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

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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

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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

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

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

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

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

public class SweetShop {

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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\")");
}

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.classBoolean.TYPEchar.classCharacter.TYPEbyte.classByte.TYPEshort.classShort.TYPEint.classInteger.TYPElong.classLong.TYPEfloat.classFloat.TYPEdouble.classDouble.TYPEvoid.classVoid.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

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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());
}
}

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