Introduction To Using Reflection

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One of Java's strengths is that it was designed with the assumption that the environment in which it was running would be changing dynamically. Classes are loaded dynamically, binding is done dynamically, and object instances are created dynamically on the fly when they are needed. What has not been very dynamic historically is the ability to manipulate "anonymous" classes. In this context, an anonymous class is one that is loaded or presented to a Java class at run time and whose type was previously unknown to the Java program. [Sun 1998]

Java programs can now "reflect" upon themselves or upon an arbitrary class to determine the methods and fields defined by that class, arguments and so on. What exactly does that mean? By utilizing the packages in the Java Reflection API to obtain complete information about any object, array, method, constructor, or field.

Fundamentally, the Reflection API consists of two components: objects that represent the various parts of a class file, and a means for extracting those objects in a safe and secure way. The latter is very important, as Java provides many security safeguards, and it would not make sense to provide a set of classes that invalidated those safeguards.

The classes that support the Java reflection API can be found in the java.lang.reflect package. Reflection is the ability for a class or object to examine itself. With reflection, you can use Java code to look at an object (actually, the class of an object) to determine its structre. Depending on the limits imposed by the security manager, it is possible to find information about constructors, methods, and fields a class has, as well as their attributes. It is even possible to change the value of fields, dynamically invoke methods, and construct new objects. This is just as you would with primitive pointers to variables and methods. This can all be done on objects that your Java code has never seen before.

Reflection is used a lot by JavaBeans to determine the capabilities of objects during runtime. Reflection is also used by object serialization to tear apart and build objects for transport over streams or into persistent storage. The internals of reflection are guarded by the security manager. You Java code is not allowed to perform anything with the reflection API that it couldn't do with static (ordinary, compiled) code. An object cannot use the reflection APIs to find out about data fields that it wouldn't normally be able to access (i.e. another objects private fields), and you cannot use it to modify data inappropriately.

The three key features of a class are:

The three methods that provide access to the above features are:

All of the above objects can be created using the Class object.

The Class class provides two pairs of methods for getting at each type of feature. One pair allows access to a class's public features (even those inherited from its superclass), while the other pair allows for access to any public or nonpublic item declared within the class (but not features that are inherited). Keep in mind that all security considerations are applied.