SpringMvc学习心得（一）浅谈spring
本人在使用springmvc框架过程中产生了一点心得和体会，在此记录下来作为参考，如有疏漏敬请指正(所有与spring有关的jar包版本均为3.1.1.RELEASE)。
1.springmvc是什么？
我认为springmvc是spring+mvc(这句话有点像废话)。换句话说，如果我们把搭建一个web项目认为是盖一座商场的话，spring是材料供应商，主要负责生产钢筋水泥等一些建筑材料。而mvc就像一个施工队，他规定了web项目的大致结构。不过此时的商场还只是一个毛坯房，还不能够使用。而springmvc的使用者就是装修队，他将毛坯房(mvc框架)转化为精美的店铺(业务逻辑)，最终供消费者使用。
综上所述，我认为在整个项目中最重要部分是spring而不是mvc。坦白的说，单从mvc的角度上来看，springmvc并不能算得上出色。但spring的IOC机制对项目的构建有着很大的帮助，这也是springmvc这么流行的原因之一。
2.关于spring的一些讨论
2.1：xsd文件的加载
一提到spring，首先想到的一般就是IOC和AOP(面试经常被问到)。spring创建JavaBean的主要包含三个步骤：(1)解析xml文件；(2)构建beandefinition；(3)根据beandefinition反射JavaBean。
spring解析xml文件使用的是dom方式进行解析，即整个xml文档都被加载进内存当中。spring本身不提供xml解析功能，该功能由第三方jar包提供。
仔细比较下面两个xml文件：
a.xml
Tom
20
UTF-8
UTF-8
XmlBeanDefinitionReader
protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
throws BeanDefinitionStoreException {
try {
int validationMode = getValidationModeForResource(resource);
Document doc = this.documentLoader.loadDocument(
inputSource, getEntityResolver(), this.errorHandler, validationMode, isNamespaceAware());
return registerBeanDefinitions(doc, resource);
}
catch (BeanDefinitionStoreException ex) {
throw ex;
}
catch (SAXParseException ex) {
throw new XmlBeanDefinitionStoreException(resource.getDescription(),
"Line " + ex.getLineNumber() + " in XML document from " + resource + " is invalid", ex);
}
catch (SAXException ex) {
throw new XmlBeanDefinitionStoreException(resource.getDescription(),
"XML document from " + resource + " is invalid", ex);
}
catch (ParserConfigurationException ex) {
throw new BeanDefinitionStoreException(resource.getDescription(),
"Parser configuration exception parsing XML from " + resource, ex);
}
catch (IOException ex) {
throw new BeanDefinitionStoreException(resource.getDescription(),
"IOException parsing XML document from " + resource, ex);
}
catch (Throwable ex) {
throw new BeanDefinitionStoreException(resource.getDescription(),
"Unexpected exception parsing XML document from " + resource, ex);
}
}
private Map getSchemaMappings() {
if (this.schemaMappings == null) {
synchronized (this) {
if (this.schemaMappings == null) {
if (logger.isDebugEnabled()) {
logger.debug("Loading schema mappings from [" + this.schemaMappingsLocation + "]");
}
try {
Properties mappings =
PropertiesLoaderUtils.loadAllProperties(this.schemaMappingsLocation, this.classLoader);
if (logger.isDebugEnabled()) {
logger.debug("Loaded schema mappings: " + mappings);
}
Map schemaMappings = new ConcurrentHashMap();
CollectionUtils.mergePropertiesIntoMap(mappings, schemaMappings);
this.schemaMappings = schemaMappings;
}
catch (IOException ex) {
throw new IllegalStateException(
"Unable to load schema mappings from location [" + this.schemaMappingsLocation + "]", ex);
}
}
}
}
return this.schemaMappings;
} 其中尤其需要注意PropertiesLoaderUtils.loadAllProperties方法，该方法能够扫描并加载包括web项目和引用jar包在内的所有具有相同路径的同名文件。(上面那句话说的比较拗口，举个简单的例子。如果web项目引用了两个jar包，这两个jar包的source folder中都有一个META-INF文件夹，而且该文件夹中都有一个叫做spring.schemas的文件。那么PropertiesLoaderUtils在调用loadAllProperties方法时会把这两个文件都进行加载。)。而spring.schemas存放的就是url和xsd文件路径的意义映射关系，spring获取了这一映射关系并加载了该文件，最后通过文件流的方式返回给XMLEntityManager供其在解析xml文件时使用。至此，校验xml文件需要的xsd文件就被成功加载了。
2.2：JavaBean的构建
成功解析xml文件只是spring运行的第一步，接下来spring将xml解析出来的element转化为beandefinition。beandefinition规定了JavaBean的组成结构。spring依旧使用PropertiesLoaderUtils进行加载，不过这一次加载的是META-INF/spring.handler。通过该文件中的信息，spring获得并注册了xml元素与元素解析器之间的映射关系。
解析META-INF/spring.handler并注册映射关系的代码：
private Map getHandlerMappings() {
if (this.handlerMappings == null) {
synchronized (this) {
if (this.handlerMappings == null) {
try {
Properties mappings =
PropertiesLoaderUtils.loadAllProperties(this.handlerMappingsLocation, this.classLoader);
if (logger.isDebugEnabled()) {
logger.debug("Loaded NamespaceHandler mappings: " + mappings);
