詳解Java反射創(chuàng)建對象

更新時(shí)間：2021年05月31日 16:27:00 作者：星夜孤帆

今天帶大家學(xué)習(xí)Java的基礎(chǔ)知識,文中對Java反射創(chuàng)建對象作了非常詳細(xì)的介紹及代碼示例,對正在學(xué)習(xí)Java的小伙伴們很有幫助,需要的朋友可以參考下

一、什么是反射

Java Reflaction in Action中的解釋：反射是運(yùn)行中的程序檢查自己和軟件運(yùn)行環(huán)境的能力，它可以根據(jù)它發(fā)現(xiàn)的進(jìn)行改變。通俗的講就是反射可以在運(yùn)行時(shí)根據(jù)指定的類名獲得類的信息個(gè)人理解：就是我們對于創(chuàng)建對象我們除了通過 new關(guān)鍵字創(chuàng)建外，還能通過什么創(chuàng)建呢？private的屬屬性真的不能獲取嗎？反射就能做到打破這些所謂的規(guī)則反射和new創(chuàng)建對象誰的效率高？ new

二、通過類對象調(diào)用newInstance()方法，適用于無參構(gòu)造方法

2.1 類名.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 對象名.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()

通過類對象的getConstructor()或getDeclaredConstructor()方法獲得構(gòu)造器（Constructor）對象并調(diào)用其newInstance()方法創(chuàng)建對象，適用于無參和有參構(gòu)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;
    }
}