這篇文章主要介紹“怎么理解spring5的@Indexed解密”，在日常操作中，相信很多人在怎么理解spring5的@Indexed解密問題上存在疑惑，小編查閱了各式資料，整理出簡單好用的操作方法，希望對大家解答”怎么理解spring5的@Indexed解密”的疑惑有所幫助！接下來，請跟著小編一起來學習吧！
隨著云原生的發展，很多技術會被重新掂量，重新定義，歷來技術的發展也是遵循天時地利，以其勢盡享其利。再云原生下，jdk的最大的問題在于笨重（幾百mb），啟動慢，而像Serverless架構，NodeJS技術?？芍^更完美。
其實在jdk9中倡導模塊化本質在于減少JVM的體積，不需要資源（Jar）不用再加載，而啟動慢的問題其實也有解決方案GraalVM （一款類似于HotSpot VM），它的先進之處在于縮短運行的成本將.java文件直接編譯成native code，而jvm則多了一個環節，首先將.java文件編譯成字節碼(.class)，再借助JVM運行時JIT技術編譯成native code。
spring5.0開始支持@Indexed來提升進應用啟動速度，通過Annotation Processing Tools API在編譯時來構建索引文件，本質是通過靜態化來解決啟動時Bean掃描加載的時間長的問題。
what is Annotation Processing Tools API？
不是什么黑科技，之前的系列也講過，有點類似lombok。
默認支持標記為Component及其派生注解（Controller、Repository、Service、Configuration等）的類，當然也可以是非spring bean（@Indexed修飾的類）。
注：如果已經是spring bean（Component修飾的類，并且Component已經被標記為@Indexed）了就沒必要再標記@Indexed，否則索引文件會再追加一個相同的，感覺這是個bug
使用非常講的，添加依賴就可以了，install后默認會生成一個META-INF/spring.components。
<dependency> <groupId>org.springframework</groupId> <artifactId>spring-context-indexer</artifactId> <optional>true</optional> </dependency>
#spring.components com.yh.rfe.lucky.day.service.impl.BasCostReportServiceImpl=org.springframework.stereotype.Component com.yh.rfe.lucky.day.service.impl.BasShopRuleDetailServiceImpl=org.springframework.stereotype.Component
而CandidateComponentsIndexer負責對符合條件的注解生成索引文件，整個源碼也不是特別復雜，通過三個組件：StereotypesProvider、MetadataCollector、MetadataStore來完成。
public class CandidateComponentsIndexer implements Processor {
@Override
public synchronized void init(ProcessingEnvironment env) {
this.stereotypesProviders = getStereotypesProviders(env);
this.typeHelper = new TypeHelper(env);
this.metadataStore = new MetadataStore(env);
this.metadataCollector = new MetadataCollector(env, this.metadataStore.readMetadata());
}
@Override
public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
this.metadataCollector.processing(roundEnv);
roundEnv.getRootElements().forEach(this::processElement);
if (roundEnv.processingOver()) {
writeMetaData();
}
return false;
}
}
//定義了哪些注解需要被索引
interface StereotypesProvider {
/**
* Return the stereotypes that are present on the given {@link Element}.
