Spring源碼分析-BeanFactoryPostProcessors 應用之 PropertyPlaceholderConfigurer

時間 2020-02-29

標籤 spring 源碼分析 beanfactorypostprocessors 應用 propertyplaceholderconfigurer 欄目 Spring 简体版

原文原文鏈接

BeanFactoryPostProcessors

介紹

BeanFactoryPostProcessors完整定義:java

/**
 * Allows for custom modification of an application context's bean definitions,
 * adapting the bean property values of the context's underlying bean factory.
 * @see BeanPostProcessor
 * @see PropertyResourceConfigurer
 */
public interface BeanFactoryPostProcessor {

    /**
     * Modify the application context's internal bean factory after its standard
     * initialization. All bean definitions will have been loaded, but no beans
     * will have been instantiated yet. This allows for overriding or adding
     * properties even to eager-initializing beans.
     * @param beanFactory the bean factory used by the application context
     * @throws org.springframework.beans.BeansException in case of errors
     */
    void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException;

}

咱們知道spring最大優勢就是其可擴展性,BeanFactoryPostProcessor接口就是spring中提供給咱們用於擴展的一個地方。咱們看該接口上的javadoc其實很是的詳細,這也是咱們看spring源碼的一個技巧，就是看一個類是幹嗎的必定要先通讀其註釋。spring

結合接口上的註釋大體描述下BeanFactoryPostProcessor：
容許用戶經過修改applicationContext 中的bean定義(就是xml中定義的bean的信息即:BeanDefinition是和xml有一對一的配置，好比是不是單利，以及propert 屬性的賦值等)
來調整applicationContext中bean工廠中bean屬性值。

也就是說執行到BeanFactoryPostProcessor時所有的BeanDefinition定義已經加載好了可是bean實例尚未被建立,咱們能夠修補或者覆蓋bean屬性值。

咱們能夠看一下ApplicationContext中BeanFactoryPostProcessor的調用位置來印證是否如此app

下面是ApplicationContext核心代碼：ide

public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            // Prepare this context for refreshing.
            prepareRefresh();

            
            //獲取beanFactory實例
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

        
            prepareBeanFactory(beanFactory);

            try {
             
                postProcessBeanFactory(beanFactory);

                //這裏正是咱們的BeanFactoryPostProcessor執行的位置
                invokeBeanFactoryPostProcessors(beanFactory);

                // Register bean processors that intercept bean creation.
                registerBeanPostProcessors(beanFactory);

                // Initialize message source for this context.
                initMessageSource();

                // Initialize event multicaster for this context.
                initApplicationEventMulticaster();

                // Initialize other special beans in specific context subclasses.
                onRefresh();

                // Check for listener beans and register them.
                registerListeners();

                // Instantiate all remaining (non-lazy-init) singletons.
                //建立非懶加載的全部單例  這裏是真正建立bean實例的地方
                finishBeanFactoryInitialization(beanFactory);

                // Last step: publish corresponding event.
                finishRefresh();
            }

            catch (BeansException ex) {
                // Destroy already created singletons to avoid dangling resources.
                destroyBeans();

                // Reset 'active' flag.
                cancelRefresh(ex);

                // Propagate exception to caller.
                throw ex;
            }
        }
    }

通過源碼分析

ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

            // Prepare the bean factory for use in this context.
            prepareBeanFactory(beanFactory);

這兩步咱們已經拿到了beanFactory實例,也就是每個bean對應的BeanDefinition已經加載好了。下面纔是執行invokeBeanFactoryPostProcessors(beanFactory)，也就印證了咱們上面的結論。
下面咱們經過一個BeanFactoryPostProcessor的典型應用PropertyPlaceholderConfigurer來詳細講解BeanFactoryPostProcessor執行原理post

PropertyPlaceholderConfigurer

簡單使用的例子

PropertyPlaceholderConfigurer相信你們都使用過,咱們在配置bean的屬性可使用佔位符來賦值，而後經過調整properties文件中對應的屬性值來修改。看一個使用PropertyPlaceholderConfigurer簡單的例子:單元測試

public class Student {
    
        private String name;
    
        private Integer age;
    
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    
        public Integer getAge() {
            return age;
        }
    
        public void setAge(Integer age) {
            this.age = age;
        }
    }

    <bean class="com.yalunwang.Student" id="student">
        <property name="name" value="${student.name}"></property>
        <property name="age" value="${student.age}"></property>
    </bean>

