Annotation

1 Annotation提供了一种为程序元素设置元数据方法，就像修饰符一样修饰包，类，构造器，方法，成员变量，参数，局部变量的声明，这些信息存储在Annotation的“name=value”对中

2 4个基本annotation

      1 限定重写父类方法  @Override

class A {
    void info() {
        
    }
}

class B extends A {
    @Override
    public void info() {
        
    }
}

      2 程序元素已过时  @Deprecated

class A {
    @Deprecated
    void info() {
        
    }
}

class B {
    public void inf() {
        new A().info();
    }
}

     3 抑制编译器警告

@SuppressWarnings(value="unchecked")
public class JDBCTest{
    public static void main(String[] args) {
        List<Object> l = new ArrayList<>();
    }
}

     4 堆污染：把不带泛型对象赋给代泛型变量

public class JDBCTest{
    @SafeVarargs
    public void a() {
        List l = new ArrayList<Integer>();
        List<String> la = l;
        System.out.println(la.get(0));
    }
}

3  4 个元Annotation

     @Retention  定义注释保留的时间

      @Target  定义注释修饰成员单元的类型

     @Documented  Annotationn 能被提取成文档

    @Inherited  Annotation 具有继承性

public class JDBCTest extends A{
    public static void main(String[] args) {
        System.out.println(JDBCTest.class.isAnnotationPresent(in.class));
    }
}
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Inherited
@interface in{
    
}
@in
class A{
    
}

4 定义Annotation

@Test(age=9)
public class JDBCTest{
    public static void main(String[] args) {
        
    }
}

@interface Test{
    String name() default "dddds";
    int age();
}

5 AnnotatedElement 接口是所有程序元素（class  method等）的父接口。

        AnnotatedElement a = JDBCTest.class;
        a.getAnnotation(Test.class);
        a.isAnnotationPresent(Test.class);
        a.getDeclaredAnnotations();

public class JDBCTest{
    @Test(age=9)
    public void info(){}
    public static void main(String[] args) throws Exception, SecurityException {
        JDBCTest t = new JDBCTest();
        Test b = t.getClass().getMethod("info").getAnnotation(Test.class);
        System.out.println(b.age());
        System.out.println(b.name());
    }
}
@Retention(RetentionPolicy.RUNTIME)
@interface Test{
    String name() default "dddds";
    int age();
}

class My {
    @Testable
    public void m1() {
        
    }
    
    public void m2() {
        
    }
    @Testable
    public void m3() {
        throw new RuntimeException("dfds");
    }
    
}
public class JDBCTest{
    public static void main(String[] args) throws Exception {
        int pass = 0;
        int field = 0;
        
        for (Method m : Class.forName("com.whe.db.My").getMethods()){
            if (m.isAnnotationPresent(Testable.class)) {
                try {
                    m.invoke(null);
                    pass++;
                } catch (Exception e) {
                    // TODO: handle exception
                    field++;
                }
            }
        }
        System.out.println("pass = " + pass + "  field = " + field);
    }
}
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
@interface Testable{
}