BeanDefinition

A BeanDefinition describes a bean instance, which has property values,
Constructor arguments values, and further information supplied by concrete implementations.
This is just a minimal interface: The main intention is to allow a BeanFactoryPostProcessor to introspect and modify property values and other bean metadata.

BeanDefinition： 顶级父接口,继承了BeanMetadataElement和AttributeAccessor.

AbstractBeanDefinition：是GenericBeanDefinition、RootBeanDefinition、ChildBeanDefinition 的通用属性的完整实现。
GenericBeanDefinition: 标准的bean定义目的的一站式服务类.
RootBeanDefinition: 根Bean定义，代表合并的BeanDefinition。
ChildBeanDefinition: 有parent的BeanDefinition

BeanDefinition是SpringFramework中定义的配置元信息接口,包含了：

• Bean 的类名
• Bean 的行为配置元素，如作用域、自动绑定模式、生命周期回调等
• 其他Bean引用，（Collaborators、Dependends)
• 配置设置（Bean的Properties）

BeanDefinition 元信息

属性	说明
Class	Bean的全限类名，必须是具体类
Name	Bean的名称或者🆔
Scope	Bean的作用域，singleton、prototype
Constructor arguments	Bean的构造器参数 for DI
Properties	Bean的属性设置 for DI
Autowiring Mode	Bean的自动绑定模式：ByName 、 byType
Lazy Initialization Mode	Bean 延迟初始化模式
initialzation method	Bean初始化回调方法名称
destruction method	Bean 销毁回调方法名称

BeanDefinition 构建

• 通过BeanDefinitionBuilder构建Beandefinition

        //bean definition builder.
BeanDefinitionBuilder beanDefinitionBuilder = BeanDefinitionBuilder.genericBeanDefinition(User.class);

beanDefinitionBuilder.addPropertyValue("id",1L);
beanDefinitionBuilder.addPropertyValue("name","lisi");

BeanDefinition beanDefinition = beanDefinitionBuilder.getBeanDefinition();

• 通过GenericBeanDefinition 构建

GenericBeanDefinition genericBeanDefinition = new GenericBeanDefinition();
genericBeanDefinition.setBeanClass(User.class);

MutablePropertyValues propertyValues = new MutablePropertyValues();
//        propertyValues.addPropertyValue("id",2L);
//        propertyValues.addPropertyValue("name","zhaowu");

propertyValues.add("id",2L)
        .add("name","zhaowu");
genericBeanDefinition.setPropertyValues(propertyValues);

SpringBean 命名

每个Bean拥有一个或者多个标识符，这些标识符在Bean所在的容器必须是唯一的。
通常，一个Bean仅有一个标识符，可以考虑使用别名来扩充。

在基于XML的配置元信息中，可以使用id或者name属性来规定Bean的标识符。通常Bean的标识符由字母组成，允许出现特殊字符。可以在Bean的name属性使用逗号“,”或者“;”或者空格来间隔名称做为别名。

Bean的id或者name属性如果为空，容器会为Bean自动生成一个唯一的名称。

BeanNameGenerator

DefaultBeanNameGenerator

/**
 * Generate a bean name for the given bean definition, unique within the
 * given bean factory.
 * @param definition the bean definition to generate a bean name for
 * @param registry the bean factory that the definition is going to be
 * registered with (to check for existing bean names)
 * @param isInnerBean whether the given bean definition will be registered
 * as inner bean or as top-level bean (allowing for special name generation
 * for inner beans versus top-level beans)
 * @return the generated bean name
 * @throws BeanDefinitionStoreException if no unique name can be generated
 * for the given bean definition
 */
public static String generateBeanName(
        BeanDefinition definition, BeanDefinitionRegistry registry, boolean isInnerBean)
        throws BeanDefinitionStoreException {

