JSP action
| JSP actions use tags in XML syntax to control the behavior of the servlet engine. JSP actions can be used to dynamically Insert Files, reuse the JavaBean Component, redirect users to another page, and generate HTML code for the Java Plug-in. JSP actions include: JSP: include: Introduce a file when the page is requested. This action inserts the specified file into the page being generated. The syntax is as follows: The include command has been introduced earlier. It introduces the file when the JSP file is converted into a servlet. the JSP: include action here is different, the file is inserted when the page is requested. The introduction time of the JSP: include action file determines that its efficiency is slightly lower, and the Referenced File cannot contain some JSP code (for example, the HTTP header cannot be set ), but it is much more flexible. For example, the following JSP page inserts four news summaries into a "What's new ?" Page. Only the four files need to be changed when the news abstract is changed, but the main JSP page can not be modified: Whatsnew. jsp <Body bgcolor = "# fdf5e6" text = "#000000" link = "# 0000ee" <Center> 13.2 JSP: usebean action JSP: usebean action is used to load a JavaBean that will be used on the JSP page. This function is very useful, because it allows us to take advantage of Java Component Reuse, but also avoids the convenience of separating jsp from servlet. The simplest syntax for JSP: usebean actions is: The meaning of this line of code is: "Create an instance of the class specified by the class property, and then bind it to the variable whose name is given by the ID property ". However, as we will see next, defining a scope attribute can associate bean with more pages. In this case, the JSP: usebean action creates an object instance only when the bean with the same ID and scope does not exist. At the same time, it is necessary to obtain the reference of the existing bean. After obtaining the bean instance, you can modify the bean attributes either through the JSP: setproperty action or by using the object variable named by the ID attribute in the scriptlet, you can call this object to explicitly modify its properties. This reminds us that when we say "a bean has a property Foo with the type of X", it means "this class has a getfoo method with the return value type of X, there is also a setfoo method that takes the value of the X type as the parameter ". Details about JSP: setproperty actions will be discussed later. However, we must understand that we can either directly provide a value through the value attribute of the JSP: setproperty action, or declare the bean property value from the specified request parameter through the param attribute, the bean attribute can also be listed to indicate that its value should come from the variable with the same name in the request parameter. To read bean attributes in a JSP expression or scriptlet, call the corresponding getxxx method, or, more generally, use the JSP: getproperty action. Note that class files containing beans should be placed in the directory where the server officially stores Java classes, rather than retaining them to the directories of classes that can be automatically loaded after modification. For example, for Java Web server, the classes used by bean and all beans should be put into the classes directory, or encapsulated into the jar file and put into the lib directory, but should not be put into the servlets. The following is a simple example. Its function is to load a bean and set/read its message attribute. Beantest. jsp <Body> <JSP: usebean id = "test" class = "Hall. simplebean"/> Simplebean. Java The beantest page uses a simplebean. The simplebean code is as follows: Public class simplebean { Public String getmessage (){ Public void setmessage (string message ){ 13.3 further description of JSP: usebean The simplest way to use bean is to load the bean with the following code: Then, modify and extract bean attributes through JSP: setproperty and JSP: getproperty. But there are two points to note. First, we can also instantiate the bean in the following format: It means that the body part is executed only when the bean is instantiated for the first time. If an existing bean instance is used, the body part is not executed. As we will introduce below, JSP: usebean does not always mean creating a new bean instance. Second, in addition to ID and class, JSP: usebean has three other attributes: scope, type, and beanname. The following table briefly describes the usage of these attributes. Attribute usage Type specifies the type of the variable that references this object. It must be the name of the bean class, the super class name, or one of the interfaces implemented by this class. Remember that the variable name is specified by the ID attribute. 13.4 JSP: setproperty action JSP: setproperty is used to set attributes of instantiated bean objects. First, you can use JSP: setproperty outside the JSP: usebean element (later), as shown below: In this case, whether JSP: usebean finds an existing bean or creates a new bean instance, JSP: setproperty will be executed. The second method is to put JSP: setproperty inside the JSP: usebean element, as shown below: In this case, JSP: setproperty is executed only when the bean instance is created. If an existing instance is used, JSP: setproperty is not executed. JSP: setproperty action has the following four attributes: attribute description Param is optional. It specifies the request parameter used as the bean attribute value. If the current request has no parameters, nothing will be done. The system will not pass null to the Set Method of the bean attribute. Therefore, you can allow the bean to provide its own default attribute value. The default attribute value can be modified only when the new value is explicitly specified in the request parameter. <JSP: setproperty name = "orderbean" If both value and Param are omitted, the effect is equivalent to providing a PARAM and its value is equal to the value of property. Further, with the idea of automatically assigning values using the same request parameter and attribute name, you can also specify "*" in the property (bean attribute name), and then omit the value and Param. In this case, the server will view all bean attributes and request parameters. If the two names are the same, the values are automatically assigned. The following is an example of using Javabean to calculate the prime number. If the request contains a numdigits parameter, the value is passed to the numdigits attribute of the bean. numprimes is similar. Jspprimes. jsp <Body> <Center> <JSP: usebean id = "primetable" class = "Hall. numberedprimes"/> Some <JSP: getproperty name = "primetable" property = "numdigits"/> </Body> Note: The numberedprimes code is omitted. 13.5 JSP: getproperty action JSP: getproperty action extracts the value of the specified bean property, converts it to a string, and then outputs it. JSP: getproperty has two required attributes: name, indicating the bean name; property, indicating the value of the attribute to be extracted. The following is an example. More examples can be found in the previous article. 13.6 JSP: forward action JSP: forward action transfers the request to another page. JSP: forward only has one property page. The page attribute contains a relative URL. The page value can be provided directly or dynamically calculated during the request, as shown in the following example: 13.7 JSP: plugin action JSP: the plugin action is used to insert the object or embed elements necessary to run the Java Applet through the Java Plug-in based on the browser type. |
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