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Spring IoC getBean 方法详解

leisurexi 人气:0

前言

本篇文章主要介绍 Spring IoC 容器 getBean() 方法。

下图是一个大致的流程图:

正文

首先定义一个简单的 POJO,如下:

public class User {
    
    private Long id;
    private String name;

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
    
    @Override
    public String toString() {
        return "User{" +
            "id=" + id +
            ", name='" + name + '\'' +
            '}';
    }
}

再编写一个 XML 文件。

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="user" class="com.leisurexi.ioc.domain.User">
        <property name="id" value="1"/>
        <property name="name" value="leisurexi"/>
    </bean>
    
</beans>

最后再来一个测试类。

@Test
public void test(){
    DefaultListableBeanFactory beanFactory = new DefaultListableBeanFactory();
    XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(beanFactory);
    reader.loadBeanDefinitions("META-INF/spring-bean.xml");
    User user = beanFactory.getBean("user", User.class);
    System.out.println(user);
}

上面的代码还是上篇文章的示例代码,这次我们主要分析 beanFactory.getBean() 方法。

AbstractBeanFactory#getBean

/**
* @param name 		  bean的名称
* @param requiredType bean的类型
*/
public <T> T getBean(String name, Class<T> requiredType) throws BeansException {
    // 调用 doGetBean 方法(方法以do开头实际做操作的方法)
    return doGetBean(name, requiredType, null, false);
}

/**
* @param name          bean的名称
* @param requiredType  bean的类型
* @param args          显示传入的构造参数
* @param typeCheckOnly 是否仅仅做类型检查
*/
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
			@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
    // 获取bean的实际名称,见下文详解
    final String beanName = transformedBeanName(name);
    Object bean;

    // Eagerly check singleton cache for manually registered singletons.
    // 直接尝试从缓存获取或 singletonFactories 中的 ObjectFactory 中获取,见下文详解
    Object sharedInstance = getSingleton(beanName);
    if (sharedInstance != null && args == null) {
        if (logger.isTraceEnabled()) {
            if (isSingletonCurrentlyInCreation(beanName)) {
                logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
                             "' that is not fully initialized yet - a consequence of a circular reference");
            }
            else {
                logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
            }
        }
        // 检查bean是否是FactoryBean的实现。不是直接返回bean,是的话首先检查beanName是否以 & 开头
        // 如果是返回FactoryBean本身,不是调用FactoryBean#getObject()返回对象
        // 见下文详解
        bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
    }

    else {
        // Fail if we're already creating this bean instance:
        // We're assumably within a circular reference.
        // 只有在单例情况下才会去尝试解决循环依赖,原型模式下,如果存在A中有
        // B属性,B中有A属性,那么当依赖注入时,就会产生当A还未创建完的时候
        // 对于B的创建而在此返回创建A,造成循环依赖
        if (isPrototypeCurrentlyInCreation(beanName)) {
            throw new BeanCurrentlyInCreationException(beanName);
        }

        // Check if bean definition exists in this factory.
        // 检查当前bean的BeanDefinition是否在当前的beanFactory,不在递归调用父工厂的getBean()去获取bean
        BeanFactory parentBeanFactory = getParentBeanFactory();
        if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
            // Not found -> check parent.
            String nameToLookup = originalBeanName(name);
            if (parentBeanFactory instanceof AbstractBeanFactory) {
                return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
                    nameToLookup, requiredType, args, typeCheckOnly);
            }
            else if (args != null) {
                // Delegation to parent with explicit args.
                return (T) parentBeanFactory.getBean(nameToLookup, args);
            }
            else if (requiredType != null) {
                // No args -> delegate to standard getBean method.
                return parentBeanFactory.getBean(nameToLookup, requiredType);
            }
            else {
                return (T) parentBeanFactory.getBean(nameToLookup);
            }
        }
        // 如果不是仅仅做类型检查,则是创建bean,这里要进行记录
        if (!typeCheckOnly) {
            // 记录bean已经创建过,见下文详解
            markBeanAsCreated(beanName);
        }

        try {
            // 合并BeanDefinition,见下文详解
            final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
            checkMergedBeanDefinition(mbd, beanName, args);

            // Guarantee initialization of beans that the current bean depends on.
            // 实例化bean前先实例化依赖bean,也就是depends-on属性中配置的beanName
            String[] dependsOn = mbd.getDependsOn();
            if (dependsOn != null) {
                for (String dep : dependsOn) {
                    // 检查是否循环依赖,即当前bean依赖dep,dep依赖当前bean,见下文详解
                    if (isDependent(beanName, dep)) {
                        throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                                        "Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
                    }
                    // 将dep和beanName的依赖关系放入到缓存中,见下文详解
                    registerDependentBean(dep, beanName);
                    try {
                        // 获取依赖dep对应的bean实例,如果还未创建实例,则先去创建
                        getBean(dep);
                    }
                    catch (NoSuchBeanDefinitionException ex) {
                        throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                                        "'" + beanName + "' depends on missing bean '" + dep + "'", ex);
                    }
                }
            }

