当运行时类装入器装入的类不能访问已经由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不允许已经由rootloader装入的类在运行时装入器地址空间中被解析。

使用'java -javaagent:trace .jar[你的java ARGS]'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env。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;
            }
        });
    }
}

虽然这可能是由于编译时和运行时之间的类路径不匹配造成的，但这并不一定是真的。

在这种情况下，记住两到三个不同的异常是很重要的:

java.lang.ClassNotFoundException This exception indicates that the class was not found on the classpath. This indicates that we were trying to load the class definition, and the class did not exist on the classpath. java.lang.NoClassDefFoundError This exception indicates that the JVM looked in its internal class definition data structure for the definition of a class and did not find it. This is different than saying that it could not be loaded from the classpath. Usually this indicates that we previously attempted to load a class from the classpath, but it failed for some reason - now we're trying to use the class again (and thus need to load it, since it failed last time), but we're not even going to try to load it, because we failed loading it earlier (and reasonably suspect that we would fail again). The earlier failure could be a ClassNotFoundException or an ExceptionInInitializerError (indicating a failure in the static initialization block) or any number of other problems. The point is, a NoClassDefFoundError is not necessarily a classpath problem.

当运行时类装入器装入的类不能访问已经由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不允许已经由rootloader装入的类在运行时装入器地址空间中被解析。

使用'java -javaagent:trace .jar[你的java ARGS]'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env。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;
            }
        });
    }
}

这也可能是因为您从具有特定包名的IDE复制了代码文件，并且希望尝试使用终端运行它。您必须首先从代码中删除包名。 这种事发生在我身上。

我从SRC库中删除了两个文件后得到了这个消息，当我把它们带回来时，我一直看到这个错误消息。

我的解决方案是:重新启动Eclipse。从那以后我就再没见过这条消息了:-)

当静态初始化器试图加载一个在运行时不可用的资源包时，也会发生NoClassDefFoundError，例如受影响的类试图从META-INF目录加载一个属性文件，但不存在。如果你没有捕获NoClassDefFoundError，有时你将无法看到完整的堆栈跟踪;为了克服这个问题，你可以临时为Throwable使用一个catch子句:

try {
    // Statement(s) that cause(s) the affected class to be loaded
} catch (Throwable t) {
    Logger.getLogger("<logger-name>").info("Loading my class went wrong", t);
}