Spring IOC容器分析(4) -- bean创建获取完整流程

上节探讨了Spring IOC容器中getBean方法，下面我们将自行编写测试用例，深入跟踪分析bean对象创建过程。

测试环境创建

测试示例代码如下：

package org.springframework.context.mytests;

import org.junit.Test;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class ApplicationContextTest {

    @Test
    public void testApplicationContext() {
        ApplicationContext applicationContext = new ClassPathXmlApplicationContext("beans.xml");
        System.out.println("numbers: " + applicationContext.getBeanDefinitionCount());
        ((Worker)applicationContext.getBean("worker")).work();
    }
}

应用ClassPathXmlApplicationContext加载解析xml文件，xml配置文件如下：

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:p="http://www.springframework.org/schema/p"
       xmlns:context="http://www.springframework.org/schema/context"
       xmlns:util="http://www.springframework.org/schema/util"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-4.1.xsd
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-4.3.xsd
        http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util-4.3.xsd">
    <bean id="worker" class="org.springframework.context.mytests.Worker"></bean>
</beans>

bean Worker代码如下：

package org.springframework.context.mytests;

public class Worker {
    public void work(){
        System.out.println("I am working");
    }
}

在IDE中对测试文件打断点，进入Debug模式，一步一步跟随程序跟踪bean创建过程。

源码跟踪

跟踪断点，进入ClassPathXmlApplicationContext源码，如下所示：

package org.springframework.context.support;

import org.springframework.beans.BeansException;
import org.springframework.context.ApplicationContext;
import org.springframework.core.io.ClassPathResource;
import org.springframework.core.io.Resource;
import org.springframework.util.Assert;

public class ClassPathXmlApplicationContext extends AbstractXmlApplicationContext {

	......

	public ClassPathXmlApplicationContext(String configLocation) throws BeansException {
		this(new String[] {configLocation}, true, null);
	}

	......

	public ClassPathXmlApplicationContext(String[] configLocations, boolean refresh, ApplicationContext parent)
			throws BeansException {

		super(parent);
		setConfigLocations(configLocations);
		if (refresh) {
			refresh();
		}
	}

	......
}

ClassPathXmlApplicationContex中都是构造器方法，表明加载解析xml文件的操作都是在实例化阶段完成的。以上是部分源码片段，在本测试中，调用第一个构造器初始化，但其实质是调用第二个构造器。从源码可以发现，第二个构造器实际上完成了两个主要功能：

1. setConfigLocations(configLocations)，设置应用上下文的配置文件路径，如果没有设置，便会使用默认路径
2. refresh()，IOC容器初始化入口

通过查找定位发现：setConfigLocations(configLocations)是继承于AbstractRefreshableConfigApplicationContext抽象类，其源码如下：

	public void setConfigLocations(String... locations) {
		if (locations != null) {
			Assert.noNullElements(locations, "Config locations must not be null");
			this.configLocations = new String[locations.length];
			for (int i = 0; i < locations.length; i++) {
				this.configLocations[i] = resolvePath(locations[i]).trim();
			}
		}
		else {
			this.configLocations = null;
		}
	}

可以看到setConfigLocations方法获取定位信息首先需要调用resolvePath方法对路径进行预处理。

refresh方法是继承于AbstractApplicationContext抽象类，是在接口ConfigurableApplicationContext中定义的。其在抽象类中源码如下：

	@Override
	public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			// Prepare this context for refreshing.
			prepareRefresh();

			// Tell the subclass to refresh the internal bean factory.
			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

			// Prepare the bean factory for use in this context.
			prepareBeanFactory(beanFactory);

			try {
				// Allows post-processing of the bean factory in context subclasses.
				postProcessBeanFactory(beanFactory);

				// Invoke factory processors registered as beans in the context.
				invokeBeanFactoryPostProcessors(beanFactory);

				// Register bean processors that intercept bean creation.
				registerBeanPostProcessors(beanFactory);

				// Initialize message source for this context.
				initMessageSource();

				// Initialize event multicaster for this context.
				initApplicationEventMulticaster();

				// Initialize other special beans in specific context subclasses.
				onRefresh();

				// Check for listener beans and register them.
				registerListeners();

				// Instantiate all remaining (non-lazy-init) singletons.
				finishBeanFactoryInitialization(beanFactory);

				// Last step: publish corresponding event.
				finishRefresh();
			}

			catch (BeansException ex) {
				if (logger.isWarnEnabled()) {
					logger.warn("Exception encountered during context initialization - " +
							"cancelling refresh attempt: " + ex);
				}

				// Destroy already created singletons to avoid dangling resources.
				destroyBeans();

				// Reset 'active' flag.
				cancelRefresh(ex);

				// Propagate exception to caller.
				throw ex;
			}

			finally {
				// Reset common introspection caches in Spring's core, since we
				// might not ever need metadata for singleton beans anymore...
				resetCommonCaches();
			}
		}
	}

refresh方法大致描述了Spring IOC容器的初始化过程，第一步prepareRefresh主要是做一些准备工作，如准备应用环境、设置启动时间、设置属性源初始化标志等。重点看第二步ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory()，这一步是获取更新后的子类Bean工厂。其源码如下：

