设计模式之工厂模式
工厂模式
工厂模式属于创建型模式，它是创建对象的最佳方式，它负责事项创建所有实例的内部逻辑。工厂类创建产品类的方法可以被外界直接调用，创建所需要的产品对象。它在spring中是非常常见的。
工厂模式例子
public interface Product{
void show();
}
创建接口实现类Moblie:
public class Mobile implement Product{
@Override
public void show(){
System.out.println("HUWEI mATE 30 Pro");
}
}
创建接口实现类Car:
public class Car implement Product{
@Override
public void show(){
System.out.println("国产车 BYD");
}
}
创建工厂ProductFactory，根据参数创建指定产品对象：
public class ProductFactory{
public Product info(String name){
if(objects.equals(name,"mobile")){
return new Mobile();
}else{
return new Car();
}else{
return null;
}
}
}
使用工厂ProductFactory创建指定对象：
public class ProductFactoryTest{
public static void main(String[] args){
//创建ProductFactory
ProductFactory factory = new ProductFactory();
//获取moblie对象
Product mobile = factory.info("mobile");
mobile.show();
//获取car对象
Product car = factory.info("car");
car.show();
}
}
或者采用getBean，采用xml方式：
public class BeanFactoryTest {
public static void main(String[] args) {
Resource resource = new ClassPathResource("spring-factory.xml");
BeanFactory beanFactory = new XmlBeanFactory(resource);
Product car1 = (Product) beanFactory.getBean("car");
Product car2 = (Product) beanFactory.getBean("car");
car1.show();
System.out.println(car1 == car2);
}
}
我们需要关注beanFactory中的加载beanDefintion的过程和getBean方法，从上面我们也看到首先需要加载xml文件，然后通过xmlBeanFactory获取bean工厂，然后获取bean。
解析xml将其变成bean的过程：
加载资源，有了资源将resource变成doucument，然后将document转换成beanDefintion，而这个过程首先经过loadBeanDefintion，这个过程首先校验DTD和xsd，然后将其变成符合spring风格的，然后将进行加载，加载的过程其实也是一个注册的过程，然后将其注册到BeanDefintionMap中，而注册中又分为自定义标签和默认标签的解析，这个过程中，我们知道解析的过程我们如果需要进行自定义标签，则会关注自定义标签是怎样实现的。然后执行获取getBean的操作，而获取bean的操作需要首先将获取的bean进行格式的转换，然后到单例池中去获取，如果没有进行单例bean的创建，这里使用到了单例模式双重校验，然后根据bean的依赖，查看循环依赖，如果存在循环依赖，解决循环依赖，然后执行createBean的过程，这个过程中，我们可以执行处理器操作，将我们需要在bean初始化的过程中对bean进行增强操作或者增加操作，这里还可以实现InitializingBean操作，重新afterPropertiesSet方法，或者在销毁bean的前置执行一些操作。
下面重点关注解析xml加载beanDefintion的过程：
Bean的加载流程
Spring中的工厂模式
public interface BeanFactory {
String FACTORY_BEAN_PREFIX = "&";
Object getBean(String var1) throws BeansException;
<T> T getBean(String var1, Class<T> var2) throws BeansException;
Object getBean(String var1, Object... var2) throws BeansException;
<T> T getBean(Class<T> var1) throws BeansException;
<T> T getBean(Class<T> var1, Object... var2) throws BeansException;
<T> ObjectProvider<T> getBeanProvider(Class<T> var1);
<T> ObjectProvider<T> getBeanProvider(ResolvableType var1);
boolean containsBean(String var1);
boolean isSingleton(String var1) throws NoSuchBeanDefinitionException;
boolean isPrototype(String var1) throws NoSuchBeanDefinitionException;
boolean isTypeMatch(String var1, ResolvableType var2) throws NoSuchBeanDefinitionException;
boolean isTypeMatch(String var1, Class<?> var2) throws NoSuchBeanDefinitionException;
@Nullable
Class<?> getType(String var1) throws NoSuchBeanDefinitionException;
String[] getAliases(String var1);
}
在BeanFactory接口中，有多个getBean方法，该方法其实就是典型的工厂设计模式特征.
