spring-framework/spring-framework-reference/src/expressions.xml

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<chapter id="expressions">
  <title>Spring Expression Language (SpEL)</title>

  <section id="expressions-intro">
    <title>Introduction</title>

    <para>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. </para>

    <para>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.</para>

    <para>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 <link
    linkend="expressions-beandef">Expression support for defining bean
    definitions.</link></para>

    <para>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. </para>
  </section>

  <section id="expressions-features">
    <title>Feature Overview</title>

    <para>The expression language support the following functionality</para>

    <itemizedlist>
      <listitem>
        <para>Literal expressions</para>
      </listitem>

      <listitem>
        <para>Boolean and relational operators</para>
      </listitem>

      <listitem>
        <para>Regular expressions</para>
      </listitem>

      <listitem>
        <para>Class expressions</para>
      </listitem>

      <listitem>
        <para>Accessing properties, arrays, lists, maps</para>
      </listitem>

      <listitem>
        <para>Method invocation</para>
      </listitem>

      <listitem>
        <para>Operators</para>
      </listitem>

      <listitem>
        <para>Assignment</para>
      </listitem>

      <listitem>
        <para>Calling constructors</para>
      </listitem>

      <listitem>
        <para>Variables</para>
      </listitem>

      <listitem>
        <para>User defined functions</para>
      </listitem>

      <listitem>
        <para>Templated expressions</para>
      </listitem>
    </itemizedlist>
  </section>

  <section id="expressions-evaluation">
    <title>Expression Evaluation using Spring's Expression Interface</title>

    <para>This section introduces the simple use of SpEL interfaces and its
    expression language. The complete language reference can be found in the
    section <link lang="" linkend="expressions-language-ref">Language
    Reference</link></para>

    <para>The following code introduces the SpEL API to evaluate the literal
    string expression 'Hello World'</para>

    <para><programlisting language="java">ExpressionParser parser = new SpelAntlrExpressionParser();
Expression exp = parser.parseExpression("<emphasis role="bold">'Hello World'</emphasis>");
String message = (String) exp.getValue();</programlisting>The value of the
    message variable is simply 'Hello World'. </para>

    <para>The SpEL classes and interfaces you are most likely to use are
    located in the packages <package>org.springframework.expression</package>
    and its subpackages <package>spel.antlr</package> and
    <package>spel.support</package>.</para>

    <para>The expression language is based on a grammar and uses ANTLR to
    construct the lexer and parser. The interface
    <interfacename>ExpressionParser</interfacename> 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
    <interfacename>Expression</interfacename> is responsible for evaluating
    the previously defined expression string. There are two exceptions that
    can be thrown, <classname>ParseException</classname> and
    <classname>EvaluationException</classname> when calling
    '<literal>parser.parseExpression</literal>' and
    '<literal>exp.getValue</literal>' respectfully.</para>

    <para>SpEL supports a wide range of features, such a calling methods,
    accessing properties and calling constructors. </para>

    <para>As an example of method invocation, we call the 'concat' method on
    the string literal</para>

    <programlisting lang="" language="java">ExpressionParser parser = new SpelAntlrExpressionParser();
Expression exp = parser.parseExpression("<emphasis role="bold">'Hello World'.concat(!)</emphasis>");
String message = (String) exp.getValue();</programlisting>

    <para>The value of message is now 'Hello World!'. </para>

    <para>As an example of calling a JavaBean property, the String property
    'Bytes' can be called as shown below</para>

    <programlisting language="java">ExpressionParser parser = new SpelAntlrExpressionParser();
Expression exp = parser.parseExpression("<emphasis role="bold">'Hello World'.bytes</emphasis>");  // invokes 'getBytes()'
byte[] bytes = (byte[]) exp.getValue();</programlisting>

