Java reflection how does it work

Improve this answer. Community Bot 1. Kilian Foth Kilian Foth k 43 43 gold badges silver badges bronze badges. You don't need reflection to get Turing completeness, so no computation ever needs reflection. Reflection is not necessarily "much slower". You can use reflection once to generate a direct calling wrapper bytecode.

You'll know it when you need it. I often wondered why outside of language generation one would need it. Then, suddenly, I did SK-logic: actually for generating bytecode you don't need reflection at all indeed reflection contains no API for bytecode manipulation at all! JoachimSauer, of course, but you'll need a reflection API to load this generated bytecode. Show 5 more comments. I was like you once, I didn't know much about reflection - still don't - but I did use it once.

I had a class with two inner classes, and each class had lots of methods. Mahmoud Hossam Mahmoud Hossam 3, 2 2 gold badges 28 28 silver badges 41 41 bronze badges. Why the downvote? Upvoting for the preamble — Dmitry Minkovsky. MahmoudHossam Perhaps not a best-practice, but your answer illustrates an important tactic one can deploy. I'd group uses of reflection into three groups: Instantiating arbitrary classes.

For example, in a dependency injection framework, you probably declare that interface ThingDoer is implemented by the class NetworkThingDoer.

The framework would then find the constructor of NetworkThingDoer and instantiate it. Marshalling and unmarshalling to some other format. For example, mapping an object with getters and settings that follow the bean convention to JSON and back again. The code doesn't actually know the names of the fields or the methods, it just examines the class. Wrapping a class in a layer of redirection perhaps that List isn't actually loaded, but just a pointer to something that knows how to fetch it from the database or faking a class entirely jMock will create a synthetic class that implements an interface for testing purposes.

Kevin Peterson Kevin Peterson 2 2 silver badges 5 5 bronze badges. This is the best explanation of reflection I've found on StackExchange.

Most answers repeat what the Java Trail says which is "you can access all these properties", but not why you would , provide examples of doing things with reflection that are much easier to do without, or give some vague answer about how Spring uses it.

It's more acceptable to use it in tests than production code. With annotations it should be fine--The main point is not to specify class or method names as "Strings" if you can possibly avoid it.

In addition to BillK's comment: Reflection is very powerful, I'd call it magic. With great power comes great responsibility. Use it only if you know what you're doing. But asking why programmers do what they do is beyond my ability to answer. Show 2 more comments. Active Oldest Votes. Update from a comment: The ability to inspect the code in the system and see object types is not reflection, but rather Type Introspection. Improve this answer. Marome 47 1 1 silver badge 11 11 bronze badges.

Matt Sheppard Matt Sheppard k 46 46 gold badges silver badges bronze badges. The null indicates there are no parameters being passed to the foo method. See docs. Just to clear up since this has so many upvotes. I love reflection but if you have control over the code then using reflection as specified in this answer is unnessary and therefore an abuse--You should use Type Introspection instanceof and strong types. If there is any way but reflection to do something, that's how it should be done.

Reflection causes serious heartache because you lose all advantages of using a statically typed language. If you need it you need it, however even then I'd consider a pre-packaged solution like Spring or something that completely encapsulates the reflection necessary--IE: let someone else have the headaches. I see the term "reflection" used in official Java documentation from Oracle to describe not only the ability to make changes at runtime but also the ability to see the type of an object.

Not to mention that most so-called "type introspection" related classes ex: Method , Constructor , Modifier , Field , Member , basically apparently all except Class are within the java. Perhaps the concept "reflection" comprehensively includes both "type introspection" and modification at run-time? Show 7 more comments. Sheharyar Liedman Liedman 9, 4 4 gold badges 32 32 silver badges 35 35 bronze badges. So in other words, you can create an instance out of it's qualified name and the compiler won't complain about it because say you use just a String for the class name.

Then, at run time, if that class is not present you get an exception. You kind of bypassed the compiler in this case. Would you give me some specific use case for this? I just can't picture when i would choose it. FernandoGabrieli while it is true that it is easy to create runtime errors with reflection, it is also perfectly possible to use reflection without risking runtime exceptions. As hinted in my answer, a common use of reflection is for libraries or frameworks, which explicitly can't know the structure of the application at compile time, since they are compiled separate from the application.

