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Java SE（注解与反射）

程序员文章站 2022-06-09 20:44:11
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注解与反射

注解

  • 内置注解

    package com.java.annotation;

//测试内置注解
public class TestAnnotation 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 void test(){
        System.out.println();
    }

    //@SuppressWarnings 用来抑制编译时的警告信息
    @SuppressWarnings("all")
    public void test02(){
        String str;//定义变量但不使用，系统提示警告信息
    }
}

  • 元注解

    package com.java.annotation;

import java.lang.annotation.*;

//测试元注解
public class TestAnnotation02 {
}

//定义一个注解
//@Target  表示可以在哪些地方使用注解
@Target(value={ElementType.TYPE,ElementType.METHOD})

//@Retention 表示注解在什么地方有效
//Runtime > class > sources
@Retention(value= RetentionPolicy.CLASS)

//@Documented  表示是否将注解生成在JavaDoc文档中
@Documented

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

}

  • 自定义注解

    package com.java.annotation;

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

public class TestAnnotation03 {


    @MyAnnotation02(age=1)//注解可以显示赋值，如果没有默认值，必须给注解赋值
    public void test(){
    }

    @MyAnnotation03("a")
    public void test02(){
    }
}

@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();
    int id() default -1;//默认值为-1，代表不存在
    String[] str() default {"default"};

}

@Target({ElementType.TYPE,ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@interface MyAnnotation03{
    String value();//变量名为value时，在给注解赋值时可以省略参数名，直接赋值
}

反射

  • 获得反射对象

    package com.java.reflection;

public class Test01 {
    public static void main(String[] args) throws ClassNotFoundException {
        //通过反射获取类的class对象
        Class c1=Class.forName("com.java.reflection.User");
        System.out.println(c1);

        Class c2=Class.forName("com.java.reflection.User");
        Class c3=Class.forName("com.java.reflection.User");
        Class c4=Class.forName("com.java.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 age;
    private int id;

    public User() {
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        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;
    }

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

  • Class类的创建方式

    package com.java.reflection;

//测试class类的创建方式
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.java.reflection.Student");
        System.out.println(c2.hashCode());

        //方式三：通过类名.class获得
        Class<Student> 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类

    package com.java.reflection;

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致，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

    package com.java.reflection;

//测试类的加载
public class Test04 {
    public static void main(String[] args) {
        A a=new A();
        System.out.println(a.m);

        /*1.类的加载，生成每个类对应的class对象
        * 2.链接
        * 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类无参构造初始化");
    }
}

  • 类发生初始化的时间

    package com.java.reflection;

//类发生初始化的时间
public class Test05 {
    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();

        //反射也会产生注定引用
        //Class.forName("com.java.reflection.Son");

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

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

        System.out.println(Son.B);
    }
}
class Father{
    static int a=1;
    static{
        System.out.println("父类被加载");
    }
}

class Son extends Father{
    static int b=10;
    static final int B=200;
    static{
        System.out.println("子类被加载");
        b=100;
    }
}

  • 类加载器

    package com.java.reflection;

public class Test06 {
    public static void main(String[] args) throws ClassNotFoundException {
        //获取系统类的加载器
        ClassLoader classLoader=ClassLoader.getSystemClassLoader();
        System.out.println(classLoader);

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

        //获取扩展类加载器的父类加载器
        ClassLoader parent2=parent.getParent();
        System.out.println(parent2);

        //测试当前类的类加载器
        ClassLoader classLoader1=Class.forName("com.java.reflection.Test06").getClassLoader();
        System.out.println(classLoader1);

        //测试JDK内置类的类加载器
        classLoader1=Class.forName("java.lang.Object").getClassLoader();
        System.out.println(classLoader1);

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

        /*E:\IdeaProjects\web-app\src\main\webapp\WEB-INF\lib;
        D:\Java\mysql-connector-java-8.0.22\mysql-connector-java-8.0.22.jar;
        E:\IdeaProjects\JavaSE\out\production\Base;
        E:\IdeaProjects\JavaSE\Bsae\src\com\lib\commons-io-2.8.0.jar
         */
    }
}

  • 获取类的运行时结构

    package com.java.reflection;

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

public class Test07 {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException, NoSuchMethodException {
        Class c1=Class.forName("com.java.reflection.User");

        //获得类名
        System.out.println(c1.getName());
        System.out.println(c1.getSimpleName());

        //获得类的属性
        System.out.println("-----------------------");
        Field[] fields=c1.getFields();//只能获取public属性
        for(Field field:fields){
            System.out.println(field);
        }

        System.out.println("-----------------------");
        Field[] fields02=c1.getDeclaredFields();//获取所有属性
        for(Field field:fields02){
            System.out.println(field);
        }

        //获取指定属性
        System.out.println("-----------------------");
        Field field=c1.getDeclaredField("name");
        System.out.println(field);

