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 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
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.
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.
- This lesson shows the various ways to obtain a
object and use it to examine properties of a class, including its
declaration and contents.
- 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
Note: 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.