标签：管理 单例模式 国庆假期 and classname 思想 conf 变量 creat
现在Springboot越来越便捷，如果简单的Spring应用，已无需再配置xml文件，基本可以实现全注解，即使是SpringCloud的那套东西，也都可以通过yaml配置完成。最近一年一直在用Springboot+JPA或者Springboot+MyBatis，基本上不用Spring和SpringMVC了，心血来潮想着趁国庆假期试着一点点实现一下Spring的基本功能（当然是会对照源码的，毕竟很多细节想不到，变量命名也会按照源码来），基本思路就是先按照Spring的类图试着自己写，争取实现相同的功能，然后再看源码的实现方式，再重构。
第一篇先实现Spring的基本组件--bean容器
定义两个接口BeanFactory和BeanDefinition
public interface BeanFactory {
BeanDefinition getBeanDefinition(String beanID)
Object getBean(String beanID);
}public interface BeanDefinition {
public String getBeanClassName();
}两个实现类DefaultBeanFactory和GenericBeanDefinition分别实现这两个接口：
public class DefaultBeanFactory implements BeanFactory {
public static final String ID_ATTRIBUTE="id";
public static final String CLASS_ATTRIBUTE="class";
private Map<String,BeanDefinition> beanDefinitionMap=new ConcurrentHashMap<String, BeanDefinition>();
public DefaultBeanFactory(String configFile) {
loadBeanDefinition(configFile);
}
private void loadBeanDefinition(String configFile) {
InputStream is= null;
ClassLoader classLoader = this.getClass().getClassLoader();
is=classLoader.getResourceAsStream(configFile);
//需要dom4j
SAXReader saxReader = new SAXReader();
try {
Document doc = saxReader.read(is);
Element root = doc.getRootElement();
Iterator iterator = root.elementIterator();
while (iterator.hasNext()){
Element element = (Element)iterator.next();
String id=element.attributeValue(ID_ATTRIBUTE);
String className=element.attributeValue(CLASS_ATTRIBUTE);
BeanDefinition beanDefinition = new GenericBeanDefinition(id, className);
beanDefinitionMap.put(id,beanDefinition);
}
} catch (DocumentException e) {
throw new BeanDefinitionStoreException("Load and parsing XML failed",new Throwable());
}finally {
if(is!=null){
try {
is.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
public BeanDefinition getBeanDefinition(String beanID) {
if(beanDefinitionMap.containsKey(beanID))
return beanDefinitionMap.get(beanID);
return null;
}
//职责2:创建bean实例
public Object getBean(String beanID) {
BeanDefinition beanDefinition = this.getBeanDefinition(beanID);
if(beanDefinition==null){
throw new BeanCreationException("Bean Definition does not exist");
}
ClassLoader classLoader = this.getClass().getClassLoader();
try {
Class<?> clz = classLoader.loadClass(beanDefinition.getBeanClassName());
return clz.newInstance();
//捕获所有异常，然后抛出自定义异常
} catch (Exception e) {
throw new BeanCreationException("create bean for "+beanDefinition.getBeanClassName()+" failed.");
}
}
}public class GenericBeanDefinition implements BeanDefinition {
private String id;
private String beanClassName;
public GenericBeanDefinition(String id, String beanClassName) {
this.id = id;
this.beanClassName = beanClassName;
}
public String getBeanClassName() {
return this.beanClassName;
}
}主要逻辑在DefaultBeanFactory中，通过解析xml来生成一个bean实例并保存到Map中。
核心思想：一个类应该有且只有一个变化的原因。
为什么引入单一职责：
在SRP中，把职责定义为变化的原因。当需求变化时，将通过更改职责相关的类来体现。如果一个类拥有多于一个的职责，则多个职责耦合在一起，会有多于一个原因来导致这个类发生变化。一个职责的变化可能会影响到其他的职责，另外，把多个职责耦合在一起，影响复用性。如：DefaultBeanFactory类目前有两个指责：1.加载和读取XML文件；2.创建bean实例
我们把读取XML的职责拆分出来给一个新类XMLBeanDefinitionReader,同时，BeanFactory是供给client使用的，而BeanDefinition是一个内部的概念，应该对client是透明的，所以不应该对外暴露，所以把getBeanDefinition和注册（即之前的添加到Map）职责分出来给一个新接口BeanDefinitionRegistry。DefaultBeanFactory实现BeanDefinitionRegistry，下一节会用一个ApplicationContext包装DefaultBeanFactory,进而对用户屏蔽getBeanDefinition()和registerBeanDefinition()。
修改后的DefaultBeanFactory
public class DefaultBeanFactory implements BeanFactory,BeanDefinitionRegistry {
private Map<String,BeanDefinition> beanDefinitionMap=new ConcurrentHashMap<String, BeanDefinition>();
public DefaultBeanFactory(){
}
public BeanDefinition getBeanDefinition(String beanID) {
if(beanDefinitionMap.containsKey(beanID))
return beanDefinitionMap.get(beanID);
return null;
}
public void registerBeanDefinition(String beanID, BeanDefinition beanDefinition) {
this.beanDefinitionMap.put(beanID,beanDefinition);
}
public Object getBean(String beanID) {
BeanDefinition beanDefinition = this.getBeanDefinition(beanID);
if(beanDefinition==null){
throw new BeanCreationException("Bean Definition does not exist");