    String generatedBeanName = definition.getBeanClassName();
    if (generatedBeanName == null) {
        if (definition.getParentName() != null) {
            generatedBeanName = definition.getParentName() + "$child";
        }
        else if (definition.getFactoryBeanName() != null) {
            generatedBeanName = definition.getFactoryBeanName() + "$created";
        }
    }
    if (!StringUtils.hasText(generatedBeanName)) {
        throw new BeanDefinitionStoreException("Unnamed bean definition specifies neither " +
                "'class' nor 'parent' nor 'factory-bean' - can't generate bean name");
    }

    String id = generatedBeanName;
    if (isInnerBean) {
        // Inner bean: generate identity hashcode suffix.
        id = generatedBeanName + GENERATED_BEAN_NAME_SEPARATOR + ObjectUtils.getIdentityHexString(definition);
    }
    else {
        // Top-level bean: use plain class name with unique suffix if necessary.
        return uniqueBeanName(generatedBeanName, registry);
    }
    return id;
}

由代码可以看出， 首先获取BeanClassName, BeanClassName 为空时，获取ParentName，ParentName为空时获取FactoryBeanName， 当三者都为空的时候名称是无法创建的。 三者不为空时生成的名称前缀为：
BeanClassName
ParentName$child
FactoryBeanName$created

在生成名字时，如果是内部类， 则生成名字为前缀+#+当前beanDefinition的hashCode， 如下：
BeanClassName#hashCode
ParentName$child#hashCode
FactoryBeanName$created#hashCode

如果不是内部类， 则调用uniqueBeanName生成唯一的BeanName，格式为 + “#” + 序号。

BeanClassName#1
ParentName$child#1
FactoryBeanName$created#1

/**
 * Turn the given bean name into a unique bean name for the given bean factory,
 * appending a unique counter as suffix if necessary.
 * @param beanName the original bean name
 * @param registry the bean factory that the definition is going to be
 * registered with (to check for existing bean names)
 * @return the unique bean name to use
 * @since 5.1
 */
public static String uniqueBeanName(String beanName, BeanDefinitionRegistry registry) {
    String id = beanName;
    int counter = -1;

    // Increase counter until the id is unique.
    while (counter == -1 || registry.containsBeanDefinition(id)) {
        counter++;
        id = beanName + GENERATED_BEAN_NAME_SEPARATOR + counter;
    }
    return id;
}

AnnotationBeanNameGeneritor

先判断是否为AnnotatedBeanDefinition, 优先按照AnnotationBeanDefinition配置的Value获取，如果没有配置，则按照缺省的方式来生成beanName(如果是mypackage.MyJdbcDao 则生成 myJdbcDao)。

@Override
public String generateBeanName(BeanDefinition definition, BeanDefinitionRegistry registry) {
    if (definition instanceof AnnotatedBeanDefinition) {
        String beanName = determineBeanNameFromAnnotation((AnnotatedBeanDefinition) definition);
        if (StringUtils.hasText(beanName)) {
            // Explicit bean name found.
            return beanName;
        }
    }
    // Fallback: generate a unique default bean name.
    return buildDefaultBeanName(definition, registry);
}

/**
 * Derive a default bean name from the given bean definition.
 * <p>The default implementation simply builds a decapitalized version
 * of the short class name: e.g. "mypackage.MyJdbcDao" -> "myJdbcDao".
 * <p>Note that inner classes will thus have names of the form
 * "outerClassName.InnerClassName", which because of the period in the
 * name may be an issue if you are autowiring by name.
 * @param definition the bean definition to build a bean name for
 * @return the default bean name (never {@code null})
 */
protected String buildDefaultBeanName(BeanDefinition definition) {
    String beanClassName = definition.getBeanClassName();
    Assert.state(beanClassName != null, "No bean class name set");
    String shortClassName = ClassUtils.getShortName(beanClassName);
    return Introspector.decapitalize(shortClassName);
}

BeanDefinition 配置

Xml

<bean id="user" class="com.github.springframework.models.User" primary="true">
    <property name="id" value="1"/>
    <property name="name" value="zhangsan"/>
</bean>

<bean id="superUser" class="com.github.springframework.models.SuperUser" parent="user" >
    <property name="addr" value="上海"/>
</bean>

