我假设以下代码是安全的,但是我在调用 hasMoreElements()
时遇到了 NPE。有什么想法可能是错的吗?
我应该补充一点,我在 Windows 64 位上使用 Java 1.7.0_55-b13。
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)
....
最佳答案
我不想暗示你的问题是如此简单,但是 pluginsUri
可以是 null
吗?至少在您的代码片段中,您创建了一个 plUri
变量,然后将未提及的 pluginsUri
传递给 ClassLoader.getResources()
。
从您的堆栈跟踪中,搜索“URLClassPath 空指针异常”未发现 this question看起来是相同的堆栈跟踪。在他们的例子中,getResources()
的参数显然为空。
查看 Java 7 codebase ,我们看到 MetaIndex:243
是:
if (entry.startsWith(conts[i])) {
entry
在此行可以是 null
。向上看堆栈,entry
看起来是您传递给 ClassLoader.getResources()
的 name
参数。
这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,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不显示指定如果 name
为 null
会发生什么。我已经提交了一个错误来建议修复此行为,或者至少澄清文档。如果/当 Oracle 接受这个问题时,我会更新这篇文章。
更新:报告跟踪为JDK-8136831 , 并已在 Java 9 中修复。
关于java - 迭代 ClassLoader.getResources() 的结果时出现奇怪的 NPE,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/23650709/