初始化器
Spring 是一个扩展性很强的容器框架，为开发者提供了丰富的扩展入口，其中一个扩展点便是 ApplicationContextInitializer （应用上下文初始化器 ）。
ApplicationContextInitializer 是 Spring 在执行 ConfigurableApplicationContext.refresh() 方法对应用上下文进行刷新之前调用的一个回调接口，用来完成对 Spring 应用上下文个性化的初始化工作，该接口定义在 org.springframework.context 包中，其内部仅包含一个 initialize() 方法，其定义代码如下。
public interface ApplicationContextInitializer<C extends ConfigurableApplicationContext> {
/**
* Initialize the given application context.
* @param applicationContext the application to configure
*/
void initialize(C applicationContext);
}
自定义初始化器
在 Springboot 中使用自定义初始化器大致可以分为以下两个步骤：
- 自定义初始化器，一般是实现 ApplicationContextInitializer 接口。
- 注册初始化器。
第一步：自定义初始化器，此处为了测试初始化器的执行顺序定义了如下3个初始化器
@Order(1)
public class Initializer1 implements ApplicationContextInitializer<ConfigurableApplicationContext> {
@Override
public void initialize(ConfigurableApplicationContext applicationContext) {
ConfigurableEnvironment environment = applicationContext.getEnvironment();
Map<String,Object> map = new HashMap<>();
map.put("key1","value1");
MapPropertySource mapPropertySource = new MapPropertySource("Initializer1", map);
environment.getPropertySources().addLast(mapPropertySource);
System.out.println("run Initializer1");
}
}
@Order(2)
public class Initializer2 implements ApplicationContextInitializer<ConfigurableApplicationContext> {
@Override
public void initialize(ConfigurableApplicationContext applicationContext) {
ConfigurableEnvironment environment = applicationContext.getEnvironment();
Map<String,Object> map = new HashMap<>();
map.put("key2","value2");
MapPropertySource mapPropertySource = new MapPropertySource("Initializer2", map);
environment.getPropertySources().addLast(mapPropertySource);
System.out.println("run Initializer2");
}
}
@Order(3)
public class Initializer3 implements ApplicationContextInitializer<ConfigurableApplicationContext> {
@Override
public void initialize(ConfigurableApplicationContext applicationContext) {
ConfigurableEnvironment environment = applicationContext.getEnvironment();
Map<String,Object> map = new HashMap<>();
map.put("key3","value3");
MapPropertySource mapPropertySource = new MapPropertySource("Initializer3", map);
environment.getPropertySources().addLast(mapPropertySource);
System.out.println("run Initializer3");
}
}
第二步：注册初始化器，有以下三种方式
- 方式一：在启动类中，使用 SpringApplication.addInitializers() 方法注册。
@SpringBootApplication
@MapperScan("com.yibo.source.code.mapper")//扫描Mapper接口
public class Application {
public static void main(String[] args) {
SpringApplication springApplication = new SpringApplication(Application.class);
springApplication.addInitializers(new Initializer2());
springApplication.run();
}
}
- 方式二：在 Springboot 核心配置文件 application.properties 中增加 context.initializer.classes = [ 初始化器全类名 ] 进行注册。
context.initializer.classes=com.yibo.source.code.initializer.Initializer3
- 方式三：通过在CLASSPATH/META-INF/spring.factories中添加 org.springframework.context.ApplicationContextInitializer 配置项进行注册。
org.springframework.context.ApplicationContextInitializer=com.yibo.source.code.initializer.Initializer1
注意：虽然可以使用 @Order 注解来控制多个初始化器的执行顺序（数值越小越先执行），但是，通过不同方式注册的初始化器的执行顺序也有所不同，若多个初始化器注册的方式不同会导致 @Order 注解顺序无效，从以上程序执行后的打印结果来看，三种方式注册的初始化器的执行顺序依次是：方式二 --> 方式一 --> 方式三。
Springboot定义的初始化器
Springboot定义的 ApplicationContextInitializer 接口的实现类有下面几个，如图所示。
DelegatingApplicationContextInitializer
DelegatingApplicationContextInitializer 初始化器负责读取核心配置文件 context.initializer.classes 配置项指定的初始化器，并调用它们的 initialize() 方法来完成对应用上下文的初始化工作。
public class DelegatingApplicationContextInitializer implements ApplicationContextInitializer<ConfigurableApplicationContext>, Ordered {
private static final String PROPERTY_NAME = "context.initializer.classes";
private int order = 0;
public DelegatingApplicationContextInitializer() {
}
/**
* 对应用上下文进行初始化
*/
public void initialize(ConfigurableApplicationContext context) {
// 获取核心配置文件中指定的初始化器类
ConfigurableEnvironment environment = context.getEnvironment();
List<Class<?>> initializerClasses = this.getInitializerClasses(environment);
if (!initializerClasses.isEmpty()) {
// 利用获取到的初始化器类对应用上下文进行初始化
this.applyInitializerClasses(context, initializerClasses);
}
}
/**
