Java之Class.forName方法详解

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一、前言
二、案例
三、详解

一、前言：

在说明Class类的静态方法forName()之前，先清楚有关Class类的几个概念：

1、 Class类封装了类或接口的运行时状态

Java程序在运行时，Java运行时系统一直对所有的对象进行所谓的运行时类型标识，这些标示纪录了每个对象所属的类。
虚拟机通常使用运行时类型信息选择正确方法去执行，用来保存这些类型信息的类是Class类。

2、Class类型的对象，是加载类时自动创建的

Class 没有公共构造方法。Class 对象是在加载类时，由Java 虚拟机以及通过调用类加载器中的 defineClass 方法自动构造的，因此不能显式地声明一个Class对象。

3、虚拟机为每种类型管理一个独一无二的Class对象

每个类（型）都有一个Class对象。
运行程序时，Java虚拟机(JVM)首先检查所要加载的类对应的Class对象是否已经加载。如果没有加载，JVM就会根据类名查找.class文件，并将其Class对象载入。

1.基本的 Java 类型（boolean、byte、char、short、int、long、float 和 double）和关键字 void 也都对应一个 Class 对象。
2.每个数组属于被映射为 Class 对象的一个类，所有具有相同元素类型和维数的数组都共享该 Class 对象。
3.一般某个类的Class对象被载入内存，它就用来创建这个类的所有对象。

以上说法查看Class源码会发现，Book.class.getName()最终调用的：

class="prism language-class="tags" href="/tags/JAVA.html" title=java>java">class="token keyword">private class="token keyword">transient class="token class-name">String nameclass="token punctuation">;
class="token keyword">public class="token class-name">String class="token function">getNameclass="token punctuation">(class="token punctuation">) class="token punctuation">{
    class="token class-name">String name class="token operator">= class="token keyword">thisclass="token punctuation">.nameclass="token punctuation">;
    class="token keyword">if class="token punctuation">(name class="token operator">== class="token keyword">nullclass="token punctuation">)
        class="token keyword">thisclass="token punctuation">.name class="token operator">= name class="token operator">= class="token function">getName0class="token punctuation">(class="token punctuation">)class="token punctuation">;
    class="token keyword">return nameclass="token punctuation">;
class="token punctuation">}

此时Book也是一个独一无二的Class对象，即对象中的对象。

二、案例：

Book.class="tags" href="/tags/JAVA.html" title=java>java类

class="prism language-class="tags" href="/tags/JAVA.html" title=java>java">class="token keyword">package class="token namespace">comclass="token punctuation">.junitclass="token punctuation">.democlass="token punctuation">;

class="token keyword">public class="token keyword">class class="token class-name">Book class="token punctuation">{
    class="token keyword">private class="token keyword">static class="token keyword">final class="token class-name">String defName class="token operator">= class="token string">"《程序猿植发》"class="token punctuation">;

    class="token keyword">static class="token punctuation">{
        class="token class-name">Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token string">"我是静态代码块，输出: " class="token operator">+ defNameclass="token punctuation">)class="token punctuation">;
    class="token punctuation">}

    class="token comment">//打印生产日期：
    class="token keyword">public class="token keyword">static class="token class-name">String class="token function">printProduceDateclass="token punctuation">(class="token class-name">String nameclass="token punctuation">) class="token punctuation">{
        class="token keyword">return class="token string">"我是静态方法printProduceDate，输出: " class="token operator">+ name class="token operator">+ class="token string">", produce is:" class="token operator">+ class="token class-name">Systemclass="token punctuation">.class="token function">currentTimeMillisclass="token punctuation">(class="token punctuation">)class="token punctuation">;
    class="token punctuation">}

    class="token keyword">private class="token class-name">String nameclass="token punctuation">;

    class="token keyword">public class="token class-name">Bookclass="token punctuation">(class="token punctuation">) class="token punctuation">{
        class="token class-name">Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token string">"我是Book声明的构造方法！"class="token punctuation">)class="token punctuation">;
        name class="token operator">= defNameclass="token punctuation">;
    class="token punctuation">}

    class="token keyword">public class="token class-name">String class="token function">toStringclass="token punctuation">(class="token class-name">String msgclass="token punctuation">) class="token punctuation">{
        class="token keyword">return name class="token operator">+ msgclass="token punctuation">;
    class="token punctuation">}
class="token punctuation">}

