Java反射创建对象详解Java反射创建对象

时间:2021-05-31 星夜孤帆人气:0

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一、通过类对象调用newInstance()方法，适用于无参构造方法

1.1 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() {
    }
}

1.2 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() {
    }
}

1.3 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() {
    }
}

二、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。

2.1 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;
    }
}

2.2 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;
    }
}

2.3 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。

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Java反射创建对象 详解Java反射创建对象