2. Introduction to Spring Framework

• Maven Central, which is the default repository that Maven queries, and does not require any special configuration to use. Many of the common libraries that Spring depends on also are available from Maven Central and a large section of the Spring community uses Maven for dependency management, so this is convenient for them. The names of the jars here are in the form spring-*-<version>.jar and the Maven groupId is org.springframework.
• In a public Maven repository hosted specifically for Spring. In addition to the final GA releases, this repository also hosts development snapshots and milestones. The jar file names are in the same form as Maven Central, so this is a useful place to get development versions of Spring to use with other libraries deployed in Maven Central. This repository also contains a bundle distribution zip file that contains all Spring jars bundled together for easy download.

Although Spring provides integration and support for a huge range of enterprise and other external tools, it intentionally keeps its mandatory dependencies to an absolute minimum: you shouldn’t have to locate and download (even automatically) a large number of jar libraries in order to use Spring for simple use cases. For basic dependency injection there is only one mandatory external dependency, and that is for logging (see below for a more detailed description of logging options).

Next we outline the basic steps needed to configure an application that depends on Spring, first with Maven and then with Gradle and finally using Ivy. In all cases, if anything is unclear, refer to the documentation of your dependency management system, or look at some sample code - Spring itself uses Gradle to manage dependencies when it is building, and our samples mostly use Gradle or Maven.

If you are using Maven for dependency management you don’t even need to supply the logging dependency explicitly. For example, to create an application context and use dependency injection to configure an application, your Maven dependencies will look like this:

<dependencies>    <dependency>        <groupId>org.springframework</groupId>        <artifactId>spring-context</artifactId>        <version>4.1.3.RELEASE</version>        <scope>runtime</scope>    </dependency></dependencies>

That’s it. Note the scope can be declared as runtime if you don’t need to compile against Spring APIs, which is typically the case for basic dependency injection use cases.

The example above works with the Maven Central repository. To use the Spring Maven repository (e.g. for milestones or developer snapshots), you need to specify the repository location in your Maven configuration. For full releases:

<repositories>    <repository>        <id>io.spring.repo.maven.release</id>        <url>http://repo.spring.io/release/</url>        <snapshots><enabled>false</enabled></snapshots>    </repository></repositories>

For milestones:

<repositories>    <repository>        <id>io.spring.repo.maven.milestone</id>        <url>http://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>http://repo.spring.io/snapshot/</url>        <snapshots><enabled>true</enabled></snapshots>    </repository></repositories>

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 declare a 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 yourdependencyManagement section to ensure that all spring dependencies (both direct and transitive) are at the same version.

<dependencyManagement>    <dependencies>        <dependency>            <groupId>org.springframework</groupId>            <artifactId>spring-framework-bom</artifactId>            <version>4.1.3.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>

To use the Spring repository with the Gradle build system, include the appropriate URL in the repositories section:

repositories {    mavenCentral()    // and optionally...    maven { url "http://repo.spring.io/release" }}

You can change the repositories URL from /release to /milestone or /snapshot as appropriate. Once a repository has been configured, you can declare dependencies in the usual Gradle way:

dependencies {    compile("org.springframework:spring-context:4.1.3.RELEASE")    testCompile("org.springframework:spring-test:4.1.3.RELEASE")}

If you prefer to use Ivy to manage dependencies then there are similar configuration options.

To configure Ivy to point to the Spring repository add the following resolver to your ivysettings.xml:

<resolvers>    <ibiblio name="io.spring.repo.maven.release"            m2compatible="true"            root="http://repo.spring.io/release/"/></resolvers>

You can change the root URL from /release/ to /milestone/ or /snapshot/ as appropriate.

Once configured, you can add dependencies in the usual way. For example (in ivy.xml):

<dependency org="org.springframework"    name="spring-core" rev="4.1.3.RELEASE" conf="compile->runtime"/>

Although using a build system that supports dependency management is the recommended way to obtain the Spring Framework, it is still possible to download a distribution zip file.

Distribution zips are published to the Spring Maven Repository (this is just for our convenience, you don’t need Maven or any other build system in order to download them).

