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

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<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
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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 be 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 integrated 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
    chapter use SpEL as if it was an independent expression language. This
    requires creating a few bootstrapping 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 syntax. 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 supports 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>Relational operators</para>
      </listitem>

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

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

      <listitem>
        <para>Ternary operator</para>
      </listitem>

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

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

      <listitem>
        <para>Collection projection</para>
      </listitem>

      <listitem>
        <para>Collection selection</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 SpelExpressionParser();
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 sub package and <package>spel.support</package>.</para>

    <para>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 as 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 SpelExpressionParser();
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 SpelExpressionParser();

// invokes 'getBytes()'
Expression exp = parser.parseExpression("<emphasis role="bold">'Hello World'.bytes</emphasis>");  

byte[] bytes = (byte[]) exp.getValue();</programlisting>

    <para>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 SpelExpressionParser();

// invokes 'getBytes().length'
Expression exp = parser.parseExpression("<emphasis role="bold">'Hello World'.bytes.length</emphasis>");  

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 SpelExpressionParser();
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 SpelExpressionParser();
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 result = 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> or passing the root object into
      the constructor. . 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>. The
      <classname>StandardEvaluationContext</classname> is also where you can
      register custom <classname>ConstructorResolver</classname>s,
      <classname>MethodResolver</classname>s, and
      <classname>PropertyAccessor</classname>s to extend how SpEL evaluates
      expressions. Please refer to the JavaDoc of these classes for more
      details.</para>

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

        <para>By default SpEL uses the conversion service available in Spring
        core
        (<literal>org.springframework.core.convert.ConversionService</literal>).
        This conversion service comes with many converters built in for common
        conversions but is also fully extensible so custom conversions between
        types can be added. Additionally it has the key capability that it is
        generics aware. This means that when working with generic types in
        expressions, SpEL will attempt conversions to maintain type
        correctness for any objects it encounters.</para>

        <para>What does this mean in practice? Suppose assignment, using
        <literal>setValue()</literal>, is being used to set a
        <literal>List</literal> property. The type of the property is actually
        <literal>List&lt;Boolean&gt;</literal>. SpEL will recognize that the
        elements of the list need to be converted to
        <literal>Boolean</literal> before being placed in it. A simple
        example:</para>

        <programlisting>class Simple {
    public List&lt;Boolean&gt; booleanList = new ArrayList&lt;Boolean&gt;();
}
    	
Simple simple = new Simple();

simple.booleanList.add(true);

StandardEvaluationContext simpleContext = new StandardEvaluationContext(simple);

// false is passed in here as a string.  SpEL and the conversion service will 
// correctly recognize that it needs to be a Boolean and convert it
parser.parseExpression("booleanList[0]").setValue(simpleContext, "false");

// b will be false
Boolean b = simple.booleanList.get(0);
        </programlisting>
      </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. Note that you do not have to prefix
      the predefined variable with the '#' symbol in this context.</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.country']"} 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 SpelExpressionParser();

// evals to "Hello World"
String helloWorld = (String) parser.parseExpression("'Hello World'").getValue(); 

double avogadrosNumber  = (Double) parser.parseExpression("6.0221415E+23").getValue();  

// evals to 2147483647
int maxValue = (Integer) parser.parseExpression("0x7FFFFFFF").getValue();  

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, Maps, Indexers</title>

      <para>Navigating with property references 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 the section <link
      linkend="expressions-example-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">// evals to 1856
int year = (Integer) parser.parseExpression("Birthdate.Year + 1900").getValue(context); 


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

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

      <programlisting language="java">ExpressionParser parser = new SpelExpressionParser();

// Inventions Array
StandardEvaluationContext teslaContext = new StandardEvaluationContext(tesla);

// evaluates to "Induction motor"
String invention = parser.parseExpression("inventions[3]").getValue(teslaContext, 
                                                                    String.class); 


// Members List
StandardEvaluationContext societyContext = new StandardEvaluationContext(ieee);

// evaluates to "Nikola Tesla"
String name = parser.parseExpression("Members[0].Name").getValue(societyContext, String.class);

