Spring --- SpEL

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一）什么是SpEL
  SpEL -- Spring Expression Language. Spring的表达式语言。举个最简单的例子：

ExpressionParser parser =new SpelExpressionParser();
Expression exp = parser.parseExpression("'Hello World'");
String message = (String) exp.getValue();

    最后 message的值就是 Hello World， 表达式中的单引号''就是表达String类型的一种格式。另外值得注意的一点时，当表达式不符合规范时， parser.parseExpression语句会抛出ParseException；而exp.getValue会抛出EvaluationException
    而为了避免最后的类型转换，我们还可以这样写：

String message = exp.getValue(String.class);

二）SpEL举例中需要使用到的类
    以下是本文介绍SpEL过程中需要使用到的类，在此列出，以供后续介绍中使用参考：
Inventor.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;}
}

PlaceOfBirth.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;}
}

Society.java

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 members = new ArrayList();
  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;
  }
}

三）EvaluationContext接口
    当在使用表达式的过程中有遇到类型转换时，我们需要EvaluationContext接口加以辅助：

class Simple {
  public List booleanList = new ArrayList();
}

Simple simple = new Simple();
simple.booleanList.add(true);
EvaluationContext 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);

注：EvaluationContext开销较大，所以多用于数据变化较少的情况。如果数据变化频繁，我们可以考虑直接引用对象（比如上例中的simple）以减小开销。

四）SpEL
  下面正式开始介绍SpEL。
1'Literal expressions 基本类的表达式

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();

2' Properties, Arrays, Lists, Maps, Indexers 属性、集合的表示
    Properties:

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

    arrays and lists:

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);

   Map:

// 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");

3' construction
    lists:

// 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);

    arrays:

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);

4' Methods

// 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);

5' Operators
    Relational operators:

// 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);
// 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);

注意关键字：instanceof、matches

    Logical operators：

// 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：

// 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

6' Assignment

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);

7' Types

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);

注： 在java.lang中的类不需要全路径。

8' Constructors

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);

9' Variables

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"

注： Spring还有两个预留的variables: #this #root   #this表示集合中的当前元素，#root表示根对象，即载入context的对象。

10' Functions
    Function的使用略微复杂。首先是调用StandardEvaluationContext的方法

public void registerFunction(String name, Method m)

进行注册，而后才能在expression中进行调用：

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();
  }
}


ExpressionParser parser = new SpelExpressionParser();
StandardEvaluationContext context = new StandardEvaluationContext();
//先注册方法
context.registerFunction("reverseString",StringUtils.class.getDeclaredMethod("reverseString",new Class[] { String.class }));
//而后才能调用
String helloWorldReversed =
parser.parseExpression("#reverseString('hello')").getValue(context,String.class);

11'Bean references

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);

没啥好多说的，知道用@就可以了~~

12'Ternary Operator

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

结果是： falseExp

13'The Elvis Operator
这是一种对三重操作符的简化写法。比如用三重操作符我们可以这样：

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

等效的用Elvis操作符实现的话就是这样：

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

省略了name的显示表达！

14'Safe Navigation operator
用?.取代. 当代码会抛出NullPointerException时，采用?.的操作仅仅会简单的返回一个null。

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!!!

15' Collection Selection
如果你要在集合内挑选符合某些条件的元素，你可以采用这样格式的表达式?[selectionExpression]

List list = (List)
parser.parseExpression("Members.?[Nationality == 'Serbian']").getValue(societyContext);
Map newMap = parser.parseExpression("map.?[value<27]").getValue();

而如果你只需要获得第一个满足条件的元素，你可以用：^[...]
而如果你只需要获得最后一个满足条件的元素，你可以用：$[...]

16' Collection Projection
那么我需要便利某些元素以组成新的集合，我们可以采用这样的表达式![projectionExpression]：

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

17' Expression templating
   template的作用便是在表达式内再嵌套表达式。比如：

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

而要实现内部嵌套的T(java.lang.Math).random()表达式。我们需要实现接口ParserContext：

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

五）SpEL在配置中的使用
  同样的语法，SpEL还能在配置文件和annotation中使用！！强大吧 ^.^ 它们的格式是#{ }
  在配置文件中：

  在annotation中使用：

public static class FieldValueTestBean{
  @Value("#{ systemProperties['user.region'] }")
  private String defaultLocale;