JAVA注解和反射

一、注解

1.1注解入门

Annotation是JDK5.0开始引入的新技术
Annotation的作用：
- 不是程序本身，可以对程序做出解释。（这一点和注释（comment）没有什么区别）
- 可以被其他程序（比如：编译器）读取
Annotation的格式
- 注解是以“@注释名”在代码中存在的，还可以添加一些参数值，例如:@SuppressWarnings(value="unchecked")
Annotation在哪里使用
- 可以附加在package，class，method，field等上面，相当于给他们添加了额外的辅助信息，我们可以通过反射机制编程对这些元数据访问

package com.Annotation;

//什么是注解
public class TestAnnotation extends  Object{
   //注解
    @Override
    public String toString() {
        return super.toString();
    }
}

1.2内置注解

package com.Annotation;

//什么是注解
public class TestAnnotation extends  Object{
    //重写注解
    @Override
    public String toString() {
        return super.toString();
    }
    //不推荐程序员使用，但是可以使用或有有更好的方式
    @Deprecated
    public boolean c(){
       return true;
    }
    //镇压警告
    @SuppressWarnings("all")
    public  void b(){

    }
}

1.3自定义注解，元注解

package com.Annotation;

import java.lang.annotation.*;

//测试元注解
public class TestAnnotation02 {
    @MyAnnotation
    public  void test(){

    }
}

//定义一个注解
//Target表示我们注解可以用在哪些地方
@Target(value = {ElementType.METHOD,ElementType.TYPE})

//Retention表示我们的注解在什么地方还有效
//runtime>class>sources
@Retention(value = RetentionPolicy.RUNTIME )
//Documented 表示是否将我们的注解生成在JAVADOC中

@Documented

//Inherited 子类可以继承父类的注解
@Inherited
@interface  MyAnnotation{

}

package com.Annotation;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

public class TestAnnotation03 {
    //注解可以显式赋值，如果没有默认值，我们必须给注解复制
    @MyAnnotation02(name = "石头",schools = {"沙子","水泥"})
    public void test(){

    }
    //一个参数可以不用写参数名和等于号
    @MyAnnotation3("房子")
    public  void test2(){

    }
}

@Target({ElementType.TYPE,ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@interface MyAnnotation02{
    //注解的参数：参数类型+参数名
    String name () default "";
    int age() default  0;
    int id() default  -1 ;//如果默认值为-1，代表不存在

    String[] schools();
}

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

二、反射机制

2.1Java反射机制概述

2.2理解Class类并获取Class实例

package com.Refiection;
//什么叫反射
public class TestRefiection {
    public static void main(String[] args) throws ClassNotFoundException {
        //通过反射获取类的Class对象
        Class c1= java.lang.Class.forName("com.Refiection.User");
        System.out.println(c1);

        Class c2= java.lang.Class.forName("com.Refiection.User");
        Class c3= java.lang.Class.forName("com.Refiection.User");
        Class c4= java.lang.Class.forName("com.Refiection.User");

        //一个类在内存中只有一个Claas对象
        //一个类被加载后，类的整个结构都会被封装在Class对象中
        System.out.println(c2.hashCode());
        System.out.println(c3.hashCode());
        System.out.println(c4.hashCode());

        System.out.println(c1.getClass());
    }
}
//实体类:poj entity
class User{
    private String name;
    private  int id;
    private  int age;

    public User() {
    }

    public User(String name, int id, int age) {
        this.name = name;
        this.id = id;
        this.age = age;
    }
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "User{" +
                "name='" + name + '\'' +
                ", id=" + id +
                ", age=" + age +
                '}';
    }
}

package com.Refiection;

import java.lang.annotation.ElementType;

//所有类型的class
public class Test {
    public static void main(String[] args) {
        Class c1 = Object.class;//类
        Class c2 = Comparable.class;//接口
        Class c3 = String[].class;//一维数组
        Class c4 = int[][].class;//二维数组
        Class c5 = Override.class;//注解
        Class c6 = ElementType.class;//枚举
        Class c7 = Integer.class;//基本数据类型
        Class c8 = void.class;//void
        Class c9 = Class.class;//Class

