这与线程无关，仅与类加载器有关。当由不同的类加载器加载相同的类定义时，JVM将其视为两个不同的类。所以
instanceof两者之间的强制转换失败。
因此，要回答您的原始问题： 在同一个类加载器加载的 线程之间传递对象是安全的，
instanceof等等。工作良好。
这是有关类加载问题的文章。
更新Romain的评论
以下是一些测试的行为的代码
instanceof：
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));输出为（在Eclipse中，Java5）：
c1.toString(): class net.torokpeter.Fooc2.toString(): class net.torokpeter.Fooc1.equals(c2): falsec1 == c2: falsefoo1: net.torokpeter.Foo@360be0foo2: net.torokpeter.Foo@45a877foo1 instanceof Foo: falsefoo2 instanceof Foo: falsec1.isAssignableFrom(c1): truec2.isAssignableFrom(c2): truec1.isAssignableFrom(c2): falsec2.isAssignableFrom(c1): falsec1.isAssignableFrom(Foo.class): falsec2.isAssignableFrom(Foo.class): falseFoo.class.isAssignableFrom(c1): falseFoo.class.isAssignableFrom(c2): false
所以一切似乎都是一致的:-)
