public class Singleton {   private static Singleton intance;   private Singleton() {}      public static Singleton getInstance() {     /*      * 一開始多線程進來，遇到鎖，一個線程進去，是為空，new對象； 后續線程進入，不為空，不操作；最后直接返回      * 對象不為空，再有多個線程進入該函數，不為空，不執行加鎖操作，直接返回      */     if (intance == null) {       synchronized (Singleton.class) {         if (intance == null) {           intance = new Singleton();         }       }     }     return intance;   } }  class Singleton1 {// 懶漢式    private static Singleton1 intance = new Singleton1();//懶的，程序運行的時候就加載出來了   private Singleton1() {}      public static Singleton1 getInstance() {     return intance;   } }  class Singleton2 {// 餓漢式   private static Singleton2 intance;   private Singleton2() {}      public static Singleton2 getInstance() {//用到的時候 才加載     if (intance == null) {       intance = new Singleton2();     }     return intance;   } }  class Singleton3 {// 餓漢式 線程安全   private static Singleton3 intance;   private Singleton3() {}      public synchronized static Singleton3 getInstance() {//用到的時候 才加載, 加鎖 多線程調用，都有一個加鎖的動作     if (intance == null) {       intance = new Singleton3();     }     return intance;   } }  class Singleton4 {// 餓漢式 線程安全   private static Singleton4 intance;   private Singleton4() {}      public static Singleton4 getInstance() {//用到的時候 才加載     synchronized (Singleton4.class) {// 加鎖 效率跟3差不多       if (intance == null) {         intance = new Singleton4();       }     }     return intance;   } }