java基础学习笔记之类加载器

public class ClassLoaderTest {
  public static void main(String[] args) {
    System.out.println(System.class.getClassLoader());
  }
}

Exception in thread "main" java.lang.NullPointerException
  at com.iot.classloader.ClassLoaderTest.main(ClassLoaderTest.java:10)
  at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
  at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
  at java.lang.reflect.Method.invoke(Method.java:483)
  at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)

protected Class<&＃63;> loadClass(String name, boolean resolve)
  throws ClassNotFoundException
{
  synchronized (getClassLoadingLock(name)) {
    // First, check if the class has already been loaded
    Class<&＃63;> c = findLoadedClass(name);
    if (c == null) {
      long t0 = System.nanoTime();
      try {
        if (parent != null) {
          c = parent.loadClass(name, false);
        } else {
          c = findBootstrapClassOrNull(name);
        }
      } catch (ClassNotFoundException e) {
        // ClassNotFoundException thrown if class not found
        // from the non-null parent class loader
      }

      if (c == null) {
        // If still not found, then invoke findClass in order
        // to find the class.
        long t1 = System.nanoTime();
        c = findClass(name);

        // this is the defining class loader; record the stats
        sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
        sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
        sun.misc.PerfCounter.getFindClasses().increment();
      }
    }
    if (resolve) {
      resolveClass(c);
    }
    return c;
  }
}

class NetworkClassLoader extends ClassLoader {
   String host;
   int port;

   public Class findClass(String name) {
     byte[] b = loadClassData(name);
     return defineClass(name, b, 0, b.length);
   }

   private byte[] loadClassData(String name) {
     // load the class data from the connection
     . . .
   }
 }