// List and Array navigation
// evaluates to "Wireless communication"
String invention = parser.parseExpression("Members[0].Inventions[6]").getValue(societyContext,
                                                                               String.class);
</programlisting>

      <para>The contents of maps are obtained by specifying the literal key
      value within the brackets. In this case, because keys for the Officers
      map are strings, we can specify string literal.</para>

      <programlisting lang="" language="java">// Officer's Dictionary

Inventor pupin = parser.parseExpression("Officers['president']").getValue(societyContext, 
                                                                          Inventor.class);

// evaluates to "Idvor"
String city = 
    parser.parseExpression("Officers['president'].PlaceOfBirth.City").getValue(societyContext,
                                                                               String.class);

// setting values
parser.parseExpression("Officers['advisors'][0].PlaceOfBirth.Country").setValue(societyContext,
                                                                                "Croatia");

</programlisting>
    </section>

    <section>
      <title>Methods</title>

      <para>Methods are invoked using typical Java programming syntax. You may
      also invoke methods on literals. Varargs are also supported.</para>

      <programlisting language="java">// string literal, evaluates to "bc"
String c = parser.parseExpression("'abc'.substring(2, 3)").getValue(String.class);

// evaluates to true
boolean isMember = parser.parseExpression("isMember('Mihajlo Pupin')").getValue(societyContext,
                                                                                Boolean.class);</programlisting>
    </section>

    <section>
      <title>Operators</title>

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

        <para>The relational operators; equal, not equal, less than, less than
        or equal, greater than, and greater than or equal are supported using
        standard operator notation.</para>

        <para><programlisting language="java">// evaluates to true
boolean trueValue = parser.parseExpression("2 == 2").getValue(Boolean.class);

// evaluates to false
boolean falseValue = parser.parseExpression("2 &lt; -5.0").getValue(Boolean.class);

// evaluates to true
boolean trueValue = parser.parseExpression("'black' &lt; 'block'").getValue(Boolean.class);</programlisting>In
        addition to standard relational operators SpEL supports the
        'instanceof' and regular expression based 'matches' operator.</para>

        <programlisting language="java">// evaluates to false
boolean falseValue = parser.parseExpression("'xyz' instanceof T(int)").getValue(Boolean.class);

// evaluates to true
boolean trueValue = 
     parser.parseExpression("'5.00' matches '^-?\\d+(\\.\\d{2})?$'").getValue(Boolean.class);

//evaluates to false
boolean falseValue = 
     parser.parseExpression("'5.0067' matches '^-?\\d+(\\.\\d{2})?$'").getValue(Boolean.class);

</programlisting>
      </section>

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

        <para>The logical operators that are supported are and, or, and not.
        Their use is demonstrated below</para>

        <para><programlisting language="java">// -- AND --

// evaluates to false
boolean falseValue = parser.parseExpression("true and false").getValue(Boolean.class);

// evaluates to true
String expression =  "isMember('Nikola Tesla') and isMember('Mihajlo Pupin')";
boolean trueValue = parser.parseExpression(expression).getValue(societyContext, Boolean.class);

// -- OR --

// evaluates to true
boolean trueValue = parser.parseExpression("true or false").getValue(Boolean.class);

// evaluates to true
String expression =  "isMember('Nikola Tesla') or isMember('Albert Einstien')";
boolean trueValue = parser.parseExpression(expression).getValue(societyContext, Boolean.class);

// -- NOT --

// evaluates to false
boolean falseValue = parser.parseExpression("!true").getValue(Boolean.class);


// -- AND and NOT --
String expression =  "isMember('Nikola Tesla') and !isMember('Mihajlo Pupin')";
boolean falseValue = parser.parseExpression(expression).getValue(societyContext, Boolean.class);</programlisting></para>
      </section>

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

        <para>The addition operator can be used on numbers, strings and dates.
        Subtraction can be used on numbers and dates. Multiplication and
        division can be used only on numbers. Other mathematical operators
        supported are modulus (%) and exponential power (^). Standard operator
        precedence is enforced. These operators are demonstrated below</para>

        <para><programlisting language="java">// Addition
int two = parser.parseExpression("1 + 1").getValue(Integer.class); // 2