        System.out.println(c1);
        System.out.println(c2);
        System.out.println(c3);
        System.out.println(c4);
        System.out.println(c5);
        System.out.println(c6);
        System.out.println(c7);
        System.out.println(c8);
        System.out.println(c9);

        //只要元素类型与维度一样，就是同一个class
        int[] a = new int[10];
        int[] b = new int[100];
        System.out.println(a.getClass().hashCode());
        System.out.println(b.getClass().hashCode());
    }
}

2.3类的加载与ClassLoader

package com.Refiection;

public class Test02 {
    public static void main(String[] args) {
        A a = new A();
        System.out.println(A.m);

        /*
        1.加载到内存，会产生一个类对应class对象
        2.链接，链接结束后 m= 0
        3.初始化
        <clinit>（）{
            System.out.println("A类静态代码块初始化");
            int m=300;
            int m= 100;
         }
         */
    }
}
class  A{
    static {
        System.out.println("A类静态代码块初始化");
        int m=300;
    }
    /*
    * m=300;
    * m=100;
    * */
    static  int m= 100;
    public  A(){
        System.out.println("A类的无参构造初始化");
    }
}

测试类的初始化

主动引用，会有初始化

package com.Refiection;
//测试类什么时候会初始化
public class Test03 {
    static {
        System.out.println("main被加载");
    }

    public static void main(String[] args) throws ClassNotFoundException {
        //1.主动引用
        Son son = new Son();

        //反射也会产生引用
       // System.out.println(Class.forName("com.Refiection.Son"));

    }
}
class Father{
    static  int b = 2;
    static {
        System.out.println("父类被加载");
    }
}
class Son extends  Father{
    static {
        System.out.println("子类被加载");
        m  = 300;
    }
    static  int m = 100;
    static  final  int N=1;
}

被动引用：不会有初始化

package com.Refiection;
//测试类什么时候会初始化
public class Test03 {
    static {
        System.out.println("main被加载");
    }

    public static void main(String[] args) throws ClassNotFoundException {
        //不会产生主动引用的方法
       // System.out.println(Son.b);

       // Son[] array = new Son[5];

        System.out.println(Son.M);


    }
}
class Father{
    static  int b = 2;
    static {
        System.out.println("父类被加载");
    }
}
class Son extends  Father{
    static {
        System.out.println("子类被加载");
        m  = 300;
    }
    static  int m = 100;
    static  final  int M=1;
}

类加载器

package com.Refiection;

public class Test04 {
    public static void main(String[] args) throws ClassNotFoundException {

        //获取系统类的加载器
        ClassLoader platformClassLoader = ClassLoader.getSystemClassLoader();
        System.out.println(platformClassLoader);

        //获取系统类加载器的父类加载器-->扩展类加载器
        ClassLoader parent = platformClassLoader.getParent();
        System.out.println(parent);

        //获取扩展类加载器的父类加载器-->更加载器（c/c++）
        ClassLoader parent1 = parent.getParent();
        System.out.println(parent1);

        //测试当前类是哪个加载器加载的
        ClassLoader classLoader = Class.forName("com.Refiection.Test04").getClassLoader();
        System.out.println(classLoader);

        //测试JDK内置的类是谁加载的
        classLoader = Class.forName("java.lang.Object").getClassLoader();
        System.out.println(classLoader);

        //如何获得系统加载器可以加载的路径
        System.out.println(System.getProperty("java.class.path"));

        /*C:\Users\XIANG\Desktop\JavaCode\out\production\java-code;
        C:\Users\XIANG\Desktop\JavaCode\src\lib\commons-io-2.6.jar
         */

        //双亲委派机制
            //java.lang.String-->

    }
}

2.4获取运行时类的完整结构

package com.Refiection;

import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.Method;

public class Test05 {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException, NoSuchMethodException {
      //  Class c1 = Class.forName("com.Refiection.User");


        User user =new User();
        Class  c1 = user.getClass();
        //获得类的名字
        System.out.println(c1.getName());//获得包名和类名
        System.out.println(c1.getSimpleName());//获得类名

        //获得类的属性
        System.out.println("==============");
        Field[] fields = c1.getFields();
        fields=c1.getDeclaredFields();
        for(Field field :fields){
        System.out.println(field);
        }
        //获得指定属性的值
        Field name = c1.getDeclaredField("name");
        System.out.println(name);

