我已经尝试了Oracle的Java教程中的两个示例。它们都可以编译，但在运行时，都会出现这个错误:
Exception in thread "main" java.lang.NoClassDefFoundError: graphics/shapes/Square
at Main.main(Main.java:7)
Caused by: java.lang.ClassNotFoundException: graphics.shapes.Square
at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
... 1 more
我想我可能把Main.java文件放在错误的文件夹里了。
下面是目录层次结构:
graphics
├ Main.java
├ shapes
| ├ Square.java
| ├ Triangle.java
├ linepoint
| ├ Line.java
| ├ Point.java
├ spaceobjects
| ├ Cube.java
| ├ RectPrism.java
这是Main.java:
import graphics.shapes.*;
import graphics.linepoint.*
import graphics.spaceobjects.*;
public class Main {
public static void main(String args[]) {
Square s = new Square(2, 3, 15);
Line l = new Line(1, 5, 2, 3);
Cube c = new Cube(13, 32, 22);
}
}
我哪里做错了?
更新
在我把Main类放入图形包之后(我添加了图形包;对于它)，将类路径设置为“_test”(包含图形的文件夹)，编译它，并使用Java图形运行它。Main(从命令行)，它工作正常。
真的很晚更新#2
我没有使用Eclipse(只有notepad++和JDK)，上面的更新解决了我的问题。然而，这些答案中似乎有许多是针对Eclipse和IntelliJ IDEA的，但它们具有类似的概念。
当运行时类装入器装入的类不能访问已由Java rootloader装入的类时，我得到NoClassFoundError。因为不同的类装入器在不同的安全域中(根据Java)， JVM不允许在运行时装入器地址空间中解析已经由rootloader装入的类。
使用'java -javaagent:trace .jar [your 'java' ARGUMENTS]'运行程序
它产生显示已加载类的输出，以及加载该类的加载器环境。追踪类为什么不能解析是非常有用的。
// ClassLoaderTracer.java
// From: https://blogs.oracle.com/sundararajan/entry/tracing_class_loading_1_5
import java.lang.instrument.*;
import java.security.*;
// manifest.mf
// Premain-Class: ClassLoadTracer
// jar -cvfm tracer.jar manifest.mf ClassLoaderTracer.class
// java -javaagent:tracer.jar [...]
public class ClassLoadTracer
{
public static void premain(String agentArgs, Instrumentation inst)
{
final java.io.PrintStream out = System.out;
inst.addTransformer(new ClassFileTransformer() {
public byte[] transform(ClassLoader loader, String className, Class classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException {
String pd = (null == protectionDomain) ? "null" : protectionDomain.getCodeSource().toString();
out.println(className + " loaded by " + loader + " at " + new java.util.Date() + " in " + pd);
// dump stack trace of the thread loading class
Thread.dumpStack();
// we just want the original .class bytes to be loaded!
// we are not instrumenting it...
return null;
}
});
}
}
当在类路径中找不到预期的类时，就会发生无类定义异常。
在编译时类:类是从Java编译器生成的，但在运行时不知道为什么找不到依赖类。
让我们来看一个简单的例子:
public class ClassA{
public static void main(String args[]){
// Some gibberish code...
String text = ClassB.getString();
System.out.println("Text is: " + text);
}
}
public class ClassB{
public static String getString(){
return "Testing some exception";
}
}
现在让我们假设上述两个Java源代码被放置在某个文件夹中，例如“NoClassDefinationFoundExceptionDemo”
现在打开一个shell(假设Java已经正确设置)
Go to folder "NoClassDefinationFoundExceptionDemo"
Compile Java source files
javac ClassB
javac ClassA
Both files are compiled successfully and generated class files in the same folder as ClassA.class and ClassB.class
Now since we are overriding ClassPath to the current working director, we execute the following command
java -cp . ClassA
and it worked successfully and you will see the output on the screen
Now let's say, you removed ClassB.class file from the present directory.
And now you execute the command again.
java -cp . ClassA Now it will greet you with NoClassDefFoundException. As ClassB which is a dependency for ClassA is not found in the classpath (i.e., the present working directory).