    <bean class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
        <property name="location" value="classpath:bean.properties"></property>
    </bean>

bean.properties配置文件:測試

student.name=anan
    student.age=2

單元測試:ui

@Test
    public void test_ioc_app(){
        ApplicationContext ac = new ClassPathXmlApplicationContext("spring-ioc.xml");

        Student student =(Student) ac.getBean("student");

        System.out.println("name: "+student.getName());
        System.out.println("age: "+student.getAge());
    }

輸出結果一切正常:this

name: anan
age: 2

源碼分析

先看一下PropertyPlaceholderConfigurer的類繼承圖:

能夠看到PropertyPlaceholderConfigurer實現了BeanFactoryPostProcessor和 PriorityOrdered。

咱們接着對上面的 invokeBeanFactoryPostProcessors(beanFactory)繼續進行分析:

/**
     * Instantiate and invoke all registered BeanFactoryPostProcessor beans,
     * respecting explicit order if given.
     * 實例化並調用全部已註冊的BeanFactoryPostProcessor Bean，
     * 若是繼承了Order接口按順序執行
     * <p>Must be called before singleton instantiation.
     */
    protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
        // Invoke BeanDefinitionRegistryPostProcessors first, if any.
        Set<String> processedBeans = new HashSet<String>();
        //若是是beanFactory實現了BeanDefinitionRegistry
        if (beanFactory instanceof BeanDefinitionRegistry) {
            BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
            List<BeanFactoryPostProcessor> regularPostProcessors = new LinkedList<BeanFactoryPostProcessor>();
            List<BeanDefinitionRegistryPostProcessor> registryPostProcessors =
                    new LinkedList<BeanDefinitionRegistryPostProcessor>();
            //遍歷硬編碼設置的beanFactory後置處理器          
            for (BeanFactoryPostProcessor postProcessor : getBeanFactoryPostProcessors()) {
                //若是是BeanDefinitionRegistryPostProcessor類型先執行postProcessBeanDefinitionRegistry方法再將其添加到registryPostProcessors集合中進行後續postProcessBeanFactory方法的執行
                if (postProcessor instanceof BeanDefinitionRegistryPostProcessor) {
                    BeanDefinitionRegistryPostProcessor registryPostProcessor =
                            (BeanDefinitionRegistryPostProcessor) postProcessor;
                    registryPostProcessor.postProcessBeanDefinitionRegistry(registry);
                    registryPostProcessors.add(registryPostProcessor);
                }
                else {
                    //同理將正常的beanFactory後置處理器添加到regularPostProcessors集合中進行後續postProcessBeanFactory方法的執行
                    regularPostProcessors.add(postProcessor);
                }
            }
            //找出配置的BeanDefinitionRegistryPostProcessor後置處理器 
            Map<String, BeanDefinitionRegistryPostProcessor> beanMap =
                    beanFactory.getBeansOfType(BeanDefinitionRegistryPostProcessor.class, true, false);
            List<BeanDefinitionRegistryPostProcessor> registryPostProcessorBeans =
                    new ArrayList<BeanDefinitionRegistryPostProcessor>(beanMap.values());
            OrderComparator.sort(registryPostProcessorBeans);
            //執行BeanDefinitionRegistryPostProcessor後置處理器的postProcessBeanDefinitionRegistry方法
            for (BeanDefinitionRegistryPostProcessor postProcessor : registryPostProcessorBeans) {
                postProcessor.postProcessBeanDefinitionRegistry(registry);
            }
            //執行上面添加的beanFactory後置處理器的集合裏的方法
            invokeBeanFactoryPostProcessors(registryPostProcessors, beanFactory);
            invokeBeanFactoryPostProcessors(registryPostProcessorBeans, beanFactory);
            invokeBeanFactoryPostProcessors(regularPostProcessors, beanFactory);
            //處理後的添加到集合裏 防止後面重複執行
            processedBeans.addAll(beanMap.keySet());
        }
        else {
            // Invoke factory processors registered with the context instance.
            invokeBeanFactoryPostProcessors(getBeanFactoryPostProcessors(), beanFactory);
        }

