Trail: The Reflection API
Reflection is commonly used by programs which require the ability to examine or modify the runtime behavior of applications running in the Java virtual machine. This is a relatively advanced feature and should be used only by developers who have a strong grasp of the fundamentals of the language. With that caveat in mind, reflection is a powerful technique and can enable applications to perform operations which would otherwise be impossible.
Extensibility Features An application may make use of external, user-defined classes by creating instances of extensibility objects using their fully-qualified names. Class Browsers and Visual Development Environments A class browser needs to be able to enumerate the members of classes. Visual development environments can benefit from making use of type information available in reflection to aid the developer in writing correct code. Debuggers and Test Tools Debuggers need to be able to examine private members on classes. Test harnesses can make use of reflection to systematically call a discoverable set APIs defined on a class, to insure a high level of code coverage in a test suite.
Drawbacks of Reflection
Reflection is powerful, but should not be used indiscriminately. If it is possible to perform an operation without using reflection, then it is preferable to avoid using it. The following concerns should be kept in mind when accessing code via reflection.
Performance Overhead Because reflection involves types that are dynamically resolved, certain Java virtual machine optimizations can not be performed. Consequently, reflective operations have slower performance than their non-reflective counterparts, and should be avoided in sections of code which are called frequently in performance-sensitive applications. Security Restrictions Reflection requires a runtime permission which may not be present when running under a security manager. This is in an important consideration for code which has to run in a restricted security context, such as in an Applet. Exposure of Internals Since reflection allows code to perform operations that would be illegal in non-reflective code, such as accessing private fields and methods, the use of reflection can result in unexpected side-effects, which may render code dysfunctional and may destroy portability. Reflective code breaks abstractions and therefore may change behavior with upgrades of the platform.
Trail Lessons
This trail covers common uses of reflection for accessing and manipulating classes, fields, methods, and constructors. Each lesson contains code examples, tips, and troubleshooting information.
Classes This lesson shows the various ways to obtain a Class object and use it to examine properties of a class, including its declaration and contents. Members This lesson describes how to use the Reflection APIs to find the fields, methods, and constructors of a class. Examples are provided for setting and getting field values, invoking methods, and creating new instances of objects using specific constructors. Arrays and Enumerated Types This lesson introduces two special types of classes: arrays, which are generated at runtime, and enum types, which define unique named object instances. Sample code shows how to retrieve the component type for an array and how to set and get fields with array or enum types.
The examples in this trail are designed for experimenting with the Reflection APIs. The handling of exceptions therefore is not the same as would be used in production code. In particular, in production code it is not recommended to dump stack traces that are visible to the user.
Reflection in Java
Reflection is an API that is used to examine or modify the behavior of methods, classes, and interfaces at runtime. The required classes for reflection are provided under java.lang.reflect package which is essential in order to understand reflection. So we are illustrating the package with visual aids to have a better understanding as follows:
- Reflection gives us information about the class to which an object belongs and also the methods of that class that can be executed by using the object.
- Through reflection, we can invoke methods at runtime irrespective of the access specifier used with them.
Reflection can be used to get information about class, constructors, and methods as depicted below in tabular format as shown:
| Class | The getClass() method is used to get the name of the class to which an object belongs. |
|---|---|
| Constructors | The getConstructors() method is used to get the public constructors of the class to which an object belongs. |
| Methods | The getMethods() method is used to get the public methods of the class to which an object belongs. |
We can invoke a method through reflection if we know its name and parameter types. We use two methods for this purpose as described below before moving ahead as follows:
Method 1: getDeclaredMethod(): It creates an object of the method to be invoked.
Syntax: The syntax for this method
Class.getDeclaredMethod(name, parametertype)
Method 2: invoke(): It invokes a method of the class at runtime we use the following method.
Method.invoke(Object, parameter)
Tip: If the method of the class doesn’t accept any parameter then null is passed as an argument.
Note: Through reflection, we can access the private variables and methods of a class with the help of its class object and invoke the method by using the object as discussed above. We use below two methods for this purpose.
Method 3: Class.getDeclaredField(FieldName): Used to get the private field. Returns an object of type Field for the specified field name.
Method 4: Field.setAccessible(true): Allows to access the field irrespective of the access modifier used with the field.
Important observations Drawn From Reflection API
- Extensibility Features: An application may make use of external, user-defined classes by creating instances of extensibility objects using their fully-qualified names.