            // Create bean instance.
            // 如果 bean 的作用域是单例
            if (mbd.isSingleton()) {
                // 创建和注册单例 bean,见下文详解
                sharedInstance = getSingleton(beanName, () -> {
                    try {
                        // 创建 bean 实例,下篇文章详解
                        return createBean(beanName, mbd, args);
                    }
                    catch (BeansException ex) {
                        // Explicitly remove instance from singleton cache: It might have been put there
                        // eagerly by the creation process, to allow for circular reference resolution.
                        // Also remove any beans that received a temporary reference to the bean.
                        destroySingleton(beanName);
                        throw ex;
                    }
                });
                // 上文解释过,这里不再赘述
                bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
            }
            // bean 的作用域是原型
            else if (mbd.isPrototype()) {
                // It's a prototype -> create a new instance.
                Object prototypeInstance = null;
                try {
                    // 原型 bean 创建前回调,默认实现是将 beanName 保存到 prototypesCurrentlyInCreation 缓存中
                    beforePrototypeCreation(beanName);
                    // 创建 bean 实例,下篇文章详解
                    prototypeInstance = createBean(beanName, mbd, args);
                }
                finally {
                    // 原型 bean 创建后回调,默认实现是将 beanName 从prototypesCurrentlyInCreation 缓存中移除
                    afterPrototypeCreation(beanName);
                }
                // 上文解释过,这里不再赘述
                bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
            }
            // 自定义作用域
            else {
                // 获取自定义作用域名称
                String scopeName = mbd.getScope();
                // 获取作用域对象
                final Scope scope = this.scopes.get(scopeName);
                // 如果为空表示作用域未注册,抛出异常
                if (scope == null) {
                    throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
                }
                try {
                    // 其他 Scope 的 bean 创建(新建了一个 ObjectFactory,并且重写了 getObject 方法)
                    Object scopedInstance = scope.get(beanName, () -> {
                        // 原型 bean 创建前回调,默认实现是将 beanName 保存到 prototypesCurrentlyInCreation 缓存中
                        beforePrototypeCreation(beanName);
                        try {
                            // 创建 bean 实例,下篇文章详解
                            return createBean(beanName, mbd, args);
                        }
                        finally {
                            // 原型 bean 创建后回调,默认实现是将 beanName 从 prototypesCurrentlyInCreation 缓存中移除
                            afterPrototypeCreation(beanName);
                        }
                    });
                    // 上文解释过,这里不再赘述
                    bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
                }
                catch (IllegalStateException ex) {
                    throw new BeanCreationException(beanName,
                                                    "Scope '" + scopeName + "' is not active for the current thread; consider " +
                                                    "defining a scoped proxy for this bean if you intend to refer to it from a singleton",ex);
                }
            }
        }
        catch (BeansException ex) {
            cleanupAfterBeanCreationFailure(beanName);
            throw ex;
        }
    }

    // Check if required type matches the type of the actual bean instance.
    // 检查所需的类型是否与实际 bean 实例的类型匹配
    if (requiredType != null && !requiredType.isInstance(bean)) {
        try {
            // 如果类型不等,进行转换,转换失败抛出异常;转换成功直接返回
            T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
            if (convertedBean == null) {
                throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
            }
            return convertedBean;
        }
        catch (TypeMismatchException ex) {
            if (logger.isTraceEnabled()) {
                logger.trace("Failed to convert bean '" + name + "' to required type '" +
                             ClassUtils.getQualifiedName(requiredType) + "'", ex);
            }
            throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
        }
    }
    // 返回 bean 实例
    return (T) bean;
}

AbstractBeanFactory#transformedBeanName

protected String transformedBeanName(String name) {
    return canonicalName(BeanFactoryUtils.transformedBeanName(name));
}

// BeanFactoryUtils.java
public static String transformedBeanName(String name) {
    Assert.notNull(name, "'name' must not be null");
    // 如果name不是&开头,直接返回
    if (!name.startsWith(BeanFactory.FACTORY_BEAN_PREFIX)) {
        return name;
    }
    // 去除name的&前缀
    return transformedBeanNameCache.computeIfAbsent(name, beanName -> {
        do {
            beanName = beanName.substring(BeanFactory.FACTORY_BEAN_PREFIX.length());
        }
        while (beanName.startsWith(BeanFactory.FACTORY_BEAN_PREFIX));
        return beanName;
    });
}

// SimpleAliasRegistry.java
public String canonicalName(String name) {
    String canonicalName = name;
    // Handle aliasing...
    String resolvedName;
    // 如果name是别名,则会循环去查找bean的实际名称
    do {
        resolvedName = this.aliasMap.get(canonicalName);
        if (resolvedName != null) {
            canonicalName = resolvedName;
        }
    }
    while (resolvedName != null);
    return canonicalName;
}

上面代码首先去除 FactoryBean 的修饰符,比如 name=&aa ,那么会首先去除 & 使 name=aa。然后取 alias 所表示的最终 beanName

我们这里简单介绍什么是 FactoryBean

一般情况下,Spring 通过反射机制利用 beanclass 属性指定实现类来实例化 bean。在某些情况下,实例化 bean 过程比较复杂,如果按照传统的方式,则需要在 <bean> 中提供大量的配置信息,配置方式的灵活性是受限的,这是采用编码的方式可能会得到一个简单的方案。Spring 为此提供了 org.springframework.bean.factory.FactoryBean 的工厂类接口,用户可以通过实现该接口定制实例化 bean 的逻辑。

