单例模式

Java中的单例

饿汉

public class Single {
  private static Signle instance = new Single();

  private Single() {}

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

懒汉

private static Single instance= null;
private Single() {}

public static Single getInstance () {
  if (instance == null) {
    instance = new Single()
  }

  return instance
}

懒汉线程安全

public static synchronized Single getInstance() {
  if (instance == null) {
    instance = new Single()
  }

  return instance
}

双检验

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

登记静态

private static class SingleHolder {
  private static final Single INSTANCE = new Signle()
}

private Single() {

}

public static final Single getInstance() {
  return SingleHolder.INSTANCE
}

枚举

public enum Single {
  INSTANCE;
  public void method() {
    
  }
}

JavaScript中的单例

闭包实现单例

function A () {
  this.data = "Class A"
}

A.getInstance = function () {
  var instance;
  return function () {
    instance = instance ? instance: new A
    return instance
  }
}()

正常单例

let A = function () {
  if (A.instance) {
    console.log("no new")
  }
  this.data = "Class A"
}

A.getInstance = function () {
  if (A.instance == null) {
    A.instance = new A()
  }
  return A.instance
}