To add a maven dependency, we have to make changes in pom.xml to include that dependency. Maven scans the pom.xml and downloads the required jar for the added dependency.
We have a sample project with one simple java class in this path : \MavenTestApp\src\main\java\com\org\j2eereference
App.java
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package com.org.j2eereference; /** * Hello world! * */ public class App { public static void main( String[] args ) { System.out.println( "Hello World!" ); } } |
This is our pom.xml in the \src folder.
pom.xml
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<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> <modelVersion>4.0.0</modelVersion> <groupId>com.org.j2eereference</groupId> <artifactId>MavenTestApp</artifactId> <version>1.0-SNAPSHOT</version> <packaging>jar</packaging> <name>MavenTestApp</name> <url>http://maven.apache.org</url> <properties> <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding> </properties> <dependencies> <dependency> <groupId>junit</groupId> <artifactId>junit</artifactId> <version>3.8.1</version> <scope>test</scope> </dependency> </dependencies> </project> |
In the pom.xml, notice that the only dependency we have is ‘junit’.
In App.java class, we will try to add ‘logging framework’ dependency. We will use the slf4j logging framework to log the test message. If we were mot using Maven, we would download the slf4j jars and add it to the classpath. Only after that we would be able to compile any class file that has dependency for those classes in the jar. With Maven we don’t have to do these steps. Maven automatically does all these steps for us.
App.java
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package com.org.j2eereference; import org.slf4j.*; /** * Hello world! * */ public class App { public static void main( String[] args ) { //System.out.println( "Hello World!" ); Logger logger = LoggerFactory.getLogger(App.class); logger.info("Hello World!"); } } |
Lets run mvn compile and see what happens
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C:\MyApp\MavenTestApp> mvn compile [INFO] Scanning for projects... [INFO] [INFO] ------------------------------------------------------------------------ [INFO] Building MavenTestApp 1.0-SNAPSHOT [INFO] ------------------------------------------------------------------------ [INFO] [INFO] --- maven-resources-plugin:2.6:resources (default-resources) @ MavenTestApp --- [INFO] Using 'UTF-8' encoding to copy filtered resources. [INFO] skip non existing resourceDirectory C:\MyApp\MavenTestApp\src\main\resources [INFO] [INFO] --- maven-compiler-plugin:3.1:compile (default-compile) @ MavenTestApp --- [INFO] Changes detected - recompiling the module! [INFO] Compiling 1 source file to C:\MyApp\MavenTestApp\target\classes [INFO] ------------------------------------------------------------- [ERROR] COMPILATION ERROR : [INFO] ------------------------------------------------------------- [ERROR] /C:/MyApp/MavenTestApp/src/main/java/com/org/j2eereference/App.java:[3,1] package org.slf4j does not exist [ERROR] /C:/MyApp/MavenTestApp/src/main/java/com/org/j2eereference/App.java:[14,9] cannot find symbol symbol: class Logger location: class com.org.j2eereference.App [ERROR] /C:/MyApp/MavenTestApp/src/main/java/com/org/j2eereference/App.java:[14,25] cannot find symbol symbol: variable LoggerFactory location: class com.org.j2eereference.App [INFO] 3 errors [INFO] ------------------------------------------------------------- [INFO] ------------------------------------------------------------------------ [INFO] BUILD FAILURE [INFO] ------------------------------------------------------------------------ [INFO] Total time: 5.826 s [INFO] Finished at: 2015-08-06T22:08:00+05:30 [INFO] Final Memory: 10M/121M [INFO] ------------------------------------------------------------------------ [ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.1:compile (default-compile) on project MavenTestApp: Compilation failure: Compilation failure: [ERROR] /C:/MyApp/MavenTestApp/src/main/java/com/org/j2eereference/App.java:[3,1] package org.slf4j does not exist [ERROR] /C:/MyApp/MavenTestApp/src/main/java/com/org/j2eereference/App.java:[14,9] cannot