创建单例模式的7种方式-Toy模板网

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饿汉式

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

懒汉式

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

双重监测

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

静态内部类(推荐)

class Singleton{
	private Singleton(){}
    private static class SingletonHelper{
        private static final Singleton INSTANCE = new Singleton();
    }
    public static Singleton getInstance(){
        return SingletonHelper.INSTANCE;
    }
}

枚举(推荐)

enum Singleton{
	INSTANCE;
}

多例模式

class Singleton{
	private static Map<String,Singleton> instances = new HashMap<String,Singleton>();
    private final int MAX_INSTANCE = 3;
    private String name;
    private Singleton(String name){
        this.name = name;
    }
    public static Singleton getinstance(String name){
        if(!instances.containKey(name) && instance.size() < MAX_INSTANCE){
            synchronized(Singleton.class){
                if(!instances.containKey(name) && instance.size() < MAX_INSTANCE){
                    instances.put(name,new Singleton(name));
                }
            }
        }
        return instances.get(name);
    }
}

线程唯一

class Singleton{
	private static ThreadLocal<Singleton> instances = new ThreadLocal<>();
    private Singleton(){}
    public static Singleton getInstance(){
        if(instances.get() == null){
            synchronized(Singleton.class){
                if(instances.get() == null){
                    instances.add(new Singleton());
                }
            }
        }
        return instances.get();
    }
}

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