String testString = 
   parser.parseExpression("'test' + ' ' + 'string'").getValue(String.class);  // 'test string'

// Subtraction
int four =  parser.parseExpression("1 - -3").getValue(Integer.class); // 4

double d = parser.parseExpression("1000.00 - 1e4").getValue(Double.class); // -9000

// Multiplication
int six =  parser.parseExpression("-2 * -3").getValue(Integer.class); // 6

double twentyFour = parser.parseExpression("2.0 * 3e0 * 4").getValue(Double.class); // 24.0

// Division
int minusTwo =  parser.parseExpression("6 / -3").getValue(Integer.class); // -2

double one = parser.parseExpression("8.0 / 4e0 / 2").getValue(Double.class); // 1.0

// Modulus
int three =  parser.parseExpression("7 % 4").getValue(Integer.class); // 3

int one = parser.parseExpression("8 / 5 % 2").getValue(Integer.class); // 1

// Operator precedence
int minusTwentyOne = parser.parseExpression("1+2-3*8").getValue(Integer.class); // -21
</programlisting></para>
      </section>
    </section>

    <section>
      <title>Assignment</title>

      <para>Setting of a property is done by using the assignment operator.
      This would typically be done within a call to
      <literal>setValue</literal> but can also be done inside a call to
      <literal>getValue</literal></para>

      <programlisting language="java">Inventor inventor = new Inventor();		
StandardEvaluationContext inventorContext = new StandardEvaluationContext(inventor);

parser.parseExpression("Name").setValue(inventorContext, "Alexander Seovic2");

// alternatively

String aleks = parser.parseExpression("Name = 'Alexandar Seovic'").getValue(inventorContext, 
                                                                            String.class);
</programlisting>

      <para></para>
    </section>

    <section>
      <title>Types</title>

      <para>The special 'T' operator can be used to specify an instance of
      java.lang.Class (the 'type'). Static methods are invoked using this
      operator as well. The <classname>StandardEvaluationContext</classname>
      uses a <classname>TypeLocator</classname> to find types and the
      <classname>StandardTypeLocator</classname> (which can be replaced) is
      built with an understanding of the java.lang package. This means T()
      references to types within java.lang do not need to be fully qualified,
      but all other type references must be.</para>

      <programlisting language="java">Class dateClass = parser.parseExpression("T(java.util.Date)").getValue(Class.class);

Class stringClass = parser.parseExpression("T(String)").getValue(Class.class);

boolean trueValue = 
   parser.parseExpression("T(java.math.RoundingMode).CEILING &lt; T(java.math.RoundingMode).FLOOR")
  .getValue(Boolean.class);
</programlisting>
    </section>

    <section>
      <title>Constructors</title>

      <para>Constructors can be invoked using the new operator. The fully
      qualified class name should be used for all but the primitive type and
      String (where int, float, etc, can be used).</para>

      <programlisting language="java">Inventor einstein = 
  p.parseExpression("new org.spring.samples.spel.inventor.Inventor('Albert Einstein', 
                                                                   'German')")
                                                                   .getValue(Inventor.class);

//create new inventor instance within add method of List
p.parseExpression("Members.add(new org.spring.samples.spel.inventor.Inventor('Albert Einstein',
                                                                   'German'))")
                                                                   .getValue(societyContext);
</programlisting>
    </section>

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

      <para>Variables can be referenced in the expression using the syntax
      #variableName. Variables are set using the method setVariable on the
      StandardEvaluationContext.</para>

      <programlisting language="java">Inventor tesla = new Inventor("Nikola Tesla", "Serbian");
StandardEvaluationContext context = new StandardEvaluationContext(tesla);
context.setVariable("newName", "Mike Tesla");

parser.parseExpression("Name = #newName").getValue(context);

System.out.println(tesla.getName()) // "Mike Tesla"</programlisting>

      <section>
        <title>The #this variable</title>

        <para>The variable #this is always defined and refers to the current
        evaluation object (the object against which unqualified references
        will be resolved).</para>

        <programlisting language="java">// create an array of integers
List&lt;Integer&gt; primes = new ArrayList&lt;Integer&gt;();
primes.addAll(Arrays.asList(2,3,5,7,11,13,17));