        //获得类的方法
        System.out.println("==================");


        //获得本类及其父类的全部public方法
        Method[] methods = c1.getMethods();
        for(Method method :methods){
            System.out.println("正常的"+method);
        }

        //获得本类的全部方法包括私有类
        methods = c1.getDeclaredMethods();
        for(Method method :methods){
            System.out.println("getDeclaredMethods："+method);
        }

        //获得指定方法
        //重载
        Method getName = c1.getMethod("getName", null);
        Method setName = c1.getMethod("setName", String.class);
        System.out.println(getName);
        System.out.println(setName);

        //获得本类及其父类的全部public构造器
        System.out.println("==================");
        Constructor[] constructors = c1.getConstructors();
        for (Constructor constructor : constructors) {
            System.out.println(constructor);
        }

        //获得本类的全部构造器包括私有
        constructors = c1.getDeclaredConstructors();
        for (Constructor constructor : constructors) {
            System.out.println("#"+constructor);
        }

        //获得指定的构造器
        Constructor declaredConstructor = c1.getDeclaredConstructor(String.class, int.class, int.class);
        System.out.println(declaredConstructor);


    }
}

newInstance

package com.Refiection;

import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

public class Test06 {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException, InvocationTargetException, InstantiationException, IllegalAccessException, NoSuchFieldException {
        //获得class对象
        Class c1 = Class.forName("com.Refiection.U");

        //构造一个对象
        U user = (U) c1.getDeclaredConstructor().newInstance();//本质调用了类的无参构造器
        System.out.println(user);

        //通过构造器创建对象
        Constructor declaredConstructor = c1.getDeclaredConstructor(String.class, int.class, int.class);
        U user2 =(U)declaredConstructor.newInstance("反射",1,2);
        System.out.println(user2);

        //通过反射调用普通方法
        U user3 =(U) c1.getDeclaredConstructor().newInstance();
        //通过反射获取一个方法
        Method setName = c1.getDeclaredMethod("setName",String.class);

        //invoke：激活的意思
        //(对象，“方法的值”)
        setName.invoke(user3,"反射");
        System.out.println(user3.getName());

        //通过反射操作属性
        System.out.println("================================");
        U user4 = (U) c1.getDeclaredConstructor().newInstance();
        Field name = c1.getDeclaredField("name");

        //不能直接操作私有属性，我们需要关闭程序的安全检查，属性或者方法的setAccessible(true);
        name.setAccessible(true);

        name.set(user4,"反射");
        System.out.println(user4.getName());
    }
}
class U{
    private String name;
    private  int id;
    private  int age;

    public U() {
    }

    public U(String name, int id, int age) {
        this.name = name;
        this.id = id;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "User{" +
                "name='" + name + '\'' +
                ", id=" + id +
                ", age=" + age +
                '}';
    }
}

分析性能问题

package com.Refiection;


import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

//分析性能问题
public class Test07 {
    //普通方式调用
    public static void test01(){
        UTest user= new UTest();

        long startTime = System.currentTimeMillis();

//        System.out.println(user.getName());
        for (int i = 0; i < 1000000000; i++) {
           user.getName();
        }

        long endTime = System.currentTimeMillis();
        System.out.println("普通方式执行10亿次："+(endTime-startTime)+"ms");
    } 


    //反射方式调用
    public static void test02() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
        UTest user= new UTest();
        Class c1 = user.getClass();
        Method getName = c1.getDeclaredMethod("getName", null);


        long startTime = System.currentTimeMillis();

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

        long endTime = System.currentTimeMillis();
        System.out.println("反射方式调用执行10亿次："+(endTime-startTime)+"ms");
    }


    //反射方式调用，关闭检测
    public static void test03() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
        UTest user= new UTest();
        Class c1 = user.getClass();
        Method getName = c1.getDeclaredMethod("getName", null);
        getName.setAccessible(true);

        long startTime = System.currentTimeMillis();

