freeCodeCamp/guide/english/java/reflection/index.md

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Reflection

Reflection

In Java, reflection is a powerful tool that can be used to do virtually anything. This can be considered a double-edged sword, as it is at times required, yet, it is often exploited and used innappropriately. It can, however, be particularly useful, if for whatever reason, a private method or private variable needs to be called, a method needs to be hooked or replaced, and so on. Reasoning for doing this may vary, for example, a developer may not have access to the original source-code for a library that they are working with and may need to work around this constraint.

Application

Assigning a private or protected variable

As previously mentioned, assigning a private or protected variable can be made possible with the use of reflection.

Consider the following class:

public class MyClass {
  private String hiddenString = "Hello!";

  public String getHiddenString() {
    return this.hiddenString;
  }
}

As we can see, the hiddenString object is marked private, meaning we cannot directly access it. It is however, accessible through the getHiddenString() method. This won't allow us to change the value of the variable, we can, however, change it using reflection.

Consider the following code:

...
MyClass myObject = new MyClass();
...

We have an instance of myObject at this point, so let's mark the private variable as accessible.

// Create an instance of the object.
MyClass myObject = new MyClass();

// Obtain a reference to the object's private member field, we know the name of the private variable is 
// 'hiddenString'.
Field hiddenStringField = myObject.getClass().getDeclaredField("hiddenString");
// Mark the field as accessible, this effectively tells the JVM that the field is not private, thus
// we can now access it.
hiddenStringField.setAccessible(true);

// The Java compiler doesn't know we have marked the field as accessible. If we wish to set the value,
// we must do so via reflection. We call the set() method on our field, supplying the target object 
// (myObject) and the target value.
// We have successfully set the value, using reflection!
hiddenStringField.set(myObject, "Hello, World!");

// This will print our newly assigned string, as we have assigned the private variable within our
// myObject instance.
System.out.println(myObject.getHiddenString());