class SuperClass{ static{ System.out.println("SuperClass init!"); } public static int value = 123; } class SubClass extends SuperClass{ static{ System.out.println("SubClass init!");//子類中援用父類的靜態(tài)字段，不會(huì)致使類初始化 } } public class Test { public static void main(String[] args) { System.out.println(SubClass.value); } }

class ConstClass{ static{ System.out.println("ConstClass init!"); } public static final String HELLOWORLD = "hello world"; } public class Test { public static void main(String[] args) { System.out.println(ConstClass.HELLOWORLD); } }

public class ClassLoaderTest { public static void main(String[] args) throws Exception { ClassLoader myLoader = new ClassLoader() { @Override public Class<?> loadClass(String name) throws ClassNotFoundException { try { String fileName = name.substring(name.lastIndexOf(".") + 1) + ".class"; InputStream is = getClass().getResourceAsStream(fileName); if (is == null) { return super.loadClass(name); } byte[] b = new byte[is.available()]; is.read(b); return defineClass(name, b, 0, b.length); } catch (IOException e) { throw new ClassNotFoundException(name); } } }; Class c = myLoader.loadClass("org.bupt.xiaoye.blog.ClassLoaderTest"); Object obj = c.newInstance(); System.out.println(obj.getClass()); System.out.println(ClassLoaderTest.class); System.out.println(obj instanceof ClassLoaderTest); } }

protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException { synchronized (getClassLoadingLock(name)) { // 首先檢查類是不是已被加載過(guò) Class c = findLoadedClass(name); if (c == null) { long t0 = System.nanoTime(); try { if (parent != null) { // 調(diào)用父類加載器加載 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. //父類加載器沒(méi)法完成加載，調(diào)用本身的加載器加載 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; } }