@@ -39,3 +39,148 @@ public final class SpringFactoriesLoader {
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4040< img src= " https://tva1.sinaimg.cn/large/008i3skNly1grkdwk98bkj31bk0py4bt.jpg" = " image-20210616203348022" = " left" = " 500" / >
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42+
43+
44+ #### 源码阅读
45+
46+ ##### 1、首先通过setInitializers#()方法设置系统初始化器
47+
48+ ```java
49+ @SuppressWarnings ({ "unchecked ", "rawtypes " })
50+ public SpringApplication (ResourceLoader resourceLoader , Class<?> ... primarySources ) {
51+ this . resourceLoader = resourceLoader;
52+ Assert . notNull(primarySources, " PrimarySources must not be null"
53+ this . primarySources = new LinkedHashSet<> (Arrays . asList(primarySources));
54+ this . webApplicationType = WebApplicationType . deduceFromClasspath();
55+ // 通过setInitializers#()方法，设置系统初始化器
56+ setInitializers((Collection ) getSpringFactoriesInstances(ApplicationContextInitializer . class));
57+ setListeners((Collection ) getSpringFactoriesInstances(ApplicationListener . class));
58+ this . mainApplicationClass = deduceMainApplicationClass();
59+ }
60+ ```
61+
62+
63+
64+ ##### 2 、获取系统初始化器实现的全路径名，为它们创建实例子，最终把它们进行排序后注入到Spring 容器集合中
65+
66+ ```java
67+ private <T > Collection<T > getSpringFactoriesInstances (Class<T > type , Class<?> [] parameterTypes , Object ... args ) {
68+ // 获得类加载器
69+ ClassLoader classLoader = getClassLoader();
70+ // 通过这个SpringFactoriesLoader#loadFactoryNames方法获取Spring中所有系统初始化器实现的全路径名
71+ Set<String > names = new LinkedHashSet<> (SpringFactoriesLoader . loadFactoryNames(type, classLoader));
72+ // 通过createSpringFactoriesInstances#()创建它们的实例
73+ List<T > instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);
74+ // 通过注解@Order进行排序
75+ AnnotationAwareOrderComparator . sort(instances);
76+ return instances;
77+ }
78+ ```
79+
80+
81+
82+ ##### 3 、获取系统初始化器实现的全路径名
83+
84+ ```java
85+ private static Map<String , List<String > > loadSpringFactories (@Nullable ClassLoader classLoader ) {
86+ MultiValueMap<String , String > result = cache. get(classLoader);
87+ // 检查是否有缓存
88+ if (result != null ) {
89+ return result;
90+ }
91+
92+ try {
93+ Enumeration<URL > urls = (classLoader != null ?
94+ // 获取 META-INF/spring.factories下所有系统初始化器的实现
95+ classLoader. getResources(FACTORIES_RESOURCE_LOCATION ) :
96+ ClassLoader . getSystemResources(FACTORIES_RESOURCE_LOCATION ));
97+ result = new LinkedMultiValueMap<> ();
98+ while (urls. hasMoreElements()) {
99+ URL url = urls. nextElement();
100+ UrlResource resource = new UrlResource (url);
101+ // 通过url路径，获取spring.factories中的配置，并将其转成Properties格式
102+ Properties properties = PropertiesLoaderUtils . loadProperties(resource);
103+ for (Map . Entry<?, ?> entry : properties. entrySet()) {
104+ String factoryClassName = ((String ) entry. getKey()). trim();
105+ for (String factoryName : StringUtils . commaDelimitedListToStringArray((String ) entry. getValue())) {
106+ // 将初始化器的包名进行返回
107+ result. add(factoryClassName, factoryName. trim());
108+ }
109+ }
110+ }
111+ // 记录到缓存中
112+ cache. put(classLoader, result);
113+ return result;
114+ }
115+ catch (IOException ex) {
116+ throw new IllegalArgumentException (" Unable to load factories from location [" +
117+ FACTORIES_RESOURCE_LOCATION + " ]"
118+ }
119+ }
120+ ```
121+
122+
123+
124+ ##### 4 、创建初始化器的实例
125+
126+ ```java
127+ @SuppressWarnings (" unchecked"
128+ private <T > List<T > createSpringFactoriesInstances (Class<T > type , Class<?> [] parameterTypes ,
129+ ClassLoader classLoader , Object [] args , Set<String > names ) {
130+ List<T > instances = new ArrayList<> (names. size());
131+ for (String name : names) {
132+ try {
133+ // 通过反射获取具体实现，并完成类的初始化，最终返回实现类的实例
134+ Class<?> instanceClass = ClassUtils . forName(name, classLoader);
135+ Assert . isAssignable(type, instanceClass);
136+ Constructor<?> constructor = instanceClass. getDeclaredConstructor(parameterTypes);
137+ T instance = (T ) BeanUtils . instantiateClass(constructor, args);
138+ instances. add(instance);
139+ }
140+ catch (Throwable ex) {
141+ throw new IllegalArgumentException (" Cannot instantiate " + type + " : " + name, ex);
142+ }
143+ }
144+ return instances;
145+ }
146+ ```
147+
148+
149+
150+ ##### 5 、按@Order 注解进行排序
151+
152+ ```java
153+ private <T > Collection<T > getSpringFactoriesInstances (Class<T > type , Class<?> [] parameterTypes , Object ... args ) {
154+ ClassLoader classLoader = getClassLoader();
155+ // Use names and ensure unique to protect against duplicates
156+ Set<String > names = new LinkedHashSet<> (SpringFactoriesLoader . loadFactoryNames(type, classLoader));
157+ List<T > instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);
158+ // 按@Order注解进行排序，值约小的越靠前
159+ AnnotationAwareOrderComparator . sort(instances);
160+ return instances;
161+ }
162+ ```
163+
164+
165+
166+ ##### 6 、将初始化器排序好的实例注册到Spring 容器中
167+
168+ ```java
169+ public void setInitializers (Collection<? extends ApplicationContextInitializer<?> > initializers ) {
170+ this . initializers = new ArrayList<> ();
171+ // 将初始化器的实例添加到Spring容器中
172+ this . initializers. addAll(initializers);
173+ }
174+ ```
175+
176+
177+
178+ #### 总结
179+
180+ - SpringFactoriesLoader 作用： SpringBoot 框架中从类路径jar包中读取特定的文件（META - INF / spring. factories）实现扩展类的载入
181+
182+ < img src= " /Users/huangyan110110114/Library/Application Support/typora-user-images/image-20210726172128392.png" = " image-20210726172128392" = " left" = " 800" / >
183+
184+ - LoadFactories 流程
185+
186+ < img src= " /Users/huangyan110110114/Library/Application Support/typora-user-images/image-20210726172754618.png" = " image-20210726172754618" = " left" = " 800" / >
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