	/**
	 * Tell the subclass to refresh the internal bean factory.
	 * @return the fresh BeanFactory instance
	 * @see #refreshBeanFactory()
	 * @see #getBeanFactory()
	 */
	protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
		refreshBeanFactory();
		ConfigurableListableBeanFactory beanFactory = getBeanFactory();
		if (logger.isDebugEnabled()) {
			logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);
		}
		return beanFactory;
	}

其内部有两个方法refreshBeanFactory()和getBeanFactory()，这两个方法均为抽象方法，如下所示：

	protected abstract void refreshBeanFactory() throws BeansException, IllegalStateException;

	public abstract ConfigurableListableBeanFactory getBeanFactory() throws IllegalStateException;

查找子类发现，这两个方法均在子类AbstractRefreshableApplicationContext中实现，该类仍然是抽象类。先来看refreshBeanFactory方法实现：

	@Override
	protected final void refreshBeanFactory() throws BeansException {
		if (hasBeanFactory()) {
			destroyBeans();
			closeBeanFactory();
		}
		try {
			DefaultListableBeanFactory beanFactory = createBeanFactory();
			beanFactory.setSerializationId(getId());
			customizeBeanFactory(beanFactory);
			loadBeanDefinitions(beanFactory);
			synchronized (this.beanFactoryMonitor) {
				this.beanFactory = beanFactory;
			}
		}
		catch (IOException ex) {
			throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
		}
	}

该方法先判断是否存在BeanFactory，若存在则直接销毁原BeanFactory，先销毁工厂中的Beans，再关闭工bean厂。之后创建新的Bean工厂，其方法为createBeanFactory，如下所示：

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

从函数返回值可以看出，重新创建的Bean工厂是默认的bean工厂DefaultListableBeanFactory类型。创建完新的bean工厂后便会根据上下文进行初始化（customizeBeanFactory），加载bean定义（loadBeanDefinitions）。其中，loadBeanDefinitions为抽象方法，如下所示：

	protected abstract void loadBeanDefinitions(DefaultListableBeanFactory beanFactory)
			throws BeansException, IOException;

该方法具体实现在AbstractXmlApplicationContext类中，该类仍然是一个抽象类，实现如下：

	@Override
	protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
		// Create a new XmlBeanDefinitionReader for the given BeanFactory.
		XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);

		// Configure the bean definition reader with this context's
		// resource loading environment.
		beanDefinitionReader.setEnvironment(this.getEnvironment());
		beanDefinitionReader.setResourceLoader(this);
		beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));

		// Allow a subclass to provide custom initialization of the reader,
		// then proceed with actually loading the bean definitions.
		initBeanDefinitionReader(beanDefinitionReader);
		loadBeanDefinitions(beanDefinitionReader);
	}

阅读源码发现，方法内部是采用XmlBeanDefinitionReader来读取加载bean对象。通过追踪源码，发现XmlBeanDefinitionReader最终调用loadBeanDefinitions方法来读取加载bean,具体实现如下：

	public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
		Assert.notNull(encodedResource, "EncodedResource must not be null");
		if (logger.isInfoEnabled()) {
			logger.info("Loading XML bean definitions from " + encodedResource.getResource());
		}

		Set<EncodedResource> currentResources = this.resourcesCurrentlyBeingLoaded.get();
		if (currentResources == null) {
			currentResources = new HashSet<EncodedResource>(4);
			this.resourcesCurrentlyBeingLoaded.set(currentResources);
		}
		if (!currentResources.add(encodedResource)) {
			throw new BeanDefinitionStoreException(
					"Detected cyclic loading of " + encodedResource + " - check your import definitions!");
		}
		try {
			InputStream inputStream = encodedResource.getResource().getInputStream();
			try {
				InputSource inputSource = new InputSource(inputStream);
				if (encodedResource.getEncoding() != null) {
					inputSource.setEncoding(encodedResource.getEncoding());
				}
				return doLoadBeanDefinitions(inputSource, encodedResource.getResource());
			}
			finally {
				inputStream.close();
			}
		}
		catch (IOException ex) {
			throw new BeanDefinitionStoreException(
					"IOException parsing XML document from " + encodedResource.getResource(), ex);
		}
		finally {
			currentResources.remove(encodedResource);
			if (currentResources.isEmpty()) {
				this.resourcesCurrentlyBeingLoaded.remove();
			}
		}
	}

到此，应该明白了IOC容器初始化bean工厂、加载bean对象的大致过程了。现在回头再来看obtainFreshBeanFactory方法中的另一个方法getBeanFactory方法。该方法在子类AbstractRefreshableApplicationContext中实现就相对简单写，如下所示：

	@Override
	public final ConfigurableListableBeanFactory getBeanFactory() {
		synchronized (this.beanFactoryMonitor) {
			if (this.beanFactory == null) {
				throw new IllegalStateException("BeanFactory not initialized or already closed - " +
						"call 'refresh' before accessing beans via the ApplicationContext");
			}
			return this.beanFactory;
		}
	}

到此为止，Spring IOC容器加载机制探讨基本上告一段落了。通过对源码分析，对Spring IOC机制能有一个大致的了解：1、解析、定位、加载xml配置文件；2、提取配置文件内容；3、新建bean工厂；4、创建Bean定义，并放入map中存储。通过追踪源码，也深刻体会到了spring对设计模式的应用。

下一节将进入对Spring另一大杀器的探讨，Spring-AOP。