2.Introduction to the Spring Framework
The Spring Framework is a Java platform that provides comprehensive infrastructure supportfor developing Java applications. Spring handles the infrastructure so you can focus onyour application.
Spring enables you to build applications from "plain old Java objects" (POJOs) and toapply enterprise services non-invasively to POJOs. This capability applies to the JavaSE programming model and to full and partial Java EE.
Examples of how you, as an application developer, can benefit from the Spring platform:
2.1Dependency Injection and Inversion of Control
A Java application — a loose term that runs the gamut from constrained, embeddedapplications to n-tier, server-side enterprise applications — typically consists ofobjects that collaborate to form the application proper. Thus the objects in anapplication have dependencies on each other.
Although the Java platform provides a wealth of application development functionality,it lacks the means to organize the basic building blocks into a coherent whole, leavingthat task to architects and developers. Although you can use design patterns suchas Factory, Abstract Factory, Builder, Decorator, and Service Locatorto compose the various classes and object instances that make up an application,these patterns are simply that: best practices given a name, with a descriptionof what the pattern does, where to apply it, the problems it addresses, and so forth.Patterns are formalized best practices that you must implement yourself in yourapplication.
The Spring Framework Inversion of Control (IoC) component addresses this concern byproviding a formalized means of composing disparate components into a fully workingapplication ready for use. The Spring Framework codifies formalized design patterns asfirst-class objects that you can integrate into your own application(s). Numerousorganizations and institutions use the Spring Framework in this manner to engineerrobust, maintainable applications.
Background
"The question is, what aspect of control are [they] inverting?" Martin Fowler posedthis question about Inversion of Control (IoC)on his site in 2004. Fowler suggestedrenaming the principle to make it more self-explanatory and came up with DependencyInjection.
2.2Framework Modules
The Spring Framework consists of features organized into about 20 modules. These modulesare grouped into Core Container, Data Access/Integration, Web, AOP (Aspect OrientedProgramming), Instrumentation, Messaging, and Test, as shown in the following diagram.
Figure2.1.Overview of the Spring Framework
The following sections list the available modules for each feature along with theirartifact names and the topics they cover. Artifact names correlate to artifact IDs usedin Dependency Management tools.
2.2.1Core Container
The Core Container consists of the spring-core,spring-beans, spring-context, spring-context-support, and spring-expression(Spring Expression Language) modules.
The spring-core and spring-beans modules provide the fundamentalparts of the framework, including the IoC and Dependency Injection features. TheBeanFactory is a sophisticated implementation of the factory pattern. It removes theneed for programmatic singletons and allows you to decouple the configuration andspecification of dependencies from your actual program logic.
The Context (spring-context) module builds on the solidbase provided by the Core and Beans modules: it is a means toaccess objects in a framework-style manner that is similar to a JNDI registry. TheContext module inherits its features from the Beans module and adds support forinternationalization (using, for example, resource bundles), event propagation, resourceloading, and the transparent creation of contexts by, for example, a Servlet container.The Context module also supports Java EE features such as EJB, JMX, and basic remoting.The ApplicationContext interface is the focal point of the Context module.spring-context-support provides support for integrating common third-party librariesinto a Spring application context for caching (EhCache, Guava, JCache), mailing(JavaMail), scheduling (CommonJ, Quartz) and template engines (FreeMarker, JasperReports,Velocity).
The spring-expression module provides a powerful ExpressionLanguage for querying and manipulating an object graph at runtime. It is an extensionof the unified expression language (unified EL) as specified in the JSP 2.1specification. The language supports setting and getting property values, propertyassignment, method invocation, accessing the content of arrays, collections and indexers,logical and arithmetic operators, named variables, and retrieval of objects by name fromSpring’s IoC container. It also supports list projection and selection as well as commonlist aggregations.
2.2.2AOP and Instrumentation
The spring-aop module provides an AOP Alliance-compliantaspect-oriented programming implementation allowing you to define, for example,method interceptors and pointcuts to cleanly decouple code that implements functionalitythat should be separated. Using source-level metadata functionality, you can alsoincorporate behavioral information into your code, in a manner similar to that of .NETattributes.
The separate spring-aspects module provides integration with AspectJ.
The spring-instrument module provides class instrumentation support and classloaderimplementations to be used in certain application servers. The spring-instrument-tomcatmodule contains Spring’s instrumentation agent for Tomcat.