- Debugging and testing tools: Debuggers use the property of reflection to examine private members of classes.
- Performance Overhead: Reflective operations have slower performance than their non-reflective counterparts, and should be avoided in sections of code that are called frequently in performance-sensitive applications.
- Exposure of Internals: Reflective code breaks abstractions and therefore may change behavior with upgrades of the platform.
Example with code:
package contains Employee class with constructors , setters and getters method
Java
//Main Method with implementation of constructor, methods and classes
Java
Name of Constructor : public org.geeksforgeeks.write.Emp(int,double,java.lang.String)
Count of constructor parameter : 3
Constructor’s parameter : int arg0
Constructor’s parameter : double arg1
Constructor’s parameter : java.lang.String arg2
Name of Constructor : org.geeksforgeeks.write.Emp()
Count of constructor parameter : 0
Length of method : 6
Method name: public int org.geeksforgeeks.write.Emp.getEid()
Method return type : int
Method parameter count: 0
Method name: public void org.geeksforgeeks.write.Emp.setEid(int,int,char)
Method return type : void
Method parameter count: 3
Method’s Parameter : int arg0
Method’s Parameter : int arg1
Method’s Parameter : char arg2
Method name: public double org.geeksforgeeks.write.Emp.getEsal()
Method return type : double
Method parameter count: 0
Method name: public void org.geeksforgeeks.write.Emp.setEsal(double,float,java.lang.String)
Method return type : void
Method parameter count: 3
Method’s Parameter : double arg0
Method’s Parameter : float arg1
Method’s Parameter : java.lang.String arg2
Method name: public java.lang.String org.geeksforgeeks.write.Emp.getEname()
Method return type : class java.lang.String
Method parameter count: 0
Method name: public void org.geeksforgeeks.write.Emp.setEname(java.lang.String)
Method return type : void
Method parameter count: 1
Method’s Parameter : java.lang.String arg0
class: class org.geeksforgeeks.write.Emp$A
Name of class: org.geeksforgeeks.write.Emp$A
class: class org.geeksforgeeks.write.Emp$B
Name of class: org.geeksforgeeks.write.Emp$B
class: interface org.geeksforgeeks.write.Emp$MyAnno
Name of class: org.geeksforgeeks.write.Emp$MyAnno
class: class org.geeksforgeeks.write.Emp$Week
Name of class: org.geeksforgeeks.write.Emp$Week
Reflections examples in java
Боже, хоть одна всеняемая статья про Рефлекшн для новичков. А то, другие авторы сразу выввливают кучу методов, без объяснений для нубов. Спасибо, автор!
Грамотно написанный класс в Java: — У меня всё как надо, всё что нужно скрыть, помечено Private, я вообще не вызываю багов вовремя работы программы. 💪😎👍 Reflection API: — 😂 Я вас умоляю, не смешите мои подковы методы).
На собеседовании : — Reflection API, для чего он нужен? — Он позволяет наиболее циничный способом попрать принцип инкапсуляции в java.
Вызов метода мяу обычным способом : Кот, мяукни! Вызов метода мяу через рефлексию : Мяу, прозвучи от того кота!
clazz = Class.forName(Cat.class.getName()); Вместо того, чтобы сразу получить объект класса, мы сначала получаем имя класса (которое нам известно), а потом по нему получаем объект класса. Или это как-то связано с полным именем (learn.javarush.Cat) ?
Я статью только начал читать, но уже появился вопрос, причём, видимо, элементарный: если класс Cat наследует класс Animal, то зачем в наследнике повторять те же поля private String name; private int age; Они же ему переходят по наследству? Вопрос второй: как понять эти строки Class clazz = Class.forName(className); Cat cat = (Cat) clazz.newInstance(); Если стоит задача создать объект класса, чьё имя на момент компиляции неизвестно, то откуда в тексте программы это самое неизвестное имя? Я бы мог представить такую строку: Object obj = clazz.newInstance(); Она была бы универсальной для любого типа. И она есть в этом же коде ниже, в методе createObject(), но он же не вызывается вроде. Вызывается createCat()
вернулся сюда из 34 уровня лекции 8 задачи 2, запомните как создать обж с конструктором который принимает аргумент.
На этом наша лекция подошла к концу! Она получилась довольно большой, но сегодня ты узнал много нового :) И сегодня, и вчера, и позавчера. Смог прожевать только за три вечера. Не уверен, что прожевал тщательно, и не случится изжоги. 🤣