JAVA学习笔记（单利模式）

单利模式只生成一个实例，减少系统性能开销，生成一个单利对象后，永久驻留内存
1.饿汉式

public class SingletonClass {
    private static SingletonClass instanceClass = new SingletonClass();
    private SingletonClass() {  
    }
    public static SingletonClass shareInstance() {
        return instanceClass;
    }
}

2.懒汉式

public class SingletonClass {
    
    private static SingletonClass instanceClass = null;
    private SingletonClass() {
    }
        //方法一
    public static synchronized SingletonClass shareInstance() {
        if (instanceClass == null) {
            instanceClass = new SingletonClass();
        }
        return instanceClass;
    }
        //方法二
     public static SingletonClass shareInstance() {
          if (instanceClass == null) {
                 synchronized(SingletonClass.class){
                      if (instanceClass == null) {
                             instanceClass = new SingletonClass();
                       }
                  }
          }
          return instanceClass;
     }
}

区别：
饿汉式编译之后在内存中就存在，线程安全
懒汉式延时对象创建，非线程安全