如何使用反射访问对象中的字段值

Posted 2021-03-30

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

扩展：我正在尝试学习反思，以使我的一些项目更通用。当我试图调用IllegalAccessException来获取该对象中该字段的值时，我遇到了field.getValue(object)。我可以得到名字和类型。

如果我将声明从private更改为public，那么这样可以正常工作。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)

答案

在你get私人领域之前，你需要在相应的领域调用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));
}

另一答案

默认情况下，您不允许读取非公共字段，但只需调用field.setAccessible(true);即可访问。换句话说，你的代码应该说

for (Field field : fields) {
  field.setAccessible(true);
  // ...
}