FactoryBean 接口对于 Spring 框架来说占有重要的地位,Spring 自身就提供了70多个 FactoryBean 的实现。它们隐藏了一下复杂 bean 的细节,给上层应用带来了便利。下面是该接口的定义:

public interface FactoryBean<T> {

    // 返回由FactoryBean创建的bean实例,如果isSingleton()返回true,
    // 则该实例会放到Spring容器中单例缓存池中
    @Nullable
    T getObject() throws Exception;
	
    // 返回FactoryBean创建的bean类型
    @Nullable
    Class<?> getObjectType();

    // 返回由FactoryBean创建的bean实例的作用域是singleton还是prototype
    default boolean isSingleton() {
        return true;
    }

}

当配置文件中 <bean>class 属性配置的实现类时 FactoryBean 时,通过 getBean() 返回的不是 FactoryBean 本身,而是 FactoryBean#getObject() 所返回的对象,相当于 FactoryBean#getObject() 代理了 getBean()。下面用简单的代码演示一下:

首先定义一个 Car 实体类:

public class Car {
	
    private Integer maxSpeed;
    private String brand;
    private Double price;

    public Integer getMaxSpeed() {
        return maxSpeed;
    }

    public void setMaxSpeed(Integer maxSpeed) {
        this.maxSpeed = maxSpeed;
    }

    public String getBrand() {
        return brand;
    }

    public void setBrand(String brand) {
        this.brand = brand;
    }

    public Double getPrice() {
        return price;
    }

    public void setPrice(Double price) {
        this.price = price;
    }
}

上面的实体类,如果用传统方式配置,每一个属性都会对应一个 <property> 元素标签。如果用 FactoryBean 的方式实现就会灵活一点,下面通过逗号分隔的方式一次性的为 Car 的所有属性配置值。

public class CarFactoryBean implements FactoryBean<Car> {
	
    private String carInfo;
	
    @Override
    public Car getObject() throws Exception {
        Car car = new Car();
        String[] infos = carInfo.split(",");
        car.setBrand(infos[0]);
        car.setMaxSpeed(Integer.valueOf(infos[1]));
        car.setPrice(Double.valueOf(infos[2]));
        return car;
    }

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

    @Override
    public boolean isSingleton() {
        return true;
    }

    public String getCarInfo() {
        return carInfo;
    }

    public void setCarInfo(String carInfo) {
        this.carInfo = carInfo;
    }
}

接下来,我们在 XML 中配置。

<bean id="car" class="com.leisurexi.ioc.domain.CarFactoryBean">
    <property name="carInfo" value="超级跑车,400,2000000"/>
</bean>

最后看下测试代码和运行结果:

@Test
public void factoryBeanTest() {
    DefaultListableBeanFactory beanFactory = new DefaultListableBeanFactory();
    XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(beanFactory);
    reader.loadBeanDefinitions("META-INF/spring-bean.xml");
    Car car = beanFactory.getBean("car", Car.class);
    System.out.println(car);
    CarFactoryBean carFactoryBean = beanFactory.getBean("&car", CarFactoryBean.class);
    System.out.println(carFactoryBean);
}

可以看到如果 beanName 前面加上 & 获取的是 FactoryBean 本身,不加获取的 getObject() 返回的对象。

AbstractBeanFactory#getSingleton

public Object getSingleton(String beanName) {
    // allowEarlyReference设置为true表示允许早期依赖
    return getSingleton(beanName, true);
}

/**
* @param allowEarlyReference  是否提前创建曝光
*/
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
    // 检查单例传中是否存在
    Object singletonObject = this.singletonObjects.get(beanName);
    if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
        // 如果为空,锁定全局变量进行处理
        synchronized (this.singletonObjects) {
            singletonObject = this.earlySingletonObjects.get(beanName);
            if (singletonObject == null && allowEarlyReference) {
                // 当某些方法需要提前初始化时则会调用addSingletonFactory方法将对应的
                // ObjectFactory 初始化策略存储在 singletonFactories
                ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
                if (singletonFactory != null) {
                    // 调用预先设定的getObject()
                    singletonObject = singletonFactory.getObject();
                    // 记录在缓存中,earlySingletonObjects和singletonFactories
                    this.earlySingletonObjects.put(beanName, singletonObject);
                    this.singletonFactories.remove(beanName);
                }
            }
        }
    }
    return singletonObject;
}

上面代码涉及到循环依赖的检测,首先尝试从 singletonObjects 里面获取实例,如果获取不到再从 earlySingletonObjects 里面获取,如果还获取不到,再尝试从 singletonFactories 里面获取 beanName 对应的 ObjectFactory,然后调用这个 ObjectFactorygetObject() 来创建 bean,并放到 earlySingletonObjects 里面去,并且从 singletonFactories 里面 remove 掉这个 ObjectFactory ,而对于后续的所有内存操作都只为了循环依赖检测时候用,也就是在 allowEarlyReferencetrue 的情况下才会使用。

这里涉及用于存储 bean 不同的 map,下面简单解释下:

