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這篇文章將為大家詳細(xì)講解有關(guān)SpringBoot留給開發(fā)者的7個(gè)啟動(dòng)擴(kuò)展點(diǎn)分別是怎樣的,文章內(nèi)容質(zhì)量較高,因此小編分享給大家做個(gè)參考,希望大家閱讀完這篇文章后對相關(guān)知識有一定的了解。
我們經(jīng)常需要在容器啟動(dòng)的時(shí)候做一些鉤子動(dòng)作,比如注冊消息消費(fèi)者,監(jiān)聽配置等,今天就總結(jié)下SpringBoot留給開發(fā)者的7個(gè)啟動(dòng)擴(kuò)展點(diǎn)。
基本用法
熟悉Spring的同學(xué)一定知道,容器刷新成功意味著所有的Bean初始化已經(jīng)完成,當(dāng)容器刷新之后Spring將會(huì)調(diào)用容器內(nèi)所有實(shí)現(xiàn)了ApplicationListener<ContextRefreshedEvent>的Bean的onApplicationEvent方法,應(yīng)用程序可以以此達(dá)到監(jiān)聽容器初始化完成事件的目的。
@Component public class StartupApplicationListenerExample implements ApplicationListener<ContextRefreshedEvent> { private static final Logger LOG = Logger.getLogger(StartupApplicationListenerExample.class); public static int counter; @Override public void onApplicationEvent(ContextRefreshedEvent event) { LOG.info("Increment counter"); counter++; } }
易錯(cuò)的點(diǎn)
這個(gè)擴(kuò)展點(diǎn)用在web容器中的時(shí)候需要額外注意,在web 項(xiàng)目中(例如spring mvc),系統(tǒng)會(huì)存在兩個(gè)容器,一個(gè)是root application context,另一個(gè)就是我們自己的context(作為root application context的子容器)。如果按照上面這種寫法,就會(huì)造成onApplicationEvent方法被執(zhí)行兩次。解決此問題的方法如下:
@Component public class StartupApplicationListenerExample implements ApplicationListener<ContextRefreshedEvent> { private static final Logger LOG = Logger.getLogger(StartupApplicationListenerExample.class); public static int counter; @Override public void onApplicationEvent(ContextRefreshedEvent event) { if (event.getApplicationContext().getParent() == null) { // root application context 沒有parent LOG.info("Increment counter"); counter++; } } }
高階玩法
當(dāng)然這個(gè)擴(kuò)展還可以有更高階的玩法:自定義事件,可以借助Spring以最小成本實(shí)現(xiàn)一個(gè)觀察者模式:
先自定義一個(gè)事件:
public class NotifyEvent extends ApplicationEvent { private String email; private String content; public NotifyEvent(Object source) { super(source); } public NotifyEvent(Object source, String email, String content) { super(source); this.email = email; this.content = content; } // 省略getter/setter方法 }
注冊一個(gè)事件監(jiān)聽器
@Component public class NotifyListener implements ApplicationListener<NotifyEvent> { @Override public void onApplicationEvent(NotifyEvent event) { System.out.println("郵件地址:" + event.getEmail()); System.out.println("郵件內(nèi)容:" + event.getContent()); } }
發(fā)布事件
@RunWith(SpringRunner.class) @SpringBootTest public class ListenerTest { @Autowired private WebApplicationContext webApplicationContext; @Test public void testListener() { NotifyEvent event = new NotifyEvent("object", "abc@qq.com", "This is the content"); webApplicationContext.publishEvent(event); } }
執(zhí)行單元測試可以看到郵件的地址和內(nèi)容都被打印出來了
當(dāng)容器上下文初始化完成之后,SpringBoot也會(huì)調(diào)用所有實(shí)現(xiàn)了CommandLineRunner接口的run方法,下面這段代碼可起到和上文同樣的作用:
@Component public class CommandLineAppStartupRunner implements CommandLineRunner { private static final Logger LOG = LoggerFactory.getLogger(CommandLineAppStartupRunner.class); public static int counter; @Override public void run(String...args) throws Exception { LOG.info("Increment counter"); counter++; } }
對于這個(gè)擴(kuò)展點(diǎn)的使用有額外兩點(diǎn)需要注意:
多個(gè)實(shí)現(xiàn)了CommandLineRunner的Bean的執(zhí)行順序可以根據(jù)Bean上的@Order注解調(diào)整
其run方法可以接受從控制臺輸入的參數(shù),跟ApplicationListener<ContextRefreshedEvent>這種擴(kuò)展相比,更加靈活
// 從控制臺輸入?yún)?shù)示例 java -jar CommandLineAppStartupRunner.jar abc abcd
