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;
            }
        });
    }
}

这是由于您的代码所依赖的类文件在编译时存在，但在运行时没有找到。寻找构建时和运行时类路径的差异。

此错误可能是由未检查的Java版本要求引起的。

在我的案例中，当我构建一个备受瞩目的开源项目时，通过使用SDKMAN从Java 9切换到Java 8，我能够解决这个错误。

sdk list java
sdk install java 8u152-zulu
sdk use java 8u152-zulu

然后按照下面的描述进行干净安装。

在使用Maven作为构建工具时，在禁用测试的情况下进行干净的“安装”构建有时是有帮助的，而且通常是令人满意的。

mvn clean install -DskipTests

现在已经构建并安装了所有内容，您可以继续运行测试。

mvn test

我发现，当使用运行时发现的不兼容的类版本编译代码时，有时会得到NoClassDefFound错误。我记得的具体实例是apache axis库。在我的运行时类路径上实际上有2个版本，它正在挑选过时和不兼容的版本，而不是正确的版本，导致NoClassDefFound错误。这是在一个命令行应用程序中，我正在使用类似的命令。

set classpath=%classpath%;axis.jar

我能够让它拿起正确的版本使用:

set classpath=axis.jar;%classpath%;

Java中的NoClassDefFoundError

定义:

Java虚拟机无法在运行时找到在编译时可用的特定类。 如果一个类在编译时存在，但在运行时在java类路径中不可用。

例子:

The class is not in Classpath, there is no sure shot way of knowing it but many times you can just have a look to print System.getproperty("java.classpath") and it will print the classpath from there you can at least get an idea of your actual runtime classpath. A simple example of NoClassDefFoundError is class belongs to a missing JAR file or JAR was not added into classpath or sometimes jar's name has been changed by someone like in my case one of my colleagues has changed tibco.jar into tibco_v3.jar and the program is failing with java.lang.NoClassDefFoundError and I were wondering what's wrong. Just try to run with explicitly -classpath option with the classpath you think will work and if it's working then it's a sure short sign that someone is overriding java classpath. Permission issue on JAR file can also cause NoClassDefFoundError in Java. Typo on XML Configuration can also cause NoClassDefFoundError in Java. when your compiled class which is defined in a package, doesn’t present in the same package while loading like in the case of JApplet it will throw NoClassDefFoundError in Java.

可能的解决方式:

The class is not available in Java Classpath. If you are working in J2EE environment than the visibility of Class among multiple Classloader can also cause java.lang.NoClassDefFoundError, see examples and scenario section for detailed discussion. Check for java.lang.ExceptionInInitializerError in your log file. NoClassDefFoundError due to the failure of static initialization is quite common. Because NoClassDefFoundError is a subclass of java.lang.LinkageError it can also come if one of it dependency like native library may not available. Any start-up script is overriding Classpath environment variable. You might be running your program using jar command and class was not defined in manifest file's ClassPath attribute.

资源:

3种方法解决NoClassDefFoundError

noclassdeffounderror问题模式

下面是演示java.lang.NoClassDefFoundError的代码。具体的解释请看Jared的回答。

NoClassDefFoundErrorDemo.java

public class NoClassDefFoundErrorDemo {
    public static void main(String[] args) {
        try {
            // The following line would throw ExceptionInInitializerError
            SimpleCalculator calculator1 = new SimpleCalculator();
        } catch (Throwable t) {
            System.out.println(t);
        }
        // The following line would cause NoClassDefFoundError
        SimpleCalculator calculator2 = new SimpleCalculator();
    }

}

SimpleCalculator.java

public class SimpleCalculator {
    static int undefined = 1 / 0;
}