单例模式demo

单例模式：目的是保证一个类只能有一个实例，而且自行实例化并向整个系统提供这个实例，避免频繁创建对象，节约内存

饿汉式：在类加载时就完成了初始化，所以类加载比较慢，但获取对象的速度快。

public class Singleton implements Serializable {

    private Singleton(){
        System.out.println("private Singleton");
    }

    //给static变量赋值最终会在static静态代码块里执行,java虚拟机会保证线程安全
    private static final Singleton INSTANCE=new Singleton();

    public static Singleton getInstance(){
        return INSTANCE;
    }

    public static void otherMethod(){
        System.out.println("otherMethod");
    } 
}


//防止反射破坏单例模式：
public class Singleton1 implements Serializable {
    
    private Singleton1(){
        if(INSTANCE!=null){
            throw new RuntimeException("单例对象不能重复创建");
        }
    }
    private static final Singleton1 INSTANCE=new Singleton1();

    public static Singleton1 getInstance(){
        return INSTANCE;
    }

    public static void otherMethod(){
        System.out.println("otherMethod");
    }

    public Object readResolve(){
        return INSTANCE;
    }
}



public enum  SingletonByEnum {

    INSTANCE;

    //枚举类的构造默认是private
     SingletonByEnum(){
        System.out.println("private SingletonByEnum()");
    }

    public static SingletonByEnum getInstance(){
        return INSTANCE;
    }

    public static void otherMethod(){
        System.out.println("otherMethod");
    }

    @Override
    public String toString(){
        return getClass().getName()+"@"+Integer.toHexString(hashCode());
    }
}

懒汉式：在类加载时不初始化，等到第一次被使用时才初始化。

public class Singleton3 implements Serializable {

    private Singleton3(){
        System.out.println("private Singleton3()");
    }

    //volatile多线程可见性和有序性，这里主要是解决有序性
    //有可能INSTANCE还没有构造完就把对象赋值给了INSTANCE，那这时候调用就会出现很多问题
    //因此加上volatile保证已经构造完了在把对象赋值给INSTANCE。
    private static volatile  Singleton3 INSTANCE=null;

    public static Singleton3 getInstance(){
        if(INSTANCE==null){
            synchronized (Singleton3.class){
                if (INSTANCE==null){
                    INSTANCE=new Singleton3();
                }
            }
        }
        return INSTANCE;
    }

    public static void otherMethod(){
        System.out.println("otherMethod");
    }
}



public class Singleton2  implements Serializable {

    private Singleton2(){
        System.out.println("private Singleton2()");
    }

    //在静态方法里，会在静态代码块里执行,jdk会保证线程安全
    private static class Holder{
        static Singleton2 INSTANCE=new Singleton2();
    }

    public static Singleton2 getInstance(){
        return Holder.INSTANCE;
    }

    public static void otherMethod(){
        System.out.println("otherMethod");
    }
}

在jdk中哪里使用了单例模式：
gc()方法:  饿汉式单例模式

   public static void gc() {
        Runtime.getRuntime().gc();
    }


  

public class Runtime {
    private static Runtime currentRuntime = new Runtime();

  public static Runtime getRuntime() {
        return currentRuntime;
    }
}

console() 双检索懒汉式:
 

  private static volatile Console cons = null;
    
     public static Console console() {
           Console  c;
         if ((c=cons) == null) {
             synchronized (System.class) {
                 if((c=cons) == null){
                   cons=c =  SharedSecrets.getJavaIOAccess().console();
                 
                } 
             }
         }
         return c;
     }