<bean id="objectFactory" class="org.springframework.beans.factory.config.ObjectFactoryCreatingFactoryBean">
     <property name="targetBeanName" value="user"/>
</bean>

Java注解 @Bean

public class AnnotationBeanDefinitionDemo {


    public static void main(String[] args) {

        //create factory bean
        AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext();

        //register config class.
        applicationContext.register(Config.class);

        //refresh
        applicationContext.refresh();

        //getUser
        System.out.println(applicationContext.getBean(User.class));
    }



    static class Config{
        @Bean
        public User user(){
            User user = new User();
            user.setId(1L);
            user.setName("Ann: zhangsan");
            return user;
        }
    }
}

Java注解 @Component

public class AnnotationBeanDefinitionDemo {
    public static void main(String[] args) {
        //create factory bean
        AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext();

        //register config class.
        applicationContext.register(Config.class);

        //refresh
        applicationContext.refresh();

        //
        System.out.println(applicationContext.getBean(Config.class));
    }
    @Component
    static class Config{
        @Bean
        public User user(){
            User user = new User();
            user.setId(1L);
            user.setName("Ann: zhangsan");
            return user;
        }
    }
}

BeanDefinition注册到容器

BeanDefinitionRegistry 提供了 BeanDefinition 的注册、删除，获取，判断是否存在等功能。

static void beanDefinitionRegistry(BeanDefinitionRegistry registry, String name) {
    BeanDefinitionBuilder beanDefinitionBuilder = BeanDefinitionBuilder.genericBeanDefinition(User.class);
    beanDefinitionBuilder.addPropertyValue("id", 1L)
            .addPropertyValue("name", "Reg: zhangsan");
    if (StringUtils.hasText(name)) {
        registry.registerBeanDefinition(name, beanDefinitionBuilder.getBeanDefinition());
    } else {
        BeanDefinitionReaderUtils.registerWithGeneratedName(beanDefinitionBuilder.getBeanDefinition(),registry);
    }
}

由于AnnotatinoConfigApplicationContext 是 BeanDefinitionRegistry的间接子类。

AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext();
applicationContext.register(Config.class);
beanDefinitionRegistry(applicationContext,"REG:ZhangSan:Name");
beanDefinitionRegistry(applicationContext,null);
applicationContext.refresh();
System.out.println(applicationContext.getBeansOfType(Config.class));
System.out.println(applicationContext.getBeansOfType(User.class));
System.out.println(applicationContext.getBeansOfType(AnnotationApplicationContextDemo.class));

Bean 实例化

常规：

• 通过构造器（配置元信息：XML、Java注解、JavaAPI）

• 静态工厂方法（配置元信息：XML、JavaAPI）

public class User2 {

    private Long id;

    private String name;
    
    public static User2 getInstance(){
        return new User2();
    }
}

<bean id="user-by-static-method" class="com.github.spring.beans.User2"  factory-method="getInstance"></bean>

ApplicationContext applicationContext = new ClassPathXmlApplicationContext("classpath:/META-INF/bean-creation-context.xml");

User2 user = applicationContext.getBean("user-by-static-method", User2.class);

• Bean工厂方法（配置元信息：XML、JavaAPI）

public interface UserFactory {
    default User2 createuser() {
        return new User2();
    }
}

public class DefaultUserFactory implements UserFactory {
}

<bean id="userFactory" class="com.github.spring.factory.DefaultUserFactory"/>
<bean id="user-by-instance-method" factory-bean="userFactory" factory-method="createuser"/>

ApplicationContext applicationContext = new ClassPathXmlApplicationContext("classpath:/META-INF/bean-creation-context.xml");
User2 user3 = applicationContext.getBean("user-by-instance-method", User2.class);

• FactoryBean（XML、Java注解、JavaAPI）

public class UserFactoryBean implements FactoryBean<User2> {

    @Override
    public User2 getObject() throws Exception {
        return User2.getInstance();
    }

    @Override
    public Class<?> getObjectType() {
        return User2.class;
    }
}

<bean id="user-by-factory-bean" class="com.github.spring.factory.UserFactoryBean"/>

