of the method into a String object in Java. The JSP specification describes in detail what Java types and Java classes will be converted into strings in JSP expressions.
Scriptlets (script fragment)
By now you have learned to use directives to introduce any Java class or Java package, you can define page-level methods or variables and use them in the page, and you can also use implicit variables that provide normal web processing capabilities. What else can be done in the JSP page is up to you, because you can write any Java code you want in scriptlets (script snippets), as shown below:
<% ...code... %>
By using the IMPORT parameter in the page directive, you can call all Java APIs from within a script fragment. Because all the JSP code you write is actually compiled to form a Java servlet, which is itself a Java class, so the language you use is Java itself, not any modified or organized version. It's like in SSJS you can write any code. Unlike SSJS, in JSP you have the right to use a rich set of Java APIs, so there are almost no limitations.
Implicit Variables
As mentioned earlier, JSP defines some implicit variables (i.e. Java objects) for you to use in expressions and script fragments. Table 2-2 of "JSP Specification Version 1.0" lists the implicit variables available in JSP1.0. Here are some commonly used objects:
The out object, of type, provides access to methods (such as print() method) to generate output results within a script fragment.
The request object directly corresponds to the class in Java, with all the properties and methods of the object of that class. For example, to get a value passed in from an HTML form or URL query string, you can call the() method to obtain parameters based on the name.
The response object corresponds to a class in Java, providing access to parameters of the HTML response generated by your web page. Therefore, to add a value to the HTML response header returned by the JSP page, you can call the () method to implement it.
Another simple example
In the following example, let's take a look at the interaction between a form and its JSP form handle. Using the script elements discussed earlier, I implemented a simple web site feedback form (see Figure 2) and a JSP form handle to validate the input and then conditionally generate feedback-based output.
Figure 2. A web site feedback form
Button in the picture: submit query－－Submit; reset―Refill
The form handle will check the name and comments columns to determine that they have been filled in, and if either or both of them are blank, the form handle will generate an error message; otherwise it will continue to see if the user comment matches the preset string. If it matches, it outputs a special message; otherwise it outputs "thank you".
Example 2
<HTML>
<HEAD>
<META NAME="GENERATOR" Content="NetObjects ScriptBuilder 2.01">
<TITLE>Feedback Results</TITLE>
</HEAD>
<%!
// Name and comment columns cannot be blank
// Check their values and return the results
boolean validateInput(String name, String comment) {
boolean result = true;
// If the name or comment is not filled in, return false to indicate that the input is invalid
if (() == 0)
result = false;
if (() == 0)
result = false;
return result;
} // End input verification validateInput
//Output results according to the comment bar on the form
String getStringCheese (String comment) {
String cheese = "I like cheese.";
String result;
if ((cheese) == 0)
result = "We like cheese too!";
else
result = "We hope someday you'll learn to like cheese.";
return result;
} //End getStringCheese
%>
<BODY BGCOLOR="#F0F0E0">
<%
// Get the data submitted through the form
String name = ("name");
String age = ("age");
String comment = ("comment");
boolean isValid;
isValid = validateInput(name, comment);
// Decide to output content based on whether the user has not filled in his name or comment column?
if (isValid) {
%>
<H2>Thank you for your feedback!</H2>
<H3>
<%
//Export the query result of the comment column
(getStringCheese(comment));
} // End if block
else {
("You didn't give us your name or a comment.");
%>
</H3>
Please <a href="feedback_form.html">try again</a>
<%
} //End else program block
%>
</BODY>
</HTML>
This example assumes that the user entered an opinion "I like cheese." (I like cheese) can be seen in the code, and this response is customized for the user who filled in the opinion. The form handle will return to the page shown in Figure 3:
Figure 3. Form handle output
The text in the picture: Thank you for your feedback! We love cheese, too.