2.2.3Messaging
Spring Framework 4 includes a spring-messaging module with key abstractions from theSpring Integration project such as Message, MessageChannel, MessageHandler, andothers to serve as a foundation for messaging-based applications. The module alsoincludes a set of annotations for mapping messages to methods, similar to the Spring MVCannotation based programming model.
2.2.4Data Access/Integration
The Data Access/Integration layer consists of the JDBC, ORM, OXM, JMS, andTransaction modules.
The spring-jdbc module provides a JDBC-abstraction layer thatremoves the need to do tedious JDBC coding and parsing of database-vendor specific errorcodes.
The spring-tx module supports programmatic and declarative transactionmanagement for classes that implement special interfaces and for all your POJOs (PlainOld Java Objects).
The spring-orm module provides integration layers for popularobject-relational mapping APIs, including JPA,JDO, and Hibernate. Using the spring-orm module you canuse all of these O/R-mapping frameworks in combination with all of the other featuresSpring offers, such as the simple declarative transaction management feature mentionedpreviously.
The spring-oxm module provides an abstraction layer that supports Object/XMLmapping implementations such as JAXB, Castor, XMLBeans, JiBX and XStream.
The spring-jms module (Java Messaging Service) contains features for producing andconsuming messages. Since Spring Framework 4.1, it provides integration with thespring-messaging module.
2.2.5Web
The Web layer consists of the spring-web, spring-webmvc, spring-websocket, andspring-webmvc-portlet modules.
The spring-web module provides basic web-oriented integration features such asmultipart file upload functionality and the initialization of the IoC container usingServlet listeners and a web-oriented application context. It also contains an HTTP clientand the web-related parts of Spring’s remoting support.
The spring-webmvc module (also known as the Web-Servlet module) contains Spring’smodel-view-controller (MVC) and REST Web Services implementationfor web applications. Spring’s MVC framework provides a clean separation between domainmodel code and web forms and integrates with all of the other features of the SpringFramework.
The spring-webmvc-portlet module (also known as the Web-Portlet module) providesthe MVC implementation to be used in a Portlet environment and mirrors the functionalityof the Servlet-based spring-webmvc module.
2.2.6Test
The spring-test module supports the unit testing andintegration testing of Spring components with JUnit or TestNG. Itprovides consistent loading of SpringApplicationContexts and caching of thosecontexts. It also provides mock objects that you can use to test yourcode in isolation.
2.3Usage scenarios
The building blocks described previously make Spring a logical choice in many scenarios,from embedded applications that run on resource-constrained devices to full-fledgedenterprise applications that use Spring’s transaction management functionality and webframework integration.
Figure2.2.Typical full-fledged Spring web application
Spring’s declarative transaction management features makethe web application fully transactional, just as it would be if you used EJBcontainer-managed transactions. All your custom business logic can be implemented withsimple POJOs and managed by Spring’s IoC container. Additional services include supportfor sending email and validation that is independent of the web layer, which lets youchoose where to execute validation rules. Spring’s ORM support is integrated with JPA,Hibernate and JDO; for example, when using Hibernate, you can continue to useyour existing mapping files and standard Hibernate SessionFactory configuration. Formcontrollers seamlessly integrate the web-layer with the domain model, removing the needfor ActionForms or other classes that transform HTTP parameters to values for yourdomain model.
Figure2.3.Spring middle-tier using a third-party web framework
Sometimes circ*mstances do not allow you to completely switch to a different framework.The Spring Framework does not force you to use everything within it; it is not anall-or-nothing solution. Existing front-ends built with Struts, Tapestry, JSFor other UI frameworks can be integrated with a Spring-based middle-tier, which allowsyou to use Spring transaction features. You simply need to wire up your business logicusing an ApplicationContext and use a WebApplicationContext to integrate your weblayer.
Figure2.4.Remoting usage scenario
When you need to access existing code through web services, you can use Spring’sHessian-, Burlap-, Rmi- or JaxRpcProxyFactory classes. Enabling remote access toexisting applications is not difficult.
Figure2.5.EJBs - Wrapping existing POJOs
The Spring Framework also provides an access and abstraction layer forEnterprise JavaBeans, enabling you to reuse your existing POJOs and wrap them instateless session beans for use in scalable, fail-safe web applications that might needdeclarative security.
