Develop Reference

Liferay 7.3 - org.hibernate.MappingException: entity class not found

I'm pretty new to Liferay development. I'm working on migrating a project from 6.2 to 7.3.
The plugin I'm working on has the -api / -service / -portlet structure that I've seen in the tutorials.
When I try to deploy the service JAR to my server, I'm getting the following error:
2021-07-07 10:30:11.759 ERROR [pipe-start 1356][LiferayServiceExtender:88] org.hibernate.MappingException: entity class not found: PhishRodLab.db.model.impl.Customer_PreferencesImpl
org.hibernate.MappingException: entity class not found: PhishRodLab.db.model.impl.Customer_PreferencesImpl
at org.hibernate.mapping.PersistentClass.getMappedClass(PersistentClass.java:125)
at org.hibernate.tuple.PropertyFactory.getGetter(PropertyFactory.java:191)
at org.hibernate.tuple.PropertyFactory.buildIdentifierProperty(PropertyFactory.java:67)
at org.hibernate.tuple.entity.EntityMetamodel.(EntityMetamodel.java:135)
at org.hibernate.persister.entity.AbstractEntityPersister.(AbstractEntityPersister.java:485)
at org.hibernate.persister.entity.SingleTableEntityPersister.(SingleTableEntityPersister.java:133)
at org.hibernate.persister.PersisterFactory.createClassPersister(PersisterFactory.java:84)
at org.hibernate.impl.SessionFactoryImpl.(SessionFactoryImpl.java:286)
at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1872)
at org.springframework.orm.hibernate3.LocalSessionFactoryBean.newSessionFactory(LocalSessionFactoryBean.java:795)
at com.liferay.portal.spring.hibernate.PortalHibernateConfiguration.newSessionFactory(PortalHibernateConfiguration.java:298)
at org.springframework.orm.hibernate3.LocalSessionFactoryBean.buildSessionFactory(LocalSessionFactoryBean.java:717)
at com.liferay.portal.spring.hibernate.PortalHibernateConfiguration.buildSessionFactory(PortalHibernateConfiguration.java:99)
at com.liferay.portal.spring.extender.internal.LiferayServiceExtender$LiferayServiceExtension.start(LiferayServiceExtender.java:148)
at com.liferay.portal.spring.extender.internal.LiferayServiceExtender.addingBundle(LiferayServiceExtender.java:83)
at com.liferay.portal.spring.extender.internal.LiferayServiceExtender.addingBundle(LiferayServiceExtender.java:61)
at org.osgi.util.tracker.BundleTracker$Tracked.customizerAdding(BundleTracker.java:475)
at org.osgi.util.tracker.BundleTracker$Tracked.customizerAdding(BundleTracker.java:1)
at org.osgi.util.tracker.AbstractTracked.trackAdding(AbstractTracked.java:256)
at org.osgi.util.tracker.AbstractTracked.track(AbstractTracked.java:229)
at org.osgi.util.tracker.BundleTracker$Tracked.bundleChanged(BundleTracker.java:450)
at org.eclipse.osgi.internal.framework.BundleContextImpl.dispatchEvent(BundleContextImpl.java:908)
at org.eclipse.osgi.framework.eventmgr.EventManager.dispatchEvent(EventManager.java:230)
at org.eclipse.osgi.framework.eventmgr.ListenerQueue.dispatchEventSynchronous(ListenerQueue.java:148)
at org.eclipse.osgi.internal.framework.EquinoxEventPublisher.publishBundleEventPrivileged(EquinoxEventPublisher.java:230)
at org.eclipse.osgi.internal.framework.EquinoxEventPublisher.publishBundleEvent(EquinoxEventPublisher.java:137)
at org.eclipse.osgi.internal.framework.EquinoxEventPublisher.publishBundleEvent(EquinoxEventPublisher.java:129)
at org.eclipse.osgi.internal.framework.EquinoxContainerAdaptor.publishModuleEvent(EquinoxContainerAdaptor.java:191)
at org.eclipse.osgi.container.Module.publishEvent(Module.java:476)
at org.eclipse.osgi.container.Module.doStart(Module.java:578)
at org.eclipse.osgi.container.Module.start(Module.java:449)
at org.eclipse.osgi.internal.framework.EquinoxBundle.start(EquinoxBundle.java:428)
at org.eclipse.osgi.internal.framework.EquinoxBundle.start(EquinoxBundle.java:447)
at org.eclipse.equinox.console.commands.EquinoxCommandProvider.start(EquinoxCommandProvider.java:243)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.apache.felix.gogo.runtime.Reflective.invoke(Reflective.java:139)
at org.apache.felix.gogo.runtime.CommandProxy.execute(CommandProxy.java:91)
at org.apache.felix.gogo.runtime.Closure.executeCmd(Closure.java:599)
at org.apache.felix.gogo.runtime.Closure.executeStatement(Closure.java:526)
at org.apache.felix.gogo.runtime.Closure.execute(Closure.java:415)
at org.apache.felix.gogo.runtime.Pipe.doCall(Pipe.java:416)
at org.apache.felix.gogo.runtime.Pipe.call(Pipe.java:229)
at org.apache.felix.gogo.runtime.Pipe.call(Pipe.java:59)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.ClassNotFoundException: PhishRodLab.db.model.impl.Customer_PreferencesImpl
at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1365)
at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1188)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:264)
at org.hibernate.util.ReflectHelper.classForName(ReflectHelper.java:200)
at org.hibernate.mapping.PersistentClass.getMappedClass(PersistentClass.java:122)
... 49 more

