使用上下文加载程序加载资源失败，出现NullPointerException

Question

我只是想知道为什么我无法使用Felix OSGi中的线程上下文加载资源？我不应该触摸上下文加载器，我做错了什么或者是一个错误？

我有一个超级简单的包用一个简单的激活：

public class Activator implements BundleActivator { 
    public void start(BundleContext context) throws Exception { 
     System.out.println("Hello World!!"); 
     String resourcePath = "META-INF/mySuperDuperResource.txt"; 

     // works 
     System.out.println(Activator.class.getClassLoader().getResource(resourcePath)); 
     // null-pointer exception 
     System.out.println(Thread.currentThread().getContextClassLoader().getResource(resourcePath)); 

    } 
    public void stop(BundleContext context) throws Exception { 
     System.out.println("Goodbye World!!"); 
    } 
} 

现在正在加载的资源用的类加载器与Activator.class.getClassLoader工作。但是不能与Thread.currentThread（）。getContextClassLoader（）。在那里，我得到：当只设置线程上下文类加载器

ERROR: Bundle info.gamlor.osgi [26] Unable to get module class path. (java.lang.NullPointerException) 
java.lang.NullPointerException 
     at org.apache.felix.framework.BundleRevisionImpl.calculateContentPath(BundleRevisionImpl.java:410) 
     at org.apache.felix.framework.BundleRevisionImpl.initializeContentPath(BundleRevisionImpl.java:347) 
     at org.apache.felix.framework.BundleRevisionImpl.getContentPath(BundleRevisionImpl.java:333) 
     at org.apache.felix.framework.BundleRevisionImpl.getResourceLocal(BundleRevisionImpl.java:472) 
     at org.apache.felix.framework.BundleWiringImpl.findClassOrResourceByDelegation(BundleWiringImpl.java:1432) 
     at org.apache.felix.framework.BundleWiringImpl.getResourceByDelegation(BundleWiringImpl.java:1360) 
     at org.apache.felix.framework.BundleWiringImpl$BundleClassLoader.getResource(BundleWiringImpl.java:2256) 
     at info.gamlor.osgi.Activator.start(Activator.java:23) 
     at org.apache.felix.framework.util.SecureAction.startActivator(SecureAction.java:641) 
     at org.apache.felix.framework.Felix.activateBundle(Felix.java:1977) 
     at org.apache.felix.framework.Felix.startBundle(Felix.java:1895) 
     at org.apache.felix.framework.BundleImpl.start(BundleImpl.java:944) 
     ... 
org.osgi.framework.BundleException: Activator start error in bundle info.gamlor.osgi [29]. 
     at org.apache.felix.framework.Felix.activateBundle(Felix.java:2027) 
     at org.apache.felix.framework.Felix.startBundle(Felix.java:1895) 
     at org.apache.felix.framework.BundleImpl.start(BundleImpl.java:944) 
     at org.apache.felix.gogo.command.Basic.start(Basic.java:729) 
     ... 
Caused by: java.lang.NullPointerException 
     at org.apache.felix.framework.BundleRevisionImpl.getResourceLocal(BundleRevisionImpl.java:474) 
     at org.apache.felix.framework.BundleWiringImpl.findClassOrResourceByDelegation(BundleWiringImpl.java:1432) 
     at org.apache.felix.framework.BundleWiringImpl.getResourceByDelegation(BundleWiringImpl.java:1360) 
     at org.apache.felix.framework.BundleWiringImpl$BundleClassLoader.getResource(BundleWiringImpl.java:2256) 
     at info.gamlor.osgi.Activator.start(Activator.java:23) 
     at org.apache.felix.framework.util.SecureAction.startActivator(SecureAction.java:641) 
     at org.apache.felix.framework.Felix.activateBundle(Felix.java:1977) 
     ... 32 more 

现在它工作得很好：

Thread.currentThread().setContextClassLoader(Activator.class.getClassLoader()); 

但是，这有一个哈克的感觉吧。这样的感觉会在稍后咬我。

Answer 1

我不知道为什么你会惊讶这发生。默认情况下，一个线程的上下文类加载器被设置为它的父类的类加载器，它在开始时被设置为系统类加载器。所以，假设你没有做任何特别的事情，上下文类加载器就是系统类加载器，它与你的bundle的类加载器不一样，因此它找不到你的资源。

我同意设置上下文classloader有一个hacky的感觉，但有些库需要这样。我会做类似的，

ClassLoader previous = Thread.currentThread().getContextClassLoader(); 
Thread.currentThread().setContextClassLoader(getClass().getClassLoader()); 
badlyBehavedLibraryCall(); 
Thread.currentThread().setContextClassLoader(previous);