    
        //獲取配置的BeanFactoryPostProcessor
        //如下按實現了 PriorityOrdered  Ordered 沒有繼承 三種進行優先級排序執行 
        String[] postProcessorNames =
                beanFactory.getBeanNamesForType(BeanFactoryPostProcessor.class, true, false);
        
        List<BeanFactoryPostProcessor> priorityOrderedPostProcessors = new ArrayList<BeanFactoryPostProcessor>();
        List<String> orderedPostProcessorNames = new ArrayList<String>();
        List<String> nonOrderedPostProcessorNames = new ArrayList<String>();
        for (String ppName : postProcessorNames) {
            //這個就是上面記錄的若是已經處理了配置的BeanFactoryPostProcessors就跳過
            if (processedBeans.contains(ppName)) {
                // skip - already processed in first phase above
            }
            else if (isTypeMatch(ppName, PriorityOrdered.class)) {
                priorityOrderedPostProcessors.add(beanFactory.getBean(ppName, BeanFactoryPostProcessor.class));
            }
            else if (isTypeMatch(ppName, Ordered.class)) {
                orderedPostProcessorNames.add(ppName);
            }
            else {
                nonOrderedPostProcessorNames.add(ppName);
            }
        }

        // First, invoke the BeanFactoryPostProcessors that implement PriorityOrdered.
        OrderComparator.sort(priorityOrderedPostProcessors);
        //上面咱們說了PropertyPlaceholderConfigurer 實現了BeanFactoryPostProcessor和 PriorityOrdered,因此會在這一步執行調用
        invokeBeanFactoryPostProcessors(priorityOrderedPostProcessors, beanFactory);

        // Next, invoke the BeanFactoryPostProcessors that implement Ordered.
        List<BeanFactoryPostProcessor> orderedPostProcessors = new ArrayList<BeanFactoryPostProcessor>();
        for (String postProcessorName : orderedPostProcessorNames) {
            orderedPostProcessors.add(getBean(postProcessorName, BeanFactoryPostProcessor.class));
        }
        OrderComparator.sort(orderedPostProcessors);
        invokeBeanFactoryPostProcessors(orderedPostProcessors, beanFactory);

        // Finally, invoke all other BeanFactoryPostProcessors.
        List<BeanFactoryPostProcessor> nonOrderedPostProcessors = new ArrayList<BeanFactoryPostProcessor>();
        for (String postProcessorName : nonOrderedPostProcessorNames) {
            nonOrderedPostProcessors.add(getBean(postProcessorName, BeanFactoryPostProcessor.class));
        }
        invokeBeanFactoryPostProcessors(nonOrderedPostProcessors, beanFactory);
    }

上面主要的執行邏輯我都添加了中文註釋方便你們理解。
總結一下改方法主要作的事情:

先判斷若是beanFactory是BeanDefinitionRegistry類型的話就添加對BeanDefinitionRegistryPostProcessor類型的調用,BeanDefinitionRegistryPostProcessor接口是BeanFactoryPostProcessor的子接口,BeanDefinitionRegistryPostProcessor的做用是方便咱們能夠手動註冊bean交給spring來管理,能夠經過擴展其惟一的方法(void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException;) 來註冊bean到spring裏，這個不是本次分析的重點後面再舉例講解。
先對硬編碼配置的BeanFactoryPostProcessor進行處理若是是BeanDefinitionRegistryPostProcessor類型會進行postProcessBeanDefinitionRegistry調用和postProcessBeanFactory調用,若是不是則只進行postProcessBeanFactory調用。
再對配置的BeanDefinitionRegistryPostProcessor進行處理(postProcessBeanDefinitionRegistry調用和postProcessBeanFactory調用)
最後對配置BeanFactoryPostProcessor的進行處理會按 PriorityOrdered/Ordered/沒有繼承任何排序接口三種進行優先級排序執行postProcessBeanFactory調用。其中咱們的PropertyPlaceholderConfigurer 實現了BeanFactoryPostProcessor和 PriorityOrdered,因此會在這一步執行調用
invokeBeanFactoryPostProcessors(priorityOrderedPostProcessors, beanFactory);

** 咱們繼續進行分析：**

invokeBeanFactoryPostProcessors(priorityOrderedPostProcessors, beanFactory);

