java - 如何使用反射访问对象中的字段值

Question

我的问题:如何克服 IllegalAccessException 以使用反射访问对象字段的值。

扩展:我正在尝试学习反射，以使我的一些项目更通用。在尝试调用 field.getValue(object) 以获取该对象中该字段的值时，我遇到了 IllegalAccessException。我可以得到名称和输入就好了。

如果我将声明从 private 更改为 public 那么这工作正常。但是为了遵循封装的“规则”，我不想这样做。任何帮助将不胜感激!谢谢!

我的代码:

package main;

import java.lang.reflect.Field;

public class Tester {

  public static void main(String args[]) throws Exception {
    new Tester().reflectionTest();
  }

  public void reflectionTest() throws Exception {
    Person person = new Person("John Doe", "555-123-4567", "Rover");
    Field[] fields = person.getClass().getDeclaredFields();
    for (Field field : fields) {
      System.out.println("Field Name: " + field.getName());
      System.out.println("Field Type: " + field.getType());
      System.out.println("Field Value: " + field.get(person));
      //The line above throws: Exception in thread "main" java.lang.IllegalAccessException: Class main.Tester can not access a member of class main.Tester$Person with modifiers "private final"
    }
  }

  public class Person {

    private final String name;
    private final String phoneNumber;
    private final String dogsName;

    public Person(String name, String phoneNumber, String dogsName) {
      this.name = name;
      this.phoneNumber = phoneNumber;
      this.dogsName = dogsName;
    }
  }
}

输出:

run:
Field Name: name
Field Type: class java.lang.String
Exception in thread "main" java.lang.IllegalAccessException: Class main.Tester can not access a member of class main.Tester$Person with modifiers "private final"
    at sun.reflect.Reflection.ensureMemberAccess(Reflection.java:95)
    at java.lang.reflect.AccessibleObject.slowCheckMemberAccess(AccessibleObject.java:261)
    at java.lang.reflect.AccessibleObject.checkAccess(AccessibleObject.java:253)
    at java.lang.reflect.Field.doSecurityCheck(Field.java:983)
    at java.lang.reflect.Field.getFieldAccessor(Field.java:927)
    at java.lang.reflect.Field.get(Field.java:372)
    at main.Tester.reflectionTest(Tester.java:17)
    at main.Tester.main(Tester.java:8)
Java Result: 1
BUILD SUCCESSFUL (total time: 0 seconds)

Answer 1

在你get一个私有(private)字段之前，你需要在对应的字段上调用setAccessible(true);:

for (Field field : fields) {
    field.setAccessible(true); // Additional line
    System.out.println("Field Name: " + field.getName());
    System.out.println("Field Type: " + field.getType());
    System.out.println("Field Value: " + field.get(person));
}