}
Map handlerMappings = new ConcurrentHashMap();
CollectionUtils.mergePropertiesIntoMap(mappings, handlerMappings);
this.handlerMappings = handlerMappings;
}
catch (IOException ex) {
throw new IllegalStateException(
"Unable to load NamespaceHandler mappings from location [" + this.handlerMappingsLocation + "]", ex);
}
}
}
}
return this.handlerMappings;
}
public BeanDefinition parseCustomElement(Element ele, BeanDefinition containingBd) {
String namespaceUri = getNamespaceURI(ele);
NamespaceHandler handler = this.readerContext.getNamespaceHandlerResolver().resolve(namespaceUri);
if (handler == null) {
error("Unable to locate Spring NamespaceHandler for XML schema namespace [" + namespaceUri + "]", ele);
return null;
}
return handler.parse(ele, new ParserContext(this.readerContext, this, containingBd));
}获取解析器并解析element的代码：
public BeanDefinition parse(Element element, ParserContext parserContext) {
return findParserForElement(element, parserContext).parse(element, parserContext);
}
private BeanDefinitionParser findParserForElement(Element element, ParserContext parserContext) {
String localName = parserContext.getDelegate().getLocalName(element);
BeanDefinitionParser parser = this.parsers.get(localName);
if (parser == null) {
parserContext.getReaderContext().fatal(
"Cannot locate BeanDefinitionParser for element [" + localName + "]", element);
}
return parser;
}BeanDefinitionParser的主要作用是创建一个BeanDefinition实例，该实例中是对JavaBean的描述，例如bean中的每个基本类型的值，bean中如果包含其他的bean，则将其他JavaBean的BeanDefinition也被注册到该JavaBean的BeanDefinition中。
spring对JavaBean的实例化：spring根据beandefinition的定义，通过反射机制产生java对象，并为其配置响应的属性(property)
spring对JavaBean实例化的代码：
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
Class beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
// Allow post-processors to modify the merged bean definition.
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
mbd.postProcessed = true;
}
}
// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
if (logger.isDebugEnabled()) {
logger.debug("Eagerly caching bean '" + beanName +
"' to allow for resolving potential circular references");
}
addSingletonFactory(beanName, new ObjectFactory() {
public Object getObject() throws BeansException {
return getEarlyBeanReference(beanName, mbd, bean);
}
});
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
populateBean(beanName, mbd, instanceWrapper);
if (exposedObject != null) {
exposedObject = initializeBean(beanName, exposedObject, mbd);
}
}
catch (Throwable ex) {
if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
throw (BeanCreationException) ex;
}
else {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
}
}
if (earlySingletonExposure) {
Object earlySingletonReference = getSingleton(beanName, false);
if (earlySingletonReference != null) {
if (exposedObject == bean) {
exposedObject = earlySingletonReference;
}
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
String[] dependentBeans = getDependentBeans(beanName);
Set actualDependentBeans = new LinkedHashSet(dependentBeans.length);
for (String dependentBean : dependentBeans) {
if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
actualDependentBeans.add(dependentBean);
}
}
if (!actualDependentBeans.isEmpty()) {
throw new BeanCurrentlyInCreationException(beanName,
"Bean with name '" + beanName + "' has been injected into other beans [" +
StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
"] in its raw version as part of a circular reference, but has eventually been " +
"wrapped. This means that said other beans do not use the final version of the " +
"bean. This is often the result of over-eager type matching - consider using " +
"'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
}
}
}
}
// Register bean as disposable.
