Java_注解和反射

注解

注解入门

//什么是注解
public class Demo01_Annotation extends Object {
    //@Override就是一个注解
    @Override
    public String toString() {
        return super.toString();
   }
}

内置注解

// 什么是注解
public class Demo01_Annotation extends Object {
    // @Override就是一个注解
    @Override
    public String toString() {
        return super.toString();
    }
    // @Deprecated不推荐程序员使用,但是可以使用,或者存在更好的更新方式
    @Deprecated
    public static void test() {
        System.out.println("Deprecated");
    }
    // @SuppressWarnings 镇压警告
    @SuppressWarnings("all")
    public void test01(){
        List<String> list = new ArrayList<String>();
    }
    public static void main(String[] args) {
        test();
    }
}

自定义注解,元注解

//测试元注解
@MyAnnotation
public class Demo02_MetaAnnotation {
    @MyAnnotation
    public void test() {
    }
}
//定义一个注解
//@Target可以用在什么地方
//ElementType.METHOD方法上有效  ElementType.TYPE类上有效
@Target(value = {ElementType.METHOD, ElementType.TYPE})
//@Retention在什么地方有效
//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 { }

//自定义注解
public class Demo03_CustomAnnotation {
    //注解可以显示赋值,如果没有默认值,就必须给注解赋值
    @MyAnnotation2(name = "张三")
    public void test() {
    }
}
@Target(value = {ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@interface MyAnnotation2 {
    //注解的参数:参数类型+参数名()
    //String name();
    String name() default "";
    int age() default 0;
    int id() default -1;//-1代表不存在
    String[] schools() default {"西部开源","清华大学"};

反射机制

Java反射机制概念

静态 & 动态语言

反射机制概念

反射机制研究与应用

反射机制优缺点

实现

//什么叫反射
public class Demo04_Reflection {
    public static void main(String[] args) throws ClassNotFoundException {
        // 通过反射获取类的class对象
        Class<?> c = Class.forName("cn.doris.reflection.User");
        System.out.println(c);
        Class<?> c1 = Class.forName("cn.doris.reflection.User");
        Class<?> c2 = Class.forName("cn.doris.reflection.User");
        Class<?> c3 = Class.forName("cn.doris.reflection.User");
        Class<?> c4 = Class.forName("cn.doris.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
        /**
         * public native int hashCode();返回该对象的hash码值
         *     注：哈希值是根据哈希算法算出来的一个值，这个值跟地址值有关，但不是实际地址值。
         */
        System.out.println(c1.hashCode());
        System.out.println(c2.hashCode());
        System.out.println(c3.hashCode());
        System.out.println(c4.hashCode());
    }
}
//实体类
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;
    }
}

理解Class类并获取Class实例

class类介绍

获取Class类的实例

//测试class类的创建方式有哪些
public class Demo05_CreateClass {
    public static void main(String[] args) throws ClassNotFoundException {
        Person person = new Student();
        System.out.println("这个人是:"+person.name);
        //方式一:通过对象查询
        Class c1 = person.getClass();
        System.out.println(c1.hashCode());
        //方式二:forname获得
        Class c2 = Class.forName("cn.doris.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,基本类型的包装类都有一个Type
        Class c4 = Integer.TYPE;
        System.out.println(c4);
        //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";
    }
}

哪些类型可以有Class对象

//所有类型的Class
public class Demo06_AllTypeClass {
    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; //美剧
        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());
    }
}

类的加载与ClassLoader

Java内存分析

类的加载

深刻理解类加载:https://blog.csdn.net/m0_38075425/article/details/81627349

//类加载
public class Demo07_ClassLoader {
    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,连接结束后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类无参构造初始化");
    }
}

分析上面代码

程序自上往下执行

什么时候会发生类初始化

//测试类什么时候会初始化
public class Demo08_ActiveReference {
    static {
        System.out.println("Main类被加载");
    }
    public static void main(String[] args) throws ClassNotFoundException {
        // 1. 主动调用
        //Son son = new Son();
        // 反射也会产生主动引用
        //Class.forName("cn.doris.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.a);
    }
}
class Father {
    static final 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 = 100;
    }
    static int m = 300;
    static final int a = 1;
}

类加载器的作用

ClassLoader systemClassLoader = ClassLoader.getSystemClassLoader();//获取系统类的加载器

ClassLoader parent = systemClassLoader.getParent();//获取系统类加载器的父类加载器—>扩展类加载器 jre1.8.0_91\lib\ext

ClassLoader parent1 = parent.getParent();//获取扩展类加载器父类加载器—>根加载器(c/c++) jre1.8.0_91\lib\rt.jar