    <para>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</para>

    <para>Public fields may also be accessed</para>

    <programlisting language="java">ExpressionParser parser = new SpelAntlrExpressionParser();
Expression exp = parser.parseExpression("<emphasis role="bold">'Hello World'.bytes.length</emphasis>");  // invokes 'getBytes().length'
int length = (Integer) exp.getValue();</programlisting>

    <para>The String's constructor can be called instead of using a string
    literal</para>

    <programlisting language="java">ExpressionParser parser = new SpelAntlrExpressionParser();
Expression exp = parser.parseExpression("<emphasis role="bold">new String('hello world').toUpperCase()</emphasis>");
String message = exp.getValue(String.class);</programlisting>

    <para>Note the use of the generic method <literal>public &lt;T&gt; T
    getValue(Class&lt;T&gt; desiredResultType)</literal>. Using this method
    removes the need to cast the value of the expression to the desired result
    type. An <classname>EvaluationException</classname> will be thrown if the
    value an not be cast to the type <literal>T</literal> or converted using
    the registered type converter.</para>

    <para>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 <literal>Name</literal> property from an instance of the
    Inventor class. </para>

    <para><programlisting language="java">// 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("<emphasis role="bold">name</emphasis>");

EvaluationContext context = new StandardEvaluationContext();
context.setRootObject(tesla);

String name = (String) exp.getValue(context);</programlisting>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. </para>

    <para><note>
        <para>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.</para>
      </note>As a final introductory example, the use of a boolean operator is
    shown using the Inventor object in the previous example</para>

    <programlisting language="java">Expression exp = parser.parseExpression("name == 'Nikola Tesla'");
boolean isEqual = exp.getValue(context, Boolean.class);  // evaluates to true</programlisting>

    <section>
      <title>The EvaluationContext interface </title>

      <para>The interface <interfacename>EvaluationContext</interfacename> is
      used when evaluating an expression to resolve properties, methods
      ,fields, and to help perform type conversion. The out-of-the-box
      implementation, <classname>StandardEvaluationContext</classname> ,uses
      reflection to manipulate the object, caching
      j<package>ava.lang.reflect</package>'s <classname>Method</classname>,
      <classname>Field</classname>, and <classname>Constructor</classname>
      instances for increased performance.</para>

      <para>The <classname>StandardEvaluationContext</classname> is where you
      specify the root object to evaluate against via the method
      <methodname>setRootObject</methodname> . You can also specify variables
      and functions that will be used in the expression using the methods
      <methodname>setVariable</methodname> and
      <methodname>registerFunction</methodname>. The use of variables and
      functions are described in the language reference sections <link
      linkend="expressions-ref-variables">Variables</link> and <link lang=""
      linkend="expressions-ref-functions">Functions</link>.</para>

      <section>
        <title>Type Conversion</title>

        <para>The StandardEvaluationContext uses an instance of
        <classname>org.springframework.expression.TypeConverter</classname>. A
        simple implementation of this interface,
        <classname>StandardTypeConverter</classname>, 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</para>
      </section>
    </section>
  </section>

  <section id="expressions-beandef">
    <title>Expression support for defining bean definitions</title>

    <para>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 <literal>#{ &lt;expression
    string&gt; }</literal>.</para>

    <section id="expressions-beandef-xml-based">
      <title>XML based configuration</title>

      <para>A property or constructor-arg value can be set using expressions
      as shown below</para>

      <programlisting language="xml">&lt;bean id="numberGuess" class="org.spring.samples.NumberGuess"&gt;
    &lt;property name="randomNumber" value="#{ T(java.lang.Math).random() * 100.0 }"/&gt;

    &lt;!-- other properties --&gt;
&lt;/bean&gt;</programlisting>

      <para>The variable 'systemProperties' is predefined, so you can use it
      in your expressions as shown below.</para>

      <programlisting language="xml">&lt;bean id="taxCalculator" class="org.spring.samples.TaxCalculator"&gt;
    &lt;property name="defaultLocale" value="#{ systemProperties['user.region'] }"/&gt;

