单例的7种写法

懒汉模式-线程不安全 12345678910111213

public class Singleton {
    private static Singleton instance;
    private Singleton (){

    }

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

懒汉模式-线程安全 12345678910111213

public class Singleton {
    private static Singleton instance;
    private Singleton (){

    }

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

饿汉模式 1234567891011

public class Singleton {
    private static Singleton instance = new Singleton();

    private Singleton (){

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

饿汉模式-变种 1234567891011121314

public class Singleton {
    private Singleton instance = null;

    static {
    instance = new Singleton();
    }
    
    private Singleton (){

    }
    public static Singleton getInstance() {
    return this.instance;
    }
}

静态内部类 12345678910111213

public class Singleton {
    private static class SingletonHolder {
    private static final Singleton INSTANCE = new Singleton();
    }

    private Singleton (){

    }

    public static final Singleton getInstance() {
    return SingletonHolder.INSTANCE;
    }
}

双重校验锁 12345678910111213141516

public class Singleton {
    private volatile static Singleton singleton;
    private Singleton (){

    }
    public static Singleton getSingleton() {
        if (singleton == null) {
            synchronized (Singleton.class) {
                if (singleton == null) {
                    singleton = new Singleton();
                }
            }
        }
        return singleton;
    }
}

枚举 12345

public enum Singleton {  
    INSTANCE;  
    public void whateverMethod() {  
    }  
}