An Applet is a Java program. It is typically run inside a Java-enabled Web browser. Because it has full Java API support, the applet is a full-featured Java application.
The important differences between standalone Java applications and applet programs are as follows:
The four methods in the Applet class provide you with a framework that you can then develop applets on the framework:
Each applet is a subclass of the Java.applet.Applet class, and the underlying applet class provides a way to invoke the derived class to get the information and services of the browser context.
These methods do the following things:
In addition, the applet class provides an interface for viewer or browser to obtain information about applets and to control the execution of applets.
The Viewer may be:
The Applet class provides a default implementation of these methods, which can be overridden when needed.
"Hello,world" applets are written according to standards. The only method that is overridden is the paint method.
An Applet is a Java program. It is typically run inside a Java-enabled Web browser. Because it has full Java API support, the Applet is a full-featured Java application.
<applet> tags are the basis for embedding applets in HTML files. The following is an example of invoking the "Hello World" applet;
Sample HTML code:<html> <Title>The Hello, World Applet</title>Ajax works by adding a middle-tier (Ajax engine) between the user and the server, making the user operation asynchronous with the server response. Not all user requests are submitted to the server, such as data validation and processing, to be done by the Ajax engine itself, only to determine the need to read new data from the server and then by the Ajax engine to submit the request to the server.
2. Comparison with traditional Web applicationsThe traditional Web application interaction triggered by the user an HTTP request to the server, the server processes it and then returns a new HTHL page to the client, each time the server processes the client-submitted request, the customer can only idle wait, and even if only a small interaction, Just get a very simple data from the server side, all to return a full HTML page, and the user each time to waste time and bandwidth to re-read the entire page. This practice wastes a lot of bandwidth, and the response time of the application depends on the response time of the server, because each application's interaction needs to send a request to the server. This causes the user interface to respond much more slowly than the local application. Unlike this, an AJAX application can send and retrieve only the necessary data to the server, using SOAP or some other XML-based Web service interface, and using JavaScript on the client to process the response from the server. Because the amount of data exchanged between the server and the browser is much reduced, the result is that we can see applications that respond faster. At the same time, a lot of processing can be done on the client machine making the request, so the processing time of the Web server is also reduced.
3. Pros and cons(1). The advantages of Ajax
<1>: No refresh update data.
The biggest advantage of Ajax is the ability to communicate with the server to maintain data without refreshing the entire page. This enables the Web application to respond more quickly to user interactions and avoids sending information that is not changed on the network, reducing user latency and bringing a very good user experience.
<2>. Asynchronously communicates with the server.
Ajax uses asynchronous methods to communicate with the server, without interrupting the user's operations and having more rapid responsiveness. Optimize communication between browser and server, reduce unnecessary data transfer, time and reduce traffic on network.
<3>: Front-end and back-end load balancing.
Ajax can pass on some of the previous server workloads to the client, take advantage of the client's idle ability to handle, reduce server and bandwidth burden, save space and broadband rental costs. and reduce the burden of the server, the principle of Ajax is "on-demand data", can minimize the redundancy request and response to the burden on the server, improve site performance.
<4> standards-based is widely supported.
Ajax is based on standardized and widely supported technologies that do not require a browser plugin or applet to be downloaded, but require the client to allow JavaScript to be executed on the browser. With the maturation of Ajax, a number of libraries that simplify the use of Ajax have been introduced. Similarly, there is another assistive programming technique that provides alternative functionality for users who do not support JavaScript.
<5>: interface and application separation.
Ajax separates the interface in the Web from the application (or the separation of data and rendering), facilitates division of labor, reduces web application errors caused by non-technical changes to the page, improves efficiency, and is more applicable to today's publishing systems.
(2). Disadvantages of Ajax
<1>. Ajax kills back and the history function, which is the destruction of the browser mechanism.
In the case of dynamically updating the page, the user cannot go back to the previous page state because the browser can only remember the static pages in the history. The difference between a fully read page and a page that has been dynamically modified is very subtle; The user will typically want to click the Back button to cancel their previous operation, but in an AJAX application this will not be possible.
The Back button is an important feature of a standard web site, but it does not work well with JS. This is a serious problem with Ajax, because users often want to be able to undo the previous operation by going backwards. So is there any way to do this question? The answer is yes, using Gmail know, Gmail under the Ajax technology to solve the problem, in Gmail can be back, but it does not change the mechanism of Ajax, it is only a relatively stupid but effective way, that is, the user click the Back button to access the history To reproduce the changes on the page by creating or using a hidden iframe. (for example, when a user clicks back in Google Maps, it searches in a hidden iframe and then reflects the search results on an AJAX element to restore the application state to its current state.) ）
However, while this problem can be solved, the cost of development is very high and deviates from the rapid development required by the AJAX framework. This is a very serious problem caused by Ajax.
A related view is that using dynamic page updates makes it difficult for users to save a particular state to a collection. The solution for this problem has also occurred, with most of the URL fragment identifiers (often referred to as anchors, which are the parts of the # URL) to keep track of, allowing the user to return to the specified application state. (many browsers allow JavaScript to dynamically update the anchor point, which allows an AJAX application to update the anchor point while updating the display.) These solutions also address many of the arguments that do not support back buttons.
<2>. Security issues with Ajax.
Ajax technology provides users with a good user experience, but also brings new security threats to it enterprises, Ajax technology is like the enterprise data set up a direct channel. This allows developers to inadvertently expose more data and server logic than ever before. The logic of Ajax can be hidden from the client's security scanning technology, allowing hackers to create new attacks from remote servers. And Ajax also makes it difficult to avoid some known security weaknesses, such as cross-site footstep attacks, SQL injection attacks, and credentials-based security vulnerabilities.
<3> Support for search engines is weak.
Support for search engines is weaker. If used improperly, Ajax can increase the flow of network data, thereby reducing the overall system performance.
<4>: The exception handling mechanism of the sabotage program.
At least for now, these Ajax frameworks, like Ajax.dll,ajaxpro.dll, can disrupt the program's exception mechanism. On this issue, had encountered in the development process, but looked at the internet there is little relevant introduction. Later, an experiment was done to delete a piece of data using Ajax and the traditional form submission mode ... It has brought us a lot of difficulties in debugging.
<5>: Violates the original intention of URL and resource location.
For example, I give you a URL address, if you use Ajax technology, perhaps you see below the URL address and what I see under this URL is different. This is contrary to the original intention of resource positioning.
<6>. Ajax does not support mobile devices very well.
Some handheld devices (such as mobile phones, PDAs, etc.) are not yet well-supported Ajax, for example, when we open a website with Ajax technology on a mobile phone's browser, it is currently unsupported.
<7> client-side fat, too many client-side code causes development costs.
Write complex, error-prone, redundant code is more (layer contains JS file is the common problem of Ajax, coupled with many of the previous service-side code is now placed on the client), destroying the original standards of the Web.
Java EE (v)
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