Spring Expression Language 简明教程

Spring SpEL - Collection Projection

SpEL 表达式支持集合投影，这是一种非常强大的表达式，允许评估子表达式并在结果中返回一个新集合。

Syntax

![projectionExpresion]

以下示例展示这种用法。

List<String> list = (List<String>)
   parser.parseExpression("employees.![country]").getValue(deptContext);

这里 SpEL 仅从 employees 列表中返回 country 为 USA 的那些 employees。

以下示例显示了各种使用案例。

Example

让我们更新在 Spring SpEL - Create Project 章节创建的项目。我们添加/更新了以下文件：

Employee.java − Employee class.
Dept.java − Department class.
MainApp.java - 要运行和测试的主应用。

以下 Employee.java 文件的内容：

package com.tutorialspoint;

public class Employee {
   private String name;
   private String country;
   public Employee(String name, String country) {
      this.name = name;
      this.country = country;
   }
   public String getName() {
      return name;
   }
   public void setName(String name) {
      this.name = name;
   }
   public String getCountry() {
      return country;
   }
   public void setCountry(String country) {
      this.country = country;
   }
   public String toString() {
      return "[" +name+ ", "+country + "]";
   }
}

以下 Dept.java 文件的内容：

package com.tutorialspoint;

import java.util.List;

public class Dept {
   private List<Employee> employees;
   public List<Employee> getEmployees() {
      return employees;
   }
   public void setEmployees(List<Employee> employees) {
      this.employees = employees;
   }
}

以下 MainApp.java 文件的内容：

package com.tutorialspoint;

import java.text.ParseException;
import java.util.ArrayList;
import java.util.List;

import org.springframework.expression.EvaluationContext;
import org.springframework.expression.ExpressionParser;
import org.springframework.expression.spel.standard.SpelExpressionParser;
import org.springframework.expression.spel.support.StandardEvaluationContext;

public class MainApp {
   public static void main(String[] args) throws ParseException {

      ExpressionParser parser = new SpelExpressionParser();

      Employee employee1 = new Employee("Robert", "USA");
      Employee employee2 = new Employee("Julie", "USA");
      Employee employee3 = new Employee("Ramesh", "India");

      List<Employee> employees = new ArrayList<Employee>();
      employees.add(employee1);
      employees.add(employee2);
      employees.add(employee3);

      Dept dept = new Dept();
      dept.setEmployees(employees);

      EvaluationContext deptContext = new StandardEvaluationContext(dept);

      // Select list of countries
      List<String> list = (List<String>)
         parser.parseExpression("employees.![country]").getValue(deptContext);
      System.out.println(list);
   }
}

Output

[USA, USA, India]