1. Bean scopesjava
Scope用來聲明 IoC 容器中對象應該處的限定場景或者說該對象的存活空間,即 IoC 容器在對象進入相應的 scope 以前生成並裝配這些對象,在該對象再也不處於 scope 的限定後,容器一般會銷燬這些對象。下面是 Scope的分類:web
Scope | Description |
singleton |
(Default) Scopes a single bean definition to a single object instance per spring Spring IoC container.bootstrap |
prototype | Scopes a single bean definition to any number of object instances. |
request | Scopes a single bean definition to the lifecycle of a single HTTP request;session that is, each HTTP request has its own instance of a bean created off the app back of a single bean definition. Only valid in the context of a web-aware ide Spring |
session | Scopes a single bean definition to the lifecycle of an HTTP Only valid in the context of a web-aware Spring |
global session |
Scopes a single bean definition to the lifecycle of a global HTTP Typically only valid when used in a portlet context. Only valid in the context of a web-aware Spring |
application | Scopes a single bean definition to the lifecycle of a in the context of a web-aware Spring |
singleton:單一實例,即一個容器中只存在一個這樣的實例,全部對該類型 Bean的依賴都是用這一單一實例 。此外,singleton 類型的 Bean 定義,從容器啓動,到它第一次被請求而實例化開始,只要容器不銷燬或者退出,該類型 Bean 的單一實例就會一直存活。
prototype:容器在接受到該類型對象的請求時,每次都會從新生成一個新的對象給請求方。但容器將對象實例返回給請求方後,就再也不擁有該對象的引用,請求方須要本身負責當前對象後繼生命週期的管理工做。
request、session、global session、application 只適用於 web程序,一般是和 XmlWebApplicationContext 共同使用。
request:Spring 容器,即 XmlWebApplicationContext 會爲每一個 HTTP 請求建立一個新的對象,當請求結束後,該對象的生命週期即結束。若是同時又10個 HTTP 請求,容器會分別針對這10個請求建立10個新的對象實例,且實例之間互不干擾。
session:Spring 容器會爲每一個獨立的 session 建立新的對象實例,比 request scope的 bean存活更長的時間。用戶登陸信息通常是用的最多。
global session:只有應用在基於porlet的web應用程序中才有意義,他映射到porlet的global範圍的session,若是普通的servlet的web 應用中使用了這個scope,容器會把它做爲普通的session的scope對待。
application:暫無。
對於singleton 和 prototype 能夠作個實驗:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"> <!-- <import resource="beans2.xml"/> --> <bean id="userDAO" class="com.dao.impl.UserDAOImpl" scope="singleton"/> <bean id="userService" class="com.service.impl.UserServiceImpl" scope="prototype"> <property name="userDAO" ref="userDAO" /> </bean> </beans>
測試 Code:
public class UserServiceTest { @Before public void setUp() throws Exception { } @Test public void testAddUser() throws Exception { ApplicationContext ctx = new ClassPathXmlApplicationContext("beans.xml"); UserDAOImpl dao1 = (UserDAOImpl) ctx.getBean("userDAO"); UserDAOImpl dao2 = (UserDAOImpl) ctx.getBean("userDAO"); UserServiceImpl service = (UserServiceImpl) ctx.getBean("userService"); UserServiceImpl service2 = (UserServiceImpl) ctx.getBean("userService"); System.out.println(dao1 == dao2); System.out.println(service == service2); } }
結果: true false
2. Bean 建立的時機:
IoC 容器初始化時會預先對非延遲加載的單例對象進行初始化,其餘都是在第一次調用 getBean 時被建立。
org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons()
public void preInstantiateSingletons() throws BeansException { if (this.logger.isDebugEnabled()) { this.logger.debug("Pre-instantiating singletons in " + this); } // Iterate over a copy to allow for init methods which in turn register new bean definitions. // While this may not be part of the regular factory bootstrap, it does otherwise work fine. List<String> beanNames = new ArrayList<String>(this.beanDefinitionNames); // Trigger initialization of all non-lazy singleton beans... for (String beanName : beanNames) { RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName); if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) { if (isFactoryBean(beanName)) { final FactoryBean<?> factory = (FactoryBean<?>) getBean(FACTORY_BEAN_PREFIX + beanName); boolean isEagerInit; if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) { isEagerInit = AccessController.doPrivileged(new PrivilegedAction<Boolean>() { @Override public Boolean run() { return ((SmartFactoryBean<?>) factory).isEagerInit(); } }, getAccessControlContext()); } else { isEagerInit = (factory instanceof SmartFactoryBean && ((SmartFactoryBean<?>) factory).isEagerInit()); } if (isEagerInit) { getBean(beanName); } } else { getBean(beanName); } } } // Trigger post-initialization callback for all applicable beans... for (String beanName : beanNames) { Object singletonInstance = getSingleton(beanName); if (singletonInstance instanceof SmartInitializingSingleton) { final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance; if (System.getSecurityManager() != null) { AccessController.doPrivileged(new PrivilegedAction<Object>() { @Override public Object run() { smartSingleton.afterSingletonsInstantiated(); return null; } }, getAccessControlContext()); } else { smartSingleton.afterSingletonsInstantiated(); } } } }
