What is reflection and why is it useful

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What is reflection  and why is it useful   I m particularly interested in Java  but I assume the principles are the same in any language

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Not every language supports reflection  but the principles are usually the same in languages that support it  Reflection is the ability to  quot reflect quot  on the structure of your program  Or more concrete  To look at the objects and classes you have and programmatically get back information on the methods  fields  and interfaces they implement  You can also look at things like annotations  It s useful in a lot of situations  Everywhere you want to be able to dynamically plug in classes into your code  Lots of object relational mappers use reflection to be able to instantiate objects from databases without knowing in advance what objects they re going to use  Plug-in architectures is another place where reflection is useful  Being able to dynamically load code and determine if there are types there that implement the right interface to use as a plugin is important in those situations

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Reflection has many uses  The one I am more familiar with  is to be able to create code on the fly       IE  dynamic classes  functions  constructors - based on any data    xml array sql results hardcoded etc

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Example   Take for example a remote application which gives your application an object which you obtain using their API Methods   Now based on the object you might need to perform some sort of computation     The provider guarantees that object can be of 3 types and we need to perform computation based on what type of object    So we might implement in 3 classes each containing a different logic  Obviously the object  information is available in runtime so you cannot statically code to perform computation hence reflection is used to instantiate the object of the class that you require to perform the computation based on the object received from the provider

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Uses of Reflection  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 in classes  Test harnesses can make use of reflection to systematically call a discoverable set APIs defined on a class  to ensure 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 cannot 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   source  The Reflection API

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IMPORTANT Starting from Java 9 you cannot longer use reflection unless the package-info java opens the module to reflection access  By default reflection access to all packages in the module is denied  See Understanding Java 9 Modules

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As I find it best to explain by example and none of the answers seem to do that     A practical example of using reflections would be a Java Language Server written in Java or a PHP Language Server written in PHP  etc  Language Server gives your IDE abilities like autocomplete  jump to definition  context help  hinting types and more  In order to have all tag names  words that can be autocompleted  to show all the possible matches as you type the Language Server has to inspect everything about the class including doc blocks and private members  For that it needs a reflection of said class   A different example would be a unit-test of a private method  One way to do so is to create a reflection and change the method s scope to public in the test s set-up phase  Of course one can argue private methods shouldn t be tested directly but that s not the point

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Reflection gives you the ability to write more generic code  It allows you to create an object at runtime and call its method at runtime  Hence the program can be made highly parameterized  It also allows introspecting the object and class to detect its variables and method exposed to the outer world

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Reflection is a set of functions which allows you to access the runtime information of your program and modify it behavior  with some limitations    It s useful because it allows you to change the runtime behavior depending on the meta information of your program  that is  you can check the return type of a function and change the way you handle the situation   In C  for example you can load an assembly  a  dll  in runtime an examine it  navigating through the classes and taking actions according to what you found  It also let you create an instance of a class on runtime  invoke its method  etc   Where can it be useful  Is not useful every time but for concrete situations  For example you can use it to get the name of the class for logging purposes  to dynamically create handlers for events according to what s specified on a configuration file and so on

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Simple example for reflection  In a chess game  you do not know what will be moved by the user at run time  reflection can be used to call methods which are already implemented at run time   public class Test        public void firstMoveChoice            System out println  First Move               public void secondMOveChoice            System out println  Second Move              public void thirdMoveChoice            System out println  Third Move               public static void main String   args  throws IllegalAccessException  IllegalArgumentException  InvocationTargetException            Test test   new Test            Method   method   test getClass   getMethods              firstMoveChoice         method 0  invoke test  null             secondMoveChoice         method 1  invoke test  null             thirdMoveChoice         method 2  invoke test  null

