spring boot 判断 bean是否存在

一、需求

今天在搭建Springboot框架的时候，又遇到一个需求：在多模块系统中，有些模块想自己管理BeanValidation的资源文件（默认是启动项目claspath下的 ValidationMessages.properties）。刚开始还天真地认为springboot会不会帮我们做了，结果并没有，于是就是撸源码了。

以下是我的实现和实现原理

二、实现

@Configuration
public class MyWebMvcConfigurer implements WebMvcConfigurer {
/**
 * 当有异常时返回默认的验证器
 * @return 返回的是org.springframework.validation.Validator,不是javax.validation.Validator
 * 所以返回时要适配一下
 */
@Override
public Validator getValidator() {
	//路径匹配
	PathMatchingResourcePatternResolver resourcePatternResolver =
			new PathMatchingResourcePatternResolver(
        MyWebMvcConfigurer.class.getClassLoader());
	try {
		//匹配属性文件，有个限制，资源文件名称必须包含Validation
		Resource[] resources = 
            resourcePatternResolver.getResources("classpath*:*Validation*.properties");

		List<String> files = Arrays.stream(resources)
				.filter(resource -> StringUtils.isNotBlank(resource.getFilename()))
				.map(resource -> {
					String fileName = resource.getFilename();
					return fileName.substring(0, fileName.indexOf("."));
				}).collect(Collectors.toList());

		javax.validation.Validator validator = Validation.byDefaultProvider()
				.configure()
            	 //这里可以加载多个文件
				.messageInterpolator(new ResourceBundleMessageInterpolator(
                    new AggregateResourceBundleLocator(files)))
				.buildValidatorFactory()
				.getValidator();
        //适配
		return new SpringValidatorAdapter(validator);
	} catch (IOException e) {
       	//发生异常，返回null，springboot框架会采用默认的validator
		return null;
	}
}

三、实现原理

源码分析

1、定位Bean在什么地方验证的

DispatcherServlet验证Bean的主要源码路径

• RequestResponseBodyMethodProcessor#resolveArgument

• AbstractMessageConverterMethodArgumentResolver#validateIfApplicable

• DataBinder#validate（核心）

源码：

/**
 * 该方法定位：org.springframework.web.servlet.mvc.method.annotation.
 			RequestResponseBodyMethodProcessor#resolveArgument
 			
 * Throws MethodArgumentNotValidException if validation fails.
 * @throws HttpMessageNotReadableException if {@link RequestBody#required()}
 * is {@code true} and there is no body content or if there is no suitable
 * converter to read the content with.
 */
@Override
public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) 
    throws Exception {

	parameter = parameter.nestedIfOptional();
	Object arg = readWithMessageConverters(webRequest, parameter, 
                                           parameter.getNestedGenericParameterType());
	String name = Conventions.getVariableNameForParameter(parameter);

	if (binderFactory != null) {
		WebDataBinder binder = binderFactory.createBinder(webRequest, arg, name);
		if (arg != null) {
            //*******校验方法参数是否符合要求*******
            //调用链：也就是验证器被调用的地方
			validateIfApplicable(binder, parameter);
			if (binder.getBindingResult().hasErrors() 
                && isBindExceptionRequired(binder, parameter)) {
				throw 
                    new MethodArgumentNotValidException(parameter, 
                                                        binder.getBindingResult());
			}
		}
		if (mavContainer != null) {
			mavContainer.addAttribute(
                BindingResult.MODEL_KEY_PREFIX + name, binder.getBindingResult());
		}
	}

	return adaptArgumentIfNecessary(arg, parameter);
}

/**
 * 该方法定位：org.springframework.web.servlet.mvc.method.annotation.
    AbstractMessageConverterMethodArgumentResolver#validateIfApplicable
 *  
 * Validate the binding target if applicable.
 * <p>The default implementation checks for {@code @javax.validation.Valid},
 * Spring's {@link org.springframework.validation.annotation.Validated},
 * and custom annotations whose name starts with "Valid".
 * @param binder the DataBinder to be used
 * @param parameter the method parameter descriptor
 * @since 4.1.5
 * @see #isBindExceptionRequired
 */
protected void validateIfApplicable(WebDataBinder binder, MethodParameter parameter) {
	Annotation[] annotations = parameter.getParameterAnnotations();
	for (Annotation ann : annotations) {
		Validated validatedAnn = AnnotationUtils.getAnnotation(ann, Validated.class);
		if (validatedAnn != null || 
            ann.annotationType().getSimpleName().startsWith("Valid")) {
			Object hints = (validatedAnn != null ? 
                            validatedAnn.value() : AnnotationUtils.getValue(ann));
			Object[] validationHints = (hints instanceof Object[] ? 
                                        (Object[]) hints : new Object[] {hints});
            //调用链
			binder.validate(validationHints);
			break;
		}
	}
}


