Class.isAssignableFrom

/**
 * Determines if the class or interface represented by this
 * {@code Class} object is either the same as, or is a superclass or
 * superinterface of, the class or interface represented by the specified
 * {@code Class} parameter. It returns {@code true} if so;
 * otherwise it returns {@code false}. If this {@code Class}
 * object represents a primitive type, this method returns
 * {@code true} if the specified {@code Class} parameter is
 * exactly this {@code Class} object; otherwise it returns
 * {@code false}.
 *
 * <p> Specifically, this method tests whether the type represented by the
 * specified {@code Class} parameter can be converted to the type
 * represented by this {@code Class} object via an identity conversion
 * or via a widening reference conversion. See <em>The Java Language
 * Specification</em>, sections 5.1.1 and 5.1.4 , for details.
 *
 * @param cls the {@code Class} object to be checked
 * @return the {@code boolean} value indicating whether objects of the
 * type {@code cls} can be assigned to objects of this class
 * @exception NullPointerException if the specified Class parameter is
 *            null.
 * @since JDK1.1
 */
public native boolean isAssignableFrom(Class<?> cls);

该 native 方法在我开发 Java Agent 的过程中进行了调试，发现对于满足实现关系的类，如果不是由相同的类加载器加载，则会返回 false。

比如以下代码：

package me.tianshuang;

public class Parent {
}

package me.tianshuang;

public class Sub extends Parent {
}

package me.tianshuang;

import java.net.URL;
import java.net.URLClassLoader;

public class CustomClassLoader extends URLClassLoader {

    public CustomClassLoader(URL[] urls) {
        super(urls, null);
    }

}

package me.tianshuang;

import java.net.MalformedURLException;
import java.net.URL;

public class Test {

    public static void main(String[] args) throws ClassNotFoundException, MalformedURLException {
        Parent parent = new Parent();
        Sub sub = new Sub();
        System.out.println(parent.getClass().isAssignableFrom(sub.getClass()));

        String path = "file:///Users/tianshuang/IdeaProjects/test/target/test-classes/";
        URL[] urls = {new URL(path)};
        Class<?> parentClass = new CustomClassLoader(urls).loadClass("me.tianshuang.Parent");
        Class<?> subClass = new CustomClassLoader(urls).loadClass("me.tianshuang.Sub");
        System.out.println(parentClass.isAssignableFrom(subClass));
    }

}

输出如下：

true
false

同理可知，如果我们对不是由同一类加载器加载的 Class 进行 java.lang.Class#asSubclass 方法调用，将会触发 ClassCastException。

/**
 * Casts this {@code Class} object to represent a subclass of the class
 * represented by the specified class object.  Checks that the cast
 * is valid, and throws a {@code ClassCastException} if it is not.  If
 * this method succeeds, it always returns a reference to this class object.
 *
 * <p>This method is useful when a client needs to "narrow" the type of
 * a {@code Class} object to pass it to an API that restricts the
 * {@code Class} objects that it is willing to accept.  A cast would
 * generate a compile-time warning, as the correctness of the cast
 * could not be checked at runtime (because generic types are implemented
 * by erasure).
 *
 * @param <U> the type to cast this class object to
 * @param clazz the class of the type to cast this class object to
 * @return this {@code Class} object, cast to represent a subclass of
 *    the specified class object.
 * @throws ClassCastException if this {@code Class} object does not
 *    represent a subclass of the specified class (here "subclass" includes
 *    the class itself).
 * @since 1.5
 */
@SuppressWarnings("unchecked")
public <U> Class<? extends U> asSubclass(Class<U> clazz) {
    if (clazz.isAssignableFrom(this))
        return (Class<? extends U>) this;
    else
        throw new ClassCastException(this.toString());
}

执行以下代码：

package me.tianshuang;

import java.net.MalformedURLException;
import java.net.URL;

public class Test {

    public static void main(String[] args) throws ClassNotFoundException, MalformedURLException {
        Parent parent = new Parent();
        Sub sub = new Sub();
        System.out.println(parent.getClass().isAssignableFrom(sub.getClass()));

        String path = "file:///Users/tianshuang/IdeaProjects/test/target/test-classes/";
        URL[] urls = {new URL(path)};
        Class<?> parentClass = new CustomClassLoader(urls).loadClass("me.tianshuang.Parent");
        Class<?> subClass = new CustomClassLoader(urls).loadClass("me.tianshuang.Sub");
        System.out.println(subClass.asSubclass(parentClass));
    }

}

输出如下：

true
Exception in thread "main" java.lang.ClassCastException: class me.tianshuang.Sub
 at java.lang.Class.asSubclass(Class.java:3404)
 at me.tianshuang.Test.main(Test.java:17)

同理，受类加载器影响的还有 instanceof 关键字，比如如下代码：

package me.tianshuang;

import java.net.MalformedURLException;
import java.net.URL;

public class Test {

    public static void main(String[] args) throws ClassNotFoundException, MalformedURLException, InstantiationException, IllegalAccessException {
        Parent parent = new Parent();
        System.out.println(parent instanceof Parent);

        String path = "file:///Users/tianshuang/IdeaProjects/test/target/test-classes/";
        URL[] urls = {new URL(path)};
        Class<?> parentClass = new CustomClassLoader(urls).loadClass("me.tianshuang.Parent");

        System.out.println(parentClass.newInstance() instanceof Parent);
    }

}

输出如下：

true
false

最后，不得不提的是我认为 java.lang.Class#isAssignableFrom 与 java.lang.Class#asSubclass 的方法名具有误导性，不太分得清操作的为参数还是调用方，StackOverflow 上也有用户提出，如果不详细查看方法注释的话确实不太分得清楚。

Reference

Generics and Class.asSubclass - Stack Overflow