  • singletonObjects:同于保存 beanNamebean 实例之间的关系。
  • singletonFactories:用于保存 beanName 和创建 bean 的工厂之间的关系。
  • earlySingletonObjects:也是保存 beanNamebean 实例之间的关系,与 singletonObjects 的不同之处在于,当一个单例 bean 被放到这里后,那么当 bean 还在创建过程中,就可以通过 getBean() 获取到了,其目的是用来检测循环引用。
  • registeredSingletons:用来保存当前所有已注册的 bean

AbstractBeanFactory#getObjectForBeanInstance

protected Object getObjectForBeanInstance(
			Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {

    // Don't let calling code try to dereference the factory if the bean isn't a factory.
    // name 是否以 & 开头
    if (BeanFactoryUtils.isFactoryDereference(name)) {
        // 如果是 null 直接返回
        if (beanInstance instanceof NullBean) {
            return beanInstance;
        }
        // beanName 以 & 开头,但又不是 FactoryBean 类型,抛出异常
        if (!(beanInstance instanceof FactoryBean)) {
            throw new BeanIsNotAFactoryException(beanName, beanInstance.getClass());
        }
        // 设置 isFactoryBean 为 true
        if (mbd != null) {
            mbd.isFactoryBean = true;
        }
        // 返回 bean 实例
        return beanInstance;
    }

    // Now we have the bean instance, which may be a normal bean or a FactoryBean.
    // If it's a FactoryBean, we use it to create a bean instance, unless the
    // caller actually wants a reference to the factory.
    // name 不是 & 开头,并且不是 FactoryBean 类型,直接返回
    if (!(beanInstance instanceof FactoryBean)) {
        return beanInstance;
    }

    Object object = null;
    if (mbd != null) {	
        mbd.isFactoryBean = true;
    }
    else {
        // 从缓存中获取实例
        object = getCachedObjectForFactoryBean(beanName);
    }
    if (object == null) {
        // Return bean instance from factory.
        // 将 beanInstance 强转成 FactoryBean
        FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
        // Caches object obtained from FactoryBean if it is a singleton.
        // 合并 BeanDefinition
        if (mbd == null && containsBeanDefinition(beanName)) {
            mbd = getMergedLocalBeanDefinition(beanName);
        }
        boolean synthetic = (mbd != null && mbd.isSynthetic());
        // 获取实例
        object = getObjectFromFactoryBean(factory, beanName, !synthetic);
    }
    return object;
}

// FactoryBeanRegistrySupport.java
protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
    // 如果是单例 bean,并且已经存在缓存中
    if (factory.isSingleton() && containsSingleton(beanName)) {
        // 加锁
        synchronized (getSingletonMutex()) {
            // 从缓存中获取
            Object object = this.factoryBeanObjectCache.get(beanName);
            if (object == null) {
                // 调用 FactoryBean 的 getObject() 获取实例
                object = doGetObjectFromFactoryBean(factory, beanName);
                // Only post-process and store if not put there already during getObject() call above
                // (e.g. because of circular reference processing triggered by custom getBean calls)
                Object alreadyThere = this.factoryBeanObjectCache.get(beanName);
                // 如果该 beanName 已经在缓存中存在,则将 object 替换成缓存中的
                if (alreadyThere != null) {
                    object = alreadyThere;
                }
                else {
                    if (shouldPostProcess) {
                        // 如果当前 bean 还在创建中,直接返回
                        if (isSingletonCurrentlyInCreation(beanName)) {
                            // Temporarily return non-post-processed object, not storing it yet.
                            return object;
                        }
                        // 单例 bean 创建前回调
                        beforeSingletonCreation(beanName);
                        try {
                            // 对从 FactoryBean 获得给定对象后处理,默认按原样返回
                            object = postProcessObjectFromFactoryBean(object, beanName);
                        }
                        catch (Throwable ex) {
                            throw new BeanCreationException(beanName,
                                                            "Post-processing of FactoryBean's singleton object failed", ex);
                        }
                        finally {
                            // 单例 bean 创建后回调
                            afterSingletonCreation(beanName);
                        }
                    }
                    if (containsSingleton(beanName)) {
                        // 将 beanName 和 object 放到 factoryBeanObjectCache 缓存中
                        this.factoryBeanObjectCache.put(beanName, object);
                    }
                }
            }
            // 返回实例
            return object;
        }
    }
    else {
        // 调用 FactoryBean 的 getObject() 获取实例
        Object object = doGetObjectFromFactoryBean(factory, beanName);
        if (shouldPostProcess) {
            try {
                // 对从 FactoryBean 获得给定对象后处理,默认按原样返回
                object = postProcessObjectFromFactoryBean(object, beanName);
            }
            catch (Throwable ex) {
                throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex);
            }
        }
        // 返回实例
        return object;
    }
}

// FactoryBeanRegistrySupport.java
private Object doGetObjectFromFactoryBean(final FactoryBean<?> factory, final String beanName) throws BeanCreationException {

    Object object;
    try {
        if (System.getSecurityManager() != null) {
            AccessControlContext acc = getAccessControlContext();
            try {
                object = AccessController.doPrivileged((PrivilegedExceptionAction<Object>) factory::getObject, acc);
            }
            catch (PrivilegedActionException pae) {
                throw pae.getException();
            }
        }
        else {
            // 调用 getObject() 获取实例
            object = factory.getObject();
        }
    }
    catch (FactoryBeanNotInitializedException ex) {
        throw new BeanCurrentlyInCreationException(beanName, ex.toString());
    }
    catch (Throwable ex) {
        throw new BeanCreationException(beanName, "FactoryBean threw exception on object creation", ex);
    }

