Language Reference - Java-Springs

Literal expressions

The types of literal expressions supported are strings, dates, numeric values (int, real, and hex), Boolean and null.Strings are delimited by single quotes. To put a single quote itself in a string use two single quote characters.The following listing shows simple usage of literals.Typically they would not be used in isolation like this, but as part of a more complex expression, for example using a literal on one side of a logical comparison operator.

ExpressionParser parser = new 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();

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

Properties, Arrays, Lists, Maps, Indexers

Navigating with property references is easy, just use a period to indicate a nested property value. To navigate "down" and get Tesla's year of birth and Pupin's city of birth the following expressions are used.

// evals to 1856
int year = (Integer) parser.parseExpression("Birthdate.Year + 1900").getValue(context);
String city = (String) parser.parseExpression("placeOfBirth.City").getValue(context);

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

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);

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 literals.

// 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");

Inline lists

Lists can be expressed directly in an expression using {} notation.

// evaluates to a Java list containing the four numbers
List numbers = (List) parser.parseExpression("{1,2,3,4}").getValue(context);
List listOfLists = (List) parser.parseExpression("{{'a','b'},{'x','y'}}").getValue(context);

{} by itself means an empty list. For performance reasons, if the list is itself entirely composed of fixed literals then a constant list is created to represent the expression, rather than building a new list on each evaluation.

Array construction

Arrays can be built using the familiar Java syntax, optionally supplying an initializer to have the array populated at construction time.

int[] numbers1 = (int[]) parser. parseExpression("new int[4]").getValue(context);
// Array with initializer
int[] numbers2 = (int[]) parser.
parseExpression("new int[]{1,2,3}").getValue(context);
// Multi dimensional array
int[][] numbers3 = (int[][]) parser.
parseExpression("new int[4][5]").getValue(context);

It is not currently allowed to supply an initializer when constructing a multi-dimensional array.

Methods

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

// 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);

Operators

Relational operators

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.

// evaluates to true
boolean trueValue = parser.parseExpression("2== 2").
getValue(Boolean.class);
// evaluates to false
boolean falseValue = parser.parseExpression("2 < -5.0").
getValue(Boolean.class);
// evaluates to true
boolean trueValue = parser.parseExpression("'black' < 'block'").
getValue(Boolean.class);

In addition to standard relational operators SpEL supports the 'instanceof' and regular expression based 'matches' operator.

// 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);

Each symbolic operator can also be specified as a purely alphabetic equivalent.This avoids problems where the symbols used have special meaning for the document type in which the expression is embedded (eg.an XML document).The textual equivalents are shown here: lt ('<'), gt ('>'), le ('<='), ge ('>='), eq ('=='), ne ('!='), div ('/'), mod ('%'), not ('!').These are case insensitive.

Logical operators

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

// -- 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);

Mathematical operators

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.

// 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

Assignment

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

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);

TYPES

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 Standard Evaluation Context uses a Type Locator to find types and the StandardTypeLocator (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.

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 < T(java.math.RoundingMode).FLOOR")
.getValue(Boolean.class);

Constructors

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).

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);

Variables

Variables can be referenced in the expression using the syntax #variableName. Variables are set using the method setVariable on the Standard Evaluation Context.

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"

The #this and #root variables

The variable #this is always defined and refers to the current evaluation object (against which unqualified references are resolved). The variable #root is always defined and refers to the root context object.

Although #this may vary as components of an expression are evaluated, #root always refers to the root.

// create an array of integers
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 SpelExpressionParser();
StandardEvaluationContext context = new StandardEvaluationContext();
context.setVariable("primes",primes);
// all prime numbers > 10 from the list (using selection ?{...})
// evaluates to [11, 13, 17]
List<Integer> primesGreaterThanTen =
(List<Integer>) parser.parseExpression("#primes.?[#this>10]").getValue(context);

Functions

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

public void registerFunction(String name, Method m)

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

public abstract class StringUtils {
public static String reverseString(String input) {
StringBuilder backwards = new StringBuilder();
for (int i = 0; i < input.length(); i++)
backwards.append(input.charAt(input.length() - 1 - i));
}
return backwards.toString();
}
}

This method is then registered with the evaluation context and can be used within an expression string.

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);

Bean references

If the evaluation context has been configured with a bean resolver it is possible to lookup beans from an expression using the (@) symbol.

ExpressionParser parser = new SpelExpressionParser();
StandardEvaluationContext context = new StandardEvaluationContext();
context.setBeanResolver(new MyBeanResolver());
// This will end up calling resolve(context,"foo") on MyBeanResolver during evaluation
Object bean = parser.parseExpression("@foo").getValue(context);

Ternary Operator (If-Then-Else)

You can use the ternary operator for performing if-then-else conditional logic inside the expression. A minimal example is:

String falseString = parser.parseExpression("false ? 'trueExp' : 'falseExp'").getValue(String.class);

In this case, the boolean false results in returning the string value 'falseExp'. A more realistic example is shown below.

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"

The Elvis Operator

The Elvis operator is a shortening of the ternary operator syntax and is used in the Groovy language. With the ternary operator syntax you usually have to repeat a variable twice, for example:

String name = "Elvis Presley"; String displayName = name != null ? name;"Unknown";

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

ExpressionParser parser = new SpelExpressionParser();
String name = parser.parseExpression("null?:'Unknown'").getValue(String.class);
System.out.println(name); // 'Unknown'

Here is a more complex example.

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

Safe Navigation operator

The Safe Navigation operator is used to avoid a NullPointerException and comes from the Groovy 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.

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!!!

Collection Selection

Selection is a powerful expression language feature that allows you to transform some source collection into another by selecting from its entries.

Selection uses the syntax ?[selectionExpression]. 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:

List<Inventor> list = (List<Inventor>) parser.parseExpression ("Members.?[Nationality == 'Serbian']").getValue(societyContext);

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 Map.Entry). Map entries have their key and value accessible as properties for use in the selection.

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

Map newMap = parser.parseExpression("map.?[value<27]").getValue();

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 ^[...] whilst to obtain the last matching selection the syntax is $[...].

Collection Projection

Projection allows a collection to drive the evaluation of a sub-expression and the result is a new collection.The syntax for projection is ![projection Expression].Most easily under stood 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:

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

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 Map.Entry). The result of a projection across a map is a list consisting of the evaluation of the projection expression against each map entry.

Expression templating

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

String randomPhrase =
parser.parseExpression("random number is #{T(java.lang.Math).random()}",
new TemplateParserContext()).getValue(String.class);
// evaluates to "random number is 0.7038186818312008"

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 parseExpression() is of the type ParserContext.The ParserContext interface is used to influence how the expression is parsed in order to support the expression templating functionality.The definition of TemplateParserContext is shown below.

public class TemplateParserContext implements ParserContext {
public String getExpressionPrefix() {
return "#{";
}
public String getExpressionSuffix() {
return "}";
}
public boolean isTemplate() {
return true;
}
}

All rights reserved © 2018 Wisdom IT Services India Pvt. Ltd DMCA.com Protection Status

Java-Springs Topics