* @param element the element to handle
* @return the stereotypes or an empty set if none were found
*/
Set<String> getStereotypes(Element element);
}
//獲取需要被索引的CandidateComponentsMetadata（元數據）
class MetadataCollector {
public CandidateComponentsMetadata getMetadata() {
CandidateComponentsMetadata metadata = new CandidateComponentsMetadata();
for (ItemMetadata item : this.metadataItems) {
metadata.add(item);
}
if (this.previousMetadata != null) {
List<ItemMetadata> items = this.previousMetadata.getItems();
for (ItemMetadata item : items) {
if (shouldBeMerged(item)) {
metadata.add(item);
}
}
}
return metadata;
}
}
//將上面的結果輸出到spring.components中
class MetadataStore {
static final String METADATA_PATH = "META-INF/spring.components";
public void writeMetadata(CandidateComponentsMetadata metadata) throws IOException {
if (!metadata.getItems().isEmpty()) {
try (OutputStream outputStream = createMetadataResource().openOutputStream()) {
PropertiesMarshaller.write(metadata, outputStream);
}
}
}
}其實在spring boot項目中絕對存在ComponentScan(在SpringBootApplication中)，而傳統的spring項目中xml中對應<context:component-scan>，通過指定的 package（路徑）來掃描注入spring bean，在掃描時通過讀取spring.components文件來讀取class（類全路徑）從而達到提升速度的目的。
CandidateComponentsIndex存儲了spring.components文件的內容
public class CandidateComponentsIndex {
private static final AntPathMatcher pathMatcher = new AntPathMatcher(".");
private final MultiValueMap<String, Entry> index;
/*返回指定的注解類型和包路徑相關候選類型
* Set<String> candidates = index.getCandidateTypes("com.example", "org.springframework.stereotype.Component");
*/
public Set<String> getCandidateTypes(String basePackage, String stereotype) {
List<Entry> candidates = this.index.get(stereotype);
if (candidates != null) {
return candidates.parallelStream()
.filter(t -> t.match(basePackage))
.map(t -> t.type)
.collect(Collectors.toSet());
}
return Collections.emptySet();
}
}CandidateComponentsIndexLoader從classloader中讀取，可以從多個jar中讀取多個索引文件。
public final class CandidateComponentsIndexLoader {
public static final String COMPONENTS_RESOURCE_LOCATION = "META-INF/spring.components";
private static final ConcurrentMap<ClassLoader, CandidateComponentsIndex> cache =
new ConcurrentReferenceHashMap<>();
@Nullable
public static CandidateComponentsIndex loadIndex(@Nullable ClassLoader classLoader) {
ClassLoader classLoaderToUse = classLoader;
if (classLoaderToUse == null) {
classLoaderToUse = CandidateComponentsIndexLoader.class.getClassLoader();
}
return cache.computeIfAbsent(classLoaderToUse, CandidateComponentsIndexLoader::doLoadIndex);
}
@Nullable
private static CandidateComponentsIndex doLoadIndex(ClassLoader classLoader) {
if (shouldIgnoreIndex) {
return null;
}
try {
Enumeration<URL> urls = classLoader.getResources(COMPONENTS_RESOURCE_LOCATION);
if (!urls.hasMoreElements()) {
return null;
}
List<Properties> result = new ArrayList<>();
while (urls.hasMoreElements()) {
URL url = urls.nextElement();
Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));
result.add(properties);
}
if (logger.isDebugEnabled()) {
logger.debug("Loaded " + result.size() + "] index(es)");
}
int totalCount = result.stream().mapToInt(Properties::size).sum();
return (totalCount > 0 ? new CandidateComponentsIndex(result) : null);
}
catch (IOException ex) {
throw new IllegalStateException("Unable to load indexes from location [" +
COMPONENTS_RESOURCE_LOCATION + "]", ex);
}
}
}ClassPathBeanDefinitionScanner非常重要，它就是spring 中scan時干最臟最累的活的終結者。而ClassPathScanningCandidateComponentProvider非常重要可以視為scan的頂級實現類。
其中ClassPathMapperScanner是mybatis的mapper掃描類。
public class ClassPathBeanDefinitionScanner extends ClassPathScanningCandidateComponentProvider {
public int scan(String... basePackages) {
int beanCountAtScanStart = this.registry.getBeanDefinitionCount();
doScan(basePackages);
// Register annotation config processors, if necessary.
