I have had an interesting issue wiht NoClassDefFoundError in JavaEE working with Liberty server. I was using IMS resource adapters and my server.xml had already resource adapter for imsudbJXA.rar. When I added new adapter for imsudbXA.rar, I would start getting this error for instance objects for DLIException, IMSConnectionSpec or SQLInteractionSpec. I could not figure why but I resolved it by creating new server.xml for my work using only imsudbXA.rar. I am sure using multiple resource adapters in server.xml is fine, I just had no time to look into that.

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;
}

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);
}

在我的情况下，由于JDK版本不匹配，我得到了这个错误。当我试图从Intelij运行应用程序时，它不工作，但从命令行运行它就工作了。这是因为Intelij试图用Java 11 JDK运行它，但在命令行上，它是用Java 8 JDK运行的。在文件>项目结构>项目设置>项目SDK下切换设置后，它为我工作了。

我通过禁用所有模块的preDexLibraries来解决我的问题:

dexOptions {
        preDexLibraries false
        ...