Spring源码分析：IoC

In this post, we are going to diving into the Spring framework in deep. The main content of this post is to analyze IoC implementation in the source code of the Spring framework.

Basics

Basic Concepts

Bean and BeanDefinition

In Spring, the objects that form the backbone of your application and that are managed by the Spring IoC container are called beans. A bean is an object that is instantiated, assembled, and otherwise managed by a Spring IoC container.

A Spring IoC container manages one or more beans. These beans are created with the configuration metadata that you supply to the container, for example, in the form of XML <bean/> definitions.

Within the container itself, these bean definitions are represented as BeanDefinition objects.

BeanWrapper

BeanWrapper is the central interface of Spring’s low-level JavaBeans infrastructure. Typically not used directly but ranther implicitly via a BeanFactory or a DataBinder. It provides operations to analyze and manipulate standard JavaBeans: get and set property values, get property descriptors, and query the readability/writability of properties.

IoC Containers

IoC container is the implementation of IoC functionality in the Spring framework. The interface org.springframework.context.ApplicationContextrepresents the Spring IoC container and is responsible for instantiating, configuring, and assembling the aforementioned beans. The container gets its instructions on what objects to instantiate, configure, and assemble by reading configuration metadata. The configuration metadata is represented in XML, Java annotations, or Java code. It allows you to express the objects that compose your application and the rich interdependencies between such objects.

Several implementations of the ApplicationContext interface are supplied out-of-the-box with Spring. In standalone applications it is common to create an instance of ClassPathXmlApplicationContext or FileSystemXmlApplicationContext.

BeanFactory vs ApplicationContext

Spring provides two kinds of IoC container, one is XMLBeanFactory and other is ApplicationContext. The ApplicationContext is a more powerful IoC container than BeanFactory.

Bean Factory

• Bean instantiation/wiring

Application Context

• Bean instantiation/wiring
• Automatic BeanPostProcessor registration
• Automatic BeanFactoryPostProcessor registration
• Convenient MessageSource access (for i18n)
• ApplicationEvent publication

WebApplicationContext vs ApplicationContext

WebApplicationContext interface provides configuration for a web application. This is read-only while the application is running, but may be reloaded if the implementation supports this.

WebApplcationContext extends ApplicationContext. This interface adds a getServletContext() method to the generic ApplicationContext interface, and defines a well-known application attribute name that the root context must be bound to in the bootstrap process.

WebApplicationContext in Spring is web aware ApplicationContext i.e it has Servlet Context information. In single web application there can be multiple WebApplicationContext. That means each DispatcherServlet associated with single WebApplicationContext.

Class Hierarchy

Bean Factories Hierarchy

I-BeanFactory
|----I-ListableBeanFactory
|--------C-StaticListableBeanFactory
|----I-HierarchicalBeanFactory
|--------I-ConfigurableBeanFactory
|------------A-AbstractBeanFactory
|------------I-ConfigurableListableBeanFactory
|----I-AutowireCappableBeanFactory
|---------A-AbstractAutowireCapableBeanFactory
|--------------C-DefaultListableBeanFactory
|-------------------C-XmlBeanFactory

ApplicationContext Hierarchy

I-ListableBeanFactory
I-HierarchicalBeanFactory
|----I-ApplicationContext
|--------I-WebApplicationContext
|--------I-ConfigurableApplicationContext
|------------A-AbstractApplicationContext
|------------I-ConfigurableWebApplicationContext		
|----------------A-AbstractRefreshableWebApplicationContext
|--------------------C-XmlWebApplicationContext

AbstractApplicationContext Hierarchy

A-AbstractApplicationContext
|----A-AbstractRefreshableApplicationContext
|--------A-AbstractRefreshableConfigApplicationContext
|------------A-AbstractRefreshableWebApplicationContext
|----------------C-AnnotationConfigWebApplicationContext
|----------------C-XmlWebApplicationContext

BeanRegister & BeanFactory Hierarchy

I-SingletonBeanRegistry
|----C-DefaultSingletonBeanRegistry
|--------A-FactoryBeanRegistrySupport
|------------A-AbstractBeanFactory

DispatherServlet Hierarchy

A-javax.servlet.http.HttpServlet
|----A-HttpServletBean
|--------A-FrameworkServlet
|------------C-DispatcherServlet

The Process of IoC Implementation

The following figure shows the process of IoC implementation.

Next, We are going to analyze the implementation in the source code of every step.

1 Java web project starting

1.1 instantiate and init servlets from web.xml

When a Java web project running in an application server like Apache Tomcat, the servlet container of the server will load, instantiate, and init Java servlets that be <load-on-startup> from web.xml file.

<servlet>
    <servlet-name>DispatcherServlet</servlet-name>
    <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
    <init-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>classpath:applicationContext.xml</param-value>
    </init-param>
    <load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
    <servlet-name>DispatcherServlet</servlet-name>
    <url-pattern>/</url-pattern>
</servlet-mapping>

2 The DispatcherServlet initialization and Calling init() in HttpServletBean

2.1 set bean properties from init parameters

Set initial parameters specified in the web.xml file for the servlet instance DispatcherServlet.

