注解与反射

注解与反射

注解（Java.Annotation）

内置注解

元注解

import java.lang.annotation.*;

//测试元注解
@MyAnnotation
public class Test01 {
    
    @MyAnnotation
    public void test(){
    }
    
}

//定义一个注解
// Target 表示注解可用在哪些地方
@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 表示子类可以继承父类的注解
@Inherited
@interface MyAnnotation{
    
}

自定义注解

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 Test02 {
    
    //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(name = "AA",schools = {"大学","高中"})
    public void test1(){ }

    //注解的参数只有一个且名为 value 时可省略
    @MyAnnotation3("BB")
    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;
    //如果默认值为-1，代表不存在
    int id() default -1;
    String[] schools() default {"清华","北大"};
}

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

反射

概述

获得反射对象

public class Test01 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.study.reflecttest.User");  //  class com.study.reflecttest.User
        System.out.println(c1);

        Class c2 = Class.forName("com.study.reflecttest.User");
        Class c3 = Class.forName("com.study.reflecttest.User");
        Class c4 = Class.forName("com.study.reflecttest.User");

        //一个类在内存中只有一个Class对象
        //一格类被加载后，类的整个结构都会被封装在Class对象中
        System.out.println(c1.hashCode());    
        System.out.println(c2.hashCode());
        System.out.println(c1.hashCode() == c2.hashCode());
        System.out.println(c3.hashCode());
        System.out.println(c3.hashCode()==c4.hashCode());
    }

}

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 +
                '}';
    }
}

得到Class类的几种方式

//测试class类的创建方式有哪些
public class Test02 {
    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.study.reflecttest.Student");
        System.out.println(c2.hashCode());

        //方式三：t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<Integer> C4 = Integer.TYPE;
        System.out.println(C4.hashCode());

        //获得父类类型
        Class c5 = c1.getSuperclass();
        System.out.println(c5);

    }

}

class Person{
    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

import java.lang.annotation.ElementType;

//所有类型的class
public class Test03 {
    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

public class Test05 {
    public static void main(String[] args) {
        A a = new A();
        System.out.println(A.m);
        
        /*
        1.加载到内存，会产生一个类对应的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;
            }
         */
        
    }
}

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

//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.study.reflecttest.Son");

        //不会产生类的引用的方法
        //System.out.println(Son.b);

        //通过数组定义类引用，不会触发此类的初始化
        //Son[] array = new Son[5];

        //引用常量不会触发此类的初始化（常量在链接阶段就存入调用类的常量池中）
        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

public class Test07 {
    public static void main(String[] args) throws ClassNotFoundException {

        //获得系统类的加载器
        ClassLoader systemClassLoader = ClassLoader.getSystemClassLoader();
        System.out.println(systemClassLoader); //sun.misc.Launcher$AppClassLoader@18b4aac2

        //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);  //sun.misc.Launcher$ExtClassLoader@1b6d3586

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

        //测试当前类是哪个加载器加载的
        ClassLoader classLoader = Class.forName("com.study.reflecttest.Test07").getClassLoader();
        System.out.println(classLoader);  //sun.misc.Launcher$AppClassLoader@18b4aac2

