详解Java反射创建对象

时间：2021-12-15 09:13:34|栏目：JAVA代码|点击：次

一、什么是反射

Java Reflaction in Action中的解释：反射是运行中的程序检查自己和软件运行环境的能力，它可以根据它发现的进行改变。通俗的讲就是反射可以在运行时根据指定的类名获得类的信息个人理解：就是我们对于创建对象我们除了通过 new关键字创建外，还能通过什么创建呢？private的属属性真的不能获取吗？反射就能做到打破这些所谓的规则反射和new创建对象谁的效率高？ new

二、通过类对象调用newInstance()方法，适用于无参构造方法

2.1 类名.class

public class Main {
 
    public static void main(String[] args) throws IllegalAccessException, InstantiationException {
        Class<Person> clazz = Person.class;
        Person person = clazz.newInstance();
        System.out.println(person instanceof Person); // true
    }
}
 
class Person {
 
    private Integer age;
 
    private String name;
 
    public Person() {
    }
}

2.2 Class.forName

public class Main {
 
    public static void main(String[] args) throws IllegalAccessException, InstantiationException, ClassNotFoundException {
        Class<?> clazz = Class.forName("com.best.test.Person");
        Person person = (Person) clazz.newInstance();
        System.out.println(person instanceof Person); // true
    }
}
 
class Person {
 
    private Integer age;
 
    private String name;
 
    public Person() {
    }
}

2.3 对象名.getClass

public class Main {
 
    public static void main(String[] args) throws IllegalAccessException, InstantiationException{
        Person person = new Person();
        Class<? extends Person> clazz = person.getClass();
        Person person1 = clazz.newInstance();
        System.out.println(person1 instanceof Person); // true
    }
}
 
class Person {
 
    private Integer age;
 
    private String name;
 
    public Person() {
    }
}

三、getConstructor()和getDeclaredConstructor()

通过类对象的getConstructor()或getDeclaredConstructor()方法获得构造器（Constructor）对象并调用其newInstance()方法创建对象，适用于无参和有参构造方法。

3.1 getConstructor()

public class Main {
 
    public static void main(String[] args) throws IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException {
        Class<Person> clazz = Person.class;
        Constructor<Person> ctor = clazz.getConstructor(Integer.class, String.class);
        Person person = ctor.newInstance(26, "jak");
        System.out.println(person instanceof Person); // true
    }
}
 
class Person {
 
    private Integer age;
 
    private String name;
 
    public Person(Integer age, String name) {
        this.age = age;
        this.name = name;
    }
}

3.2 getDeclaredConstructor()

public class Main {
 
    public static void main(String[] args) throws IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException {
        Class<Person> clazz = Person.class;
        Constructor<Person> ctor = clazz.getDeclaredConstructor(String.class);
        Person person = ctor.newInstance("jak");
        
        System.out.println(person instanceof Person); // true
    }
}
 
class Person {
 
    private Integer age;
 
    private String name;
 
    public Person(Integer age, String name) {
        this.age = age;
        this.name = name;
    }
 
    public Person(String name) {
        this.name = name;
    }
}