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定义

单例模式是指在一个应用（JVM）中，对于某个类的实例只能有一个，所有对于某个类实例的访问都是对于同一个对象的访问

框架实例

ServletContext, ServletConfig, ApplicationContext, DBPool

常见实现方式

代码实现

1. 饿汉式单例模式

package com.architecture.pattern.singleton;/** * @Auth yangyongp * @Create 2020/7/3 22:58 */public class HungrySingletonDemo {  private static final HungrySingletonDemo instance;  static {    instance = new HungrySingletonDemo();  }  private HungrySingletonDemo(){  }  public static HungrySingletonDemo getInstance(){    return instance;  }}

package com.architecture.pattern.singleton;/** * @Auth yangyongp * @Create 2020/7/3 23:01 */public class HungrySingletonDemo2 {  private static final HungrySingletonDemo2 instance = new HungrySingletonDemo2();  private HungrySingletonDemo2(){  }    public static HungrySingletonDemo2 getInstance(){    return instance;  }}

2.懒汉式单例模式

package com.architecture.pattern.singleton;/** * @Auth yangyongp * @Create 2020/7/3 23:06 */public class LazySingletonDemo {  private static LazySingletonDemo instance = null;  private LazySingletonDemo(){  }  public static LazySingletonDemo getInstance(){    if(instance == null){      synchronized (LazySingletonDemo.class){        if(instance == null){          instance = new LazySingletonDemo();        }      }    }    return instance;  }}

package com.architecture.pattern.singleton;/** * @Auth yangyongp * @Create 2020/7/14 21:42 */public class LazyInnerClasssSingletonDemo {  private LazyInnerClasssSingletonDemo(){  }  /**   * 懒汉式单例   * LazyHolder里面的逻辑需要等到外部方法调用时才执行   * 巧妙利用了内部的特性   * JVM底层执行逻辑，避免了线程安全问题   * @return   */  public static LazyInnerClasssSingletonDemo getInstance(){    return LazyHolder.LAZY;  }  private static class LazyHolder{    private static final LazyInnerClasssSingletonDemo LAZY = new LazyInnerClasssSingletonDemo();  }}

3. 注册式单例
   a.枚举式单例
   从JDK层面就为枚举不被序列化和反射破坏来保驾护航

package com.architecture.pattern.singleton;/** * @Auth yangyongp * @Create 2020/7/3 23:10 */public enum EnumSingletonDemo {  INSTANCE;  private Object data;  public Object getData() {    return data;  }  public void setData(Object data) {    this.data = data;  }  public static EnumSingletonDemo getInstance(){    return INSTANCE;  }}

b.容器式

package com.architecture.pattern.singleton;import java.util.Map;import java.util.concurrent.ConcurrentHashMap;/** * @Auth yangyongp * 对象方便管理，其实也是属于懒加载 * @Create 2020/7/14 22:25 */public class ContainerSingletonDemo {  private ContainerSingletonDemo(){  }  private static Map
   
     ioc = new ConcurrentHashMap<>();  public static Object getBean(String className){    if(!ioc.containsKey(className)){      synchronized (ioc){        if(!ioc.containsKey(className)){          Object obj = null;          try{            obj = Class.forName(className).newInstance();            ioc.put(className, obj);          }catch (Exception ex){            ex.printStackTrace();          }          return obj;        }      }    }    return ioc.get(className);  }}
   

相关知识点

1.私有构造方法

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