一、  判断.class是否存在

  is(‘.classname’)的方法

$('div').is('.redColor')；

二、添加.class

$('div').addClass("redColor");

三、删除.class

$('div').removeClass("redColor")

 

 

了解下这两个文件类型：.java文件是给人看的，.class是给计算机看的。程序员编写的是.java文件，经编译后生成.class文件给计算执行。
相信这两句话很形象的说明了这两个文件的关系了。
1，首先反编译器就是jd-gui这个软件了，可以把jar包直接拖进这个软件就能够看到代码了。当然jd-gui不能编译.class文件。
2，怎么把.class转成.java文件呢？也是在jd-gui文件里面。点击要转的文件，然后file-->save Source,如下图：

保存后就是.java文件了。当然可以直接保存整个jar包，也可以是单个文件。
3，eclipse里面怎么看.class文件？新建的一个.java文件有.class的前提是要编译成功，比我常用的项目是maven工程，要先maven install编译才能生成“完整的”.class文件，单单保存是不行的，这里下面会有说明。eclipse可以通过ctrl+shift+R，然后输入类名，可同时看到.class和.java文件。

4，怎么编译单个的java文件呢？方法有很多，这里我介绍个在dos命令中生成.class文件的办法。
首先在eclipse中新建个aa.java类，里面就啥都不写了，最好也别带汉字，不然后面编译的时候会报编码错误，找不到符合等各种错误。然后拷到桌面上，打开dos命令，输入以下命令：
E:\jdk1.7\bin\javac.exe  C:\Users\Administrator\Desktop\aa.java
解释：E:\jdk1.7\bin\javac.exe 是javac.exe的安装目录，其实找到jdk就行了。C:\Users\Administrator\Desktop\aa.java是刚才写的测试类，把他拷到桌面的位置。
最后Enter运行命令，运行成功的话就是可以再aa的目录下生成同名的.class文件了。如图：



这里对3解说下，编译单个文件的时候，如果这个类引入的有其他新建的类，就会编译报错，显示找不到符号，这个也很容易理解，因为实在javac虚拟机上执行的，我们写的测试类内容只能包含jdk自带的代码，对于我们引入自己新建类肯定是不识别的。也就是在eclipse中虽然保存后有.class文件，但是不一定编译成功。

本博客已迁移，本文对应新的地址如下：http://blog.321aiyi.com/article/387

异常摘要
org.aspectj.apache.bcel.classfile.ClassFormatException: Invalid byte tag in constant pool: 18
	at org.aspectj.apache.bcel.classfile.Constant.readConstant(Constant.java:133)
	at org.aspectj.apache.bcel.classfile.ConstantPool.<init>(ConstantPool.java:45)
	at org.aspectj.apache.bcel.classfile.ClassParser.readConstantPool(ClassParser.java:186)
	at org.aspectj.apache.bcel.classfile.ClassParser.parse(ClassParser.java:131)
	at org.aspectj.apache.bcel.util.NonCachingClassLoaderRepository.loadJavaClass(NonCachingClassLoaderRepository.java:262)
	at org.aspectj.apache.bcel.util.NonCachingClassLoaderRepository.loadClass(NonCachingClassLoaderRepository.java:242)
	at org.aspectj.apache.bcel.util.NonCachingClassLoaderRepository.loadClass(NonCachingClassLoaderRepository.java:249)
	at org.aspectj.weaver.reflect.Java15AnnotationFinder.getAnnotations(Java15AnnotationFinder.java:202)
	at org.aspectj.weaver.reflect.ReflectionBasedResolvedMemberImpl.unpackAnnotations(ReflectionBasedResolvedMemberImpl.java:211)
	at org.aspectj.weaver.reflect.ReflectionBasedResolvedMemberImpl.hasAnnotation(ReflectionBasedResolvedMemberImpl.java:163)
	at org.aspectj.weaver.patterns.ExactAnnotationTypePattern.matches(ExactAnnotationTypePattern.java:109)
	at org.aspectj.weaver.patterns.ExactAnnotationTypePattern.matches(ExactAnnotationTypePattern.java:96)
	at org.aspectj.weaver.patterns.AnnotationPointcut.matchInternal(AnnotationPointcut.java:156)
	at org.aspectj.weaver.patterns.Pointcut.match(Pointcut.java:137)
	at org.aspectj.weaver.internal.tools.PointcutExpressionImpl.getShadowMatch(PointcutExpressionImpl.java:239)
	at org.aspectj.weaver.internal.tools.PointcutExpressionImpl.matchesExecution(PointcutExpressionImpl.java:105)
	at org.aspectj.weaver.internal.tools.PointcutExpressionImpl.matchesMethodExecution(PointcutExpressionImpl.java:96)
	at org.springframework.aop.aspectj.AspectJExpressionPointcut.getShadowMatch(AspectJExpressionPointcut.java:416)
	at org.springframework.aop.aspectj.AspectJExpressionPointcut.matches(AspectJExpressionPointcut.java:271)
	at org.springframework.aop.support.AopUtils.canApply(AopUtils.java:210)
	at org.springframework.aop.support.AopUtils.canApply(AopUtils.java:248)
	at org.springframework.aop.support.AopUtils.findAdvisorsThatCanApply(AopUtils.java:280)
	at org.springframework.aop.framework.autoproxy.AbstractAdvisorAutoProxyCreator.findAdvisorsThatCanApply(AbstractAdvisorAutoProxyCreator.java:118)
	at org.springframework.aop.framework.autoproxy.AbstractAdvisorAutoProxyCreator.findEligibleAdvisors(AbstractAdvisorAutoProxyCreator.java:88)
	at org.springframework.aop.framework.autoproxy.AbstractAdvisorAutoProxyCreator.getAdvicesAndAdvisorsForBean(AbstractAdvisorAutoProxyCreator.java:69)
	at org.springframework.aop.framework.autoproxy.AbstractAutoProxyCreator.wrapIfNecessary(AbstractAutoProxyCreator.java:346)
	at org.springframework.aop.framework.autoproxy.AbstractAutoProxyCreator.postProcessAfterInitialization(AbstractAutoProxyCreator.java:298)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyBeanPostProcessorsAfterInitialization(AbstractAutowireCapableBeanFactory.java:422)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1583)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:545)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:482)
	at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306)
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:772)
	at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:839)
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:538)
	at org.springframework.web.context.ContextLoader.configureAndRefreshWebApplicationContext(ContextLoader.java:444)
	at org.springframework.web.context.ContextLoader.initWebApplicationContext(ContextLoader.java:326)
	at org.springframework.web.context.ContextLoaderListener.contextInitialized(ContextLoaderListener.java:107)
	at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4842)
	at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5303)
	at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:147)
	at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1407)
	at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1397)
	at java.util.concurrent.FutureTask.run(Unknown Source)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
	at java.lang.Thread.run(Unknown Source)