        //获取类的方法
        System.out.println("-----------------------");
        Method[] methods=c1.getMethods();//获取本类及父类的全部public方法
        for (Method method:methods) {
            System.out.println("getMethods的方法："+method);
        }
        Method[] methods02=c1.getDeclaredMethods();//获取本类的全部方法
        for (Method method:methods02) {
            System.out.println("getDeclaredMethods的方法："+method);
        }

        //获取指定方法
        System.out.println("-----------------------");
        Method getName=c1.getDeclaredMethod("getName", null);
        Method setName=c1.getDeclaredMethod("setName", String.class);
        System.out.println(getName);
        System.out.println(setName);

        //获取构造器
        System.out.println("-----------------------");
        Constructor[] constructors=c1.getConstructors();
        for (Constructor constructor:constructors) {
            System.out.println(constructor);
        }
        Constructor[] constructors02=c1.getDeclaredConstructors();
        for (Constructor constructor:constructors02) {
            System.out.println("***-->"+constructor);
        }

        //获取指定构造器
        System.out.println("-----------------------");
        Constructor constructor03=c1.getDeclaredConstructor(String.class, int.class, i
                                                            nt.class);
        System.out.println(constructor03);
    }
}

  • 动态创建对象执行方

    package com.java.reflection;

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

public class Test08 {
    public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException, NoSuchFieldException {
        //获得class对象
        Class c1=Class.forName("com.java.reflection.User");

        //构造一个对象
        System.out.print("构造一个对象:");
        User user=(User) c1.newInstance();
        System.out.println(user);

        //通过构造器创建对象
        System.out.print("通过构造器创建对象:");
        Constructor constructor=c1.getDeclaredConstructor(String.class,int.class,int.class);
        User user1=(User) constructor.newInstance("xuexi",002,23);
        System.out.println(user1);

        //通过反射调用普通方法
        System.out.print("反射调用普通方法：");
        User user2=(User) c1.newInstance();
        //通过反射获取方法
        Method setName=c1.getDeclaredMethod("setName", String.class);

        //invoke(对象,"方法值"): **
        setName.invoke(user2,"芝士");
        System.out.println(user2.getName());

        //通过反射获取属性
        System.out.print("反射获取属性：");
        User user3=(User) c1.newInstance();
        Field name = c1.getDeclaredField("name");

        //不能直接操作私有属性，需要关闭程序的安全检测，属性或方法的setAccessible(true)
        name.setAccessible(true);
        name.set(user3,"圣灵");
        System.out.println(user3.getName());
    }
}

  • 性能对比分析

    package com.java.reflection;

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

public class Test09 {
    //普通方法调用
    public static void test01(){
        User user=new User();

        long startTime=System.currentTimeMillis();

        for (int i = 0; i < 10_0000_0000; i++) {
            user.getName();
        }

        long endTime=System.currentTimeMillis();

        System.out.println("普通方法执行10亿次："+(endTime-startTime)+"ms");
    }

    //反射方法调用
    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 < 10_0000_0000; 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 < 10_0000_0000; i++) {
            getName.invoke(user,null);
        }

        long endTime=System.currentTimeMillis();

        System.out.println("反射方法(关闭检测)执行10亿次："+(endTime-startTime)+"ms");
    }

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

  • 获取泛型信息

    package com.java.reflection;

import java.lang.reflect.MalformedParametersException;
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 Test10 {
    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=Test10.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=Test10.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.java.reflection;

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

//练习：ORM-->对象关系映射
//利用注解和反射完成类和表结构的映射关系
public class Test11 {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException {
        Class c1=Class.forName("com.java.reflection.Student01");
        
        //通过反射获取注解
        Annotation[] annotations = c1.getAnnotations();
        for (Annotation annotation:annotations) {
            System.out.println(annotation);
        }

        //获得注解的value值
        TableName tableName=(TableName) c1.getAnnotation(TableName.class);
        String value = tableName.value();
        System.out.println(value);

        //获得类指定的注解
        Field f=c1.getDeclaredField("name");
        FieldInterface annotation=f.getAnnotation(FieldInterface.class);
        System.out.println(annotation.colName());
        System.out.println(annotation.type());
        System.out.println(annotation.length());
    }
}

@TableName("db-student01")
class Student01{
    @FieldInterface(colName = "db-name",type = "varchar",length = 20)
    private String name;
    @FieldInterface(colName = "db-id",type = "int",length = 10)
    private int id;
    @FieldInterface(colName = "db-age",type = "int",length = 10)
    private int age;

    public Student01() {
    }

    public Student01(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 "student{" +
                "name='" + name + '\'' +
                ", id=" + id +
                ", age=" + age +
                '}';
    }
}

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

//属性的注解
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface FieldInterface{
    String colName();
    String type();
    int length();
}
相关标签： 自学 学习 反射 java class

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