* 读取核心配置文件中 context.initializer.classes 指定的初始化器类
*/
private List<Class<?>> getInitializerClasses(ConfigurableEnvironment env) {
String classNames = env.getProperty("context.initializer.classes");
List<Class<?>> classes = new ArrayList();
if (StringUtils.hasLength(classNames)) {
String[] var4 = StringUtils.tokenizeToStringArray(classNames, ",");
int var5 = var4.length;
for(int var6 = 0; var6 < var5; ++var6) {
String className = var4[var6];
classes.add(this.getInitializerClass(className));
}
}
return classes;
}
/**
* 使用指定的初始化器类对应用上下文进行初始化
*/
private void applyInitializerClasses(ConfigurableApplicationContext context, List<Class<?>> initializerClasses) {
Class<?> contextClass = context.getClass();
List<ApplicationContextInitializer<?>> initializers = new ArrayList();
Iterator var5 = initializerClasses.iterator();
while(var5.hasNext()) {
Class<?> initializerClass = (Class)var5.next();
initializers.add(this.instantiateInitializer(contextClass, initializerClass));
}
this.applyInitializers(context, initializers);
}
/**
* 使用指定的初始化器对应用上下文进行初始化
*/
private void applyInitializers(ConfigurableApplicationContext context, List<ApplicationContextInitializer<?>> initializers) {
// 对初始化器进行 Order 排序
initializers.sort(new AnnotationAwareOrderComparator());
Iterator var3 = initializers.iterator();
while(var3.hasNext()) {
ApplicationContextInitializer initializer = (ApplicationContextInitializer)var3.next();
initializer.initialize(context);
}
}
......
}
ContextIdApplicationContextInitializer
ContextIdApplicationContextInitializer 初始化器的作用是给应用上下文设置一个ID，这个ID通过environment中的"spring.application.name"获取，如果不存在则直接返回默认"application"
public class ContextIdApplicationContextInitializer implements ApplicationContextInitializer<ConfigurableApplicationContext>, Ordered {
private int order = 2147483637;
public ContextIdApplicationContextInitializer() {
}
public void setOrder(int order) {
this.order = order;
}
public int getOrder() {
return this.order;
}
public void initialize(ConfigurableApplicationContext applicationContext) {
ContextIdApplicationContextInitializer.ContextId contextId = this.getContextId(applicationContext);
applicationContext.setId(contextId.getId());
applicationContext.getBeanFactory().registerSingleton(ContextIdApplicationContextInitializer.ContextId.class.getName(), contextId);
}
private String getApplicationId(ConfigurableEnvironment environment) {
String name = environment.getProperty("spring.application.name");
return StringUtils.hasText(name) ? name : "application";
}
}
ConfigurationWarningsApplicationContextInitializer
ConfigurationWarningsApplicationContextInitializer 初始化器用来对常见的由于配置错误而引起的警告进行打印报告。
public class ConfigurationWarningsApplicationContextInitializer implements ApplicationContextInitializer<ConfigurableApplicationContext> {
private static final Log logger = LogFactory.getLog(ConfigurationWarningsApplicationContextInitializer.class);
public void initialize(ConfigurableApplicationContext context) {
// 添加一个 ConfigurationWarningsPostProcessor 用来打印警告信息
context.addBeanFactoryPostProcessor(new ConfigurationWarningsApplicationContextInitializer.ConfigurationWarningsPostProcessor(this.getChecks()));
}
}
其中的 ConfigurationWarningsPostProcessor 是一个静态内部类，用来打印注册 BeanDefinition 过程中产生的配置错误警告信息。
protected static final class ConfigurationWarningsPostProcessor implements PriorityOrdered, BeanDefinitionRegistryPostProcessor {
private ConfigurationWarningsApplicationContextInitializer.Check[] checks;
public ConfigurationWarningsPostProcessor(ConfigurationWarningsApplicationContextInitializer.Check[] checks) {
this.checks = checks;
}
public int getOrder() {
return 2147483646;
}
public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
}
public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException {
ConfigurationWarningsApplicationContextInitializer.Check[] var2 = this.checks;
int var3 = var2.length;
for(int var4 = 0; var4 < var3; ++var4) {
ConfigurationWarningsApplicationContextInitializer.Check check = var2[var4];
String message = check.getWarning(registry);
if (StringUtils.hasLength(message)) {
this.warn(message);
}
}
}
private void warn(String message) {
