【spring源码分析】IOC容器初始化（一）

前言：spring主要就是对bean进行管理，因此IOC容器的初始化过程非常重要，搞清楚其原理不管在实际生产或面试过程中都十分的有用。在【spring源码分析】准备工作中已经搭建好spring的环境，并利用xml配置形式对类进行了实例化。在test代码中有一个非常关键的类ClassPathXmlApplicationContext，在这个类中实现了IOC容器的初始化，因此我们从ClassPathXmlApplicationContext着手开始研究IOC的初始化过程。

ClassPathXmlApplicationContext类继承关系

ClassPathXmlApplicationContext类的继承关系非常的庞大，在IOC容器初始化的过程中，经常使用委派的方式进行函数的调用，因此需特别注意类之间的继承关系。通过阅读源码，可粗略的将IOC容器初始化过程分为两步：

① 解析xml文件，导入bean

② 通过反射生成bean

因此下面将从这两大步对IOC初始化进行分析。

导入bean阶段程序调用链

首先给出导入bean的调用链：

通过程序调用链可知：

#1.导入bean的过程大致分为三个阶段：

①导入bean（loadBeanDefinitions）

②解析bean（parseBeanDefinition）

③注册bean（registerBeanDefinition）

#2.最终bean是存储在beanDefinitionMap中：键为类名（beanName），值为GenericBeanDefinition（BeanDefinition）。

下面对导入bean的三个阶段进行分析。

导入bean阶段源码分析

首先来看ClassPathXmlApplicationContext构造函数，具体代码如下：

 1 public ClassPathXmlApplicationContext(
 2             String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
 3             throws BeansException {
 4         // 初始化父类相关资源
 5         super(parent);
 6         // 解析配置文件路径，并设置资源路径
 7         setConfigLocations(configLocations);
 8         if (refresh) {
 9             // 核心方法 ioc容器初始化在此方法中实现
10             refresh();
11         }
12     }

看似寥寥的几行代码，其实也非常重要，从这里我们可以了解到spring是如何解析配置文件中的占位符信息的，这里关注PropertyResolver接口。

首先我们看PropertyResolver接口源码：

  1 public interface PropertyResolver {
  2 
  3     /**
  4      * 是否包含某个属性<br/>
  5      * Return whether the given property key is available for resolution,
  6      * i.e. if the value for the given key is not {@code null}.
  7      */
  8     boolean containsProperty(String key);
  9 
 10     /**
 11      * 获取属性值 如果找不到则返回null<br/>
 12      * Return the property value associated with the given key,
 13      * or {@code null} if the key cannot be resolved.
 14      *
 15      * @param key the property name to resolve
 16      * @see #getProperty(String, String)
 17      * @see #getProperty(String, Class)
 18      * @see #getRequiredProperty(String)
 19      */
 20     @Nullable
 21     String getProperty(String key);
 22 
 23     /**
 24      * 获取属性值，如果找不到则返回默认值<br/>
 25      * Return the property value associated with the given key, or
 26      * {@code defaultValue} if the key cannot be resolved.
 27      *
 28      * @param key          the property name to resolve
 29      * @param defaultValue the default value to return if no value is found
 30      * @see #getRequiredProperty(String)
 31      * @see #getProperty(String, Class)
 32      */
 33     String getProperty(String key, String defaultValue);
 34 
 35     /**
 36      * 获取指定类型的属性值，找不到则返回null<br/>
 37      * Return the property value associated with the given key,
 38      * or {@code null} if the key cannot be resolved.
 39      *
 40      * @param key        the property name to resolve
 41      * @param targetType the expected type of the property value
 42      * @see #getRequiredProperty(String, Class)
 43      */
 44     @Nullable
 45     <T> T getProperty(String key, Class<T> targetType);
 46 
 47     /**
 48      * 获取指定类型的属性值，找不到则返回默认值<br/>
 49      * Return the property value associated with the given key,
 50      * or {@code defaultValue} if the key cannot be resolved.
 51      *
 52      * @param key          the property name to resolve
 53      * @param targetType   the expected type of the property value
 54      * @param defaultValue the default value to return if no value is found
 55      * @see #getRequiredProperty(String, Class)
 56      */
 57     <T> T getProperty(String key, Class<T> targetType, T defaultValue);
 58 
 59     /**
 60      * 获取属性值，找不到则抛出异常IllegalStateException<br/>
 61      * Return the property value associated with the given key (never {@code null}).
 62      *
 63      * @throws IllegalStateException if the key cannot be resolved
 64      * @see #getRequiredProperty(String, Class)
 65      */
 66     String getRequiredProperty(String key) throws IllegalStateException;
 67 
 68     /**
 69      * 获取指定类型的属性值，找不到则抛出异常IllegalStateException<br/>
 70      * Return the property value associated with the given key, converted to the given
 71      * targetType (never {@code null}).
 72      *
 73      * @throws IllegalStateException if the given key cannot be resolved
 74      */
 75     <T> T getRequiredProperty(String key, Class<T> targetType) throws IllegalStateException;
 76 
 77     /**
 78      * 替换文本中的占位符(${key})到属性值，找不到则不解析
 79      * Resolve ${...} placeholders in the given text, replacing them with corresponding
 80      * property values as resolved by {@link #getProperty}. Unresolvable placeholders with
 81      * no default value are ignored and passed through unchanged.
 82      *
 83      * @param text the String to resolve
 84      * @return the resolved String (never {@code null})
 85      * @throws IllegalArgumentException if given text is {@code null}
 86      * @see #resolveRequiredPlaceholders
 87      * @see org.springframework.util.SystemPropertyUtils#resolvePlaceholders(String)
 88      */
 89     String resolvePlaceholders(String text);
 90 
 91     /**
 92      * 替换文本中占位符(${key})到属性值，找不到则抛出异常IllegalArgumentException
 93      * Resolve ${...} placeholders in the given text, replacing them with corresponding
 94      * property values as resolved by {@link #getProperty}. Unresolvable placeholders with
 95      * no default value will cause an IllegalArgumentException to be thrown.
 96      *
 97      * @return the resolved String (never {@code null})
 98      * @throws IllegalArgumentException if given text is {@code null}
 99      *                                  or if any placeholders are unresolvable
100      * @see org.springframework.util.SystemPropertyUtils#resolvePlaceholders(String, boolean)
101      */
102     String resolveRequiredPlaceholders(String text) throws IllegalArgumentException;
103 
104 }

