投射到同一对象时出现 ClassCastException

Question

Java在动态加载类时会抛出Cannot Cast Exception的可能原因是什么？

例子：

    String classToLoad = null;
    try {
        classToLoad = extractMainClassManifest(jar);
        LOGGER.info("Class to load: " + classToLoad);
        JarByteClassloader loader = new JarByteClassloader(jar); 
        // e.g. class MyJarEntryObject extends JarEntryObject
        Class c = loader.loadClass(classToLoad); 
        JarEntryObject jarEntry = (JarEntryObject) c.newInstance();
        // other stuff
    } catch (Exception e) {
        e.printStackTrace();
    }

在这段代码抛出错误的地方，我不认为这里有另一个类加载器（多个类加载器的情况）。

此代码在使用 JUnit 或仅在 main() 方法中运行时运行良好。 在运行时出现转换问题Servlet 容器（例如mvn jetty:run）

日志：

java.lang.ClassCastException: com.mywebapp.example.MyJarEntryObject cannot be cast to com.mysdk.JarEntryObject
    at org.restlet.routing.Filter.doHandle(Filter.java:150)
    at org.restlet.routing.Filter.handle(Filter.java:197)
    at org.restlet.routing.Router.doHandle(Router.java:422)
    at org.restlet.routing.Router.handle(Router.java:641)
    at org.restlet.routing.Filter.doHandle(Filter.java:150)
    at org.restlet.routing.Filter.handle(Filter.java:197)
    at org.restlet.routing.Filter.doHandle(Filter.java:150)
    at org.restlet.routing.Filter.handle(Filter.java:197)
    at org.restlet.routing.Filter.doHandle(Filter.java:150)
    at org.restlet.routing.Filter.handle(Filter.java:197)
    at org.restlet.routing.Filter.doHandle(Filter.java:150)
    at org.restlet.engine.application.StatusFilter.doHandle(StatusFilter.java:140)
    at org.restlet.routing.Filter.handle(Filter.java:197)
    at org.restlet.routing.Filter.doHandle(Filter.java:150)
    at org.restlet.routing.Filter.handle(Filter.java:197)
    at org.restlet.engine.CompositeHelper.handle(CompositeHelper.java:202)
    at org.restlet.engine.application.ApplicationHelper.handle(ApplicationHelper.java:77)
    at org.restlet.Application.handle(Application.java:385)
    at org.restlet.routing.Filter.doHandle(Filter.java:150)
    at org.restlet.routing.Filter.handle(Filter.java:197)
    at org.restlet.routing.Router.doHandle(Router.java:422)
    at org.restlet.routing.Router.handle(Router.java:641)
    at org.restlet.routing.Filter.doHandle(Filter.java:150)
    at org.restlet.routing.Filter.handle(Filter.java:197)
    at org.restlet.routing.Router.doHandle(Router.java:422)
    at org.restlet.routing.Router.handle(Router.java:641)
    at org.restlet.routing.Filter.doHandle(Filter.java:150)
    at org.restlet.routing.Filter.handle(Filter.java:197)
    at org.restlet.engine.CompositeHelper.handle(CompositeHelper.java:202)
    at org.restlet.Component.handle(Component.java:408)
    at org.restlet.Server.handle(Server.java:507)
    at org.restlet.engine.connector.ServerHelper.handle(ServerHelper.java:63)
    at org.restlet.engine.adapter.HttpServerHelper.handle(HttpServerHelper.java:143)
    at org.restlet.ext.servlet.ServerServlet.service(ServerServlet.java:1117)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
    at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:487)
    at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:362)
    at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
    at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:181)
    at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:726)
    at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:405)
    at org.mortbay.jetty.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:206)
    at org.mortbay.jetty.handler.HandlerCollection.handle(HandlerCollection.java:114)
    at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
    at org.mortbay.jetty.Server.handle(Server.java:324)
    at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:505)
    at org.mortbay.jetty.HttpConnection$RequestHandler.headerComplete(HttpConnection.java:829)
    at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:514)
    at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:211)
    at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:380)
    at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:395)
    at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:488)

Answer 1

您的JarByteClassloader 似乎没有加入默认类加载器的层次结构。确保构造函数调用super(ClassLoader)。这样的事情可能会奏效：

public class JarByteClassloader extends ClassLoader {
  public JarByteClassloader(Object objectLoadedByDefaultClassLoader) {
    super(objectLoadedByDefaultClassLoader.getClass().getClassLoader());
  }
}

由于您已经传入了由默认类加载器加载的jar，所以您已经完成了！

实际上，我只记得getSystemClassLoader()，所以这段代码可能更好，而且更能自我记录：

public class JarByteClassloader extends ClassLoader {
  public JarByteClassloader() {
    super(ClassLoader.getSystemClassLoader());
  }
}