Spring SpEL - 集合选择

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SpEL 表达式支持集合选择，这是一个非常强大的表达式，允许通过从源集合中选择条目来将源集合转换为另一个集合。

句法

?[selectionExpresion]

以下示例显示了用法。

List<Employee> list = (List<Employee>)
   parser.parseExpression("employees.?[country == 'USA']").getValue(deptContext);

此处 SpEL 将仅返回来自美国的员工列表中的员工。

以下示例显示了各种用例。

例子

让我们更新在Spring SpEL - 创建项目章节中创建的项目。我们正在添加/更新以下文件 -

• Employee.java - 员工类。

• Dept.java - 部门类。

• MainApp.java - 要运行和测试的主应用程序。

这是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 + "]";
   }
}

这是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;
   }
}

这是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 employees who are living in USA
      List<Employee> list = (List<Employee>)
         parser.parseExpression("employees.?[country == 'USA']").getValue(deptContext);
      System.out.println(list);
   }
}

输出

[[Robert, USA], [Julie, USA]]