一、前言
- Springboot源码解析是一件大工程,逐行逐句的去研究代码,会很枯燥,也不容易坚持下去。
- 我们不追求大而全,而是试着每次去研究一个小知识点,最终聚沙成塔,这就是我们的springboot源码管中窥豹系列。
二、Initializer
- 上一节我们介绍了Runner,它是在项目加载完成之后执行的
- 有后就有前,有没有在项目加载之前执行的呢?
我们今天介绍的ApplicationContextInitializer就是在spring的bean加载之前执行的
public interface ApplicationContextInitializer {
void initialize(C applicationContext);
}
- 使用很简单,实现ApplicationContextInitializer接口就可以了
- 它是先于普通bean加载的,所以不能用@Component的方式
- 具体怎么被springboot加载,往下看,我们分析源码的时候说
三、源码解析
如果对springboot源码一点都不了解的,推荐先看第一节:整体架构
1、获取ApplicationContextInitializer
我们直接先看SpringApplication的构造方法
public SpringApplication(ResourceLoader resourceLoader, Class... primarySources) {
...
setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
...
}
我们先看setInitializers方法,再看里面的getSpringFactoriesInstances方法
public void setInitializers(Collection> initializers) {
this.initializers = new ArrayList<>(initializers);
}
- 很简单,把查询的initializers集合赋值到本地变量上
- 接着看getSpringFactoriesInstances方法,这个initializers集合怎么查的
private Collection getSpringFactoriesInstances(Class type) {
return getSpringFactoriesInstances(type, new Class[] {});
}
private Collection getSpringFactoriesInstances(Class type, Class[] parameterTypes, Object... args) {
ClassLoader classLoader = getClassLoader();
// Use names and ensure unique to protect against duplicates
Set names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader));
List instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);
AnnotationAwareOrderComparator.sort(instances);
return instances;
}
- (1) 获取ApplicationContextInitializer实现类的名称集合
- (2) 加载成实例instances
- (3) 排序,返回
我们先研究下SpringFactoriesLoader.loadFactoryNames(type, classLoader)这个方法:
public static List loadFactoryNames(Class factoryType, @Nullable ClassLoader classLoader) {
String factoryTypeName = factoryType.getName();
return loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());
}
private static Map> loadSpringFactories(@Nullable ClassLoader classLoader) {
MultiValueMap result = cache.get(classLoader);
if (result != null) {
return result;
}
try {
Enumeration urls = (classLoader != null ?
classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
result = new LinkedMultiValueMap<>();
while (urls.hasMoreElements()) {
URL url = urls.nextElement();
UrlResource resource = new UrlResource(url);
Properties properties = PropertiesLoaderUtils.loadProperties(resource);
for (Map.Entry entry : properties.entrySet()) {
String factoryTypeName = ((String) entry.getKey()).trim();
for (String factoryImplementationName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
result.add(factoryTypeName, factoryImplementationName.trim());
}
}
}
cache.put(classLoader, result);
return result;
}
catch (IOException ex) {
throw new IllegalArgumentException("Unable to load factories from location [" +
FACTORIES_RESOURCE_LOCATION + "]", ex);
}
}
- 注意:factoryType就是我们传入的参数 ApplicationContextInitializer.class
- 这里有个新的map结构:MultiValueMap
,它和Map 是一样的 - 我们先看一下这里:classLoader.getResources(FACTORIES_RESOURCE_LOCATION)
public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
- 加载所有的META-INF/spring.factories,按接口名称放入MultiValueMap
,并cache - 注意,这类文件不止一个,它们分布在不同的jar包里面
- 这么说你可能不懂,我们看一下这类文件长什么样,我们看一个系统自带的
# PropertySource Loaders
org.springframework.boot.env.PropertySourceLoader=\
org.springframework.boot.env.PropertiesPropertySourceLoader,\
org.springframework.boot.env.YamlPropertySourceLoader
# Run Listeners
org.springframework.boot.SpringApplicationRunListener=\
org.springframework.boot.context.event.EventPublishingRunListener
# Error Reporters
org.springframework.boot.SpringBootExceptionReporter=\
org.springframework.boot.diagnostics.FailureAnalyzers
# Application Context Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,\
org.springframework.boot.context.ContextIdApplicationContextInitializer,\
org.springframework.boot.context.config.DelegatingApplicationContextInitializer,\
org.springframework.boot.rsocket.context.RSocketPortInfoApplicationContextInitializer,\
org.springframework.boot.web.context.ServerPortInfoApplicationContextInitializer
# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.ClearCachesApplicationListener,\
org.springframework.boot.builder.ParentContextCloserApplicationListener,\
