1. Overview
(1) Overview of Notes
- Java annotation (Annotation), also known as Java annotation, is an annotation mechanism introduced by JDK5.
- Classes, constructors, methods, member variables, parameters, etc. in the Java language can be annotated in the form of annotations.
(2) The role of annotations
- Mark the classes, methods, and variables in Java, and then perform special processing.
2. Custom annotation
(1) Custom annotation format
public @interface 注解名称 {
public 属性类型 属性名() default 默认值;
}
(2) Example of custom annotation
public @interface MyBook {
String id();
String[] names();
double price();
//特殊的属性value，当只定义一个属性且名称为value则调用时可不写value，若有多个，则必须填写
String value();
}
3. Meta annotations
(1) Concept: meta-annotation is a form of annotation that acts on annotations in the form of annotations.
(2) Common meta annotations
- @Target - restrict the scope of use of custom annotations
- @Retention - declares the life cycle of annotations
(3) Common values of common meta-annotation parameters
@Target - ElementType enumeration class
| value | scope |
|---|---|
| TYPE | class/interface |
| FIELD | Member variables |
| METHOD | member method |
| PARAMETER | method parameters |
| CONSTRUCTOR | constructor |
| LOCAL_VARIABLE | local variable |
@Retention - RententionPolicy enumeration class
| value | scope |
|---|---|
| SOURCE | Acting on the source code stage, the generated bytecode file does not exist |
| CLASS | Acts on the source code stage, the bytecode file stage, does not exist in the runtime stage, the default value |
| RUNTIME | Acting on the source code stage, bytecode file stage, running stage, commonly used in development |
(4) Using meta annotations
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface Book{
String value();
}
Four, annotation analysis
(1) Concept: Annotation parsing is to determine whether an annotation exists and analyze its content.
(2) Related APIs
- Annotation interface: the original interface of the annotation
- AnnotationElement: defines the annotation parsing method
| method | illustrate |
|---|---|
| Annotation[] getDeclaredAnnotations() | Get all annotations used on the current object and return an array of annotations |
| T getDeclaredAnnotation(Class<T> annotationClass) | Obtain the corresponding annotation object according to the annotation type |
| boolean isAnnotationPresent(Class<Annotation> annotationClass) | Determine whether the object uses the specified interface |
(3) Skills for parsing annotations: Which component is the annotation on, which component object should be parsed first
(4) Case Analysis
@Target({
ElementType.TYPE,ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@interface Book{
String value(); //书名
double price() default 100; //价格 默认为100
String[] authors(); //多位作者
}
@Book(value = "《Java深度学习》",price = 100,authors = {
"xxx","xxx","xxx"})
class BookStore{
private String name;
private double price;
private String[] authors;
@Book(value = "《Java浅度学习》",price = 50,authors = {
"佚名"})
public void text(){
}
@Override
public String toString() {
return "BookStore{" +
"name='" + name + '\'' +
", price=" + price +
", authors=" + Arrays.toString(authors) +
'}';
}
}
public class AnnotationDemo{
@Test
public void parseClass(){
//1、先得到类对象
Class c = BookStore.class;
//2、判断是否存在
if (c.isAnnotationPresent(Book.class)){
//3、直接获取注解对象
Book annotation = (Book) c.getDeclaredAnnotation(Book.class);
System.out.println(annotation.value());
System.out.println(annotation.price());
System.out.println(Arrays.toString(annotation.authors()));
}
}
@Test
public void parseMethod() throws NoSuchMethodException {
//1、先得到类对象
Method c = BookStore.class.getMethod("text");
//2、判断是否存在
if (c.isAnnotationPresent(Book.class)){
//3、直接获取注解对象
Book annotation = (Book) c.getDeclaredAnnotation(Book.class);
System.out.println(annotation.value());
System.out.println(annotation.price());
System.out.println(Arrays.toString(annotation.authors()));
}
}
}
/*测试结果*/
《Java深度学习》
100.0
[xxx, xxx, xxx]
======================
《Java浅度学习》
50.0
[佚名]
5. The application of annotations - simulating the Junit test framework
/*
* 需求：定义若干个方法，只要加了MyTest注解，就可以触发执行
* 分析：获取类中成员方法，判断是否有注解，若有注解则执行，否则不执行
*/
public class JunitDemo {
@MyJunit
public static void test1(){
System.out.println("=======test1========");
}
public static void test2(){
System.out.println("=======test2========");
}
@MyJunit
public static void test3(){
System.out.println("=======test3========");
}
public static void main(String[] args) throws InvocationTargetException, IllegalAccessException {
Class anno = JunitDemo.class;
Method[] methods = anno.getMethods();
for (Method method :methods) {
if (method.isAnnotationPresent(MyJunit.class)){
method.invoke(anno);
}else System.out.println(method.getName() + "未执行");
}
}
}
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
@interface MyJunit{
}