无法在 jar 文件中加载类答案

【问题标题】：Unable to load classes in a jar file无法在 jar 文件中加载类
【发布时间】：2014-11-01 18:57:58
【问题描述】：

我目前正在从事一个旨在接受插件的项目。为了从他们的 jar 文件中加载插件，我使用了ClassLoader。这是我用来加载插件的代码。

URLClassLoader loader = URLClassLoader.newInstance(new URL[] {pluginJar.toURI().toURL()},      this.getClass().getClassLoader());
    try {
        Class<?> clazz= Class.forName("msciplugin.PluginDriver", true, loader);

        Constructor<?> constructor=clazz.getConstructor(ProgramManager.class);
        MSCIPlugin plugin=(MSCIPlugin) constructor.newInstance(programManager);

        plugins.add(plugin);
        plugin.addMSCIPluginListener(this);
        this.pluginAdded(plugin);
    } catch (Exception e) {
        e.printStackTrace();
        return false;
    }
loader.close();

当我运行程序时，我能够加载我创建的测试插件，但在程序的后面，我要求插件执行一个操作，该操作使用插件 jar 中最初未使用的类。然后程序抛出由ClassNotFoundException引起的NoClassDefFoundError

Exception in thread "AWT-EventQueue-0" java.lang.NoClassDefFoundError: msciplugin/TimeUtils
    at   msciplugin.ServerControllerConfigurationWindow.refreashEditor(ServerControllerConfigurationWindow.java:195)
    at msciplugin.ServerControllerConfigurationWindow.valueChanged(ServerControllerConfigurationWindow.java:299)
    at javax.swing.JList.fireSelectionValueChanged(Unknown Source)
    at javax.swing.JList$ListSelectionHandler.valueChanged(Unknown Source)
    at javax.swing.DefaultListSelectionModel.fireValueChanged(Unknown Source)
    at javax.swing.DefaultListSelectionModel.fireValueChanged(Unknown Source)
    at javax.swing.DefaultListSelectionModel.fireValueChanged(Unknown Source)
    at javax.swing.DefaultListSelectionModel.changeSelection(Unknown Source)
    at javax.swing.DefaultListSelectionModel.changeSelection(Unknown Source)
    at javax.swing.DefaultListSelectionModel.setSelectionInterval(Unknown Source)
    at javax.swing.JList.setSelectionInterval(Unknown Source)
    at javax.swing.plaf.basic.BasicListUI$Handler.adjustSelection(Unknown Source)
    at javax.swing.plaf.basic.BasicListUI$Handler.mousePressed(Unknown Source)
    at java.awt.AWTEventMulticaster.mousePressed(Unknown Source)
    at java.awt.Component.processMouseEvent(Unknown Source)
    at javax.swing.JComponent.processMouseEvent(Unknown Source)
    at java.awt.Component.processEvent(Unknown Source)
    at java.awt.Container.processEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Window.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$400(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)
Caused by: java.lang.ClassNotFoundException: msciplugin.TimeUtils
    at java.net.URLClassLoader$1.run(Unknown Source)
    at java.net.URLClassLoader$1.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at java.net.FactoryURLClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    ... 45 more

任何帮助将不胜感激。

【问题讨论】：

标签： java plugins classloader

【解决方案1】：

我发现了问题所在。加载类 msciplugin.PluginDriver 时，URLClassLoader 没有加载所有其他需要的类。我通过加载 jar 文件中的所有类解决了这个问题。我使用here的方法找到了jar文件中的所有类。

【讨论】：