The Spring Expression Language (SpEL for short) is a powerful expression language that supports querying and manipulating an object graph at runtime. The language syntax is similar to Unified EL but offers additional features, most notably method invocation and basic string templating functionality. While there are several other Java expression languages available, OGNL, MVEL, and JBoss EL, to name a few, the Spring Expression Language was created to provide the Spring community with a single well supported expression language that can used across all the products in the Spring portfolio. Its language features are driven by the requirements of the projects in the Spring portfolio, including tooling requirements for code completion support within the eclipse based SpringSource Tool Suite. That said, SpEL is based on an technology agnostic API allowing other expression language implementations to be integreated should the need arise. While SpEL serves as the foundation for expression evaluation within the Spring portfolio, it is not directly tied to Spring and can be used independently. In order to be self contained, many of the examples in this chatper use SpEL as if it was an independent expression language. This requires creating a few boostrapping infrastructure classes such as the parser. Most Spring users will not need to deal with this infrastructure and will instead only author expression strings for evaluation. An example of this typical use is the integration of SpEL into creating XML or annotated based bean definitions as shown in the section Expression support for defining bean definitions. This chapter covers the features of the expression language, its API, and its language sytnax. In several places an Inventor and Inventor's Society class are used as the target objects for expression evaluation. These class declarations and the data used to populate them are listed at the end of the chapter.

The expression language support the following functionality Literal expressions Boolean and relational operators Regular expressions Class expressions Accessing properties, arrays, lists, maps Method invocation Operators Assignment Calling constructors Variables User defined functions Templated expressions

This section introduces the simple use of SpEL interfaces and its expression language. The complete language reference can be found in the section Language Reference The following code introduces the SpEL API to evaluate the literal string expression 'Hello World' ExpressionParser parser = new SpelAntlrExpressionParser(); Expression exp = parser.parseExpression("'Hello World'"); String message = (String) exp.getValue();The value of the message variable is simply 'Hello World'. The SpEL classes and interfaces you are most likely to use are located in the packages org.springframework.expression and its subpackages spel.antlr and spel.support. The expression language is based on a grammar and uses ANTLR to construct the lexer and parser. The interface ExpressionParser is responsible for parsing an expression string. In this example the expression string is a string literal denoted by the surrounding single quotes. The interface Expression is responsible for evaluating the previously defined expression string. There are two exceptions that can be thrown, ParseException and EvaluationException when calling 'parser.parseExpression' and 'exp.getValue' respectfully. SpEL supports a wide range of features, such a calling methods, accessing properties and calling constructors. As an example of method invocation, we call the 'concat' method on the string literal ExpressionParser parser = new SpelAntlrExpressionParser(); Expression exp = parser.parseExpression("'Hello World'.concat(!)"); String message = (String) exp.getValue(); The value of message is now 'Hello World!'. As an example of calling a JavaBean property, the String property 'Bytes' can be called as shown below ExpressionParser parser = new SpelAntlrExpressionParser(); Expression exp = parser.parseExpression("'Hello World'.bytes"); // invokes 'getBytes()' byte[] bytes = (byte[]) exp.getValue(); Upper or lowercase can be used to specify the property name. SpEL also supports nested properties using standard 'dot' notation, i.e. prop1.prop2.prop3 and the setting of property values Public fields may also be accessed ExpressionParser parser = new SpelAntlrExpressionParser(); Expression exp = parser.parseExpression("'Hello World'.bytes.length"); // invokes 'getBytes().length' int length = (Integer) exp.getValue(); The String's constructor can be called instead of using a string literal ExpressionParser parser = new SpelAntlrExpressionParser(); Expression exp = parser.parseExpression("new String('hello world').toUpperCase()"); String message = exp.getValue(String.class); Note the use of the generic method public <T> T getValue(Class<T> desiredResultType). Using this method removes the need to cast the value of the expression to the desired result type. An EvaluationException will be thrown if the value an not be cast to the type T or converted using the registered type converter. The more common usage of SpEL is provide an expression string that is evaluated against a specific object instance. In the following example we retrieve the Name property from an instance of the Inventor class. // Create and set a calendar GregorianCalendar c = new GregorianCalendar(); c.set(1856, 7, 9); // The constructor arguments are name, birthday, and nationaltiy. Inventor tesla = new Inventor("Nikola Tesla", c.getTime(), "Serbian"); ExpressionParser parser = new SpelAntlrExpressionParser(); Expression exp = parser.parseExpression("name"); EvaluationContext context = new StandardEvaluationContext(); context.setRootObject(tesla); String name = (String) exp.getValue(context);In the last line, the value of the string variable 'name' will be set to "Nikola Tesla". The class StandardEvaluationContext is where you can specify which object the "Name" property will be evaluated against. You can reuse the same expression over and over again and set a new root object on the evaluation context. Expressions are evaluated using reflection. In standalone usage of SpEL you will need to create the parser as well as provide an evaluation context. However, more common usage is to provide only the SpEL expression string as part of a configuration file, for example for Spring bean or Spring Web Flow definitions. In this case, the parser, evaluation context, root object and any predefined variables will be set up for you implicitly. As a final introductory example, the use of a boolean operator is shown using the Inventor object in the previous example Expression exp = parser.parseExpression("name == 'Nikola Tesla'"); boolean isEqual = exp.getValue(context, Boolean.class); // evaluates to true

