最近的一次項目引入了MongoDB，擼完代碼，啟動服務，Junit單元測試一跑，報錯：
java.lang.NoClassDefFoundError: org/springframework/core/DefaultParameterNameDiscoverer
at org.springframework.data.mapping.model.PreferredConstructorDiscoverer.<clinit>(PreferredConstructorDiscoverer.java:38)
at org.springframework.data.mapping.model.BasicPersistentEntity.<init>(BasicPersistentEntity.java:90)
at org.springframework.data.mongodb.core.mapping.BasicMongoPersistentEntity.<init>(BasicMongoPersistentEntity.java:67)
at org.springframework.data.mongodb.core.mapping.MongoMappingContext.createPersistentEntity(MongoMappingContext.java:89)
at org.springframework.data.mongodb.core.mapping.MongoMappingContext.createPersistentEntity(MongoMappingContext.java:37)
at org.springframework.data.mapping.context.AbstractMappingContext.addPersistentEntity(AbstractMappingContext.java:279)
at org.springframework.data.mapping.context.AbstractMappingContext.getPersistentEntity(AbstractMappingContext.java:180)
at org.springframework.data.mapping.context.AbstractMappingContext.getPersistentEntity(AbstractMappingContext.java:140)
at org.springframework.data.mapping.context.AbstractMappingContext.getPersistentEntity(AbstractMappingContext.java:67)
at org.springframework.data.mongodb.core.MongoTemplate.assertUpdateableIdIfNotSet(MongoTemplate.java:1138)
at org.springframework.data.mongodb.core.MongoTemplate.doInsert(MongoTemplate.java:708)
at org.springframework.data.mongodb.core.MongoTemplate.insert(MongoTemplate.java:672)
at com.isz.app.baseDao.MongodbBaseDao.insert(MongodbBaseDao.java:250)
at com.isz.app.push.mapper.impl.PushMessageTaskMapperImpl.insertPushMessageTask(PushMessageTaskMapperImpl.java:64)
at com.isz.app.push.biz.PushServiceBiz.insertPushMessageTask(PushServiceBiz.java:136)
at com.isz.app.push.biz.PushServiceBiz.singlePush(PushServiceBiz.java:44)
at com.isz.app.push.service.impl.PushServiceFacade.singlePush(PushServiceFacade.java:22)
at com.isz.app.push.service.impl.PushServiceFacadeTest.test1(PushServiceFacadeTest.java:31)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.springframework.test.context.junit4.statements.RunBeforeTestMethodCallbacks.evaluate(RunBeforeTestMethodCallbacks.java:74)
at org.springframework.test.context.junit4.statements.RunAfterTestMethodCallbacks.evaluate(RunAfterTestMethodCallbacks.java:83)
at org.springframework.test.context.junit4.statements.SpringRepeat.evaluate(SpringRepeat.java:72)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:231)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:88)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229)
at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)
at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:71)
at org.junit.runners.ParentRunner.run(ParentRunner.java:309)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:174)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:675)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)
Caused by: java.lang.ClassNotFoundException: org.springframework.core.DefaultParameterNameDiscoverer
at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
... 46 more
看到ClassNotFoundException（注意ClassNotFoundException和NoClassDefFoundError的區別），主要是因為org.springframework.core.DefaultParameterNameDiscoverer這個類找不到，下面開始分析過程：
1.先看了下項目中spring-core的jar包用的是3.2.4.release版本，打開源碼，確實沒有DefaultParameterNameDiscoverer這個類
2.判斷是用於MongoDB中哪個Spring的jar包和我現在項目中Spring的jar包沖突了
3.debug一步步找到報錯的地方（Spring代碼的風格只要是以do開頭都是具體的實現類），在new PreferredConstructorDiscoverer這個類的時候報錯，打開這個類，根據報錯信息，這個類的38行：
4.這里確實是用到了DefaultParameterNameDiscoverer，這個類所在的jar包是，spring-data-commons-1.9.3-RELEAS版本，說明1.9.3版本和3.2.4core是不兼容的， 1.9.3版本是要對應spring-core-4.0.0.0以上的（下載了4.0的core源碼，是有DefaultParameterNameDiscoverer這個類的）
5.現在要確定的是這個項目是怎么引入spring-data-commons-1.9.3-RELEAS這個jar包的，這個時候就用到了maven的一個依賴樹命令mvn:dependency:tree,這個命令可以查看到整個項目的所有依賴
這種屬於一級依賴，去掉后，再查看項目依賴樹，原來的1.9.3依賴已經去掉了，現在用的是1.8.2的jar包
6.啟動項目，OK，完美運行。