该接口定义了一些与属性（解析占位符/获取属性）相关的方法，以ClassPathXmlApplicationContext构造函数的第7行代码为Debug入口，下面会通过调试的形式进行分析。

PropertyResolver继承关系如下，注意AbstractPropertyResolver与StandardEnvironment都间接的实现了PropertyResolver接口。

在setConfigLocations(String)打断点，进行Debug，会走到AbstractRefreshableConfigApplicationContext#resolvePath处：

1 protected String resolvePath(String path) {
2         return getEnvironment().resolveRequiredPlaceholders(path);
3     }

这里getEnvironment()调用的是父类AbstractApplicationContext的方法：

 1     public ConfigurableEnvironment getEnvironment() {
 2         if (this.environment == null) {
 3             // 创建一个ConfigurableEnvironment对象
 4             this.environment = createEnvironment();
 5         }
 6         return this.environment;
 7     }
 8     protected ConfigurableEnvironment createEnvironment() {
 9         return new StandardEnvironment();
10     }

注意这里返回的是一个ConfigurableEnvironment 对象，继续debug，进入resolveRequiredPlaceholders(String)函数：

1    private final ConfigurablePropertyResolver propertyResolver =new PropertySourcesPropertyResolver(this.propertySources);
2     
3     public String resolveRequiredPlaceholders(String text) throws IllegalArgumentException {
4         // 委派给AbstractPropertyResolver执行
5         return this.propertyResolver.resolveRequiredPlaceholders(text);
6     }