3 initServletBean() in FrameworkServlet

3.1 initWebApplicationContext()

Initialize and publish the WebApplicationContext for this servlet.

• Check is there any WebApplicationContext instance or root application context, if not create a new webApplicationContext instance (See #3.1.1).

3.1.1 createWebApplicationContext()

Using reflection to create a WebApplicationContext instance, and configure it (see #3.1.1.1).

FrameworkServlet.java

protected WebApplicationContext createWebApplicationContext(@Nullable ApplicationContext parent) {
    //...
    ConfigurableWebApplicationContext wac =
    	(ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
    //...go to #3.1.1.1
    configureAndRefreshWebApplicationContext(wac);
}

3.1.1.1 configureAndRefreshWebApplicationContext()

Set WebApplication instance properties and go to refresh (See #4).

FrameworkServlet.java

protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac) {
    //...
    wac.setServletContext(getServletContext());
    wac.setServletConfig(getServletConfig());
    wac.setNamespace(getNamespace());
    postProcessWebApplicationContext(wac);
    applyInitializers(wac);
    //...go to #4
    wac.refresh();
}

4 refresh() in AbstractApplicationContext

As this is a startup method, it should destroy already created singletons if it fails, to avoid dangling resources. In other words, after invocation of that method, either all or no singletons at all should be instantiated.

AbstractApplicationContext.java

public void refresh() throws BeansException, IllegalStateException {
    //...
    // 1. Create a BeanFactory instance. Refer to #4.1
    ConfigurableListableBeanFactory beanFactory = this.obtainFreshBeanFactory();
    // 2. Configure properties of AbstractApplicationContext, prepare for bean initialization and context refresh
    this.postProcessBeanFactory(beanFactory);
    this.invokeBeanFactoryPostProcessors(beanFactory);
    this.registerBeanPostProcessors(beanFactory);
    this.initMessageSource();
    this.initApplicationEventMulticaster();
    this.onRefresh();
    this.registerListeners();
    // 3. bean initialization. Refer to #4.2
    this.finishBeanFactoryInitialization(beanFactory);
    // 4. context refresh.
    this.finishRefresh();
    // ...
}

4.1 obtainFreshBeanFactory()

Create a BeanFactory instance in AbstractRefreshableApplicationContext.

AbstractRefreshableApplicationContext.java

protected final void refreshBeanFactory() throws BeansException {
    //...
    // 1. Creating a BeanFactory instance.
    DefaultListableBeanFactory beanFactory = this.createBeanFactory();
    // 2. Set BeanFactory instance's properties
    beanFactory.setSerializationId(this.getId());
    this.customizeBeanFactory(beanFactory);
    // 3. load bean definitions
    this.loadBeanDefinitions(beanFactory);
    // 4. assign this new BeanFactory instance to the AbstractRefreshableApplicationContext's field beanFactory
    synchronized(this.beanFactoryMonitor) {
        this.beanFactory = beanFactory;
    }
    //...
}

protected DefaultListableBeanFactory createBeanFactory() {
    return new DefaultListableBeanFactory(this.getInternalParentBeanFactory());
}

4.2 finishBeanFactoryInitialization()

Initializate beanFactory, and go to instantiate bean instances (see #5).

AbstractApplicationContext.java

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
    // 1. initialize bean factory
    //...
    // 2. instantiate bean instances, go to #5
    beanFactory.preInstantiateSingletons();
}

5 preInstantiateSingletons() in DefaultListableBeanFactory

Using Iterator of beanNames to get bean instances.

5.1 to create all bean instances

AbstractApplicationContext.java

public void preInstantiateSingletons() throws BeansException {
    // 1. get beanNames' iterator
    List<String> beanNames = new ArrayList(this.beanDefinitionNames);
    Iterator var2 = beanNames.iterator();
	
    // 2. for loop to create bean instances, go to #6
    while (true){
        //...
        beanName = (String)var2.next();
        if (this.isFactoryBean(beanName)) {
            bean = this.getBean("&" + beanName);
            break;
        }
        this.getBean(beanName);
        //...
    }
}

6 getBean(name) in AbstractBeanFactory

To get the bean instances.

AbstractBeanFactory.java

public Object getBean(String name) throws BeansException {
    return this.doGetBean(name, (Class)null, (Object[])null, false);
}

6.1 doGetBean()

To get the bean instance by bean name.

AbstractBeanFactory.java

protected <T> T doGetBean(String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly) throws BeansException {
    String beanName = this.transformedBeanName(name);
    Object sharedInstance = this.getSingleton(beanName);
    Object bean;

    // 1. check is the bean instance in creation
    if (sharedInstance != null && args == null) {
        //...
    } else {
        // 2. get and check beanDefinition by beanName
        RootBeanDefinition mbd = this.getMergedLocalBeanDefinition(beanName);
        this.checkMergedBeanDefinition(mbd, beanName, args);
        // 3. to get different type bean instance: singleton, prototype, and so on.
        if (mbd.isSingleton()) {
            // to create singleton bean instance go to #7
            sharedInstance = this.getSingleton(beanName, () -> {
                try {
                    return this.createBean(beanName, mbd, args);
                } catch (BeansException var5) {
                    this.destroySingleton(beanName);
                    throw var5;
                }
            });
            bean = this.getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
        } else if (mbd.isPrototype()) {
            //...
        } else{
            //...
        }
    }
}

7 getSingleton(beanName, singletonFactory) in DefaultSingletonBeanRegistry

To get singleton bean instance.