2.3.1Dependency Management and Naming Conventions
Dependency management and dependency injection are different things. To get those nicefeatures of Spring into your application (like dependency injection) you need toassemble all the libraries needed (jar files) and get them onto your classpath atruntime, and possibly at compile time. These dependencies are not virtual componentsthat are injected, but physical resources in a file system (typically). The process ofdependency management involves locating those resources, storing them and adding them toclasspaths. Dependencies can be direct (e.g. my application depends on Spring atruntime), or indirect (e.g. my application depends on commons-dbcp which depends oncommons-pool). The indirect dependencies are also known as "transitive" and it isthose dependencies that are hardest to identify and manage.
If you are going to use Spring you need to get a copy of the jar libraries that comprisethe pieces of Spring that you need. To make this easier Spring is packaged as a set ofmodules that separate the dependencies as much as possible, so for example if you don’twant to write a web application you don’t need the spring-web modules. To refer toSpring library modules in this guide we use a shorthand naming convention spring-* orspring-*.jar, where * represents the short name for the module(e.g. spring-core, spring-webmvc, spring-jms, etc.). The actual jar file name thatyou use is normally the module name concatenated with the version number(e.g. spring-core-4.3.30.RELEASE.jar).
Each release of the Spring Framework will publish artifacts to the following places:
- Maven Central, which is the default repository that Maven queries, and does notrequire any special configuration to use. Many of the common libraries that Springdepends on also are available from Maven Central and a large section of the Springcommunity uses Maven for dependency management, so this is convenient for them. Thenames of the jars here are in the form
spring-*-<version>.jarand the Maven groupIdisorg.springframework. - In a public Maven repository hosted specifically for Spring. In addition to the finalGA releases, this repository also hosts development snapshots and milestones. The jarfile names are in the same form as Maven Central, so this is a useful place to getdevelopment versions of Spring to use with other libraries deployed in Maven Central.This repository also contains a bundle distribution zip file that contains all Springjars bundled together for easy download.
So the first thing you need to decide is how to manage your dependencies: we generallyrecommend the use of an automated system like Maven, Gradle or Ivy, but you can also doit manually by downloading all the jars yourself.
Below you will find the list of Spring artifacts. For a more complete description ofeach module, see Section2.2, “Framework Modules”.
Table2.1.Spring Framework Artifacts
| GroupId | ArtifactId | Description |
|---|---|---|
org.springframework | spring-aop | Proxy-based AOP support |
org.springframework | spring-aspects | AspectJ based aspects |
org.springframework | spring-beans | Beans support, including Groovy |
org.springframework | spring-context | Application context runtime, including scheduling and remoting abstractions |
org.springframework | spring-context-support | Support classes for integrating common third-party libraries into a Spring application context |
org.springframework | spring-core | Core utilities, used by many other Spring modules |
org.springframework | spring-expression | Spring Expression Language (SpEL) |
org.springframework | spring-instrument | Instrumentation agent for JVM bootstrapping |
org.springframework | spring-instrument-tomcat | Instrumentation agent for Tomcat |
org.springframework | spring-jdbc | JDBC support package, including DataSource setup and JDBC access support |
org.springframework | spring-jms | JMS support package, including helper classes to send/receive JMS messages |
org.springframework | spring-messaging | Support for messaging architectures and protocols |
org.springframework | spring-orm | Object/Relational Mapping, including JPA and Hibernate support |
org.springframework | spring-oxm | Object/XML Mapping |
org.springframework | spring-test | Support for unit testing and integration testing Spring components |
org.springframework | spring-tx | Transaction infrastructure, including DAO support and JCA integration |
org.springframework | spring-web | Foundational web support, including web client and web-based remoting |
org.springframework | spring-webmvc | HTTP-based Model-View-Controller and REST endpoints for Servlet stacks |
org.springframework | spring-webmvc-portlet | MVC implementation to be used in a Portlet environment |
org.springframework | spring-websocket | WebSocket and SockJS infrastructure, including STOMP messaging support |
Spring Dependencies and Depending on Spring
Although Spring provides integration and support for a huge range of enterprise andother external tools, it intentionally keeps its mandatory dependencies to an absoluteminimum: you shouldn’t have to locate and download (even automatically) a large numberof jar libraries in order to use Spring for simple use cases. For basic dependencyinjection there is only one mandatory external dependency, and that is for logging (seebelow for a more detailed description of logging options).
