深入Java虚拟机（番外篇）ClassLoader 初探

news/2024/5/19 3:02:08 标签: jvm, java, class, 反射
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本篇文章是在深入Java虚拟机（四）的基础上产生的一些想法，从另一个方面研究下类加载问题。

面向接口编程

在项目工程目录定义了一个获取钱包余额的接口，而作为普通人会返回50大洋，供你吃喝玩乐。

class="prism language-class="tags" href="/tags/JAVA.html" title=java>java">class="token keyword">public class="token keyword">interface class="token class-name">Pocket class="token punctuation">{
    class="token keyword">int class="token function">getBalanceclass="token punctuation">(class="token punctuation">)class="token punctuation">;
class="token punctuation">}
class="token keyword">public class="token keyword">class class="token class-name">NormalUser class="token keyword">implements class="token class-name">Pocketclass="token punctuation">{
    class="token annotation punctuation">@Override
    class="token keyword">public class="token keyword">int class="token function">getBalanceclass="token punctuation">(class="token punctuation">) class="token punctuation">{
        class="token keyword">return class="token number">50class="token punctuation">;
    class="token punctuation">}
class="token punctuation">}

我在桌面编译了另一个版本的NormalUser，给我的钱包提提额度。

class="prism language-class="tags" href="/tags/JAVA.html" title=java>java">class="token keyword">public class="token keyword">class class="token class-name">NormalUser class="token keyword">implements class="token class-name">Pocketclass="token punctuation">{
    class="token annotation punctuation">@Override
    class="token keyword">public class="token keyword">int class="token function">getBalanceclass="token punctuation">(class="token punctuation">) class="token punctuation">{
        class="token keyword">return class="token number">50000class="token punctuation">;
    class="token punctuation">}
class="token punctuation">}

现在我想改善下生活品质，于是我做了下面的事情:

在项目中自定义一个类加载器：

class="prism language-class="tags" href="/tags/JAVA.html" title=java>java">class="token keyword">public class="token keyword">class class="token class-name">LocalClassLoader class="token keyword">extends class="token class-name">ClassLoader class="token punctuation">{
    class="token annotation punctuation">@Override
    class="token keyword">protected Classclass="token operator"><class="token operator">?class="token operator">> class="token function">findClassclass="token punctuation">(String nameclass="token punctuation">) class="token keyword">throws ClassNotFoundException class="token punctuation">{
        String path class="token operator">= class="token string">"/Users/lijie/Desktop/NormalUser.class"class="token punctuation">;
        class="token keyword">try class="token punctuation">{
            FileInputStream ins class="token operator">= class="token keyword">new class="token class-name">FileInputStreamclass="token punctuation">(pathclass="token punctuation">)class="token punctuation">;
            class="token keyword">int size class="token operator">= insclass="token punctuation">.class="token function">availableclass="token punctuation">(class="token punctuation">)class="token punctuation">;
            class="token keyword">byteclass="token punctuation">[class="token punctuation">] clazzBytes class="token operator">= class="token keyword">new class="token class-name">byteclass="token punctuation">[sizeclass="token punctuation">]class="token punctuation">;
            class="token keyword">if class="token punctuation">(insclass="token punctuation">.class="token function">readclass="token punctuation">(clazzBytesclass="token punctuation">) class="token operator">> class="token number">0class="token punctuation">) class="token punctuation">{
                class="token keyword">return class="token function">defineClassclass="token punctuation">(class="token string">"hua.lee.classloader.NormalUser"class="token punctuation">, clazzBytesclass="token punctuation">, class="token number">0class="token punctuation">, sizeclass="token punctuation">)class="token punctuation">;
            class="token punctuation">} class="token keyword">else class="token punctuation">{
                class="token keyword">throw class="token keyword">new class="token class-name">ClassNotFoundExceptionclass="token punctuation">(class="token punctuation">)class="token punctuation">;
            class="token punctuation">}
        class="token punctuation">} class="token keyword">catch class="token punctuation">(class="token class-name">IOException eclass="token punctuation">) class="token punctuation">{
            eclass="token punctuation">.class="token function">printStackTraceclass="token punctuation">(class="token punctuation">)class="token punctuation">;
        class="token punctuation">}
        class="token keyword">throw class="token keyword">new class="token class-name">ClassNotFoundExceptionclass="token punctuation">(class="token punctuation">)class="token punctuation">;
    class="token punctuation">}
class="token punctuation">}