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I just want to add some point to all that was listed   With Reflection API you can write universal toString   method for any object   It is useful at debugging   Here is some example   class ObjectAnalyzer       private ArrayList lt Object gt  visited   new ArrayList lt Object gt                   Converts an object to a string representation that lists all fields         param obj an object        return a string with the object s class name and all field names and       values           public String toString Object obj          if  obj    null  return  null         if  visited contains obj   return              visited add obj         Class cl   obj getClass          if  cl    String class  return  String  obj        if  cl isArray               String r   cl getComponentType                     for  int i   0  i  lt  Array getLength obj   i                  if  i  gt  0  r                     Object val   Array get obj  i               if  cl getComponentType   isPrimitive    r    val              else r    toString val                       return r                       String r   cl getName             inspect the fields of this class and all superclasses       do            r                  Field   fields   cl getDeclaredFields             AccessibleObject setAccessible fields  true               get the names and values of all fields          for  Field f   fields                if   Modifier isStatic f getModifiers                      if   r endsWith       r                        r    f getName                         try                     Class t   f getType                      Object val   f get obj                     if  t isPrimitive    r    val                    else r    toString val                    catch  Exception e                      e printStackTrace                                                       r                  cl   cl getSuperclass            while  cl    null          return r

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The name reflection is used to describe code which is able to inspect other code in the same system  or itself    For example  say you have an object of an unknown type in Java  and you would like to call a  doSomething  method on it if one exists  Java s static typing system isn t really designed to support this unless the object conforms to a known interface  but using reflection  your code can look at the object and find out if it has a method called  doSomething  and then call it if you want to   So  to give you a code example of this in Java  imagine the object in question is foo     Method method   foo getClass   getMethod  doSomething   null   method invoke foo  null     One very common use case in Java is the usage with annotations  JUnit 4  for example  will use reflection to look through your classes for methods tagged with the  Test annotation  and will then call them when running the unit test   There are some good reflection examples to get you started at http   docs oracle com javase tutorial reflect index html  And finally  yes  the concepts are pretty much similar in other statically typed languages which support reflection  like C    In dynamically typed languages  the use case described above is less necessary  since the compiler will allow any method to be called on any object  failing at runtime if it does not exist   but the second case of looking for methods which are marked or work in a certain way is still common   Update from a comment      The ability to inspect the code in the system and see object types is   not reflection  but rather Type Introspection  Reflection is then the   ability to make modifications at runtime by making use of   introspection  The distinction is necessary here as some languages   support introspection  but do not support reflection  One such example   is C

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As name itself suggest it reflects what it holds for example class method etc apart from providing feature to invoke method creating instance dynamically at runtime   It is used by many frameworks and application under the wood to invoke services without actually knowing the code

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Java Reflection is quite powerful and can be very useful  Java Reflection makes it possible to inspect classes  interfaces  fields and methods at runtime  without knowing the names of the classes  methods etc  at compile time  It is also possible to instantiate new objects  invoke methods and get set field values using reflection   A quick Java Reflection example to show you what using reflection looks like   Method   methods   MyObject class getMethods         for Method method   methods           System out println  method       method getName             This example obtains the Class object from the class called MyObject  Using the class object the example gets a list of the methods in that class  iterates the methods and print out their names   Exactly how all this works is explained here  Edit  After almost 1 year I am editing this answer as while reading about reflection I got few more uses of Reflection    Spring uses bean configuration such as       lt bean id  someID  class  com example Foo  gt       lt property name  someField  value  someValue    gt   lt  bean gt    When the Spring context processes this  lt  bean   element  it will use Class forName String  with the argument  com example Foo  to instantiate that Class    It will then again use reflection to get the appropriate setter for the  lt  property   element and set its value to the specified value    Junit uses Reflection especially for testing Private Protected methods    For Private methods   Method method   targetClass getDeclaredMethod methodName  argClasses   method setAccessible true   return method invoke targetObject  argObjects     For private fields   Field field   targetClass getDeclaredField fieldName   field setAccessible true   field set object  value

