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Spring源码分析—spring bean创建和初始化
2017-07-17 10:43:20       个评论    来源:谢杨易的博客  
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1 介绍

创建并初始化spring容器中,refresh()方法中解析xml配置文件,注册容器后处理器,bean后处理器,初始化MessageSource,ApplicationEventMulticaster广播器,注册完ApplicationListener监听器后,关键一步就是创建和初始化其他非lazy-init的singleton beans。这样在容器初始化好的时候,这些singleton beans就已经创建和初始化好了,可以大大提高bean的访问效率。这个过程比较复杂,本文将详细分析整个流程。先看涉及到的关键类。

AbstractApplicationContext: 定义了spring容器初始化的大部分流程方法,子类必须遵循这个流程,但可以修改流程中的方法,典型的模板模式。bean创建的入口方法finishBeanFactoryInitialization也在这个方法中。

DefaultListableBeanFactory:一种BeanFactory容器实现,实现了ConfigurableListableBeanFactory接口

BeanDefinition:描述bean结构,对应XML中的或者注解中的@Component

AbstractBeanFactory:继承了BeanFactory容器,主要负责getBean创建Bean实例。

2 流程

2.1 finishBeanFactoryInitialization

初始化spring容器中的refresh()方法中,会调用finishBeanFactoryInitialization()方法,它是创建和初始化其他非lazy-init的singleton的bean的入口。下面从这个方法开始分析。

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
   // 初始化conversionService类型转换bean,它可以服务于其他bean的类型转换
   if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
         beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
      beanFactory.setConversionService(
            beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
   }

   // 注册字符串解析器,用来解析注解中的属性
   if (!beanFactory.hasEmbeddedValueResolver()) {
      beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
   }

   // 初始化LoadTimeWeaverAware bean
   String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
   for (String weaverAwareName : weaverAwareNames) {
      getBean(weaverAwareName);
   }

   // 停止使用临时的ClassLoader,
   beanFactory.setTempClassLoader(null);
   beanFactory.freezeConfiguration();

   // 这儿才是最关键的一步,创建和初始化非lazy-init的singleton beans
   beanFactory.preInstantiateSingletons();
}

finishBeanFactoryInitialization()做了初始化conversionService类型转换器等的工作,这些不是关键点。关键点在preInstantiateSingletons()方法中,它会做创建和初始化singleton bean的工作。下面接着分析

public void preInstantiateSingletons() throws BeansException {
   // 获取XML配置文件解析时,解析到的所有beanname
   List beanNames = new ArrayList<>(this.beanDefinitionNames);

   // 遍历所有没有标注lazy-init的singleton的beanname,创建bean
   for (String beanName : beanNames) {
      // 利用beanname获取BeanDefinition,在XML解析时会生成BeanDefinition对象,将XML中的各属性添加到BeanDefinition的相关标志位中,比如abstractFlag,scope等
      RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
      // 非abstract,非lazy-init的singleton bean才需要在容器初始化阶段创建
      if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
         // 处理FactoryBean
         if (isFactoryBean(beanName)) {
            // 获取FactoryBean实例,FactoryBean前面会加一个&符号
            final FactoryBean factory = (FactoryBean) getBean(FACTORY_BEAN_PREFIX + beanName);
            boolean isEagerInit;
            if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
               isEagerInit = AccessController.doPrivileged((PrivilegedAction) () ->
                     ((SmartFactoryBean) factory).isEagerInit(),
                     getAccessControlContext());
            }
            else {
               isEagerInit = (factory instanceof SmartFactoryBean &&
                     ((SmartFactoryBean) factory).isEagerInit());
            }
            if (isEagerInit) {
               getBean(beanName);
            }
         }
         // 非Factorybean,直接调用getBean方法,关键所在,后续分析
         else {
            getBean(beanName);
         }
      }
   }