ApplicationContext applicationContext = new ClassPathXmlApplicationContext("classpath:/META-INF/bean-creation-context.xml");
User2 user2 = applicationContext.getBean("user-by-factory-method", User2.class);

特殊方式：

• ServiceLoaderFactoryBean

Java ServiceLoader: 会在classpath:/META-INF/services下加载接口配置文件：

文件名名称为全限定接口名称，内容为全限定实现类

com.github.spring.factory.DefaultUserFactory

public static void demoServiceLoader() {
    ServiceLoader<UserFactory> serviceLoader = ServiceLoader.load(UserFactory.class, Thread.currentThread().getContextClassLoader());
    displayServiceLoad(serviceLoader);
}

public static void displayServiceLoad(ServiceLoader<UserFactory> serviceLoader) {
    Iterator<UserFactory> iterator = serviceLoader.iterator();
    while (iterator.hasNext()) {
        UserFactory userFactory = iterator.next();
        System.out.println(userFactory.createuser());
    }
}

ServiceLoaderFactoryBean的实现

<bean id="userFactoryServiceLoader" class="org.springframework.beans.factory.serviceloader.ServiceLoaderFactoryBean">
    <property name="serviceType" value="com.github.spring.factory.UserFactory"/>
</bean>

ApplicationContext applicationContext = new ClassPathXmlApplicationContext("classpath:/META-INF/sp-bean-creation-context.xml");
ServiceLoader<UserFactory> serviceLoader = applicationContext.getBean("userFactoryServiceLoader", ServiceLoader.class);
displayServiceLoad(serviceLoader);

• AutowireCapableBeanFactory#createBean(java.lang.Class,int, boolean);

AutowireCapableBeanFactory beanFactory = applicationContext.getAutowireCapableBeanFactory();
UserFactory userFactory = beanFactory.createBean(DefaultUserFactory.class);
System.out.println(userFactory.createuser());

• BeanDefinitionRegistry#registerBeanDefinition(String,BeanDefinition);

Bean的初始化

• @PostConstructor

• InitializingBean 的 afterPropertiesSet

• Bean的initMethod
  @Bean(initMethod=””)

  AbstractBeanDefinition.setInitMethodName("");

public class DefaultUserFactory implements UserFactory, InitializingBean {

    public DefaultUserFactory() {
        System.out.println("DefaultUserFactory Constructor Called.");
    }

    @PostConstruct
    public void init(){
        System.out.println("DefaultUserFactory PostConstructor Called.");
    }

    public void initUserFactory(){
        System.out.println("DefaultUserFactory initUserFactory Called.");
    }

    @Override
    public void afterPropertiesSet() throws Exception {
        System.out.println("DefaultUserFactory afterPropertiesSet called.");
    }
}

@Configuration
public class BeanInitializationDemo {
    public static void main(String[] args) {
        AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext();
        applicationContext.register(BeanInitializationDemo.class);
        applicationContext.refresh();
        UserFactory userFactory = applicationContext.getBean(UserFactory.class)
        System.out.println(userFactory.createuser());
        applicationContext.close();
    }

    @Bean(initMethod = "initUserFactory")
    public DefaultUserFactory userFactory(){
        return new DefaultUserFactory();
    }
}

延迟初始化

• xml:
• annotation: @Lazy

对于延迟初始化，是在applicationContext初始化之后进行初始化。
非延迟初始化，是在applicationContext初始化之前进行初始化。

Bean 的销毁

• @PreDestroy
• 实现DisposableBean接口的destoy()方法
• Bean的 destroy方法 @Bean(destroy="") AbstractBeanDefinition.setDestroyMethodName("");

• 如何注册一个Spring Bean

可以通过BeanDefinition和外部单体对象来注册。

UserFactory userFactory = new DefaultUserFactory();
ConfiguableListableBeanFactory factory = applicationContext.getBeanFactory();
factory.registerSingleton("userFactory", userFactory);

UserFactory userFactory = applicationContext.getBean("UserFactory",UserFactory.class);

• Spring BeanDefinition 是什么？

• Spring 容器是如何注册Bean的？