if (ConfigurationWarningsApplicationContextInitializer.logger.isWarnEnabled()) {
ConfigurationWarningsApplicationContextInitializer.logger.warn(String.format("%n%n** WARNING ** : %s%n%n", message));
}
}
}
ServerPortInfoApplicationContextInitializer
ServerPortInfoApplicationContextInitializer 初始化器通过监听 EmbeddedServletContainerInitializedEvent 事件，来对内部服务器实际要监听的端口号进行属性设置。
public class ServerPortInfoApplicationContextInitializer implements ApplicationContextInitializer<ConfigurableApplicationContext>, ApplicationListener<WebServerInitializedEvent> {
public ServerPortInfoApplicationContextInitializer() {
}
public void initialize(ConfigurableApplicationContext applicationContext) {
applicationContext.addApplicationListener(this);
}
public void onApplicationEvent(WebServerInitializedEvent event) {
String propertyName = "local." + this.getName(event.getApplicationContext()) + ".port";
this.setPortProperty((ApplicationContext)event.getApplicationContext(), propertyName, event.getWebServer().getPort());
}
private String getName(WebServerApplicationContext context) {
String name = context.getServerNamespace();
return StringUtils.hasText(name) ? name : "server";
}
private void setPortProperty(ApplicationContext context, String propertyName, int port) {
if (context instanceof ConfigurableApplicationContext) {
this.setPortProperty(((ConfigurableApplicationContext)context).getEnvironment(), propertyName, port);
}
if (context.getParent() != null) {
this.setPortProperty(context.getParent(), propertyName, port);
}
}
private void setPortProperty(ConfigurableEnvironment environment, String propertyName, int port) {
MutablePropertySources sources = environment.getPropertySources();
PropertySource<?> source = sources.get("server.ports");
if (source == null) {
source = new MapPropertySource("server.ports", new HashMap());
sources.addFirst((PropertySource)source);
}
((Map)((PropertySource)source).getSource()).put(propertyName, port);
}
}
SharedMetadataReaderFactoryContextInitializer
SharedMetadataReaderFactoryContextInitializer 初始化器用来创建一个可以在 ConfigurationClassPostProcessor 和Spring Boot 之间共享的CachingMetadataReaderFactory。
class SharedMetadataReaderFactoryContextInitializer implements ApplicationContextInitializer<ConfigurableApplicationContext>, Ordered {
public static final String BEAN_NAME = "org.springframework.boot.autoconfigure.internalCachingMetadataReaderFactory";
SharedMetadataReaderFactoryContextInitializer() {
}
public void initialize(ConfigurableApplicationContext applicationContext) {
applicationContext.addBeanFactoryPostProcessor(new SharedMetadataReaderFactoryContextInitializer.CachingMetadataReaderFactoryPostProcessor());
}
}
看下SpringBoot是如何加载初始化器的
入口类
@SpringBootApplication
@MapperScan("com.yibo.source.code.mapper")//扫描Mapper接口
public class Application {
public static void main(String[] args) {
SpringApplication.run(Application.class,args);
}
}
@SpringBootApplication我们上一篇文章https://blog.51cto.com/u_14014612/6007650 中大概的讲过了，有兴趣的可以看看我第一篇关于SpringBoot的文章，本篇文章主要关注SpringApplication.run(Application.class, args);，我们跟进去看看
// 调用静态类，参数对应的就是HelloWorldMainApplication.class以及main方法中的args
public static ConfigurableApplicationContext run(Class<?> primarySource, String... args) {
return run(new Class[]{primarySource}, args);
}
public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) {
return (new SpringApplication(primarySources)).run(args);
}
它实际上会构造一个SpringApplication的实例，并把我们的启动类Application.class作为参数传进去，然后运行它的run方法 SpringApplication构造器
public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
this.sources = new LinkedHashSet();
this.bannerMode = Mode.CONSOLE;
this.logStartupInfo = true;
this.addCommandLineProperties = true;
this.addConversionService = true;
this.headless = true;
this.registerShutdownHook = true;
this.additionalProfiles = new HashSet();
this.isCustomEnvironment = false;
this.resourceLoader = resourceLoader;
Assert.notNull(primarySources, "PrimarySources must not be null");
//把Application.class设置为属性存储起来
this.primarySources = new LinkedHashSet(Arrays.asList(primarySources));
//设置应用类型是Standard还是Web
this.webApplicationType = WebApplicationType.deduceFromClasspath();