Next we outline the basic steps needed to configure an application that depends onSpring, first with Maven and then with Gradle and finally using Ivy. In all cases, ifanything is unclear, refer to the documentation of your dependency management system, orlook at some sample code - Spring itself uses Gradle to manage dependencies when it isbuilding, and our samples mostly use Gradle or Maven.
Maven Dependency Management
If you are using Maven for dependency management you don’t evenneed to supply the logging dependency explicitly. For example, to create an applicationcontext and use dependency injection to configure an application, your Maven dependencieswill look like this:
<dependencies> <dependency> <groupId>org.springframework</groupId> <artifactId>spring-context</artifactId> <version>4.3.30.RELEASE</version> <scope>runtime</scope> </dependency></dependencies>
That’s it. Note the scope can be declared as runtime if you don’t need to compileagainst Spring APIs, which is typically the case for basic dependency injection usecases.
The example above works with the Maven Central repository. To use the Spring Mavenrepository (e.g. for milestones or developer snapshots), you need to specify therepository location in your Maven configuration. For full releases:
<repositories> <repository> <id>io.spring.repo.maven.release</id> <url>https://repo.spring.io/release/</url> <snapshots><enabled>false</enabled></snapshots> </repository></repositories>
For milestones:
<repositories> <repository> <id>io.spring.repo.maven.milestone</id> <url>https://repo.spring.io/milestone/</url> <snapshots><enabled>false</enabled></snapshots> </repository></repositories>
And for snapshots:
<repositories> <repository> <id>io.spring.repo.maven.snapshot</id> <url>https://repo.spring.io/snapshot/</url> <snapshots><enabled>true</enabled></snapshots> </repository></repositories>
Maven "Bill Of Materials" Dependency
It is possible to accidentally mix different versions of Spring JARs when using Maven.For example, you may find that a third-party library, or another Spring project,pulls in a transitive dependency to an older release. If you forget to explicitly declarea direct dependency yourself, all sorts of unexpected issues can arise.
To overcome such problems Maven supports the concept of a "bill of materials" (BOM)dependency. You can import the spring-framework-bom in your dependencyManagementsection to ensure that all spring dependencies (both direct and transitive) are atthe same version.
<dependencyManagement> <dependencies> <dependency> <groupId>org.springframework</groupId> <artifactId>spring-framework-bom</artifactId> <version>4.3.30.RELEASE</version> <type>pom</type> <scope>import</scope> </dependency> </dependencies></dependencyManagement>
An added benefit of using the BOM is that you no longer need to specify the <version>attribute when depending on Spring Framework artifacts:
<dependencies> <dependency> <groupId>org.springframework</groupId> <artifactId>spring-context</artifactId> </dependency> <dependency> <groupId>org.springframework</groupId> <artifactId>spring-web</artifactId> </dependency><dependencies>
Gradle Dependency Management
To use the Spring repository with the Gradle build system,include the appropriate URL in the repositories section:
repositories { mavenCentral() // and optionally... maven { url "https://repo.spring.io/release" }}You can change the repositories URL from /release to /milestone or /snapshot asappropriate. Once a repository has been configured, you can declare dependencies in theusual Gradle way:
dependencies { compile("org.springframework:spring-context:4.3.30.RELEASE") testCompile("org.springframework:spring-test:4.3.30.RELEASE")}Ivy Dependency Management
If you prefer to use Ivy to manage dependencies then thereare similar configuration options.
To configure Ivy to point to the Spring repository add the following resolver to yourivysettings.xml:
<resolvers> <ibiblio name="io.spring.repo.maven.release" m2compatible="true" root="https://repo.spring.io/release/"/></resolvers>
You can change the root URL from /release/ to /milestone/ or /snapshot/ asappropriate.
Once configured, you can add dependencies in the usual way. For example (in ivy.xml):
<dependency org="org.springframework" name="spring-core" rev="4.3.30.RELEASE" conf="compile->runtime"/>
Distribution Zip Files
Although using a build system that supports dependency management is the recommendedway to obtain the Spring Framework, it is still possible to download a distributionzip file.
Distribution zips are published to the Spring Maven Repository (this is just for ourconvenience, you don’t need Maven or any other build system in order to download them).
To download a distribution zip open a web browser tohttps://repo.spring.io/release/org/springframework/spring and select the appropriatesubfolder for the version that you want. Distribution files end -dist.zip, for examplespring-framework-{spring-version}-RELEASE-dist.zip. Distributions are also publishedfor milestones andsnapshots.
