spring boot实战(第九篇)Application创建源码分析

前言

通过前面的文章了解到在spring boot的启动时，利用的是编写的Application类，使用了注解@SpringBootApplication，本篇将阐述该Bean的加载过程。

@SpringBootApplication
public class Application {
public static void main(String[] args) {
SpringApplication app = new SpringApplication(Application.class);
app.addListeners(new MyApplicationStartedEventListener());
app.run(args);
}
}

Application

上篇中讲述了上下文的创建，在run方法中接下来会执行

load(context, sources.toArray(new Object[sources.size()]));

这个的sources表示的为Application类，在创建SpringApplication时手动传递

SpringApplication app = new SpringApplication(Application.class);

load方法如下：

protected void load(ApplicationContext context, Object[] sources) {
if (this.log.isDebugEnabled()) {
this.log.debug("Loading source "
+ StringUtils.arrayToCommaDelimitedString(sources));
}
BeanDefinitionLoader loader = createBeanDefinitionLoader(
getBeanDefinitionRegistry(context), sources);
if (this.beanNameGenerator != null) {
loader.setBeanNameGenerator(this.beanNameGenerator);
}
if (this.resourceLoader != null) {
loader.setResourceLoader(this.resourceLoader);
}
if (this.environment != null) {
loader.setEnvironment(this.environment);
}
loader.load();
}

调用loader.load()；

private int load(Object source) {
Assert.notNull(source, "Source must not be null");
if (source instanceof Class<?>) {
return load((Class<?>) source);
}
if (source instanceof Resource) {
return load((Resource) source);
}
if (source instanceof Package) {
return load((Package) source);
}
if (source instanceof CharSequence) {
return load((CharSequence) source);
}
throw new IllegalArgumentException("Invalid source type " + source.getClass());
}

执行load((Class<?>) source)

private int load(Class<?> source) {
if (isGroovyPresent()) {
// Any GroovyLoaders added in beans{} DSL can contribute beans here
if (GroovyBeanDefinitionSource.class.isAssignableFrom(source)) {
GroovyBeanDefinitionSource loader = BeanUtils.instantiateClass(source,
GroovyBeanDefinitionSource.class);
load(loader);
}
}
if (isComponent(source)) {
this.annotatedReader.register(source);
return 1;
}
return 0;
}

isComponent判断Application是否存在注解Compent

private boolean isComponent(Class<?> type) {
// This has to be a bit of a guess. The only way to be sure that this type is
// eligible is to make a bean definition out of it and try to instantiate it.
if (AnnotationUtils.findAnnotation(type, Component.class) != null) {
return true;
}
// Nested anonymous classes are not eligible for registration, nor are groovy
// closures
if (type.getName().matches(".*\\$_.*closure.*") || type.isAnonymousClass()
|| type.getConstructors() == null || type.getConstructors().length == 0) {
return false;
}
return true;
}

AnnotationUtils.findAnnotation(type, Component.class) 工具类获取执行类对应的注解信息，该工具类在自己编码代码时可用得到

由于Application使用注解@SpringBootApplication，其定义如下

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Configuration
@EnableAutoConfiguration
@ComponentScan
public @interface SpringBootApplication {

/**
* Exclude specific auto-configuration classes such that they will never be applied.
* @return the classes to exclude
*/
Class<?>[] exclude() default {};

}

发现不存在Compoment注解，是不是表明Application不是一个Component呢？其实不然，来看下@Configuration注解

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Configuration {

/**
* Explicitly specify the name of the Spring bean definition associated
* with this Configuration class.  If left unspecified (the common case),
* a bean name will be automatically generated.
* <p>The custom name applies only if the Configuration class is picked up via
* component scanning or supplied directly to a {@link AnnotationConfigApplicationContext}.
* If the Configuration class is registered as a traditional XML bean definition,
* the name/id of the bean element will take precedence.
* @return the specified bean name, if any
* @see org.springframework.beans.factory.support.DefaultBeanNameGenerator
*/
String value() default "";

}

发现Configuration注解上存在Component注解，表明Application为Component

接下来执行
this.annotatedReader.register(source);

public void register(Class<?>... annotatedClasses) {
for (Class<?> annotatedClass : annotatedClasses) {
registerBean(annotatedClass);
}
}

调用registerBean注册Application对应的bean信息

public void registerBean(Class<?> annotatedClass, String name,
@SuppressWarnings("unchecked") Class<? extends Annotation>... qualifiers) {

AnnotatedGenericBeanDefinition abd = new AnnotatedGenericBeanDefinition(annotatedClass);
if (this.conditionEvaluator.shouldSkip(abd.getMetadata())) {
return;
}

ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(abd);
abd.setScope(scopeMetadata.getScopeName());
String beanName = (name != null ? name : this.beanNameGenerator.generateBeanName(abd, this.registry));
AnnotationConfigUtils.processCommonDefinitionAnnotations(abd);
if (qualifiers != null) {
for (Class<? extends Annotation> qualifier : qualifiers) {
if (Primary.class.equals(qualifier)) {
abd.setPrimary(true);
}
else if (Lazy.class.equals(qualifier)) {
abd.setLazyInit(true);
}
else {
abd.addQualifier(new AutowireCandidateQualifier(qualifier));
}
}
}

BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(abd, beanName);
definitionHolder = AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
BeanDefinitionReaderUtils.registerBeanDefinition(definitionHolder, this.registry);
}

首先来看

if (this.conditionEvaluator.shouldSkip(abd.getMetadata())) {
return;
}

判断是否需要跳过，其代码如下：

public boolean shouldSkip(AnnotatedTypeMetadata metadata, ConfigurationPhase phase) {
if (metadata == null || !metadata.isAnnotated(Conditional.class.getName())) {
return false;
}

if (phase == null) {
if (metadata instanceof AnnotationMetadata &&
ConfigurationClassUtils.isConfigurationCandidate((AnnotationMetadata) metadata)) {
return shouldSkip(metadata, ConfigurationPhase.PARSE_CONFIGURATION);
}
return shouldSkip(metadata, ConfigurationPhase.REGISTER_BEAN);
}

List<Condition> conditions = new ArrayList<Condition>();
for (String[] conditionClasses : getConditionClasses(metadata)) {
for (String conditionClass : conditionClasses) {
Condition condition = getCondition(conditionClass, this.context.getClassLoader());
conditions.add(condition);
}
}

AnnotationAwareOrderComparator.sort(conditions);

for (Condition condition : conditions) {
ConfigurationPhase requiredPhase = null;
if (condition instanceof ConfigurationCondition) {
requiredPhase = ((ConfigurationCondition) condition).getConfigurationPhase();
}
if (requiredPhase == null || requiredPhase == phase) {
if (!condition.matches(this.context, metadata)) {
return true;
}
}
}

return false;
}

该代码判断Application上是否存在Conditional注解，如果不满足Conditional对应条件则该bean不被创建；

Conditional注解

代码分析到这里可以先看看Conditional注解的使用，其定义为：

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE, ElementType.METHOD})
public @interface Conditional {
/**
* All {@link Condition}s that must {@linkplain Condition#matches match}
* in order for the component to be registered.
*/
Class<? extends Condition>[] value();

}

从源码可以看出，首先判断Application上是否存在Conditional，如果存在，则获取Conditional注解中的value数组值，对应的Class必须实现Condition接口：

public interface Condition {
boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata);

}

如果matches返回true 表明该bean需要被创建，否则表明该bean不需要被创建。

明白了该注解的用法后，来一个实际案例

package com.u51.lkl.springboot.condition;

import org.springframework.context.annotation.Conditional;
import org.springframework.stereotype.Component;

@Component("MyCondition")
@Conditional(MyCondition.class)
public class ConditionBean {
}

创建ConditionBean，使用注解@Conditional(MyCondition.class)调用MyCondition类

/**
* 自定义condition  修改返回值，查看bean是否创建
*
* @author liaokailin
*/
public class MyCondition implements Condition {

/**
* 返回true 生成bean
* 返回false 不生成bean
*/
@Override
public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
Map<String, Object> map = metadata.getAnnotationAttributes(Component.class.getName());
return "MyCondition".equals(map.get("value").toString());
}

}

MyCondition实现接口Condition，在matches方法中获取bean上注解Component信息，如果bean名称等于MyCondition返回true，否则返回false，bean不会被创建。

回到前面Application的分析，Application上不存在Conditional，因此shouldSkip返回false，代码继续执行

ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(abd);

处理Scope注解信息，默认是单例bean

执行

AnnotationConfigUtils.processCommonDefinitionAnnotations(abd);

处理一些常见注解信息

static void processCommonDefinitionAnnotations(AnnotatedBeanDefinition abd, AnnotatedTypeMetadata metadata) {
if (metadata.isAnnotated(Lazy.class.getName())) {
abd.setLazyInit(attributesFor(metadata, Lazy.class).getBoolean("value"));
}
else if (abd.getMetadata() != metadata && abd.getMetadata().isAnnotated(Lazy.class.getName())) {
abd.setLazyInit(attributesFor(abd.getMetadata(), Lazy.class).getBoolean("value"));
}

if (metadata.isAnnotated(Primary.class.getName())) {
abd.setPrimary(true);
}
if (metadata.isAnnotated(DependsOn.class.getName())) {
abd.setDependsOn(attributesFor(metadata, DependsOn.class).getStringArray("value"));
}

if (abd instanceof AbstractBeanDefinition) {
AbstractBeanDefinition absBd = (AbstractBeanDefinition) abd;
if (metadata.isAnnotated(Role.class.getName())) {
absBd.setRole(attributesFor(metadata, Role.class).getNumber("value").intValue());
}
if (metadata.isAnnotated(Description.class.getName())) {
absBd.setDescription(attributesFor(metadata, Description.class).getString("value"));
}
}
}

处理Lazy、Primary、DependsOn、Role、Description等注解

最后调用

BeanDefinitionReaderUtils.registerBeanDefinition(definitionHolder, this.registry);

注册bean信息，在注册bean信息之前通过

String beanName = (name != null ? name : this.beanNameGenerator.generateBeanName(abd, this.registry));

获取bean名称

bean的注册调用为

registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());

该代码在上篇中已有说明。

此时Application对应bean已创建完成。