使用 Spring @Lazy 和 @PostConstruct 注解

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()
    }
}

假设 Spring 初始化 bean 如下:
1. 创建第一个 bean B.在创建 bean B 时，由于 @Lazy 注释，字段 a 将不会被初始化.
2. 下一个 bean D 被创建.然后 method3() 将被执行，因为它被标记为 @PostConstruct，但是 bean A 还没有被 Spring 触及.那么当 a.method1() 被调用时，Spring 会创建 bean A 并将其注入字段 a 还是会抛出 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，当您指定 @Lazy 作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:

除了用于组件初始化的作用外，@Lazy注解也可以放在标有的注入点上@Autowired 或 @Inject.在这种情况下，它导致注入一个延迟解析代理.

这意味着在启动时 Spring 将注入代理类的实例而不是 A 类的实例.代理类是自动生成的类，与A 类具有相同的接口.任何方法代理的第一次调用都会在 self 内部创建类 A 的实例.之后，所有方法调用都将重定向到代理内部的 A 类实例.

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.

所以没有理由害怕任何问题.

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

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