To download a distribution zip open a web browser to http://repo.spring.io/release/org/springframework/spring and select the appropriate subfolder for the version that you want. Distribution files end -dist.zip, for example spring-framework-4.1.3.RELEASE-RELEASE-dist.zip. Distributions are also published for milestones andsnapshots.

Unfortunately, the runtime discovery algorithm in commons-logging, while convenient for the end-user, is problematic. If we could turn back the clock and start Spring now as a new project it would use a different logging dependency. The first choice would probably be the Simple Logging Facade for Java ( SLF4J), which is also used by a lot of other tools that people use with Spring inside their applications.

There are basically two ways to switch off commons-logging:

To exclude commons-logging, add the following to your dependencyManagement section:

<dependencies>    <dependency>        <groupId>org.springframework</groupId>        <artifactId>spring-core</artifactId>        <version>4.1.3.RELEASE</version>        <exclusions>            <exclusion>                <groupId>commons-logging</groupId>                <artifactId>commons-logging</artifactId>            </exclusion>        </exclusions>    </dependency></dependencies>

Now this application is probably broken because there is no implementation of the JCL API on the classpath, so to fix it a new one has to be provided. In the next section we show you how to provide an alternative implementation of JCL using SLF4J as an example.

SLF4J is a cleaner dependency and more efficient at runtime than commons-logging because it uses compile-time bindings instead of runtime discovery of the other logging frameworks it integrates. This also means that you have to be more explicit about what you want to happen at runtime, and declare it or configure it accordingly. SLF4J provides bindings to many common logging frameworks, so you can usually choose one that you already use, and bind to that for configuration and management.

SLF4J provides bindings to many common logging frameworks, including JCL, and it also does the reverse: bridges between other logging frameworks and itself. So to use SLF4J with Spring you need to replace the commons-logging dependency with the SLF4J-JCL bridge. Once you have done that then logging calls from within Spring will be translated into logging calls to the SLF4J API, so if other libraries in your application use that API, 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 binding from SLF4J to Log4J. You need to supply 4 dependencies (and exclude the existingcommons-logging): the bridge, the SLF4J API, the binding to Log4J, and the Log4J implementation itself. In Maven you would do that like this

<dependencies>    <dependency>        <groupId>org.springframework</groupId>        <artifactId>spring-core</artifactId>        <version>4.1.3.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>1.5.8</version>    </dependency>    <dependency>        <groupId>org.slf4j</groupId>        <artifactId>slf4j-api</artifactId>        <version>1.5.8</version>    </dependency>    <dependency>        <groupId>org.slf4j</groupId>        <artifactId>slf4j-log4j12</artifactId>        <version>1.5.8</version>    </dependency>    <dependency>        <groupId>log4j</groupId>        <artifactId>log4j</artifactId>        <version>1.2.14</version>    </dependency></dependencies>

That might seem like a lot of dependencies just to get some logging. Well it is, but it is optional, and it should behave better than the vanilla commons-logging with respect to classloader issues, notably if you are in a strict container like an OSGi platform. Allegedly there is also a performance benefit because the bindings are at compile-time not runtime.

A more common choice amongst SLF4J users, which uses fewer steps and generates fewer dependencies, is to bind directly to Logback. This removes the extra binding step because Logback implements SLF4J directly, so you only need to depend on two libraries not four ( jcl-over-slf4j and logback). If you do that you might also need to exclude the slf4j-api dependency from other external dependencies (not Spring), because you only want one version of that API on the classpath.

Many people use Log4j as a logging framework for configuration and management purposes. It’s efficient and well-established, and in fact it’s what we use at runtime when we build and test Spring. Spring also provides some utilities for configuring and initializing Log4j, so it has an optional compile-time dependency on Log4j in some modules.

To make Log4j work with the default JCL dependency ( commons-logging) all you need 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 users this is your dependency declaration:

<dependencies>    <dependency>        <groupId>org.springframework</groupId>        <artifactId>spring-core</artifactId>        <version>4.1.3.RELEASE</version>    </dependency>    <dependency>        <groupId>log4j</groupId>        <artifactId>log4j</artifactId>        <version>1.2.14</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=DEBUG