65 lines
2.3 KiB
Markdown
65 lines
2.3 KiB
Markdown
|
---
|
||
|
|
title: 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());
|
||
|
|
```
|