}
ClassLoader classLoader = this.getClass().getClassLoader();
try {
Class<?> clz = classLoader.loadClass(beanDefinition.getBeanClassName());
return clz.newInstance();
//捕获所有异常，然后抛出自定义异常
} catch (Exception e) {
throw new BeanCreationException("create bean for "+beanDefinition.getBeanClassName()+" failed.");
}
}
}BeanDefinitionRegistry接口:
public interface BeanDefinitionRegistry {
BeanDefinition getBeanDefinition(String beanID);
void registerBeanDefinition(String beanID,BeanDefinition beanDefinition);
}
XmlBeanDefinitionReader类：用来读取XML并调用BeanDefinitionRegistry的registerBeanDefinition方法注册beanDefinition。
public class XmlBeanDefinitionReader {
public static final String ID_ATTRIBUTE = "id";
public static final String CLASS_ATTRIBUTE = "class";
public static final String SCOPE_ATTRIBUTE = "scope";
BeanDefinitionRegistry registry;
public XmlBeanDefinitionReader(BeanDefinitionRegistry registry) {
this.registry = registry;
}
public void loadBeanDefinition(String configFile) {
InputStream is = null;
ClassLoader classLoader = this.getClass().getClassLoader();
is = classLoader.getResourceAsStream(configFile);
SAXReader saxReader = new SAXReader();
try {
Document doc = saxReader.read(is);
Element root = doc.getRootElement();
Iterator iterator = root.elementIterator();
while (iterator.hasNext()) {
Element element = (Element) iterator.next();
String id = element.attributeValue(ID_ATTRIBUTE);
String className = element.attributeValue(CLASS_ATTRIBUTE);
BeanDefinition beanDefinition = new GenericBeanDefinition(id, className);
registry.registerBeanDefinition(id, beanDefinition);
}
} catch (DocumentException e) {
throw new BeanDefinitionStoreException("Load and parsing XML failed", new Throwable());
} finally {
if (is != null) {
try {
is.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
}Spring中通常不会直接访问BeanFactory，而是通过ApplicationContext来得到bean，即通过ApplicationContext调用BeanFactory方法。
定义一个接口ApplicationContext继承BeanFactory:
public interface ApplicationContext extends BeanFactory {
}创建一个实现类ClassPathXmlApplicationContext,从ClassPath下读取XML，内部持有一个DefaultBeanFactory实例，对外只暴露getBean()方法，屏蔽了getBeanDefinition()和registerBeanDefinition()：
public class ClassPathXmlApplicationContext implements ApplicationContext {
private DefaultBeanFactory factory=null;
public ClassPathXmlApplicationContext(String configFile) {
factory=new DefaultBeanFactory();
XmlBeanDefinitionReader reader=new XmlBeanDefinitionReader(factory);
reader.loadBeanDefinition(configFile);
}
public Object getBean(String beanID) {
return factory.getBean(beanID);
}
}使用Resource来抽象资源
除了从ClassPath读取XML，还可以从FileSystem读取，最终都是要转换成为一个InputStream，所以抽象出一个Resource接口，并创建两个实现类来分别处理从两种途径读取XML。
public interface Resource {
InputStream getInputStream() throws IOException;
String getDescription();
}public class ClassPathResource implements Resource {
private String path;
private ClassLoader classLoader;
public ClassPathResource(String path) {
this(path, (ClassLoader) null);
}
public ClassPathResource(String path, ClassLoader classLoader) {
this.path = path;
this.classLoader = (classLoader != null ? classLoader : ClassUtils.getDefaultClassLoader());
}
public InputStream getInputStream() throws IOException {
InputStream is = this.classLoader.getResourceAsStream(this.path);
if (is == null) {
throw new FileNotFoundException(path + " cannot be opened");
}
return is;
}
public String getDescription(){
return this.path;
}
}public class FileSystemResource implements Resource {
private final String path;
private final File file;
public FileSystemResource(String path) {
//这里的Assert不是junit的Assert，是自定义的一个工具类,就是判空处理并提示指定信息，逻辑简单不贴代码了
Assert.notNull(path, "Path must not be null");
this.file = new File(path);
this.path = path;
}
public InputStream getInputStream() throws IOException {
return new FileInputStream(this.file);
}
public String getDescription() {
return "file [" + this.file.getAbsolutePath() + "]";
}
}现在DefaultBeanFactory中的loadBeanDefinition可以接收一个Resource对象，从中获取InputStream，而不用管是从classpath还是从FileSystem读取的。同时可以创建一个与ClassPathXmlApplicationContext相对应的FileSystemXmlApplicationContext类来完成从FileSystem读取XML并获取bean：
