The Reflection API

The reflection API represents, or reflects, the classes, interfaces, and objects in the current Java Virtual Machine.

With the reflection API you can:

• Determine the class of an object.
• Get information about a class's modifiers, fields, methods, constructors, and superclasses.
• Find out what constants and method declarations belong to an interface.
• Create an instance of a class whose name is not known until runtime.
• Get and set the value of an object's field, even if the field name is unknown to your program until runtime.
• Invoke a method on an object, even if the method is not known until runtime.
• Create a new array, whose size and component type are not known until runtime, and then modify the array's components.

For each class, the Java Runtime Environment (JRE) maintains an immutable Class object that contains information about the class. A Class object represents, or reflects, the class. With the reflection API, you can invoke methods on a Class object which return Constructor, Method, and Fieldobjects. You can use these objects to get information about the corresponding constructors, methods, and fields defined in the class.

Class objects also represent interfaces. You invoke Class methods to find out about an interface's modifiers, methods, and public constants. Not all of the Class methods are appropriate when a Class object reflects an interface. For example, it doesn't make sense to invoke getConstructors when the Class object represents an interface.

 

Examining Classes

explains how to determine the class of an object, and how to get information about classes and interfaces.

Retrieving Class Objects

 

First things first. Before you can find out anything about a class, you must first retrieve its corresponding Class object.

Getting the Class Name

 

It's easy to find out the name of a Class object. All you have to do is invoke the getName method.

Discovering Class Modifiers

 

This section shows you the methods you need to call to find out what modifiers a particular class has.

Finding Superclasses

 

In this section you'll learn how to retrieve all of the Class objects for the ancestors of a given class.

Identifying the Interfaces Implemented by a Class

 

If you want to find out what interfaces a class implements, then check out this section.

Examining Interfaces

 

In this section you'll learn how to tell if a Class object represents an interface or a class. You'll also get some tips on how to get more information about an interface.

Identifying Class Fields

 

This section shows you how to discover what fields belong to a class, and how to find out more about these fields by accessing Field objects.

Discovering Class Constructors

 

This section, which introduces the Constructor class, explains how to get information about a class's contructors.

Obtaining Method Information

 

To find out about a class's methods, you need to retrieve the corresponding Method objects. This section shows you how to do this.

 

Manipulating Objects shows you how to instantiate classes, get or set field values, and invoke methods. With the reflection API, you can perform these tasks even if the names of the classes, fields, and methods are unknown until runtime.

Creating Objects

 

How can you create an object if you don't know its class name until runtime? You'll find the answer in this section.

Getting Field Values

 

In this section you'll learn how to get the values of an object's fields, even if you don't know the name of the object, or even its class, until runtime.

Setting Field Values

 

Not only can you get field values at runtime, you can also set them. This section shows you how.

Invoking Methods

 

This section shows you how to dynamically invoke methods. Given an object, you can find out what methods its class defines, and then invoke the methods.

 Working with Arrays describes the APIs used to create and to modify arrays whose names are not known until runtime.

Identifying Arrays

 

This section shows you how to determine if an object really is an array.

Retrieving Component Types

 

If you want to find out the component type of an array, you'll want to check out the programming example in this section.

Creating Arrays

 

This section shows you how simple it is to create arrays at run time.

Getting and Setting Element Values

 

Even if you don't know the name of an array until run time, you can examine or modify the values of its elements. This section shows you how.

 Summary of Classes lists the classes that comprise the reflection API, and provides links to the apppropriate API documentation.

The following table summarizes the classes that compose the reflection API. The Class and Object classes are in the java.langpackage. The other classes are contained in the java.lang.reflectpackage.

 

Class	Description
Array	Provides static methods to dynamically create and access arrays.
Class	Represents, or reflects, classes and interfaces.
Constructor	Provides information about, and access to, a constructor for a class. Allows you to instantiate a class dynamically.
Field	Provides information about, and dynamic access to, a field of a class or an interface.
Method	Provides information about, and access to, a single method on a class or interface. Allows you to invoke the method dynamically.
Modifier	Provides static methods and constants that allow you to get information about the access modifiers of a class and its members.
Object	Provides the getClass method.