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java注解和反射讲义

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java注解

什么是注解

元注解

自定义注解

使用@interface自定义注解时,自动继承了java.lang.annotation.Annotation接口

public class Test03 {
   //注解可以显示赋值,如果没有默认值,一定要给注解赋值
   @Myannotation2(name = "aj",schloos = {"机电学院"})
   public void test(){

   }

   @MyAnnotation3("")
   public void test2(){

   }
}


@Target({ElementType.METHOD,ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@interface Myannotation2{
   // 注解的参数,参数类型+参数名
   String name() default  "";

   int age() default  0;

   //如果默认值为-1 代表不存在
   int id() default -1;

   String[] schloos() ;
}


@Target({ElementType.METHOD,ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@interface  MyAnnotation3{

   String value();
}

给代码加注解其实就是这么多,关键还是我们如何去读取注解,这就需要用到反射,下面重点介绍java反射

java反射

反射是java被视为动态语言的关键,反射机制允许程序在执行期借助Reflection API取得任何类的内部信息,并能直接操作任意对象内部熟悉及方法

Class c = Class.forName("java.lang.String")

加载完类之后,在堆内存的方法区就产生了一个Class类型的对象(一个类只有一个Class对象),这个对象就包含了完整的类的结构信息。我们可以通过这个对象看到类的结构。这个对象就像一面镜子,透过这个镜子看到类的结构,所以我们称之为:反射

Class类

对于每个类而言,JRE都为其保留一个不变的Class类型的对象,一个Class对象包含了特定某个结构的有关信息。

Class类的常用方法

![image-20210502212459742](/Users/cb/Library/Application Support/typora-user-images/image-20210502212459742.png)

反射获取对象

public class Test02 {
    public static void main(String[] args) throws ClassNotFoundException {
        Person person = new Student();
        System.out.println("这个人是"+person.name);

        //通过对象获取
        Class c1 = person.getClass();
        System.out.println(c1.hashCode());

        //通过forname获取
        Class c2 = Class.forName("reflection.Student");
        System.out.println(c2.hashCode());

        //通过类名获取
        Class c3 = Student.class;
        System.out.println(c3.hashCode());

        //获得父类类型
        Class c4 = c1.getSuperclass();
        System.out.println(c4);
    }
}



@Data
class Person{
    public String name;
    public int age;
}


class Student extends  Person{
    public Student(){
        this.name = "学生";
    }
}

class Teacher extends  Person{
    public Teacher(){
        this.name = "老师";
    }
}

反射操作方法、属性

public class Test03 {
    public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException, NoSuchFieldException {
        Class c1 = Class.forName("reflection.Student");

        Student student = (Student) c1.newInstance();
        System.out.println(student.getName());

        // 通过反射操作方法
        Method setName = c1.getDeclaredMethod("setName", String.class);
        setName.invoke(student, "zhangshan");
        System.out.println(student.getName());


        Student student1 = (Student) c1.newInstance();
        Field name = c1.getDeclaredField("name");
        //反射不能直接操作私有属性,需要手动关掉程序的安全检测,setAccessible(true)
        name.setAccessible(true);
        name.set(student1,"lisi");
        System.out.println(student1.getName());

    }
}

性能检测

public class Test04 {

    public static void test01(){
        User user = new User();
        long startTime = System.currentTimeMillis();

        for (int i = 0; i <1000000000 ; i++) {
            user.getName();
        }
        long endTime = System.currentTimeMillis();

        System.out.println(endTime - startTime +"ms");
    }


    public static void test02() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
        User user = new User();
        long startTime = System.currentTimeMillis();

        Class c1 = user.getClass();
        Method getName = c1.getDeclaredMethod("getName", null);

        for (int i = 0; i <1000000000 ; i++) {
            getName.invoke(user, null);
        }
        long endTime = System.currentTimeMillis();

        System.out.println(endTime - startTime +"ms");
    }


    public static void test03() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
        User user = new User();
        long startTime = System.currentTimeMillis();

        Class c1 = user.getClass();
        Method getName = c1.getDeclaredMethod("getName", null);
        getName.setAccessible(true);

        for (int i = 0; i <1000000000 ; i++) {
            getName.invoke(user, null);
        }
        long endTime = System.currentTimeMillis();

        System.out.println(endTime - startTime +"ms");
    }

    public static void main(String[] args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException {
        test01();
        test02();
        test03();
    }
}

反射操作注解

public class Test05 {

    public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException {
        Class<?> c1 = Class.forName("reflection.Customer");

        // 通过反射获取注解
        Annotation[] annotations = c1.getAnnotations();
        for (Annotation annotation:annotations){
            System.out.println(annotation);
        }

        // 获取注解的值
        TableAnnotation annotation = c1.getAnnotation(TableAnnotation.class);
        System.out.println(annotation.value());

        //获取类指定注解
        Field id = c1.getDeclaredField("id");
        FiledAnnotation annotation1 = id.getAnnotation(FiledAnnotation.class);
        System.out.println(annotation1.columnName());
        System.out.println(annotation1.length());
        System.out.println(annotation1.type());


    }
}


//类注解
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface TableAnnotation{
    String value();
}


@Data
@TableAnnotation("db_customer")
class Customer {

    @FiledAnnotation(columnName="id",type = "Long",length =10)
    private Long id;

    @FiledAnnotation(columnName="age",type = "int",length =10)
    private int age;

    @FiledAnnotation(columnName="name",type = "String",length =10)
    private String name;


}


//方法注解
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface FiledAnnotation{
    String columnName();

    String type();

    int length();
}

标签:Class,java,String,System,public,讲义,注解,class
来源: https://www.cnblogs.com/bangaj/p/14748372.html