//设置初始化器(Initializer),最后会调用这些初始化器
this.setInitializers(this.getSpringFactoriesInstances(ApplicationContextInitializer.class));
//设置监听器(Listener)
this.setListeners(this.getSpringFactoriesInstances(ApplicationListener.class));
this.mainApplicationClass = this.deduceMainApplicationClass();
}
设置应用类型
public enum WebApplicationType {
NONE,
SERVLET,
REACTIVE;
private static final String[] SERVLET_INDICATOR_CLASSES = new String[]{"javax.servlet.Servlet", "org.springframework.web.context.ConfigurableWebApplicationContext"};
private static final String WEBMVC_INDICATOR_CLASS = "org.springframework.web.servlet.DispatcherServlet";
private static final String WEBFLUX_INDICATOR_CLASS = "org.springframework.web.reactive.DispatcherHandler";
private static final String JERSEY_INDICATOR_CLASS = "org.glassfish.jersey.servlet.ServletContainer";
private static final String SERVLET_APPLICATION_CONTEXT_CLASS = "org.springframework.web.context.WebApplicationContext";
private static final String REACTIVE_APPLICATION_CONTEXT_CLASS = "org.springframework.boot.web.reactive.context.ReactiveWebApplicationContext";
private WebApplicationType() {
}
static WebApplicationType deduceFromClasspath() {
if (ClassUtils.isPresent("org.springframework.web.reactive.DispatcherHandler", (ClassLoader)null) && !ClassUtils.isPresent("org.springframework.web.servlet.DispatcherServlet", (ClassLoader)null) && !ClassUtils.isPresent("org.glassfish.jersey.servlet.ServletContainer", (ClassLoader)null)) {
return REACTIVE;
} else {
String[] var0 = SERVLET_INDICATOR_CLASSES;
int var1 = var0.length;
for(int var2 = 0; var2 < var1; ++var2) {
String className = var0[var2];
if (!ClassUtils.isPresent(className, (ClassLoader)null)) {
return NONE;
}
}
return SERVLET;
}
}
}
这里主要是通过类加载器判断REACTIVE相关的Class是否存在，如果不存在，则web环境即为SERVLET类型。这里设置好web环境类型，在后面会根据类型初始化对应环境。大家还记得我们第一篇文章中引入的依赖吗？
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
spring-boot-starter-web 的pom又会引入Tomcat和spring-webmvc，如下
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter</artifactId>
<version>2.1.10.RELEASE</version>
<scope>compile</scope>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-json</artifactId>
<version>2.1.10.RELEASE</version>
<scope>compile</scope>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-tomcat</artifactId>
<version>2.1.10.RELEASE</version>
<scope>compile</scope>
</dependency>
<dependency>
<groupId>org.hibernate.validator</groupId>
<artifactId>hibernate-validator</artifactId>
<version>6.0.18.Final</version>
<scope>compile</scope>
</dependency>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-web</artifactId>
<version>5.1.11.RELEASE</version>
<scope>compile</scope>
</dependency>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-webmvc</artifactId>
<version>5.1.11.RELEASE</version>
<scope>compile</scope>
</dependency>
</dependencies>
很明显spring-webmvc中存在DispatcherServlet这个类，也就是我们以前SpringMvc的核心Servlet，通过类加载能加载DispatcherServlet这个类，那么我们的应用类型自然就是WebApplicationType.SERVLET
设置初始化器(Initializer)
this.setInitializers(this.getSpringFactoriesInstances(ApplicationContextInitializer.class));
我们先来看看this.getSpringFactoriesInstances(ApplicationContextInitializer.class)
private <T> Collection<T> getSpringFactoriesInstances(Class<T> type) {
return this.getSpringFactoriesInstances(type, new Class[0]);
}
// 这里的入参type就是ApplicationContextInitializer.class
private <T> Collection<T> getSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, Object... args) {
ClassLoader classLoader = this.getClassLoader();
// 使用Set保存names来避免重复元素
Set<String> names = new LinkedHashSet(SpringFactoriesLoader.loadFactoryNames(type, classLoader));
// 根据names来进行实例化
List<T> instances = this.createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);
// 对实例进行排序
AnnotationAwareOrderComparator.sort(instances);
return instances;
}
这里面首先会根据入参type读取所有的names(是一个String集合)，然后根据这个集合来完成对应的实例化操作：
public final class SpringFactoriesLoader {
public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
// 入参就是ApplicationContextInitializer.class
public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {
String factoryClassName = factoryClass.getName();