   // bean创建后,对SmartInitializingSingleton回调afterSingletonsInstantiated()方法,这儿不用太care
   for (String beanName : beanNames) {
      Object singletonInstance = getSingleton(beanName);
      if (singletonInstance instanceof SmartInitializingSingleton) {
         final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
         if (System.getSecurityManager() != null) {
            AccessController.doPrivileged((PrivilegedAction) () -> {
               smartSingleton.afterSingletonsInstantiated();
               return null;
            }, getAccessControlContext());
         }
         else {
            smartSingleton.afterSingletonsInstantiated();
         }
      }
   }
}

preInstantiateSingletons流程稍微复杂点,主要有

获取XML解析时的beanNames 遍历beanNames,获取BeanDefinition。对非abstract,非lazy-init的singleton bean的进行实例化 如果是FactoryBean,则需要判断isEagerInit,来确定是否调用getBean创建对应的bean。 如果不是,则直接调用getBean创建对应bean bean创建后,对SmartInitializingSingleton回调afterSingletonsInstantiated()方法。

2.2 getBean 创建和初始化bean实例

下面我们着重来分析bean的创建,也就是getBean()方法。

public Object getBean(String name) throws BeansException {
   return doGetBean(name, null, null, false);
}
protected  T doGetBean(final String name, @Nullable final Class requiredType,
      @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {

   // beanname转换,去掉FactoryBean的&前缀,处理alias声明。细节可自行分析
   final String beanName = transformedBeanName(name);
   Object bean;

   Object sharedInstance = getSingleton(beanName);
   if (sharedInstance != null && args == null) {
      // 判断singleton bean是否已经创建好了,创建好了则直接从内存取出。
      bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
   } else {
      // 之前没创建的,则需要创建。
      // 正在创建,则直接异常返回
      if (isPrototypeCurrentlyInCreation(beanName)) {
         throw new BeanCurrentlyInCreationException(beanName);
      }

      // 检查是否有beanname对应的BeanDefinition
      BeanFactory parentBeanFactory = getParentBeanFactory();
      if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
         // 没有找到BeanDefinition,看看parent工厂中有没有,调用parent工厂的getBean
         // 获取原始的name,包含了FactoryBean前缀,&符号
         String nameToLookup = originalBeanName(name);
         if (parentBeanFactory instanceof AbstractBeanFactory) {
            return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
                  nameToLookup, requiredType, args, typeCheckOnly);
         }
         else if (args != null) {
            return (T) parentBeanFactory.getBean(nameToLookup, args);
         }
         else {
            return parentBeanFactory.getBean(nameToLookup, requiredType);
         }
      }

      if (!typeCheckOnly) {
         markBeanAsCreated(beanName);
      }

      try {
         // 找到了beanname对应的BeanDefinition,合并parent的BeanDefinition(XML中的parent属性)
         final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
         checkMergedBeanDefinition(mbd, beanName, args);

         // 处理dependsOn属性
         String[] dependsOn = mbd.getDependsOn();
         if (dependsOn != null) {
            // 遍历所有的dependOn bean,要先注册和创建依赖的bean
            for (String dep : dependsOn) {
               // check是否两个bean是循环依赖,spring不能出现bean的循环依赖
               if (isDependent(beanName, dep)) {
                  throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                        "Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
               }
               // 注册并创建依赖的bean
               registerDependentBean(dep, beanName);
               getBean(dep);
            }
         }

         // 处理scope属性
         if (mbd.isSingleton()) {
            // singleton, 必须保证线程安全情况下创建bean,保证单例
            sharedInstance = getSingleton(beanName, () -> {
               try {
                  // 反射创建bean实例,这个过程很复杂,稍后分析
                  return createBean(beanName, mbd, args);
               }
               catch (BeansException ex) {
                  // 异常处理,清除掉bean
                  destroySingleton(beanName);
                  throw ex;
               }
            });
            // 获取bean实例
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
         }
         else if (mbd.isPrototype()) {
            // prototype,创建一个全新的实例
            Object prototypeInstance = null;
            try {
               // 创建前的回调
               beforePrototypeCreation(beanName);
               // 反射创建bean实例,稍后详细分析
               prototypeInstance = createBean(beanName, mbd, args);
            }
            finally {
               // 创建后的回调,清除inCreation的标志
               afterPrototypeCreation(beanName);
            }
            bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
         }