执行方法：

class="prism language-class="tags" href="/tags/JAVA.html" title=java>java">class="token keyword">public class="token keyword">static class="token keyword">void class="token function">mainclass="token punctuation">(class="token class-name">Stringclass="token punctuation">[class="token punctuation">] argsclass="token punctuation">) class="token keyword">throws class="token class-name">ClassNotFoundExceptionclass="token punctuation">, class="token class-name">NoSuchMethodExceptionclass="token punctuation">, class="token class-name">InvocationTargetExceptionclass="token punctuation">, class="token class-name">IllegalAccessExceptionclass="token punctuation">, class="token class-name">InstantiationException class="token punctuation">{
    class="token comment">//1-将指定类加载到JVM中（ClassNotFoundException）
    class="token class-name">Class aClass class="token operator">= class="token class-name">Classclass="token punctuation">.class="token function">forNameclass="token punctuation">(class="token string">"com.junit.demo.Book"class="token punctuation">)class="token punctuation">;
    class="token class-name">Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(aClassclass="token punctuation">)class="token punctuation">;
    class="token comment">//2.1-访问静态方法：NoSuchMethodException，InvocationTargetException
    class="token class-name">Method method class="token operator">= aClassclass="token punctuation">.class="token function">getMethodclass="token punctuation">(class="token string">"printProduceDate"class="token punctuation">, class="token class-name">Stringclass="token punctuation">.class="token keyword">classclass="token punctuation">)class="token punctuation">;
    class="token class-name">String result class="token operator">= class="token punctuation">(class="token class-name">Stringclass="token punctuation">) methodclass="token punctuation">.class="token function">invokeclass="token punctuation">(aClassclass="token punctuation">, class="token string">"《程序猿的颈椎自传》"class="token punctuation">)class="token punctuation">;
    class="token class-name">Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(resultclass="token punctuation">)class="token punctuation">;
    class="token class-name">Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token string">"---------------------------\n"class="token punctuation">)class="token punctuation">;
    class="token comment">//2.2-初始化对象：
    class="token class-name">Book obj class="token operator">= class="token punctuation">(class="token class-name">Bookclass="token punctuation">) aClassclass="token punctuation">.class="token function">newInstanceclass="token punctuation">(class="token punctuation">)class="token punctuation">;
    class="token class-name">Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token string">"得到对象后访问get方法：" class="token operator">+ objclass="token punctuation">.class="token function">toStringclass="token punctuation">(class="token string">""class="token punctuation">)class="token punctuation">)class="token punctuation">;
    class="token class-name">Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token string">"---------------------------\n"class="token punctuation">)class="token punctuation">;
    class="token comment">//2.3-初始化对象后访问方法：
    class="token class-name">Method method3 class="token operator">= aClassclass="token punctuation">.class="token function">getMethodclass="token punctuation">(class="token string">"toString"class="token punctuation">, class="token class-name">Stringclass="token punctuation">.class="token keyword">classclass="token punctuation">)class="token punctuation">;
    class="token class-name">String result3 class="token operator">= class="token punctuation">(class="token class-name">Stringclass="token punctuation">) method3class="token punctuation">.class="token function">invokeclass="token punctuation">(aClassclass="token punctuation">.class="token function">newInstanceclass="token punctuation">(class="token punctuation">)class="token comment">/*obj*/class="token punctuation">, class="token string">"这本书是我的伙伴！"class="token punctuation">)class="token punctuation">;
    class="token class-name">Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(result3class="token punctuation">)class="token punctuation">;
    class="token class-name">Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token string">"---------------------------\n"class="token punctuation">)class="token punctuation">;

    class="token class-name">Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token class-name">Bookclass="token punctuation">.class="token keyword">classclass="token punctuation">.class="token function">getNameclass="token punctuation">(class="token punctuation">)class="token punctuation">)class="token punctuation">;
class="token punctuation">}

