在Threads之间传递对象时是否可以使用instanceof？

Question

我遇到了一个问题,其中instanceof工作,然后它没有.进入细节很困难,但我认为这可能是问题所在:

阅读本文:http://www.theserverside.com/news/thread.tss？ thread_id = 40229(搜索Thread.currentThread),似乎暗示即使两个对象是同一个类,如果你传递它们在具有不同类加载器的线程之间,instanceof(和isAssignableFrom)可能仍会失败.

这当然可以解释我的行为,但我想知道是否有人可以验证它？

(我希望在讨论开始时链接的文章仍然可用,但它似乎不是.)

Answer 1

这与线程无关,只与类加载器有关.当由不同的类加载器加载时,相同的类定义被JVM视为两个不同的类.所以instanceof或两者之间的演员都失败了.

所以回答你原来的问题:在同一个类加载器加载的线程之间传递对象是安全的等等instanceof.工作良好.

这是一篇关于类加载问题的文章.

另请参阅我之前的回答,以了解验证游戏中哪些类加载器的方法.

更新Romain的评论

以下是一些测试行为的代码instanceof,其中包括:

URL[] urls = new URL[] {new File("build/classes/").toURL()};
ClassLoader loader1 = new URLClassLoader(urls, null);
ClassLoader loader2 = new URLClassLoader(urls, null);
Class<?> c1 = loader1.loadClass("net.torokpeter.Foo");
Class<?> c2 = loader2.loadClass("net.torokpeter.Foo");
Object foo1 = c1.newInstance();
Object foo2 = c2.newInstance();

System.out.println("c1.toString(): " + c1);
System.out.println("c2.toString(): " + c2);
System.out.println("c1.equals(c2): " + c1.equals(c2));
System.out.println("c1 == c2: " + (c1 == c2));
System.out.println("foo1: " + foo1);
System.out.println("foo2: " + foo2);
System.out.println("foo1 instanceof Foo: " + (foo1 instanceof Foo));
System.out.println("foo2 instanceof Foo: " + (foo2 instanceof Foo));
System.out.println("c1.isAssignableFrom(c1): " + c1.isAssignableFrom(c1));
System.out.println("c2.isAssignableFrom(c2): " + c2.isAssignableFrom(c2));
System.out.println("c1.isAssignableFrom(c2): " + c1.isAssignableFrom(c2));
System.out.println("c2.isAssignableFrom(c1): " + c2.isAssignableFrom(c1));
System.out.println("c1.isAssignableFrom(Foo.class): " + c1.isAssignableFrom(Foo.class));
System.out.println("c2.isAssignableFrom(Foo.class): " + c2.isAssignableFrom(Foo.class));
System.out.println("Foo.class.isAssignableFrom(c1): " + Foo.class.isAssignableFrom(c1));
System.out.println("Foo.class.isAssignableFrom(c2): " + Foo.class.isAssignableFrom(c2));

输出是(在Eclipse中,Java5):

c1.toString(): class net.torokpeter.Foo
c2.toString(): class net.torokpeter.Foo
c1.equals(c2): false
c1 == c2: false
foo1: net.torokpeter.Foo@360be0
foo2: net.torokpeter.Foo@45a877
foo1 instanceof Foo: false
foo2 instanceof Foo: false
c1.isAssignableFrom(c1): true
c2.isAssignableFrom(c2): true
c1.isAssignableFrom(c2): false
c2.isAssignableFrom(c1): false
c1.isAssignableFrom(Foo.class): false
c2.isAssignableFrom(Foo.class): false
Foo.class.isAssignableFrom(c1): false
Foo.class.isAssignableFrom(c2): false

所以一切似乎都是一致的:-)