Reflection
Advanced Programming
SUT • Spring 2019
Contents
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What is RTTI?
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Why we need it?
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Type information
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Java and Reflection
Reflection
A Challenge
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Suppose you want to implement RPC or RMI
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What do you need?
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Socket Programming
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Serialization
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How do you invoke methods in other side?
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Problem
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Suppose you should write a program
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It reads the name of a class
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And the name of one of its methods
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And all of its parameters
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The program
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creates and object from the specified class
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and invokes the specified method
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with specified parameters
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Problem (2)
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How can you implement it?
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What is its application?
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RPC and RMI
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Object transfer to a web service
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RTTI
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Runtime type information (RTTI)
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Allows you to discover and use type information while a program is running
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This feature is also called Reflection in java
RTTI (2)
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With RTTI, you can ask an object reference the exact type that it’s referring to.
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And you can get information about it its characteristics and capabilities
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Methods, constructors, fields, …
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And you can call its methods and get its properties
Solve the Problem
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How can you implement the requested program with RTTI?
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How can you simulate RPC and RMI?
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How can you send an object via web-service?
The Class Object
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How type information is represented at run time?
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This is accomplished through a special kind of object
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It is called the Class object
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it contains information about the class
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Java performs its RTTI using the Class object
Class Loader
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There’s one Class object for each class that is part of your program
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Each time you write and compile a new class, a single Class object is also created
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and stored, appropriately enough, in an identically named .class file
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To make an object of that class, JVM uses a subsystem called a class loader
How Classes are Loaded?
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A classes is loaded into the JVM dynamically
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upon the first use of the class
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When?
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when the program makes the first reference to a static member of that class
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The constructor is also a static method of a class!
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Even though the static keyword is not declared
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Instantiation: using the new operator = reference to a static member (constructor)
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Dynamic Loading
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A Java program isn’t completely loaded before it begins
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Pieces of it are loaded when necessary
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This is called Dynamic loading
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Different from many traditional languages
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Enables difficult or impossible behavior
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to duplicate in a statically loaded language like C++.
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Default Class Loader
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The class loader first checks:
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Is the Class object for that type loaded?
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If not, class loader finds the .class file and loads it
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A customized class loader may load the class from a DB
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First Example
Method method = String.class.getMethod("substring", int.class);
Object value = method.invoke("Taghi Taghavi", 6);
System.out.println((String)value);
Example
Class c = Class.forName(args[0]);
Method m[] = c.getDeclaredMethods();
for (int i = 0; i < m.length; i++)
System.out.println(m[i].toString());More Reflection
Class clazz = object.getClass();
Annotation[] annotations = clazz.getAnnotations();
Field[] fields = clazz.getFields();
Constructor[] constructors = clazz.getConstructors();Example
package drawing;
class MyClass{
String name;
public MyClass(String name) {
this.name = name;
}
public String getName() {
return name;
}
}
Example
Class c = Class.forName("drawing.MyClass");
Constructor constructor = c.getConstructor(String.class);
MyClass instance = (MyClass) constructor.newInstance("Ali Alavi");
Field field = instance.getClass().getDeclaredField("name");
field.set(instance, "Taghi Taghavi");
System.out.println(instance.getName());
instanceof Operator
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Tells you if an object is an instance of a particular type
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Use instanceof before a downcast
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when you don’t have other information that tells you the type of the object;
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Otherwise, you may end up with a …?
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ClassCastException
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if(x instanceof Dog)
((Dog)x).bark();
instanceof
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instanceof returns false if the reference is null
void f(Object c){
if(c instanceof Serializable && c instanceof String)
System.out.println("YES!");
}
instanceof vs. Class equivalence
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There’s an important difference between instanceof and the direct comparison of the Class objects
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But instanceof and islnstance() produce equivalent results
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if(c instanceof String)
...
if(c.getClass().equals(String.class))
...How to Retrieve Class Object
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Compile time code (Hard coded)
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1. ClassName.class
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Class clazz = Person.class;
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Runtime
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2. Class.forName
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Class clazz = Class.forName("edu.sharif.ce.Rectangle");
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3. reference.getClass
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Object o = new Person();
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Class clazz= o.getClass();
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Class is a Generic Class
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Example1
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Class<Person> clazz = Person.class;
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Person p = clazz.newInstance();
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No cast is needed
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Example2
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Object o = new Person();
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Class<? extends Object> c = o.getClass();
Class and Generic Types
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What is wrong with this code?
class GenericType<T>{
private T element;
public void f(){
Class c2 = element.getClass();
Class c1 = T.class;
}
}TYPE in Wrapper Classes
Changing the Accessibility!
class MyClass{
private void privateMethod(){
}
}
...
MyClass instance = new MyClass();
Method method = instance.getClass().
getDeclaredMethod("privateMethod");
method.setAccessible(true);
method.invoke(instance);Swap two integers
public static void swap(Integer i, Integer j) {
try {
Integer lastJ = new Integer(j);
Field value = Integer.class.getDeclaredField("value");
value.setAccessible(true);
value.set(j, i);
value.set(i, lastJ);
value.setAccessible(false);
} catch (Exception e) {
e.printStackTrace();
}
} Title Text
Reflection
By Behnam Hatami
Reflection
Reflection / Advanced Programming Course @ SUT, Spring 2019