2.3.2Logging
Logging is a very important dependency for Spring because a) it is the only mandatoryexternal dependency, b) everyone likes to see some output from the tools they areusing, and c) Spring integrates with lots of other tools all of which have also madea choice of logging dependency. One of the goals of an application developer is often tohave unified logging configured in a central place for the whole application, includingall external components. This is more difficult than it might have been since there are somany choices of logging framework.
The mandatory logging dependency in Spring is the Jakarta Commons Logging API (JCL). Wecompile against JCL and we also make JCL Log objects visible for classes that extendthe Spring Framework. It’s important to users that all versions of Spring use the samelogging library: migration is easy because backwards compatibility is preserved evenwith applications that extend Spring. The way we do this is to make one of the modulesin Spring depend explicitly on commons-logging (the canonical implementation of JCL),and then make all the other modules depend on that at compile time. If you are usingMaven for example, and wondering where you picked up the dependency on commons-logging,then it is from Spring and specifically from the central module called spring-core.
The nice thing about commons-logging is that you don’t need anything else to make yourapplication work. It has a runtime discovery algorithm that looks for other loggingframeworks in well known places on the classpath and uses one that it thinks isappropriate (or you can tell it which one if you need to). If nothing else is availableyou get pretty nice looking logs just from the JDK (java.util.logging or JUL for short).You should find that your Spring application works and logs happily to the console outof the box in most situations, and that’s important.
Using Log4j 1.2 or 2.x
 | Note |
|---|---|
Log4j 1.2 is EOL in the meantime. Also, Log4j 2.3 is the last Java 6 compatiblerelease, with newer Log4j 2.x releases requiring Java 7+. |
Many people use Log4j as a logging framework forconfiguration and management purposes. It is efficient and well-established, and infact it is what we use at runtime when we build Spring. Spring also provides someutilities for configuring and initializing Log4j, so it has an optional compile-timedependency on Log4j in some modules.
To make Log4j 1.2 work with the default JCL dependency (commons-logging) all youneed to do is put Log4j on the classpath, and provide it with a configuration file(log4j.properties or log4j.xml in the root of the classpath). So for Maven usersthis is your dependency declaration:
<dependencies> <dependency> <groupId>org.springframework</groupId> <artifactId>spring-core</artifactId> <version>4.3.30.RELEASE</version> </dependency> <dependency> <groupId>log4j</groupId> <artifactId>log4j</artifactId> <version>1.2.17</version> </dependency></dependencies>
And here’s a sample log4j.properties for logging to the console:
log4j.rootCategory=INFO, stdoutlog4j.appender.stdout=org.apache.log4j.ConsoleAppenderlog4j.appender.stdout.layout=org.apache.log4j.PatternLayoutlog4j.appender.stdout.layout.ConversionPattern=%d{ABSOLUTE} %5p %t %c{2}:%L - %m%nlog4j.category.org.springframework.beans.factory=DEBUGTo use Log4j 2.x with JCL, all you need to do is put Log4j on the classpath andprovide it with a configuration file (log4j2.xml, log4j2.properties, or othersupported configurationformats). For Maven users, the minimal dependencies needed are:
<dependencies> <dependency> <groupId>org.apache.logging.log4j</groupId> <artifactId>log4j-core</artifactId> <version>2.6.2</version> </dependency> <dependency> <groupId>org.apache.logging.log4j</groupId> <artifactId>log4j-jcl</artifactId> <version>2.6.2</version> </dependency></dependencies>
If you also wish to enable SLF4J to delegate to Log4j, e.g. for other librarieswhich use SLF4J by default, the following dependency is also needed:
<dependencies> <dependency> <groupId>org.apache.logging.log4j</groupId> <artifactId>log4j-slf4j-impl</artifactId> <version>2.6.2</version> </dependency></dependencies>
Here is an example log4j2.xml for logging to the console:
<?xml version="1.0" encoding="UTF-8"?><Configuration status="WARN"> <Appenders> <Console name="Console" target="SYSTEM_OUT"> <PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n"/> </Console> </Appenders> <Loggers> <Logger name="org.springframework.beans.factory" level="DEBUG"/> <Root level="error"> <AppenderRef ref="Console"/> </Root> </Loggers></Configuration>
Avoiding Commons Logging
Unfortunately, the runtime discovery algorithm in the standard commons-logging API,while convenient for the end-user, can be problematic. If you’d like to avoid JCL’sstandard lookup, there are basically two ways to switch it off:
- Exclude the dependency from the
spring-coremodule (as it is the only module thatexplicitly depends oncommons-logging) - Depend on a special
commons-loggingdependency that replaces the library withan empty jar (more details can be found in theSLF4J FAQ)
To exclude commons-logging, add the following to your dependencyManagement section:
<dependencies> <dependency> <groupId>org.springframework</groupId> <artifactId>spring-core</artifactId> <version>4.3.30.RELEASE</version> <exclusions> <exclusion> <groupId>commons-logging</groupId> <artifactId>commons-logging</artifactId> </exclusion> </exclusions> </dependency></dependencies>
Now this application is currently broken because there is no implementation of the JCLAPI on the classpath, so to fix it a new one has to be provided. In the next section weshow you how to provide an alternative implementation of JCL using SLF4J.