/**
 * 该方法定位：org.springframework.validation.DataBinder#validate(java.lang.Object...)
 *
 * Invoke the specified Validators, if any, with the given validation hints.
 * <p>Note: Validation hints may get ignored by the actual target Validator.
 * @param validationHints one or more hint objects to be passed to a {@link SmartValidator}
 * @see #setValidator(Validator)
 * @see SmartValidator#validate(Object, Errors, Object...)
 * 核心方法
 */
public void validate(Object... validationHints) {
	for (Validator validator : getValidators()) {
		if (!ObjectUtils.isEmpty(validationHints) && validator instanceof SmartValidator) {
			((SmartValidator) validator).validate(getTarget(), 
                                                  getBindingResult(), validationHints);
		}
		else if (validator != null) {
			validator.validate(getTarget(), getBindingResult());
		}
	}
}
/**
 * 获取验证器（关键就在：this.validators怎么初始化的？）
 */
public List<Validator> getValidators() {
	return Collections.unmodifiableList(this.validators);
}

发现：在DataBinder#validate中有验证Bean的核心代码validator.validate(...)

分析到这里关键就是validator在哪赋值的？

2、validators赋值

• DataBinder属性validators赋值
  private final List validators = new ArrayList<>();

//断点跟踪发现：
public void setValidator(@Nullable Validator validator) {
    assertValidators(validator);
    this.validators.clear();
    if (validator != null) {
        this.validators.add(validator);
    }

}

• DataBinder#setValidator被调用的位置

• org.springframework.web.bind.support.ConfigurableWebBindingInitializer#initBinder

源码：

@Override
public void initBinder(WebDataBinder binder) {
	binder.setAutoGrowNestedPaths(this.autoGrowNestedPaths);
	if (this.directFieldAccess) {
		binder.initDirectFieldAccess();
	}
	if (this.messageCodesResolver != null) {
		binder.setMessageCodesResolver(this.messageCodesResolver);
	}
	if (this.bindingErrorProcessor != null) {
		binder.setBindingErrorProcessor(this.bindingErrorProcessor);
	}
	if (this.validator != null && binder.getTarget() != null &&
			this.validator.supports(binder.getTarget().getClass())) {
		//发现是在这里调用的，下面的问题就是ConfigurableWebBindingInitializer
		//中的validator属性在哪初始化的？
		//在对应的setValidator方法打断点
		binder.setValidator(this.validator);
	}
	if (this.conversionService != null) {
		binder.setConversionService(this.conversionService);
	}
	if (this.propertyEditorRegistrars != null) {
		for (PropertyEditorRegistrar propertyEditorRegistrar : 		   
             this.propertyEditorRegistrars) {
			propertyEditorRegistrar.registerCustomEditors(binder);
		}
	}
}

• ConfigurableWebBindingInitializer#initBinder被调用的位置
  研究发现：ConfigurableWebBindingInitializer#initBinder是在springboot初始化时被调用的
  调用链如下：
  调用链1：初始化springmvc的requestMappingHandlerAdapter
  EnableWebMvcConfiguration#requestMappingHandlerAdapter

• super.requestMappingHandlerAdapter();—>WebMvcConfigurationSupport
  调用链2：
  WebMvcConfigurationSupport#requestMappingHandlerAdapter
• adapter.setWebBindingInitializer(getConfigurableWebBindingInitializer());
  调用链3：
  EnableWebMvcConfiguration#ConfigurableWebBindingInitializer
• super.getConfigurableWebBindingInitializer();
  调用链4：
  WebMvcConfigurationSupport#ConfigurableWebBindingInitializer
• mvcValidator(),这个是核心

源码：

/**
 * @see org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration.
 * EnableWebMvcConfiguration#requestMappingHandlerAdapter
 */
@Bean
@Override
public RequestMappingHandlerAdapter requestMappingHandlerAdapter() {
    //调用链1：调用父类WebMvcConfigurationSupport#requestMappingHandlerAdapter
	RequestMappingHandlerAdapter adapter = super.requestMappingHandlerAdapter();
	adapter.setIgnoreDefaultModelOnRedirect(this.mvcProperties == null
			|| this.mvcProperties.isIgnoreDefaultModelOnRedirect());
	return adapter;
}

/**
 * @seeorg.springframework.web.servlet.config.annotation.
 * WebMvcConfigurationSupport#requestMappingHandlerAdapter
 *
 */
public RequestMappingHandlerAdapter requestMappingHandlerAdapter() {
	RequestMappingHandlerAdapter adapter = createRequestMappingHandlerAdapter();
	adapter.setContentNegotiationManager(mvcContentNegotiationManager());
	adapter.setMessageConverters(getMessageConverters());
    //调用链2：EnableWebMvcConfiguration#getConfigurableWebBindingInitializer
	adapter.setWebBindingInitializer(getConfigurableWebBindingInitializer());
	adapter.setCustomArgumentResolvers(getArgumentResolvers());
	adapter.setCustomReturnValueHandlers(getReturnValueHandlers());
	...
}

