Java学习路线（23）——反射机制

一、概述
（一）什么是反射： 反射指的是任何一个Class类，在“运行时”都可以直接得到全部成分。
（二）动态可获取的对象： 构造器对象——Constructor，成员变量对象——Field，成员方法对象——Method。
（三）反射关键： 第一步都是得到编译后的Class对象，然后可以获得Class全部成分。

---

二、反射相关对象的获取
（一）获取Class对象

//过程：Hello.java -> javac -> Hello.class
Class c = Hello.class;

1、Object.forName(String className) -> 源码阶段获取
2、Object.class -> 内存阶段获取
3、Object.class() -> 实例化阶段获取

public class getClassInfoDemo {
    
    
    public static void main(String[] args) {
    
    
        try {
    
    
            /*forName(类全名)*/
            Class c = Class.forName("com.zengoo.reflected.Student");
            System.out.println(c);
        } catch (ClassNotFoundException e) {
    
    
            throw new RuntimeException(e);
        }

        /*类获取*/
        System.out.println(Student.class);

        /*对象获取*/
        System.out.println(new Student().getClass());
    }
}

class Student{
    
    
    private String id;
    private String name;

    public Student() {
    
    
    }

    public Student(String id, String name) {
    
    
        this.id = id;
        this.name = name;
    }

    public String getId() {
    
    
        return id;
    }

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

    public String getName() {
    
    
        return name;
    }

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

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

/*打印输出*/
class com.zengoo.reflected.Student
class com.zengoo.reflected.Student
class com.zengoo.reflected.Student

（二）获取构造器

1、相关API

方法	说明
Constructor<?>[] getConstructors()	获取public修饰的所有构造器对象
Constructor<?>[] getDecalaredConstructors()	获取所有构造器对象
Constructor<?> getConstructor(Class<T>… parameterTypes)	根据参数类型返回单个public构造器
Constructor<?> getDecalaredConstructor(Class<T>… parameterTypes)	根据参数类型获取单个构造器对象

2、获取构造器示例

public class getClassConstructorDemo {
    
    
    public static void main(String[] args) {
    
    

        Class c = Student.class;

        //获取所有public构造器
        Constructor[] c1 = c.getConstructors();
        System.out.println("=============获取所有public构造器=============");
        for (Constructor constructor :c1) {
    
    
            System.out.println(constructor.getName() + "===>" + constructor.getParameterCount());
        }
        //获取所有构造器
        System.out.println("=============获取所有构造器=============");
        Constructor[] c2 = c.getDeclaredConstructors();
        for (Constructor constructor :c2) {
    
    
            System.out.println(constructor.getName() + "===>" + constructor.getParameterCount());
        }

        //获取单个public构造器对象
        System.out.println("=============获取单个public构造器对象=============");
        try {
    
    
            Constructor c3 = c.getConstructor();
            System.out.println(c3.getName() + "===>" + c3.getParameterCount());
        } catch (NoSuchMethodException e) {
    
    
            throw new RuntimeException(e);
        }

        //获取单个构造器对象
        System.out.println("=============获取单个构造器对象=============");
        try {
    
    
            Constructor c4 = c.getDeclaredConstructor(String.class,String.class);
            System.out.println(c4.getName() + "===>" + c4.getParameterCount());
        } catch (NoSuchMethodException e) {
    
    
            throw new RuntimeException(e);
        }
    }
}

/*打印输出*/
=============获取所有public构造器=============
com.zengoo.reflected.Student===>0
com.zengoo.reflected.Student===>2
=============获取所有构造器=============
com.zengoo.reflected.Student===>0
com.zengoo.reflected.Student===>2
=============获取单个public构造器对象=============
com.zengoo.reflected.Student===>0
=============获取单个构造器对象=============
com.zengoo.reflected.Student===>2

3、获取构造器并调用

（1）作用： 获取构造器的作用是为了初始化一个对象返回。
（2）相关API

方法	说明
T newInstance(Object initargs)	根据指定构造器创建对象
void setAccessible(boolean flag)	设置true表示无视访问限制，暴力反射

（3）使用示例

class useConstructor{
    
    
    public static void main(String[] args) throws Exception {
    
    
        Constructor constructor = Student.class.getConstructor(String.class,String.class);
        constructor.setAccessible(true); //暴力破解私有权限，若构造器为公开权限，则不需要
        Student student = (Student) constructor.newInstance(UUID.randomUUID().toString(),"金牙齿");
        System.out.println(student);
    }
}

