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(xù)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(shù)，不為空，不執(zhí)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(diào)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; 
  } 
}