I've encountered that error when migrating from previous versions when the DTD of the service.xml file is not the right one. You must make sure you are using the latest version available for the version you are using (7.3 in this case).
For example, the header of the service.xml file should start like this:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE service-builder PUBLIC "-//Liferay//DTD Service Builder 7.3.0//EN" "http://www.liferay.com/dtd/liferay-service-builder_7_3_0.dtd">
<service-builder...>
The best option is to generate a test servicebuilder project using the tutorial, and then compare the generated code with yours, making note of the differences.
More info: https://help.liferay.com/hc/es/articles/360029028211-Introduction-to-Service-Builder

Axiom throwing error for Axis2 generated WSDL

I have auto generated classes from a WSDL file using Axis2 wsdl2java script. After adding the generated Java files to a Maven project the dependencies were only axis2-adb and axis2-kernel. However during runtime it is throwing the following:
Caused by: java.lang.ClassNotFoundException: org.apache.axiom.util.UIDGenerator
at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
and
Caused by: java.lang.NoClassDefFoundError: org/apache/axiom/util/UIDGenerator
at org.apache.axis2.description.AxisOperation.<init>(AxisOperation.java:90)
at org.apache.axis2.description.AxisOperation.<init>(AxisOperation.java:95)
at org.apache.axis2.description.TwoChannelAxisOperation.<init>(TwoChannelAxisOperation.java:52)
at org.apache.axis2.description.OutInAxisOperation.<init>(OutInAxisOperation.java:65)
at org.apache.axis2.description.RobustOutOnlyAxisOperation.<init>(RobustOutOnlyAxisOperation.java:47)
at org.apache.axis2.client.Stub.addAnonymousOperations(Stub.java:233)
at com.tobeprecise.sms.webservices.HttpReceiverStub.populateAxisService(HttpReceiverStub.java:42)
at com.tobeprecise.sms.webservices.HttpReceiverStub.<init>(HttpReceiverStub.java:184)
at com.tobeprecise.sms.webservices.HttpReceiverStub.<init>(HttpReceiverStub.java:173)
at com.tobeprecise.sms.webservices.HttpReceiverStub.<init>(HttpReceiverStub.java:222)
at com.tobeprecise.sms.webservices.HttpReceiverStub.<init>(HttpReceiverStub.java:214)
at in.capillary.nsadmin.gateway.tobeprecisebulk.TobepreciseBulkGatewayChannel.sendMessages(TobepreciseBulkGatewayChannel.java:122)
at in.capillary.nsadmin.gateway.BufferedGatewayChannelProcessor.process(BufferedGatewayChannelProcessor.java:150)
at in.capillary.nsadmin.gateway.BaseGateway.process(BaseGateway.java:207)
at org.apache.camel.processor.DelegateSyncProcessor.process(DelegateSyncProcessor.java:63)
... 18 more
While looking at the dependencies I have found the axiom-api-1.2.11 as a dependency and it very well contains the UIDGenerator class. Is this due to version mismatch?
Found a similar issue ClassNotFoundException axiom-api-1.2.7.jar but I coudn't make much sense out of it(my manifest file: http://pastebin.com/n4jKWPck). Any help?

