Java Reflection Field Access

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We can get or set a field using reflection in two steps.

• get the reference of the field.
• To read the field's value, call the getXxx() method on the field, where Xxx is the data type of the field.
• To set the value of a field, you call the corresponding setXxx() method.

Static and instance fields are accessed the same way.

Example

import java.lang.reflect.Field;
//from ww  w. j a va  2s .  co  m
class MyClass {
  public String name = "Unknown";
  public MyClass() {
  }
  public String toString() {
    return "name=" + this.name;
  }
}
public class Main {
  public static void main(String[] args) {
    Class<MyClass> ppClass = MyClass.class;
    try {
      MyClass p = ppClass.newInstance();
      Field name = ppClass.getField("name");
      String nameValue = (String) name.get(p);
      System.out.println("Current name is " + nameValue);
      name.set(p, "abc");
      nameValue = (String) name.get(p);
      System.out.println("New  name is " + nameValue);
    } catch (InstantiationException | IllegalAccessException
        | NoSuchFieldException | SecurityException | IllegalArgumentException e) {
      System.out.println(e.getMessage());
    }
  }
}

The code above generates the following result.

Bypassing Accessibility Check

To access non-accessible fields, methods, and constructors of a class using reflection call setAccessible(boolean flag) method from AccessibleObject class.

We need to call this method with a true argument to make that field, method, and constructor accessible.

import java.lang.reflect.Field;
// ww w  .j  a  v a  2  s  . c o  m
class MyClass {
  private String name = "Unknown";

  public MyClass() {
  }

  public String toString() {
    return "name=" + this.name;
  }
}

public class Main {
  public static void main(String[] args) {
    Class<MyClass> my = MyClass.class;
    try {
      MyClass p = my.newInstance();
      Field nameField = my.getDeclaredField("name");
      nameField.setAccessible(true);
      String nameValue = (String) nameField.get(p);
      System.out.println("Current name is " + nameValue);
      nameField.set(p, "abc");
      nameValue = (String) nameField.get(p);
      System.out.println("New name is " + nameValue);
    } catch (InstantiationException | IllegalAccessException
        | NoSuchFieldException | SecurityException | IllegalArgumentException e) {
      System.out.println(e.getMessage());
    }
  }
}

The code above generates the following result.

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