會執行PropertyPlaceholderConfigurer父類PropertyResourceConfigurer中的方法

/**
     * {@linkplain #mergeProperties Merge}, {@linkplain #convertProperties convert} and
     * {@linkplain #processProperties process} properties against the given bean factory.
     * @throws BeanInitializationException if any properties cannot be loaded
     */
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
        try {
            Properties mergedProps = mergeProperties();

            // Convert the merged properties, if necessary.
            convertProperties(mergedProps);

            // Let the subclass process the properties.
            processProperties(beanFactory, mergedProps);
        }
        catch (IOException ex) {
            throw new BeanInitializationException("Could not load properties", ex);
        }
    }

mergeProperties()方法會先將配置的properties加載到mergedProps裏
後面調用** processProperties(beanFactory, mergedProps)**;進行處理

processProperties(beanFactory, mergedProps)分析

protected void processProperties(ConfigurableListableBeanFactory beanFactoryToProcess, Properties props)
            throws BeansException {

        StringValueResolver valueResolver = new PlaceholderResolvingStringValueResolver(props);

        this.doProcessProperties(beanFactoryToProcess, valueResolver);
    }

這裏只有兩行代碼,第一行是建立StringValueResolver實例(用於替換佔位符的真正方法)
咱們跨過千山萬水終於要到立刻要進行佔位符替換了，繼續分析

protected void doProcessProperties(ConfigurableListableBeanFactory beanFactoryToProcess,
            StringValueResolver valueResolver) {
       
        
        //將建立的valueResolver設置到BeanDefinitionVisitor裏 用於最終替換邏輯 (替換佔位符爲對應properties裏配置的值)
        BeanDefinitionVisitor visitor = new BeanDefinitionVisitor(valueResolver);
        //拿出ioc容器裏註冊的beans
        String[] beanNames = beanFactoryToProcess.getBeanDefinitionNames();
        for (String curName : beanNames) {
            
            //排除掉當前beanName也就是 PropertyPlaceholderConfigurer Bean 且beanFactoryToProcess必須是當前容器
            if (!(curName.equals(this.beanName) && beanFactoryToProcess.equals(this.beanFactory))) {
                //獲取一個bean對應的bean定義
                BeanDefinition bd = beanFactoryToProcess.getBeanDefinition(curName);
                try {
                    //進行bean定義元數據的替換操做
                    visitor.visitBeanDefinition(bd);
                }
                catch (Exception ex) {
                    throw new BeanDefinitionStoreException(bd.getResourceDescription(), curName, ex.getMessage());
                }
            }
        }

        // New in Spring 2.5: resolve placeholders in alias target names and aliases as well.
        beanFactoryToProcess.resolveAliases(valueResolver);

        // New in Spring 3.0: resolve placeholders in embedded values such as annotation attributes.
        beanFactoryToProcess.addEmbeddedValueResolver(valueResolver);
    }

上面我已經使用中文註釋寫的很清楚了接着進行分析

visitor.visitBeanDefinition(bd);

public void visitBeanDefinition(BeanDefinition beanDefinition) {
        visitParentName(beanDefinition);
        visitBeanClassName(beanDefinition);
        visitFactoryBeanName(beanDefinition);
        visitFactoryMethodName(beanDefinition);
        visitScope(beanDefinition);
        visitPropertyValues(beanDefinition.getPropertyValues());
        ConstructorArgumentValues cas = beanDefinition.getConstructorArgumentValues();
        visitIndexedArgumentValues(cas.getIndexedArgumentValues());
        visitGenericArgumentValues(cas.getGenericArgumentValues());
    }

這裏能夠對應 bean的parentName beanClassName property等進行替換操做咱們這裏只關注屬性的替換操做

visitPropertyValues(beanDefinition.getPropertyValues());

protected void visitPropertyValues(MutablePropertyValues pvs) {
        PropertyValue[] pvArray = pvs.getPropertyValues();
        for (PropertyValue pv : pvArray) {
            Object newVal = resolveValue(pv.getValue());
            if (!ObjectUtils.nullSafeEquals(newVal, pv.getValue())) {
                pvs.add(pv.getName(), newVal);
            }
        }
    }

Object newVal = resolveValue(pv.getValue());