//类加载器
public class Demo09_ClassLoader1 {
    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    jre1.8.0_91\lib\ext
        ClassLoader parent = systemClassLoader.getParent();
        System.out.println(parent);
        //获取扩展类加载器父类加载器-->根加载器(c/c++)  jre1.8.0_91\lib\rt.jar
        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("cn.doris.reflection.Demo09_ClassLoader1").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  检测安全性 你写的类和跟加载器一样的不会用你写的类
            //java.lang.String -->往上推
        /**
         * D:\Environment\java\jdk1.8.0_91\jre\lib\charsets.jar;
         * D:\Environment\java\jdk1.8.0_91\jre\lib\deploy.jar;
         * D:\Environment\java\jdk1.8.0_91\jre\lib\ext\access-bridge-64.jar;
         * D:\Environment\java\jdk1.8.0_91\jre\lib\ext\cldrdata.jar;
         * D:\Environment\java\jdk1.8.0_91\jre\lib\ext\dnsns.jar;
         * D:\Environment\java\jdk1.8.0_91\jre\lib\ext\jaccess.jar;
         * D:\Environment\java\jdk1.8.0_91\jre\lib\ext\jfxrt.jar;
         * D:\Environment\java\jdk1.8.0_91\jre\lib\ext\localedata.jar;
         * D:\Environment\java\jdk1.8.0_91\jre\lib\ext\nashorn.jar;
         * D:\Environment\java\jdk1.8.0_91\jre\lib\ext\sunec.jar;
         * D:\Environment\java\jdk1.8.0_91\jre\lib\ext\sunjce_provider.jar;
         * D:\Environment\java\jdk1.8.0_91\jre\lib\ext\sunmscapi.jar;
         * D:\Environment\java\jdk1.8.0_91\jre\lib\ext\sunpkcs11.jar;
         * D:\Environment\java\jdk1.8.0_91\jre\lib\ext\zipfs.jar;
         * D:\Environment\java\jdk1.8.0_91\jre\lib\javaws.jar;
         * D:\Environment\java\jdk1.8.0_91\jre\lib\jce.jar;
         * D:\Environment\java\jdk1.8.0_91\jre\lib\jfr.jar;
         * D:\Environment\java\jdk1.8.0_91\jre\lib\jfxswt.jar;
         * D:\Environment\java\jdk1.8.0_91\jre\lib\jsse.jar;
         * D:\Environment\java\jdk1.8.0_91\jre\lib\management-agent.jar;
         * D:\Environment\java\jdk1.8.0_91\jre\lib\plugin.jar;
         * D:\Environment\java\jdk1.8.0_91\jre\lib\resources.jar;
         * D:\Environment\java\jdk1.8.0_91\jre\lib\rt.jar;
         * E:\StudyProject\annotation_reflection\out\production\annotation_reflection;
         * D:\WorkingSoftware\IntelliJ IDEA 2020.3.1\lib\idea_rt.jar
         */
    }
}

创建运行时类的对象

获取运行类的完整结构

方法:

Class c1 = Class.forName(“cn.doris.reflection.User”); //获取当前对象的Class

//获得类的名字

c1.getName();// 获得包名 + 类名

c1.getSimpleName();// 获得类名

//获得类的属性

c1.getFields();//只能找到public属性

c1.getDeclaredFields();//找到全部的属性

c1.getDeclaredField(“name”); //获得指定属性的值

//获得类的方法

c1.getMethods(); //获得本类及父类的全部public方法

c1.getDeclaredMethods(); //获得本类的所有方法

c1.getMethod(“getName”, null);//获得指定的方法

//获得类的构造器

c1.getConstructors();

c1.getDeclaredConstructors();

