Spring Expression Language Guide

1. Panoramica

Spring Expression Language (SpEL) è un potente linguaggio di espressione che supporta l'interrogazione e la manipolazione di un oggetto grafico in fase di esecuzione. Può essere utilizzato con XML o configurazioni Spring basate su annotazioni.

Sono disponibili diversi operatori nella lingua:

genere Operatori
Aritmetica +, -, *, /,%, ^, div, mod
Relazionale , ==,! =, =, lt, gt, eq, ne, le, ge
Logico e, o, no, &&, ||,!
Condizionale ?:
Regex partite

2. Operatori

Per questi esempi, useremo la configurazione basata sull'annotazione. Maggiori dettagli sulla configurazione XML sono disponibili nelle sezioni successive di questo articolo.

Le espressioni SpEL iniziano con il simbolo # e sono racchiuse tra parentesi graffe: # {espressione} . È possibile fare riferimento alle proprietà in modo simile, iniziando con un simbolo $ e racchiuso tra parentesi graffe: $ {property.name} . I segnaposto di proprietà non possono contenere espressioni SpEL, ma le espressioni possono contenere riferimenti a proprietà:

#{${someProperty} + 2}

Nell'esempio sopra, si supponga che someProperty abbia valore 2, quindi l'espressione risultante sarebbe 2 + 2, che verrebbe valutata su 4.

2.1. Operatori aritmetici

Sono supportati tutti gli operatori aritmetici di base.

@Value("#{19 + 1}") // 20 private double add; @Value("#{'String1 ' + 'string2'}") // "String1 string2" private String addString; @Value("#{20 - 1}") // 19 private double subtract; @Value("#{10 * 2}") // 20 private double multiply; @Value("#{36 / 2}") // 19 private double divide; @Value("#{36 div 2}") // 18, the same as for / operator private double divideAlphabetic; @Value("#{37 % 10}") // 7 private double modulo; @Value("#{37 mod 10}") // 7, the same as for % operator private double moduloAlphabetic; @Value("#{2 ^ 9}") // 512 private double powerOf; @Value("#{(2 + 2) * 2 + 9}") // 17 private double brackets; 

Le operazioni di divisione e modulo hanno alias alfabetici, div per / e mod per % . L' operatore + può essere utilizzato anche per concatenare le stringhe.

2.2. Operatori relazionali e logici

Sono supportate anche tutte le operazioni logiche e relazionali di base.

@Value("#{1 == 1}") // true private boolean equal; @Value("#{1 eq 1}") // true private boolean equalAlphabetic; @Value("#{1 != 1}") // false private boolean notEqual; @Value("#{1 ne 1}") // false private boolean notEqualAlphabetic; @Value("#{1 < 1}") // false private boolean lessThan; @Value("#{1 lt 1}") // false private boolean lessThanAlphabetic; @Value("#{1  1}") // false private boolean greaterThan; @Value("#{1 gt 1}") // false private boolean greaterThanAlphabetic; @Value("#{1 >= 1}") // true private boolean greaterThanOrEqual; @Value("#{1 ge 1}") // true private boolean greaterThanOrEqualAlphabetic; 

Tutti gli operatori relazionali hanno anche alias alfabetici. Ad esempio, nelle configurazioni basate su XML non possiamo utilizzare operatori contenenti parentesi angolari ( < , <=, > , > = ). Invece, possiamo usare lt (minore di), le (minore o uguale), gt (maggiore di) o ge (maggiore o uguale).

2.3. Operatori logici

SpEL supporta tutte le operazioni logiche di base:

@Value("#") // true private boolean orAlphabetic; @Value("#{!true}") // false private boolean not; @Value("#{not true}") // false private boolean notAlphabetic;

Come per gli operatori aritmetici e relazionali, anche tutti gli operatori logici hanno cloni alfabetici.

2.4. Operatori condizionali

Gli operatori condizionali vengono utilizzati per inserire valori diversi a seconda di alcune condizioni:

@Value("#{2 > 1 ? 'a' : 'b'}") // "a" private String ternary;

L'operatore ternario viene utilizzato per eseguire la logica condizionale compatta if-then-else all'interno dell'espressione. In questo esempio proviamo a verificare se era vero o no.

Un altro uso comune dell'operatore ternario è controllare se una variabile è nulla e quindi restituire il valore della variabile o un valore predefinito:

@Value("#{someBean.someProperty != null ? someBean.someProperty : 'default'}") private String ternary;

L'operatore Elvis è un modo per abbreviare la sintassi dell'operatore ternario per il caso precedente utilizzato nel linguaggio Groovy. È disponibile anche in SpEL. Il codice seguente è equivalente al codice sopra:

@Value("#{someBean.someProperty ?: 'default'}") // Will inject provided string if someProperty is null private String elvis;

2.5. Utilizzo di Regex in SpEL

The matches operator can be used to check whether or not a string matches a given regular expression.