输出：

class="prism language-class="tags" href="/tags/JAVA.html" title=java>java">我是静态代码块，输出class="token operator">: 《程序猿植发》
class="token keyword">class class="token class-name">class="token namespace">comclass="token punctuation">.junitclass="token punctuation">.democlass="token punctuation">.Book
我是静态方法printProduceDate，输出class="token operator">: 《程序猿的颈椎自传》class="token punctuation">, produce isclass="token operator">:class="token number">1626682894095
class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">-

我是class="token class-name">Book声明的构造方法！
得到对象后访问get方法：《程序猿植发》
class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">-

我是class="token class-name">Book声明的构造方法！
《程序猿植发》这本书是我的伙伴！
class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">--class="token operator">-

class="token class-name">class="token namespace">comclass="token punctuation">.junitclass="token punctuation">.democlass="token punctuation">.Book

三、详解：

对于任何一个类，都能知道这个类所有的属性和方法；对于任何一个对象，都能够调用它的任意方法和属性；这种动态获取信息以及动态调用对象方法的功能称为Java语言的class="tags" href="/tags/FanShe.html" title=反射>反射机制。

getMethod()返回某个类的所有公用（public）方法包括其继承类的公用方法，当然也包括它所实现接口的方法。

getDeclaredMethod()对象表示的类或接口声明的所有方法,包括公共、保护、默认（包）访问和私有方法，但不包括继承的方法。当然也包括它所实现接口的方法。

详细举例：

class="prism language-class="tags" href="/tags/JAVA.html" title=java>java">class="token class-name">Class aClass class="token operator">= class="token class-name">Classclass="token punctuation">.class="token function">forNameclass="token punctuation">(class="token string">"com.junit.demo.Book"class="token punctuation">)class="token punctuation">;

1、访问静态方法：

class="prism language-class="tags" href="/tags/JAVA.html" title=java>java">class="token comment">// 由Class获取方法：第一个参数为方法名，第二个参数为方法的参数类型。
class="token comment">// 如add(int a,int b)则getMethod("add",int.class,int.class)。当然，也可以是Java对象。
class="token class-name">Method method class="token operator">= aClassclass="token punctuation">.class="token function">getMethodclass="token punctuation">(class="token string">"printProduceDate"class="token punctuation">, class="token class-name">Stringclass="token punctuation">.class="token keyword">classclass="token punctuation">)class="token punctuation">;
class="token comment">// 引用方法：(引用实例/调用静态方法可为null，参数值/有多个用逗号隔开)，参数值要和参数类型的数量匹配！
class="token class-name">String result class="token operator">= class="token punctuation">(class="token class-name">Stringclass="token punctuation">) methodclass="token punctuation">.class="token function">invokeclass="token punctuation">(aClassclass="token punctuation">, class="token string">"《程序猿的颈椎自传》"class="token punctuation">)class="token punctuation">;

简写：

class="prism language-class="tags" href="/tags/JAVA.html" title=java>java">aClassclass="token punctuation">.class="token function">getMethodclass="token punctuation">(class="token string">"printProduceDate"class="token punctuation">, class="token class-name">Stringclass="token punctuation">.class="token keyword">classclass="token punctuation">)class="token punctuation">.class="token function">invokeclass="token punctuation">(class="token keyword">nullclass="token punctuation">, class="token string">"《程序猿的颈椎自传》"class="token punctuation">)class="token punctuation">;

2、访问实例方法：

重要：aClass.newInstance();，实例化指定对象。
和 new Book() 效果一样。

class="prism language-class="tags" href="/tags/JAVA.html" title=java>java">class="token comment">//方法一：直接转化实例化后的对象，直接调用方法
class="token class-name">Book bookclass="token operator">= class="token punctuation">(class="token class-name">Bookclass="token punctuation">) aClassclass="token punctuation">.class="token function">newInstanceclass="token punctuation">(class="token punctuation">)class="token punctuation">;
class="token comment">// book.setName('xxx'); or book.getName(); or more...

