java - 为什么 Class.newInstance 总是抛出异常？

Question

我想定义一个自定义类加载器来在运行时加载我自己的类。

但是，Class.newInstance即使我定义了一个零参数的构造函数，也总是失败。

异常消息是:

java.lang.IllegalAccessException: 类 Hello 无法使用修饰符“public”访问 Test 类的成员

为什么？

import java.io.BufferedInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.HashMap;
import java.util.Map;

class CustomClassLoader extends ClassLoader {

    private Map<String, Class<?>> classes = new HashMap<String, Class<?>>();

    public String toString() {
        return CustomClassLoader.class.getName();
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {

        if (classes.containsKey(name)) {
            return classes.get(name);
        }

        byte[] classData;

        try {
            classData = loadClassData(name);
        } catch (IOException e) {
            throw new ClassNotFoundException("Class [" + name
                    + "] could not be found", e);
        }

        Class<?> c = defineClass(name, classData, 0, classData.length);
        resolveClass(c);
        classes.put(name, c);

        return c;
    }

    private byte[] loadClassData(String name) throws IOException {
        BufferedInputStream in = new BufferedInputStream(
                ClassLoader.getSystemResourceAsStream(name.replace(".", "/")
                        + ".class"));
        ByteArrayOutputStream out = new ByteArrayOutputStream();
        int i;

        while ((i = in.read()) != -1) {
            out.write(i);
        }

        in.close();
        byte[] classData = out.toByteArray();
        out.close();

        return classData;
    }
}

class Test
{
    public Test()
    {}

    public void Hello()
    {}
}

public class Hello {

    public static void main(String[] args)
    {
        try {
            CustomClassLoader loader = new CustomClassLoader();
            Class<?> c = loader.findClass("Test"); // OK!
            Object o = c.newInstance(); // ALWAYS FAIL!
        }
        catch (Exception e)
        { 
            String s = e.getMessage();
            // s is "java.lang.IllegalAccessException: Class Hello can not access"
            // " a member of class Test with modifiers "public""
        }
    }
}

Answer 1

问题是您的 Test类以默认(包)范围声明，但在运行时，您的 Hello和 Test类在不同的包中。

Java 中类的名称(唯一标识符)是完全限定的类名和加载它的类加载器的组合:

At run time, a class or interface is determined not by its name alone, but by a pair: its binary name (§4.2.1) and its defining class loader. Each such class or interface belongs to a single run-time package. The run-time package of a class or interface is determined by the package name and defining class loader of the class or interface. (JVMS 5.3)

在这种情况下， Test类正在由下游类加载器加载，因此它的(默认)包不被视为与 Hello 相同的(默认)包(由系统类加载器加载)在其中。因此，您无权访问构造函数，因为类本身不是公共(public)的。

如果您使用 Test，则此示例将有效。一个单独的公共(public)顶级类或使用反射使其可用。