For example, the Class.isInstance method can be used to simulate the instanceof operator: Once Class information is in hand, often the next step is to ask basic questions about the Class object. In the examples below, the three steps are combined to present self contained illustrations of how to tackle specific applications using reflection. Will display a textual representation of the first method declared in String. For example, the sequence:Ĭlass c = Class.forName("") Method m = c.getDeclaredMethods() (m.toString()) Once this information is in hand, then the third step is to use the reflection API to manipulate the information. The second step is to call a method such as getDeclaredMethods, to get a list of all the methods declared by the class. The latter approach accesses the predefined TYPE field of the wrapper (such as Integer) for the fundamental type. To obtain Class information on fundamental types. One way of obtaining a Class object is to say: Class c = Class.forName("") is used to represent classes and interfaces in a running Java program. The first step is to obtain a object for the class that you want to manipulate. There are three steps that must be followed to use these classes. The reflection classes, such as Method, are found in. is a class representing a single class method. This program loads the specified class using class.forName, and then calls getDeclaredMethods to retrieve the list of methods defined in the class. ![]() That is, the method names of class are listed, along with their fully qualified parameter and return types. Sample code shows how to retrieve the component type for an array and how to set and get fields with array or enum types.Copy public ( 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. Examples are provided for setting and getting field values, invoking methods, and creating new instances of objects using specific constructors. Members This lesson describes how to use the Reflection APIs to find the fields, methods, and constructors of a class. Classes This lesson shows the various ways to obtain aĬlass object and use it to examine properties of a class, including its declaration and contents. Each lesson contains code examples, tips, and troubleshooting information. This trail covers common uses of reflection for accessing and manipulating classes, fields, methods, and constructors. Reflective code breaks abstractions and therefore may change behavior with upgrades of the platform. 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. This is in an important consideration for code which has to run in a restricted security context, such as in an Applet. Security Restrictions Reflection requires a runtime permission which may not be present when running under a security manager. 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. Performance Overhead Because reflection involves types that are dynamically resolved, certain Java virtual machine optimizations can not be performed. The following concerns should be kept in mind when accessing code via reflection. ![]() If it is possible to perform an operation without using reflection, then it is preferable to avoid using it. Reflection is powerful, but should not be used indiscriminately. 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 suite. Debuggers and Test Tools Debuggers need to be able to examine private members on classes. Visual development environments can benefit from making use of type information available in reflection to aid the developer in writing correct code. Class Browsers and Visual Development Environments A class browser needs to be able to enumerate the members of classes. Extensibility Features An application may make use of external, user-defined classes by creating instances of extensibility objects using their fully-qualified names. With that caveat in mind, reflection is a powerful technique and can enable applications to perform operations which would otherwise be impossible. This is a relatively advanced feature and should be used only by developers who have a strong grasp of the fundamentals of the language. 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.
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