Java中的单例模式

第一种（懒汉，线程不安全）

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

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

第二种（懒汉，线程安全）

pulic class Singleton {
    private static Singleton instance;
    private Singleton () {}
    public static synchronized Singleton getInstace() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

第三种（饿汉）

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

第四种（饿汉，变种）

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

第五种（静态内部类）

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

第六种（枚举）

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

第七中（双重校验锁）

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;
    }
}