Spring spel expression usage example

Spring in action third edition reading notes

spring3. 0 introduces spring expression language (spel), which we can implement through spel

1. Reference the bean by its ID

2. Calling methods and properties in reference objects

3. Calculate the value of the expression

4. Regular expression matching

5. Collection operation

The ultimate goal of spel is to get the calculated value of the expression. These expressions may be some enumerated values, some attributes of the reference object, or some constants in the class. Complex spel expressions are usually composed of some simple elements. The simplest thing is to get the values of some given elements, for example:

。 This situation seems silly. Spel is not needed at all, but complex expressions are composed of simple expressions

References to other beans

Through spel, we can also reference other beans in the context

Equivalent to

Reference another bean with ID Saxophone as the value of the instrument to reference a property in other beans

Take the song field of the bean with ID Kenny as the reference of the song value to a method in other beans

Call the selectsong () method of the bean with ID songselector and use its return value as the value of song, which also brings a problem. If the selectsong () method returns null, a null pointer exception will be thrown

< property name = "song" value = "#{songselector. Selectsong()?. touppercase()}" / >, expression (?.) You can ensure that the touppercase () method is called when the return of selectsong () is not empty. If the return is empty, the subsequent methods will not be called

Reference to class

If a class is an external class rather than a bean defined in spring, how do you reference it?

Use the expression t (), for example:

Spel evaluates an expression

Spel expressions support a variety of operators. We can use these operators to calculate the value of the expression

Filter elements from a collection using spel:

Define a collection using the element < util: list > in spring's util namespace

Filter collections using spel

，

[] operator can also filter the map, assuming that citis is a map type < property name = "chosencity" value = "#{cities [" keyname "]}" / > [] operate on the properties type

< util: properties id = "Settings" location = "classpath: settings. Properties" / > use the < util: Properties > tag to read a properties file

summary

The above is all about the spring spel expression usage example in this article. I hope it will be helpful to you.