The interface EvaluationContext is used when evaluating an expression to resolve properties, methods ,fields, and to help perform type conversion. The out-of-the-box implementation, StandardEvaluationContext ,uses reflection to manipulate the object, caching java.lang.reflect's Method, Field, and Constructor instances for increased performance. The StandardEvaluationContext is where you specify the root object to evaluate against via the method setRootObject . You can also specify variables and functions that will be used in the expression using the methods setVariable and registerFunction. The use of variables and functions are described in the language reference sections Variables and Functions.

The StandardEvaluationContext uses an instance of org.springframework.expression.TypeConverter. A simple implementation of this interface, StandardTypeConverter, that converts basic types, primitive values, booleans and characters is used but will be replaced with an updated type conversion framework that is to be included as part of Spring 3.0

SpEL expressions can be used with XML or annotation based configuration metadata for defining BeanDefinitions. In both cases the syntax to define the expression is of the form #{ <expression string> }.

A property or constructor-arg value can be set using expressions as shown below <bean id="numberGuess" class="org.spring.samples.NumberGuess"> <property name="randomNumber" value="#{ T(java.lang.Math).random() * 100.0 }"/> <!-- other properties --> </bean> The variable 'systemProperties' is predefined, so you can use it in your expressions as shown below. <bean id="taxCalculator" class="org.spring.samples.TaxCalculator"> <property name="defaultLocale" value="#{ systemProperties['user.region'] }"/> <!-- other properties --> </bean> You can also refer to other bean properties by name, for example <bean id="numberGuess" class="org.spring.samples.NumberGuess"> <property name="randomNumber" value="#{ T(java.lang.Math).random() * 100.0 }"/> <!-- other properties --> </bean> <bean id="shapeGuess" class="org.spring.samples.ShapeGuess"> <property name="initialShapeSeed" value="#{ numberGuess.randomNumber }"/> <!-- other properties --> </bean>

The @Value annotation can be placed on fields, methods and method/constructor parameters to specify a default value. Here is an example to set the default value of a field variable public static class FieldValueTestBean @Value("#{ systemProperties['user.region'] }") private String defaultLocale; public void setDefaultLocale(String defaultLocale) { this.defaultLocale = defaultLocale; } public String getDefaultLocale() { return this.defaultLocale; } } The equivalent but on a property setter method is shown below public static class PropertyValueTestBean private String defaultLocale; @Value("#{ systemProperties['user.region'] }") public void setDefaultLocale(String defaultLocale) { this.defaultLocale = defaultLocale; } public String getDefaultLocale() { return this.defaultLocale; } } Autowired methods and constructors can also use the @Value annotation. public class SimpleMovieLister { private MovieFinder movieFinder; private String defaultLocale; @Autowired public void configure(MovieFinder movieFinder, @Value("#{ systemProperties['user.region'] } String defaultLocale) { this.movieFinder = movieFinder; this.defaultLocale = defaultLocale; } // ... } public class MovieRecommender { private String defaultLocale; private CustomerPreferenceDao customerPreferenceDao; @Autowired public MovieRecommender(CustomerPreferenceDao customerPreferenceDao, @Value("#{ systemProperties['user.region'] } String defaultLocale) { this.customerPreferenceDao = customerPreferenceDao; this.defaultLocale = defaultLocale; } // ... }

The types of literal expressions supported are strings, dates, numeric values (int, real, and hex), boolean and null. String are delimited by single quotes. To put a single quote itself in a string use the backslash character. The following listing shows simple usage of literals. Typically they would not be used in isolation like this, but as part of a more complex expression, for example using a literal on one side of a logical comparison operator. ExpressionParser parser = new SpelAntlrExpressionParser(); String helloWorld = (String) parser.parseExpression("'Hello World'").getValue(); // evals to "Hello World" double avogadrosNumber = (Double) parser.parseExpression("6.0221415E+23").getValue(); int maxValue = (Integer) parser.parseExpression("0x7FFFFFFF").getValue(); // evals to 2147483647 boolean trueValue = (Boolean) parser.parseExpression("true").getValue(); Object nullValue = parser.parseExpression("null").getValue(); Numbers support the use of the negative sign, exponential notation, and decimal points. By default real numbers are parsed using Double.parseDouble().

Navigating through properties is easy, just use a period to indicate a nested property value. The instances of Inventor class, pupin and tesla, were populated with data listed in section Section Classes used in the examples. To navigate "down" and get Tesla's year of birth and Pupin's city of birth the following expressions are used int year = (Integer) parser.parseExpression("Birthdate.Year + 1900").getValue(context); // 1856 String city = (String) parser.parseExpression("placeOfBirth.City").getValue(context); Case insensitivty is allowed for the first letter of proprety names. The contents of arrays and lists are obtained using square bracket notation.

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