Singleton模式的两种实现形式

（1）第一种形式

public class AClass {

    private AClass() {
    }

    private static AClass instance = new AClass();

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

（1）第二种形式

public class AClass {
    private static AClass instance = null;

    public static synchronized AClass getInstance() {

        if (instance == null)
            instance = new AClass();
        return instance;
    }
}