/*打印输出*/
Student{
    
    id='77430b37-a65f-4379-b181-437dc5544032', name='金牙齿'}

（三）获取成员变量

1、相关API

方法	说明
Field<?>[] getFields()	获取public修饰的所有成员变量
Field<?>[] getDeclaredFields()	获取所有成员变量
Field<?> getField(String name)	根据参数名称返回单个public成员变量
Field<?> getDecalaredField(String name)	根据参数名称返回单个成员变量
void set(Object obj,Object value)	给对象注入某个成员变量数据
Object get(Object obj)	根据参数名称返回单个成员变量
void setAccessible(boolean b)	暴力开权限
Class getType()	获取属性的类型，返回Class对象
String getName()	获取属性名称

2、使用示例

public class getClassFieldDemo {
    
    
    public static void main(String[] args) throws NoSuchFieldException {
    
    
        Class c = Student.class;

        //获取所有public成员变量
        Field[] fields = c.getFields();
        for (Field f :fields) {
    
    
            System.out.println(f.getType() + " " + f.getName());
        }

        //获取所有成员变量
        Field[] fields1 = c.getDeclaredFields();
        for (Field f :fields1) {
    
    
            System.out.println(f.getType() + " " + f.getName());
        }

        //获取单个public成员变量
        Field field = c.getDeclaredField("name");
        System.out.println(field.getType() + " " + field.getName());

        //获取单个成员变量
        Field field1 = c.getDeclaredField("id");
        System.out.println(field1.getType() + " " + field1.getName());
    }
}

/*打印变量*/
class java.lang.String id
class java.lang.String name
class java.lang.String name
class java.lang.String id

class useField{
    
    
    public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException {
    
    
        Class c = Student.class;

        Field id = c.getDeclaredField("id");
        Field name = c.getDeclaredField("name");

        id.setAccessible(true);
        name.setAccessible(true);

        Student s = new Student();
        id.set(s, UUID.randomUUID().toString());
        name.set(s, "金钟罩");

        System.out.println(id.getType() + " " + id.get(s));
        System.out.println(name.getType() + " " + name.get(s));
    }
}

/*打印输出*/
class java.lang.String d368bc5c-76c9-413a-ad2d-565729b7afe9
class java.lang.String 金钟罩

（四）获取成员方法

1、相关API

方法	说明
Method[] getMethods()	返回所有public修饰的成员方法对象的数组
Method[] getDeclaredMethods()	返回所有成员方法对象的数组
Method getMethod(String name，Class<?>… parameterTypes)	根据方法名，参数列表属性，返回单个public修饰的成员方法对象
Method getDecalaredField(String name，Class<?>… parameterTypes)	根据方法名，参数列表属性，返回单个修饰的成员方法对象
void setAccessible(boolean b)	暴力开权限

2、使用示例

public class getClassMethodDemo {
    
    
    public static void main(String[] args) throws NoSuchMethodException {
    
    
        Class c = Student.class;


        //获取所有public成员方法
        Method[] methods = c.getMethods();
        for (Method method :methods) {
    
    
            System.out.println(method.getGenericParameterTypes().toString() + " " + method.getName());
        }

        //获取所有成员方法
        Method[] methods1 = c.getDeclaredMethods();
        for (Method method :methods1) {
    
    
            System.out.println(method.getGenericParameterTypes().toString() + " " + method.getName());
        }

        //获取单个public成员变量
        Method method = c.getMethod("getName");
        System.out.println(method.getGenericParameterTypes().toString() + " " + method.getName());

        //获取单个成员变量
        Method method1 = c.getMethod("setId", String.class);
        System.out.println(method1.getGenericParameterTypes().toString() + " " + method1.getName());
    }
}

/*打印输出*/
[Ljava.lang.Class;@67b64c45 getName
[Ljava.lang.Class;@68837a77 toString
[Ljava.lang.Class;@6be46e8f setName
[Ljava.lang.Class;@3567135c getId
[Ljava.lang.Class;@327471b5 setId
[Ljava.lang.Class;@4157f54e wait
[Ljava.lang.Class;@90f6bfd wait
[Ljava.lang.Class;@47f6473 wait
[Ljava.lang.Class;@15975490 equals
[Ljava.lang.Class;@6b143ee9 hashCode
[Ljava.lang.reflect.Type;@9f70c54 getClass
[Ljava.lang.Class;@234bef66 notify
[Ljava.lang.Class;@737996a0 notifyAll
[Ljava.lang.Class;@61dc03ce getName
[Ljava.lang.Class;@50f8360d toString
[Ljava.lang.Class;@2cb4c3ab setName
[Ljava.lang.Class;@13c78c0b getId
[Ljava.lang.Class;@12843fce setId
[Ljava.lang.Class;@3dd3bcd getName
[Ljava.lang.Class;@97e1986 setId

3、方法调用

class useMethod{
    
    
    public static void main(String[] args) throws NoSuchMethodException, InvocationTargetException, InstantiationException, IllegalAccessException {
    
    
        Class c = Student.class;
        Constructor constructor = c.getConstructor(String.class,String.class);
        constructor.setAccessible(true);