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Reflection allows instantiation of new objects  invocation of methods  and get set operations on class variables dynamically at run time without having prior knowledge of its implementation   Class myObjectClass   MyObject class  Method   method   myObjectClass getMethods       Here the method takes a string parameter if there is no param  put null  Method method   aClass getMethod  method name   String class     Object returnValue   method invoke null   parameter-value1      In above example the null parameter is the object you want to invoke the method on  If the method is static you supply null  If the method is not static  then while invoking you need to supply a valid MyObject instance instead of null   Reflection also allows you to access private member methods of a class   public class A     private String str  null     public A String str      this str  str             A obj  new A  Some value     Field privateStringField   A class getDeclaredField  privateString       Turn off access check for this field privateStringField setAccessible true    String fieldValue    String  privateStringField get obj   System out println  fieldValue       fieldValue      For inspection of classes  also know as introspection  you don t need to import the reflection package  java lang reflect   Class metadata can be accessed through java lang Class    Reflection is a very powerful API but it may slow down the application if used in excess  as it resolves all the types at runtime

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As per my understanding   Reflection allows programmer to access entities in program dynamically  i e  while coding an application if programmer is unaware about a class or its methods  he can make use of such class dynamically  at run time  by using reflection   It is frequently used in scenarios where a class name changes frequently  If such a situation arises  then it is complicated for the programmer to rewrite the application and change the name of the class again and again   Instead  by using reflection  there is need to worry about a possibly changing class name

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Reflection is a key mechanism to allow an application or framework to work with code that might not have even been written yet   Take for example your typical web xml file  This will contain a list of servlet elements  which contain nested servlet-class elements  The servlet container will process the web xml file  and create new a new instance of each servlet class through reflection   Another example would be the Java API for XML Parsing  JAXP   Where an XML parser provider is  plugged-in  via well-known system properties  which are used to construct new instances through reflection   And finally  the most comprehensive example is Spring which uses reflection to create its beans  and for its heavy use of proxies

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I want to answer this question by example  First of all Hibernate project uses Reflection API to generate CRUD statements to bridge the chasm between the running application and the persistence store  When things change in the domain  the Hibernate has to know about them to persist them to the data store and vice versa   Alternatively works Lombok Project  It just injects code at compile time  result in code being inserted into your domain classes   I think it is OK for getters and setters   Hibernate chose reflection because it has minimal impact on the build process for an application   And from Java 7 we have MethodHandles  which works as Reflection API  In projects  to work with loggers we just copy-paste the next code   Logger LOGGER   Logger getLogger MethodHandles lookup   lookupClass   getName       Because it is hard to make typo-error in this case

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Reflection is to let object to see their appearance  This argument seems nothing to do with reflection  In fact  this is the  self-identify  ability   Reflection itself is a word for such languages that lack the capability of self-knowledge and self-sensing as Java and C   Because they do not have the capability of self-knowledge  when we want to observe how it looks like  we must have another thing to reflect on how it looks like  Excellent dynamic languages such as Ruby and Python can perceive the reflection of their own without the help of other individuals  We can say that the object of Java cannot perceive how it looks like without a mirror  which is an object of the reflection class  but an object in Python can perceive it without a mirror  So that s why we need reflection in Java

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Reflection is an API which is used to examine or modify the behaviour of methods  classes  interfaces at runtime    The required classes for reflection are provided under java lang reflect package  Reflection gives us information about the class to which an object belongs and also the methods of that class which can be executed by using the object  Through reflection we can invoke methods at runtime irrespective of the access specifier used with them    The java lang and java lang reflect packages provide classes for java reflection   Reflection can be used to get information about       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 objects belongs    The Reflection API is mainly used in   IDE  Integrated Development Environment  e g  Eclipse  MyEclipse  NetBeans etc  Debugger and Test Tools etc     Advantages of Using Reflection   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 on classes   Drawbacks   Performance Overhead  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   Exposure of Internals  Reflective code breaks abstractions and therefore may change behaviour with upgrades of the platform   Ref  Java Reflection  javarevisited blogspot in