return (List)loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
}
private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
MultiValueMap<String, String> result = (MultiValueMap)cache.get(classLoader);
if (result != null) {
return result;
} else {
try {
//从类路径的META-INF/spring.factories中加载所有默认的自动配置类
Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");
LinkedMultiValueMap result = new LinkedMultiValueMap();
while(urls.hasMoreElements()) {
URL url = (URL)urls.nextElement();
UrlResource resource = new UrlResource(url);
Properties properties = PropertiesLoaderUtils.loadProperties(resource);
Iterator var6 = properties.entrySet().iterator();
while(var6.hasNext()) {
Entry<?, ?> entry = (Entry)var6.next();
//获取ApplicationContextInitializer.class的所有值
String factoryClassName = ((String)entry.getKey()).trim();
String[] var9 = StringUtils.commaDelimitedListToStringArray((String)entry.getValue());
int var10 = var9.length;
for(int var11 = 0; var11 < var10; ++var11) {
String factoryName = var9[var11];
result.add(factoryClassName, factoryName.trim());
}
}
}
cache.put(classLoader, result);
return result;
} catch (IOException var13) {
throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", var13);
}
}
}
}
这个方法会尝试从类路径的META-INF/spring.factories处读取相应配置文件，然后进行遍历，读取配置文件中Key为：org.springframework.context.ApplicationContextInitializer的value。以spring-boot-autoconfigure这个包为例，它的META-INF/spring.factories部分定义如下所示：
# Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\
org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener
这两个类名会被读取出来，然后放入到Set<String>集合中，准备开始下面的实例化操作：
// parameterTypes: 上一步得到的names集合
private <T> List<T> createSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, ClassLoader classLoader, Object[] args, Set<String> names) {
List<T> instances = new ArrayList(names.size());
Iterator var7 = names.iterator();
while(var7.hasNext()) {
String name = (String)var7.next();
try {
Class<?> instanceClass = ClassUtils.forName(name, classLoader);
//确认被加载类是ApplicationContextInitializer的子类
Assert.isAssignable(type, instanceClass);
Constructor<?> constructor = instanceClass.getDeclaredConstructor(parameterTypes);
//反射实例化对象
T instance = BeanUtils.instantiateClass(constructor, args);
//加入List集合中
instances.add(instance);
} catch (Throwable var12) {
throw new IllegalArgumentException("Cannot instantiate " + type + " : " + name, var12);
}
}
return instances;
}
确认被加载的类确实是org.springframework.context.ApplicationContextInitializer的子类，然后就是得到构造器进行初始化，最后放入到实例列表中。
因此，所谓的初始化器就是org.springframework.context.ApplicationContextInitializer的实现类，这个接口是这样定义的：
public interface ApplicationContextInitializer<C extends ConfigurableApplicationContext> {
/**
* Initialize the given application context.
* @param applicationContext the application to configure
*/
void initialize(C applicationContext);
}
在Spring上下文被刷新之前进行初始化的操作。即在SpringApplication.prepareContext方法中调用applyInitializers方法进行初始化操作，典型地比如在Web应用中，注册Property Sources或者是激活Profiles。Property Sources比较好理解，就是配置文件。Profiles是Spring为了在不同环境下(如DEV，TEST，PRODUCTION等)，加载不同的配置项而抽象出来的一个实体。
public class SpringApplication {
public ConfigurableApplicationContext run(String... args) {
......
this.prepareContext(context, environment, listeners, applicationArguments, printedBanner);
......
}
private void prepareContext(ConfigurableApplicationContext context, ConfigurableEnvironment environment, SpringApplicationRunListeners listeners, ApplicationArguments applicationArguments, Banner printedBanner) {
......
this.applyInitializers(context);
......
}
}
SpringFactoriesLoader介绍
- 1、框架内部使用的通用工厂加载机制
- 2、从classpath下多个jar包特定的位置读取文件并初始化类
- 3、文件内容必须是kv形式,即properties类型
- 4、key是全限定名,value是实现,多个实现用逗号分隔
SpringFactoriesLoader的作用
SpringBoot框架中从类路径jar包中读取特定文件实现扩展类的载入
系统初始化器调用流程
文章开始的三种自定义初始化器实现原理
- 定义在spring.factories文件中被SpringFactoriesLoader发现注册
- SpringApplication初始化完毕后手动添加
- 定义成环境变量被DelegatingApplicationContextInitializer发现注册
推荐使用在spring.factories文件中定义系统初始化器被SpringFactoriesLoader发现注册的方式
在spring.factories文件中定义系统初始化器，被SpringFactoriesLoader载入，成为SpringBoot框架的SpringApplicationInitializer属性，在运行的时候会遍历这个属性依次调用initialize方法完成向容器内注册属性。
参考： https://blog.csdn.net/pengjunlee/article/details/79394735