Wildfly Failed To Define Class
By March 8, 2018 October 17, 2018Every developer has the goal of building the most resilient application possible. Due to the distributed nature of microservices, resiliency and handling failures gracefully is mandatory. The Java ecosystem has some nice frameworks for fault tolerance, such as.
However, none of these provide a standard API, so using them means your application will be tightly coupled to that framework. The primary motivation for the MicroProfile specifications is to provide standard APIs that eliminates the tight coupling and improves deployment flexibility. This article will describe the main features of the MicroProfile Fault Tolerance specification, and then demonstrate how it was implemented in, the Red Hat MicroProfile implementation.About Eclipse MicroProfile Fault Toleranceis one of the specifications that provides a standard and easy way to add resiliency to your microservice or other Java EE development.Note: see Jeff Mesnil’s article onLike most MicroProfile specifications, Fault Tolerance is based on Contexts and Dependency Injection (CDI) and more precisely on the CDI interceptor implementation. It also relies on the MicroProfile Config specification to allow external configuration for Fault Tolerance policies.The main idea of the spec is to decouple business logic from Fault Tolerance boilerplate code. To achieve that, the spec defines interceptor binding annotations to apply Fault Tolerance policies on a method execution or on a class (in that case all class methods have the same policy).Policies included in the Fault Tolerance specification are the following:. Timeout: applied with @Timeout annotation. It adds a timeout to the current operation.
Retry: applied with @Retry annotation. It adds retry behavior and allows its configuration on the current operation. Fallback: applied with @Fallback annotation. It defines the code to execute, should the current operation fail. Bulkhead: applied with @Bulkhead annotation. It isolates failures in the current operation to preserve execution of other operations.
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Circuit Breaker: applied with @CircuitBreaker annotation. It provides an automatic fast failing execution to prevent overloading the system. Asynchronous: applied with @Asynchronous annotation. It makes the current operation asynchronous (i.e. Code will be invoked asynchronously).Applying one or more of these policies is as easy as adding the required annotations on the method (or the class) for which you’d like to have these policies enabled.
So, using Fault Tolerance is rather simple. But this simplicity doesn’t prevent flexibility thanks to all the configuration parameters available for each policy.Right now the following vendors are providing an implementation for the specification. Red Hat in WildFly Swarm. IBM in WebSphere Liberty. Payara in Payara Server. Apache Safeguard for Hammock and TomEE.
KumuluzEE for KumuluzEE frameworkIn this post we will focus on the implementation and usage.
.WildFly 8 Developer Guide Target AudienceJava Developers Prerequisites Class loading in WildFly 8Since JBoss AS7, Class loading is considerably different to previous versions of JBoss AS. Class loading is based on the project. Instead of the more familiar hierarchical class loading environment, WildFly's class loading is based on modules that have to define explicit dependencies on other modules. Deployments in WildFly are also modules, and do not have access to classes that are defined in jars in the application server unless an explicit dependency on those classes is defined.
Deployment Module NamesModule names for top level deployments follow the format deployment.myarchive.war while sub deployments are named like deployment.myear.ear.mywar.war.This means that it is possible for a deployment to import classes from another deployment using the other deployments module name, the details of how to add an explicit module dependency are explained below. Automatic DependenciesEven though in WildFly modules are isolated by default, as part of the deployment process some dependencies on modules defined by the application server are set up for you automatically.
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For instance, if you are deploying a Java EE application a dependency on the Java EE API's will be added to your module automatically. Similarly if your module contains a beans.xml file a dependency on will be added automatically, along with any supporting modules that weld needs to operate.For a complete list of the automatic dependencies that are added, please see.Automatic dependencies can be excluded through the use of jboss-deployment-structure.xml. Class Loading PrecedenceA common source of errors in Java applications is including API classes in a deployment that are also provided by the container.
This can result in multiple versions of the class being created and the deployment failing to deploy properly. To prevent this in WildFly, module dependencies are added in a specific order that should prevent this situation from occurring.In order of highest priority to lowest priority. System Dependencies - These are dependencies that are added to the module automatically by the container, including the Java EE api's.
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User Dependencies - These are dependencies that are added through jboss-deployment-structure.xml or through the Dependencies: manifest entry. Local Resource - Class files packaged up inside the deployment itself, e.g. Class files from WEB-INF/classes or WEB-INF/lib of a war. Inter deployment dependencies - These are dependencies on other deployments in an ear deployment. This can include classes in an ear's lib directory, or classes defined in other ejb jars.WAR Class LoadingThe war is considered to be a single module, so classes defined in WEB-INF/lib are treated the same as classes in WEB-INF/classes. All classes packaged in the war will be loaded with the same class loader.