// create parser and set variable 'primes' as the array of integers
ExpressionParser parser = new SpelExpressionParser();
StandardEvaluationContext context = new StandardEvaluationContext();
context.setVariable("primes",primes);

// all prime numbers &gt; 10 from the list (using selection ?{...})
// evaluates to [11, 13, 17]
List&lt;Integer&gt; primesGreaterThanTen = 
             (List&lt;Integer&gt;) parser.parseExpression("#primes.?[#this&gt;10]").getValue(context);

</programlisting>
      </section>

      <!--
        <section>
        <title>The #root variable</title>

        <para>The variable #root is always defined and refers to the
        root evaluation object.  This is the object against which the first unqualified 
        reference to a property or method is resolved.</para>
        
        <para>It differs from #this in that #this typically varies throughout the 
        evaluation of an expression, whilst #root remains constant.
        It can be useful when writing a selection criteria, where the decision
        needs to be made based on some property of the root object rather than the
        current collection element.  For example:</para>

        <programlisting language="java">List selection = (List)parser.parseExpression("#someList.?[#root.supports(#this)]").getValue();
</programlisting>
      </section>
      -->
    </section>

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

      <para>You can extend SpEL by registering user defined functions that can
      be called within the expression string. The function is registered with
      the <classname>StandardEvaluationContext</classname> using the
      method</para>

      <programlisting language="java">public void registerFunction(String name, Method m)</programlisting>

      <para>A reference to a Java Method provides the implementation of the
      function. For example, a utility method to reverse a string is shown
      below.</para>

      <programlisting>public abstract class StringUtils {

  public static String reverseString(String input) {
    StringBuilder backwards = new StringBuilder();
    for (int i = 0; i &lt; input.length(); i++) 
      backwards.append(input.charAt(input.length() - 1 - i));
    }
    return backwards.toString();
  }
}</programlisting>

      <para>This method is then registered with the evaluation context and can
      be used within an expression string</para>

      <programlisting language="java">ExpressionParser parser = new SpelExpressionParser();
StandardEvaluationContext context = new StandardEvaluationContext();

context.registerFunction("reverseString", 
                         StringUtils.class.getDeclaredMethod("reverseString", 
                                                             new Class[] { String.class }));

String helloWorldReversed = 
          parser.parseExpression("#reverseString('hello')").getValue(context, String.class);</programlisting>
    </section>

    <section>
      <title>Ternary Operator (If-Then-Else)</title>

      <para>You can use the ternary operator for performing if-then-else
      conditional logic inside the expression. A minimal example is;</para>

      <programlisting language="java">String falseString = 
             parser.parseExpression("false ? 'trueExp' : 'falseExp'").getValue(String.class);</programlisting>

      <para>In this case, the boolean false results in returning the string
      value 'falseExp'. A less artificial example is shown below.</para>

      <programlisting language="java">parser.parseExpression("Name").setValue(societyContext, "IEEE");
societyContext.setVariable("queryName", "Nikola Tesla");

expression = "isMember(#queryName)? #queryName + ' is a member of the ' " + 
             "+ Name + ' Society' : #queryName + ' is not a member of the ' + Name + ' Society'";

String queryResultString = 
                    parser.parseExpression(expression).getValue(societyContext, String.class);
// queryResultString = "Nikola Tesla is a member of the IEEE Society"</programlisting>

      <para>Also see the next section on the Elvis operator for an even
      shorter syntax for the ternary operator</para>
    </section>

    <section>
      <title>The Elvis Operator</title>

      <para>The Elvis operator is a shortening of the ternary operator syntax
      and is used in the <ulink
      url="http://groovy.codehaus.org/Operators#Operators-ElvisOperator(%3F%3A)">Groovy</ulink>
      language. The ternary operator syntax you usually have to repeat a
      variable twice, for example</para>

      <programlisting>String name = "Elvis Presley";
String displayName = name != null ? name : "Unknown";</programlisting>

      <para>Instead you can use the Elvis operator, named for the resemblance
      to Elvis' hair style. </para>

      <programlisting>ExpressionParser parser = new SpelExpressionParser();

