protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
{
  synchronized (getClassLoadingLock(name)) {
    // 首先检查类是否已经加载过
    Class<?> c = findLoadedClass(name);
    if (c == null) {
      // 没有加载过，则委托父加载器加载
      long t0 = System.nanoTime();
      try {
        if (parent != null) {
          // 若有父加载器，则委托父加载加载
          c = parent.loadClass(name, false);
        } else {
          // 若没有父加载器（最上层父加载器也未加载此类），则委托BootStrap加载器加载
          c = findBootstrapClassOrNull(name);
        }
      } catch (ClassNotFoundException e) {
        // ClassNotFoundException thrown if class not found
        // from the non-null parent class loader
      }

      if (c == null) {
        // If still not found, then invoke findClass in order
        // to find the class.
        // 这种情况代表Bootstrap加载器也未加载此类，则委托给本加载器加载。
        long t1 = System.nanoTime();
        c = findClass(name);
        // this is the defining class loader; record the stats
        PerfCounter.getParentDelegationTime().addTime(t1 - t0);
        PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
        PerfCounter.getFindClasses().increment();
      }
    }
    if (resolve) {
      resolveClass(c);
    }
    return c;
  }

测试双亲委派机制

• 新建一个测试用的类

  package misc;
  
  public class Model
  {
      static
      {
          System.out.println("类被加载了");
      }
  
      public static void sayHello()
      {
          System.out.println("hello");
      }
  }
  

• 自己定义一个类加载器

  public class MyClassLoader extends ClassLoader
  {
      private final String classPath;
  
      public MyClassLoader(String classPath)
      {
          this.classPath = classPath;
      }
  
      @Override
      protected Class<?> findClass(String name) throws ClassNotFoundException
      {
        	// 如果使用MyClassLoader加载，那么这句话将会输出至控制台
          System.out.println("使用MyClassLoader加载");
          var classFilePath = classPath + "/" + name.replace(".","/").concat(".class");
          var bao = new ByteArrayOutputStream();
          var readByte = 1;
          try
          {
              var fis = new FileInputStream(classFilePath);
              while ((readByte = fis.read()) != -1)
              {
                  bao.write(readByte);
              }
              var bytesArray = bao.toByteArray();
              return defineClass(name, bytesArray, 0, bytesArray.length);
          } catch (IOException ex)
          {
              ex.printStackTrace();
              throw new ClassNotFoundException(ex.getMessage());
          }
      }
  }
  

• 测试代码

  public static void loadClassViaMyClassLoader() throws Exception
  {
      var classLoader = new MyClassLoader("/Users/huobingnan/code/java/misc/out/production/misc");
      var modelClass = classLoader.loadClass("misc.Model");
    	System.out.println("Model的类加载器是：" + modelClass.getClassLoader());
      var sayHelloMethod = modelClass.getDeclaredMethod("sayHello");
      sayHelloMethod.invoke(null);
  }
  

• 测试输出

  Model的类加载器是：jdk.internal.loader.ClassLoaders$AppClassLoader@7c53a9eb
  类被加载了
  hello
  

  通过输出可以发现，Model类的加载并未使用我们自定义的MyClassLoader，而是使用了JDK中的应用程序类加载器，这就是双亲委派机制的体现，你也可以对上述代码进行DEBUG运行，从中便可得知类加载的途径是

  MyClassLoader -> AppClassLoader -> PlatformClassLoader -> BootstrapClassLoader -> PlatformClassLoader -> AppClassLoader 。注意：不同的JDK可能加载器的名称会有所不同，笔者这里使用的是zulu-jdk-arm64。

打破双亲委派机制

通过上文的测试用例可以得知，尽管我们自定义了ClassLoader，但是由于双亲委派机制的存在，字节码文件没有使用我们自定义的ClassLoader加载。那么如何强制字节码文件使特定ClassLoader加载呢？我们可以通过重写CLassLoader.loadClass(String name, boolean resovle)方法进行。例如下面的代码：

public class MyClassLoader extends ClassLoader
{
    private final String classPath;

    public MyClassLoader(String classPath)
    {
        this.classPath = classPath;
    }

    @Override
    protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException
    {
        if (name.startsWith("misc"))
        {
          	// 如果包名以misc开头，我们使用MyClassLoader加载
            return findClass(name); 
        }
        return super.loadClass(name, resolve);
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException
    {
        System.out.println("使用MyClassLoader加载");
        var classFilePath = classPath + "/" + name.replace(".", "/").concat(".class");
        var bao = new ByteArrayOutputStream();
        var readByte = 1;
        try
        {
            var fis = new FileInputStream(classFilePath);
            while ((readByte = fis.read()) != -1)
            {
                bao.write(readByte);
            }
            var bytesArray = bao.toByteArray();
            return defineClass(name, bytesArray, 0, bytesArray.length);
        } catch (IOException ex)
        {
            ex.printStackTrace();
            throw new ClassNotFoundException(ex.getMessage());
        }
    }
}

使用MyClassLoader加载
Model的类加载器是：misc.MyClassLoader@d041cf
类被加载了
hello

 public static void testClassLoaderCastBehavior() throws Exception
 {
   var classLoader = new MyClassLoader("/Users/huobingnan/code/java/misc/lib");
   var modelClass = classLoader.loadClass("misc.Model");
   var object = modelClass.getConstructor().newInstance();
   var model = (Model)object;
 }

使用MyClassLoader加载
类被加载了
Exception in thread "main" java.lang.ClassCastException: class misc.Model cannot be cast to class misc.Model (misc.Model is in unnamed module of loader misc.MyClassLoader @d041cf; misc.Model is in unnamed module of loader 'app')
	at misc.ClassLoaderTest.testClassLoaderCastBehavior(ClassLoaderTest.java:19)
	at misc.ClassLoaderTest.main(ClassLoaderTest.java:85)