目前缺枚舉類實現(xiàn)：

package pattern;public class SingleInstance{private static SingleInstance sInstance = new SingleInstance();private SingleInstance(){}public static SingleInstance getInstance(){return sInstance;}public static void main(String[] arg) {System.out.println(SingleInstance.getInstance());System.out.println(SingleInstance.getInstance());System.out.println(SingleInstance2.getInstance());System.out.println(SingleInstance2.getInstance());System.out.println(SingleInstance3.getInstance());System.out.println(SingleInstance3.getInstance());System.out.println(SingleInstance4.getInstance());System.out.println(SingleInstance4.getInstance());System.out.println(SingleInstance5.getInstance());System.out.println(SingleInstance5.getInstance());} }class SingleInstance2 {private static SingleInstance2 sInstance;private SingleInstance2(){}public static SingleInstance2 getInstance(){if( null == sInstance){sInstance = new SingleInstance2();}return sInstance;}/* 上述的代碼在多個線程密集調(diào)用getInstance時，存在創(chuàng)建多個實例的可能。* 比如線程A進(jìn)入null == sInstance這段代碼塊，而在A線程未創(chuàng)建完成實例時，* 如果線程B也進(jìn)入了該代碼塊，必然會造成兩個實例的產(chǎn)生。* */ }class SingleInstance3 {private static SingleInstance3 sInstance;private SingleInstance3() {}public static synchronized SingleInstance3 getInstance(){if (null == sInstance){sInstance = new SingleInstance3();}return sInstance;} } /* Volatile是輕量級的synchronized，它在多處理器開發(fā)中保證了共享變量的“可見性”。* 可見性的意思是當(dāng)一個線程修改一個共享變量時，另外一個線程能讀到這個修改的值。* 使用volatile修飾sInstance變量之后，可以確保多個線程之間正確處理sInstance變量。* */ class SingleInstance4 {private static volatile SingleInstance4 sInstance;private SingleInstance4(){}public static SingleInstance4 getInstance(){if(null==sInstance){synchronized(SingleInstance4.class){if(null == sInstance){sInstance = new SingleInstance4();}}}return sInstance;}}/* 在Java中，類的靜態(tài)初始化會在類被加載時觸發(fā)，我們利用這個原理，可以實現(xiàn)利用這一特性，* 結(jié)合內(nèi)部類，可以實現(xiàn)如下的代碼，進(jìn)行懶漢式創(chuàng)建實例。* */ class SingleInstance5 {private SingleInstance5(){}public static SingleInstance5 getInstance() {return SingleInstanceHolder.sInstance;}private static class SingleInstanceHolder {private static SingleInstance5 sInstance = new SingleInstance5();}}

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