class="token comment">//方法二：使用invoke调用指定实例a的指定方法b
class="token class-name">Method method3 class="token operator">= aClassclass="token punctuation">.class="token function">getMethodclass="token punctuation">(class="token string">"toString"class="token punctuation">, class="token class-name">Stringclass="token punctuation">.class="token keyword">classclass="token punctuation">)class="token punctuation">;
class="token comment">//这里的book可以是已实例化的对象，或者使用 aClass.newInstance() 传入，详见简写：
class="token class-name">String result3 class="token operator">= class="token punctuation">(class="token class-name">Stringclass="token punctuation">) method3class="token punctuation">.class="token function">invokeclass="token punctuation">(bookclass="token punctuation">, class="token string">"这本书是我的伙伴！"class="token punctuation">)class="token punctuation">;

简写：

class="prism language-class="tags" href="/tags/JAVA.html" title=java>java">aClassclass="token punctuation">.class="token function">getMethodclass="token punctuation">(class="token string">"toString"class="token punctuation">, class="token class-name">Stringclass="token punctuation">.class="token keyword">classclass="token punctuation">)class="token punctuation">.class="token function">invokeclass="token punctuation">(aClassclass="token punctuation">.class="token function">newInstanceclass="token punctuation">(class="token punctuation">)class="token punctuation">, class="token string">"xxx"class="token punctuation">)class="token punctuation">;

值得注意的是，如果是类似于工具类可用于全部类访问的，可以使用一个实例化对象，而不需要每次都newInstance。

另外，方法一适用于需要映射的类是已知或少数时，反之需要统一按指定字符串class="tags" href="/tags/FanShe.html" title=反射>反射调用方法的话，需使用方法二。

3、访问私有方法：

一睹为快：

class="prism language-class="tags" href="/tags/JAVA.html" title=java>java">

举例：

class="prism language-class="tags" href="/tags/JAVA.html" title=java>java">class="token keyword">package class="token namespace">comclass="token punctuation">.junitclass="token punctuation">.democlass="token punctuation">;

class="token keyword">public class="token keyword">class class="token class-name">Netbar class="token punctuation">{
    class="token keyword">private class="token class-name">Double random class="token operator">= class="token class-name">Mathclass="token punctuation">.class="token function">randomclass="token punctuation">(class="token punctuation">)class="token punctuation">;

    class="token keyword">public class="token class-name">Double class="token function">getRandomclass="token punctuation">(class="token punctuation">) class="token punctuation">{
        class="token keyword">return randomclass="token punctuation">;
    class="token punctuation">}

    class="token keyword">private class="token class-name">Netbar class="token function">createNewclass="token punctuation">(class="token punctuation">) class="token punctuation">{
        class="token keyword">return class="token keyword">new class="token class-name">Netbarclass="token punctuation">(class="token punctuation">)class="token punctuation">;
    class="token punctuation">}

    class="token keyword">private class="token keyword">static class="token class-name">Netbar class="token function">getInstanceclass="token punctuation">(class="token punctuation">) class="token punctuation">{
        class="token keyword">return class="token keyword">new class="token class-name">Netbarclass="token punctuation">(class="token punctuation">)class="token punctuation">;
    class="token punctuation">}

    class="token keyword">public class="token keyword">static class="token keyword">void class="token function">mainclass="token punctuation">(class="token class-name">Stringclass="token punctuation">[class="token punctuation">] argsclass="token punctuation">) class="token punctuation">{
        class="token class-name">Netbar netbar class="token operator">= class="token keyword">new class="token class-name">Netbarclass="token punctuation">(class="token punctuation">)class="token punctuation">;
        class="token class-name">Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token string">"Random:: " class="token operator">+ netbarclass="token punctuation">.class="token function">getRandomclass="token punctuation">(class="token punctuation">)class="token punctuation">)class="token punctuation">;
        class="token class-name">Netbar netbar2 class="token operator">= netbarclass="token punctuation">.class="token function">createNewclass="token punctuation">(class="token punctuation">)class="token punctuation">;
        class="token class-name">Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token string">"Random2:: " class="token operator">+ netbar2class="token punctuation">.class="token function">getRandomclass="token punctuation">(class="token punctuation">)class="token punctuation">)class="token punctuation">;
        class="token class-name">Netbar netbar3 class="token operator">= class="token class-name">Netbarclass="token punctuation">.class="token function">getInstanceclass="token punctuation">(class="token punctuation">)class="token punctuation">;
        class="token class-name">Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token string">"Random3:: " class="token operator">+ netbar3class="token punctuation">.class="token function">getRandomclass="token punctuation">(class="token punctuation">)class="token punctuation">)class="token punctuation">;
    class="token punctuation">}
class="token punctuation">}

