java 运行时类型识别(RTTI) - 1 - Class与instanceof

三种方式:

1.传统类型转换

2.查询Class对象

3.instanceof

1.传统类型转换

String string = (String) s;

2.查询Class对象

首先需要了解一下java.lang.Class这个类

类型信息在运行时的表示是有Class这个类的实例完成的

java使用Class对象来执行其RTTI,即使是类似转型的这样的操作

每个类都有一个Class对象(被保存在一个同名的.class文件中),为了生成这个类的对象,运行这个程序的JVM将使用被称为"类加载器"的子系统

(有关类加载器的内容,稍后介绍)

一旦某个类的Class对象被载入内存,它就被用来创建这个类的所有对象

static{
//...
}

static{
    //...
}

这些代码在类第一次加载时执行

下面看看think in java中的示例代码

publicclassCandy{
static{
System.out.println("LoadingCandy");
}
}

public class Candy {
    static {
        System.out.println("Loading Candy");
    }
}

publicclassGum{
static{
System.out.println("LoadingGum");
}
}

public class Gum {
    static {
        System.out.println("Loading Gum");
    }
}

public class SweetShop {

publicstaticvoidmain(String[]args){
System.out.println("insidemain");
newCandy();
System.out.println("AftercreatingCandy");
try{
Class.forName("Gum");
}catch(ClassNotFoundExceptione){
e.printStackTrace();
}
System.out.println("AfterClass.forName(\"Gum\")");
}

	public static void main(String[] args) {
		System.out.println("inside main");
		new Candy();
		System.out.println("After creating Candy");
		try {
			Class.forName("Gum");
		} catch(ClassNotFoundException e) {
			e.printStackTrace();
		}
		System.out.println("After Class.forName(\"Gum\")");
	}
}

运行结果

inside main

Loading Candy

After creating Candy

Loading Gum

After Class.forName("Gum")

Class仅在需要时加载,不然Loading Candy和Loading Gum应该在前面就输出了

这里要注意下Class.forName("Gum");

从log中看出,这句话也使得类Gum被加载了

生成Class引用可以用下面两种方式

1.newInstance()

newInstance使用的时候需要注意:

a.类需要有默认的构造器才可以,不然则会出现java.lang.InstantiationException

b.默认构造器必须能访问才行,不然会出现java.lang.IllegalAccessException

2.类字面常量

使用类字面量常量更简单,更安全:因为它在编译时被检查,所以也不需要try catch

并且使用类字面量不会初始化该Class

类字面量可以用于类、接口、数组、基本类型

对于基本数据类型的包装器类,有一个标准字段TYPE.TYPE是一个引用,指向对应的基本数据类型的Class

...等价于...
boolean.class	Boolean.TYPE
char.class	Character.TYPE
byte.class	Byte.TYPE
short.class	Short.TYPE
int.class	Integer.TYPE
long.class	Long.TYPE
float.class	Float.TYPE
double.class	Double.TYPE
void.class	Void.TYPE

3.instanceof

我们可以使用关键字instanceof来做类型检查(x instanceof Gum),它返回一个boolean

我们也可以使用动态的boolean java.lang.Class.isInstance(Object obj)来做类型检查

instanceof与Class的等价性

publicclassTest{
staticclassBase{}
staticclassDerivedextendsBase{}

staticvoidtest(Objectx){
System.out.println("Testingxoftype"+x.getClass());
System.out.println("xinstanceofBase"+(xinstanceofBase));
System.out.println("xinstanceofDerived"+(xinstanceofDerived));
System.out.println("Base.isInstance(x)"+(Base.class.isInstance(x)));
System.out.println("Derived.isInstance(x)"+(Derived.class.isInstance(x)));
System.out.println("x.getClass()==Base.class"+(x.getClass()==Base.class));
System.out.println("x.getClass()==Derived.class"+(x.getClass()==Derived.class));
System.out.println("x.getClass().equals(Base.class)"+(x.getClass().equals(Base.class)));
System.out.println("x.getClass().equals(Derived.class)"+(x.getClass().equals(Derived.class)));
}

publicstaticvoidmain(String[]args){
Test.test(newBase());
System.err.println("----------------------------");
Test.test(newDerived());
}
}

public class Test {
	static class Base{}
	static class Derived extends Base{}
	
	static void test(Object x){
		System.out.println("Testing x of type " + x.getClass());
		System.out.println("x instanceof Base " + (x instanceof Base));
		System.out.println("x instanceof Derived " + (x instanceof Derived));
		System.out.println("Base.isInstance(x) " + (Base.class.isInstance(x)));
		System.out.println("Derived.isInstance(x) " + (Derived.class.isInstance(x)));
		System.out.println("x.getClass() == Base.class " + (x.getClass() == Base.class));
		System.out.println("x.getClass() == Derived.class " + (x.getClass() == Derived.class));
		System.out.println("x.getClass().equals(Base.class) " + (x.getClass().equals(Base.class)));
		System.out.println("x.getClass().equals(Derived.class) " + (x.getClass().equals(Derived.class)));
	}
	
	public static void main(String[] args) {
		Test.test(new Base());
		System.err.println("----------------------------");
		Test.test(new Derived());
	}
}

结果

Testing x of type class Test$Base

x instanceof Base true

x instanceof Derived false

Base.isInstance(x) true

Derived.isInstance(x) false

x.getClass() == Base.class true

x.getClass() == Derived.class false

x.getClass().equals(Base.class) true

x.getClass().equals(Derived.class) false

----------------------------

Testing x of type class Test$Derived

x instanceof Base true

x instanceof Derived true

Base.isInstance(x) true

Derived.isInstance(x) true

x.getClass() == Base.class false

x.getClass() == Derived.class true

x.getClass().equals(Base.class) false

x.getClass().equals(Derived.class) true

可以看出:instanceof与isInstance()结果一样,equals和==的结果一样

instanceof是类型检查,==是对象比较,不考虑继承等关系.

个人觉得比较好理解

类加载器

这里只贴出笔记,之后在总结虚拟机的时候再详细说明,会在这里给出链接