         else {
            // 其他scope值
            String scopeName = mbd.getScope();
            final Scope scope = this.scopes.get(scopeName);
            if (scope == null) {
               // scope属性不能接收空值
               throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
            }
            try {
               Object scopedInstance = scope.get(beanName, () -> {
                  beforePrototypeCreation(beanName);
                  try {
                     return createBean(beanName, mbd, args);
                  }
                  finally {
                     afterPrototypeCreation(beanName);
                  }
               });
               bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
            }
            catch (IllegalStateException ex) {
               throw new BeanCreationException(beanName,
                     "Scope '" + scopeName + "' is not active for the current thread; consider " +
                     "defining a scoped proxy for this bean if you intend to refer to it from a singleton",
                     ex);
            }
         }
      }
      catch (BeansException ex) {
         cleanupAfterBeanCreationFailure(beanName);
         throw ex;
      }
   }
   // check 创建的bean是否是requiredType指明的类型。如果不是,先做转换,转换不成的话只能类型不匹配抛出异常了
   if (requiredType != null && bean != null && !requiredType.isInstance(bean)) {
        try {
             // 尝试将创建的bean转换为requiredType指明的类型
            return getTypeConverter().convertIfNecessary(bean, requiredType);
        } catch (TypeMismatchException ex) {
             // 转换不成功,抛出异常
            throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
        }
   } 

   return (T) bean;
}

doGetBean概括了bean创建和初始化的主要流程,十分复杂,步骤主要为

beanname转换,去掉FactoryBean的&前缀,处理alias声明 判断singleton bean是否已经创建好了,创建好了则直接从内存取出 没有创建好,则检查是否有beanname对应的BeanDefinition,没有则到parent工厂中查找,命中则使用parent工厂再次调用getBean以及doGetBean创建 有BeanDefinition,则合并parent属性指向的中的属性,这主要是处理bean的parent属性。子bean会继承parent bean的属性。 处理dependsOn属性。必须先创建好所有的依赖的bean 处理scope属性,如果是singleton的,则必须保证线程安全情况下创建单例。如果是prototype,则必须保证创建一个全新的bean。创建bean通过createBean()反射创建。

2.3 createBean 反射创建bean实例

下面来分析createBean()方法,这个过程也是相当复杂的。

protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
      throws BeanCreationException {
   RootBeanDefinition mbdToUse = mbd;

   // 拷贝一个新的RootBeanDefinition供创建bean使用
   Class resolvedClass = resolveBeanClass(mbd, beanName);
   if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
      mbdToUse = new RootBeanDefinition(mbd);
      mbdToUse.setBeanClass(resolvedClass);
   }

   // 处理bean中定义的覆盖方法,主要是xml:lookup-method或replace-method。标记override的方法为已经加载过的,避免不必要的参数检查开销。这儿不详细展开了。
   try {
      mbdToUse.prepareMethodOverrides();
   }
   catch (BeanDefinitionValidationException ex) {
      throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
            beanName, "Validation of method overrides failed", ex);
   }

   // 调用BeanPostProcessors bean后处理器,使得bean后处理器可以返回一个proxy bean,从而代替我们要创建的bean。回调后处理器的postProcessBeforeInstantiation()方法,如果这个方法中返回了一个bean,也就是使用了proxy,则再回调postProcessAfterInitialization()方法。之后返回这个Proxy bean即可。
   try {
      Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
      if (bean != null) {
         return bean;
      }
   }
   catch (Throwable ex) {
      throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
            "BeanPostProcessor before instantiation of bean failed", ex);
   }

   // doCreateBean创建bean实例,后面详细分析
   try {
      Object beanInstance = doCreateBean(beanName, mbdToUse, args);
      return beanInstance;
   }
   // 各种异常,省略
   ... 
}

createBean()方法大概步骤如下

拷贝一个新的RootBeanDefinition供创建bean使用 处理lookup-method或replace-method 调用BeanPostProcessors后处理器 doCreateBean创建bean实例

下面我们重点分析doCreateBean方法

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
      throws BeanCreationException {

   // 创建bean实例,如果是singleton,先尝试从缓存中取,取不到则创建
   BeanWrapper instanceWrapper = null;
   if (mbd.isSingleton()) {
      instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
   }
   if (instanceWrapper == null) {
      // 反射创建bean实例,后面详细说
      instanceWrapper = createBeanInstance(beanName, mbd, args);
   }
   final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
   Class beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
   mbd.resolvedTargetType = beanType;

