Spring--原理--@Component

桃扇骨 2023-02-14 10:06 32阅读 0赞

原文网址：[Spring--原理--@Component\_IT利刃出鞘的博客-CSDN博客][Spring--_--_Component_IT_-CSDN]

# **其他网址** #

> [Spring注解Component原理源码解析 - Griez - 博客园][Spring_Component_ - Griez -]

# **简介** #

> 我们经常使用Spring的@Component、@Service、@Repository以及 @Controller等注解来实现bean托管给Spring容器管理。Spring是怎么样实现的呢？我们一起跟着源码看看整个过程吧！
> 
> 先看调用时序图：
> 
> ![format_png][]
> 
> 概述@Component到Spring bean容器管理过程。
> 
> 第一步：初始化时设置了Component类型过滤器；  
> 第二步：根据指定扫描包扫描.class文件，生成Resource对象；  
> 第三步：解析.class文件并注解归类，生成MetadataReader对象；  
> 第四步：使用第一步的注解过滤器过滤出有@Component类；  
> 第五步：生成BeanDefinition对象；  
> 第六步：把BeanDefinition注册到Spring容器。  
> 以上是@Component注解原理，@Service、@Controller和@Repository上都有@Component修饰，所以原理是一样的。

## **第1步：**扫描指定包下的文件 ##

> 对应时序图方法1，ClassPathBeanDefinitionScanner\#scan。交给ClassPathBeanDefinitionScanner处理。

>     public AnnotationConfigApplicationContext(String... basePackages) {
>        this();
>        scan(basePackages);
>        refresh();
>     }

> Spring启动时，会去扫描指定包下的文件。

>     public void scan(String... basePackages) {
>        Assert.notEmpty(basePackages, "At least one base package must be specified");
>        this.scanner.scan(basePackages);
>     }

## **第2步：**把class注册到容器 ##

> 对应时序图方法2，ClassPathBeanDefinitionScanner\#doScan。该方法对包下class文件解析，若类符合Spring容器管理条件，则生成BeanDefinition，并注册到容器中。

> ClassPathBeanDefinitionScanner 初始化时设置了注解过滤器

>     public ClassPathBeanDefinitionScanner(BeanDefinitionRegistry registry, boolean useDefaultFilters,Environment environment, @Nullable ResourceLoader resourceLoader) {
>     	Assert.notNull(registry, "BeanDefinitionRegistry must not be null");
>     	this.registry = registry;
>     	if (useDefaultFilters) {
>         // 注册注解过滤器
>     		registerDefaultFilters();
>     	}
>     	setEnvironment(environment);
>     	setResourceLoader(resourceLoader);
>     }

>     protected void registerDefaultFilters() {
>        // 添加Component类型
>        this.includeFilters.add(new AnnotationTypeFilter(Component.class));
>        ClassLoader cl = ClassPathScanningCandidateComponentProvider.class.getClassLoader();
>        try {
>           this.includeFilters.add(new AnnotationTypeFilter(
>                 ((Class<? extends Annotation>) ClassUtils.forName("javax.annotation.ManagedBean", cl)), false));
>        }
>        catch (ClassNotFoundException ex) {
>        }
>        try {
>           this.includeFilters.add(new AnnotationTypeFilter(
>                 ((Class<? extends Annotation>) ClassUtils.forName("javax.inject.Named", cl)), false));
>        }
>        catch (ClassNotFoundException ex) {
>        }
>     }

> 在includeFilters添加了Component，ManagedBean两种注解类型。后面用来过滤加载到的class文件是否需要交给Spring容器管理。

>     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) {
>           // 扫描包下有Spring Component注解，并且生成BeanDefinition
>           Set<BeanDefinition> candidates = findCandidateComponents(basePackage);
>           for (BeanDefinition candidate : candidates) {
>              // 设置scope，默认是singleton
>              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);
>                 // 生成代理类信息
>                 definitionHolder = AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
>                 beanDefinitions.add(definitionHolder);
>                 // 注册到Spring容器
>                 registerBeanDefinition(definitionHolder, this.registry);
>              }
>           }
>        }
>        return beanDefinitions;
>     }

## **第3步：返回**有@Component封装BeanDefinition列表 ##

对应时序图方法3，ClassPathScanningCandidateComponentProvider\#findCandidateComponents。

> 扫描包下的class文件，把有Component注解的封装BeanDefinition列表返回。

>     public Set<BeanDefinition> findCandidateComponents(String basePackage) {
>        if (this.componentsIndex != null && indexSupportsIncludeFilters()) {
>           return addCandidateComponentsFromIndex(this.componentsIndex, basePackage);
>        }
>        else {
>           return scanCandidateComponents(basePackage);
>        }
>     }