String name = parser.parseExpression("null?:'Unknown'").getValue(String.class);

System.out.println(name);  // 'Unknown'

</programlisting>

      <para>Here is a more complex example </para>

      <programlisting>ExpressionParser parser = new SpelExpressionParser();

Inventor tesla = new Inventor("Nikola Tesla", "Serbian");
StandardEvaluationContext context = new StandardEvaluationContext(tesla);

String name = parser.parseExpression("Name?:'Elvis Presley'").getValue(context, String.class);

System.out.println(name); // Mike Tesla

tesla.setName(null);

name = parser.parseExpression("Name?:'Elvis Presley'").getValue(context, String.class);

System.out.println(name); // Elvis Presley</programlisting>
    </section>

    <section>
      <title>Safe Navigation operator</title>

      <para>The Safe Navigation operator is used to avoid a
      <literal>NullPointerException</literal> and comes from the <ulink
      url="http://groovy.codehaus.org/Operators#Operators-SafeNavigationOperator(%3F.)">Groovy</ulink>
      language. Typically when you have a reference to an object you might
      need to verify that it is not null before accessing methods or
      properties of the object. To avoid this, the safe navigation operator
      will simply return null instead of throwing an exception.</para>

      <programlisting>ExpressionParser parser = new SpelExpressionParser();

Inventor tesla = new Inventor("Nikola Tesla", "Serbian");
tesla.setPlaceOfBirth(new PlaceOfBirth("Smiljan"));

StandardEvaluationContext context = new StandardEvaluationContext(tesla);

String city = parser.parseExpression("PlaceOfBirth?.City").getValue(context, String.class);
System.out.println(city); // Smiljan

tesla.setPlaceOfBirth(null);

city = parser.parseExpression("PlaceOfBirth?.City").getValue(context, String.class);

System.out.println(city); // null - does not throw NullPointerException!!!</programlisting>
    </section>

    <section>
      <title>Collection Selection</title>

      <para>Selection is a powerful expression language feature that allow you
      to transform some source collection into another by selecting from its
      entries.</para>

      <para>Selection uses the syntax
      <literal>?[selectionExpression]</literal>. This will filter the
      collection and return a new collection containing a subset of the
      original elements. For example, selection would allow us to easily get a
      list of Serbian inventors:</para>

      <programlisting language="java">List&lt;Inventor&gt; list = (List&lt;Inventor&gt;) 
      parser.parseExpression("Members.?[Nationality == 'Serbian']").getValue(societyContext);</programlisting>

      <para>Selection is possible upon both lists and maps. In the former case
      the selection criteria is evaluated against each individual list element
      whilst against a map the selection criteria is evaluated against each
      map entry (objects of the Java type <literal>Map.Entry</literal>). Map
      entries have their key and value accessible as properties for use in the
      selection.</para>

      <para>This expression will return a new map consisting of those elements
      of the original map where the entry value is less than 27.</para>

      <programlisting language="java">Map newMap = parser.parseExpression("map.?[value&lt;27]").getValue();</programlisting>

      <para>In addition to returning all the selected elements, it is possible
      to retrieve just the first or the last value. To obtain the first entry
      matching the selection the syntax is <literal>^[...]</literal> whilst to
      obtain the last matching selection the syntax is
      <literal>$[...]</literal>.</para>
    </section>

    <section>
      <title>Collection Projection</title>

      <para>Projection allows a collection to drive the evaluation of a
      sub-expression and the result is a new collection. The syntax for
      projection is <literal>![projectionExpression]</literal>. Most easily
      understood by example, suppose we have a list of inventors but want the
      list of cities where they were born. Effectively we want to evaluate
      'placeOfBirth.city' for every entry in the inventor list. Using
      projection:</para>

      <programlisting language="java">// returns [ 'Smiljan', 'Idvor' ]
List placesOfBirth = (List)parser.parseExpression("Members.![placeOfBirth.city]");</programlisting>

      <para>A map can also be used to drive projection and in this case the
      projection expression is evaluated against each entry in the map
      (represented as a Java <literal>Map.Entry</literal>). The result of a
      projection across a map is a list consisting of the evaluation of the
      projection expression against each map entry.</para>
    </section>