This example is very simple and easy to understand. Even if you are just a JavaScript programmer, you should be able to understand it. I would also point out some features that are not very obvious in the JSP specification as reflected in this example. First, note that I define some methods in the declaration section (the part in <%'...%>), exactly the same as defining methods in Java classes. This is because the JSP engine transforms these methods into the underlying Java servlets, which are executed by the server when the browser makes a request to the web page. Therefore, any definition of variables and methods must comply with standard Java syntax.
It should also be noted that in my script fragment code, I separate an if...else statement, which spans two different script fragment segments. This is totally legal! Not only is it legal, but it is also a good way to conditionally generate HTML, just like I did in this case.
Finally, you can see that I get the value of the form element by calling the() method and assign it to a temporary variable. This is a standard way to handle values entered from a form or query string.
Whether you have Java programming knowledge or not, you can use JSP. JSP contains some server-side tags, so that dynamic data can be displayed without writing a line of Java code. You can directly access the bean to complete the operation, and then use the JSP tag to display the results as dynamic content. You can also use servlets to generate beans, and the calculation results of the servlets operation are stored in it, and then use JSP tags to display the results. There is no need to write Java code in the JSP page.
There are three ways to add Java code to your web page:
1. Use declarations (declarations), you can define global variables or Java methods that can be accessed anywhere in the page. The declaration is included in the tag <%!...%>.
2. Using scriptlets (script fragments), you can write any logic required for processing within the page, which is included in the <%...%> tag.
3. Expressions, included in <%=...%>. It provides an easy way to display the results of Java expressions. The attached expression will be calculated and displayed on the page as if you have explicitly written the value of the operation result in the code.
In your own code, you can use some implicit variables - predefined Java objects provided by JSP. In addition, by using JSP directives, non-Java code modules can also be included, such as HTML text from other files.
Let's take a closer look at these script elements, which will be used frequently when writing your own JSP scripts.
Directives
JSP defines three in-page instructions for setting JSP parameters or augmenting code. They are page, include and taglib and must be written on the first line of the JSP page. The syntax is as follows:
<%@ directive attribute="value" ... %>
The page directive allows you to set some basic parameters for the web page, including setting parameters of the script language used (default is Java), Java classes introduced in your script fragments, setting output buffers, etc. For the complete page directive parameter table, see Chapter 2.8.1 of "JSP Specification Version 1.0" ("JSP Specification 1.0").
Use the include directive to include the contents of other files, such as HTML headers and footers stored in separate files.
The taglib directive is used to extend the standard JSP tag set, which is beyond the scope of this article. However, it is beneficial to understand that JSP defines a way to expand its tag set, which is especially important when you are a software vendor who wants to expand the original functionality of JSP without breaking its compatibility.
Declarations (Declaration)
Using declarations, you can define methods or variables in a JSP page that can be accessed by other code in the same page. In most cases, you might define methods in your own bean. However, sometimes it may be easier to define methods within web pages, especially when the code is only used on a single page. Regardless of defining methods or variables, declarations are included in the <%! %> tag.
Note that the declaration does not produce any output within the JSP page. They are used only for definitions and do not generate output results. To generate output results, you should use JSP expressions or script fragments.
Expressions
Expressions is a very simple JSP tag that is used to convert the value of a JSP expression defined in <%= %> into a string and send this value as dynamic text. Expression is indeed a shortcut to generating text. With it, you don't have to call the print() method every time you want to display a piece of dynamic text. A typical application is that you can use expressions to display the return value of a simple variable value or method in a bean.
For example, the following code will generate the return value of the getName() method:
<H2>Welcome, <%= () %></H2>
In fact, before generating dynamic output, JSP must convert the return value of the method into a String object in Java. The JSP specification describes in detail what Java types and Java classes will be converted into strings in JSP expressions.
Scriptlets (script fragment)
By now you have learned to use directives to introduce any Java class or Java package, you can define page-level methods or variables and use them in the page, and you can also use implicit variables that provide normal web processing capabilities. What else can be done in the JSP page is up to you, because you can write any Java code you want in scriptlets (script snippets), as shown below:
<% ...code... %>
By using the IMPORT parameter in the page directive, you can call all Java APIs from within a script fragment. Because all the JSP code you write is actually compiled to form a Java servlet, which is itself a Java class, so the language you use is Java itself, not any modified or organized version. It's like in SSJS you can write any code. Unlike SSJS, in JSP you have the right to use a rich set of Java APIs, so there are almost no limitations.