        Student student = (Student) constructor.newInstance(UUID.randomUUID().toString(),"金环法");

        System.out.println(student);

        Method method = c.getMethod("setName", String.class);

        method.setAccessible(true);
        method.invoke(student,"新名称");

        System.out.println(student);
    }
}

/*打印输出*/
Student{
    
    id='d001810b-88e7-468d-bb8c-bc4129970191', name='金环法'}
Student{
    
    id='d001810b-88e7-468d-bb8c-bc4129970191', name='新名称'}

---

三、反射的作用

（一）绕过编译阶段为集合添加数据（原理是泛型擦除，即强制执行）

class useClassDemo{
    
    
    public static void main(String[] args) throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
    
    
        List<Integer> list1 = new ArrayList<>();
        List<Integer> list2 = new ArrayList<>();

        list1.add(1);
        list1.add(2);
        list1.add(3);

        list2.add(1);
        list2.add(2);
        list2.add(3);

        System.out.println(list1.getClass());
        System.out.println(list2.getClass());

        System.out.println(list1.getClass() == list2.getClass());

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

        Class c = list2.getClass();

        Method method = c.getMethod("add", Object.class);

        method.invoke(list2,"字符串");

        System.out.println(list1.getClass() == list2.getClass());
        System.out.println(list1);
        System.out.println(list2);
    }
}

/*打印输出*/
class java.util.ArrayList
class java.util.ArrayList
true
===========================
true
[1, 2, 3]
[1, 2, 3, 字符串]

（二）通用框架的底层原理（重点）

用反射做通用框架示例

/**
* 需求：给定任意对象，在不清楚对象字段的情况下，将对象的字段名和对应值存储到文件中去。
*/

public class useReflectedToFrame {
    
    
    public static void main(String[] args) {
    
    
      Shop shop = new Shop(UUID.randomUUID().toString(),"店名","xxx市xxx区xxx街道xxx");
      Goods goods = new Goods(UUID.randomUUID().toString(),"商品名",28.5);

      String parentPath = "D:\\JavaBase\\JavaSEpro\\src\\com\\zengoo\\outDir\\";

      TestUtil.save(shop,parentPath+"shop.txt");
      TestUtil.save(goods,parentPath+"goods.txt");
      System.out.println("打印完毕");
    }
}

/*自定义通用框架*/
class TestUtil{
    
    
    public static void save(Object obj,String pathname) {
    
    
        try(PrintStream printStream = new PrintStream(new FileOutputStream(pathname))){
    
    
            //1、提取对象的所有成员变量
            Class c = obj.getClass();

            Field[] fields = c.getDeclaredFields();
            for (Field field:fields) {
    
    
                field.setAccessible(true);
                //2、获取变量名
                String name = field.getName();
                //3、获取变量名对应的值
                String value = " = " + field.get(obj);

                printStream.println(name + value);
            }
        } catch (IllegalAccessException | FileNotFoundException e) {
    
    
            throw new RuntimeException(e);
        }
    }
}

class Shop{
    
    
    private String id;
    private String shopName;
    private String address;

    public Shop() {
    
    
    }

    public Shop(String id, String shopName, String address) {
    
    
        this.id = id;
        this.shopName = shopName;
        this.address = address;
    }

    public String getId() {
    
    
        return id;
    }

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

    public String getShopName() {
    
    
        return shopName;
    }

    public void setShopName(String shopName) {
    
    
        this.shopName = shopName;
    }

    public String getAddress() {
    
    
        return address;
    }

    public void setAddress(String address) {
    
    
        this.address = address;
    }

    @Override
    public String toString() {
    
    
        return "Shop{" +
                "id='" + id + '\'' +
                ", shopName='" + shopName + '\'' +
                ", address='" + address + '\'' +
                '}';
    }
}

class Goods{
    
    
    private String id;
    private String goodsName;
    private double price;

    public Goods() {
    
    
    }

    public Goods(String id, String goodsName, double price) {
    
    
        this.id = id;
        this.goodsName = goodsName;
        this.price = price;
    }

    public String getId() {
    
    
        return id;
    }

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

    public String getGoodsName() {
    
    
        return goodsName;
    }

    public void setGoodsName(String goodsName) {
    
    
        this.goodsName = goodsName;
    }

    public double getPrice() {
    
    
        return price;
    }

    public void setPrice(double price) {
    
    
        this.price = price;
    }


    @Override
    public String toString() {
    
    
        return "Goods{" +
                "id='" + id + '\'' +
                ", goodsName='" + goodsName + '\'' +
                ", price=" + price +
                '}';
    }
}

shop.txt

id = cd23f6eb-91c3-4f19-adf5-654ef4602b2e
shopName = 店名
address = xxx市xxx区xxx街道xxx

goods.txt

id = 77d46cea-04ec-45dd-b806-f39179bfa37e
goodsName = 商品名
price = 28.5