结果为打印的3个值都是不同的，这是正常的访问方式，下面用class="tags" href="/tags/FanShe.html" title=反射>反射来访问。

获取私有非静态方法：

class="prism language-class="tags" href="/tags/JAVA.html" title=java>java">class="token keyword">public class="token keyword">static class="token keyword">void class="token function">mainclass="token punctuation">(class="token class-name">Stringclass="token punctuation">[class="token punctuation">] argsclass="token punctuation">) class="token keyword">throws class="token class-name">Exception class="token punctuation">{
    class="token class-name">Class aClass class="token operator">= class="token class-name">Classclass="token punctuation">.class="token function">forNameclass="token punctuation">(class="token string">"com.junit.demo.Netbar"class="token punctuation">)class="token punctuation">;
    class="token class-name">Method method class="token operator">= aClassclass="token punctuation">.class="token function">getDeclaredMethodclass="token punctuation">(class="token string">"createNew"class="token punctuation">)class="token punctuation">;class="token comment">//这里可以带参数
    methodclass="token punctuation">.class="token function">setAccessibleclass="token punctuation">(class="token boolean">trueclass="token punctuation">)class="token punctuation">;class="token comment">//对所有属性设置访问权限  当类中的成员变量为private时 必须设置此项
    class="token class-name">Netbar netbar class="token operator">= class="token punctuation">(class="token class-name">Netbarclass="token punctuation">) methodclass="token punctuation">.class="token function">invokeclass="token punctuation">(aClassclass="token punctuation">.class="token function">newInstanceclass="token punctuation">(class="token punctuation">)class="token punctuation">)class="token punctuation">;
    class="token class-name">Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token string">"Random2:: " class="token operator">+ netbarclass="token punctuation">.class="token function">getRandomclass="token punctuation">(class="token punctuation">)class="token punctuation">)class="token punctuation">;
class="token punctuation">}

获取私有静态方法：

class="prism language-class="tags" href="/tags/JAVA.html" title=java>java">class="token keyword">public class="token keyword">static class="token keyword">void class="token function">mainclass="token punctuation">(class="token class-name">Stringclass="token punctuation">[class="token punctuation">] argsclass="token punctuation">) class="token keyword">throws class="token class-name">Exception class="token punctuation">{
    class="token class-name">Class aClass class="token operator">= class="token class-name">Classclass="token punctuation">.class="token function">forNameclass="token punctuation">(class="token string">"com.junit.demo.Netbar"class="token punctuation">)class="token punctuation">;
    class="token class-name">Method method class="token operator">= aClassclass="token punctuation">.class="token function">getDeclaredMethodclass="token punctuation">(class="token string">"getInstance"class="token punctuation">)class="token punctuation">;class="token comment">//这里可以带参数
    methodclass="token punctuation">.class="token function">setAccessibleclass="token punctuation">(class="token boolean">trueclass="token punctuation">)class="token punctuation">;class="token comment">//对所有属性设置访问权限  当类中的成员变量为private时 必须设置此项
    class="token class-name">Netbar netbar class="token operator">= class="token punctuation">(class="token class-name">Netbarclass="token punctuation">) methodclass="token punctuation">.class="token function">invokeclass="token punctuation">(class="token keyword">nullclass="token punctuation">)class="token punctuation">;
    class="token class-name">Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token string">"Random2:: " class="token operator">+ netbarclass="token punctuation">.class="token function">getRandomclass="token punctuation">(class="token punctuation">)class="token punctuation">)class="token punctuation">;
class="token punctuation">}