Introduction to Java Design Pattern Memento Pattern

Memento definition: memento is an object that saves a copy of the internal state of another object, so that the object can be restored to its originally saved state in the future.

The Memento mode is relatively easy to understand. Let’s look at the following code:

public class Originator {
　　 private int number;
　　private File file = null;
　　public Originator(){}
　　// 创建一个Memento
　　public Memento getMemento(){
　　　　return new Memento(this);
　　}
　　// 恢复到原始值
　　public void setMemento(Memento m){
　　　　 number = m.number;
　　　　 file = m.file;
　　}
}

Let’s look at the Memento class again:

private class Memento implements java.io.Serializable{
　　private int number;
　　private File file = null;
　　public Memento( Originator o){
　　　　number = o.number;
　　　　file = o.file;
　　}
}

It can be seen that Memento saves the number and number in Originator. The value of file. If the number and file values ​​​​in the Originator are changed, they can be restored by calling the setMemento() method.

The disadvantage of Memento mode is that it is very expensive. If there are many internal states, saving another copy will unintentionally waste a lot of memory.

Application of Memento pattern in Jsp+Javabean

In Jsp applications, we usually have many forms that require user input, such as user registration, where you need to enter your name and email, etc. If some table items The user has not filled in or filled in incorrectly. We hope that after the user presses "Submit", through the Jsp program check, if it is found that there are indeed unfilled items, a warning or error will be displayed in red under the item, and at the same time, it will also display what the user has just entered. table entry.

In the picture below, the First Name has been entered by the user, but the Last Name has not been entered. We will prompt a red text warning

The implementation of this technology is to use The scope="request" or scope="session" characteristics of Javabeans are the Memento pattern.

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