Java:反射

1.获得反射对象

public class Reflection1 {
    
    
    public static void main(String[] args) throws ClassNotFoundException {
    
    
        Class c1 = Class.forName("com.test.Reflection.User");
        System.out.println(c1);
    }
}

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;
    }
}

2.得到Class类的几种方式

//得到class类的几种方式
public class Reflection2 {
    
    
    public static void main(String[] args) throws ClassNotFoundException {
    
    
        Person person = new Student1();
        System.out.println(person.name);

        //通过对象获得
        Class c1 = person.getClass();
        System.out.println(c1.hashCode());

        //通过forName获得
        Class c2 = Class.forName("com.test.Reflection.Student1");
        System.out.println(c2.hashCode());

        //通过类名获得
        Class c3 = Student1.class;
        System.out.println(c3.hashCode());

        //基本内置包装类都有一个Type属性
        Class c4 = Integer.TYPE;
        System.out.println(c4);

        //获取父类类型
        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 Student1 extends Person {
    
    
    public Student1() {
    
    
        this.name = "学生";
    }
}

class Teacher1 extends Person {
    
    
    public Teacher1() {
    
    
        this.name = "老师";
    }
}

3.所有类型的Class对象

import java.lang.annotation.ElementType;

public class Reflection3 {
    
    
    public static void main(String[] args) {
    
    
        //类
        Class c1 = Object.class;
        //接口
        Class c2 = Comparable.class;
        //一维数组
        Class c3 = String[].class;
        //二维数组
        Class c4 = int[][].class;
        //注解
        Class c5 = Override.class;
        //枚举
        Class c6 = ElementType.class;
        //基本数据类型
        Class c7 = Integer.class;
        //void
        Class c8 = void.class;
        //Class
        Class c9 = 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);

        //只要元素类型和维度一样，就是同一个class
        int[] i1 = new int[100];
        int[] i2 = new int[1000];
        System.out.println(i1.getClass().hashCode());
        System.out.println(i2.getClass().hashCode());
    }
}

4.类加载内存分析

public class Reflection4 {
    
    
    public static void main(String[] args) {
    
    
        A a = new A();
        System.out.println(a.m);
    }
}

class A {
    
    
    static {
    
    
        System.out.println("静态代码块初始化");
        m = 300;
    }

    static int m = 100;

    public A() {
    
    
        System.out.println("构造代码块初始化");
    }

}

运行结果如图：

5.分析类初始化

public class Reflection5 {
    
    
    static {
    
    
        System.out.println("main函数被加载");
    }

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

        //反射会产生主动引用
        Class.forName("com.test.Reflection.Son");

        //不会产生主动引用的方法
        //System.out.println(son.a);

        Son[] s = new Son[5];

        //System.out.println(son.M);
    }
}

class Father {
    
    
    static int a = 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;
}

运行结果如图：

6.类加载器

public class Reflection6 {
    
    
    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);

        //获取系统类的加载器的父类加载器-扩展类加载器
        ClassLoader parent1 = systemClassLoader.getParent();
        System.out.println(parent1);

        //获取扩展类加载器的父类加载器
        ClassLoader parent2 = parent1.getParent();
        System.out.println(parent2);

        //测试当前类是哪个加载器加载的
        ClassLoader classLoader1 = Class.forName("com.test.Reflection.Reflection6").getClassLoader();
        System.out.println(classLoader1);

        //测试JDK内置的类是谁加载的
        ClassLoader classLoader2 = Class.forName("java.lang.Object").getClassLoader();
        System.out.println(classLoader2);

        //获取系统类加载器可以加载的路径
        String property = System.getProperty("java.class.path");
        System.out.println(property);
    }
}

7.获取类的运行时结构

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

//获得类的信息
public class Reflection7 {
    
    
    public static void main(String[] args) throws Exception {
    
    
        Class c1 = Class.forName("com.test.Reflection.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.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("------------------");
        Method[] methods1 = c1.getMethods();//获得本类及其父类的全部public方法
        for (Method method : methods1) {
    
    
            System.out.println("getMethods:" + method);
        }
        System.out.println("------------------");
        Method[] methods2 = c1.getDeclaredMethods();//获得本类的所有方法
        for (Method method : methods2) {
    
    
            System.out.println("getDeclaredMethods:" + method);
        }
        //获取指定方法
        System.out.println("------------------");
        Method getName = c1.getMethod("getName", null);
        System.out.println(getName);
        Method setName = c1.getMethod("setName", String.class);
        System.out.println(setName);
        //获取构造器
        System.out.println("------------------");
        Constructor[] constructors1 = c1.getConstructors();//获得本类及其父类的全部构造器
        for (Constructor constructor : constructors1) {
    
    
            System.out.println("getConstructors:" + constructor);
        }

        Constructor[] constructors2 = c1.getDeclaredConstructors();//获得本类的构造器
        for (Constructor constructor : constructors2) {
    