   // 回调MergedBeanDefinitionPostProcessor.postProcessMergedBeanDefinition,它可以修改bean属性
   if (beanType != null) {
      synchronized (mbd.postProcessingLock) {
         if (!mbd.postProcessed) {
            try {
               applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
            }
            catch (Throwable ex) {
               throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                     "Post-processing of merged bean definition failed", ex);
            }
            mbd.postProcessed = true;
         }
      }
   }

   // 曝光单例对象的引用,主要是为了解决单例间的循环依赖问题,以及依赖的bean比较复杂时的初始化性能问题
   boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
         isSingletonCurrentlyInCreation(beanName));
   if (earlySingletonExposure) {
      addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
   }

   // 初始化bean,后面详细介绍
   Object exposedObject = bean;
   try {
      populateBean(beanName, mbd, instanceWrapper);
      if (exposedObject != null) {
         exposedObject = initializeBean(beanName, exposedObject, mbd);
      }
   }
   // 省略异常处理

   // 单例曝光对象的处理,不用太在意
   if (earlySingletonExposure) {
      Object earlySingletonReference = getSingleton(beanName, false);
      if (earlySingletonReference != null) {
         if (exposedObject == bean) {
            exposedObject = earlySingletonReference;
         }
         else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
            String[] dependentBeans = getDependentBeans(beanName);
            Set actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
            for (String dependentBean : dependentBeans) {
               if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
                  actualDependentBeans.add(dependentBean);
               }
            }
            if (!actualDependentBeans.isEmpty()) {
               // 抛出异常,省略代码
            }
         }
      }
   }

  // 注册bean为可销毁的bean,bean销毁时,会回调destroy-method
   if (bean != null) {
      try {
         registerDisposableBeanIfNecessary(beanName, bean, mbd);
      }
      catch (BeanDefinitionValidationException ex) {
         throw new BeanCreationException(
               mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
      }
   }

   return exposedObject;
}

doCreateBean方法主要流程为

createBeanInstance() 创建bean实例 回调postProcessMergedBeanDefinition(), 可以修改bean属性 initializeBean() 初始化bean实例,包括后处理器的调用,init-method的调用等 注册bean为可销毁的,这样在bean销毁时,就可以回调到destroy-method.

2.3.1 createBeanInstance 反射创建bean实例

我们先分析如何创建bean实例的。

个protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
   // 先创建class对象,反射的套路。利用bean的class属性进行反射,所以class属性一定要是bean的实现类
   Class beanClass = resolveBeanClass(mbd, beanName);

   // class如果不是public的,则抛出异常。因为没法进行实例化
   if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
      throw new BeanCreationException(mbd.getResourceDescription(), beanName,
            "Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
   }

   // 
   Supplier instanceSupplier = mbd.getInstanceSupplier();
   if (instanceSupplier != null) {
      return obtainFromSupplier(instanceSupplier, beanName);
   }

   // 使用FactoryBean的factory-method来创建,支持静态工厂和实例工厂
   if (mbd.getFactoryMethodName() != null)  {
      return instantiateUsingFactoryMethod(beanName, mbd, args);
   }

   // 无参数情况时,创建bean。调用无参构造方法
   boolean resolved = false;
   boolean autowireNecessary = false;
   if (args == null) {
      synchronized (mbd.constructorArgumentLock) {
         if (mbd.resolvedConstructorOrFactoryMethod != null) {
            resolved = true;
            autowireNecessary = mbd.constructorArgumentsResolved;
         }
      }
   }
   if (resolved) {
      if (autowireNecessary) {
         // autoWire创建 自动装配
         return autowireConstructor(beanName, mbd, null, null);
      }
      else {
         // 普通创建
         return instantiateBean(beanName, mbd);
      }
   }

   // 有参数情况时,创建bean。先利用参数个数,类型等,确定最精确匹配的构造方法。
   Constructor[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
   if (ctors != null ||
         mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
         mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))  {
      return autowireConstructor(beanName, mbd, ctors, args);
   }