org.springframework.boot.cloud.CloudFoundryVcapEnvironmentPostProcessor,\
org.springframework.boot.context.FileEncodingApplicationListener,\
org.springframework.boot.context.config.AnsiOutputApplicationListener,\
org.springframework.boot.context.config.ConfigFileApplicationListener,\
org.springframework.boot.context.config.DelegatingApplicationListener,\
org.springframework.boot.context.logging.ClasspathLoggingApplicationListener,\
org.springframework.boot.context.logging.LoggingApplicationListener,\
org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener
# Environment Post Processors
org.springframework.boot.env.EnvironmentPostProcessor=\
org.springframework.boot.cloud.CloudFoundryVcapEnvironmentPostProcessor,\
org.springframework.boot.env.SpringApplicationJsonEnvironmentPostProcessor,\
org.springframework.boot.env.SystemEnvironmentPropertySourceEnvironmentPostProcessor,\
org.springframework.boot.reactor.DebugAgentEnvironmentPostProcessor
# Failure Analyzers
org.springframework.boot.diagnostics.FailureAnalyzer=\
org.springframework.boot.diagnostics.analyzer.BeanCurrentlyInCreationFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.BeanDefinitionOverrideFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.BeanNotOfRequiredTypeFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.BindFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.BindValidationFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.UnboundConfigurationPropertyFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.ConnectorStartFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.NoSuchMethodFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.NoUniqueBeanDefinitionFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.PortInUseFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.ValidationExceptionFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.InvalidConfigurationPropertyNameFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.InvalidConfigurationPropertyValueFailureAnalyzer
# FailureAnalysisReporters
org.springframework.boot.diagnostics.FailureAnalysisReporter=\
org.springframework.boot.diagnostics.LoggingFailureAnalysisReporter
- 接口 = 实现类1,实现类2,实现类3
- 我们假如有了自己的ApplicationContextInitializer实现类,我们在resource下面新建/META-INF/spring.factories文件,按上面的格式写上就可以被加载了
org.springframework.context.ApplicationContextInitializer=\
org.my.zb.MyApplicationContextInitializer
- 我们把思维拉回去,讲完了怎么取的实现类名称集合
- 回去看 createSpringFactoriesInstances();
private List createSpringFactoriesInstances(Class type, Class[] parameterTypes,
ClassLoader classLoader, Object[] args, Set names) {
List instances = new ArrayList<>(names.size());
for (String name : names) {
try {
Class instanceClass = ClassUtils.forName(name, classLoader);
Assert.isAssignable(type, instanceClass);
Constructor constructor = instanceClass.getDeclaredConstructor(parameterTypes);
T instance = (T) BeanUtils.instantiateClass(constructor, args);
instances.add(instance);
}
catch (Throwable ex) {
throw new IllegalArgumentException("Cannot instantiate " + type + " : " + name, ex);
}
}
return instances;
}
- (1) 获取Class
- (2) 获取构造函数
- (3) 利用反射新建instance对象
- (4) 加入集合
至此,我们就得到了:List instances
2、执行ApplicationContextInitializer
我们看SpringApplication的run方法:
public ConfigurableApplicationContext run(String... args) {
...
try {
...
prepareContext(context, environment, listeners, applicationArguments, printedBanner);
...
}
catch (Throwable ex) {
...
}
...
return context;
}
进入到prepareContext方法:
private void prepareContext(ConfigurableApplicationContext context, ConfigurableEnvironment environment,
SpringApplicationRunListeners listeners, ApplicationArguments applicationArguments, Banner printedBanner) {
...
applyInitializers(context);
...
}
定位到了applyInitializers():
protected void applyInitializers(ConfigurableApplicationContext context) {
for (ApplicationContextInitializer initializer : getInitializers()) {
Class requiredType = GenericTypeResolver.resolveTypeArgument(initializer.getClass(),
ApplicationContextInitializer.class);
Assert.isInstanceOf(requiredType, context, "Unable to call initializer.");
initializer.initialize(context);
}
}
- 前两行判断类型
- 最后一行回调执行
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