Document Elvis Operator (?: ) and Safe Navigation Operator (?.)
remove references to ANTLR
This commit is contained in:
Mark Pollack
parent
ea2ece4516
commit
bf08c3ffbf
|
|
@ -122,24 +122,22 @@
|
|||
<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();
|
||||
<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 packages <package>spel.antlr</package> and
|
||||
<package>spel.support</package>.</para>
|
||||
and its sub 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
|
||||
<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>
|
||||
|
|
@ -150,7 +148,7 @@ String message = (String) exp.getValue();</programlisting>The value of the
|
|||
<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();
|
||||
<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>
|
||||
|
||||
|
|
@ -159,7 +157,7 @@ String message = (String) exp.getValue();</programlisting>
|
|||
<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();
|
||||
<programlisting language="java">ExpressionParser parser = new SpelExpressionParser();
|
||||
|
||||
// invokes 'getBytes()'
|
||||
Expression exp = parser.parseExpression("<emphasis role="bold">'Hello World'.bytes</emphasis>");
|
||||
|
|
@ -171,7 +169,7 @@ byte[] bytes = (byte[]) exp.getValue();</programlisting>
|
|||
|
||||
<para>Public fields may also be accessed</para>
|
||||
|
||||
<programlisting language="java">ExpressionParser parser = new SpelAntlrExpressionParser();
|
||||
<programlisting language="java">ExpressionParser parser = new SpelExpressionParser();
|
||||
|
||||
// invokes 'getBytes().length'
|
||||
Expression exp = parser.parseExpression("<emphasis role="bold">'Hello World'.bytes.length</emphasis>");
|
||||
|
|
@ -181,7 +179,7 @@ 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();
|
||||
<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>
|
||||
|
||||
|
|
@ -204,7 +202,7 @@ 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();
|
||||
ExpressionParser parser = new SpelExpressionParser();
|
||||
Expression exp = parser.parseExpression("<emphasis role="bold">name</emphasis>");
|
||||
|
||||
EvaluationContext context = new StandardEvaluationContext();
|
||||
|
|
@ -244,8 +242,9 @@ boolean result = exp.getValue(context, Boolean.class); // evaluates to true</pr
|
|||
|
||||
<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>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
|
||||
|
|
@ -446,7 +445,7 @@ Boolean b = simple.booleanList.get(0);
|
|||
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();
|
||||
<programlisting language="java">ExpressionParser parser = new SpelExpressionParser();
|
||||
|
||||
// evals to "Hello World"
|
||||
String helloWorld = (String) parser.parseExpression("'Hello World'").getValue();
|
||||
|
|
@ -486,11 +485,10 @@ String city = (String) parser.parseExpression("placeOfBirth.City").getValue(cont
|
|||
names. The contents of arrays and lists are obtained using square
|
||||
bracket notation.</para>
|
||||
|
||||
<programlisting language="java">ExpressionParser parser = new SpelAntlrExpressionParser();
|
||||
<programlisting language="java">ExpressionParser parser = new SpelExpressionParser();
|
||||
|
||||
// Inventions Array
|
||||
StandardEvaluationContext teslaContext = new StandardEvaluationContext();
|
||||
teslaContext.setRootObject(tesla);
|
||||
StandardEvaluationContext teslaContext = new StandardEvaluationContext(tesla);
|
||||
|
||||
// evaluates to "Induction motor"
|
||||
String invention = parser.parseExpression("inventions[3]").getValue(teslaContext,
|
||||
|
|
@ -498,8 +496,7 @@ String invention = parser.parseExpression("inventions[3]").getValue(teslaContext
|
|||
|
||||
|
||||
// Members List
|
||||
StandardEvaluationContext societyContext = new StandardEvaluationContext();
|
||||
societyContext.setRootObject(ieee);
|
||||
StandardEvaluationContext societyContext = new StandardEvaluationContext(ieee);
|
||||
|
||||
// evaluates to "Nikola Tesla"
|
||||
String name = parser.parseExpression("Members[0].Name").getValue(societyContext, String.class);
|
||||
|
|
@ -665,8 +662,7 @@ int minusTwentyOne = parser.parseExpression("1+2-3*8").getValue(Integer.class);
|
|||
<literal>getValue</literal></para>
|
||||
|
||||
<programlisting language="java">Inventor inventor = new Inventor();
|
||||
StandardEvaluationContext inventorContext = new StandardEvaluationContext();
|
||||
inventorContext.setRootObject(inventor);
|
||||
StandardEvaluationContext inventorContext = new StandardEvaluationContext(inventor);
|
||||
|
||||
parser.parseExpression("Name").setValue(inventorContext, "Alexander Seovic2");
|
||||
|
||||
|
|
@ -728,12 +724,9 @@ p.parseExpression("Members.add(new org.spring.samples.spel.inventor.Inventor('Al
|
|||
StandardEvaluationContext.</para>
|
||||
|
||||
<programlisting language="java">Inventor tesla = new Inventor("Nikola Tesla", "Serbian");
|
||||
StandardEvaluationContext context = new StandardEvaluationContext();
|
||||
StandardEvaluationContext context = new StandardEvaluationContext(tesla);
|
||||
context.setVariable("newName", "Mike Tesla");
|
||||
|
||||
|
||||
context.setRootObject(tesla);
|
||||
|
||||
parser.parseExpression("Name = #newName").getValue(context);
|
||||
|
||||
System.out.println(tesla.getName()) // "Mike Tesla"</programlisting>
|
||||
|
|
@ -750,7 +743,7 @@ List<Integer> primes = new ArrayList<Integer>();
|
|||
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 SpelAntlrExpressionParser();
|
||||
ExpressionParser parser = new SpelExpressionParser();
|
||||
StandardEvaluationContext context = new StandardEvaluationContext();
|
||||
context.setVariable("primes",primes);
|
||||
|
||||
|
|
@ -810,7 +803,7 @@ List<Integer> primesGreaterThanTen =
|
|||
<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 SpelAntlrExpressionParser();
|
||||
<programlisting language="java">ExpressionParser parser = new SpelExpressionParser();
|
||||
StandardEvaluationContext context = new StandardEvaluationContext();
|
||||
|
||||
context.registerFunction("reverseString",
|
||||
|
|
@ -842,6 +835,78 @@ expression = "isMember(#queryName)? #queryName + ' is a member of the ' " +
|
|||
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>
|
||||
|
|
|
|||
Loading…
Reference in New Issue