    
            System.out.println("getDeclaredConstructors:" + constructor);
        }
        System.out.println("------------------");
        //获取指定的构造器
        Constructor constructor1 = c1.getDeclaredConstructor(String.class, int.class, int.class);
        System.out.println(constructor1);
        Constructor constructor2 = c1.getDeclaredConstructor(null);
        System.out.println(constructor2);
    }
}

8.动态创建对象执行方法

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

public class Reflection8 {
    
    
    public static void main(String[] args) throws Exception {
    
    
        //获得Class对象
        Class c1 = Class.forName("com.test.Reflection.User");
        //构造一个对象
        User user1 = (User) c1.newInstance();
        System.out.println(user1);

        //通过构造器创建对象
        Constructor constructor = c1.getDeclaredConstructor(String.class, int.class, int.class);
        User user2 = (User) constructor.newInstance("张三", 1, 20);
        System.out.println(user2);

        User user3 = (User) c1.newInstance();
        //通过反射获取一个方法
        Method setName = c1.getDeclaredMethod("setName", String.class);

        Method setAge = c1.getDeclaredMethod("setAge", int.class);
        //invoke:激活
        setName.invoke(user3, "李四");
        System.out.println(user3.getName());

        setAge.invoke(user3, 20);
        System.out.println(user3.getAge());

        //通过反射操作属性
        User user4 = (User) c1.newInstance();
        Field name = c1.getDeclaredField("name");

        //访问权限开启
        name.setAccessible(true);
        name.set(user4, "王五");
        System.out.println(user4.getName());
    }
}

9.性能对比分析

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

//分析性能问题
public class Reflection9 {
    
    
    //普通方式调用
    public static void test1() {
    
    
        User user = new User();
        long startTime = System.currentTimeMillis();
        for (int i = 0; i < 1000000000; i++) {
    
    
            user.getName();
        }
        long endTime = System.currentTimeMillis();
        System.out.println("普通运行耗时：" + (endTime - startTime) + "ms");
    }

    //通过反射调用
    public static void test2() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
    
    
        User user = new User();
        Class c1 = user.getClass();
        Method getName = c1.getMethod("getName", null);
        long startTime = System.currentTimeMillis();
        for (int i = 0; i < 1000000000; i++) {
    
    
            getName.invoke(user, null);
        }
        long endTime = System.currentTimeMillis();
        System.out.println("通过反射调用耗时：" + (endTime - startTime) + "ms");
    }

    //通过反射调用,关闭检测
    public static void test3() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
    
    
        User user = new User();
        Class c1 = user.getClass();
        Method getName = c1.getMethod("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("通过反射并关闭检测调用耗时：" + (endTime - startTime) + "ms");
    }

    public static void main(String[] args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException {
    
    
        test1();
        test2();
        test3();
    }
}

运行结果如图：

10.获取泛型类型

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 Reflection10 {
    
    
    public void test1(Map<String, User> map, List<User> list) {
    
    
        System.out.println("test1");
    }

    public Map<String, User> test2() {
    
    
        System.out.println("test2");
        return null;
    }

    public static void main(String[] args) throws NoSuchMethodException {
    
    
        Method method = Reflection10.class.getMethod("test1", Map.class, List.class);
        //获得泛型的参数类型
        Type[] genericParameterTypes = method.getGenericParameterTypes();
        for (Type genericParameterType : genericParameterTypes) {
    
    
            System.out.println("#" + genericParameterType);
            //泛型参数类型是否等于结构化参数类型
            if (genericParameterType instanceof ParameterizedType) {
    
    
                Type[] actualTypeArguments = ((ParameterizedType) genericParameterType).getActualTypeArguments();
                for (Type actualTypeArgument : actualTypeArguments) {
    
    
                    System.out.println(actualTypeArgument);
                }
            }
        }

        method = Reflection10.class.getMethod("test2", null);
        Type genericReturnType = method.getGenericReturnType();
        if (genericReturnType instanceof ParameterizedType) {
    
    
            Type[] actualTypeArguments = ((ParameterizedType) genericReturnType).getActualTypeArguments();
            for (Type actualTypeArgument : actualTypeArguments) {
    
    
                System.out.println(actualTypeArgument);
            }
        }


    }
}

运行结果如图：

11.获取注解信息

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

public class Reflectiion11 {
    
    
    public static void main(String[] args) throws Exception {
    
    
        Class c1 = Class.forName("com.test.Reflection.student2");
        System.out.println(c1);
        //通过反射获得注解
        Annotation[] annotations = c1.getAnnotations();
        for (Annotation annotation : annotations) {
    
    
            System.out.println(annotation);
        }

        //获取注解的value的值
        TableTest table = (TableTest) c1.getAnnotation(TableTest.class);
        String value = table.value();
        System.out.println(value);

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

    }
}

@TableTest("db_student")
class student2 {
    
    
    @FieldTest(columnName = "db_name", type = "varchar", length = 3)
    public String name;
    @FieldTest(columnName = "db_id", type = "int", length = 10)
    private int id;
    @FieldTest(columnName = "db_age", type = "int", length = 10)
    private int age;

    public student2() {
    
    
    }

    public student2(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;
    }
}

运行结果如图：