Oozie Java Action : Passing Hbase classpath

I'm running a test hbase java program via oozie java action. The following error is encountered :
Failing Oozie Launcher, Main class [HbaseTest], main() threw exception, org/apache/hadoop/hbase/HBaseConfiguration
java.lang.NoClassDefFoundError: org/apache/hadoop/hbase/HBaseConfiguration
at HbaseTest.main(HbaseTest.java:28)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.apache.oozie.action.hadoop.LauncherMapper.map(LauncherMapper.java:495)
at org.apache.hadoop.mapred.MapRunner.run(MapRunner.java:50)
at org.apache.hadoop.mapred.MapTask.runOldMapper(MapTask.java:417)
at org.apache.hadoop.mapred.MapTask.run(MapTask.java:332)
at org.apache.hadoop.mapred.Child$4.run(Child.java:268)
at java.security.AccessController.doPrivileged(Native Method)
at javax.security.auth.Subject.doAs(Subject.java:396)
at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1438)
at org.apache.hadoop.mapred.Child.main(Child.java:262)
Caused by: java.lang.ClassNotFoundException: org.apache.hadoop.hbase.HBaseConfiguration
at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
... 14 more
The program runs correctly from command line :
java -cp `hbase classpath` HbaseTest
Is there a way I can pass output of 'hbase classpath' to the oozie java action.
I dont want to copy hbase jars to workflow's lib directory as that will be a maintenance overhead.
Following is the java action from workflow.xml :
<java>
<job-tracker>${jobTracker}</job-tracker>
<name-node>${nameNode}</name-node>
<configuration>
<property>
<name>mapred.job.queue.name</name>
<value>${queueName}</value>
</property>
</configuration>
<main-class>HbaseTest</main-class>
<java-opts></java-opts>
<arg>HELLO</arg>
</java>

Since Oozie 2.3 you can use Share Libraries:
Oozie supports job and system share libraries for workflow jobs.
Share libraries can simplify the deployment and management of common components across workflow applications.
For example, if a workflow job uses a share library with the Streaming, Pig & Har JARs files it does not have to bundled those JARs files in the workflow application lib/ path.
If workflow job uses a share library, Oozie will include all the JAR/SO files in the library in the classpath/libpath for all its actions.
A workflow job can specify a share library path using the job property oozie.libpath .
A workflow job can use the system share library by setting the job property oozie.use.system.libpath to true .
http://oozie.apache.org/docs/3.2.0-incubating/WorkflowFunctionalSpec.html#a17_HDFS_Share_Libraries_for_Workflow_Applications_since_Oozie_2.3
How to install:
http://oozie.apache.org/docs/4.0.0/DG_QuickStart.html#OozieShareLib

Using hibernate and ms sql server driver

Microsoft doesn't supply jdbc jar to central maven repository. It needs register driver manually.
But I make something wrong. I saw same exception trace:
Initial SessionFactory creation failed.org.hibernate.service.classloading.spi.ClassLoadingException: Specified JDBC Driver "com.microsoft.sqlserver.jdbc.SQLServerDriver" class not found
Exception in thread "main" java.lang.ExceptionInInitializerError
at logic.HibernateUtil.buildSessionFactory(HibernateUtil.java:22)
at logic.HibernateUtil.<clinit>(HibernateUtil.java:8)
at logic.Main.main(Main.java:12)
Caused by: org.hibernate.service.classloading.spi.ClassLoadingException: Specified JDBC Driver "com.microsoft.sqlserver.jdbc.SQLServerDriver" class not found
at org.hibernate.service.jdbc.connections.internal.DriverManagerConnectionProviderImpl.configure(DriverManagerConnectionProviderImpl.java:107)
at org.hibernate.service.internal.StandardServiceRegistryImpl.configureService(StandardServiceRegistryImpl.java:75)
at org.hibernate.service.internal.AbstractServiceRegistryImpl.initializeService(AbstractServiceRegistryImpl.java:159)
at org.hibernate.service.internal.AbstractServiceRegistryImpl.getService(AbstractServiceRegistryImpl.java:131)
at org.hibernate.engine.jdbc.internal.JdbcServicesImpl.buildJdbcConnectionAccess(JdbcServicesImpl.java:223)
at org.hibernate.engine.jdbc.internal.JdbcServicesImpl.configure(JdbcServicesImpl.java:89)
at org.hibernate.service.internal.StandardServiceRegistryImpl.configureService(StandardServiceRegistryImpl.java:75)
at org.hibernate.service.internal.AbstractServiceRegistryImpl.initializeService(AbstractServiceRegistryImpl.java:159)
at org.hibernate.service.internal.AbstractServiceRegistryImpl.getService(AbstractServiceRegistryImpl.java:131)
at org.hibernate.cfg.SettingsFactory.buildSettings(SettingsFactory.java:71)
at org.hibernate.cfg.Configuration.buildSettingsInternal(Configuration.java:2277)
at org.hibernate.cfg.Configuration.buildSettings(Configuration.java:2273)
at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1742)
at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1782)
at logic.HibernateUtil.buildSessionFactory(HibernateUtil.java:17)
... 2 more
Caused by: org.hibernate.service.classloading.spi.ClassLoadingException: Unable to load class ["com.microsoft.sqlserver.jdbc.SQLServerDriver"]
at org.hibernate.service.classloading.internal.ClassLoaderServiceImpl.classForName(ClassLoaderServiceImpl.java:141)
at org.hibernate.service.jdbc.connections.internal.DriverManagerConnectionProviderImpl.configure(DriverManagerConnectionProviderImpl.java:104)
... 16 more
Caused by: java.lang.ClassNotFoundException: Could not load requested class : "com.microsoft.sqlserver.jdbc.SQLServerDriver"
at org.hibernate.service.classloading.internal.ClassLoaderServiceImpl$1.findClass(ClassLoaderServiceImpl.java:99)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:247)
at org.hibernate.service.classloading.internal.ClassLoaderServiceImpl.classForName(ClassLoaderServiceImpl.java:138)
... 17 more
I use STS(eclipse product)
in maven dependencies :
What do I make wrong?