由于函数调用过程太细，所以这里给出解析配置文件中占位符的最终核心点：PropertyPlaceholderHelper#parseStringValue方法上：

 1 protected String parseStringValue(
 2             String value, PlaceholderResolver placeholderResolver, Set<String> visitedPlaceholders) {
 3 
 4         StringBuilder result = new StringBuilder(value);
 5         // 获取前缀"${"的索引位置
 6         int startIndex = value.indexOf(this.placeholderPrefix);
 7         while (startIndex != -1) {
 8             // 获取后缀"}"的索引位置
 9             int endIndex = findPlaceholderEndIndex(result, startIndex);
10             if (endIndex != -1) {
11                 // 截取"${"和"}"中间的内容，即配置文件中对应的值
12                 String placeholder = result.substring(startIndex + this.placeholderPrefix.length(), endIndex);
13                 String originalPlaceholder = placeholder;
14                 if (!visitedPlaceholders.add(originalPlaceholder)) {
15                     throw new IllegalArgumentException(
16                             "Circular placeholder reference '" + originalPlaceholder + "' in property definitions");
17                 }
18                 // Recursive invocation, parsing placeholders contained in the placeholder key.
19                 // 解析占位符键中包含的占位符，真正的值
20                 placeholder = parseStringValue(placeholder, placeholderResolver, visitedPlaceholders);
21                 // Now obtain the value for the fully resolved key...
22                 // 从Properties中获取placeHolder对应的propVal
23                 String propVal = placeholderResolver.resolvePlaceholder(placeholder);
24                 // 如果不存在
25                 if (propVal == null && this.valueSeparator != null) {
26                     // 查询":"的位置
27                     int separatorIndex = placeholder.indexOf(this.valueSeparator);
28                     // 如果存在
29                     if (separatorIndex != -1) {
30                         // 截取":"前面部分的actualPlaceholder
31                         String actualPlaceholder = placeholder.substring(0, separatorIndex);
32                         // 截取":"后面的defaulValue
33                         String defaultValue = placeholder.substring(separatorIndex + this.valueSeparator.length());
34                         // 从Properties中获取actualPlaceholder对应的值
35                         propVal = placeholderResolver.resolvePlaceholder(actualPlaceholder);
36                         // 如果不存在，则返回defaultValue
37                         if (propVal == null) {
38                             propVal = defaultValue;
39                         }
40                     }
41                 }
42                 if (propVal != null) {
43                     // Recursive invocation, parsing placeholders contained in the
44                     // previously resolved placeholder value.
45                     propVal = parseStringValue(propVal, placeholderResolver, visitedPlaceholders);
46                     result.replace(startIndex, endIndex + this.placeholderSuffix.length(), propVal);
47                     if (logger.isTraceEnabled()) {
48                         logger.trace("Resolved placeholder '" + placeholder + "'");
49                     }
50                     startIndex = result.indexOf(this.placeholderPrefix, startIndex + propVal.length());
51                 }
52                 else if (this.ignoreUnresolvablePlaceholders) {
53                     // Proceed with unprocessed value.
54                     // 忽略值
55                     startIndex = result.indexOf(this.placeholderPrefix, endIndex + this.placeholderSuffix.length());
56                 }
57                 else {
58                     throw new IllegalArgumentException("Could not resolve placeholder '" +
59                             placeholder + "'" + " in value \"" + value + "\"");
60                 }
61                 visitedPlaceholders.remove(originalPlaceholder);
62             }
63             else {
64                 startIndex = -1;
65             }
66         }
67         // 返回propVal，就是替换之后的值
68         return result.toString();
69     }

分析：

该函数的主要作用就是取占位符"${}"或":"中的值进行赋值，比如在配置文件中直接使用xxx="${xxx.xx.xx}"或使用注解扫描时使用@Value("${xxx.xx.xx}")进行属性值注入的时候，都会走该函数进行解析。

接下来看非常重要的AbstractApplicationContext#refresh()函数：

 1 public void refresh() throws BeansException, IllegalStateException {
 2         synchronized (this.startupShutdownMonitor) {
 3             // Prepare this context for refreshing.
 4             // 准备刷新上下文环境
 5             prepareRefresh();
 6 
 7             // Tell the subclass to refresh the internal bean factory.
 8             // 创建并初始化BeanFactory
 9             ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
10 
11             // Prepare the bean factory for use in this context.
12             // 填充BeanFactory
13             prepareBeanFactory(beanFactory);
14 
15             try {
16                 // Allows post-processing of the bean factory in context subclasses.
17                 // 提供子类覆盖的额外处理，即子类处理定义的BeanFactoryPostProcess
18                 postProcessBeanFactory(beanFactory);
19 
20                 // Invoke factory processors registered as beans in the context.
21                 // 激活各种BeanFactory处理器
22                 invokeBeanFactoryPostProcessors(beanFactory);
23 
24                 // Register bean processors that intercept bean creation.
25                 // 注册拦截Bean创建的Bean处理器，即注册BeanPostProcessor
26                 registerBeanPostProcessors(beanFactory);
27 
28                 // Initialize message source for this context.
29                 // 初始化上下文中的资源文件，如国际化文件的处理
30                 initMessageSource();
31 
32                 // Initialize event multicaster for this context.
33                 // 初始化上下文事件广播器
34                 initApplicationEventMulticaster();
35 
36                 // Initialize other special beans in specific context subclasses.
37                 // 给子类扩展初始化其他bean
38                 onRefresh();
39 
40                 // Check for listener beans and register them.
41                 // 在所有bean中查找listener bean，然后注册到广播器中
42                 registerListeners();
43 
44                 // Instantiate all remaining (non-lazy-init) singletons.
45                 // 初始化剩下的单例Bean(非延迟加载的)
46                 finishBeanFactoryInitialization(beanFactory);
47 
48                 // Last step: publish corresponding event.
49                 // 完成刷新过程，通知声明周期处理器lifecycleProcessor刷新过程，同时发出ContextRefreshEvent事件通知别人
50                 finishRefresh();
51             } catch (BeansException ex) {
52                 if (logger.isWarnEnabled()) {
53                     logger.warn("Exception encountered during context initialization - " +
54                                         "cancelling refresh attempt: " + ex);
55                 }
56 
57                 // Destroy already created singletons to avoid dangling resources.
58                 // 销毁已经创建的bean
59                 destroyBeans();
60 
61                 // Reset 'active' flag.
62                 // 重置容器激活标签
63                 cancelRefresh(ex);
64 
65                 // Propagate exception to caller.
66                 throw ex;
67             } finally {
68                 // Reset common introspection caches in Spring's core, since we
69                 // might not ever need metadata for singleton beans anymore...
70                 resetCommonCaches();
71             }
72         }
73     }