##触发场景

JDK1.8环境+Spring4+Mybatis3
##异常分析

这个异常有点儿莫名其妙，我也是第一次遇见，一般这个时候我会看一下这个异常会引起的其他异常，或许可以找到原因，下面是该异常引起的一个Service实例化失败的异常报文
[org.springframework.web.context.support.XmlWebApplicationContext] - Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'appUserServiceImpl' defined in file [F:\workspace_n\.metadata\.plugins\org.eclipse.wst.server.core\tmp0\wtpwebapps\usermanager\WEB-INF\classes\com\aiyi\base\app\user\impl\AppUserServiceImpl.class]: Initialization of bean failed; nested exception is org.aspectj.apache.bcel.classfile.ClassFormatException: File: 'com.aiyi.base.app.user.impl.AppUserServiceImpl': Invalid byte tag in constant pool: 18
[org.springframework.beans.factory.support.DisposableBeanAdapter] - Invocation of destroy method 'close' failed on bean with name 'sqlSession': java.lang.UnsupportedOperationException: Manual close is not allowed over a Spring managed SqlSession
[org.springframework.web.context.ContextLoader] - Context initialization failed
org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'appUserServiceImpl' defined in file [F:\workspace_n\.metadata\.plugins\org.eclipse.wst.server.core\tmp0\wtpwebapps\usermanager\WEB-INF\classes\com\aiyi\base\app\user\impl\AppUserServiceImpl.class]: Initialization of bean failed; nested exception is org.aspectj.apache.bcel.classfile.ClassFormatException: File: 'com.aiyi.base.app.user.impl.AppUserServiceImpl': Invalid byte tag in constant pool: 18
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:553)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:482)
	at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306)
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:772)
	at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:839)
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:538)
	at org.springframework.web.context.ContextLoader.configureAndRefreshWebApplicationContext(ContextLoader.java:444)
	at org.springframework.web.context.ContextLoader.initWebApplicationContext(ContextLoader.java:326)
	at org.springframework.web.context.ContextLoaderListener.contextInitialized(ContextLoaderListener.java:107)
	at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4842)
	at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5303)
	at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:147)
	at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1407)
	at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1397)
	at java.util.concurrent.FutureTask.run(Unknown Source)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
	at java.lang.Thread.run(Unknown Source)
Caused by: org.aspectj.apache.bcel.classfile.ClassFormatException: File: 'com.aiyi.base.app.user.impl.AppUserServiceImpl': Invalid byte tag in constant pool: 18
	at org.aspectj.apache.bcel.classfile.ClassParser.readConstantPool(ClassParser.java:192)
	at org.aspectj.apache.bcel.classfile.ClassParser.parse(ClassParser.java:131)
	at org.aspectj.apache.bcel.util.NonCachingClassLoaderRepository.loadJavaClass(NonCachingClassLoaderRepository.java:262)
	at org.aspectj.apache.bcel.util.NonCachingClassLoaderRepository.loadClass(NonCachingClassLoaderRepository.java:242)
	at org.aspectj.apache.bcel.util.NonCachingClassLoaderRepository.loadClass(NonCachingClassLoaderRepository.java:249)
	at org.aspectj.weaver.reflect.Java15AnnotationFinder.getAnnotations(Java15AnnotationFinder.java:202)
	at org.aspectj.weaver.reflect.ReflectionBasedResolvedMemberImpl.unpackAnnotations(ReflectionBasedResolvedMemberImpl.java:211)
	at org.aspectj.weaver.reflect.ReflectionBasedResolvedMemberImpl.hasAnnotation(ReflectionBasedResolvedMemberImpl.java:163)
	at org.aspectj.weaver.patterns.ExactAnnotationTypePattern.matches(ExactAnnotationTypePattern.java:109)
	at org.aspectj.weaver.patterns.ExactAnnotationTypePattern.matches(ExactAnnotationTypePattern.java:96)
	at org.aspectj.weaver.patterns.AnnotationPointcut.matchInternal(AnnotationPointcut.java:156)
	at org.aspectj.weaver.patterns.Pointcut.match(Pointcut.java:137)
	at org.aspectj.weaver.internal.tools.PointcutExpressionImpl.getShadowMatch(PointcutExpressionImpl.java:239)
	at org.aspectj.weaver.internal.tools.PointcutExpressionImpl.matchesExecution(PointcutExpressionImpl.java:105)
	at org.aspectj.weaver.internal.tools.PointcutExpressionImpl.matchesMethodExecution(PointcutExpressionImpl.java:96)
	at org.springframework.aop.aspectj.AspectJExpressionPointcut.getShadowMatch(AspectJExpressionPointcut.java:416)
	at org.springframework.aop.aspectj.AspectJExpressionPointcut.matches(AspectJExpressionPointcut.java:271)
	at org.springframework.aop.support.AopUtils.canApply(AopUtils.java:210)
	at org.springframework.aop.support.AopUtils.canApply(AopUtils.java:248)
	at org.springframework.aop.support.AopUtils.findAdvisorsThatCanApply(AopUtils.java:280)
	at org.springframework.aop.framework.autoproxy.AbstractAdvisorAutoProxyCreator.findAdvisorsThatCanApply(AbstractAdvisorAutoProxyCreator.java:118)
	at org.springframework.aop.framework.autoproxy.AbstractAdvisorAutoProxyCreator.findEligibleAdvisors(AbstractAdvisorAutoProxyCreator.java:88)
	at org.springframework.aop.framework.autoproxy.AbstractAdvisorAutoProxyCreator.getAdvicesAndAdvisorsForBean(AbstractAdvisorAutoProxyCreator.java:69)
	at org.springframework.aop.framework.autoproxy.AbstractAutoProxyCreator.wrapIfNecessary(AbstractAutoProxyCreator.java:346)
	at org.springframework.aop.framework.autoproxy.AbstractAutoProxyCreator.postProcessAfterInitialization(AbstractAutoProxyCreator.java:298)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyBeanPostProcessorsAfterInitialization(AbstractAutowireCapableBeanFactory.java:422)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1583)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:545)
	... 20 more

