单例模式的三种最佳方式

• 1,提前初始化

  public class Singleton {
  private static Singleton instance = new Singleton();
  private Singleton() {
  }

  public static Singleton getInstance() {
  return instance;
  }
  }

• 2.双重检查锁定 + volatile。

  public class Singleton {
  private static volatile Singleton instance;

  private Singleton() {
  }

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

• 3.延迟初始化占位类模式。

  public class Singleton {
  private static class InstanceHolder {
  public static Singleton instance = new Singleton();
  }

  private Singleton() {
  }

  public static Singleton getInstance() {
  return InstanceHolder.instance;
  }
  }