分析：

该函数中进行了IOC容器的初始化工作，以该函数为切入点，进行相应源码的分析，一步一步来力求搞清楚。

AbstractApplicationContext#prepareRefresh()

 1 protected void prepareRefresh() {
 2         // Switch to active.
 3         // 设置启动时间
 4         this.startupDate = System.currentTimeMillis();
 5         // 设置context当前状态
 6         this.closed.set(false);
 7         this.active.set(true);
 8 
 9         if (logger.isDebugEnabled()) {
10             if (logger.isTraceEnabled()) {
11                 logger.trace("Refreshing " + this);
12             } else {
13                 logger.debug("Refreshing " + getDisplayName());
14             }
15         }
16 
17         // Initialize any placeholder property sources in the context environment.
18         // 初始化context environment(上下文环境)中的占位符属性来源，该函数主要提供给子类进行扩展使用
19         initPropertySources();
20 
21         // Validate that all properties marked as required are resolvable:
22         // see ConfigurablePropertyResolver#setRequiredProperties
23         // 对属性值进行必要的验证
24         getEnvironment().validateRequiredProperties();
25 
26         // Store pre-refresh ApplicationListeners...
27         if (this.earlyApplicationListeners == null) {
28             this.earlyApplicationListeners = new LinkedHashSet<>(this.applicationListeners);
29         } else {
30             // Reset local application listeners to pre-refresh state.
31             this.applicationListeners.clear();
32             this.applicationListeners.addAll(this.earlyApplicationListeners);
33         }
34 
35         // Allow for the collection of early ApplicationEvents,
36         // to be published once the multicaster is available...
37         this.earlyApplicationEvents = new LinkedHashSet<>();
38     }

分析：

prepareRefresh()函数，作用较为简单，主要是做一些设置操作，这里不做过多赘述。

AbstractApplicationContext#obtainFreshBeanFactory()

1 protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
2         // 刷新BeanFactory
3         refreshBeanFactory();
4         // 返回BeanFactory
5         return getBeanFactory();
6     }

分析：

该函数主要作用：创建并初始化BeanFactory。

这里简单介绍一下BeanFactory：它是一个基本的Bean容器，其中BeanDefinition是它的基本结构，BeanFactory内部维护了一个BeanDefinitionMap（要点），BeanFactory可根据BeanDefinition的描述进行bean的创建与管理。

注：DefaultListableBeanFactory为最终默认实现，它实现了所有接口。

进入AbstractRefreshableApplicationContext#refreshBeanFactory()函数

 1 @Override
 2     protected final void refreshBeanFactory() throws BeansException {
 3         // 若已有BeanFactory，则销毁bean，并销毁BeanFactory
 4         if (hasBeanFactory()) {
 5             destroyBeans();
 6             closeBeanFactory();
 7         }
 8         try {
 9             // 创建BeanFactory对象
10             DefaultListableBeanFactory beanFactory = createBeanFactory();
11             // 指定序列化编号
12             beanFactory.setSerializationId(getId());
13             // 定制BeanFactory 设置相关属性
14             customizeBeanFactory(beanFactory);
15             // 加载BeanDefinition
16             loadBeanDefinitions(beanFactory);
17             // 设置Context的BeanFactory
18             synchronized (this.beanFactoryMonitor) {
19                 this.beanFactory = beanFactory;
20             }
21         } catch (IOException ex) {
22             throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
23         }
24     }

分析：

相应代码已经给出了基本注释，这里我们主要关注第16行代码：loadBeanDefinitions(DefaultListableBeanFactory)，从该函数可引申非常多重要的知识点，介于篇幅原因，将在后面进行详细分析。

总结

这里再次总结本文重点：

PropertyResolver，以及引申出来的PropertyPlaceholderHelper#parseStringValue（占位符解析重要函数）与日常开发也息息相关。
BeanFactory以及其最终实现类DefaultListableBeanFactory，基础的IoC容器，提供与bean相关的方法。
loadBeanDefinitions方法，这里再次强调一下，该方法非常重要。

by Shawn Chen，2018.11.24日，晚。

posted @ 2018-11-24 22:06 developer_chan 阅读(1624) 评论(0) 编辑收藏举报

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