LocalClassLoader写的比较死板，但是挣钱的思路是有了的。。。。

LocalClassLoader重写了 findClass方法，没有重写loadClass，这样双亲委托机制还有效。对于系统中查找不到的类型，都会走到这里，而在findClass方法中，我就会把桌面的NormalUser加载进来。

我们看下测试代码：

class="prism language-class="tags" href="/tags/JAVA.html" title=java>java">class="token keyword">public class="token keyword">class class="token class-name">ClassLoaderTest class="token punctuation">{
    class="token keyword">public class="token keyword">static class="token keyword">void class="token function">mainclass="token punctuation">(Stringclass="token punctuation">[class="token punctuation">] argsclass="token punctuation">) class="token keyword">throws ClassNotFoundExceptionclass="token punctuation">, IllegalAccessExceptionclass="token punctuation">, InstantiationException class="token punctuation">{
        Classclass="token operator"><class="token operator">?class="token operator">> clazz class="token operator">= Classclass="token punctuation">.class="token function">forNameclass="token punctuation">(class="token string">"hua.lee.classloader.NormalUser"class="token punctuation">)class="token punctuation">;
        Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token string">"系统自带 ClassLoader=" class="token operator">+ clazzclass="token punctuation">.class="token function">getClassLoaderclass="token punctuation">(class="token punctuation">)class="token punctuation">)class="token punctuation">;
        Pocket pocket class="token operator">= class="token punctuation">(Pocketclass="token punctuation">) clazzclass="token punctuation">.class="token function">newInstanceclass="token punctuation">(class="token punctuation">)class="token punctuation">;
        Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token string">"系统自带："class="token operator">+pocketclass="token punctuation">.class="token function">getBalanceclass="token punctuation">(class="token punctuation">)class="token punctuation">)class="token punctuation">;

        class="token comment">//没有重写 loadClass，维持双亲委派机制
        LocalClassLoader lcl class="token operator">= class="token keyword">new class="token class-name">LocalClassLoaderclass="token punctuation">(class="token punctuation">)class="token punctuation">;
        class="token comment">//我们加个后缀名，这样双亲委托查找失败了，就调用LocalClassLoader.findClass 方法加载
        class="token comment">//其实 findClass 方法还是加载 NormalUser类，只是换了个位置和方式：
        class="token comment">//defineClass("hua.lee.classloader.NormalUser", clazzBytes, 0, size);
        Classclass="token operator"><class="token operator">?class="token operator">> clazzOut02 class="token operator">= lclclass="token punctuation">.class="token function">loadClassclass="token punctuation">(class="token string">"hua.lee.classloader.NormalUser_Temp"class="token punctuation">)class="token punctuation">;

        class="token comment">//双亲委托机制先检查当前类加载器中有没有这个类的 Class 实例，没有的话再搜寻父类，
        class="token comment">//查找到 hua.lee.classloader.NormalUser 的 Class 实例，就返回该对象
        Classclass="token operator"><class="token operator">?class="token operator">> clazzOut01 class="token operator">= lclclass="token punctuation">.class="token function">loadClassclass="token punctuation">(class="token string">"hua.lee.classloader.NormalUser"class="token punctuation">)class="token punctuation">;


        Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token string">"clazzOut01 自定义 ClassLoader=" class="token operator">+ clazzOut01class="token punctuation">.class="token function">getClassLoaderclass="token punctuation">(class="token punctuation">)class="token punctuation">)class="token punctuation">;
        pocket class="token operator">= class="token punctuation">(Pocketclass="token punctuation">) clazzOut01class="token punctuation">.class="token function">newInstanceclass="token punctuation">(class="token punctuation">)class="token punctuation">;
        Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token string">"clazzOut01 自定义加载器获取余额："class="token operator">+pocketclass="token punctuation">.class="token function">getBalanceclass="token punctuation">(class="token punctuation">)class="token punctuation">)class="token punctuation">;

        Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token string">"clazzOut02 自定义 ClassLoader=" class="token operator">+ clazzOut02class="token punctuation">.class="token function">getClassLoaderclass="token punctuation">(class="token punctuation">)class="token punctuation">)class="token punctuation">;
        pocket class="token operator">= class="token punctuation">(Pocketclass="token punctuation">) clazzOut02class="token punctuation">.class="token function">newInstanceclass="token punctuation">(class="token punctuation">)class="token punctuation">;
        Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token string">"clazzOut02 自定义加载器获取余额："class="token operator">+pocketclass="token punctuation">.class="token function">getBalanceclass="token punctuation">(class="token punctuation">)class="token punctuation">)class="token punctuation">;

        Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(clazzclass="token operator">==clazzOut01class="token punctuation">)class="token punctuation">;
        Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(clazzclass="token operator">==clazzOut02class="token punctuation">)class="token punctuation">;
        Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(clazzOut01class="token operator">==clazzOut02class="token punctuation">)class="token punctuation">;
    class="token punctuation">}

控制台输出是这样的：

系统自带 ClassLoader=sun.misc.Launcher$AppClassLoader@18b4aac2
系统自带：50
clazzOut01 自定义 ClassLoader=hua.lee.classloader.LocalClassLoader@60e53b93
clazzOut01 自定义加载器获取余额：50000
clazzOut02 自定义 ClassLoader=hua.lee.classloader.LocalClassLoader@60e53b93
clazzOut02 自定义加载器获取余额：50000
false   //clazz==clazzOut01
false   //clazz==clazzOut02
true    //clazzOut01==clazzOut02

感觉找到了致富的方向。虚拟机里的加载约束没看到呢.

面向抽象类编程

上面的致富之路好像很顺利，那我们尝试用如下的方式：

class="prism language-class="tags" href="/tags/JAVA.html" title=java>java">class="token keyword">public class="token keyword">abstract class="token keyword">class class="token class-name">Pocket class="token punctuation">{
    class="token keyword">abstract class="token keyword">int class="token function">getBalanceclass="token punctuation">(class="token punctuation">)class="token punctuation">;
class="token punctuation">}
class="token keyword">public class="token keyword">class class="token class-name">NormalUser class="token keyword">extends class="token class-name">Pocketclass="token punctuation">{
    class="token annotation punctuation">@Override
    class="token keyword">public class="token keyword">int class="token function">getBalanceclass="token punctuation">(class="token punctuation">) class="token punctuation">{
        class="token keyword">return class="token number">50class="token punctuation">;
    class="token punctuation">}
class="token punctuation">}

同样，我在桌面编译了一个富人版本：

class="prism language-class="tags" href="/tags/JAVA.html" title=java>java">class="token keyword">public class="token keyword">class class="token class-name">NormalUser class="token keyword">extends class="token class-name">Pocketclass="token punctuation">{
    class="token annotation punctuation">@Override
    class="token keyword">public class="token keyword">int class="token function">getBalanceclass="token punctuation">(class="token punctuation">) class="token punctuation">{
        class="token keyword">return class="token number">50000class="token punctuation">;
    class="token punctuation">}
class="token punctuation">}

LocalClassLoader和测试代码没变，但是打印结果确实这样的：

系统自带 ClassLoader=sun.misc.Launcher$AppClassLoader@18b4aac2
系统自带：50
clazzOut01 自定义 ClassLoader=hua.lee.classloader.LocalClassLoader@60e53b93
Exception in thread "main" class="tags" href="/tags/JAVA.html" title=java>java.lang.AbstractMethodError
	at hua.lee.classloader.ClassLoaderTest.main(ClassLoaderTest.class="tags" href="/tags/JAVA.html" title=java>java:25)