对应时序图方法4，ClassPathScanningCandidateComponentProvider\#scanCandidateComponents。

>     private Set<BeanDefinition> scanCandidateComponents(String basePackage) {
>        Set<BeanDefinition> candidates = new LinkedHashSet<>();
>        try {
>           // classpath*:basePackage/**/*.class
>           String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX +
>                 resolveBasePackage(basePackage) + '/' + this.resourcePattern;
>           // 获取 basePackage 包下的 .class 文件资源
>           Resource[] resources = getResourcePatternResolver().getResources(packageSearchPath);
>           for (Resource resource : resources) {
>              // 判断是否可读
>              if (resource.isReadable()) {
>                 try {
>                    // 获取.class文件类信息
>                    MetadataReader metadataReader = getMetadataReaderFactory().getMetadataReader(resource);
>                    if (isCandidateComponent(metadataReader)) {
>                       ScannedGenericBeanDefinition sbd = new ScannedGenericBeanDefinition(metadataReader);
>                       sbd.setResource(resource);
>                       sbd.setSource(resource);
>                       if (isCandidateComponent(sbd)) {
>                          candidates.add(sbd);
>                       }
>                    }
>                 } catch (Throwable ex) {
>                    throw new BeanDefinitionStoreException("Failed to read candidate component class: " + resource, ex);
>                 }
>              }
>           }
>        }
>        catch (IOException ex) {
>           throw new BeanDefinitionStoreException("I/O failure during classpath scanning", ex);
>        }
>        return candidates;
>     }

## 第4步：获得元数据读取器 ##

对应时序图方法5，CachingMetadataReaderFactory\#getMetadataReader。 super.getMetadataReader(resource) 调用的是 SimpleMetadataReaderFactory\#getMetadataReader。

>     public MetadataReader getMetadataReader(Resource resource) throws IOException {
>        if (this.metadataReaderCache instanceof ConcurrentMap) {
>           // No synchronization necessary...
>           MetadataReader metadataReader = this.metadataReaderCache.get(resource);
>           if (metadataReader == null) {
>              // 获取.class类元信息
>              metadataReader = super.getMetadataReader(resource);
>              this.metadataReaderCache.put(resource, metadataReader);
>           }
>           return metadataReader;
>        }
>        else if (this.metadataReaderCache != null) {
>           synchronized (this.metadataReaderCache) {
>              MetadataReader metadataReader = this.metadataReaderCache.get(resource);
>              if (metadataReader == null) {
>                 metadataReader = super.getMetadataReader(resource);
>                 this.metadataReaderCache.put(resource, metadataReader);
>              }
>              return metadataReader;
>           }
>        }
>        else {
>           return super.getMetadataReader(resource);
>        }
>     }

对应时序图方法6，SimpleMetadataReader\#SimpleMetadataReader。 组装SimpleMetadataReader。

>     public MetadataReader getMetadataReader(Resource resource) throws IOException {
>        // 默认是SimpleMetadataReader实例
>        return new SimpleMetadataReader(resource, this.resourceLoader.getClassLoader());
>     }

>     SimpleMetadataReader(Resource resource, @Nullable ClassLoader classLoader) throws IOException {
>        // 加载.class文件
>        InputStream is = new BufferedInputStream(resource.getInputStream());
>        ClassReader classReader;
>        try {
>           classReader = new ClassReader(is);
>        }
>        catch (IllegalArgumentException ex) {
>           throw new NestedIOException("ASM ClassReader failed to parse class file - " +
>                 "probably due to a new Java class file version that isn't supported yet: " + resource, ex);
>        }
>        finally {
>           is.close();
>        }
>        AnnotationMetadataReadingVisitor visitor = new AnnotationMetadataReadingVisitor(classLoader);
>        // 解析.class元信息
>        classReader.accept(visitor, ClassReader.SKIP_DEBUG);
>        this.annotationMetadata = visitor;
>        this.classMetadata = visitor;
>        this.resource = resource;
>     }

## 第5步：把.class解析并组装出来 ##

对应时序图方法7，ClassReader\#accept。该方法把二进制的.class文件解析组装到AnnotationMetadataReadingVisitor

>     public void accept(
>         final ClassVisitor classVisitor,
>         final Attribute[] attributePrototypes,
>         final int parsingOptions) {
>       Context context = new Context();
>       context.attributePrototypes = attributePrototypes;
>       context.parsingOptions = parsingOptions;
>       context.charBuffer = new char[maxStringLength];
>     
>       ... 省略代码
>         
>       // Visit the RuntimeVisibleAnnotations attribute.
>       if (runtimeVisibleAnnotationsOffset != 0) {
>         int numAnnotations = readUnsignedShort(runtimeVisibleAnnotationsOffset);
>         int currentAnnotationOffset = runtimeVisibleAnnotationsOffset + 2;
>         while (numAnnotations-- > 0) {
>           // Parse the type_index field.
>           String annotationDescriptor = readUTF8(currentAnnotationOffset, charBuffer);
>           currentAnnotationOffset += 2;
>           // 这里面封装Spring Component注解
>           currentAnnotationOffset =
>               readElementValues(classVisitor.visitAnnotation(annotationDescriptor,true),
>                   currentAnnotationOffset,true,charBuffer);
>         }
>       }
>     
>      ... 省略代码
>     }

