"The methods and constructors of objects created by a class loader may reference other classes" 是什么意思？

Question

问题("The methods and constructors of objects created by a class loader may reference other classes")中的内容引用自JavaDoc of ClassLoader.

后面虽然有例子，还是看不懂。能否请你举个例子，因为这个会导致错误？

非常感谢。

Answer 1

"The methods and constructors of objects created by a class loader may reference other classes"

这只是意味着 class 可能引用了其他 classes。因此，classloader 还必须负责加载引用的 classes。

让我们定义两个简单的 classes。 class TestReference 方法 foo:

public class TestReference {
    public void foo() {
        System.out.println("Hello world");
    }
}

和一个 class TestClass 在其构造函数中实例化 TestReference：

public class TestClass {
    public TestClass() {
        TestReference reference = new TestReference();
        reference.foo();
    }
}

现在我们定义一个自定义 classloader 从 .class 文件加载 classes:

public class MyClassLoader extends ClassLoader {

    @Override
    public Class<?> loadClass(String name) throws ClassNotFoundException {
        // Ignore this for the example as Object is always implicitly referenced by a class
        if("java.lang.Object".equals(name)) {
            return super.loadClass(name);
        }
        System.out.println("Loading class: " + name);

        // Load class from "/path/to/my/classes/TestClass.class"
        Path fileLocation = Paths.get("/path/to/my/classes/" + name + ".class");
        try {
            byte[] classData = Files.readAllBytes(fileLocation);
            Class<?> clazz = defineClass(name, classData, 0, classData.length);
            // Class has been successfully loaded and defined by the classloader
            System.out.println("Loaded: " + clazz.getSimpleName());
            return clazz;
        } catch (IOException e) {
            // Class could not be found
            throw new ClassNotFoundException();
        }
    }
}

使用 javac 编译 TestReference.java 和 TestClass.java，仅将 TestClass.class 移动到 /path/to/my/classes/

"To determine the class(es) referred to, the Java virtual machine invokes the loadClass method of the class loader that originally created the class."

以下将失败，因为 classloader 传递性地尝试查找和加载 TestReference.class 但它不会在指定位置找到它：

public static void main(String[] args) {
    ClassLoader classLoader = new MyClassLoader();
    try {
        Object o = classLoader.loadClass("TestClass").newInstance();
    } catch (ClassNotFoundException | IllegalAccessException | InstantiationException e) {
        e.printStackTrace();
    }
}

控制台输出：

Loading class: TestClass
Loaded: TestClass
Loading class: TestReference
Exception in thread "main" java.lang.NoClassDefFoundError: TestReference

要修复此错误，请将 TestReference.class 移动到 /path/to/my/classes/ 并调整 classloader 的 loadClass 如下：

if("java.lang.Object".equals(name) ||
   "java.lang.System".equals(name) || 
   "java.io.PrintStream".equals(name)) {
    return super.loadClass(name);
}

"The methods and constructors of objects created by a class loader may reference other classes" 是什么意思？

what does "The methods and constructors of objects created by a class loader may reference other classes" mean?

java

jvm