c1.getDeclaredConstructor(String.class, int.class, int.class);//获得指定的构造器

//获取类的信息
public class Demo10_ClassInfo {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException, NoSuchMethodException {
        Class c1 = Class.forName("cn.doris.reflection.User");
        User user = new User();
        c1 = user.getClass();
        //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
        System.out.println("=======================");
        //获得类的属性
        Field[] fields = c1.getFields();//只能找到public属性
        for (Field field : fields) {
            System.out.println("getFields:" + field);
        }
        fields = c1.getDeclaredFields();//找到全部的属性
        for (Field field : fields) {
            System.out.println("getDeclaredFields:" + field);
        }
        //获得指定属性的值
        Field name = c1.getDeclaredField("name");
        System.out.println(name);
        System.out.println("=======================");
        //获得类的方法
        Method[] methods = c1.getMethods(); //获得本类及父类的全部public方法
        for (Method method : methods) {
            System.out.println("getMethods:" + method);
        }
        methods = c1.getDeclaredMethods(); //获得本类的所有方法
        for (Method method : methods) {
            System.out.println("getDeclaredMethods:" + method);
        }
        System.out.println("=======================");
        //获得指定的方法
        //重载
        Method getName = c1.getMethod("getName", null);
        Method setName = c1.getMethod("setName", String.class);
        System.out.println(getName);
        System.out.println(setName);
        //获得类的构造器
        System.out.println("=======================");
        Constructor[] constructors = c1.getConstructors();
        for (Constructor constructor : constructors) {
            System.out.println("getConstructors:" + constructor);
        }
        constructors = c1.getDeclaredConstructors();
        for (Constructor constructor : constructors) {
            System.out.println("getDeclaredConstructors:" + constructor);
        }
        //获得指定的构造器
        Constructor declaredConstructor = c1.getDeclaredConstructor(String.class, int.class, int.class);
        System.out.println("指定构造器" + declaredConstructor);
    }
}

调用运行时类的指定结构

有Class对象,能做什么

方法

//获得Class对象

Class c1 = Class.forName(“cn.doris.reflection.User”);

//本质上调用了类的无参构造器

User user = (User) c1.newInstance();

//构造器创建对象

Constructor constructor=c1.getDeclaredConstructor(String.class, int.class, int.class);
User user1 = (User) constructor.newInstance(“长歌”,001,17);

//invoke:激活
// (对象,”方法值”)
setName.invoke(user2, “doris”);

//设置安全检测
name.setAccessible(true);

//动态的创建对象,通过反射
public class Demo11_DynamicCreateObject {
    public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException, NoSuchFieldException {
        //获得Class对象
        Class c1 = Class.forName("cn.doris.reflection.User");
        //构造一个对象
        /*User user = (User) c1.newInstance();//本质上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(String.class, int.class, int.class);
        User user1 = (User) constructor.newInstance("长歌",001,17);
        System.out.println(user1);*/
        //通过反射调用普通方法
        User user2 = (User) c1.newInstance();
        //通过反射获取一个方法
        Method setName = c1.getDeclaredMethod("setName", String.class);
        //invoke:激活
        // (对象,"方法值")
        setName.invoke(user2, "doris");
        System.out.println(user2.getName());
        //通过反射操作属性
        User user3 = (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,属性或方法的setAccessible(true)
        //设置安全检测
        name.setAccessible(true);
        name.set(user3, "doris2");
        System.out.println(user3.getName());
    }
}

性能检测:

//分析性能问题
public class Demo12_Performance {
    //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("普通方式执行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("反射方式执行10亿次,关闭检测:" + (endTime - startTime) + "ms");
   }
    public static void main(String[] args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException {
        test01();
        test02();
        test03();
    }
}

反射操作泛型

//分析性能问题
public class Demo12_Performance {
    //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("普通方式执行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("反射方式执行10亿次,关闭检测:" + (endTime - startTime) + "ms");
   }
    public static void main(String[] args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException {
        test01();
        test02();
        test03();
    }
}

反射操作注解

//练习反射操作注解
public class Demo14_ORM {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException {
        Class c1 = Class.forName("cn.doris.reflection.Student2");
        //通过反射获取注解
        Annotation[] annotations = c1.getAnnotations();
        for (Annotation annotation : annotations) {
            System.out.println(annotation);
        }
        //获得注解value
        TableDoris tableDoris = (TableDoris) c1.getAnnotation(TableDoris.class);
        String value = tableDoris.value();
        System.out.println(value);
        //获得类指定的注解
        Field name = c1.getDeclaredField("name");
        FiledDoris annotation = name.getAnnotation(FiledDoris.class);
        System.out.println(annotation.columnName());
        System.out.println(annotation.type());
        System.out.println(annotation.length());
    }
}
@TableDoris("db_student")
class Student2 {
    @FiledDoris(columnName = "db_id", type = "int", length = 10)
    private int id;
    @FiledDoris(columnName = "db_age", type = "int", length = 3)
    private int age;
    @FiledDoris(columnName = "db_name", type = "varchar", length = 200)
    private String name;
    public Student2() {
    }
    public Student2(int id, int age, String name) {
        this.id = id;
        this.age = age;
        this.name = name;
    }
    @Override
    public String toString() {
        return "Student2{" +
                "id=" + id +
                ", age=" + age +
                ", 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 TableDoris {
    String value();
}
//属性注解
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface FiledDoris {
    String columnName();
    String type();
    int length();
}