/**
 * @see springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration.
 * EnableWebMvcConfiguration#getConfigurableWebBindingInitializer
 *
 */
@Override
protected ConfigurableWebBindingInitializer getConfigurableWebBindingInitializer() {
	try {
        //这里是不存在实例，报异常
		return this.beanFactory.getBean(ConfigurableWebBindingInitializer.class);
	}
	catch (NoSuchBeanDefinitionException ex) {
        //调用链3：WebMvcConfigurationSupport#getConfigurableWebBindingInitializer
		return super.getConfigurableWebBindingInitializer();
	}
}

/**
 * @seespringframework.web.servlet.config.annotation.WebMvcConfigurationSupport
 * #getConfigurableWebBindingInitializer
 *
 */
protected ConfigurableWebBindingInitializer getConfigurableWebBindingInitializer() {
	ConfigurableWebBindingInitializer initializer = 
	new ConfigurableWebBindingInitializer();
	initializer.setConversionService(mvcConversionService());
    //调用链4：核心方法mvcValidator()
	initializer.setValidator(mvcValidator());
	MessageCodesResolver messageCodesResolver = getMessageCodesResolver();
	if (messageCodesResolver != null) {
		initializer.setMessageCodesResolver(messageCodesResolver);
	}
	return initializer;
}

3、validator是什么

通过源码分析，找到了关键点就是mvcValidator()，现在对其分析，找出其返回的validator到底是什么？

断点调试时发现mvcValidator()进入了

org.springframework.context.annotation.ConfigurationClassEnhancer.BeanMethodInterceptor#intercept

• org.springframework.context.annotation.ConfigurationClassEnhancer.BeanMethodInterceptor#resolveBeanReference
  resolveBeanReference方法里有个关键的代码

//关键在于 beanFactory.getBean(beanName)，name = "mvcValidator"，创建该实例
//从而会找到EnableWebMvcConfiguration的mvcValidator方法
//（因为mvcValidator方法上有@Bean,方法名称又与beanName相同，故调用）
Object beanInstance = (useArgs ? beanFactory.getBean(beanName, beanMethodArgs) :
                       beanFactory.getBean(beanName));

• org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration.EnableWebMvcConfiguration#mvcValidator
  终于找到创建validator对象的点了，以下就是如何自己扩展？
  继续研究创建validator的源码，寻找关键点

@Bean
@Override
public Validator mvcValidator() {
	if (!ClassUtils.isPresent("javax.validation.Validator",
                              getClass().getClassLoader())) {
		return super.mvcValidator();
	}
    //关键在于getValidator()方法
    //真正调用的是父类DelegatingWebMvcConfiguration#getValidator
	return ValidatorAdapter.get(getApplicationContext(), getValidator());
}

4、关键点：分析getValidator()方法

注意：这里就是我们可以扩展的地方

/**
 * springframework.web.servlet.config.annotation.DelegatingWebMvcConfiguration#getValidator
 */
@Override
@Nullable
protected Validator getValidator() {
	//configurers属性是WebMvcConfigurerComposite的对象
	return this.configurers.getValidator();
}

/**
 *@see springframework.web.servlet.config.annotation.WebMvcConfigurerComposite#getValidator
 */
@Override
public Validator getValidator() {
	Validator selected = null;
    //看到WebMvcConfigurer这个东西，我是很激动呀！终于看到曙光了,激动半天
    //于是我就自定义了MyWebMvcConfigurer实现WebMvcConfigurer，并重写
    //其中的getValidator方法，哈哈，终于找到扩展点了
	for (WebMvcConfigurer configurer : this.delegates) {
		Validator validator = configurer.getValidator();
		if (validator != null) {
			if (selected != null) {
				throw new IllegalStateException("No unique Validator found: {" +
						selected + ", " + validator + "}");
			}
			selected = validator;
		}
	}
	return selected;
}

通过getValidator()获取自定义的validator后

ValidatorAdapter.get(getApplicationContext(), getValidator());对其包装如下：

/**
 * @see springframework.boot.autoconfigure.validation.ValidatorAdapter#get
 */
public static Validator get(ApplicationContext applicationContext,
		Validator validator) {
    //如果为null（自定义的validator发生异常），返回默认的
	if (validator != null) {
        //因为非空，会执行该行代码
		return wrap(validator, false);
	}
	return getExistingOrCreate(applicationContext);
}

private static Validator wrap(Validator validator, boolean existingBean) {
	if (validator instanceof javax.validation.Validator) {
        //执行该代码
		if (validator instanceof SpringValidatorAdapter) {
			return new ValidatorAdapter((SpringValidatorAdapter) validator,
					existingBean);
		}
		return new ValidatorAdapter(
				new SpringValidatorAdapter((javax.validation.Validator) validator),
				existingBean);
	}
	return validator;
}

总结：在分析源码的过程中犯了最大的错误就是：总想什么都搞明白，跟踪每个源码的实现，结果发现还是没搞懂，白白浪费了很多时间。其实在分析源码的过程中，不需要钻牛角尖，把每个都搞懂。你要搞明白你的“关注点“在哪？，不要走着走着就走偏了。很多源码“观其大意”就行，没必要深究，不然就呵呵了。