轉換爲同一對象時發生ClassCastException

Question

Java會拋出什麼可能的原因動態加載類時無法投射異常？

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(); 
    } 

如果這個代碼拋出的錯誤，我不認爲有另一個類加載器在這裏（多類加載器的情況下）。與的JUnit運行，或者只是在一個的main（）方法運行時，

此代碼工作正常。

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

它看起來像你的JarByteClassloader不加入默認的類加載器的層次：一個Servlet容器中運行時（例如mvn jetty:run）

登錄出現偏色問題。確保構造函數調用super(ClassLoader)。像這樣的東西可能會奏效：

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

既然你已經傳入，是由默認的類加載器加載一個jar，大功告成！

其實，我只是記得getSystemClassLoader()，所以這段代碼可能是更好的，更自我記錄：

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