转自http://blog.arganzheng.me/posts/how-to-init-right-after-system-startup.html
背景
在我们的应用中，经常有这样的场景，就是需要在应用启动的时候就预先加载某些资源，如果加载失败，应用都不需要启动了。这些资源的加载，我们都会把他们封装在一个类的loadResource方法中。关键是这个方法如何在系统初始化的时候被调用。一般有两种方式。
法一：通过Spring触发loadResource方法
如果这个类是通过Spring加载的，那么可以让Spring初始化之后直接调用该类的loadResource方法。因为Spring框架基本都是系统启动的时候就加载的，而且对于ApplicationContext来说，其bean都是pre-loading的Lazy loading vs. pre-loading beans with Spring Framework。
方法有如下四种
1. Spring的@PostConstruct注解
这个注解会导致Spring在初始化完Bean之后，调用该bean的@PostConstruct注解的方法。系统启动时候加载Spring，Spring的ApplicationContext会pre-loading所有的bean，@PostConstruct会在bean loading完成后触发init方法。
@Component
public class MetadataService {
@Value("${metadata.source}")
private String source = SOURCE_YIXUN;
public static final String SOURCE_YIXUN = "yixun";
public static final String SOURCE_WANGGOU = "wanggou";
public static final String SOURCE_PAIPAI = "paipai";
private volatile ApiMetadataDTO apiMetadata = null;
private volatile Map<String, ApiDTO> apiMetadataMap = null;
@PostConstruct
public void loadMetadata() {
// 调用IDL得到最新元数据
if (apiMetadataMap == null) {
synchronized (this) {
if (apiMetadataMap == null) {
try {
ApiMetadataIdlClient apiMetadataIdlCilent = new ApiMetadataIdlClient(StringUtils.EMPTY, TIMEOUT);
apiMetadata = apiMetadataIdlCilent.getMetadata(source, true, true);
apiMetadataMap = transformToMap(apiMetadata);
if (CollectionUtils.isEmpty(apiMetadataMap)) {
logger.warn("apiMetadataMap for source " + source + " is Empty!");
} else {
logger.info("success loading api metadata to map! source: " + source + ",MapSize: "
+ apiMetadataMap.size());
}
} catch (Exception ex) {
logger.error("getMetadata failed for source: " + source, ex);
System.exit(-1); // 获取不到元数据，相当于废物一个，直接应用退出。
// throw new RuntimeException("getMetadata failed！", ex);
}
}
}
}
}
}
By default, Spring will not aware of the @PostConstruct and @PreDestroy annotation. To enable it, you have to either register
CommonAnnotationBeanPostProcessoror specify the<context:annotation-config />in bean configuration file,
2. Spring Bean的init-method
<bean class="me.arganzheng.study.api.service.MetadataService" init-method="loadMetadata" />
这个跟上面的@PostConstruct方式一样，前提是你的bean是通过xml配置的。
3. Spring的InitializingBean接口
@Component
public class MetadataService implements InitializingBean {
@Value("${metadata.source}")
private String source = SOURCE_YIXUN;
/**
* 元数据Map。key: methodName
*/
private volatile ApiMetadataDTO apiMetadata = null;
private volatile Map<String, ApiDTO> apiMetadataMap = null;
public void loadMetadata() {
// 调用IDL得到最新元数据
if (apiMetadataMap == null) {
synchronized (this) {
if (apiMetadataMap == null) {
try {
ApiMetadataIdlClient apiMetadataIdlCilent = new ApiMetadataIdlClient(StringUtils.EMPTY, TIMEOUT);
apiMetadata = apiMetadataIdlCilent.getMetadata(source, true, true);
apiMetadataMap = transformToMap(apiMetadata);
if (CollectionUtils.isEmpty(apiMetadataMap)) {
logger.warn("apiMetadataMap for source " + source + " is Empty!");
} else {
logger.info("success loading api metadata to map! source: " + source + ",MapSize: "
+ apiMetadataMap.size());
}
} catch (Exception ex) {
logger.error("getMetadata failed for source: " + source, ex);
System.exit(-1); // 获取不到元数据，相当于废物一个，直接应用退出。
// throw new RuntimeException("getMetadata failed！", ex);
}
}
}
}
}
@Override
public void afterPropertiesSet() throws Exception {
loadMetadata();
}
}
4. 通过BeanPostProcessor接口
这个其实就是上面@PostConstruct注解的底层实现机制，Spring通过CommonAnnotationBeanPostProcessor来实现@PostConstruct和@PreDestroy：
org.springframework.beans.factory.config.BeanPostProcessorimplementation that supports common Java annotations out of the box, in particular the JSR-250 annotations in the javax.annotation package. These common Java annotations are supported in many Java EE 5 technologies (e.g. JSF 1.2), as well as in Java 6's JAX-WS.This post-processor includes support for the
javax.annotation.PostConstructandjavax.annotation.PreDestroyannotations - as init annotation and destroy annotation, respectively - through inheriting fromInitDestroyAnnotationBeanPostProcessorwith pre-configured annotation types.
