annotation手册（annotation手册）.doc

annotation手册（annotation手册） What exactly is Annotation? Annotation provides a way to associate any information with program elements or any metadata (metadata). In some ways, annotation is used as a modifier and applied to packages, types, constructs, methods, methods, member variables, parameters, declarations of local variables. This information is stored in the name=value structure of the annotation. The annotation type is an interface that provides access to its information by way of java reflection of API. Annotation can be used to correlate any information to a program element (class, method, member variable, etc.). It is important to note that there is a basic underlying rule: annotaion does not affect the execution of program code; the execution of code is consistent regardless of the addition or deletion of annotation. In addition, although some annotation are reflected by the Java, the API method is accessed at run time, and the Java interpreter ignores these annotation at work. It is because of the Java virtual machine to ignore annotation, leading to the annotation type is no effect in the code; only through a supporting tool can access and processing of information in the annotation type. The standard annotation and meta-annotation types will be covered in this article, and the tools that accompany these annotation types are Java compilers (of course, to handle them in some special way). For these reasons, annotation is very handy in use. A local variable can be marked as a annotation type named after NonNull as an assertion that this local variable cannot be assigned a null value. And we can write a annotation code analysis tool to parse the code with the previous variables and try to validate the assertion. Of course, the code doesnt have to be written by itself. In the JDK after installation, can be found in the JDK/bin directory called apt tool, it provides a framework for dealing with annotation: it started scanning the source code annotation, we de