上面的例子会调用：
BeanFactory beanFactory = new XmlBeanFactory(resource);
调用XmlBeanFactory
public XmlBeanFactory(Resource resource, BeanFactory parentBeanFactory) throws BeansException {
super(parentBeanFactory);
this.reader.loadBeanDefinitions(resource);
}
加载beanDefintion
public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
Assert.notNull(encodedResource, "EncodedResource must not be null");
if (logger.isTraceEnabled()) {
logger.trace("Loading XML bean definitions from " + encodedResource);
}
Set<EncodedResource> currentResources = this.resourcesCurrentlyBeingLoaded.get();
if (currentResources == null) {
currentResources = new HashSet<>(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();
}
}
}
进行加载
protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
throws BeanDefinitionStoreException {
try {
//加载文件
Document doc = doLoadDocument(inputSource, resource);
int count = registerBeanDefinitions(doc, resource);
if (logger.isDebugEnabled()) {
logger.debug("Loaded " + count + " bean definitions from " + resource);
}
return count;
}
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);
}
}
注册BeanDefintion
public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
int countBefore = getRegistry().getBeanDefinitionCount();
documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
return getRegistry().getBeanDefinitionCount() - countBefore;
}
注册beanDefiontions:
@Override
public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
this.readerContext = readerContext;
doRegisterBeanDefinitions(doc.getDocumentElement());
}
执行注册BeanDefintion
/**
* Register each bean definition within the given root {@code <beans/>} element.
*/
@SuppressWarnings("deprecation") // for Environment.acceptsProfiles(String...)
protected void doRegisterBeanDefinitions(Element root) {
// Any nested <beans> elements will cause recursion in this method. In
// order to propagate and preserve <beans> default-* attributes correctly,
// keep track of the current (parent) delegate, which may be null. Create
// the new (child) delegate with a reference to the parent for fallback purposes,
// then ultimately reset this.delegate back to its original (parent) reference.
// this behavior emulates a stack of delegates without actually necessitating one.
BeanDefinitionParserDelegate parent = this.delegate;
this.delegate = createDelegate(getReaderContext(), root, parent);
if (this.delegate.isDefaultNamespace(root)) {
String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
if (StringUtils.hasText(profileSpec)) {
String[] specifiedProfiles = StringUtils.tokenizeToStringArray(
profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
// We cannot use Profiles.of(...) since profile expressions are not supported
// in XML config. See SPR-12458 for details.
if (!getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {
if (logger.isDebugEnabled()) {
logger.debug("Skipped XML bean definition file due to specified profiles [" + profileSpec +
"] not matching: " + getReaderContext().getResource());
}
return;
}
}
}
preProcessXml(root);
parseBeanDefinitions(root, this.delegate);
postProcessXml(root);
this.delegate = parent;
}
创建委托
protected BeanDefinitionParserDelegate createDelegate(
XmlReaderContext readerContext, Element root, @Nullable BeanDefinitionParserDelegate parentDelegate) {
BeanDefinitionParserDelegate delegate = new BeanDefinitionParserDelegate(readerContext);
delegate.initDefaults(root, parentDelegate);
return delegate;
}
进行解析
protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
if (delegate.isDefaultNamespace(root)) {
NodeList nl = root.getChildNodes();
for (int i = 0; i < nl.getLength(); i++) {
Node node = nl.item(i);
if (node instanceof Element) {
Element ele = (Element) node;
if (delegate.isDefaultNamespace(ele)) {
parseDefaultElement(ele, delegate);
}
else {
delegate.parseCustomElement(ele);
}
}
}
}
else {
delegate.parseCustomElement(root);
}
}
解析默认元素
private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
importBeanDefinitionResource(ele);
}
else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
processAliasRegistration(ele);
}
else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
processBeanDefinition(ele, delegate);
}
else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
// recurse
doRegisterBeanDefinitions(ele);
}
}
解析自定义元素
@Nullable
public BeanDefinition parseCustomElement(Element ele) {
return parseCustomElement(ele, null);
}
/**
* Parse a custom element (outside of the default namespace).
* @param ele the element to parse
* @param containingBd the containing bean definition (if any)
* @return the resulting bean definition
*/
@Nullable
public BeanDefinition parseCustomElement(Element ele, @Nullable BeanDefinition containingBd) {
String namespaceUri = getNamespaceURI(ele);
if (namespaceUri == null) {
return null;
}
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));
}
进行解析操作：
@Override
@Nullable
public BeanDefinition parse(Element element, ParserContext parserContext) {
BeanDefinitionParser parser = findParserForElement(element, parserContext);
return (parser != null ? parser.parse(element, parserContext) : null);
}
为元素查找解析
/**
* Locates the {@link BeanDefinitionParser} from the register implementations using
* the local name of the supplied {@link Element}.
