详解Java反射创建对象

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;
    }
}

3.3 getConstructor()和getDeclaredConstructor()区别

getDeclaredConstructor(Class<?>… parameterTypes) 

这个方法会返回指定参数类型的所有构造器，包括public的和非public的，当然也包括private的。getDeclaredConstructors()的返回结果就没有参数类型的过滤了。

再来看getConstructor(Class<?>… parameterTypes)

这个方法返回的是上面那个方法返回结果的子集，只返回指定参数类型访问权限是public的构造器。getConstructors()的返回结果同样也没有参数类型的过滤。

参考文章