這個(gè)擴(kuò)展和SpringBoot的CommandLineRunner接口的擴(kuò)展類似,只不過接受的參數(shù)是一個(gè)ApplicationArguments類,對控制臺輸入的參數(shù)提供了更好的封裝,以--開頭的被視為帶選項(xiàng)的參數(shù),否則是普通的參數(shù)
@Component public class AppStartupRunner implements ApplicationRunner { private static final Logger LOG = LoggerFactory.getLogger(AppStartupRunner.class); public static int counter; @Override public void run(ApplicationArguments args) throws Exception { LOG.info("Application started with option names : {}", args.getOptionNames()); LOG.info("Increment counter"); counter++; } }
比如:
java -jar CommandLineAppStartupRunner.jar abc abcd --autho=mark verbose
前面的內(nèi)容總結(jié)了針對容器初始化的擴(kuò)展點(diǎn),在有些場景,比如監(jiān)聽消息的時(shí)候,我們希望Bean初始化完成之后立刻注冊監(jiān)聽器,而不是等到整個(gè)容器刷新完成,Spring針對這種場景同樣留足了擴(kuò)展點(diǎn):
@PostConstruct注解一般放在Bean的方法上,被@PostConstruct修飾的方法會(huì)在Bean初始化后馬上調(diào)用: @Component public class PostConstructExampleBean { private static final Logger LOG = Logger.getLogger(PostConstructExampleBean.class); @Autowired private Environment environment; @PostConstruct public void init() { LOG.info(Arrays.asList(environment.getDefaultProfiles())); } }
InitializingBean的用法基本上與@PostConstruct一致,只不過相應(yīng)的Bean需要實(shí)現(xiàn)afterPropertiesSet方法
@Component public class InitializingBeanExampleBean implements InitializingBean { private static final Logger LOG = Logger.getLogger(InitializingBeanExampleBean.class); @Autowired private Environment environment; @Override public void afterPropertiesSet() throws Exception { LOG.info(Arrays.asList(environment.getDefaultProfiles())); } }
通過@Bean注入Bean的時(shí)候可以指定初始化方法:
Bean的定義
public class InitMethodExampleBean { private static final Logger LOG = Logger.getLogger(InitMethodExampleBean.class); @Autowired private Environment environment; public void init() { LOG.info(Arrays.asList(environment.getDefaultProfiles())); } }
Bean注入
@Bean(initMethod="init") public InitMethodExampleBean initMethodExampleBean() { return new InitMethodExampleBean(); }
Spring也支持通過構(gòu)造函數(shù)注入,我們可以把搞事情的代碼寫在構(gòu)造函數(shù)中,同樣能達(dá)到目的
@Component public class LogicInConstructorExampleBean { private static final Logger LOG = Logger.getLogger(LogicInConstructorExampleBean.class); private final Environment environment; @Autowired public LogicInConstructorExampleBean(Environment environment) { this.environment = environment; LOG.info(Arrays.asList(environment.getDefaultProfiles())); } }
可以用一個(gè)簡單的測試:
@Component @Scope(value = "prototype") public class AllStrategiesExampleBean implements InitializingBean { private static final Logger LOG = Logger.getLogger(AllStrategiesExampleBean.class); public AllStrategiesExampleBean() { LOG.info("Constructor"); } @Override public void afterPropertiesSet() throws Exception { LOG.info("InitializingBean"); } @PostConstruct public void postConstruct() { LOG.info("PostConstruct"); } public void init() { LOG.info("init-method"); } }
實(shí)例化這個(gè)Bean后輸出:
[main] INFO o.b.startup.AllStrategiesExampleBean - Constructor [main] INFO o.b.startup.AllStrategiesExampleBean - PostConstruct [main] INFO o.b.startup.AllStrategiesExampleBean - InitializingBean [main] INFO o.b.startup.AllStrategiesExampleBean - init-method
關(guān)于SpringBoot留給開發(fā)者的7個(gè)啟動(dòng)擴(kuò)展點(diǎn)分別是怎樣的就分享到這里了,希望以上內(nèi)容可以對大家有一定的幫助,可以學(xué)到更多知識。如果覺得文章不錯(cuò),可以把它分享出去讓更多的人看到。
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