@SuppressWarnings("rawtypes")
    protected Object resolveValue(Object value) {
        if (value instanceof BeanDefinition) {
            visitBeanDefinition((BeanDefinition) value);
        }
        else if (value instanceof BeanDefinitionHolder) {
            visitBeanDefinition(((BeanDefinitionHolder) value).getBeanDefinition());
        }
        else if (value instanceof RuntimeBeanReference) {
            RuntimeBeanReference ref = (RuntimeBeanReference) value;
            String newBeanName = resolveStringValue(ref.getBeanName());
            if (!newBeanName.equals(ref.getBeanName())) {
                return new RuntimeBeanReference(newBeanName);
            }
        }
        else if (value instanceof RuntimeBeanNameReference) {
            RuntimeBeanNameReference ref = (RuntimeBeanNameReference) value;
            String newBeanName = resolveStringValue(ref.getBeanName());
            if (!newBeanName.equals(ref.getBeanName())) {
                return new RuntimeBeanNameReference(newBeanName);
            }
        }
        else if (value instanceof Object[]) {
            visitArray((Object[]) value);
        }
        else if (value instanceof List) {
            visitList((List) value);
        }
        else if (value instanceof Set) {
            visitSet((Set) value);
        }
        else if (value instanceof Map) {
            visitMap((Map) value);
        }
        else if (value instanceof TypedStringValue) {
            TypedStringValue typedStringValue = (TypedStringValue) value;
            String stringValue = typedStringValue.getValue();
            if (stringValue != null) {
                String visitedString = resolveStringValue(stringValue);
                typedStringValue.setValue(visitedString);
            }
        }
        else if (value instanceof String) {
            return resolveStringValue((String) value);
        }
        return value;
    }

這裏有不少類型,是由於spring支持不少類型的配置好比property的值咱們能夠配置爲ref=xxxbean那麼value就是RuntimeBeanReference類型,
若是配置

<list>
                <value>343</value>
                <value>45</value>
            </list>

那麼value就是List類型等等。這裏咱們例子中配置的類型是TypedStringValue，那麼執行

else if (value instanceof TypedStringValue) {
            TypedStringValue typedStringValue = (TypedStringValue) value;
            //這裏拿到的值是原始的帶有佔位符的好比例子裏就是${student.name}、${student.age}這種
            String stringValue = typedStringValue.getValue();
            if (stringValue != null) {
                //這一步就是去進行替換
                String visitedString = resolveStringValue(stringValue);
                typedStringValue.setValue(visitedString);
            }
        }

String visitedString = resolveStringValue(stringValue);

/**
     * Resolve the given String value, for example parsing placeholders.
     * @param strVal the original String value
     * @return the resolved String value
     */
    protected String resolveStringValue(String strVal) {
        if (this.valueResolver == null) {
            throw new IllegalStateException("No StringValueResolver specified - pass a resolver " +
                    "object into the constructor or override the 'resolveStringValue' method");
        }
        //調用咱們以前傳進來的valueResolver也就是上面 (BeanDefinitionVisitor visitor = new BeanDefinitionVisitor(valueResolver); 這裏傳進來的) 進行替換操做
        String resolvedValue = this.valueResolver.resolveStringValue(strVal);
        // Return original String if not modified.
        return (strVal.equals(resolvedValue) ? strVal : resolvedValue);
    }

valueResolver其實就是PlaceholderResolvingStringValueResolver實例,它又委託PropertyPlaceholderHelper進行操做

也就是

/**
     * Replaces all placeholders of format {@code ${name}} with the value returned
     * from the supplied {@link PlaceholderResolver}.
     * @param value the value containing the placeholders to be replaced.
     * @param placeholderResolver the {@code PlaceholderResolver} to use for replacement.
     * @return the supplied value with placeholders replaced inline.
     */
    public String replacePlaceholders(String value, PlaceholderResolver placeholderResolver) {
        Assert.notNull(value, "Argument 'value' must not be null.");
        return parseStringValue(value, placeholderResolver, new HashSet<String>());
    }

    protected String parseStringValue(
            String strVal, PlaceholderResolver placeholderResolver, Set<String> visitedPlaceholders) {