EAR Class LoadingEar deployments are multi-module deployments. This means that not all classes inside an ear will necessarily have access to all other classes in the ear, unless explicit dependencies have been defined. By default the EAR/lib directory is a single module, and every WAR or EJB jar deployment is also a separate module. Sub deployments (wars and ejb-jars) always have a dependency on the parent module, which gives them access to classes in EAR/lib, however they do not always have an automatic dependency on each other. This behaviour is controlled via the ear-subdeployments-isolated setting in the ee subsystem configuration. The ear-subdeployments-isolated element value has no effect on the isolated classloader of the.war file(s). Irrespective of whether this flag is set to true or false, the.war within a.ear will have a isolated classloader and other sub-deployments within that.ear will not be able to access classes from that.war.
This is as per spec.If the ear-subdeployments-isolated is set to true then no automatic module dependencies between the sub-deployments are set up. User must manually setup the dependency with Class-Path entries, or by setting up explicit module dependencies.
It is also possible to override the ear-subdeployments-isolated element value at a per deployment level. See the section on jboss-deployment-structure.xml below.Dependencies: Manifest EntriesDeployments (or more correctly modules within a deployment) may set up dependencies on other modules by adding a Dependencies: manifest entry. This entry consists of a comma separated list of module names that the deployment requires. The available modules can be seen under the modules directory in the application server distribution. If your deployment is a jar you must use the maven-jar-plugin rather than the maven-war-plugin.Class Path EntriesIt is also possible to add module dependencies on other modules inside the deployment using the Class-Path manifest entry.
This can be used within an ear to set up dependencies between sub deployments, and also to allow modules access to additional jars deployed in an ear that are not sub deployments and are not in the EAR/lib directory. If a jar in the EAR/lib directory references a jar via Class-Path: then this additional jar is merged into the parent ear's module, and is accessible to all sub deployments in the ear. Global ModulesIt is also possible to set up global modules, that are accessible to all deployments. This is done by modifying the configuration file (standalone/domain.xml).For example, to add javassist to all deployments you can use the following XML. Note that the slot field is optional and defaults to main. JBoss Deployment Structure Filejboss-deployment-structure.xml is a JBoss specific deployment descriptor that can be used to control class loading in a fine grained manner. It should be placed in the top level deployment, in META-INF (or WEB-INF for web deployments).
It can do the following:. Prevent automatic dependencies from being added.
Add additional dependencies. Define additional modules. Change an EAR deployments isolated class loading behaviour. Add additional resource roots to a moduleAn example of a complete jboss-deployment-structure.xml file for an ear deployment is as follows. Implicit module dependencies for deploymentsAs explained in the article, WildFly 8 is based on module classloading. A class within a module B isn't visible to a class within a module A, unless module B adds a dependency on module A.
Module dependencies can be explicitly (as explained in that classloading article) or can be 'implicit'. This article will explain what implicit module dependencies mean and how, when and which modules are added as implicit dependencies.
What's an implicit module dependency?Consider an application deployment which contains EJBs. EJBs typically need access to classes from the javax.ejb. package and other Java EE API packages.
The jars containing these packages are already shipped in WildFly and are available as 'modules'. The module which contains the javax.ejb. classes has a specific name and so does the module which contains all the Java EE API classes. For an application to be able to use these classes, it has to add a dependency on the relevant modules.
Forcing the application developers to add module dependencies like these (i.e. Dependencies which can be 'inferred') isn't a productive approach. Hence, whenever an application is being deployed, the deployers within the server, which are processing this deployment 'implicitly' add these module dependencies to the deployment so that these classes are visible to the deployment at runtime.
This way the application developer doesn't have to worry about adding them explicitly. How and when these implicit dependencies are added is explained in the next section.
How and when is an implicit module dependency added?When a deployment is being processed by the server, it goes through a chain of 'deployment processors'. Each of these processors will have a way to check if the deployment meets a certain criteria and if it does, the deployment processor adds a implicit module dependency to that deployment. Let's take an example - Consider (again) an EJB3 deployment which has the following class. As can be seen, we have a simple @Stateless EJB. When the deployment containing this class is being processed, the EJB deployment processor will see that the deployment contains a class with the @Stateless annotation and thus identifies this as a EJB deployment.
This is just one of the several ways, various deployment processors can identify a deployment of some specific type. The EJB deployment processor will then add an implicit dependency on the Java EE API module, so that all the Java EE API classes are visible to the deployment.Some subsystems will always add a API classes, even if the trigger condition is not met. These are listed separately below.In the next section, we'll list down the implicit module dependencies that are added to a deployment, by various deployers within WildFly. Which are the implicit module dependencies?