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<?> loadClass(String name, boolean resolve)  throws ClassNotFoundException{  synchronized (getClassLoadingLock(name)) {    // First, check if the class has already been loaded    Class<?> 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     . . .   } }