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
} 需要尤其注意的是代码中的三个函数：createBeanInstance，addSingletonFactory以及populateBean。其中 createBeanInstance从beandefinition当中获取对象的构造函数，并使用spring的工具类BeanUtils完成java对象的实例化工作。
addSingletonFactory则将反射出来的JavaBean注册到beanfactory中，具体代码如下：
protected void addSingletonFactory(String beanName, ObjectFactory singletonFactory) {
Assert.notNull(singletonFactory, "Singleton factory must not be null");
synchronized (this.singletonObjects) {
if (!this.singletonObjects.containsKey(beanName)) {
this.singletonFactories.put(beanName, singletonFactory);
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}
}
protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {
if (pvs == null || pvs.isEmpty()) {
return;
}
MutablePropertyValues mpvs = null;
List original;
if (System.getSecurityManager()!= null) {
if (bw instanceof BeanWrapperImpl) {
((BeanWrapperImpl) bw).setSecurityContext(getAccessControlContext());
}
}
if (pvs instanceof MutablePropertyValues) {
mpvs = (MutablePropertyValues) pvs;
if (mpvs.isConverted()) {
// Shortcut: use the pre-converted values as-is.
try {
bw.setPropertyValues(mpvs);
return;
}
catch (BeansException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Error setting property values", ex);
}
}
original = mpvs.getPropertyValueList();
}
else {
original = Arrays.asList(pvs.getPropertyValues());
}
TypeConverter converter = getCustomTypeConverter();
if (converter == null) {
converter = bw;
}
BeanDefinitionValueResolver valueResolver = new BeanDefinitionValueResolver(this, beanName, mbd, converter);
// Create a deep copy, resolving any references for values.
List deepCopy = new ArrayList(original.size());
boolean resolveNecessary = false;
for (PropertyValue pv : original) {
if (pv.isConverted()) {
deepCopy.add(pv);
}
else {
String propertyName = pv.getName();
Object originalValue = pv.getValue();
Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);
Object convertedValue = resolvedValue;
boolean convertible = bw.isWritableProperty(propertyName) &&
!PropertyAccessorUtils.isNestedOrIndexedProperty(propertyName);
if (convertible) {
convertedValue = convertForProperty(resolvedValue, propertyName, bw, converter);
}
// Possibly store converted value in merged bean definition,
// in order to avoid re-conversion for every created bean instance.
if (resolvedValue == originalValue) {
if (convertible) {
pv.setConvertedValue(convertedValue);
}
deepCopy.add(pv);
}
else if (convertible && originalValue instanceof TypedStringValue &&
!((TypedStringValue) originalValue).isDynamic() &&
!(convertedValue instanceof Collection || ObjectUtils.isArray(convertedValue))) {
pv.setConvertedValue(convertedValue);
deepCopy.add(pv);
}
else {
resolveNecessary = true;
deepCopy.add(new PropertyValue(pv, convertedValue));
}
}
}
if (mpvs != null && !resolveNecessary) {
mpvs.setConverted();
}
// Set our (possibly massaged) deep copy.
try {
bw.setPropertyValues(new MutablePropertyValues(deepCopy));
}
catch (BeansException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Error setting property values", ex);
}
} 至此，spring完成了解析xml文件以及实例化java对象的全部过程。之后的博客将对spring的注入功能进行分析