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Initial pass of docs for he Spring expression language.

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Mark Pollack 2009-04-05 09:38:19 +00:00
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<?xml version="1.0" encoding="UTF-8"?>
<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 integreatd 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 shown
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 be authoring expression strings for evaluation. An example of
this typical use is the integration of SpEL into the definition of XML or
annotated based bean definitions 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 and its
synax. 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 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>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 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 case 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 to method invocation, we call the 'concat' method on
the string literal</para>
<programlisting>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>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. </para>
<para>Public fields may also be accessed</para>
<programlisting>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 the string
literal</para>
<programlisting>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>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);</programlisting>In the last
line, the value of the string variable 'name' will be set to "Nikola
Tesla". </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
would be to provide only the SpEL expression string as part of a
configuration file, for example for Spring bean or web flow
definitions. The parser, evaluation context and root object will be
set up for you implicitly. </para>
</note></para>
<section>
<title>The EvaluationContext interface </title>
<para>The interface <interfacename>EvaluationContext</interfacename> is
used when evaluating an expression to resolve properties, methods
,fields, and help perform type conversion. The out-of-the-box
implementation, <classname>StandardEvaluationContext</classname> ,uses
reflection to manipulate the object, caching java.lang.reflect Method,
Fields, and Constructor 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 using the methods <methodname>setVariable</methodname>
and, <methodname>registerFunction</methodname>. The use of variables and
functions are described in the language reference section.</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>&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>&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>&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 @Value 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>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>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>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>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>
<para></para>
<section>
<title>Literal expressions</title>
<para>blah blah</para>
</section>
<section>
<title>Properties, Arrays, Lists, Dictionaries, Indexers</title>
<para>blah blah</para>
</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>
<title>Variables</title>
<para>blah blah</para>
<section>
<title>The #this and #root variables</title>
<para>blah blah </para>
</section>
</section>
<section>
<title>Functions</title>
<para>blah blah</para>
</section>
<section>
<title>Expression templating</title>
<para>blah blah</para>
</section>
</section>
<section>
<title>Classes used in the examples</title>
<para></para>
</section>
</chapter>

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@ -7,6 +7,7 @@
<!ENTITY beans SYSTEM "beans.xml"> <!ENTITY beans SYSTEM "beans.xml">
<!ENTITY resources SYSTEM "resources.xml"> <!ENTITY resources SYSTEM "resources.xml">
<!ENTITY validation SYSTEM "validation.xml"> <!ENTITY validation SYSTEM "validation.xml">
<!ENTITY expressions SYSTEM "expressions.xml">
<!ENTITY aop SYSTEM "aop.xml"> <!ENTITY aop SYSTEM "aop.xml">
<!ENTITY aop-api SYSTEM "aop-api.xml"> <!ENTITY aop-api SYSTEM "aop-api.xml">
<!ENTITY transaction SYSTEM "transaction.xml"> <!ENTITY transaction SYSTEM "transaction.xml">
@ -196,6 +197,9 @@
<listitem> <listitem>
<para><xref linkend="validation"/></para> <para><xref linkend="validation"/></para>
</listitem> </listitem>
<listitem>
<para><xref linkend="expressions"/></para>
</listitem>
<listitem> <listitem>
<para><xref linkend="aop"/></para> <para><xref linkend="aop"/></para>
</listitem> </listitem>
@ -210,6 +214,7 @@
&beans; &beans;
&resources; &resources;
&validation; &validation;
&expressions;
&aop; &aop;
&aop-api; &aop-api;
&testing; &testing;