原型模式
原型模式屬於對象的創建模式。通過給出一個原型對象來指明所有創建的對象的類型，然後用複製這個原型對象的辦法創建出更多同類型的對象。這就是原型模式的用意。
這種形式涉及到三個角色：
-
客戶(Client)角色：客戶類提出創建對象的請求。
-
抽象原型(Prototype)角色：這是一個抽象角色，通常由一個Java接口或Java抽象類實現。此角色給出所有的具體原型類所需的接口。
-
具體原型（Concrete Prototype）角色：被複制的對象。此角色需要實現抽象的原型角色所要求的接口。
原型模式的優點
原型模式允許在運行時動態改變具體的實現類型。原型模式可以在運行期間，由客戶來註冊符合原型接口的實現類型，也可以動態地改變具體的實現類型，看起來接口沒有任何變化，但其實運行的已經是另外一個類實例了。因爲克隆一個原型就類似於實例化一個類。
原型模式的缺點
原型模式最主要的缺點是每一個類都必須配備一個克隆方法。配備克隆方法需要對類的功能進行通盤考慮，這對於全新的類來說不是很難，而對於已經有的類不一定很容易，特別是當一個類引用不支持序列化的間接對象，或者引用含有循環結構的時候。
原型模式即在當前類的基礎上創建出一個新的類，說到底就是怎麼克隆出一個對象，可以通過淺拷貝和深拷貝方式。
案例
1、測試demo
/**
* @Author: jiangcw
* @Date: 2019-6-29 下午 3:24
* @Version 1.0
*
* 原型模式：創建型模式
*
* java中天生支持原型模式，通過實現Cloneable接口，重寫clone方法，實現拷貝
* java中的clone方法默認的拷貝是淺拷貝，如果想實現深拷貝，需要重寫clone方法。
*
* 注：使用原型模式可以將當前對象的狀態複製到新創建的對象上。
*/
public class PrototypeBase {
public static void main(String[] args) throws CloneNotSupportedException {
M m = new M();
m.elementA = new ElementA();
m.elementB = new ElementB();
M cloneObject = (M)m.clone();
System.out.println(m.elementA == cloneObject.elementA);
System.out.println(m.elementB == cloneObject.elementB);
System.out.println(m == cloneObject);
ElementA elementA = new ElementA();
ElementB elementB = new ElementB();
elementB.clone();
}
static class M implements Cloneable {
private ElementA elementA;
private ElementB elementB;
@Override
protected Object clone() throws CloneNotSupportedException {
M m = (M)super.clone();
m.elementB = new ElementB();
return m;
}
}
static class ElementA implements Cloneable {
String a;
@Override
protected Object clone() throws CloneNotSupportedException {
return new ElementA();
}
}
static class ElementB implements Cloneable {
@Override
protected Object clone() throws CloneNotSupportedException {
return super.clone();
}
}
}
2、spring的prototype和singleton
在spring中一個bean可以是單例的，scope定義爲singleton即可。也可以定義爲原型模式，此時bean就是多例了，在需要使用相同的類型的bean的定義，都是重新創建一個狀態相同的bean。
關鍵類：AbstractBeanFactory，關鍵方法：doGetBean
protected <T> T doGetBean(
final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException {
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
if (logger.isDebugEnabled()) {
if (isSingletonCurrentlyInCreation(beanName)) {
logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
"' that is not fully initialized yet - a consequence of a circular reference");
}
else {
logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
}
}
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
String nameToLookup = originalBeanName(name);
if (args != null) {
// Delegation to parent with explicit args.
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
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.
// 如果是單例走下面的邏輯，保證一個類的實例是唯一的
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
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);
}
// 如果是原型模式，會採用深度拷貝的方式，在注入實例的地方都會重新創建一個類的實例。
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, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
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 && bean != null && !requiredType.isInstance(bean)) {
try {
return getTypeConverter().convertIfNecessary(bean, requiredType);
}
catch (TypeMismatchException ex) {
if (logger.isDebugEnabled()) {
logger.debug("Failed to convert bean '" + name + "' to required type '" +
ClassUtils.getQualifiedName(requiredType) + "'", ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
}