为什么这个异常没有被捕获
我有以下代码，
try {
xpathInstance = XPath.newInstance(xpathExpr);
list = (Text) xpathInstance.selectSingleNode(doc);
} catch (JDOMException e) {
throw new Exception(e);
}
但我忘记包含一个依赖于 jdom.jar 库的库。当我运行该应用程序时，我看到了这个错误。
Exception in thread "main" java.lang.NoClassDefFoundError: org/jaxen/NamespaceContext
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:141)
at org.jdom.xpath.XPath.newInstance(XPath.java:134)
at com.myapp.parser.GenericXMLParser.getSingleNodeValue(GenericXMLParser.java:63)
根据 JDOM 文档， newInsance() 方法会抛出 JDOMEXCeption ，所以它不应该捕获错误吗？
另外，如何避免添加 catch(Exception e) 子句以避免未知异常。
谢谢
I have the following code
try {
xpathInstance = XPath.newInstance(xpathExpr);
list = (Text) xpathInstance.selectSingleNode(doc);
} catch (JDOMException e) {
throw new Exception(e);
}
I had forgotten to include a library that was a dependency of the jdom.jar library. When i run the application i saw this error.
Exception in thread "main" java.lang.NoClassDefFoundError: org/jaxen/NamespaceContext
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:141)
at org.jdom.xpath.XPath.newInstance(XPath.java:134)
at com.myapp.parser.GenericXMLParser.getSingleNodeValue(GenericXMLParser.java:63)
According to the JDOM documentation, the newInsance() method throws a JDOMEXCeption so shouldnt it have caught the error?
Also, how can i avoid having to add a catch(Exception e) clause to avoid unknown exceptions.
Thanks
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评论（3）
您在运行时错误中看到的错误，该错误是针对预期位于 CLASSPATH 中但未找到的类而引发的。如果 jdom.jar 确实包含 org/jaxen/NamespaceContext 类，那么应该可以解决此问题。
不，这不是
JDOMException，而是NoClassDefFoundError，因此它不会捕获它。 最重要的是，这发生在 JDOM 类出现之前 - 发生在类加载期间。一般来说，您不应该尝试捕获 NoClassDefFoundError，因为它是一种属于
无法恢复的失败类别的错误。您可以尝试通过使用反射并捕获 ClassNotFoundException 来解决这个问题，但正如我一般所说，这是一个您无法从中恢复的异常，因此尝试捕获它可能是一个没有实际意义的问题。The error that you saw in a runtime error thrown for a class that was expected to be in the CLASSPATH but was not found. If
jdom.jardoes indeed includeorg/jaxen/NamespaceContextclass then that should fix this issue.No this is not a
JDOMException, it's aNoClassDefFoundError, therefore it does not catch it. Most importantly, this happens before JDOM class is in the picture - happens during class loading.In general you should not try to catch NoClassDefFoundError since it is a type of error that falls under the category of
failures from which recovery is not feasible. You can try to work around it by using Reflection and catchingClassNotFoundExceptionbut as I said in general this is an exception you cannot recover from so attempts to catch it is probably a moot point.此异常不是由构造函数引发的。它是由类加载器引发的。当它尝试在构造函数运行之前加载您的类时，找不到该类，并且引发了您尚未处理的异常（未找到类定义错误）（顺便说一句，无法处理错误）。
This exception is not raised by the constructor. It's raised by the class loader. When it tries to load your class long before the constructor runs, the class was not found and this exception (No Class Defination Found Error) was raised which you have not handled (BTW errors can't be handled).
它没有被抓住，因为它没有被抛出。抛出的异常是
java.lang.NoClassDefFoundError并且如果你想捕获异常，你必须捕获它。您无法采取任何措施来避免这种情况，这会破坏整个例外点。
It wasn't caught because it wasn't thrown. The exception thrown was a
java.lang.NoClassDefFoundErrorAnd if you want to catch an Exception, you have to catch it. There is nothing you can do to avoid this, that would kind of defeat the whole point of exceptions.