    &lt;!-- other properties --&gt;
&lt;/bean&gt;</programlisting>

      <para>You can also refer to other bean properties by name, for
      example</para>

      <para><programlisting language="xml">&lt;bean id="numberGuess" class="org.spring.samples.NumberGuess"&gt;
    &lt;property name="randomNumber" value="#{ T(java.lang.Math).random() * 100.0 }"/&gt;

    &lt;!-- other properties --&gt;
&lt;/bean&gt;


&lt;bean id="shapeGuess" class="org.spring.samples.ShapeGuess"&gt;
    &lt;property name="initialShapeSeed" value="#{ numberGuess.randomNumber }"/&gt;

    &lt;!-- other properties --&gt;
&lt;/bean&gt;</programlisting></para>
    </section>

    <section id="expressions-beandef-annotation-based">
      <title>Annotation-based configuration</title>

      <para>The <literal>@Value</literal> annotation can be placed on fields,
      methods and method/constructor parameters to specify a default
      value.</para>

      <para>Here is an example to set the default value of a field
      variable</para>

      <programlisting language="java">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;
  }

}

</programlisting>

      <para>The equivalent but on a property setter method is shown
      below</para>

      <programlisting language="java">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;
  }

}</programlisting>

      <para>Autowired methods and constructors can also use the
      <literal>@Value</literal> annotation.</para>

      <programlisting language="java">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;
    }

    // ...
}</programlisting>

      <para><programlisting language="java">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;
    }

    // ...
}</programlisting></para>
    </section>
  </section>

  <section id="expressions-language-ref">
    <title>Language Reference</title>

    <section id="expressions-ref-literal">
      <title>Literal expressions</title>

      <para>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. </para>

      <programlisting language="java">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();
</programlisting>

      <para>Numbers support the use of the negative sign, exponential
      notation, and decimal points. By default real numbers are parsed using
      Double.parseDouble().</para>
    </section>

    <section>
      <title>Properties, Arrays, Lists, Dictionaries, Indexers</title>

      <para>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 <link
      linkend="expressions-examples-classes">Classes used in the
      examples</link>. To navigate "down" and get Tesla's year of birth and
      Pupin's city of birth the following expressions are used </para>

      <programlisting lang="" language="java">int year = (Integer) parser.parseExpression("Birthdate.Year + 1900").getValue(context); // 1856


String city = (String) parser.parseExpression("placeOfBirth.City").getValue(context);</programlisting>

      <para>Case insensitivty is allowed for the first letter of proprety
      names. The contents of arrays and lists are obtained using square
      bracket notation. </para>

      <programlisting></programlisting>
    </section>

    <section>
      <title>Methods</title>

      <para>blah blah varargs</para>
    </section>

    <section>
      <title>Operators</title>

      <para>blah blah</para>

      <section>
        <title>Relational operators</title>

        <para>blah blah</para>

        <para></para>
      </section>

      <section>
        <title>Logical operators</title>

        <para></para>

        <para></para>
      </section>

      <section>
        <title>Mathematical operators</title>

        <para>blah blah</para>

        <para></para>
      </section>
    </section>

    <section>
      <title>Assignment</title>

      <para>blah blah</para>

      <para></para>
    </section>

    <section>
      <title>Types</title>

      <para>blah blah</para>
    </section>

    <section>
      <title>Constructors</title>

      <para>blah blah</para>
    </section>

    <section id="expressions-ref-variables">
      <title>Variables</title>

      <para>blah blah</para>

      <section>
        <title>The #this and #root variables</title>

        <para>blah blah </para>
      </section>
    </section>

    <section id="expressions-ref-functions">
      <title>Functions</title>

      <para>blah blah</para>
    </section>

    <section>
      <title>Expression templating</title>

      <para>blah blah</para>
    </section>
  </section>

  <section id="expressions-example-classes">
    <title>Classes used in the examples</title>

    <para></para>
  </section>
</chapter>