If you are using JRE version 6.0 or 7.0 then use sqljdbc4.jar file. If the JRE version is 5.0 then use sqljdbc.jar file.
These jar files can be downloaded from here
Also you can get more clarity about System Requirements for the JDBC Driver from here

Please check following properties are correctly configured or not
<property name="hibernate.connection.url">jdbc:sqlserver://localhost:1433;database=<dbname>;</property>
<property name="hibernate.dialect">org.hibernate.dialect.SQLServer2008Dialect</property>

Jenkins Plugin having trouble using Class?

I'm trying to integrate scribe-java and write a Jenkins Plugin for it, but it seems Jenkins can't be used with Class? Does anyone know why is this?
Jenkins Plugin code:
OAuthService service =
new ServiceBuilder().provider( com.lookout.jenkins.publishoverbox.BoxApi20.class ).apiKey( clientId ).apiSecret(clientSecret ).callback(callbackUrl).grantType( OAuthConstants.AUTHORIZATION_CODE ).build();
Log:
SEVERE: Failed to create Guice container from all the plugins
com.google.common.collect.ComputationException: java.lang.NoClassDefFoundError: Lorg/scribe/oauth/OAuthService;
at com.google.common.collect.ComputingConcurrentHashMap$ComputingMapAdapter.get(ComputingConcurrentHashMap.java:397)
at com.google.inject.internal.FailableCache.get(FailableCache.java:49)
at com.google.inject.internal.MembersInjectorStore.get(MembersInjectorStore.java:66)
at com.google.inject.internal.ConstructorInjectorStore.createConstructor(ConstructorInjectorStore.java:73)
at com.google.inject.internal.ConstructorInjectorStore.access$000(ConstructorInjectorStore.java:29)
at com.google.inject.internal.ConstructorInjectorStore$1.create(ConstructorInjectorStore.java:36)
at com.google.inject.internal.ConstructorInjectorStore$1.create(ConstructorInjectorStore.java:33)
at com.google.inject.internal.FailableCache$1.apply(FailableCache.java:38)
at com.google.common.collect.ComputingConcurrentHashMap$ComputingValueReference.compute(ComputingConcurrentHashMap.java:355)
at com.google.common.collect.ComputingConcurrentHashMap$ComputingSegment.compute(ComputingConcurrentHashMap.java:184)
at com.google.common.collect.ComputingConcurrentHashMap$ComputingSegment.getOrCompute(ComputingConcurrentHashMap.java:153)
at com.google.common.collect.ComputingConcurrentHashMap.getOrCompute(ComputingConcurrentHashMap.java:69)
at com.google.common.collect.ComputingConcurrentHashMap$ComputingMapAdapter.get(ComputingConcurrentHashMap.java:393)
at com.google.inject.internal.FailableCache.get(FailableCache.java:49)
at com.google.inject.internal.ConstructorInjectorStore.get(ConstructorInjectorStore.java:49)
at com.google.inject.internal.ConstructorBindingImpl.initialize(ConstructorBindingImpl.java:127)
at com.google.inject.internal.InjectorImpl.initializeBinding(InjectorImpl.java:506)
at com.google.inject.internal.AbstractBindingProcessor$Processor$1.run(AbstractBindingProcessor.java:171)
at com.google.inject.internal.ProcessedBindingData.initializeBindings(ProcessedBindingData.java:44)
at com.google.inject.internal.InternalInjectorCreator.initializeStatically(InternalInjectorCreator.java:123)
at com.google.inject.internal.InternalInjectorCreator.build(InternalInjectorCreator.java:107)
at com.google.inject.Guice.createInjector(Guice.java:94)
at com.google.inject.Guice.createInjector(Guice.java:71)
at hudson.ExtensionFinder$GuiceFinder.<init>(ExtensionFinder.java:282)
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 java.lang.Class.newInstance0(Class.java:357)
at java.lang.Class.newInstance(Class.java:310)
at net.java.sezpoz.IndexItem.instance(IndexItem.java:181)
