运行junit测试报javax/servlet/ServletContext : Unsupported major.minor version 51.0错误

问题描述：

在运行junit测试的时候，发生一下异常：

java.lang.UnsupportedClassVersionError: javax/servlet/ServletContext : Unsupported major.minor version 51.0
	at java.lang.ClassLoader.defineClass1(Native Method)
	at java.lang.ClassLoader.defineClassCond(ClassLoader.java:631)
	at java.lang.ClassLoader.defineClass(ClassLoader.java:615)
	at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141)
	at java.net.URLClassLoader.defineClass(URLClassLoader.java:283)
	at java.net.URLClassLoader.access$000(URLClassLoader.java:58)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:197)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
	at java.lang.Class.getDeclaredConstructors0(Native Method)
	at java.lang.Class.privateGetDeclaredConstructors(Class.java:2398)
	at java.lang.Class.getConstructor0(Class.java:2708)
	at java.lang.Class.getDeclaredConstructor(Class.java:1987)
	at org.springframework.beans.BeanUtils.instantiateClass(BeanUtils.java:105)
	at org.springframework.test.context.TestContextManager.retrieveTestExecutionListeners(TestContextManager.java:232)
	at org.springframework.test.context.TestContextManager.(TestContextManager.java:120)
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.createTestContextManager(SpringJUnit4ClassRunner.java:120)
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.(SpringJUnit4ClassRunner.java:109)
	at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
	at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
	at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
	at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
	at org.junit.internal.builders.AnnotatedBuilder.buildRunner(AnnotatedBuilder.java:29)
	at org.junit.internal.builders.AnnotatedBuilder.runnerForClass(AnnotatedBuilder.java:21)
	at org.junit.runners.model.RunnerBuilder.safeRunnerForClass(RunnerBuilder.java:59)
	at org.junit.internal.builders.AllDefaultPossibilitiesBuilder.runnerForClass(AllDefaultPossibilitiesBuilder.java:26)
	at org.junit.runners.model.RunnerBuilder.safeRunnerForClass(RunnerBuilder.java:59)
	at org.junit.internal.requests.ClassRequest.getRunner(ClassRequest.java:26)
	at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.(JUnit4TestReference.java:33)
	at org.eclipse.jdt.internal.junit4.runner.JUnit4TestClassReference.(JUnit4TestClassReference.java:25)
	at org.eclipse.jdt.internal.junit4.runner.JUnit4TestLoader.createTest(JUnit4TestLoader.java:48)
	at org.eclipse.jdt.internal.junit4.runner.JUnit4TestLoader.loadTests(JUnit4TestLoader.java:38)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:452)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)

原因是由于工程中使用到了servlet-api的jar包依赖，如下

  javax.servlet
  javax.servlet-api
  3.1.0
  provided

由于这个jar包是用jdk1.7编译的，我的工程是用jdk1.6，于是发生了以上错误。

ps：之前没写单元测试，开发环境都是部署在tomcat下测试的，由于这个jar包的scope是provider，意味着servlet-api不会部署到tomcat低下(打war包也不会打进去)，而是用tomcat自身的servlet-api，所以没有发现以上错误。

解决方案：

把servlet-api的版本换成用jdk1.6编译的版本就可以了，我用的是

  javax.servlet
  javax.servlet-api
  3.0.1
  provided