定义

单例模式确保一个类只有一个实例, 并提供一个全局访问点

案例

实现单例模式只需要 私有的构造器, 一个静态方法和一个静态变量

BaseSingleton延迟加载, 存在线程安全问题

public class BaseSingleton {
  private static BaseSingleton instanse;
  private BaseSingleton() {
  }
  public static BaseSingleton getInstance() {
    if (instanse == null) {
      instanse = new BaseSingleton();
    }
    return instanse;
  }
}

解决一: 通过饿汉式, 一开始就初始化

public class HungrySingleton {
  private static HungrySingleton instance = new HungrySingleton();
  private HungrySingleton() {
  }
  public HungrySingleton getInstance() {
    return instance;
  }
}

解决二
SyncSingleton加synchronized关键字
问题: 每次经过getInstance方法都会有同步的性能损耗, 实际只需要在第一次instance为null的时候保证只创建一次就行了

public class SyncSingleton {
  private static SyncSingleton instance;
  private SyncSingleton() {
  }
  public synchronized SyncSingleton getInstance() {
    if (instance == null) {
      instance = new SyncSingleton();
    }
    return instance;
  }
}

解决三
DoubleCheckLockSingleton利用volatile和synchronized双重检查锁, 这样性能上损耗最低, 又是延迟加载线程安全的

public class DoubleCheckLockSingleton {
  private volatile static DoubleCheckLockSingleton instance;
  private DoubleCheckLockSingleton() {
  }
  public DoubleCheckLockSingleton getInstance() {
    if (instance == null) {
      synchronized (DoubleCheckLockSingleton.class) {
        if (instance == null) {
          instance = new DoubleCheckLockSingleton();
        }
      }
    }
    return instance;
  }
}