Using SLF4J with Log4j or Logback
The Simple Logging Facade for Java (SLF4J) is a popular APIused by other libraries commonly used with Spring. It is typically used withLogback which is a native implementation of the SLF4J API.
SLF4J provides bindings to many common logging frameworks, including Log4j, and it alsodoes the reverse: bridges between other logging frameworks and itself. So to use SLF4Jwith Spring you need to replace the commons-logging dependency with the SLF4J-JCLbridge. Once you have done that then logging calls from within Spring will be translatedinto logging calls to the SLF4J API, so if other libraries in your application use thatAPI, then you have a single place to configure and manage logging.
A common choice might be to bridge Spring to SLF4J, and then provide explicit bindingfrom SLF4J to Log4j. You need to supply several dependencies (and exclude the existingcommons-logging): the JCL bridge, the SLF4j binding to Log4j, and the Log4j provideritself. In Maven you would do that like this
<dependencies> <dependency> <groupId>org.springframework</groupId> <artifactId>spring-core</artifactId> <version>4.3.30.RELEASE</version> <exclusions> <exclusion> <groupId>commons-logging</groupId> <artifactId>commons-logging</artifactId> </exclusion> </exclusions> </dependency> <dependency> <groupId>org.slf4j</groupId> <artifactId>jcl-over-slf4j</artifactId> <version>1.7.21</version> </dependency> <dependency> <groupId>org.slf4j</groupId> <artifactId>slf4j-log4j12</artifactId> <version>1.7.21</version> </dependency> <dependency> <groupId>log4j</groupId> <artifactId>log4j</artifactId> <version>1.2.17</version> </dependency></dependencies>
A more common choice amongst SLF4J users, which uses fewer steps and generates fewerdependencies, is to bind directly to Logback. This removes theextra binding step because Logback implements SLF4J directly, so you only need to dependon just two libraries, namely jcl-over-slf4j and logback):
<dependencies> <dependency> <groupId>org.slf4j</groupId> <artifactId>jcl-over-slf4j</artifactId> <version>1.7.21</version> </dependency> <dependency> <groupId>ch.qos.logback</groupId> <artifactId>logback-classic</artifactId> <version>1.1.7</version> </dependency></dependencies>
Using JUL (java.util.logging)
Commons Logging will delegate to java.util.logging by default, provided that noLog4j is detected on the classpath. So there is no special dependency to set up:just use Spring with no external dependency for log output to java.util.logging,either in a standalone application (with a custom or default JUL setup at the JDKlevel) or with an application server’s log system (and its system-wide JUL setup).
Commons Logging on WebSphere
Spring applications may run on a container that itself provides an implementation ofJCL, e.g. IBM’s WebSphere Application Server (WAS). This does not cause issues per sebut leads to two different scenarios that need to be understood:
In a "parent first" ClassLoader delegation model (the default on WAS), applicationswill always pick up the server-provided version of Commons Logging, delegating to theWAS logging subsystem (which is actually based on JUL). An application-provided variantof JCL, whether standard Commons Logging or the JCL-over-SLF4J bridge, will effectivelybe ignored, along with any locally included log provider.
With a "parent last" delegation model (the default in a regular Servlet container butan explicit configuration option on WAS), an application-provided Commons Loggingvariant will be picked up, enabling you to set up a locally included log provider,e.g. Log4j or Logback, within your application. In case of no local log provider,regular Commons Logging will delegate to JUL by default, effectively logging toWebSphere’s logging subsystem like in the "parent first" scenario.
All in all, we recommend deploying Spring applications in the "parent last" modelsince it naturally allows for local providers as well as the server’s log subsystem.
