spring中bean的构造函数，Autowired(Value)注入与@PostConstruct调用顺序

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最近在项目开发中遇到这样一个需求，由于元数据在短时间内被客户端多次读取，因此希望直接将数据存储到内存，以减少网络开销，借助guava cache于是有了下面这个类

/**
 * Created on 2018/10/18
 */
@Component
public class CacheUtil {
<span class="token annotation punctuation">@Autowired</span>
CaseGraphService caseGraphService<span class="token punctuation">;</span>

<span class="token annotation punctuation">@Value</span><span class="token punctuation">(</span><span class="token string">"${cache.expire.duration}"</span><span class="token punctuation">)</span>
<span class="token keyword">long</span> expireDuration<span class="token punctuation">;</span>

<span class="token keyword">private</span> Cache<span class="token generics function"><span class="token punctuation">&lt;</span>Long<span class="token punctuation">,</span> CaseGraphDTO<span class="token punctuation">&gt;</span></span> metaNodeCache <span class="token operator">=</span> CacheBuilder
            <span class="token punctuation">.</span><span class="token function">newBuilder</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
            <span class="token punctuation">.</span><span class="token function">maximumSize</span><span class="token punctuation">(</span><span class="token number">1000</span><span class="token punctuation">)</span>
            <span class="token punctuation">.</span><span class="token function">expireAfterAccess</span><span class="token punctuation">(</span>expireDuration<span class="token punctuation">,</span> TimeUnit<span class="token punctuation">.</span>MINUTES<span class="token punctuation">)</span> <span class="token comment">//设置过期时间</span>
            <span class="token punctuation">.</span><span class="token function">build</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">public</span> CaseGraphDTO <span class="token function">getMetaNode</span><span class="token punctuation">(</span><span class="token keyword">long</span> caseId<span class="token punctuation">)</span> <span class="token keyword">throws</span> ExecutionException <span class="token punctuation">{</span>
    <span class="token keyword">return</span> metaNodeCache<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span>caseId<span class="token punctuation">,</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-</span><span class="token operator">&gt;</span> caseGraphService<span class="token punctuation">.</span><span class="token function">getCaseGraph</span><span class="token punctuation">(</span>caseId<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">removeMetaNodeByKey</span><span class="token punctuation">(</span><span class="token keyword">long</span> caseId<span class="token punctuation">)</span><span class="token punctuation">{</span>
    metaNodeCache<span class="token punctuation">.</span><span class="token function">invalidate</span><span class="token punctuation">(</span>caseId<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">removeMetaNodeAll</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    metaNodeCache<span class="token punctuation">.</span><span class="token function">invalidateAll</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}
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我们在另一个类中注入CacheUtil并调用它的getMetaNode方法，类似于这样：

@Component
public class TestComponent {
	@Autowired
	CacheUtil cacheUtil;
<span class="token keyword">public</span> CaseGraphDTO <span class="token function">getMetaData</span><span class="token punctuation">(</span><span class="token keyword">long</span> caseId<span class="token punctuation">)</span><span class="token punctuation">{</span>
	<span class="token keyword">return</span> cacheUtil<span class="token punctuation">.</span><span class="token function">getMetaNode</span><span class="token punctuation">(</span>caseId<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}
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当我们第一次调用getMetaNode时，cache使用caseGraphService获取元数据，而后将这一元数据存放的cache中，我们在CacheUtil的getMetaNode方法中添加两行代码来测试一下：

public CaseGraphDTO getMetaNode(long caseId) throws ExecutionException {
        //metaNodeCache的get方法，如果缓存中已有数据，直接返回数据，如果没有则通过Callable方法获取存入缓存再返回数据
        CaseGraphDTO caseGraphDTO = metaNodeCache.get(caseId, () -> caseGraphService.getCaseGraph
                (caseId));
        //getIfPresent方法，如果存在数据即返回
        CaseGraphDTO caseGraphDTO1 = metaNodeCache.getIfPresent(caseId);
        System.out.println(caseGraphDTO1);
        return caseGraphDTO;
    }
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启动application， 打开断点调试：

可以看到caseGraphDTO1为null，而不是我们预想的元数据；metaNodeCache中localCache的大小为0，也就是根本没有缓存任何数据。

而后我采用另一种方式@PostConstruct的方式来初始化metaNodeCache, 代码如下：

@Component
public class CacheUtil {
<span class="token annotation punctuation">@Autowired</span>
CaseGraphService caseGraphService<span class="token punctuation">;</span>

