jaxb 在new instance时候的调用classloader的方式

	private static JAXBContext createContext(Class<?> clazz) throws JAXBException {
		return JAXBContext.newInstance(clazz);
	}




        final String jaxbContextFQCN = JAXBContext.class.getName();
        String factoryClassName;

        // search for jaxb.properties in the class loader of each class first
        for (final Class c : classes) {
            // this classloader is used only to load jaxb.properties, so doing this should be safe.
            ClassLoader classLoader = AccessController.doPrivileged(new PrivilegedAction<ClassLoader>() {
                public ClassLoader run() {
                    return c.getClassLoader();
                }
            });
            Package pkg = c.getPackage();
            if(pkg==null)
                continue;       // this is possible for primitives, arrays, and classes that are loaded by poorly implemented ClassLoaders
            String packageName = pkg.getName().replace('.', '/');

            // TODO: do we want to optimize away searching the same package?  org.Foo, org.Bar, com.Baz
            //       classes from the same package might come from different class loades, so it might be a bad idea

            // TODO: it's easier to look things up from the class
            // c.getResourceAsStream("jaxb.properties");

            // build the resource name and use the property loader code
            String resourceName = packageName+"/jaxb.properties";
            logger.fine("Trying to locate "+resourceName);
            Properties props = loadJAXBProperties(classLoader, resourceName);
            if (props == null) {
                logger.fine("  not found");
            } else {
                logger.fine("  found");
                if (props.containsKey(JAXB_CONTEXT_FACTORY)) {
                    // trim() seems redundant, but adding to satisfy customer complaint
                    factoryClassName = props.getProperty(JAXB_CONTEXT_FACTORY).trim();
                    return newInstance(classes, properties, factoryClassName);
                } else {
                    throw new JAXBException(Messages.format(Messages.MISSING_PROPERTY, packageName, JAXB_CONTEXT_FACTORY));
                }
            }
        }

        // search for a system property second (javax.xml.bind.JAXBContext)
        logger.fine("Checking system property "+jaxbContextFQCN);
        factoryClassName = AccessController.doPrivileged(new GetPropertyAction(jaxbContextFQCN));
        if(  factoryClassName != null ) {
            logger.fine("  found "+factoryClassName);
            return newInstance( classes, properties, factoryClassName );
        }
        logger.fine("  not found");

        // search META-INF services next
        logger.fine("Checking META-INF/services");
        BufferedReader r;
        try {
            final String resource = new StringBuilder("META-INF/services/").append(jaxbContextFQCN).toString();
            ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
            URL resourceURL;
            if(classLoader==null)
                resourceURL = ClassLoader.getSystemResource(resource);
            else
                resourceURL = classLoader.getResource(resource);

            if (resourceURL != null) {
                logger.fine("Reading "+resourceURL);
                r = new BufferedReader(new InputStreamReader(resourceURL.openStream(), "UTF-8"));
                factoryClassName = r.readLine().trim();
                return newInstance(classes, properties, factoryClassName);
            } else {
                logger.fine("Unable to find: " + resource);
            }
        } catch (UnsupportedEncodingException e) {
            // should never happen
            throw new JAXBException(e);
        } catch (IOException e) {
            throw new JAXBException(e);
        }

        // else no provider found
        logger.fine("Trying to create the platform default provider");
        return newInstance(classes, properties, PLATFORM_DEFAULT_FACTORY_CLASS);




contextFinder

  private static Class safeLoadClass(String className, ClassLoader classLoader) throws ClassNotFoundException {
       logger.fine("Trying to load "+className);
       try {
          // make sure that the current thread has an access to the package of the given name.
          SecurityManager s = System.getSecurityManager();
          if (s != null) {
              int i = className.lastIndexOf('.');
              if (i != -1) {
                  s.checkPackageAccess(className.substring(0,i));
              }
          }
 
          if (classLoader == null) {
              return Class.forName(className);
          } else {
              return classLoader.loadClass(className);
          }
       } catch (SecurityException se) {
           // anyone can access the platform default factory class without permission
           if (PLATFORM_DEFAULT_FACTORY_CLASS.equals(className)) {
              return Class.forName(className);
           }
           throw se;
       }
    }