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

        //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"));
        //C:\Program Files\Java\jdk1.8.0_351\jre\lib\charsets.jar;
        // C:\Program Files\Java\jdk1.8.0_351\jre\lib\deploy.jar;
        // C:\Program Files\Java\jdk1.8.0_351\jre\lib\ext\access-bridge-64.jar;
        // C:\Program Files\Java\jdk1.8.0_351\jre\lib\ext\cldrdata.jar;
        // C:\Program Files\Java\jdk1.8.0_351\jre\lib\ext\dnsns.jar;
        // C:\Program Files\Java\jdk1.8.0_351\jre\lib\ext\jaccess.jar;
        // C:\Program Files\Java\jdk1.8.0_351\jre\lib\ext\jfxrt.jar;
        // C:\Program Files\Java\jdk1.8.0_351\jre\lib\ext\localedata.jar;
        // C:\Program Files\Java\jdk1.8.0_351\jre\lib\ext\nashorn.jar;
        // C:\Program Files\Java\jdk1.8.0_351\jre\lib\ext\sunec.jar;
        // C:\Program Files\Java\jdk1.8.0_351\jre\lib\ext\sunjce_provider.jar;
        // C:\Program Files\Java\jdk1.8.0_351\jre\lib\ext\sunmscapi.jar;
        // C:\Program Files\Java\jdk1.8.0_351\jre\lib\ext\sunpkcs11.jar;
        // C:\Program Files\Java\jdk1.8.0_351\jre\lib\ext\zipfs.jar;
        // C:\Program Files\Java\jdk1.8.0_351\jre\lib\javaws.jar;
        // C:\Program Files\Java\jdk1.8.0_351\jre\lib\jce.jar;
        // C:\Program Files\Java\jdk1.8.0_351\jre\lib\jfr.jar;
        // C:\Program Files\Java\jdk1.8.0_351\jre\lib\jfxswt.jar;
        // C:\Program Files\Java\jdk1.8.0_351\jre\lib\jsse.jar;
        // C:\Program Files\Java\jdk1.8.0_351\jre\lib\management-agent.jar;
        // C:\Program Files\Java\jdk1.8.0_351\jre\lib\plugin.jar;
        // C:\Program Files\Java\jdk1.8.0_351\jre\lib\resources.jar;
        // C:\Program Files\Java\jdk1.8.0_351\jre\lib\rt.jar;
        // E:\Sophomore\Java\JavaSE\project\out\production\annotation;
        // D:\idea\2019\ideaIU-2019.2.win\lib\idea_rt.jar

    }

}

创建运行时类的对象

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

public class Test08 {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException, NoSuchMethodException {
        Class<?> c1 = Class.forName("com.study.reflecttest.User");

        //获得类的名字
        System.out.println(c1.getName());  //获得包名 + 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  getDeclaredField:private  getField：public
        Field name = c1.getDeclaredField("name");
        System.out.println(name);

        //获得类的方法
        System.out.println("===================================================");
        Method[] methods = c1.getMethods();  // 获得本类及其父类的全部public方法

        for (Method method : methods) {
            System.out.println("正常的："+method);
        }
        methods = c1.getDeclaredMethods();  // 获得本类的所有方法
        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);

        //获得指定的构造器
        System.out.println("===================================================");
        Constructor[] constructors = c1.getConstructors();  //获得public方法
        for (Constructor constructor : constructors) {
            System.out.println(constructor);
        }
        constructors = c1.getDeclaredConstructors();  // 获得全部方法
        for (Constructor constructor : constructors) {
            System.out.println(constructor);
        }

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

    }

}

动态的创建对象执行方法

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, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException, NoSuchFieldException {
        //获得Class对象
        Class c1 = Class.forName("com.study.reflecttest.User");

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

        //通过构造器创造对象
        Constructor constructor = c1.getDeclaredConstructor(String.class, int.class, int.class);
        Object user2 = constructor.newInstance("中文", 001, 12);
        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,"注解名字");
        System.out.println(user3.getName());

        //通过反射操作属性
        System.out.println("2222222222222222222222222222222222222222222222");
        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测，属性或者方法的setAccessible(true)
        name.setAccessible(true);
        name.set(user4,"user4：name名字字段");
        System.out.println(user4.getName());
    }

}

性能对比分析

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

//分析性能问题
public class Test10 {

    //普通方式调用
    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("反射方式执行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("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 NoSuchMethodException, IllegalAccessException, InvocationTargetException {
        test01();  //2ms
        test02();  //1564ms
        test03();  //：947ms
    }
}

获取泛型信息

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

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);
                }
            }
        }

        System.out.println("===========================================================================");
        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

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

//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.study.reflecttest.Student2");

        //通过反射获得注解
        Annotation[] annotations = c1.getAnnotations();
        for (Annotation annotation : annotations) {
            System.out.println(annotation);  //@com.study.reflecttest.Table(value=db_student)
        }

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

        //获得类指定的注解
        Field name = c1.getDeclaredField("name");
        FieldStu annotation = name.getAnnotation(FieldStu.class);
        System.out.println(annotation.columnName()); //db_name
        System.out.println(annotation.type());  //varchar
        System.out.println(annotation.length());  //10

    }
}

@Table("db_student")
class Student2{
    @FieldStu(columnName = "db_id",type = "int",length = 10)
    private int id;
    @FieldStu(columnName = "db_age",type = "int",length = 10)
    private int age;
    @FieldStu(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;
    }
}

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

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