java.lang.ClassNotFoundException: Could not load requested class : org.postgresql.Driver in Wildfly

Recently I was trying migrate our application from wildfly 10 to wildfly 20. We faced the following error:

Unable to create requested service [org.hibernate.engine.jdbc.env.spi.JdbcEnvironment]
        at org.jboss.as.jpa.service.PersistenceUnitServiceImpl$1$1.run(PersistenceUnitServiceImpl.java:198)
        at org.jboss.as.jpa.service.PersistenceUnitServiceImpl$1$1.run(PersistenceUnitServiceImpl.java:128)
        at org.wildfly.security.manager.WildFlySecurityManager.doChecked(WildFlySecurityManager.java:658)
        at org.jboss.as.jpa.service.PersistenceUnitServiceImpl$1.run(PersistenceUnitServiceImpl.java:212)
        at org.jboss.threads.ContextClassLoaderSavingRunnable.run(ContextClassLoaderSavingRunnable.java:35)
        at org.jboss.threads.EnhancedQueueExecutor.safeRun(EnhancedQueueExecutor.java:1982)
        at org.jboss.threads.EnhancedQueueExecutor$ThreadBody.doRunTask(EnhancedQueueExecutor.java:1486)
        at org.jboss.threads.EnhancedQueueExecutor$ThreadBody.run(EnhancedQueueExecutor.java:1377)
        at java.lang.Thread.run(Thread.java:748)
        at org.jboss.threads.JBossThread.run(JBossThread.java:485)
Caused by: org.hibernate.service.spi.ServiceException: Unable to create requested service [org.hibernate.engine.jdbc.env.spi.JdbcEnvironment]
        at org.hibernate.service.internal.AbstractServiceRegistryImpl.createService(AbstractServiceRegistryImpl.java:275)
        at org.hibernate.service.internal.AbstractServiceRegistryImpl.initializeService(AbstractServiceRegistryImpl.java:237)
        at org.hibernate.service.internal.AbstractServiceRegistryImpl.getService(AbstractServiceRegistryImpl.java:214)
        at org.hibernate.id.factory.internal.DefaultIdentifierGeneratorFactory.injectServices(DefaultIdentifierGeneratorFactory.java:152)
        at org.hibernate.service.internal.AbstractServiceRegistryImpl.injectDependencies(AbstractServiceRegistryImpl.java:286)
        at org.hibernate.service.internal.AbstractServiceRegistryImpl.initializeService(AbstractServiceRegistryImpl.java:243)
        at org.hibernate.service.internal.AbstractServiceRegistryImpl.getService(AbstractServiceRegistryImpl.java:214)
        at org.hibernate.boot.internal.InFlightMetadataCollectorImpl.<init>(InFlightMetadataCollectorImpl.java:179)
        at org.hibernate.boot.model.process.spi.MetadataBuildingProcess.complete(MetadataBuildingProcess.java:119)
        at org.hibernate.jpa.boot.internal.EntityManagerFactoryBuilderImpl.metadata(EntityManagerFactoryBuilderImpl.java:1215)
        at org.hibernate.jpa.boot.internal.EntityManagerFactoryBuilderImpl.build(EntityManagerFactoryBuilderImpl.java:1246)
        at org.jboss.as.jpa.hibernate5.TwoPhaseBootstrapImpl.build(TwoPhaseBootstrapImpl.java:44)
        at org.jboss.as.jpa.service.PersistenceUnitServiceImpl$1$1.run(PersistenceUnitServiceImpl.java:170)
        ... 9 more
Caused by: org.hibernate.boot.registry.classloading.spi.ClassLoadingException: HHH010003: JDBC Driver class not found: org.postgresql.Driver
        at org.hibernate.c3p0.internal.C3P0ConnectionProvider.configure(C3P0ConnectionProvider.java:130)
        at org.hibernate.boot.registry.internal.StandardServiceRegistryImpl.configureService(StandardServiceRegistryImpl.java:100)
        at org.hibernate.service.internal.AbstractServiceRegistryImpl.initializeService(AbstractServiceRegistryImpl.java:246)
        at org.hibernate.service.internal.AbstractServiceRegistryImpl.getService(AbstractServiceRegistryImpl.java:214)
        at org.hibernate.engine.jdbc.env.internal.JdbcEnvironmentInitiator.buildJdbcConnectionAccess(JdbcEnvironmentInitiator.java:145)
        at org.hibernate.engine.jdbc.env.internal.JdbcEnvironmentInitiator.initiateService(JdbcEnvironmentInitiator.java:66)
        at org.hibernate.engine.jdbc.env.internal.JdbcEnvironmentInitiator.initiateService(JdbcEnvironmentInitiator.java:35)
        at org.hibernate.boot.registry.internal.StandardServiceRegistryImpl.initiateService(StandardServiceRegistryImpl.java:94)
        at org.hibernate.service.internal.AbstractServiceRegistryImpl.createService(AbstractServiceRegistryImpl.java:263)
        ... 21 more
Caused by: org.hibernate.boot.registry.classloading.spi.ClassLoadingException: Unable to load class [org.postgresql.Driver]
        at org.hibernate.boot.registry.classloading.internal.ClassLoaderServiceImpl.classForName(ClassLoaderServiceImpl.java:134)
        at org.hibernate.c3p0.internal.C3P0ConnectionProvider.configure(C3P0ConnectionProvider.java:127)
        ... 29 more
Caused by: java.lang.ClassNotFoundException: Could not load requested class : org.postgresql.Driver
        at org.hibernate.boot.registry.classloading.internal.AggregatedClassLoader.findClass(AggregatedClassLoader.java:210)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
        at java.lang.Class.forName0(Native Method)
        at java.lang.Class.forName(Class.java:348)
        at org.hibernate.boot.registry.classloading.internal.ClassLoaderServiceImpl.classForName(ClassLoaderServiceImpl.java:131)
        ... 30 more

Here I am going to provide solution to the problem. For loading postgresql classes you need to create a module for postgres and reference that module in your application jar.

Steps for creating modue is:

1. Copy postgres jar to /home/wildfly/modules/system/layers/base/org/postgres/main/postgresql-42.2.2.jar
2. Create /home/wildfly/modules/system/layers/base/org/postgres/main/module.xml file with following content.

<module xmlns="urn:jboss:module:1.7" name="org.postgres">
        <resources>
                <resource-root path="postgresql-42.2.2.jar" />
        </resources>
        <dependencies>
                <module name="javax.api"/>
                <module name="javax.transaction.api"/>
        </dependencies>
</module>

Now you need to refer the module from your application JAR. For doing that you need to create jboss-deployment-structure.xml file with following content:

<?xml version="1.0" encoding="UTF-8"?>
  <jboss-deployment-structure>
      <ear-subdeployments-isolated>false</ear-subdeployments-isolated>
          <deployment>
                <resources>
                        <resource-root path="myapplication.jar" />
                </resources>
                <dependencies>
                    <module name="org.hibernate" export="true"/>
                    <module name="javax.api" />
                    <module name="org.postgres" export="true"/>
                    <module name="org.infinispan" />
                    <module name="org.apache.commons.lang" />
                </dependencies>
                <exclusions>
                        <module name="org.jboss.ejb-client"/>
                </exclusions>

          </deployment>
  </jboss-deployment-structure>

Please note that export="true" is required, without this class will not be loaded.