从这里可以看到Spring在实例化AppUserServiceImpl这个类的时候失败了，具体失败原因是aspectj.weaver这个包引起的，在这个异常中有一段儿有意思的话：

“Invalid byte tag in constant pool（说的是在反射这个Bean的时候遇到无效的字节）”。

那么我想我大概知道异常原因了，估计是因为我在这个类里面使用了JDK1.8的一些新的特性，而aspectj.weaver不支持这个特性导致Invalid byte tag in constant pool。

##异常解决
既然知道病因了就该对症下药，我特意看了一下我的pom文件，aspectj weaver这个包我引用的是1.6版，我去Maven中央仓库看了一下，我了个乖乖，都更新到1.9X了，果然是版本太老了。于是修改pom.xml将aspectj weaver的版本改为1.8.9。
<!-- 旧版本，不支持JDK1.7以上的新特性 -->
<!-- <dependency>
    <groupId>org.aspectj</groupId>
    <artifactId>aspectjweaver</artifactId>
    <version>1.6.11</version>
</dependency> -->

<!-- 该版本完美解决 -->
<!-- https://mvnrepository.com/artifact/org.aspectj/aspectjweaver -->
<dependency>
    <groupId>org.aspectj</groupId>
    <artifactId>aspectjweaver</artifactId>
    <version>1.8.9</version>
</dependency>

再次运行，问题解决。
后来我又在网上查阅过这方面的资料，可惜的是国内的网络上目前并没有过多的介绍。找来找去在一篇博客上看到了类似的问题，当然并没有得到有效地解决，而是退而求次将JDK1.8换成了JDK7来避免了这个异常。
在这里有点儿小感慨，想想现在可能是因为用1.6和1.7版本JDK的人比较多吧。即使用了更新的版本，也不一定会使用它的新特性，所以这个异常出现的几率目前来说不大。不过随着IT的更新迭代，可能接下来的一段儿时间里，会有很多人面临这个问题，所以特此将这个异常的解决方案写上来分享该大家。

 

.class1.class2（中间没有空格）匹配的是同时拥有class1和class2的元素

<div class="class1 class2">...</div>



.class1 .class2匹配的是父节点有.class1，子节点有.class2的元素


 


	
<div class="class1">
	
	

	
<div class="class2">
	
	

	
...
	
	

	
</div>
	
	

	
</div>