Any library that uses "code by convention" is likely to use reflection, but not necessarily using magic strings. Add a comment. Array; import java. Ben Williams Ben Williams 2, 2 2 gold badges 19 19 silver badges 15 15 bronze badges. What should Callcount be set to? StackOverflowError when I ran this. Tom callCount should be set to zero. It's value is used to determine how many tabs should precede each line of output: each time dump needs to dump a "subobject", the output would print as nested in the parent.

That method proves useful when wrapped in another. StackOverflowError might be created in case of circular references, because of the unchecked recursion: buffer. Can you specifically release your code to Public Domain, pretty please?

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. Extensibility Features An application may make use of external, user-defined classes by creating instances of extensibility objects using their fully-qualified names. Drawbacks of Reflection Reflection is powerful, but should not be used indiscriminately.

Performance Overhead Because reflection involves types that are dynamically resolved, certain Java virtual machine optimizations cannot be performed.

Security Restrictions Reflection requires a runtime permission which may not be present when running under a security manager. 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. Gary Sheppard 4, 3 3 gold badges 21 21 silver badges 33 33 bronze badges.

Desmond Smith Desmond Smith 1, 9 9 silver badges 13 13 bronze badges. Pang 8, gold badges 82 82 silver badges bronze badges. Mendelt Mendelt Class metadata can be accessed through java. Nikhil Shekhar Nikhil Shekhar 4 4 silver badges 4 4 bronze badges. 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. VeKe VeKe 8, 5 5 gold badges 57 57 silver badges 67 67 bronze badges.

Example: Take for example a remote application which gives your application an object which you obtain using their API Methods. Ihor Patsian 1, 2 2 gold badges 15 15 silver badges 24 24 bronze badges.

I need something similar.. An example would help me a lot as I am new to reflection concepts.. I'm confused: can't you use instanceof to determine object type at runtime? Isuru Jayakantha Isuru Jayakantha 2 2 silver badges 3 3 bronze badges. The required classes for reflection are provided under java. 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. 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. 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.

How to access the private method from outside the class? Next Topics of Reflection API Tutorial newInstance method Understanding javap tool creating javap tool creating appletviewer tool Call private method from another class. Next Topic New Instance Method. Reinforcement Learning. R Programming. React Native. Python Design Patterns. Python Pillow. Python Turtle.

Verbal Ability. Interview Questions. Company Questions. Artificial Intelligence. Cloud Computing. Data Science. Angular 7. Machine Learning. Note: Do not try to use a 1. As you can see above, the first thing the code does is import the Reflection API classes. Next, it jumps right into the main method, which starts out as shown below. The method main declares arrays of constructors, fields, and methods. If you recall, these are three of the four fundamental parts of the class file.

The fourth part is the attributes, which the Reflection API unfortunately does not give you access to. After the arrays, I've done some command-line processing. If the user has typed a class name, the code attempts to load it using the forName method of class Class. The forName method takes Java class names, not file names, so to look inside the java. BigInteger class, you simply type "java ReflectClass java. BigInteger," rather than point out where the class file actually is stored.

In this step, the name of the class is retrieved using the getName method in class Class. This method returns the fully qualified name, and if the name contains dots, we can presume that the class was defined as part of a package. So Step 0 is to separate the package name part from the class name part, and print out the package name part on a line that starts with "package With the package statement taken care of, we proceed to Step 1, which is to collect all of the other class names that are referenced by this class.

This collection process is shown in the code below. Remember that the three most common places where class names are referenced are as types for fields instance variables , return types for methods, and as the types of the parameters passed to methods and constructors.

In the above code, the array ff is initialized to be an array of Field objects. The loop collects the type name from each field and process it through the tName method.

The tName method is a simple helper that returns the shorthand name for a type. So java. String becomes String. And it notes in a hashtable which objects have been seen. At this stage, the code is more interested in collecting class references than in printing. The next source of class references are the parameters supplied to constructors. The next piece of code, shown below, processes each declared constructor and collects the references from the parameter lists.