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Reflection is a language s ability to inspect and dynamically call classes  methods  attributes  etc  at runtime    For example  all objects in Java have the method getClass    which lets you determine the object s class even if you don t know it at compile time  e g  if you declared it as an Object  - this might seem trivial  but such reflection is not possible in less dynamic languages such as C    More advanced uses lets you list and call methods  constructors  etc   Reflection is important since it lets you write programs that do not have to  know  everything at compile time  making them more dynamic  since they can be tied together at runtime  The code can be written against known interfaces  but the actual classes to be used can be instantiated using reflection from configuration files   Lots of modern frameworks use reflection extensively for this very reason  Most other modern languages use reflection as well  and in scripting languages  such as Python  they are even more tightly integrated  since it feels more natural within the general programming model of those languages

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One of my favorite uses of reflection is the below Java dump method  It takes any object as a parameter and uses the Java reflection API to print out every field name and value   import java lang reflect Array  import java lang reflect Field   public static String dump Object o  int callCount        callCount        StringBuffer tabs   new StringBuffer        for  int k   0  k  lt  callCount  k              tabs append   t              StringBuffer buffer   new StringBuffer        Class oClass   o getClass        if  oClass isArray              buffer append   n            buffer append tabs toString             buffer append               for  int i   0  i  lt  Array getLength o   i                  if  i  lt  0                  buffer append                   Object value   Array get o  i               if  value getClass   isPrimitive                          value getClass      java lang Long class                        value getClass      java lang String class                        value getClass      java lang Integer class                        value getClass      java lang Boolean class                                         buffer append value                 else                   buffer append dump value  callCount                                    buffer append tabs toString             buffer append    n          else           buffer append   n            buffer append tabs toString             buffer append    n            while  oClass    null                Field   fields   oClass getDeclaredFields                for  int i   0  i  lt  fields length  i                      buffer append tabs toString                     fields i  setAccessible true                   buffer append fields i  getName                     buffer append                       try                       Object value   fields i  get o                       if  value    null                            if  value getClass   isPrimitive                                      value getClass      java lang Long class                                    value getClass      java lang String class                                    value getClass      java lang Integer class                                    value getClass      java lang Boolean class                                                                 buffer append value                             else                               buffer append dump value  callCount                                                                      catch  IllegalAccessException e                        buffer append e getMessage                                       buffer append   n                              oClass   oClass getSuperclass                      buffer append tabs toString             buffer append    n              return buffer toString

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From java documentation page     java lang reflect package provides classes and interfaces for obtaining reflective information about classes and objects  Reflection allows programmatic access to information about the fields  methods and constructors of loaded classes  and the use of reflected fields  methods  and constructors to operate on their underlying counterparts  within security restrictions    AccessibleObject allows suppression of access checks if the necessary ReflectPermission is available   Classes in this package  along with java lang Class accommodate applications such as debuggers  interpreters  object inspectors  class browsers  and services such as Object Serialization and JavaBeans that need access to either the public members of a target object  based on its runtime class  or the members declared by a given class  It includes following functionality    Obtaining Class objects  Examining properties of a class  fields  methods  constructors   Setting and getting field values  Invoking methods  Creating new instances of objects    Have a look at this documentation link for the methods exposed by Class class   From this article  by Dennis Sosnoski  President  Sosnoski Software Solutions  Inc   and this article  security-explorations pdf    I can see considerable drawbacks than uses of using Reflection  User of Reflection    It provides very versatile way of dynamically linking program components It is useful for creating libraries that work with objects in very general ways   Drawbacks of Reflection    Reflection is much slower than direct code when used for field and method access  It can obscure what s actually going on inside your code It bypasses the source code can create maintenance problems Reflection code is also more complex than the corresponding direct code It  allows violation of key Java security constraints such as data access protection and type safety   General abuses    Loading of restricted classes  Obtaining references to constructors  methods or fields of a restricted class  Creation of new object instances  methods invocation  getting or setting field values of a restricted class    Have a look at this SE question regarding abuse of reflection feature   How do I read a private field in Java   Summary   Insecure use of its functions conducted from within a system code can also easily lead to the compromise of a Java security model  So use this feature sparingly

[java] What is reflection and why is it useful?

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