Implicit Variables
As mentioned earlier, JSP defines some implicit variables (i.e. Java objects) for you to use in expressions and script fragments. Table 2-2 of "JSP Specification Version 1.0" lists the implicit variables available in JSP1.0. Here are some commonly used objects:
The out object, of type, provides access to methods (such as print() method) to generate output results within a script fragment.
The request object directly corresponds to the class in Java, with all the properties and methods of the object of that class. For example, to get a value passed in from an HTML form or URL query string, you can call the() method to obtain parameters based on the name.
The response object corresponds to a class in Java, providing access to parameters of the HTML response generated by your web page. Therefore, to add a value to the HTML response header returned by the JSP page, you can call the () method to implement it.
Another simple example
In the following example, let's take a look at the interaction between a form and its JSP form handle. Using the script elements discussed earlier, I implemented a simple web site feedback form (see Figure 2) and a JSP form handle to validate the input and then conditionally generate feedback-based output.
Figure 2. A web site feedback form
Button in the picture: submit query－－Submit; reset―Refill
The form handle will check the name and comments columns to determine that they have been filled in, and if either or both of them are blank, the form handle will generate an error message; otherwise it will continue to see if the user comment matches the preset string. If it matches, it outputs a special message; otherwise it outputs "thank you".
Example 2
<HTML>
<HEAD>
<META NAME="GENERATOR" Content="NetObjects ScriptBuilder 2.01">
<TITLE>Feedback Results</TITLE>
</HEAD>
<%!
// Name and comment columns cannot be blank
// Check their values and return the results
boolean validateInput(String name, String comment) {
boolean result = true;
// If the name or comment is not filled in, return false to indicate that the input is invalid
if (() == 0)
result = false;
if (() == 0)
result = false;
return result;
} // End input verification validateInput
//Output results according to the comment bar on the form
String getStringCheese (String comment) {
String cheese = "I like cheese.";
String result;
if ((cheese) == 0)
result = "We like cheese too!";
else
result = "We hope someday you'll learn to like cheese.";
return result;
} //End getStringCheese
%>
<BODY BGCOLOR="#F0F0E0">
<%
// Get the data submitted through the form
String name = ("name");
String age = ("age");
String comment = ("comment");
boolean isValid;
isValid = validateInput(name, comment);
// Decide to output content based on whether the user has not filled in his name or comment column?
if (isValid) {
%>
<H2>Thank you for your feedback!</H2>
<H3>
<%
//Export the query result of the comment column
(getStringCheese(comment));
} // End if block
else {
("You didn't give us your name or a comment.");
%>
</H3>
Please <a href="feedback_form.html">try again</a>
<%
} //End else program block
%>
</BODY>
</HTML>
This example assumes that the user entered an opinion "I like cheese." (I like cheese) can be seen in the code, and this response is customized for the user who filled in the opinion. The form handle will return to the page shown in Figure 3:
Figure 3. Form handle output
The text in the picture: Thank you for your feedback! We love cheese, too.
This example is very simple and easy to understand. Even if you are just a JavaScript programmer, you should be able to understand it. I would also point out some features that are not very obvious in the JSP specification as reflected in this example. First, note that I define some methods in the declaration section (the part in <%'...%>), exactly the same as defining methods in Java classes. This is because the JSP engine transforms these methods into the underlying Java servlets, which are executed by the server when the browser makes a request to the web page. Therefore, any definition of variables and methods must comply with standard Java syntax.
It should also be noted that in my script fragment code, I separate an if...else statement, which spans two different script fragment segments. This is totally legal! Not only is it legal, but it is also a good way to conditionally generate HTML, just like I did in this case.
Finally, you can see that I get the value of the form element by calling the() method and assign it to a temporary variable. This is a standard way to handle values entered from a form or query string.