迭代ClassLoader.getResources()的结果时出现奇怪的NPE

Question

我假设下面的代码是安全的,但是我在调​​用时得到了一个N​​PE hasMoreElements().什么想法可能是错的？

我应该补充一点,我在Windows上使用Java 1.7.0_55-b13,64位.

final List<URL> urls = new ArrayList<URL>();
final String plUri = "META-INF/plugin.xml";
Enumeration<URL> urlsEn =
   Thread.currentThread().getContextClassLoader().getResources(pluginsUri);
if (urlsEn != null) {
  while (urlsEn.hasMoreElements()) {  //  NPE happens here
    final URL u = urlsEn.nextElement();
    urls.add(u);
  }
}

堆栈跟踪:

java.lang.NullPointerException
    at sun.misc.MetaIndex.mayContain(MetaIndex.java:243)
    at sun.misc.URLClassPath$JarLoader.getResource(URLClassPath.java:830)
    at sun.misc.URLClassPath$2.next(URLClassPath.java:273)
    at sun.misc.URLClassPath$2.hasMoreElements(URLClassPath.java:283)
    at java.lang.ClassLoader$2.hasMoreElements(ClassLoader.java:1322)
    at sun.misc.CompoundEnumeration.next(CompoundEnumeration.java:45)
    at sun.misc.CompoundEnumeration.hasMoreElements(CompoundEnumeration.java:54)
    at sun.misc.CompoundEnumeration.next(CompoundEnumeration.java:45)
    at sun.misc.CompoundEnumeration.hasMoreElements(CompoundEnumeration.java:54)
    at com.github.jochen.afw.core.guice.GuiceComponentFactoryBuilder.getComponentUrls(GuiceComponentFactoryBuilder.java:256)
    at com.github.jochen.afw.core.guice.GuiceComponentFactoryBuilder.build(GuiceComponentFactoryBuilder.java:160)
    at com.github.jochen.afw.core.guice.GuiceComponentFactoryBuilderTest.testSuccessfullConfiguration(GuiceComponentFactoryBuilderTest.java:20)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    ....

Answer 1

我不想说你的问题是如此简单，但可能pluginsUri在null这里吗？至少在您的代码片段中，您创建了一个plUri变量，但随后传入了一个未提及的pluginsUrito ClassLoader.getResources()。

从您的堆栈跟踪中，搜索“URLClassPath 空指针异常”发现了这个问题，它看起来是相同的堆栈跟踪。在他们的例子中， 的论点getResources()显然是无效的。

查看Java 7 代码库，我们看到MetaIndex:243：

if (entry.startsWith(conts[i])) {

并且entry可能null在这条线上。查看堆栈的较高位置，entry看起来是name您传递给 的参数ClassLoader.getResources()。

本次SSCCE：

public class ClassLoaderNPE {
  public static void main(String[] args) throws IOException {
      Enumeration<URL> urls = ClassLoader.getSystemClassLoader().getResources(null);
      System.out.println(urls.hasMoreElements());
  }
}

复制您的堆栈跟踪（在 Java 8 中也同样如此）：

$ java -version
java version "1.8.0_45"
Java(TM) SE Runtime Environment (build 1.8.0_45-b15)
Java HotSpot(TM) 64-Bit Server VM (build 25.45-b02, mixed mode)

$ java -cp . ClassLoaderNPE
Exception in thread "main" java.lang.NullPointerException
        at sun.misc.MetaIndex.mayContain(MetaIndex.java:242)
        at sun.misc.URLClassPath$JarLoader.getResource(URLClassPath.java:995)
        at sun.misc.URLClassPath$2.next(URLClassPath.java:288)
        at sun.misc.URLClassPath$2.hasMoreElements(URLClassPath.java:298)
        at java.lang.ClassLoader$2.hasMoreElements(ClassLoader.java:1278)
        at sun.misc.CompoundEnumeration.next(CompoundEnumeration.java:45)
        at sun.misc.CompoundEnumeration.hasMoreElements(CompoundEnumeration.java:54)
        at sun.misc.CompoundEnumeration.next(CompoundEnumeration.java:45)
        at sun.misc.CompoundEnumeration.hasMoreElements(CompoundEnumeration.java:54)
        at ClassLoaderNPE.main(ClassLoaderNPE.java:9)

JDK没有出现指定如果是的话会发生什么。我已经提交了一个错误，建议修复此行为，或者至少澄清文档。如果/当 Oracle 接受该问题时，我将更新这篇文章。namenull

更新：该报告被跟踪为JDK-8136831，并已在 Java 9 中修复。