注解和反射

注解

1.内置注解

分别有@SuppressWarnings、@Override、@Deprecated

package com.tian.annotation;

import java.util.ArrayList;
import java.util.List;

//什么是注解
//@SuppressWarnings("all")镇压警告的注解
@SuppressWarnings("all")
public class Test01 extends Object{
    //@Override重写的注解
    @Override
    public String toString() {
        return super.toString();
    }
    //@Deprecated 不推荐程序员使用，但是可以使用，或者存在更好的方式
    @Deprecated
    public static void test(){
        System.out.println("Deprecated");
    }
    public void test02(){
        List list = new ArrayList();
    }
    public static void main(String[] args) {
        test();
    }
}

2.元注解

作用：负责注解其他注解

package com.tian.annotation;

import java.lang.annotation.*;

//测试元注解
@MyAnnotation
public class Test02 {


    public void test() {

    }
}
//定义一个注解
//Target 表示我们的注解可以在哪些地方可以使用
@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{

}

3.自定义注解

package com.tian.annotation;

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

//自定义注解
public class Test03 {
    //注解可以显示赋值，如果没有默认值,我们就必须给注解赋值
    @MyAnnotation2(age = 18, name = "秦疆")
    public void test(){}

    @MyAnnotation3("秦疆")
    public void test2(){}
}
@Target({ElementType.TYPE,ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@interface  MyAnnotation2{
    //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() default {"西部开源","清华大学"};
}
@Target({ElementType.TYPE,ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@interface MyAnnotation3{
    String value();
}

反射

反射概述

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助于Reflection API取得任何类的内部信息，并能直接操作任意对象的内部属性及方法

获得反射对象

package com.tian.reflection;

import java.util.concurrent.Callable;

//什么叫反射
public class Test02 extends Object{
    public static void main(String[] args) throws ClassNotFoundException {
       //通过反射获取类的Class对象
        Class c1 = Class.forName("com.tian.reflection.User");
        System.out.println(c1);

        Class c2 = Class.forName("com.tian.reflection.User");
        Class c3 = Class.forName("com.tian.reflection.User");
        Class c4 = Class.forName("com.tian.reflection.User");

        //一个类在内存中，只有一个Class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());
    }
}
//实体类:pojo ,entity
class User{
    private String name;
    private int id;
    private int age;

    public User() {
    }

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

    public int getAge() {
        return age;
    }

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

    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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

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

得到class类的几种方式

package com.tian.reflection;

import com.sun.scenario.effect.impl.sw.java.JSWBlend_SRC_OUTPeer;

import java.util.concurrent.Callable;

//测试Class类的创建方式有哪些
public class Test03 {
    public static void main(String[] args) throws ClassNotFoundException{
        Person person = new Student();
        System.out.println("这个人是："+person.name);
        
        //方式一：通过对象获得
        Class c1 = person.getClass();
        System.out.println(c1.hashCode());
        //方式二：forname获得
        Class c2 = Class.forName("com.tian.reflection.Student");
        System.out.println(c2.hashCode());
        //方式三：通过类名.class获得
        Class c3 = Student.class;
        System.out.println(c3.hashCode());
        //方式四：基本内置类型的包装类都有一个Type属性
        Class c4 = Integer.TYPE;
        System.out.println(c4);
        //获得父类类型
        Class c5 = c1.getSuperclass();
        System.out.println(c5);
    }
}

class Person{
    public String name;

    public Person() {
    }
    public Person(String name){
        this.name = name;
    }

    @Override
    public String toString() {
        return "Person{"+"name="+name+'\''+'}';
    }
}
class Student extends Person{
    public Student(){
        this.name = "学生";
    }
}
class Teacher extends Person{
    public Teacher(){
        this.name = "老师";
    }
}

所有类型的class对象

package com.tian.reflection;

import javax.xml.bind.Element;
import java.lang.annotation.ElementType;

//所有类型的Class
public class Test04 {
    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());
    }
}

类加载内存分析

package com.tian.reflection;

public class Test05 {
    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")
                         m = 300;
                         m = 100;
             }
             m = 100
         */
    }
}
class A{
    static {
        System.out.println("A类静态代码块初始化");
        m = 300;
    }
    static int m = 100;
    public A(){
        System.out.println("A类的无参构造初始化");
    }
}

分析类初始化

package com.tian.reflection;

//测试类什么时候会初始化
public class Test06 {
    static {
        System.out.println("Main类被加载");
    }

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

        //反射也会产生主动引用
        //Class.forName("com.tian.reflection.Son");