虚拟机在解析抽象类实现和接口实现的时候有什么区别么？

不成熟的推测

首先面向接口中的模式应该是躲过了装载约束的，或者虚拟机设计上是支持接口这么搞的。

既然提示AbstractMethodError，那我把测试代码改一下，不再用抽象类Pocket，直接指向NormalUser:

class="prism language-class="tags" href="/tags/JAVA.html" title=java>java">class="token keyword">public class="token keyword">class class="token class-name">ClassLoaderTest class="token punctuation">{
    class="token keyword">public class="token keyword">static class="token keyword">void class="token function">mainclass="token punctuation">(Stringclass="token punctuation">[class="token punctuation">] argsclass="token punctuation">) class="token keyword">throws ClassNotFoundExceptionclass="token punctuation">, IllegalAccessExceptionclass="token punctuation">, InstantiationException class="token punctuation">{

        Classclass="token operator"><class="token operator">?class="token operator">> clazz class="token operator">= Classclass="token punctuation">.class="token function">forNameclass="token punctuation">(class="token string">"hua.lee.classloader.NormalUser"class="token punctuation">)class="token punctuation">;
        Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token string">"系统自带 ClassLoader=" class="token operator">+ clazzclass="token punctuation">.class="token function">getClassLoaderclass="token punctuation">(class="token punctuation">)class="token punctuation">)class="token punctuation">;
                    class="token comment">//请注意此处⬇️
        NormalUser pocket class="token operator">= class="token punctuation">(NormalUserclass="token punctuation">) clazzclass="token punctuation">.class="token function">newInstanceclass="token punctuation">(class="token punctuation">)class="token punctuation">;
                    class="token comment">//请注意此处⬆️
        Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token string">"系统自带："class="token operator">+pocketclass="token punctuation">.class="token function">getBalanceclass="token punctuation">(class="token punctuation">)class="token punctuation">)class="token punctuation">;

        class="token comment">//没有重写 loadClass，维持双亲委派机制
        LocalClassLoader lcl class="token operator">= class="token keyword">new class="token class-name">LocalClassLoaderclass="token punctuation">(class="token punctuation">)class="token punctuation">;
        class="token comment">//我们加个后缀名，这样双亲委托查找失败了，就调用LocalClassLoader.findClass 方法加载
        class="token comment">//其实 findClass 方法还是加载 NormalUser类，只是换了个位置和方式：
        class="token comment">//defineClass("hua.lee.classloader.NormalUser", clazzBytes, 0, size);
        Classclass="token operator"><class="token operator">?class="token operator">> clazzOut02 class="token operator">= lclclass="token punctuation">.class="token function">loadClassclass="token punctuation">(class="token string">"hua.lee.classloader.NormalUser_Temp"class="token punctuation">)class="token punctuation">;

        class="token comment">//双亲委托机制会自动搜寻父类，
        class="token comment">//查找到 hua.lee.classloader.NormalUser 的 Class 实例，就返回该对象
        Classclass="token operator"><class="token operator">?class="token operator">> clazzOut01 class="token operator">= lclclass="token punctuation">.class="token function">loadClassclass="token punctuation">(class="token string">"hua.lee.classloader.NormalUser"class="token punctuation">)class="token punctuation">;


        Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token string">"clazzOut01 自定义 ClassLoader=" class="token operator">+ clazzOut01class="token punctuation">.class="token function">getClassLoaderclass="token punctuation">(class="token punctuation">)class="token punctuation">)class="token punctuation">;
        pocket class="token operator">= class="token punctuation">(NormalUserclass="token punctuation">) clazzOut01class="token punctuation">.class="token function">newInstanceclass="token punctuation">(class="token punctuation">)class="token punctuation">;
        Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token string">"clazzOut01 自定义加载器获取余额："class="token operator">+pocketclass="token punctuation">.class="token function">getBalanceclass="token punctuation">(class="token punctuation">)class="token punctuation">)class="token punctuation">;

        Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token string">"clazzOut02 自定义 ClassLoader=" class="token operator">+ clazzOut02class="token punctuation">.class="token function">getClassLoaderclass="token punctuation">(class="token punctuation">)class="token punctuation">)class="token punctuation">;
        pocket class="token operator">= class="token punctuation">(NormalUserclass="token punctuation">) clazzOut02class="token punctuation">.class="token function">newInstanceclass="token punctuation">(class="token punctuation">)class="token punctuation">;
        Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(class="token string">"clazzOut02 自定义加载器获取余额："class="token operator">+pocketclass="token punctuation">.class="token function">getBalanceclass="token punctuation">(class="token punctuation">)class="token punctuation">)class="token punctuation">;

        Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(clazzclass="token operator">==clazzOut01class="token punctuation">)class="token punctuation">;
        Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(clazzclass="token operator">==clazzOut02class="token punctuation">)class="token punctuation">;
        Systemclass="token punctuation">.outclass="token punctuation">.class="token function">printlnclass="token punctuation">(clazzOut01class="token operator">==clazzOut02class="token punctuation">)class="token punctuation">;
    class="token punctuation">}
class="token punctuation">}

惊喜的事情来了，输出变成了下面这样：

系统自带 ClassLoader=sun.misc.Launcher$AppClassLoader@18b4aac2
系统自带：50
clazzOut01 自定义 ClassLoader=hua.lee.classloader.LocalClassLoader@60e53b93
Exception in thread "main" class="tags" href="/tags/JAVA.html" title=java>java.lang.ClassCastException: hua.lee.classloader.NormalUser cannot be cast to hua.lee.classloader.NormalUser
	at hua.lee.classloader.ClassLoaderTest.main(ClassLoaderTest.class="tags" href="/tags/JAVA.html" title=java>java:24)

这个异常说明了一个问题我们自定义加载进来的hua.lee.classloader.NormalUser不是虚拟机想要的hua.lee.classloader.NormalUser。

第一个问题：虚拟机怎么知道这两个NormalUser不一样的？

和定义类装载器有关系，同一个全限定名的类型，如果不是同一个定义类装载器，虚拟机就认为不是同一个类型。（按照装载约束应该报错的）

啥是定义类装载器？就是真正把 class 文件装载进来的那个ClassLoader

第二个问题：为什么接口可以这样跑呢？

那是因为面向接口实现的版本没有明确指名类型NormalUser，类型转换都是按照Pocket类型执行方法，虚拟机自动查找接口类型的实现。只要是Pocket的实现就行。

如果你把pocket = (Pocket) clazzOut01.newInstance();换成pocket = (NormalUser) clazzOut01.newInstance();，此时，面向接口的版本也会报ClassCastException。

对于虚拟机来说，在接口的实现类，它并不 care 你的实现类的来源（哪个加载器加载的无所谓啦），符合接口定义就可以了。

但是当你指定类型转换的时候，它就要去做类型检查了。

第三个问题：抽象类为什么不行？

首先 YY 一下class="tags" href="/tags/JAVA.html" title=java>java.lang.AbstractMethodError：

当Pocket为抽象类时，根据抽象类的定义，要有特定实现类来执行。
而对于pocket = (Pocket) clazzOut01.newInstance();这句话，Pocket的强制类型转换是成功（因为解析Poket类型需要执行双亲委派逻辑，解析后发现大家都是同一个Poket），但是当寻找实现类时就发现这个clazzOut01.newInstance()对象对应的类型NormalUser不是自己想要的，虚拟机以为没有对应的实现，所以当执行到pocket.getBalance()时，虚拟机以为程序要单纯的执行Pocket的抽象方法，于是报错class="tags" href="/tags/JAVA.html" title=java>java.lang.AbstractMethodError。

如果上面的理解了，那么class="tags" href="/tags/JAVA.html" title=java>java.lang.ClassCastException也就好理解了：

对于pocket = (NormalUser) clazzOut01.newInstance();这句话，虚拟机会把clazzOut01.newInstance()产生的对象强制转换为NormalUser，到这里就回到第一个问题的答案了：clazzOut01所代表的NormalUser和要强转的NormalUser不是一个类型，虚拟机就直接报错了

结语

为啥这个要有结语呢？

因为不成熟的推测真的是本人 YY 的结果，不敢保证真实有效。如有错误之处，还望不吝赐教。

下一篇会续上Java虚拟机（五）垃圾收集（老是感觉要写写垃圾分类啥的，猪都不能吃的那种）


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