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

      <para>Expression templates allow a mixing of literal text with one or
      more evaluation blocks. Each evaluation block is delimited with a prefix
      and suffix characters that you can define, a common choice is to use
      <literal>${} </literal>as the delimiters. For example,</para>

      <programlisting language="java">String randomPhrase = 
   parser.parseExpression("random number is ${T(java.lang.Math).random()}", 
                          new TemplatedParserContext()).getValue(String.class);

// evaluates to "random number is 0.7038186818312008"</programlisting>

      <para>The string is evaluated by concatenating the literal text 'random
      number is' with the result of evaluating the expression inside the ${}
      delimiter, in this case the result of calling that random() method. The
      second argument to the method <literal>parseExpression()</literal> of
      the type <interfacename>ParserContext</interfacename>. The
      <interfacename>ParserContext</interfacename> interface is used to
      influence how the expression is parsed in order to support the
      expression templating functionality. The definition of
      <classname>TemplatedParserContext</classname> is shown below</para>

      <programlisting language="java">public class TemplatedParserContext implements ParserContext {

  public String getExpressionPrefix() {
    return "${";
  }

  public String getExpressionSuffix() {
    return "}";
  }
  
  public boolean isTemplate() {
    return true;
  }
}</programlisting>
    </section>
  </section>

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

    <para>Inventor.java</para>

    <programlisting language="java">package org.spring.samples.spel.inventor;

import java.util.Date;
import java.util.GregorianCalendar;

public class Inventor {

  private String name;
  private String nationality;
  private String[] inventions;
  private Date birthdate;
  private PlaceOfBirth placeOfBirth;
  
  
  public Inventor(String name, String nationality)
  {
    GregorianCalendar c= new GregorianCalendar();
    this.name = name;
    this.nationality = nationality;
    this.birthdate = c.getTime();
  }
  public Inventor(String name, Date birthdate, String nationality) {
    this.name = name;
    this.nationality = nationality;
    this.birthdate = birthdate;
  }
  
  public Inventor() {
  }

  public String getName() {
    return name;
  }
  public void setName(String name) {
    this.name = name;
  }
  public String getNationality() {
    return nationality;
  }
  public void setNationality(String nationality) {
    this.nationality = nationality;
  }
  public Date getBirthdate() {
    return birthdate;
  }
  public void setBirthdate(Date birthdate) {
    this.birthdate = birthdate;
  }
  public PlaceOfBirth getPlaceOfBirth() {
    return placeOfBirth;
  }
  public void setPlaceOfBirth(PlaceOfBirth placeOfBirth) {
    this.placeOfBirth = placeOfBirth;
  }
  public void setInventions(String[] inventions) {
    this.inventions = inventions;
  }
  public String[] getInventions() {
    return inventions;
  }       
}
</programlisting>

    <para>PlaceOfBirth.java</para>

    <programlisting language="java">package org.spring.samples.spel.inventor;

public class PlaceOfBirth {

	private String city;
	private String country;
	
	public PlaceOfBirth(String city) {
		this.city=city;
	}
	public PlaceOfBirth(String city, String country)
	{
		this(city);
		this.country = country;
	}
	
	
	public String getCity() {
		return city;
	}
	public void setCity(String s) {
		this.city = s;
	}
	public String getCountry() {
		return country;
	}
	public void setCountry(String country) {
		this.country = country;
	}

	
	
}
</programlisting>

    <para>Society.java</para>

    <programlisting lang="">package org.spring.samples.spel.inventor;

import java.util.*;

public class Society {

	private String name;
	
	public static String Advisors = "advisors";
	public static String President = "president";
	
	private List&lt;Inventor&gt; members = new ArrayList&lt;Inventor&gt;();
	private Map officers = new HashMap();

	public List getMembers() {
		return members;
	}

	public Map getOfficers() {
		return officers;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public boolean isMember(String name)
	{
		boolean found = false;
		for (Inventor inventor : members) {
			if (inventor.getName().equals(name))
			{
				found = true;
				break;
			}
		}		
		return found;
	}

	
}
</programlisting>
  </section>
</chapter>