   // 有参数时,又没获取到构造方法,则只能调用无参构造方法来创建实例了(兜底方法)
   return instantiateBean(beanName, mbd);
}

instantiateBean,使用无参构造方法,反射创建bean实例代码如下

protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
   try {
      Object beanInstance;
      final BeanFactory parent = this;
      if (System.getSecurityManager() != null) {
         beanInstance = AccessController.doPrivileged((PrivilegedAction) () ->
               getInstantiationStrategy().instantiate(mbd, beanName, parent),
               getAccessControlContext());
      }
      else {
         // 创建实例,关键点,其他都不用care
         beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
      }
      BeanWrapper bw = new BeanWrapperImpl(beanInstance);
      initBeanWrapper(bw);
      return bw;
   }
   catch (Throwable ex) {
      throw new BeanCreationException(
            mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
   }
}

这个方法没什么要注意的,关键点在instantiate方法

public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
   // Don't override the class with CGLIB if no overrides.
   if (bd.getMethodOverrides().isEmpty()) {
      Constructor constructorToUse;
      // 保证线程安全情况下,获取Constructor
      synchronized (bd.constructorArgumentLock) {
         // 获取构造方法或factory-method
         constructorToUse = (Constructor) bd.resolvedConstructorOrFactoryMethod;
         if (constructorToUse == null) {
            // BeanDefinition中如果没有Constructor或者factory-method,则直接使用默认无参构造方法。
            final Class clazz = bd.getBeanClass();
            if (clazz.isInterface()) {
               throw new BeanInstantiationException(clazz, "Specified class is an interface");
            }
            try {
               if (System.getSecurityManager() != null) {
                  constructorToUse = AccessController.doPrivileged(
                        (PrivilegedExceptionAction>) () ->
                              clazz.getDeclaredConstructor());
               }
               else {
                  // 获取默认无参构造方法
                  constructorToUse = clazz.getDeclaredConstructor();
               }
               bd.resolvedConstructorOrFactoryMethod = constructorToUse;
            }
            catch (Throwable ex) {
               throw new BeanInstantiationException(clazz, "No default constructor found", ex);
            }
         }
      }
      // 使用上一步得到的Constructor,反射获取bean实例
      return BeanUtils.instantiateClass(constructorToUse);
   }
   else {
      // Must generate CGLIB subclass.
      return instantiateWithMethodInjection(bd, beanName, owner);
   }
}

instantiate方法主要做两件事

确定Constructor或者factory-method 利用Constructor,反射创建bean实例。

分析到这儿Bean的创建就结束了,这个过程实在是太复杂了!

2.3.2 initializeBean 初始化bean实例

bean创建完后,容器会对它进行初始化,包括后处理的调用,init-method的调用等。请看下面详解。

protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {
   // 回调各种aware method,如BeanNameAware, BeanFactoryAware等
   if (System.getSecurityManager() != null) {
      AccessController.doPrivileged((PrivilegedAction) () -> {
         invokeAwareMethods(beanName, bean);
         return null;
      }, getAccessControlContext());
   }
   else {
      invokeAwareMethods(beanName, bean);
   }

   // 回调beanPostProcessor的postProcessBeforeInitialization()方法
   Object wrappedBean = bean;
   if (mbd == null || !mbd.isSynthetic()) {
      wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
   }

   // init-method 和 postProcessAfterInitialization
   if (wrappedBean != null) {
      try {
         // 回调init-method
         invokeInitMethods(beanName, wrappedBean, mbd);
      }
      catch (Throwable ex) {
         throw new BeanCreationException(
               (mbd != null ? mbd.getResourceDescription() : null),
               beanName, "Invocation of init method failed", ex);
      }
      // 回调beanPostProcessor的postProcessAfterInitialization()方法
      if (mbd == null || !mbd.isSynthetic()) {
         wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
      }
   }

   return wrappedBean;
}

由此可见,initializeBean(),也就是bean的初始化流程为

回调各种aware method,如BeanNameAware,将容器中相关引用注入到bean中,供bean使用 回调beanPostProcessor的postProcessBeforeInitialization(), 后处理器的初始化前置调用 回调init-method, 注解和XML中都可以声明 回调beanPostProcessor的postProcessAfterInitialization()方法,后处理器的初始化后置调用。

从这个流程,我们也能清晰的分析出容器后处理器两个方法的调用时机。分析源码可以大大加深我们对spring API的理解。

3 总结

Bean实例的创建和初始化流程还是十分复杂的。从源码中可以清晰的分析出spring bean的各种特性。如factory-method, BeanPostProcessor等。有助于我们spring bean行为的理解。所以分析源码还是十分值得的。

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