if (this.includeAnnotationConfig) {
AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
}
return (this.registry.getBeanDefinitionCount() - beanCountAtScanStart);
}
protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
Assert.notEmpty(basePackages, "At least one base package must be specified");
Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<>();
for (String basePackage : basePackages) {
Set<BeanDefinition> candidates = findCandidateComponents(basePackage);//看這里吧
for (BeanDefinition candidate : candidates) {
ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
candidate.setScope(scopeMetadata.getScopeName());
String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
if (candidate instanceof AbstractBeanDefinition) {
postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName);
}
if (candidate instanceof AnnotatedBeanDefinition) {
AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate);
}
if (checkCandidate(beanName, candidate)) {
BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
definitionHolder =
AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
beanDefinitions.add(definitionHolder);
registerBeanDefinition(definitionHolder, this.registry);
}
}
}
return beanDefinitions;
}
}
public class ClassPathScanningCandidateComponentProvider implements EnvironmentCapable, ResourceLoaderAware {
private MetadataReaderFactory metadataReaderFactory;//這個之前講過類元數據讀取
private CandidateComponentsIndex componentsIndex;//前面講過
public Set<BeanDefinition> findCandidateComponents(String basePackage) {
if (this.componentsIndex != null && indexSupportsIncludeFilters()) {
return addCandidateComponentsFromIndex(this.componentsIndex, basePackage);
}
else {
return scanCandidateComponents(basePackage);
}
}
private Set<BeanDefinition> addCandidateComponentsFromIndex(CandidateComponentsIndex index, String basePackage) {
Set<BeanDefinition> candidates = new LinkedHashSet<>();
try {
Set<String> types = new HashSet<>();
for (TypeFilter filter : this.includeFilters) {
String stereotype = extractStereotype(filter);
if (stereotype == null) {
throw new IllegalArgumentException("Failed to extract stereotype from "+ filter);
}
types.addAll(index.getCandidateTypes(basePackage, stereotype));
}
boolean traceEnabled = logger.isTraceEnabled();
boolean debugEnabled = logger.isDebugEnabled();
for (String type : types) {
MetadataReader metadataReader = getMetadataReaderFactory().getMetadataReader(type);
if (isCandidateComponent(metadataReader)) {
AnnotatedGenericBeanDefinition sbd = new AnnotatedGenericBeanDefinition(
metadataReader.getAnnotationMetadata());
if (isCandidateComponent(sbd)) {
if (debugEnabled) {
logger.debug("Using candidate component class from index: " + type);
}
candidates.add(sbd);
}
else {
if (debugEnabled) {
logger.debug("Ignored because not a concrete top-level class: " + type);
}
}
}
else {
if (traceEnabled) {
logger.trace("Ignored because matching an exclude filter: " + type);
}
}
}
}
catch (IOException ex) {
throw new BeanDefinitionStoreException("I/O failure during classpath scanning", ex);
}
return candidates;
}
}AnnotationConfigApplicationContext#scan你一定不陌生吧，這可是開發用戶級的API，其實它的scanner就是ClassPathBeanDefinitionScanner
public class AnnotationConfigApplicationContext extends GenericApplicationContext implements AnnotationConfigRegistry {
private final AnnotatedBeanDefinitionReader reader;
private final ClassPathBeanDefinitionScanner scanner;
public AnnotationConfigApplicationContext() {
this.reader = new AnnotatedBeanDefinitionReader(this);
this.scanner = new ClassPathBeanDefinitionScanner(this);
}
public AnnotationConfigApplicationContext(String... basePackages) {
this();
scan(basePackages);
refresh();
}
public void register(Class<?>... annotatedClasses) {
Assert.notEmpty(annotatedClasses, "At least one annotated class must be specified");
this.reader.register(annotatedClasses);
}
}其實關于@Indexed個人覺得實現上還是有一定局限性（只是針對當前maven的一個module，換言之是基于jar的），要基于當前整個工程文件特別是org.springframework包（這個下面有很多待加載到ioc的bean的jar）工作量還是不少的，官方還沒考慮吧。
到此，關于“怎么理解spring5的@Indexed解密”的學習就結束了，希望能夠解決大家的疑惑。理論與實踐的搭配能更好的幫助大家學習，快去試試吧！若想繼續學習更多相關知識，請繼續關注億速云網站，小編會繼續努力為大家帶來更多實用的文章！
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