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The Spring Expression Language (SpEL) is a powerful expression language that supports querying and manipulating an object graph at runtime. We can use it with XML or annotation-based Spring configurations.
There are several operators available in the language:
For these examples, we will use annotation-based configuration. Find more details about XML configuration in later sections of this article.
SpEL expressions begin with the # symbol and are wrapped in braces: #{expression} .
Properties can be referenced in a similar fashion, starting with a $ symbol and wrapped in braces: ${property.name} .
Property placeholders cannot contain SpEL expressions, but expressions can contain property references:
In the example above, assume someProperty has value 2, so the resulting expression would be 2 + 2, which would be evaluated to 4.
SpEL supports all basic arithmetic operators:
Divide and modulo operations have alphabetic aliases, div for / and mod for % . The + operator can also be used to concatenate strings.
SpEL also supports all basic relational and logical operations:
All relational operators have alphabetic aliases as well. For example, in XML-based configs we can't use operators containing angle brackets ( < , <= , > , >= ). Instead, we can use lt (less than), le (less than or equal), gt (greater than) or ge (greater than or equal).
SpEL also supports all basic logical operations:
As with arithmetic and relational operators, all logical operators also have alphabetic clones.
We use conditional operators for injecting different values depending on some condition:
We use the ternary operator for performing compact if-then-else conditional logic inside the expression. In this example, we're trying to check if there was true or not.
Another common use for the ternary operator is to check if some variable is null and then return the variable value or a default:
The Elvis operator is a way of shortening of the ternary operator syntax for the case above used in the Groovy language. It is also available in SpEL.
This code is equivalent to the code above:
We can use the matches operator to check whether or not a string matches a given regular expression:
With the help of SpEL, we can access the contents of any Map or List in the context.
We'll create new bean workersHolder that will store information about some workers and their salaries in a List and a Map :
Now we can access the values of the collections using SpEL:
In this example, we'll look at how to use SpEL in XML-based configuration. We can use expressions to reference beans or bean fields/methods.
For example, suppose we have the following classes:
Now we create an application context in which expressions are used to inject values:
Take a look at the someCar bean. The engine and horsePower fields of someCar use expressions that are bean references to the engine bean and horsePower field respectively.
To do the same with annotation-based configurations, use the @Value(“#{expression}”) annotation.
Each operator from the first section of this article can be used in XML and annotation-based configurations.
However, remember that in XML-based configuration, we can't use the angle bracket operator “<“. Instead, we should use the alphabetic aliases, such as lt (less than) or le (less than or equals).
For annotation-based configurations, there are no such restrictions:
Now we will add a spelOperators bean to the application context:
Retrieving that bean from the context, we can then verify that values were injected properly:
Here we can see the output of the toString method of spelOperators bean:
At times, we may want to parse expressions outside the context of configuration. Fortunately, this is possible using SpelExpressionParser .
We can use all operators that we saw in previous examples but should use them without braces and hash symbol. That is, if we want to use an expression with the + operator when used in Spring configuration, the syntax is #{1 + 1} ; when used outside of configuration, the syntax is simply 1 + 1 .
In the following examples, we will use the Car and Engine beans defined in the previous section.
ExpressionParser is responsible for parsing expression strings. In this example, SpEL parser will simply evaluate the string ‘Any String' as an expression. Unsurprisingly, the result will be ‘Any String' .
As with using SpEL in configuration, we can use it to call methods, access properties or call constructors:
Additionally, instead of directly operating on the literal, we could call the constructor:
We can also access the bytes property of String class, in the same way, resulting in the byte[] representation of the string:
We can chain method calls, just as in normal Java code:
In this case, the result will be 9 because we have replaced whitespace with the empty string.
If we don't want to cast the expression result, we can use the generic method T getValue(Class desiredResultType) , in which we can provide the desired type of class that we want to be returned.
Note that EvaluationException will be thrown if the returned value cannot be cast to desiredResultType :
The most common usage is to provide an expression string that is evaluated against a specific object instance:
In this case, the result will be equal to the value of the model field of the car object, “ Model 3 “. The StandardEvaluationContext class specifies which object the expression will be evaluated against.
It cannot be changed after the context object is created. StandardEvaluationContext is expensive to construct, and during repeated usage, it builds up cached state that enables subsequent expression evaluations to be performed more quickly. Because of caching, it is good practice to reuse StandardEvaluationContext where possible if the root object does not change.
However, if the root object is changed repeatedly, we can use the mechanism shown in the example below:
Here we call the getValue method with an argument that represents the object to which we want to apply a SpEL expression.
We can also use the generic getValue method, just as before:
Using the setValue method on the Expression object returned by parsing an expression, we can set values on objects. SpEL will take care of type conversion. By default, SpEL uses org.springframework.core.convert.ConversionService . We can create our own custom converter between types. ConversionService is generics aware, so we can use it with generics.
Let's take a look how we do that in practice:
The resulting car object will have model “ Other model “, which was changed from “ Model 3 “.
In the following example, we will use this class:
It is possible to configure ExpressionParser by calling the constructor with a SpelParserConfiguration object .
For example, if we try to add car object into the cars array of CarPark class without configuring the parser, we will get an error like this:
We can change the behavior of the parser, to allow it to automatically create elements if the specified index is null ( autoGrowNullReferences , the first parameter to the constructor), or to automatically grow an array or list to accommodate elements beyond its initial size ( autoGrowCollections , the second parameter):
The resulting car object will be equal to the car object that was set as the first element of the cars array of carPark object from the previous example.
SpEL is a powerful, well-supported expression language that we can use across all the products in the Spring portfolio. We can use it to configure Spring applications or to write parsers to perform more general tasks in any application.
The code samples in this article are available in the linked GitHub repository .
<, >, ==, !=, <=, >=, lt, gt, eq, ne, le, ge
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SpEL is an exression language supporting the features of querying and manipulating an object graph at runtime.
There are many expression languages available such as JSP EL, OGNL, MVEL and JBoss EL. SpEL provides some additional features such as method invocation and string templating functionality.
The SpEL API provides many interfaces and classes. They are as follows:
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