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How to access Private Field and Method Using Reflection in Java? Example Tutorial

Reflection in Java is a very powerful feature and allows you to access private methods and fields which is not possible by any other means in Java and because of this feature of reflection many code coverage tools, static analysis tools,s and Java IDE like Eclipse and Netbeans has been so helpful. In the last article, we have seen details about private keywords in Java and learned why we should always make fields and methods private in Java. There we have mentioned that private fields and methods are only accessible in the class they are declared but with reflection, you can call the private method and access private fields outside the class. In this article, we will see a simple example of accessing a private field using reflection and invoking a private method using reflection in Java.

Accessing private fields in Java using reflection

In order to access a private field using reflection, you need to know the name of the field than by calling getDeclaredFields(String name) you will get a java.lang.reflect.Field instance representing that field. remember using the getDclaredFields() method and not getFields() method which returns all non-private fields both from sub class and super class. 

while getDeclaredFields() returns both private and non-private fields declared in the class. Once you get the field reference you need to make it accessible by calling Field.setAccessible(true) because you are going to access the private field. 

Now you can get value or private field by calling Field.get(String field_name). if you don't call setAccessible(true) and try to access the private field using reflection you will get an Exception as shown in the below example

java.lang.IllegalAccessException: Class test.ReflectionTest can not access a member of class test.Person with modifiers "private"

        at sun.reflect.Reflection.ensureMemberAccess(Reflection.java:65)
        at java.lang.reflect.Field.doSecurityCheck(Field.java:960)
        at java.lang.reflect.Field.getFieldAccessor(Field.java:896)
        at java.lang.reflect.Field.get(Field.java:358)
        at test.ReflectionTest.main(ReflectionTest.java:31)

Calling private methods in Java using reflection

In our last Java tutorial on Reflection, we have seen how to call a method by its String name and we will use that information here for invoking the private method. Calling the private method using reflection is similar to accessing private fields reflectively. Use getDeclaredMethods(String name, Class.. parameter) to get the declared private method. 

pass all the argument types needed by method or nothing if the method doesn't accept any argument. This will give you an instance of java.lang.reflect.Method which can then be used to call the private method using reflection, as shown in the code example.

invoke private method and field in java with reflection code example

Code example of accessing private field and method using reflection

package test;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.Arrays;
import java.util.logging.Level;
import java.util.logging.Logger;

public class ReflectionTest {

    public static void main(String args[]) throws ClassNotFoundException {
        Class<Person> person = (Class<Person>) Class.forName("test.Person");
        //getFields() does not return private field
        System.out.println("Fields : " + Arrays.toString(person.getFields()));
        //getDeclaredFields() return both private and non private fields using reflection
        System.out.println("Declared Fields : " + Arrays.toString(person.getDeclaredFields()));      
        //getDeclaredMethods() return both private and non private methods using reflection
        System.out.println("Declared methods : " + Arrays.toString(person.getDeclaredMethods()));
        try {
            //accessing value of private field using reflection in Java
            Person privateRyan = new Person("John" , "8989736353");
            Field privateField = person.getDeclaredField("phone");
            //this call allows private fields to be accessed via reflection
            //getting value of private field using reflection
            String value = (String) privateField.get(privateRyan);          
            //print value of private field using reflection
            System.out.println("private field: " + privateField + " value: " + value);
            //accessing private method using reflection
            Method privateMethod = person.getDeclaredMethod("call");
            //making private method accessible using reflection
            //calling private method using reflection in java
        } catch (InvocationTargetException ex) {
            Logger.getLogger(ReflectionTest.class.getName()).log(Level.SEVERE, null, ex);
        } catch (NoSuchMethodException ex) {
            Logger.getLogger(ReflectionTest.class.getName()).log(Level.SEVERE, null, ex);
        } catch (IllegalArgumentException ex) {
            Logger.getLogger(ReflectionTest.class.getName()).log(Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            Logger.getLogger(ReflectionTest.class.getName()).log(Level.SEVERE, null, ex);
        } catch (NoSuchFieldException ex) {
            Logger.getLogger(ReflectionTest.class.getName()).log(Level.SEVERE, null, ex);
        } catch (SecurityException ex) {
            Logger.getLogger(ReflectionTest.class.getName()).log(Level.SEVERE, null, ex);

class Person{
    public String name;
    private String phone;
    public Person(String name, String phone){
        this.name = name;
        this.phone = phone;
    private void call(){
        System.out.println("Calling " + this.name +" at " + this.phone);
    public String getName(){
        return name;

Fields : [public java.lang.String test.Person.name]
Declared Fields : [public java.lang.String test.Person.name, private java.lang.String test.Person.phone]
Declared methods : [public java.lang.String test.Person.getName(), private void test.Person.call()]
private field: private java.lang.String test.Person.phone value: 8989736353
Calling John at 8989736353

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Hemanth said...

I think Reflection breaks the purpose of using private variable. So, is Reflection a bad thing ? Is there any way we can avoid this ?

Samuel Grigolato said...

I think you should provide a paragraph explaining why you should never use Reflection to access private fields/methods if you plan to write maintainable code. Reflection's primary purpose is not to be used in everyday common development tasks. Your article, as is, might result in a poor decision by a junior developer without giving him/her the chance to take a step back and wonder why the need to such a maneuver in the first place. Other than that, very nicely written piece of tutorial. :)

Anonymous said...

I think there is a slight typo, we pass the object to Field.get instead of Field.get(String field_name)

Higinio said...

Very good article!
With this possibility we can always follow the rule of making private the "fields" of a class, and after we could use a dynamic system to use the fields name (.getName())and the fields type (.getType()) without the need of knowing the name of the fields of a given class.

Unknown said...

Good article.
Kindly give an example to call private methods with arguments and non-void return types.

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