    // Do not accept a null value for a FactoryBean that's not fully
    // initialized yet: Many FactoryBeans just return null then.
    // 如果 object 为 null,并且当前 singleton bean 正在创建中,抛出异常
    if (object == null) {
        if (isSingletonCurrentlyInCreation(beanName)) {
            throw new BeanCurrentlyInCreationException(
                beanName, "FactoryBean which is currently in creation returned null from getObject");
        }
        object = new NullBean();
    }
    // 返回 object 实例
    return object;
}

上面代码总结起来就是:如果 beanName& 开头,直接返回 FactoryBean 实例;否则调用 getObject() 方法获取实例,然后执行 postProcessObjectFromFactoryBean() 回调,可以在回调方法中修改实例,默认按原样返回。

AbstractBeanFactory#getMergedLocalBeanDefinition

下文将合并后的 BeanDefinition 简称为 MergedBeanDefinition

protected RootBeanDefinition getMergedLocalBeanDefinition(String beanName) throws BeansException {
    // Quick check on the concurrent map first, with minimal locking.
    // 获取当前bean合并后的BeanDefinition
    RootBeanDefinition mbd = this.mergedBeanDefinitions.get(beanName);
    // 如果存在合并后的BeanDefinition,并且不是过期的,直接返回
    if (mbd != null && !mbd.stale) {
        return mbd;
    }
    // 获取已经注册的BeanDefinition然后去合并
    return getMergedBeanDefinition(beanName, getBeanDefinition(beanName));
}

protected RootBeanDefinition getMergedBeanDefinition(String beanName, BeanDefinition bd)
		throws BeanDefinitionStoreException {
    // 顶级bean获取合并后的BeanDefinition
    return getMergedBeanDefinition(beanName, bd, null);
}

/**
* @param containingBd 如果是嵌套bean该值为顶级bean,如果是顶级bean该值为null
*/
protected RootBeanDefinition getMergedBeanDefinition(
		String beanName, BeanDefinition bd, @Nullable BeanDefinition containingBd)
		throws BeanDefinitionStoreException {

    synchronized (this.mergedBeanDefinitions) {
        // 本次的RootBeanDefinition
        RootBeanDefinition mbd = null;
        // 以前的RootBeanDefinition
        RootBeanDefinition previous = null;

        // Check with full lock now in order to enforce the same merged instance.
        // 如果bean是顶级bean,直接获取合并后的BeanDefinition
        if (containingBd == null) {
            mbd = this.mergedBeanDefinitions.get(beanName);
        }
		// 没有合并后的BeanDefinition || BeanDefinition过期了
        if (mbd == null || mbd.stale) {
            previous = mbd;
            // 如果bean没有parent
            if (bd.getParentName() == null) {
                // Use copy of given root bean definition.
                // 如果bd本身就是RootBeanDefinition直接复制一份,否则创建一个
                if (bd instanceof RootBeanDefinition) {
                    mbd = ((RootBeanDefinition) bd).cloneBeanDefinition();
                }
                else {
                    mbd = new RootBeanDefinition(bd);
                }
            }
            else {	
                // Child bean definition: needs to be merged with parent.
                // bean有parent
                BeanDefinition pbd;
                try {
                    // 获取parent bean的实际名称
                    String parentBeanName = transformedBeanName(bd.getParentName());
                    if (!beanName.equals(parentBeanName)) {
                        // 当前beanName不等于它的parent beanName
                      	// 获取parent合并后的BeanDefinition
                        pbd = getMergedBeanDefinition(parentBeanName);
                    }
                    else {
                        // 如果父定义的beanName与bd的beanName相同,则拿到父BeanFactory
                        // 只有在存在父BeanFactory的情况下,才允许父定义beanName与自己相同
                        BeanFactory parent = getParentBeanFactory();
                        if (parent instanceof ConfigurableBeanFactory) {
                            // 如果父BeanFactory是ConfigurableBeanFactory
                            // 则通过父BeanFactory获取parent合并后的BeanDefinition
                            pbd = ((ConfigurableBeanFactory) parent).getMergedBeanDefinition(parentBeanName);
                        }
                        else {
                            // 如果父BeanFactory不是ConfigurableBeanFactory,抛出异常
                            throw new NoSuchBeanDefinitionException(parentBeanName,
                                                                    "Parent name '" + parentBeanName + "' is equal to bean name '" + beanName +
                                                                    "': cannot be resolved without an AbstractBeanFactory parent");
                        }
                    }
                }
                catch (NoSuchBeanDefinitionException ex) {
                    throw new BeanDefinitionStoreException(bd.getResourceDescription(), beanName,
                                                           "Could not resolve parent bean definition '" + bd.getParentName() + "'", ex);
                }
                // Deep copy with overridden values.
                // 使用父定义pbd构建一个新的RootBeanDefinition对象(深拷贝)
                mbd = new RootBeanDefinition(pbd);
                // 覆盖与parent相同的属性,
                mbd.overrideFrom(bd);
            }
            