##  **第6步：读取元素的值** ##

> 对应时序图方法8，ClassReader\#readElementValues。

>     private int readElementValues(
>         final AnnotationVisitor annotationVisitor,
>         final int annotationOffset,
>         final boolean named,
>         final char[] charBuffer) {
>       ... 省略代码
>       if (annotationVisitor != null) {
>         // 主要逻辑在这里面
>         annotationVisitor.visitEnd();
>       }
>       return currentOffset;
>     }

## **第7步：去掉非Spring的注解** ##

> 对应时序图方法9，AnnotationAttributesReadingVisitor\#visitEnd。过滤掉 java.lang.annotation 包下的注解，然后把剩下的注解放到metaAnnotationMap。

>     public void visitEnd() {
>        super.visitEnd();
>     
>        Class<? extends Annotation> annotationClass = this.attributes.annotationType();
>        if (annotationClass != null) {
>           ... 省略代码
>           // 过滤java.lang.annotation包下的注解，及保留Spring注解
>           if (!AnnotationUtils.isInJavaLangAnnotationPackage(annotationClass.getName())) {
>              try {
>                 // 获取该类上的所有注解
>                 Annotation[] metaAnnotations = annotationClass.getAnnotations();
>                 if (!ObjectUtils.isEmpty(metaAnnotations)) {
>                    Set<Annotation> visited = new LinkedHashSet<>();
>                    for (Annotation metaAnnotation : metaAnnotations) {
>                       // 过滤java.lang.annotation包下的注解，及保留Spring注解
>                       recursivelyCollectMetaAnnotations(visited, metaAnnotation);
>                    }
>                    // 封装需要的注解
>                    if (!visited.isEmpty()) {
>                       Set<String> metaAnnotationTypeNames = new LinkedHashSet<>(visited.size());
>                       for (Annotation ann : visited) {
>                          metaAnnotationTypeNames.add(ann.annotationType().getName());
>                       }
>                       this.metaAnnotationMap.put(annotationClass.getName(), metaAnnotationTypeNames);
>                    }
>                 }
>              }
>              catch (Throwable ex) {
>              }
>           }
>        }
>     }

对应时序图方法10，ClassPathScanningCandidateComponentProvider\#isCandidateComponent。使用前面提过的ClassPathBeanDefinitionScanner初始化时设置的注解类型过滤器，includeFilters 包含ManagedBean和Component类型。

>     protected boolean isCandidateComponent(MetadataReader metadataReader) throws IOException {
>        for (TypeFilter tf : this.excludeFilters) {
>           if (tf.match(metadataReader, getMetadataReaderFactory())) {
>              return false;
>           }
>        }
>        for (TypeFilter tf : this.includeFilters) {
>           if (tf.match(metadataReader, getMetadataReaderFactory())) {
>              return isConditionMatch(metadataReader);
>           }
>        }
>        return false;
>     }

对应时序图方法11，AbstractTypeHierarchyTraversingFilter\#match。

>     public boolean match(MetadataReader metadataReader, MetadataReaderFactory metadataReaderFactory)
>           throws IOException {
>     
>        if (matchSelf(metadataReader)) {
>           return true;
>        }
>     
>       ... 省略代码
>     
>        return false;
>     }

对应时序图方法12，AnnotationTypeFilter\#matchSelf。判断类的metadata中是否包含Component。

>     protected boolean matchSelf(MetadataReader metadataReader) {
>        AnnotationMetadata metadata = metadataReader.getAnnotationMetadata();
>        return metadata.hasAnnotation(this.annotationType.getName()) ||
>              (this.considerMetaAnnotations && metadata.hasMetaAnnotation(this.annotationType.getName()));
>     }

第8步：

[Spring--_--_Component_IT_-CSDN]: https://knife.blog.csdn.net/article/details/106484664
[Spring_Component_ - Griez -]: https://www.cnblogs.com/wolf-bin/p/11667208.html
[format_png]: https://imgconvert.csdnimg.cn/aHR0cHM6Ly9pbWcyMDE4LmNuYmxvZ3MuY29tL2Jsb2cvMTMwNjkyNS8yMDE5MTAvMTMwNjkyNS0yMDE5MTAxMzE3MzAyOTIwMC05OTE3MDgyMTcucG5n?x-oss-process=image/format,png