spring中bean的構造函數，Autowired(Value)注入與@PostConstruct調用順序

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最近在項目開發中遇到這樣一個需求，由於元數據在短時間內被客戶端多次讀取，因此希望直接將數據存儲到內存，以減少網絡開銷，借助guava cache於是有了下面這個類

/** * 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">//設置過期時間</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>

}

    
    
    
            

   
   
   
           

我們在另一個類中注入CacheUtil並調用它的getMetaNode方法，類似於這樣：

@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>

}

    
    
    
            

   
   
   
           

當我們第一次調用getMetaNode時，cache使用caseGraphService獲取元數據，而后將這一元數據存放的cache中，我們在CacheUtil的getMetaNode方法中添加兩行代碼來測試一下：

public CaseGraphDTO getMetaNode(long caseId) throws ExecutionException {
        //metaNodeCache的get方法，如果緩存中已有數據，直接返回數據，如果沒有則通過Callable方法獲取存入緩存再返回數據
        CaseGraphDTO caseGraphDTO = metaNodeCache.get(caseId, () -> caseGraphService.getCaseGraph
                (caseId));
        //getIfPresent方法，如果存在數據即返回
        CaseGraphDTO caseGraphDTO1 = metaNodeCache.getIfPresent(caseId);
        System.out.println(caseGraphDTO1);
        return caseGraphDTO;
    }

    
    
    
            

   
   
   
           

啟動application， 打開斷點調試：

可以看到caseGraphDTO1為null，而不是我們預想的元數據；metaNodeCache中localCache的大小為0，也就是根本沒有緩存任何數據。

而后我采用另一種方式@PostConstruct的方式來初始化metaNodeCache, 代碼如下：

@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，如果緩存中已有數據，直接返回數據，如果沒有則通過Callable方法獲取存入緩存再返回數據</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，如果存在數據即返回</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>

}

    
    
    
            

   
   
   
           

再進行調試：

這時我們看到caseGraphDTO1中有了數據，並且metaNodeCache中localCache的大小也變成了1.

那么直接初始化成員變量和使用postConstruct來初始化二者的區別是什么呢？

要將對象p注入到對象a，那么首先就必須得生成對象p與對象a，才能執行注入。所以，如果一個類A中有個成員變量p被@Autowired注解，那么@Autowired注入是發生在A的構造方法執行完之后的。
如果想在生成對象時候完成某些初始化操作，而偏偏這些初始化操作又依賴於依賴注入，那么就無法在構造函數中實現。為此，可以使用@PostConstruct注解一個方法來完成初始化，@PostConstruct注解的方法將會在依賴注入完成后被自動調用。

事情似乎明朗起來了，雖然我們在初始化metaNodeCache時沒有使用autowired進來的caseGraphService，但我們使用了@Value來注入緩存過期時間（配置中這個值為60）。讓我們再來斷點調試一下，當使用直接初始化成員變量的方式時，@Value("${cache.expire.duration}")注入的過期時間是多少，如下：

我們看到expireDuration的值為0，而不是配置中的60，對於cache的含義即為緩存中的數據被讀取后0分鍾使其失效，等同於立即失效。所以導致上文我們的緩存中始終沒有數據。

讓我們再來看看使用PostConstruct初始化時，這個過期時間的值：

此時看到expireDuration的值為60.

因此我們需要記住，構造函數，Autowired(Value)，PostConstruct的執行順序為：

Constructor >> Autowired >> PostConstruct
如果初始化成員變量需要使用注入進來的對象或者值，那么應該放在被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