            // Set default singleton scope, if not configured before.
            // 如果bean没有设置scope属性,默认是singleton
            if (!StringUtils.hasLength(mbd.getScope())) {
                mbd.setScope(RootBeanDefinition.SCOPE_SINGLETON);
            }

            // A bean contained in a non-singleton bean cannot be a singleton itself.
            // Let's correct this on the fly here, since this might be the result of
            // parent-child merging for the outer bean, in which case the original inner bean
            // definition will not have inherited the merged outer bean's singleton status.
            // 当前bean是嵌套bean && 顶级bean的作用域不是单例 && 当前bean的作用域是单例
            // 这里总结起来就是,如果顶层bean不是单例的,那么嵌套bean也不能是单例的
            if (containingBd != null && !containingBd.isSingleton() && mbd.isSingleton()) {
                // 设置当前bean的作用域和顶级bean一样
                mbd.setScope(containingBd.getScope());
            }

            // Cache the merged bean definition for the time being
            // (it might still get re-merged later on in order to pick up metadata changes)
            // 当前bean是顶级bean && 缓存bean的元数据(该值默认为true)
            if (containingBd == null && isCacheBeanMetadata()) {
                // 将当前bean合并后的RootBeanDefinition缓存起来
                this.mergedBeanDefinitions.put(beanName, mbd);
            }
        }
        // 以前的RootBeanDefinition不为空,拷贝相关的BeanDefinition缓存
        if (previous != null) {
            copyRelevantMergedBeanDefinitionCaches(previous, mbd);
        }
        return mbd;
    }
}	

上面代码主要是获取 MergedBeanDefinition ,主要步骤如下:

  1. 首先从缓存中获取 beanMergedBeanDefinition,如果存在并且未过期直接返回。

  2. 不存在或者已过期的 MergedBeanDefinition ,获取已经注册的 BeanDefinition 去作为顶级 bean 合并。

  3. bean 没有 parent (就是 XML 中的 parent 属性),直接封装成 RootBeanDefinition

  4. beanparent ,先去获取父 MergedBeanDefinition ,然后覆盖和合并与 parent 相同的属性。

    注意:这里只有 abstractscopelazyInitautowireModedependencyCheckdependsOnfactoryBeanNamefactoryMethodNameinitMethodNamedestroyMethodName会覆盖,而 constructorArgumentValuespropertyValuesmethodOverrides 会合并。

  5. 如果没有设置作用域,默认作用域为 singleton

  6. 缓存 MergedBeanDefinition

上文中提到如果 beanparent,会合并一些属性,这里我们稍微展示一下合并后的 propertyValues:

首先定义一个 SuperUser 继承上面定义的 User,如下:

public class SuperUser extends User {

    private String address;

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }

    @Override
    public String toString() {
        return "SuperUser{" +
            "address='" + address + '\'' +
            '}';
    }

}

然后我们在 XML 文件中配置一下,如下:

<bean id="superUser" class="com.leisurexi.ioc.domain.SuperUser" parent="user">
    <property name="address" value="北京"/>
 </bean>

然后下图是我 Debug 的截图,可以看到 superUserpropertyValues 合并了 useridname 属性。

上文还提到了嵌套 bean ,下面我们简单看一下什么是嵌套 bean

在 Spring 中,如果某个 bean 所依赖的 bean 不想被 Spring 容器直接访问,可以使用嵌套 bean。和普通的 bean 一样,使用 bean 元素来定义嵌套的 bean,嵌套 bean 只对它的外部 bean 有效,Spring 无法直接访问嵌套 bean ,因此定义嵌套 bean 也无需指定 id 属性。如下配置片段是一个嵌套 bean 示例:

采用上面的配置形式可以保证嵌套 bean 不能被容器访问,因此不用担心其他程序修改嵌套 bean。外部 bean 的用法和使用结果和以前没有区别。

嵌套 bean 提高了 bean 的内聚性,但是降低了程序的灵活性。只有在确定无需通过 Spring 容器访问某个 bean 实例时,才考虑使用嵌套 bean 来定义。

DefaultSingletonBeanRegistry#isDependent

protected boolean isDependent(String beanName, String dependentBeanName) {
    // 加锁
    synchronized (this.dependentBeanMap) {
        // 检测beanName和dependentBeanName是否有循环依赖
        return isDependent(beanName, dependentBeanName, null);
    }
}

private boolean isDependent(String beanName, String dependentBeanName, @Nullable Set<String> alreadySeen) {
    // 如果当前bean已经检测过,直接返回false
    if (alreadySeen != null && alreadySeen.contains(beanName)) {
        return false;
    }
    // 解析别名
    String canonicalName = canonicalName(beanName);
    // 获取canonicalName所依赖beanName集合
    Set<String> dependentBeans = this.dependentBeanMap.get(canonicalName);
    // 如果为空,两者还未确定依赖关系,返回false
    if (dependentBeans == null) {
        return false;
    }
    // 如果dependentBeanName已经存在于缓存中,两者已经确定依赖关系,返回true
    if (dependentBeans.contains(dependentBeanName)) {
        return true;
    }
    // 循环检查,即检查依赖canonicalName的所有beanName是否被dependentBeanName依赖(即隔层依赖)
    for (String transitiveDependency : dependentBeans) {
        if (alreadySeen == null) {
            alreadySeen = new HashSet<>();
        }
        // 将已经检查过的记录下来,下次直接跳过
        alreadySeen.add(beanName);
        if (isDependent(transitiveDependency, dependentBeanName, alreadySeen)) {
            return true;
        }
    }
    return false;
}