public class FileSystemXmlApplicationContext implements ApplicationContext {
DefaultBeanFactory factory=null;
public FileSystemXmlApplicationContext(String path) {
factory=new DefaultBeanFactory();
XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(factory);
//这是与ClassPathXmlApplicationContext唯一的区别
Resource resource=new FileSystemResource(path);
reader.loadBeanDefinition(resource);
}
public Object getBean(String beanID){
return factory.getBean(beanID);
}
}可以发现这个类和ClassPathXmlApplicationContext唯一的区别就是Resource不同，为了避免重复代码，用模板方法重构，新建一个抽象类AbstractApplicationContext,然后两个ApplicationContext类继承并实现getResourceByPath。
public abstract class AbstractApplicationContext implements ApplicationContext {
private DefaultBeanFactory factory = null;
private ClassLoader beanClassLoader=null;
public AbstractApplicationContext(String configFile){
factory = new DefaultBeanFactory();
XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(factory);
Resource resource = this.getResourceByPath(configFile);
reader.loadBeanDefinition(resource);
}
public Object getBean(String beanID) {
return factory.getBean(beanID);
}
protected abstract Resource getResourceByPath(String path);
}Spring中的bean有一个scope属性用来指定bean是否是单例。而Spring是如何管理单例对象的呢？肯定不是把类设计成单例模式，而是Spring统一管理bean，然后根据scope属性来提供bean实例。
先定义一个接口SingletonBeanRegistry：
public interface SingletonBeanRegistry {
void registerSingleton(String beanName, Object singletonObject);
Object getSingleton(String beanName);
}它的实现类DefaultSingletonBeanRegistry，通过一个Map管理单例对象：
public class DefaultSingletonBeanRegistry implements SingletonBeanRegistry {
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<String, Object>(64);
public void registerSingleton(String beanName, Object singletonObject) {
Assert.notNull(beanName, "‘beanName‘ must not be null");
Object oldObject = this.singletonObjects.get(beanName);
if (oldObject != null) {
throw new IllegalStateException("Could not register object [" + singletonObject +
"] under bean name ‘" + beanName + "‘: there is already object [" + oldObject + "] bound");
}
this.singletonObjects.put(beanName, singletonObject);
}
public Object getSingleton(String beanName) {
return this.singletonObjects.get(beanName);
}
}咱们的DefaultBeanFactory要继承DefaultSingletonBeanRegistry（也可以内部持有一个DefaultSingletonBeanRegistry对象，采用组合模式），修改getBean()方法：
public Object getBean(String beanID) {
BeanDefinition beanDefinition = this.getBeanDefinition(beanID);
if(beanDefinition==null){
throw new BeanCreationException("Bean Definition does not exist");
}
if(beanDefinition.isSingleton()){
Object bean = this.getSingleton(beanID);
if(bean == null){
bean = createBean(beanDefinition);
this.registerSingleton(beanID, bean);
}
return bean;
}
return createBean(beanDefinition);
}同时我们的BeanDefinition和GenericBeanDefinition也要修改，增加Singleton相关的属性：
public interface BeanDefinition {
public static final String SCOPE_SINGLETON = "singleton";
public static final String SCOPE_PROTOTYPE = "prototype";
public static final String SCOPE_DEFAULT = "";
public boolean isSingleton();
public boolean isPrototype();
String getScope();
void setScope(String scope);
public String getBeanClassName();
}public class GenericBeanDefinition implements BeanDefinition {
private String id;
private String beanClassName;
private boolean singleton = true;
private boolean prototype = false;
private String scope = SCOPE_DEFAULT;
public GenericBeanDefinition(String id, String beanClassName) {
this.id = id;
this.beanClassName = beanClassName;
}
public String getBeanClassName() {
return this.beanClassName;
}
public boolean isSingleton() {
return this.singleton;
}
public boolean isPrototype() {
return this.prototype;
}
public String getScope() {
return this.scope;
}
public void setScope(String scope) {
this.scope = scope;
this.singleton = SCOPE_SINGLETON.equals(scope) || SCOPE_DEFAULT.equals(scope);
this.prototype = SCOPE_PROTOTYPE.equals(scope);
}
}XmlBeanDefinitionReader类中的loadBeanDefinition（）也要修改，使其能读取XML文件中的scope属性。
至此，基本的BeanFactory就实现了。我们可以通过Xml文件装载Bean了。
标签：管理 单例模式 国庆假期 and classname 思想 conf 变量 creat
原文地址：https://www.cnblogs.com/yzykkpl/p/10356228.html
