注册式单例
將每一個實例都緩存到統(tǒng)一的容器中，使用唯一標識獲取實例
//常量中去使用，常量不就是用來大家都能夠共用嗎？ //通常在通用API中使用 public enum EnumSingleton {INSTANCE;private Object data;public Object getData() {return data;}public void setData(Object data) {this.data = data;}public static EnumSingleton getInstance(){return INSTANCE;} } public class EnumSingletonTest { // public static void main(String[] args) { // try { // EnumSingleton instance1 = null; // // EnumSingleton instance2 = EnumSingleton.getInstance(); // instance2.setData(new Object()); // // FileOutputStream fos = new FileOutputStream("EnumSingleton.obj"); // ObjectOutputStream oos = new ObjectOutputStream(fos); // oos.writeObject(instance2); // oos.flush(); // oos.close(); // // FileInputStream fis = new FileInputStream("EnumSingleton.obj"); // ObjectInputStream ois = new ObjectInputStream(fis); // instance1 = (EnumSingleton) ois.readObject(); // ois.close(); // // System.out.println(instance1.getData()); // System.out.println(instance2.getData()); // System.out.println(instance1.getData() == instance2.getData()); // // }catch (Exception e){ // e.printStackTrace(); // } // }public static void main(String[] args) {try {Class clazz = EnumSingleton.class;Constructor c = clazz.getDeclaredConstructor(String.class,int.class);c.setAccessible(true);EnumSingleton enumSingleton = (EnumSingleton)c.newInstance("Tom",666);}catch (Exception e){e.printStackTrace();}}} static {INSTANCE = new EnumSingleton("INSTANCE", 0);$VALUES = (new EnumSingleton[] {INSTANCE}); }原來，枚舉式單例在靜態(tài)代碼塊中就給INSTANCE 進行了賦值，是餓漢式單例的實現。至此，我們還可以試想，序列化我們能否破壞枚舉式單例呢？我們不妨再來看一下JDK源碼，還是回到ObjectInputStream 的readObject0()方法：
private Object readObject0(boolean unshared) throws IOException {...case TC_ENUM:return checkResolve(readEnum(unshared));... }我們看到在readObject0()中調用了readEnum()方法，來看readEnum()中代碼實現：
case TC_ENUM:return checkResolve(readEnum(unshared)); private Enum<?> readEnum(boolean unshared) throws IOException {if (bin.readByte() != TC_ENUM) {throw new InternalError();}ObjectStreamClass desc = readClassDesc(false);if (!desc.isEnum()) {throw new InvalidClassException("non-enum class: " + desc);}int enumHandle = handles.assign(unshared ? unsharedMarker : null);ClassNotFoundException resolveEx = desc.getResolveException();if (resolveEx != null) {handles.markException(enumHandle, resolveEx);}String name = readString(false);Enum<?> result = null;Class<?> cl = desc.forClass();if (cl != null) {try {@SuppressWarnings("unchecked")Enum<?> en = Enum.valueOf((Class)cl, name);result = en;} catch (IllegalArgumentException ex) {throw (IOException) new InvalidObjectException("enum constant " + name + " does not exist in " +cl).initCause(ex);}if (!unshared) {handles.setObject(enumHandle, result);}}handles.finish(enumHandle);passHandle = enumHandle;return result; }我們發(fā)現枚舉類型其實通過類名和Class 對象類找到一個唯一的枚舉對象。因此，枚舉對象不可能被類加載器加載多次。那么反射是否能破壞枚舉式單例呢？來看一段測試代碼：
Enum<?> en = Enum.valueOf((Class)cl, name); public static void main(String[] args) {try {Class clazz = EnumSingleton.class;Constructor c = clazz.getDeclaredConstructor(String.class,int.class);c.setAccessible(true);EnumSingleton enumSingleton = (EnumSingleton)c.newInstance("Tom",666);}catch (Exception e){e.printStackTrace();} }報的是java.lang.NoSuchMethodException 異常，意思是沒找到無參的構造方法。這時候，我們打開java.lang.Enum 的源碼代碼，查看它的構造方法，只有一個protected的構造方法，代碼如下：
protected Enum(String name, int ordinal) {this.name = name;this.ordinal = ordinal; }這時錯誤已經非常明顯了，告訴我們Cannot reflectively create enum objects，不能用反射來創(chuàng)建枚舉類型。還是習慣性地想來看看JDK 源碼，進入Constructor 的newInstance()方法：
@CallerSensitive public T newInstance(Object ... initargs)throws InstantiationException, IllegalAccessException,IllegalArgumentException, InvocationTargetException {if (!override) {if (!Reflection.quickCheckMemberAccess(clazz, modifiers)) {Class<?> caller = Reflection.getCallerClass();checkAccess(caller, clazz, null, modifiers);}}if ((clazz.getModifiers() & Modifier.ENUM) != 0)throw new IllegalArgumentException("Cannot reflectively create enum objects");ConstructorAccessor ca = constructorAccessor; // read volatileif (ca == null) {ca = acquireConstructorAccessor();}@SuppressWarnings("unchecked")T inst = (T) ca.newInstance(initargs);return inst; }在newInstance()方法中做了強制性的判斷，如果修飾符是Modifier.ENUM 枚舉類型，直接拋出異常。到這為止，我們是不是已經非常清晰明了呢？枚舉式單例也是《Effective Java》書中推薦的一種單例實現寫法。在JDK 枚舉的語法特殊性，以及反射也為枚舉保駕護航，讓枚舉式單例成為一種比較優(yōu)雅的實現。
接下來看注冊式單例還有另一種寫法，容器緩存的寫法，創(chuàng)建ContainerSingleton 類：
容器式寫法適用于創(chuàng)建實例非常多的情況，便于管理。但是，是非線程安全的。到此，注冊式單例介紹完畢。我們還可以來看看Spring 中的容器式單例的實現代碼：
public class ContainerSingletonTest {public static void main(String[] args) {try {long start = System.currentTimeMillis();ConcurrentExecutor.execute(new ConcurrentExecutor.RunHandler() {public void handler() {Object obj = ContainerSingleton.getInstance("com.leon.pattern.singleton.Pojo");;System.out.println(System.currentTimeMillis() + ": " + obj);}}, 10,6);long end = System.currentTimeMillis();System.out.println("總耗時：" + (end - start) + " ms.");}catch (Exception e){e.printStackTrace();}} } public class Pojo { } public class ConcurrentExecutor {/*** @param runHandler* @param executeCount 發(fā)起請求總數* @param concurrentCount 同時并發(fā)執(zhí)行的線程數* @throws Exception*/public static void execute(final RunHandler runHandler,int executeCount,int concurrentCount) throws Exception {ExecutorService executorService = Executors.newCachedThreadPool();//控制信號量，此處用于控制并發(fā)的線程數final Semaphore semaphore = new Semaphore(concurrentCount);//閉鎖，可實現計數量遞減final CountDownLatch countDownLatch = new CountDownLatch(executeCount);for (int i = 0; i < executeCount; i ++){executorService.execute(new Runnable() {public void run() {try{//執(zhí)行此方法用于獲取執(zhí)行許可，當總計未釋放的許可數不超過executeCount時,//則允許同性，否則線程阻塞等待，知道獲取到許可semaphore.acquire();runHandler.handler();//釋放許可semaphore.release();}catch (Exception e){e.printStackTrace();}countDownLatch.countDown();}});}countDownLatch.await();//線程阻塞，知道閉鎖值為0時，阻塞才釋放，繼續(xù)往下執(zhí)行executorService.shutdown();}public interface RunHandler{void handler();} }?
總結
