Spring中Bean的作用域Scope你知道多少？如何自定義作用域？
作者：Springboot實戰案例錦集
如果當前你配置的@Scope不是singleton及prototype那么從scopes集合中取（這個集合是通過AbstractBeanFactory#registerScope方法進行注冊的，一般我們可以通過BeanDefinitionRegistryPostProcessor進行注冊），如果集合中也不存在那么就會拋出異常。如果存在就會執行Scope#get方法。
1 Scope作用
通過@Scope注解可以指定Bean的作用域，默認情況都是單例的（ConfigurableBeanFactory.SCOPE_SINGLETON=singleton）
在創建bean實例時就是根據當前定義BeanDefinition中的Scope來做不同的創建，源碼如下：
protected <T> T doGetBean(
String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
throws BeansException {
String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
// other code
} else {
// other code
try {
RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
// other code
// Create bean instance.
// 根據BeanDefinition中定義的Scope創建實例
// 判斷如果是單例
if (mbd.isSingleton()) {
// 如果是單例Bean會將Bean保存到緩存中singletonObjects
sharedInstance = getSingleton(beanName, () -> {
try {
return createBean(beanName, mbd, args);
} catch (BeansException ex) {
destroySingleton(beanName);
throw ex;
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
// 判斷如果是原型（多例）
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();
if (!StringUtils.hasLength(scopeName)) {
throw new IllegalStateException("No scope name defined for bean 麓" + beanName + "'");
}
Scope scope = this.scopes.get(scopeName);
// 當集合中也不存在時拋出異常
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;
}
}
// other code
return (T) bean;
}從上面源碼看到分別判斷是了 是否是 Singleton及Proptotype，如果都不是則會從Map<String, Scope> scopes中獲取。如果當前你配置的@Scope不是singleton及prototype那么從scopes集合中取（這個集合是通過AbstractBeanFactory#registerScope方法進行注冊的，一般我們可以通過BeanDefinitionRegistryPostProcessor進行注冊），如果集合中也不存在那么就會拋出異常。如果存在就會執行Scope#get方法。
Scope scope = this.scopes.get(scopeName);
Object scopedInstance = scope.get(beanName, () -> {
beforePrototypeCreation(beanName);
try {
return createBean(beanName, mbd, args);
} finally {
afterPrototypeCreation(beanName);
}
});2 自定義Scope
自定義Scope
public class CustomScope implements Scope {
private Object target ;
@Override
public Object get(String name, ObjectFactory<?> objectFactory) {
return target != null ? target : objectFactory.getObject() ;
}
// 如果調用了這個方法，那么下次在注入有@Scope("custom")的bean時 將會重寫調用objectFactory.getObject()方法。
@Override
public Object remove(String name) {
target = null ;
return "success" ;
}
@Override
public void registerDestructionCallback(String name, Runnable callback) {
}
@Override
public Object resolveContextualObject(String key) {
return null;
}
@Override
public String getConversationId() {
return null;
}
}注冊Scope
@Component
public class CustomScopeRegistryPostProcessor implements BeanDefinitionRegistryPostProcessor {
@Override
public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
beanFactory.registerScope("custom", new CustomScope()) ;
}
@Override
public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException {
}
}使用Scope
@Component
@Scope("custom")
public class ApplyScopeBean {
}示例
@RestController
@RequestMapping("/refresh")
@Scope(value = ConfigurableBeanFactory.SCOPE_PROTOTYPE)
public class RefreshController implements ApplicationContextAware{
@Resource
private ApplyScopeBean scopeBean ;
@Resource
private CustomScope customScope ;
@GetMapping("/custom")
public String custom() {
return scopeBean.getCustom() ;
}
@GetMapping("/remove")
public Object remove() {
return customScope.remove("applyScopeBean") ;
}
}這里將Controller設置為多例，以便查看效果。交替執行上面的接口，只要刪除了就會創建新的實例。
3 多例注入
如果一個Bean 設置了@Scope(value =ConfigurableBeanFactory.SCOPE_PROTOTYPE) 當這個Bean需要在一個單例Bean中被注入時，需要如下配置才可
@Component
@Scope(value = ConfigurableBeanFactory.SCOPE_PROTOTYPE, proxyMode = ScopedProxyMode.TARGET_CLASS)
public class ApplyScopeBean {
}這樣才能正確地注入Bean，否則因為本身使用者是單例的，屬性只會被初始化一次。也可以在每次使用前調用BeanFactory#getBean()。
責任編輯：武曉燕
來源：
Spring全家桶實戰案例源碼