*/
@Nullable
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;
}
进行解析操作：
@Override
@Nullable
public final BeanDefinition parse(Element element, ParserContext parserContext) {
AbstractBeanDefinition definition = parseInternal(element, parserContext);
if (definition != null && !parserContext.isNested()) {
try {
String id = resolveId(element, definition, parserContext);
if (!StringUtils.hasText(id)) {
parserContext.getReaderContext().error(
"Id is required for element '" + parserContext.getDelegate().getLocalName(element)
+ "' when used as a top-level tag", element);
}
String[] aliases = null;
if (shouldParseNameAsAliases()) {
String name = element.getAttribute(NAME_ATTRIBUTE);
if (StringUtils.hasLength(name)) {
aliases = StringUtils.trimArrayElements(StringUtils.commaDelimitedListToStringArray(name));
}
}
BeanDefinitionHolder holder = new BeanDefinitionHolder(definition, id, aliases);
registerBeanDefinition(holder, parserContext.getRegistry());
if (shouldFireEvents()) {
BeanComponentDefinition componentDefinition = new BeanComponentDefinition(holder);
postProcessComponentDefinition(componentDefinition);
parserContext.registerComponent(componentDefinition);
}
}
catch (BeanDefinitionStoreException ex) {
String msg = ex.getMessage();
parserContext.getReaderContext().error((msg != null ? msg : ex.toString()), element);
return null;
}
}
return definition;
}
AbstractBeanFactory
@Override
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}
第一步：AbstractBeanFactory#transformedBeanName(String name)
返回Bean名称，必要时去除工厂取消引用前缀，并将别名解析为规范名称。
protected String transformedBeanName(String name) {
return canonicalName(BeanFactoryUtils.transformedBeanName(name));
}
//经过bean工厂处理前缀操作，也即对bean工厂进行规则化，通过别名map拿到解析名称
public String canonicalName(String name) {
String canonicalName = name;
String resolvedName;
do {
resolvedName = (String)this.aliasMap.get(canonicalName);
if (resolvedName != null) {
canonicalName = resolvedName;
}
} while(resolvedName != null);
return canonicalName;
}
调用bean工具类BeanFactoryUtils
public static String transformedBeanName(String name) {
Assert.notNull(name, "'name' must not be null");
if (!name.startsWith(BeanFactory.FACTORY_BEAN_PREFIX)) {
return name;
}
return transformedBeanNameCache.computeIfAbsent(name, beanName -> {
do {
beanName = beanName.substring(BeanFactory.FACTORY_BEAN_PREFIX.length());
}
while (beanName.startsWith(BeanFactory.FACTORY_BEAN_PREFIX));
return beanName;
});
}
执行doGetBean:重点
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
//转换bean名称
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
//为自然注册单例预先检查单例缓存
Object sharedInstance = getSingleton(beanName);
//单例实例是否为空，如果不为空，则
if (sharedInstance != null && args == null) {
if (logger.isTraceEnabled()) {
//是否是Singleton当前正在创建中
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 + "'");
}
}
//为bean实例获取对象
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
//如果已经在创建此bean实例，则失败：大概在循环参考之内,需要考虑循环依赖的情况
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
//检查如果beanDefintion存在在这个工厂
//获取父bean工厂
BeanFactory parentBeanFactory = getParentBeanFactory();
//如果父bean工厂不为空，同时不包含beanDefintion，则检查parent
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
//如果么有找到，则检查父类
String nameToLookup = originalBeanName(name);
//如果父类bean工厂是抽象bean工厂类型，执行getBean操作，进行返回
if (parentBeanFactory instanceof AbstractBeanFactory) {
return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
nameToLookup, requiredType, args, typeCheckOnly);
}
//如果参数不为空，则
else if (args != null) {
// Delegation to parent with explicit args.
//使用显式参数委派给父级
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else if (requiredType != null) {
// No args -> delegate to standard getBean method.
//没有参数，委托给标准getBean方法
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
else {
return (T) parentBeanFactory.getBean(nameToLookup);
}
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
// 确保当前bean依赖的bean的初始化
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dep : dependsOn) {
if (isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
registerDependentBean(dep, beanName);
try {
getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
}
// Create bean instance.
//创建bean实例
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, () -> {
try {
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;
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
Object prototypeInstance = null;
try {
beforePrototypeCreation(beanName);
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
else {
String scopeName = mbd.getScope();
final Scope scope = this.scopes.get(scopeName);
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
}
try {
Object scopedInstance = scope.get(beanName, () -> {
beforePrototypeCreation(beanName);
try {
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
});
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.
if (requiredType != null && !requiredType.isInstance(bean)) {
try {
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());
}
}
return (T) bean;
}本文参与 腾讯云自媒体分享计划，分享自微信公众号。
原始发表：2020-11-22，如有侵权请联系 cloudcommunity@tencent.com 删除
评论
登录后参与评论
推荐阅读
目录