这里的 dependentBeanMap 其实是 beanName 和其依赖的 dependentBeanName 反过来存的。比如,A 依赖 B,B 依赖 A;那么首先调用 getBean() 获取 A,然后到 isDependent() ,因为是第一次进来所以 dependentBeans 是空的直接返回 false,接着到下面 registerDepenndentBean() ,这里先将 dependentBeanName 作为 keyvalue 是添加了 beanNameLinkedHashSet ,添加进 dependentBeanMap;然后因为依赖 B,所以去实例化 B,又由于 B 依赖 A,到了 isDepnedent(),接着 dependentBeans.contains(dependentBeanName) 这行代码会返回 true (因为在实例化 A 的过程中,已经将 B 作为 key 放入了 dependentBeanMap),最后直接抛出 循环引用 的异常。

DefaultSingletonBeanRegistry#registerDependentBean

public void registerDependentBean(String beanName, String dependentBeanName) {
    // 解析别名
    String canonicalName = canonicalName(beanName);
    // 加锁
    synchronized (this.dependentBeanMap) {
        // 获取canonicalName依赖beanName集合,如果为空默认创建一个LinkedHashSet当做默认值
        Set<String> dependentBeans =
            this.dependentBeanMap.computeIfAbsent(canonicalName, k -> new LinkedHashSet<>(8));
        // 如果dependentBeanName已经记录过了,直接返回
        if (!dependentBeans.add(dependentBeanName)) {
            return;
        }
    }
    // 加锁
    synchronized (this.dependenciesForBeanMap) {
        // 这里是和上面的dependentBeanMap倒过来,key为dependentBeanName
        Set<String> dependenciesForBean =
            this.dependenciesForBeanMap.computeIfAbsent(dependentBeanName, k -> new LinkedHashSet<>(8));
        dependenciesForBean.add(canonicalName);
    }
}

这个方法又引入了一个跟 dependentBeanMap 类似的缓存 dependenciesForBeanMap。这两个缓存很容易搞混,这里再举一个简单的例子:A 依赖 B,那么 dependentBeanMap 存放的是 key 为 B,value 为含有 A 的 Set;而 dependenciesForBeanMap 存放的是key 为 A,value 为含有 B 的 Set

DefaultSingletonBeanRegistry#getSingleton

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
    Assert.notNull(beanName, "Bean name must not be null");
    // 加锁
    synchronized (this.singletonObjects) {
        Object singletonObject = this.singletonObjects.get(beanName);
        // 缓存中不存在当前 bean,也就是当前 bean 第一次创建
        if (singletonObject == null) {
            // 如果当前正在销毁 singletons,抛出异常
            if (this.singletonsCurrentlyInDestruction) {
                throw new BeanCreationNotAllowedException(beanName,
                                                          "Singleton bean creation not allowed while singletons of this factory are in destruction " +
                                                          "(Do not request a bean from a BeanFactory in a destroy method implementation!)");
            }
            if (logger.isDebugEnabled()) {
                logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
            }
            // 创建单例 bean 之前的回调
            beforeSingletonCreation(beanName);
            boolean newSingleton = false;
            boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
            if (recordSuppressedExceptions) {
                this.suppressedExceptions = new LinkedHashSet<>();
            }
            try {
                // 获取 bean 实例,在此处才会去真正调用创建 bean 的方法,也就是 createBean 方法
                singletonObject = singletonFactory.getObject();
                newSingleton = true;
            }
            catch (IllegalStateException ex) {
                // Has the singleton object implicitly appeared in the meantime ->
                // if yes, proceed with it since the exception indicates that state.
                singletonObject = this.singletonObjects.get(beanName);
                if (singletonObject == null) {
                    throw ex;
                }
            }
            catch (BeanCreationException ex) {
                if (recordSuppressedExceptions) {
                    for (Exception suppressedException : this.suppressedExceptions) {
                        ex.addRelatedCause(suppressedException);
                    }
                }
                throw ex;
            }
            finally {
                if (recordSuppressedExceptions) {
                    this.suppressedExceptions = null;
                }
                // 创建单例 bean 之后的回调
                afterSingletonCreation(beanName);
            }
            if (newSingleton) {
                // 将 singletonObject 放入缓存
                addSingleton(beanName, singletonObject);
            }
        }
        // 返回 bean 实例
        return singletonObject;
    }
}

// 单例 bean 创建前的回调方法,默认实现是将 beanName 加入到当前正在创建 bean 的缓存中
protected void beforeSingletonCreation(String beanName) {
    if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {
        throw new BeanCurrentlyInCreationException(beanName);
    }
}

// 单例 bean 创建后的回调方法,默认实现是将 beanName 从当前正在创建 bean 的缓存中移除
protected void afterSingletonCreation(String beanName) {
    if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.remove(beanName)) {
        throw new IllegalStateException("Singleton '" + beanName + "' isn't currently in creation");
    }
}

protected void addSingleton(String beanName, Object singletonObject) {
    synchronized (this.singletonObjects) {
        // 将 bean 实例缓存起来
        this.singletonObjects.put(beanName, singletonObject);
        // 移除 bean 的工厂
        this.singletonFactories.remove(beanName);
        // bean 已经实际创建完毕,这里从早起单例缓存中删除
        this.earlySingletonObjects.remove(beanName);
        // 将 beanName 添加到已注册 bean 缓存中
        this.registeredSingletons.add(beanName);
    }
}