at hudson.ExtensionFinder$Sezpoz._find(ExtensionFinder.java:625)
at hudson.ExtensionFinder$Sezpoz.find(ExtensionFinder.java:600)
at hudson.ExtensionFinder._find(ExtensionFinder.java:151)
at hudson.ClassicPluginStrategy.findComponents(ClassicPluginStrategy.java:318)
at hudson.ExtensionList.load(ExtensionList.java:295)
at hudson.ExtensionList.ensureLoaded(ExtensionList.java:248)
at hudson.ExtensionList.iterator(ExtensionList.java:138)
at hudson.ClassicPluginStrategy.findComponents(ClassicPluginStrategy.java:311)
at hudson.ExtensionList.load(ExtensionList.java:295)
at hudson.ExtensionList.ensureLoaded(ExtensionList.java:248)
at hudson.ExtensionList.get(ExtensionList.java:153)
at hudson.PluginManager$PluginUpdateMonitor.getInstance(PluginManager.java:1066)
at hudson.maven.PluginImpl.init(PluginImpl.java:54)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at hudson.init.InitializerFinder.invoke(InitializerFinder.java:120)
at hudson.init.InitializerFinder$TaskImpl.run(InitializerFinder.java:184)
at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:259)
at jenkins.model.Jenkins$7.runTask(Jenkins.java:888)
at org.jvnet.hudson.reactor.Reactor$2.run(Reactor.java:187)
at org.jvnet.hudson.reactor.Reactor$Node.run(Reactor.java:94)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918)
at java.lang.Thread.run(Thread.java:680)
Caused by: java.lang.NoClassDefFoundError: Lorg/scribe/oauth/OAuthService;
at java.lang.Class.getDeclaredFields0(Native Method)
at java.lang.Class.privateGetDeclaredFields(Class.java:2300)
at java.lang.Class.getDeclaredFields(Class.java:1745)
at com.google.inject.spi.InjectionPoint.getInjectionPoints(InjectionPoint.java:651)
at com.google.inject.spi.InjectionPoint.forInstanceMethodsAndFields(InjectionPoint.java:358)
at com.google.inject.internal.MembersInjectorStore.createWithListeners(MembersInjectorStore.java:91)
at com.google.inject.internal.MembersInjectorStore.access$000(MembersInjectorStore.java:35)
at com.google.inject.internal.MembersInjectorStore$1.create(MembersInjectorStore.java:43)
at com.google.inject.internal.MembersInjectorStore$1.create(MembersInjectorStore.java:40)
at com.google.inject.internal.FailableCache$1.apply(FailableCache.java:38)
at com.google.common.collect.ComputingConcurrentHashMap$ComputingValueReference.compute(ComputingConcurrentHashMap.java:355)
at com.google.common.collect.ComputingConcurrentHashMap$ComputingSegment.compute(ComputingConcurrentHashMap.java:184)
at com.google.common.collect.ComputingConcurrentHashMap$ComputingSegment.getOrCompute(ComputingConcurrentHashMap.java:153)
at com.google.common.collect.ComputingConcurrentHashMap.getOrCompute(ComputingConcurrentHashMap.java:69)
at com.google.common.collect.ComputingConcurrentHashMap$ComputingMapAdapter.get(ComputingConcurrentHashMap.java:393)
... 56 more
Caused by: java.lang.ClassNotFoundException: org.scribe.oauth.OAuthService
at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
... 71 more
Jun 3, 2013 11:37:17 AM jenkins.InitReactorRunner$1 onAttained

What ever you need in your plugin, you specify the same in dependencies.
Reason 1:
Jenkins loads each plugin into a separate class loader to avoid conflicts. Plugins can then participate to the system activities just like other Jenkins built-in classes do. They can participate in XStream-based persistence, they can provide "views" by Jelly, they can provide static resources like images, and from users, everything works seamlessly --- there's no distinction between functionalities that are built-in vs those from plugins.
Reason 2: You don't want to change your code when jenkins change their dependencies.