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

        long endTime = System.currentTimeMillis();
        System.out.println("反射方式调用，关闭检测执行10亿次："+(endTime-startTime)+"ms");
    }
    public static void main(String[] args) throws InvocationTargetException, NoSuchMethodException, IllegalAccessException {
        test01();
        test02();
        test03();
    }
}
class UTest{
    private String name;
    private  int id;
    private  int age;

    public UTest() {
    }

    public UTest(String name, int id, int age) {
        this.name = name;
        this.id = id;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "User{" +
                "name='" + name + '\'' +
                ", id=" + id +
                ", age=" + age +
                '}';
    }
}

2.5调用运行时类的指定结构

获取泛型信息

package com.Refiection;

import java.lang.reflect.Method;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.List;
import java.util.Map;

//通过反射获取泛型
public class Test08 {


    public  void test01(Map<String,User> map, List<User>list){
        System.out.println("test01");
    }
    public  Map<String,User> test02(){
        System.out.println("test02");
        return null;
    }

    public static void main(String[] args) throws NoSuchMethodException {
        Method method = Test08.class.getMethod("test01", Map.class, List.class);
        Type[] genericParameterTypes = method.getGenericParameterTypes();
        for (Type genericParameterType : genericParameterTypes) {
            System.out.println("###:"+genericParameterType);
            if(genericParameterType instanceof ParameterizedType){
                Type[] actualTypeArguments = ((ParameterizedType) genericParameterType).getActualTypeArguments();
                for (Type actualTypeArgument : actualTypeArguments) {
                    System.out.println(actualTypeArgument);
                }
            }
        }
        method = Test08.class.getMethod("test02");
        Type genericReturnType = method.getGenericReturnType();
        if(genericReturnType instanceof  ParameterizedType){
            Type[] actualTypeArgyments = ((ParameterizedType)genericReturnType).getActualTypeArguments();
            for (Type actualTypeArgyment : actualTypeArgyments) {
                System.out.println("--->"+actualTypeArgyment);
            }
        }

    }
}
//实体类:poj entity
class User{
    private String name;
    private  int id;
    private  int age;

    public User() {
    }

    public User(String name, int id, int age) {
        this.name = name;
        this.id = id;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "User{" +
                "name='" + name + '\'' +
                ", id=" + id +
                ", age=" + age +
                '}';
    }
}

反射操作注解

getAnnotations
getAnnotation

package com.Refiection;

import java.lang.annotation.*;
import java.lang.reflect.Method;

//联系发射操作注解
public class Test09 {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException {
        Class c1 = Class.forName("com.Refiection.Student2");

        //通过反射获得注解

        Annotation[] annotations = c1.getAnnotations();
        for (Annotation annotation : annotations) {
            System.out.println(annotation);
        }

        //获得注解的value的值
        Table table= (Table)c1.getAnnotation(Table.class);
        String value = table.value();
        System.out.println(value);

        //获得指定的注解
        java.lang.reflect.Field name = c1.getDeclaredField("name");
        Field annotation = name.getAnnotation(Field.class);
        System.out.println(annotation.columnName());
        System.out.println(annotation.type());
        System.out.println(annotation.length());


    }

}
@Table("db_student")
class  Student2{
    @Field(columnName = "db_id",type = "int",length = 10)
    private  int id;
    @Field(columnName = "db_age",type = "int",length = 10)
    private  int age;
    @Field(columnName = "db_name",type = "varchar",length = 10)
    private  String name;

    public Student2() {
    }

    public Student2(int id, int age, String name) {
        this.id = id;
        this.age = age;
        this.name = name;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Student2{" +
                "id=" + id +
                ", age=" + age +
                ", name='" + name + '\'' +
                '}';
    }
}


//类名的注解
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface  Table{
    String value();
}

//属性的注解
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface Field{
    String columnName();
    String type();
    int length();
}

posted @ 2022-01-15 20:03 项sir 阅读(32) 评论(0) 编辑收藏举报

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XIANGSIR

项sir

博客

JAVA注解和反射

一、注解

1.1注解入门

1.2内置注解

1.3自定义注解，元注解

二、反射机制

2.1Java反射机制概述

2.2理解Class类并获取Class实例

2.3类的加载与ClassLoader

测试类的初始化

类加载器

2.4获取运行时类的完整结构

newInstance

分析性能问题

2.5调用运行时类的指定结构

获取泛型信息

反射操作注解

公告

常用链接

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