public class InitMetadataServicePostProcessor implements BeanPostProcessor {
@Override
public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
return bean;
}
@Override
public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
if (bean instanceof MetadataService) {
System.out.println("postProcessAfterInitialization with MetadataService!");
MetadataService metaDataService = (MetadataService) bean;
metaDataService.loadMetadata();
}
return bean;
}
}
需要在xml中注册一下：
<bean class="me.arganzheng.study.api.tools.common.InitMetadataServicePostProcessor" />
TIPS Spring的注解和AOP基本都是通过实现BeanPostProcessor接口完成的，可以在bean初始化之后做一些事情。
法二：通过servlet来完成资源初始化
如果你的bean不是通过Spring容器加载（一般情况下是通过static块来执行资源加载，但是加载的时间是这个类第一次被引用的时候（即被classloader加载的时候）），那么可以通过servlet来完成资源初始化。因为web容器在启动的时候会加载servlet，可以通过servlet来完成应用启动时的资源加载。（其实法一的Spring框架就是通过servlet/ContextListener完成加载的）。
public class MetadataService {
private volatile ApiMetadataDTO apiMetadata = null;
private volatile Map<String, ApiDTO> apiMetadataMap = null;
private volatile Map<String, Service> idlMetadataMap = null;
private MetadataService(String source){
checkUpdate(source);
}
private static MetadataService instance = new MetadataService(SOURCE_YIXUN);
public static MetadataService getInstance() {
return instance;
}
private void checkUpdate(String source) {
if (apiMetadataMap == null || idlMetadataMap == null) {
synchronized (this) {
if (apiMetadataMap == null || idlMetadataMap == null) {
try {
ApiMetadataIdlClient apiMetadataIdlCilent = new ApiMetadataIdlClient(StringUtils.EMPTY, TIMEOUT);
IdlMetadataIdlClient idlMetadataIdlClient = new IdlMetadataIdlClient(StringUtils.EMPTY, TIMEOUT);
apiMetadata = apiMetadataIdlCilent.getMetadata(source, true, true);
apiMetadataMap = transformToMap(apiMetadata);
if (CollectionUtils.isEmpty(apiMetadataMap)) {
logger.warn("apiMetadataMap is Empty!");
} else {
logger.info("success loading api metadata to map! MapSize: " + apiMetadataMap.size());
}
GetIdlMetadataResp idlMetadata = idlMetadataIdlClient.getMetadata(source, 0, Integer.MAX_VALUE);
idlMetadataMap = trasformToMap(idlMetadata);
if (CollectionUtils.isEmpty(idlMetadataMap)) {
logger.warn("idlMetadataMap is Empty!");
} else {
logger.info("success loading idl metadata to map! MapSize: " + idlMetadataMap.size());
}
} catch (Exception ex) {
logger.error("getMetadata failed for source: " + source, ex);
throw new RuntimeException("getMetadata failed！", ex);
}
}
}
}
}
}