        //不会产生类的引用的方法
        //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.tian.reflection;

public class Test07 {
    public static void main(String[] args) throws ClassNotFoundException {
        //获取系统类的加载器
        ClassLoader systemClassLoader = ClassLoader.getSystemClassLoader();
        System.out.println(systemClassLoader);

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

        //获取扩展类加载器的父类加载器-->根加载器
        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.tian.reflection.Test07").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);

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

        //双亲委派机制(知道即可)
    }
}

获取类的运行时结构

package com.tian.reflection;

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

//获取类的信息
public class Test08 {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException, NoSuchMethodException {
        Class c1 = Class.forName("com.tian.reflection.User");

        //获取类的名字
        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
        Field[] fields = c1.getFields();//只能找到public属性

        fields = c1.getDeclaredFields();//找到全部的属性
        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
        Method[] methods = c1.getMethods();//获得本类和父类的全部public方法
        for (Method method : methods){
            System.out.println("正常的："+method);
        }
        methods = c1.getDeclaredMethods();//获得本类的所有的方法
        for (Method method : methods){
            System.out.println("getDeclaredMethods:"+method);
        }

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

        //获得构造器
        Constructor[] constructors = c1.getConstructors();//获得本类和父类的全部public构造器
        for (Constructor constructor : constructors){
            System.out.println(constructor);
        }
        constructors = c1.getDeclaredConstructors();//获得本类的所有的构造器
        for (Constructor constructor : constructors){
            System.out.println("#"+constructor);
        }

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

    }
}

动态创建对象执行方法

package com.tian.reflection;

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

//动态的创建对象，通过反射
public class Test09 {
    public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException, NoSuchMethodException, InvocationTargetException, NoSuchFieldException {
        //获得Class对象
        Class c1 = Class.forName("com.tian.reflection.User");

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

        //通过构造器创建对象
        //Constructor constructor = c1.getDeclaredConstructor(int.class, int.class, String.class);
        //User user2 = (User) constructor.newInstance(001,18,"秦疆");
        //System.out.println(user2);

        //通过反射调用普通方法
        User user3 =  (User)c1.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,"狂神");
        System.out.println(user3.getName());

        //通过反射操作属性
        User user4 =  (User)c1.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,"狂神2");
        System.out.println(user4.getName());
    }
}

性能对比分析

package com.tian.reflection;

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

//分析性能问题
public class Test10 {

    //普通方法调用
    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("普通方法执行10亿次："+(endTime-startTime)+"ms");
    }
    //反射方式调用
    public static void test02() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
        User user = new User();
        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("反射方法执行10亿次："+(endTime-startTime)+"ms");
    }

    //反射方式调用 关闭检测
    public static void test03() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
        User user = new User();
        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("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();
    }
}

获取泛型信息

package com.tian.reflection;

import com.tian.annotation.Test01;

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

//通过反射获取泛型
public class Test11 {
    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 = Test11.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 = Test11.class.getMethod("test02",null);
        Type genericReturnType = method.getGenericReturnType();
        if (genericReturnType instanceof ParameterizedType){
            Type[] actualTypeArguments = ((ParameterizedType) genericReturnType).getActualTypeArguments();
            for (Type actualTypeArgument : actualTypeArguments){
                System.out.println(actualTypeArgument);
            }
        }
    }
}

获取注解信息

package com.tian.reflection;

import java.lang.annotation.*;
import java.lang.reflect.Field;
import java.util.concurrent.Callable;

//练习反射操作注解
public class Test12 {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException {
        Class c1 = Class.forName("com.tian.reflection.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
        Tablekuang tablekuang = (Tablekuang)c1.getAnnotation(Tablekuang.class);
        String value = tablekuang.value();
        System.out.println(value);
        
        //获得类指定的注解
        Field f = c1.getDeclaredField("name");
        Fieldkuang annotation = f.getAnnotation(Fieldkuang.class);
        System.out.println(annotation.columnName());
        System.out.println(annotation.type());
        System.out.println(annotation.length());
    }
}
@Tablekuang("db_student")
class Student2 {
    @Fieldkuang(columnName = "db_id",type = "int",length = 10)
    private int id;
    @Fieldkuang(columnName = "db_age",type = "int",length = 10)
    private int age;
    @Fieldkuang(columnName = "db_name",type = "varcher",length = 3)
    private String name;

    public Student2() {
    }

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

    public int getAge() {
        return age;
    }

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

    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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

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

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

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