<span class="token annotation punctuation">@Value</span><span class="token punctuation">(</span><span class="token string">"${cache.expire.duration}"</span><span class="token punctuation">)</span>
<span class="token keyword">long</span> expireDuration<span class="token punctuation">;</span>

<span class="token keyword">private</span> Cache<span class="token generics function"><span class="token punctuation">&lt;</span>Long<span class="token punctuation">,</span> CaseGraphDTO<span class="token punctuation">&gt;</span></span> metaNodeCache<span class="token punctuation">;</span>

<span class="token annotation punctuation">@PostConstruct</span>
<span class="token keyword">private</span> <span class="token keyword">void</span> <span class="token function">init</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    metaNodeCache <span class="token operator">=</span> CacheBuilder
            <span class="token punctuation">.</span><span class="token function">newBuilder</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
            <span class="token punctuation">.</span><span class="token function">maximumSize</span><span class="token punctuation">(</span><span class="token number">1000</span><span class="token punctuation">)</span>
            <span class="token punctuation">.</span><span class="token function">expireAfterAccess</span><span class="token punctuation">(</span>expireDuration<span class="token punctuation">,</span> TimeUnit<span class="token punctuation">.</span>MINUTES<span class="token punctuation">)</span>
            <span class="token punctuation">.</span><span class="token function">build</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">public</span> CaseGraphDTO <span class="token function">getMetaNode</span><span class="token punctuation">(</span><span class="token keyword">long</span> caseId<span class="token punctuation">)</span> <span class="token keyword">throws</span> ExecutionException <span class="token punctuation">{</span>
    <span class="token comment">//metaNodeCache的get方法，如果缓存中已有数据，直接返回数据，如果没有则通过Callable方法获取存入缓存再返回数据</span>
    CaseGraphDTO caseGraphDTO <span class="token operator">=</span> metaNodeCache<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span>caseId<span class="token punctuation">,</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-</span><span class="token operator">&gt;</span> caseGraphService<span class="token punctuation">.</span>getCaseGraph
            <span class="token punctuation">(</span>caseId<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">//getIfPresent方法，如果存在数据即返回</span>
    CaseGraphDTO caseGraphDTO1 <span class="token operator">=</span> metaNodeCache<span class="token punctuation">.</span><span class="token function">getIfPresent</span><span class="token punctuation">(</span>caseId<span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>caseGraphDTO1<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> caseGraphDTO<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">removeMetaNodeByKey</span><span class="token punctuation">(</span><span class="token keyword">long</span> caseId<span class="token punctuation">)</span><span class="token punctuation">{</span>
    metaNodeCache<span class="token punctuation">.</span><span class="token function">invalidate</span><span class="token punctuation">(</span>caseId<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">removeMetaNodeAll</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    metaNodeCache<span class="token punctuation">.</span><span class="token function">invalidateAll</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}
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再进行调试：

这时我们看到caseGraphDTO1中有了数据，并且metaNodeCache中localCache的大小也变成了1.

那么直接初始化成员变量和使用postConstruct来初始化二者的区别是什么呢？

要将对象p注入到对象a，那么首先就必须得生成对象p与对象a，才能执行注入。所以，如果一个类A中有个成员变量p被@Autowired注解，那么@Autowired注入是发生在A的构造方法执行完之后的。
如果想在生成对象时候完成某些初始化操作，而偏偏这些初始化操作又依赖于依赖注入，那么就无法在构造函数中实现。为此，可以使用@PostConstruct注解一个方法来完成初始化，@PostConstruct注解的方法将会在依赖注入完成后被自动调用。

事情似乎明朗起来了，虽然我们在初始化metaNodeCache时没有使用autowired进来的caseGraphService，但我们使用了@Value来注入缓存过期时间（配置中这个值为60）。让我们再来断点调试一下，当使用直接初始化成员变量的方式时，@Value("${cache.expire.duration}")注入的过期时间是多少，如下：

我们看到expireDuration的值为0，而不是配置中的60，对于cache的含义即为缓存中的数据被读取后0分钟使其失效，等同于立即失效。所以导致上文我们的缓存中始终没有数据。

让我们再来看看使用PostConstruct初始化时，这个过期时间的值：

此时看到expireDuration的值为60.

因此我们需要记住，构造函数，Autowired(Value)，PostConstruct的执行顺序为：

Constructor >> Autowired >> PostConstruct
如果初始化成员变量需要使用注入进来的对象或者值，那么应该放在被PostConstruct注解的方法中去做

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原文地址：https://blog.csdn.net/yyysylvia/article/details/83177345