DefaultSingletonBeanRegistry.java

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
    // 1. check conditions and logging
    //...
    // 2. to get a singleton bean instance, go to #8
    singletonObject = singletonFactory.getObject();
    newSingleton = true;
    // 3. add created singleton bean instance to the bean list
    if (newSingleton) {
        this.addSingleton(beanName, singletonObject);
    }
}

protected void addSingleton(String beanName, Object singletonObject) {
    synchronized(this.singletonObjects) {
        // add bean instance to the bean list "singletonObjects"
        this.singletonObjects.put(beanName, singletonObject);
        this.singletonFactories.remove(beanName);
        this.earlySingletonObjects.remove(beanName);
        this.registeredSingletons.add(beanName);
    }
}

8 createBean(beanName, beanDefinition, args) in AbstractAutowireCapableBeanFactory

To get a bean instance, create a beanWrapper, initialize the bean, add add the bean to the beanFactory.

AbstractAutowireCapableBeanFactory.java

protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException {
    // ...
    // 1. construct a beanDefinition for use.
    // ...
    // 2. try to create bean instance
    Object beanInstance = this.resolveBeforeInstantiation(beanName, mbdToUse);
    if (beanInstance != null) {
        return beanInstance;
    }
    // 3. try to create bean instance again if fail to create in first time
    beanInstance = this.doCreateBean(beanName, mbdToUse, args);
    return beanInstance;
}

protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException {
    // 1. to construct a beanWrapper and create the bean instance
    BeanWrapper instanceWrapper = this.createBeanInstance(beanName, mbd, args);
    // 2. post processing 
    //...
    // 3. Add bean to the beanFactory and some bean instances record list
    this.addSingletonFactory(beanName, () -> {
        return this.getEarlyBeanReference(beanName, mbd, bean);
    });
	// 4. autowired and handle property values
    this.populateBean(beanName, mbd, instanceWrapper);
	// 5. initialization bean
    exposedObject = this.initializeBean(beanName, exposedObject, mbd);
	// 6. return bean
    return exposedObject;
}

protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
    // ...
    ctors = mbd.getPreferredConstructors();
    return ctors != null ? this.autowireConstructor(beanName, mbd, ctors, (Object[])null) : this.instantiateBean(beanName, mbd);
}

protected BeanWrapper instantiateBean(String beanName, RootBeanDefinition mbd) {
    // 1. to create the bean instance
    Object beanInstance = this.getInstantiationStrategy().instantiate(mbd, beanName, this);

    // 2. create the BeanWrapper by bean instance
    BeanWrapper bw = new BeanWrapperImpl(beanInstance);
    this.initBeanWrapper(bw);
    return bw;
}

9 instantiate() in SimpleInstantiationStrategy

Get the Constructor of the bean class and to get the bean instance.

SimpleInstantiationStrategy.java

public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
    // 1. get the Constructor of the bean class
    Constructor constructorToUse = (Constructor)bd.resolvedConstructorOrFactoryMethod;
    Class<?> clazz = bd.getBeanClass();
    constructorToUse = (Constructor)AccessController.doPrivileged(() -> {
        return clazz.getDeclaredConstructor();
    });
    // 2. to craete bean instance
    return BeanUtils.instantiateClass(constructorToUse, new Object[0]);
}

10 instantiateClass() in BeanUtils

Using reflection to create a instance.

BeanUtils.java

public static <T> T instantiateClass(Constructor<T> ctor, Object... args) throws BeanInstantiationException {
    //...
    // Using reflection to create instance
    return ctor.newInstance(argsWithDefaultValues);
}

Conclusion

Spring framework uses the servlet container loads the DispatcherServlet when server startup to trigger the automatic beans instantiation.

The ApplicationContext is responsible for the whole process of beans instantiation. There is a BeanFactory as a field in the ApplicationContext, it is responsible to create bean instances.

The important steps of beans instantiation are: load bean definitions, create and initialize beanFactory, create and initialize bean instances, keep bean records in beanFactory.

References

[1] Spring Framework Source Code

[2] Spring Framework API - Current 5.2.7.RELEASE

[3] Spring Framework Documentation - 5.2.7.RELEASE

[4] How does spring Dispatcher Servlet create default beans without any XML configuration? - Stack Overflow

[5] Difference between ApplicationContext and WebApplicationContext in Spring MVC

[6] Spring life-cycle when we refresh the application context - Stack Overflow

[7] BeanFactory vs ApplicationContext - Stack Overflow