我试图根据JFileChooser返回给我的文件来实例化一个类。所以到目前为止，我有JFileChooser工作，我可以成功获取我正在寻找的.class文件。但是，我实际上无法获取.class文件并使用它创建对象。
这是我写作的方法......
public static Agent loadAgentFromFile(File file){
Agent agent = null;
String fileName = file.getName();
String filePath = file.getAbsolutePath();
String parentDirectory = file.getParentFile().getName();
String agentClassName = fileName.substring(0, fileName.lastIndexOf('.'));
Object object = null;
//System.out.println(filePath.contains(parentDirectory));
//System.out.println(filePath.substring(0, filePath.indexOf(parentDirectory)));
try {
File f = new File(filePath.substring(0, filePath.indexOf(parentDirectory)));
URL[] cp = {f.toURI().toURL()};
URLClassLoader urlcl = new URLClassLoader(cp);
Class agentDefinition = null;
if (parentDirectory.equals("Agents"))
{
System.out.println(PACKAGE + agentClassName);
agentDefinition = urlcl.loadClass(PACKAGE + agentClassName);
//agentDefinition = Class.forName(PACKAGE + agentClassName);
}
else
{
System.out.println(PACKAGE + parentDirectory + "." + agentClassName);
agentDefinition = urlcl.loadClass(parentDirectory + "." + agentClassName);
}
object = agentDefinition.newInstance();
agent = (Agent)object;
}
catch (InstantiationException e) {
JOptionPane.showMessageDialog(THIS,
"The chosen class must not be an interface or be abstract, " +
"and it must not have any arguments in its constructor.");
}
catch (IllegalAccessException e) {
JOptionPane.showMessageDialog(THIS,
"The chosen class's constructor must be public.");
}
catch (ClassNotFoundException e) {
JOptionPane.showMessageDialog(THIS,
"The chosen class cannot be found. Make sure the .java file is compiled.");
}
catch (ClassCastException e) {
JOptionPane.showMessageDialog(THIS,
"The chosen class does not extend from the Agent class.");
}
catch (Exception e) {
e.printStackTrace(); //prints the stack to see what exception occurred that we didn't catch
JOptionPane.showMessageDialog(THIS,
"The chosen class does not conform to the \"Agent\" class.");
}
return agent;
}
然而，当这个方法被调用时，我实际上得到了一个NoClassDefFoundError，它被我的捕获所有异常所捕获：
Exception in thread "AWT-EventQueue-0" java.lang.NoClassDefFoundError: TestAgents/TestDFSAgent (wrong name: WumpusEnvironment/Model/Agent/TestAgents/TestDFSAgent)
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(Unknown Source)
at java.security.SecureClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.access$100(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at WumpusEnvironment.Model.Agent.AgentLoader.loadAgentFromFile(AgentLoader.java:41)
at WumpusEnvironment.View.MainWindow.ApplicationWindow.actionPerformed(ApplicationWindow.java:270)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.AbstractButton.doClick(Unknown Source)
at javax.swing.plaf.basic.BasicMenuItemUI.doClick(Unknown Source)
at javax.swing.plaf.basic.BasicMenuItemUI$Handler.mouseReleased(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$200(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)
我已经查看过相同类型的先前答案了，我最初在整个包结构中都有.class个文件，所以我移动了它们。我的类文件现在与源文件位于同一文件夹中，但我仍然收到此错误。可能有必要注意我尝试在Eclipse中运行它，尽管将来整个项目将被导出到.jar文件，该文件将实例化不会在其中的类。包结构（因为此时所有内容都在.jar内）。
有人能指出我在正确的方向吗？我还非常感谢您在导出到.jar文件时如何使建议继续有效。我意识到Eclipse设置一些相对于.java文件的.class文件的方式有时会弄乱这些东西。
谢谢！
答案 0 :(得分：1)
有人能指出我在正确的方向吗？
我认为正确的方向不是这样做的。我建议你：
将插件代理类放入JAR文件中（在JAR中使用正确的路径）。
为每个JAR文件创建单独的类加载器或为所有JAR文件创建一个类加载器...取决于是否需要将代理类彼此隔离。
您遇到的异常发生是因为类包名称与＆＃34; .class＆＃34;的路径不匹配。来自classloader的根目录的文件。在最糟糕的情况下，你不能让它匹配...没有移动＆＃34; .class＆＃34;文件，或编写一个棘手的自定义类加载器。即使在那时，你也有（潜在的）问题，即处理其他＆＃34; .class＆＃34;文件。
将插件放入JAR可以避免这些问题。