上面方法是单例 bean 的处理逻辑,主要做的就是创建 bean 实例,然后将实例放入到缓存中;然后下次再获取该 bean 是直接从缓存中获取返回。

在创建 bean 实例的前后提供了两个扩展点,分别是 beforeSingletonCreation()afterSingletonCreation() ,我们可以继承 DefaultSingletonBeanRegistry 来扩展这两个方法。

自定义作用域示例

我们实现一个 ThreadLocal 级别的作用域,也就是同一个线程内 bean 是同一个实例,不同线程的 bean 是不同实例。首先我们继承 Scope 接口实现,其中方法。如下:

public class ThreadLocalScope implements Scope {

    /** scope 名称,在 XML 中的 scope 属性就配置此名称 */
    public static final String SCOPE_NAME = "thread-local";

    private final NamedThreadLocal<Map<String, Object>> threadLocal = new NamedThreadLocal<>("thread-local-scope");

    /**
    * 返回实例对象,该方法被 Spring 调用
    */
    @Override
    public Object get(String name, ObjectFactory<?> objectFactory) {
        Map<String, Object> context = getContext();
        Object object = context.get(name);
        if (object == null) {
            object = objectFactory.getObject();
            context.put(name, object);
        }
        return object;
    }

    /**
    * 获取上下文 map
    */
    @NonNull
    private Map<String, Object> getContext() {
        Map<String, Object> map = threadLocal.get();
        if (map == null) {
            map = new HashMap<>();
            threadLocal.set(map);
        }
        return map;
    }

    @Override
    public Object remove(String name) {	
        return getContext().remove(name);
    }

    @Override
    public void registerDestructionCallback(String name, Runnable callback) {
        // TODO
    }
	
    @Override
    public Object resolveContextualObject(String key) {
        Map<String, Object> context = getContext();
        return context.get(key);
    }

    @Override
    public String getConversationId() {
        return String.valueOf(Thread.currentThread().getId());
    }

}

上面的 ThreadLocalScope 重点关注下 get() 即可,该方法是被 Spring 调用的。

然后在 XML 中配置 beanscopethread-local。如下:

<bean id="user" name="user" class="com.leisurexi.ioc.domain.User" scope="thread-local">
    <property name="id" value="1"/>
    <property name="name" value="leisurexi"/>
</bean>

接着我们测试一下。测试类:

@Test
public void test() throws InterruptedException {
    DefaultListableBeanFactory beanFactory = new DefaultListableBeanFactory();
    // 注册自定义作用域
    beanFactory.registerScope(ThreadLocalScope.SCOPE_NAME, new ThreadLocalScope());
    XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(beanFactory);
    reader.loadBeanDefinitions("META-INF/spring-bean.xml");
    for (int i = 0; i < 3; i++) {
        Thread thread = new Thread(() -> {
            User user = beanFactory.getBean("user", User.class);
            System.err.printf("[Thread id :%d] user = %s%n", Thread.currentThread().getId(), user.getClass().getName() + "@" + Integer.toHexString(user.hashCode()));
            User user1 = beanFactory.getBean("user", User.class);
            System.err.printf("[Thread id :%d] user1 = %s%n", Thread.currentThread().getId(), user1.getClass().getName() + "@" + Integer.toHexString(user1.hashCode()));
        });
        thread.start();
        thread.join();
    }
}

说一下我们这里的主要思路,新建了三个线程,查询线程内 user bean 是否相等,不同线程是否不等。

结果如下图:

总结

本文主要介绍了通过 getBean() 流程,我们可以重新梳理一下思路:

  1. 获取 bean 实际名称,如果缓存中存在直接取出实际 bean 返回。
  2. 缓存中不存在,判断当前工厂是否有 BeanDefinition ,没有递归去父工厂创建 bean
  3. 合并 BeanDefinition ,如果 depends-on 不为空,先去初始化依赖的 bean
  4. 如果 bean 的作用域是单例,调用 createBean() 创建实例,这个方法会执行 bean 的其它生命周期回调,以及属性赋值等操作;接着执行单例 bean 创建前后的生命周期回调方法,并放入 singletonObjects 缓存起来。
  5. 如果 bean 的作用域是原型,调用 createBean() 创建实例,并执行原型 bean 前后调用生命周期回调方法。
  6. 如果 bean 的作用域是自定义的,获取对应的 Scope 对象,调用重写的 get() 获取实例,并执行原型 bean 前后调用生命周期回调方法。
  7. 最后检查所需的类型是否与实际 bean 实例的类型匹配,如果不等进行转换,最后返回实例。

最后,我模仿 Spring 写了一个精简版,代码会持续更新,现在是 0.0.1 版本。地址:https://github.com/leisurexi/tiny-spring。访问新博客地址,观看效果更佳 https://leisurexi.github.io/

参考

  • 《Spring 源码深度解析》—— 郝佳

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