        StringBuilder buf = new StringBuilder(strVal);

        int startIndex = strVal.indexOf(this.placeholderPrefix);
        while (startIndex != -1) {
            int endIndex = findPlaceholderEndIndex(buf, startIndex);
            if (endIndex != -1) {
                //將ex. ${student.name} 轉成 student.name
                String placeholder = buf.substring(startIndex + this.placeholderPrefix.length(), endIndex);
                String originalPlaceholder = placeholder;
                if (!visitedPlaceholders.add(originalPlaceholder)) {
                    throw new IllegalArgumentException(
                            "Circular placeholder reference '" + originalPlaceholder + "' in property definitions");
                }
                // 遞歸調用直到沒有佔位符
                placeholder = parseStringValue(placeholder, placeholderResolver, visitedPlaceholders);
                // 獲取properties配置文件對應的值  就是獲取student.name 對應在 properties裏的值
                String propVal = placeholderResolver.resolvePlaceholder(placeholder);
                if (propVal == null && this.valueSeparator != null) {
                    int separatorIndex = placeholder.indexOf(this.valueSeparator);
                    if (separatorIndex != -1) {
                        String actualPlaceholder = placeholder.substring(0, separatorIndex);
                        String defaultValue = placeholder.substring(separatorIndex + this.valueSeparator.length());
                        propVal = placeholderResolver.resolvePlaceholder(actualPlaceholder);
                        if (propVal == null) {
                            propVal = defaultValue;
                        }
                    }
                }
                if (propVal != null) {
                    //再次遞歸調用 確保最後的值沒有佔位符
                    propVal = parseStringValue(propVal, placeholderResolver, visitedPlaceholders);
                    //將原先的StringBuff的原始值替換爲拿到的值
                    buf.replace(startIndex, endIndex + this.placeholderSuffix.length(), propVal);
                    if (logger.isTraceEnabled()) {
                        logger.trace("Resolved placeholder '" + placeholder + "'");
                    }
                    //這裏由於已經替換成真正的值 拿不到佔位符(${)因此值就是-1 會跳出循環返回
                    startIndex = buf.indexOf(this.placeholderPrefix, startIndex + propVal.length());
                }
                else if (this.ignoreUnresolvablePlaceholders) {
                    // Proceed with unprocessed value.
                    startIndex = buf.indexOf(this.placeholderPrefix, endIndex + this.placeholderSuffix.length());
                }
                else {
                    throw new IllegalArgumentException("Could not resolve placeholder '" +
                            placeholder + "'" + " in string value \"" + strVal + "\"");
                }
                visitedPlaceholders.remove(originalPlaceholder);
            }
            else {
                startIndex = -1;
            }
        }

        return buf.toString();
    }

該方法是真正替換的操做所在,改方法已經添加中文註釋應該很好理解了，總結就是作了一下兩件事:
1.首先會將ex. ${student.name} 轉成 student.name
2.將student.name經過 tring propVal = placeholderResolver.resolvePlaceholder(placeholder);獲取真正的值而後返回。

protected String resolvePlaceholder(String placeholder, Properties props, int systemPropertiesMode) {
        String propVal = null;
        if (systemPropertiesMode == SYSTEM_PROPERTIES_MODE_OVERRIDE) {
            propVal = resolveSystemProperty(placeholder);
        }
        if (propVal == null) {
            propVal = resolvePlaceholder(placeholder, props);
        }
        if (propVal == null && systemPropertiesMode == SYSTEM_PROPERTIES_MODE_FALLBACK) {
            propVal = resolveSystemProperty(placeholder);
        }
        return propVal;
    }

該方法就比較簡單了就是根據不一樣的模式作處理,systemPropertiesMode默認是SYSTEM_PROPERTIES_MODE_FALLBACK

若是systemPropertiesMode = SYSTEM_PROPERTIES_MODE_OVERRIDE 是指系統配置文件優先級大於咱們的配置文件。
拿不到或者配置不等於SYSTEM_PROPERTIES_MODE_OVERRIDE 就去咱們配置文件裏進行獲取
若是還獲取不到且若是systemPropertiesMode=SYSTEM_PROPERTIES_MODE_FALLBACK就再嘗試去系統文件裏查找。

結論

通過以上各個步驟最終BeanDefinition裏的parentName beanClassName property中的佔位符都會被咱們propertis配置文件中對應的值所替換掉，這就爲後續實例化bean後作bean實例屬性填充時作好了準備。
咱們再進行 Student student =(Student) ac.getBean("student"); 時就能夠正常打印對應的值了。