@Value("#{'100' matches '\\d+' }") // true private boolean validNumericStringResult; @Value("#{'100fghdjf' matches '\\d+' }") // false private boolean invalidNumericStringResult; @Value("#{'valid alphabetic string' matches '[a-zA-Z\\s]+' }") // true private boolean validAlphabeticStringResult; @Value("#{'invalid alphabetic string #$1' matches '[a-zA-Z\\s]+' }") // false private boolean invalidAlphabeticStringResult; @Value("#{someBean.someValue matches '\d+'}") // true if someValue contains only digits private boolean validNumericValue;

2.6. Accessing List and Map Objects

With help of SpEL, we can access the contents of any Map or List in the context. We will create new bean workersHolder that will store information about some workers and their salaries in a List and a Map:

@Component("workersHolder") public class WorkersHolder { private List workers = new LinkedList(); private Map salaryByWorkers = new HashMap(); public WorkersHolder() { workers.add("John"); workers.add("Susie"); workers.add("Alex"); workers.add("George"); salaryByWorkers.put("John", 35000); salaryByWorkers.put("Susie", 47000); salaryByWorkers.put("Alex", 12000); salaryByWorkers.put("George", 14000); } //Getters and setters }

Now we can access the values of the collections using SpEL:

@Value("#{workersHolder.salaryByWorkers['John']}") // 35000 private Integer johnSalary; @Value("#{workersHolder.salaryByWorkers['George']}") // 14000 private Integer georgeSalary; @Value("#{workersHolder.salaryByWorkers['Susie']}") // 47000 private Integer susieSalary; @Value("#{workersHolder.workers[0]}") // John private String firstWorker; @Value("#{workersHolder.workers[3]}") // George private String lastWorker; @Value("#{workersHolder.workers.size()}") // 4 private Integer numberOfWorkers;

3. Use in Spring Configuration

3.1. Referencing a Bean

In this example, we'll look at how to use SpEL in XML-based configuration. Expressions can be used to reference beans or bean fields/methods. For example, suppose we have the following classes:

public class Engine { private int capacity; private int horsePower; private int numberOfCylinders; // Getters and setters } public class Car { private String make; private int model; private Engine engine; private int horsePower; // Getters and setters }

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.

3.2. Using Operators in Configuration

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.

public class SpelOperators { private boolean equal; private boolean notEqual; private boolean greaterThanOrEqual; private boolean and; private boolean or; private String addString; // Getters and setters
 @Override public String toString() { // toString which include all fields }

Now we will add a spelOperators bean to the application context:

   = 6}"/>   300 or someCar.engine.capacity > 3000}"/>

Retrieving that bean from the context, we can then verify that values were injected properly:

ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml"); SpelOperators spelOperators = (SpelOperators) context.getBean("spelOperators"); 

Here we can see the output of the toString method of spelOperators bean:

[equal=true, notEqual=false, greaterThanOrEqual=true, and=true, or=true, addString=Some model manufactured by Some make] 

4. Parsing Expressions Programmatically

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.

4.1. Using ExpressionParser

Let's look at a simple example:

ExpressionParser expressionParser = new SpelExpressionParser(); Expression expression = expressionParser.parseExpression("'Any string'"); String result = (String) expression.getValue(); 

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.

Expression expression = expressionParser.parseExpression("'Any string'.length()"); Integer result = (Integer) expression.getValue();

Additionally, instead of directly operating on the literal, we could call the constructor:

Expression expression = expressionParser.parseExpression("new String('Any string').length()");

We can also access the bytes property of String class, in the same way, resulting in the byte[] representation of the string:

Expression expression = expressionParser.parseExpression("'Any string'.bytes"); byte[] result = (byte[]) expression.getValue();

We can chain method calls, just as in normal Java code:

Expression expression = expressionParser.parseExpression("'Any string'.replace(\" \", \"\").length()"); Integer result = (Integer) expression.getValue();

In this case, the result will be 9, because we have replaced whitespace with the empty string. If we don't wish 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:

Integer result = expression.getValue(Integer.class);

The most common usage is to provide an expression string that is evaluated against a specific object instance:

Car car = new Car(); car.setMake("Good manufacturer"); car.setModel("Model 3"); car.setYearOfProduction(2014); ExpressionParser expressionParser = new SpelExpressionParser(); Expression expression = expressionParser.parseExpression("model"); EvaluationContext context = new StandardEvaluationContext(car); String result = (String) expression.getValue(context);

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 it 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:

Expression expression = expressionParser.parseExpression("model"); String result = (String) expression.getValue(car);

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:

Expression expression = expressionParser.parseExpression("yearOfProduction > 2005"); boolean result = expression.getValue(car, Boolean.class);

4.2. Using ExpressionParser to Set a Value

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 it can be used with generics. Let's take a look how we can use it in practice:

Car car = new Car(); car.setMake("Good manufacturer"); car.setModel("Model 3"); car.setYearOfProduction(2014); CarPark carPark = new CarPark(); carPark.getCars().add(car); StandardEvaluationContext context = new StandardEvaluationContext(carPark); ExpressionParser expressionParser = new SpelExpressionParser(); expressionParser.parseExpression("cars[0].model").setValue(context, "Other model");

The resulting car object will have modelOther model” which was changed from “Model 3“.

4.3. Parser Configuration

In the following example, we will use the following class:

public class CarPark { private List cars = new ArrayList(); // Getter and setter }

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:

EL1025E:(pos 4): The collection has '0' elements, index '0' is invalid

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).

SpelParserConfiguration config = new SpelParserConfiguration(true, true); StandardEvaluationContext context = new StandardEvaluationContext(carPark); ExpressionParser expressionParser = new SpelExpressionParser(config); expressionParser.parseExpression("cars[0]").setValue(context, car); Car result = carPark.getCars().get(0);

The resulting car object will be equal to the car object which was set as the first element of the cars array of carPark object from the previous example.

5. Conclusion

SpEL è un linguaggio di espressione potente e ben supportato che può essere utilizzato in tutti i prodotti del portafoglio Spring. Può essere utilizzato per configurare le applicazioni Spring o per scrivere parser per eseguire attività più generali in qualsiasi applicazione.

Gli esempi di codice in questo articolo sono disponibili nel repository GitHub collegato.