使用Spring @Lazy和@PostConstruct批注 [英] Using Spring @Lazy and @PostConstruct annotations

问题描述

我有以下课程:

@Repository
class A {

    public void method1() {
        ...
    }
}

@Component
class B implements C {

    @Autowired
    @Lazy
    private A a;

    public void method2() {
        a.method1();
    }
}

@Component
class D {

    @Autowired
    private List<C> c;

    @PostConstruct
    public void method3() {
        // iterate on list c and call method2()
    }
}

让我们假设Spring如下初始化bean:
1.创建第一个beanB.创建bean B时，由于@Lazy批注，字段a将不会初始化.
2.创建下一个beanD.然后method3()将被执行，因为它已被@PostConstruct标记，但是Spring尚未触及beanA.因此，当调用a.method1()时，Spring将创建bean A并将其注入到字段a中，或者它将抛出NullPointerException?

Let's suppose Spring initializes the beans as following:
1. First bean B is created. When bean B is being created, field a will not be initialized because of the @Lazy annotation.
2. Next bean D is created. Then method3() will get executed as it is marked with @PostConstruct, but bean A is not yet touched by Spring. So when a.method1() will be called, then will Spring create bean A and inject it into the field a or will it throw a NullPointerException?

推荐答案

您需要了解，将@Lazy指定为注入的一部分时会发生什么.根据文档:

You need to understand, what's going on when you're specifying @Lazy as part of injection. According to documentation:

@Lazy 除了其用于组件初始化的作用 注释也可以放在标有的注入点上 @Autowired或@Inject.在这种情况下，它会导致注入 惰性解析代理.

In addition to its role for component initialization, the @Lazy annotation may also be placed on injection points marked with @Autowired or @Inject. In this context, it leads to the injection of a lazy-resolution proxy.

这意味着在启动时，Spring将注入代理类的实例而不是类A的实例.代理类是自动生成的类，具有与类A相同的接口.第一次调用任何方法时，代理都会在self内创建类A的实例.之后，所有方法调用将重定向到代理内部的类A的该实例.

This means that on start Spring will inject instance of proxy class instead of instance of class A . Proxy class is automatically generated class that have same interface as class A. On first call of any method proxy will create instance of class A inside of self. After that all calls of methods will be redirected to this instance of class A inside of proxy.

因此没有理由担心任何问题.

So there is no reason to be afraid of any problems.

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