已知代码如下

Bean.java

public class Bean {
    private String beanName;
    public String getBeanName() {
        return beanName;
    }
    public void setBeanName(String beanName) {
        this.beanName = beanName;
    }
    public Bean(String beanName) {
        this.beanName = beanName;
    }
}

BeanFactory.java

public class BeanFactory {
    private static BeanFactory instance = new BeanFactory();
    private static Bean bean = null;
    public BeanFactory() {
        bean = new Bean("mybean");
    }
    public static BeanFactory getInstance(){
        return instance;
    }
    public static Bean getBean(){
        return bean;
    }
}

App.java

public class App {
    public static void main(String[] args) {
        Bean bean = BeanFactory.getInstance().getBean();
        System.out.println(bean.getBeanName());
    }
}

运行结果如下【1】

Exception in thread "main" java.lang.NullPointerException
    at com.xy.factory_simple.App.main(App.java:6)

更改BeanFactory.java

public class BeanFactory {
    private static BeanFactory instance = new BeanFactory();
    private static Bean bean;
    public BeanFactory() {
        bean = new Bean("mybean");
    }
    public static BeanFactory getInstance(){
        return instance;
    }
    public static Bean getBean(){
        return bean;
    }
}

运行结果如下【2】

mybean

更改BeanFactory.java

    private static Bean bean = null;
    private static BeanFactory instance = new BeanFactory();
    public BeanFactory() {
        bean = new Bean("mybean");
    }
    public static BeanFactory getInstance(){
        return instance;
    }
    public static Bean getBean(){
        return bean;
    }
}

运行结果如下【3】

mybean

你知道上面是为什么吗？