设计模式学习（五）：单例模式 （上）

2018年08月30日

目录

​​1、单例模式概念​​

​​2、饿汉模式​​

​​3、懒汉模式​​

---

 

1、单例模式概念

• 确保某一个类只有一个实例，并且自行实例化，并且向整个系统提供这个实例。
• 与全局变量的区别：全局变量不能实现继承，而单例模式可以；

 

2、饿汉模式

package designModel.SingleExampleModel;

public class testSingleExample {
  
  

  public testSingleExample() {}
  
  private static testSingleExample test = new testSingleExample();
  
  public static testSingleExample getInstance(){
    if(test == null){
      System.out.println("未实例化。。");
    }
    return test;
  }

  public static void main(String[] args) {
    // TODO Auto-generated method stub
    System.out.println("1 "+testSingleExample.getInstance());
    Thread thread = new Thread(new Runnable(){

      @Override
      public void run() {
        // TODO Auto-generated method stub
        for(int i =1; i<11; i++){          
          System.out.println(i+" "+testSingleExample.getInstance());
        }
      }
      
    });
    thread.start();
  }

}

 

说明：饿汉模式，即对象实例化发生在编译时，static 静态变量被初始化了。

 

console：

1 designModel.SingleExampleModel.testSingleExample@5ce345c2
1 designModel.SingleExampleModel.testSingleExample@5ce345c2
2 designModel.SingleExampleModel.testSingleExample@5ce345c2
3 designModel.SingleExampleModel.testSingleExample@5ce345c2
4 designModel.SingleExampleModel.testSingleExample@5ce345c2
5 designModel.SingleExampleModel.testSingleExample@5ce345c2
6 designModel.SingleExampleModel.testSingleExample@5ce345c2
7 designModel.SingleExampleModel.testSingleExample@5ce345c2
8 designModel.SingleExampleModel.testSingleExample@5ce345c2
9 designModel.SingleExampleModel.testSingleExample@5ce345c2
10 designModel.SingleExampleModel.testSingleExample@5ce345c2
 

可见上面获取到的对象都是同一个，单例模式效果。

 

3、懒汉模式

package designModel.SingleExampleModel;

public class testSingleExampleOfLazyType {

  public testSingleExampleOfLazyType() {}
  
  public static testSingleExampleOfLazyType test = null;
  
  public static testSingleExampleOfLazyType getInstance(){
    if(test == null){
      System.out.println("懒汉模式==实例化。。。");
      test = new testSingleExampleOfLazyType();
      return test;
    }else {
      return test;
    }
  }
  
  public static void main(String [] args){
    System.out.println(testSingleExampleOfLazyType.getInstance());
    
    Thread thread = new Thread(new Runnable(){

      @Override
      public void run() {
        // TODO Auto-generated method stub
        for(int i = 1;i<11;i++){
          System.out.println(i+" "+testSingleExampleOfLazyType.getInstance());          
        }
      }
      
    });
    
    thread.start();
    
  }
}

 

说明：懒汉模式即，对象的实例化（内存分配）发生在被调用的那一刻。

 

懒汉模式==实例化。。。
designModel.SingleExampleModel.testSingleExampleOfLazyType@5ce345c2
1 designModel.SingleExampleModel.testSingleExampleOfLazyType@5ce345c2
2 designModel.SingleExampleModel.testSingleExampleOfLazyType@5ce345c2
3 designModel.SingleExampleModel.testSingleExampleOfLazyType@5ce345c2
4 designModel.SingleExampleModel.testSingleExampleOfLazyType@5ce345c2
5 designModel.SingleExampleModel.testSingleExampleOfLazyType@5ce345c2
6 designModel.SingleExampleModel.testSingleExampleOfLazyType@5ce345c2
7 designModel.SingleExampleModel.testSingleExampleOfLazyType@5ce345c2
8 designModel.SingleExampleModel.testSingleExampleOfLazyType@5ce345c2
9 designModel.SingleExampleModel.testSingleExampleOfLazyType@5ce345c2
10 designModel.SingleExampleModel.testSingleExampleOfLazyType@5ce345c2
 

只有当单例模式提供的外部接口被调用，对象才发生实例化。