3. 延遲初始化 Bean
指在 IoC 容器啓動時,並不初始化 延遲初始化的 Bean,而是在使用時纔會建立以及初始化 Bean。
配置方式:在 <bean> 標籤上指定 lazy-init 屬性值爲 "true",如: <bean id="xxx" class="xxx" lazy-init="true" />
Spring 容器預先初始化 Bean 一般能幫助咱們提早發現配置錯誤,因此通常狀況下不建議開啓lazy-init。除非有某個 Bean 可能須要加載很大資源,並且在整個應用程序生命週期中極可能使用不到,那麼能夠設置爲 延遲初始化。
延遲初始化的 Bean 一般會在第一次使用時被初始化;或者在 被非延遲初始化 Bean 做爲依賴對象注入時,隨着該 Bean 的初始化而初始化 lazy-init Bean。
4. 自動裝配(Autowire)
Autowire 指由 spring 容器自動地注入依賴對象。
Mode | Explanation |
no |
(Default) No autowiring. Bean references must be defined via a Changing the default setting is not recommended for larger deployments, because specifying collaborators explicitly gives greater control and clarity. To some extent, it documents the structure of a system. |
byName | Autowiring by property name. Spring looks for a bean with the same name as the property that needs to be autowired. For example, if a bean definition is set to autowire by name, and it contains a master property (that is, it has a setMaster(..) method), Spring looks for a bean definition named and uses it to set the property. |
byType | Allows a property to be autowired if exactly one bean of the property type exists in the container. If more than one exists, a fatal exception is thrown, which indicates that you may not use byType autowiring for that bean. If there are no matching beans, nothing happens; the property is not set. |
constructor | Analogous to byType, but applies to constructor arguments. If there is not exactly one bean of the constructor argument type in the container, a fatal error is raised. |
目前 Spring 4.2.2 支持 "no","byName","byType","constructor" 四種自動裝配,默認是 "no" 指不支持自動裝配的。其中Spring 3.0 開始已經不推薦使用以前版本的 "autodetect" 自動裝配,推薦使用 Java 5+支持的(@Autowired)註解方式代替;自動裝配的好處是:減小構造器注入和setter注入配置,減小配置文件。
配置方式:經過配置<bean>標籤的「autowire」屬性來改變自動裝配方式
<bean id="userService" class="com.service.impl.UserServiceImpl" autowire="byType"> <!-- <property name="userDAO" ref="userDAO" /> --> </bean>
(1)default:表示使用默認的自動裝配,默認的自動裝配須要在<beans>標籤中使用default-autowire屬性指定,其支持「no」、「byName 」、「byType」、「constructor」四種自動裝配。
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd" default-autowire="byName">
(2)no:意思是不支持自動裝配,必須明確指定依賴。
(3)byName:經過設置Bean定義屬性autowire="byName",意思是根據名字進行自動裝配,只能用於setter注入。
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"> <!-- <import resource="beans2.xml"/> --> <bean id="userDAO" class="com.dao.impl.UserDAOImpl" > <property name="daoId" value="1"></property> </bean> <bean id="userDAO2" class="com.dao.impl.UserDAOImpl" > <property name="daoId" value="2"></property> </bean> <bean id="userService" class="com.service.impl.UserServiceImpl" autowire="byName"> <!-- <property name="userDAO" ref="userDAO" /> --> </bean> </beans>
public class UserServiceImpl implements UserService { private UserDAO userDAO; public UserDAO getUserDAO() { return userDAO; } public void setUserDAO(UserDAO userDAO) { this.userDAO = userDAO; } //constructor // public UserServiceImpl(UserDAO userDAO) { // super(); // this.userDAO = userDAO; // } @Override public void addUser(User user) { this.userDAO.saveUser(user); } }
根據 byName 自動裝配會去找 UserServiceImpl 中的 userDAO 這個name的 Bean,而配置文件中有 userDAO 和 userDAO2 兩個,其中裝配的是 userDAO。若是 userDAO 這個 Bean沒有配置,則會報錯。
(4)byType:經過設置Bean定義屬性autowire="byType",意思是指根據類型注入,用於setter注入。好比若是指定自動裝配方式爲「byType」,而 setUserDAO() 方法須要注入 UserDAO 類型數據,則Spring容器將查找 UserDAO 類Bean,若是找到一個則注入該Bean,若是找不到將什麼也不注入;若是找到多個Bean將優先注入<bean>標籤「primary」屬性爲 true 的 Bean,不然拋出異常來代表有個多個Bean發現但不知道使用哪一個。
(5)constructor:經過設置 Bean 定義屬性autowire="constructor",功能和「byType」功能同樣,根據類型注入構造器參數,只是用於構造器注入方式。
5. init-method 和 destroy-method
(1)init-method="init" :指定初始化方法,在構造器注入和setter注入完畢後執行。
(2)destroy-method="destroy":指定銷燬方法,只有「singleton」做用域能銷燬;「prototype」做用域的必定不能,由於容器不會監控 prototype Bean的死亡,推薦不要和 prototype 一塊兒使用;其餘做用域不必定能;
<bean id="userService" class="com.service.impl.UserServiceImpl" autowire="byName" init-method="init" destroy-method="destroy"> <!-- <property name="userDAO" ref="userDAO" /> --> </bean>