find symbol [ERROR] symbol: class Logger [ERROR] location: class com.org.j2eereference.App [ERROR] /C:/MyApp/MavenTestApp/src/main/java/com/org/j2eereference/App.java:[14,25] cannot find symbol [ERROR] symbol: variable LoggerFactory [ERROR] location: class com.org.j2eereference.App [ERROR] -> [Help 1] [ERROR] [ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch. [ERROR] Re-run Maven using the -X switch to enable full debug logging. [ERROR] [ERROR] For more information about the errors and possible solutions, please read the following articles: [ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoFailureException |
We get the above error. Note that the error message says it failed to execute the maven compiler plugin. It says that org.slf4j package does not exist because it cannot do an import as there is no such class in the classpath and hence it cannot resolve the symbols ‘Logger’ and ‘LoggerFactory’. This problem is because we don’t have the jar file in place. So, we need to tell Maven to pull up the jar files from central repository. To do that, we need to add a dependency for slf4j in pom.xml with proper groupId, artifactId and version of slf4j. These are the coordinates for Maven to identify any jar file. How do we find this information?? A simple google search for ‘maven repository search’ would help us here. It will list out Maven repositories. Pick any one and search for slf4j inside the repository. It will give all the information we need. It will also show the maven coordinates to be included in the pom.xml for this dependency. It is here : <dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-api</artifactId>
<version>1.7.12</version>
</dependency>
After adding this dependency in the pom.xml, run mvn compile.
What happens now is, before MAven issues the compile command, it scans the pom.xml and finds that new dependency added and downloads slf4j jars from maven repository. Once all the jars are downloaded, Maven includes them in the classpath to makes it available for compile.
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C:\MyApp\MavenTestApp> mvn compile [INFO] Scanning for projects... [INFO] [INFO] ------------------------------------------------------------------------ [INFO] Building MavenTestApp 1.0-SNAPSHOT [INFO] ------------------------------------------------------------------------ Downloading: https://repo.maven.apache.org/maven2/org/slf4j/slf4j-api/1.7.12/slf4j-api-1.7.12.pom 3/3 KB Downloaded: https://repo.maven.apache.org/maven2/org/slf4j/slf4j-api/1.7.12/slf4j-api-1.7.12.pom (3 KB at 1.0 KB/sec) Downloading: https://repo.maven.apache.org/maven2/org/slf4j/slf4j-parent/1.7.12/slf4j-parent-1.7.12.pom 4/12 KB 8/12 KB 12/12 KB Downloaded: https://repo.maven.apache.org/maven2/org/slf4j/slf4j-parent/1.7.12/slf4j-parent-1.7.12.pom (12 KB at 18.7 KB/sec) Downloading: https://repo.maven.apache.org/maven2/org/slf4j/slf4j-api/1.7.12/slf4j-api-1.7.12.jar 4/32 KB 8/32 KB 12/32 KB 16/32 KB 20/32 KB 24/32 KB 28/32 KB 32/32 KB 32/32 KB Downloaded: https://repo.maven.apache.org/maven2/org/slf4j/slf4j-api/1.7.12/slf4j-api-1.7.12.jar (32 KB at 27.4 KB/sec) [INFO] [INFO] --- maven-resources-plugin:2.6:resources (default-resources) @ MavenTestApp --- [INFO] Using 'UTF-8' encoding to copy filtered resources. [INFO] skip non existing resourceDirectory C:\MyApp\MavenTestApp\src\main\resources [INFO] [INFO] --- maven-compiler-plugin:3.1:compile (default-compile) @ MavenTestApp --- [INFO] Changes detected - recompiling the module! [INFO] Compiling 1 source file to C:\MyApp\MavenTestApp\target\classes [INFO] ------------------------------------------------------------------------ [INFO] BUILD SUCCESS [INFO] ------------------------------------------------------------------------ [INFO] Total time: 7.876 s [INFO] Finished at: 2015-08-06T23:04:42+05:30 [INFO] Final Memory: 13M/159M [INFO] ------------------------------------------------------------------------ |
In the log we can notice that, it connects to the Maven repository and downloads the slf4j jars. This time build has been successful. The /target/classes folder is generated that contains the .class files.