If everything works fine, we should never have to worry about troubleshooting. However, most of the time this is not the case. A Maven build can fail for many reasons, some of which are under your control and also out of your control. Knowing proper troubleshooting tips helps you to pinpoint the exact problem. The following section lists some of the most used troubleshooting tips. We will expand the list as we proceed in this book.
Once the Maven debug level logging is enabled, it will print all the actions that it takes during the build process. To enable debug level logging, use the following command:
$ mvn clean install –X
If you find any issue with any dependency in your Maven project, the first step is to build a dependency tree. This shows where each dependency comes from. To build the dependency tree, run the following command against your project POM file:
$ mvn dependency:tree
The following shows the truncated output of the previous command executed against the Apache Rampart project:
[INFO] -------------------------------------------------------------- [INFO] Building Rampart - Trust 1.6.1-wso2v12 [INFO] -------------------------------------------------------------- [INFO] [INFO] --- maven-dependency-plugin:2.1:tree (default-cli) @ rampart-trust --- [INFO] org.apache.rampart:rampart-trust:jar:1.6.1-wso2v12 [INFO] +- org.apache.rampart:rampart-policy:jar:1.6.1-wso2v12:compile [INFO] +- org.apache.axis2:axis2-kernel:jar:1.6.1-wso2v10:compile [INFO] | +- org.apache.ws.commons.axiom:axiom-api:jar:1.2.11-wso2v4:compile (version managed from 1.2.11) [INFO] | | \- jaxen:jaxen:jar:1.1.1:compile [INFO] | +- org.apache.ws.commons.axiom:axiom-impl:jar:1.2.11-wso2v4:compile (version managed from 1.2.11) [INFO] | +- org.apache.geronimo.specs:geronimo-ws-metadata_2.0_spec:jar:1.1.2:compile [INFO] | +- org.apache.geronimo.specs:geronimo-jta_1.1_spec:jar:1.1:compile [INFO] | +- javax.servlet:servlet-api:jar:2.3:compile [INFO] | +- commons-httpclient:commons-httpclient:jar:3.1:compile [INFO] | | \- commons-codec:commons-codec:jar:1.2:compile [INFO] | +- commons-fileupload:commons-fileupload:jar:1.2:compile
If you have multiple JDKs installed in your system, you may wonder what is being used by Maven. The following command will display all the environment variables and system properties set for a given Maven project:
$ mvn help:system
The following is the truncated output of the previous command:
======================Platform Properties Details====================
=====================================================================
System Properties
=====================================================================
java.runtime.name=Java(TM) SE Runtime Environment
sun.boot.library.path= /Library/Java/JavaVirtualMachines/jdk1.7.0_75.jdk/Contents/Home/jre/lib
java.vm.version= 24.75-b04
awt.nativeDoubleBuffering=true
gopherProxySet=false
mrj.build=11M4609
java.vm.vendor=Apple Inc.
java.vendor.url=http://www.apple.com/
guice.disable.misplaced.annotation.check=true
path.separator=:
java.vm.name=Java HotSpot(TM) 64-Bit Server VM
file.encoding.pkg=sun.io
sun.java.launcher=SUN_STANDARD
user.country=US
sun.os.patch.level=unknown
========================================================
Environment Variables
========================================================
JAVA_HOME=/System/Library/Frameworks/JavaVM.framework/Versions/CurrentJDK/Home
HOME=/Users/prabath
TERM_SESSION_ID=C2CEFB58-4705-4C67-BE1F-9E4179F96391
M2_HOME=/usr/share/maven/maven-3.3.3/
COMMAND_MODE=unix2003
Apple_PubSub_Socket_Render=/tmp/launch-w7NZbG/Render
LOGNAME=prabath
USER=prabath
Maven uses default values for configuration parameters when they are not overridden in the configuration. This is exactly what we discussed under the Convention over configuration section. If we take the same sample POM file that we used before in this chapter, we can see how the effective POM file would look using the following command. This is also the best way to see what default values are being used by Maven:
$ mvn help:effective-pom
Note
More details about the effective-pom command are discussed in Chapter 2, Understanding the Project Object Model (POM).
The following command will list all the JAR files and directories in the build classpath:
$ mvn dependency:build-classpath
The following shows the truncated output of the previous command executed against the Apache Rampart project:
[INFO] --------------------------------------------------------------
[INFO] Building Rampart - Trust 1.6.1-wso2v12
[INFO] --------------------------------------------------------------
[INFO]
[INFO] --- maven-dependency-plugin:2.1:build-classpath (default-cli) @ rampart-trust ---
[INFO] Dependencies classpath:
/Users/prabath/.m2/repository/bouncycastle/bcprov-jdk14/140/bcprov-jdk14-140.jar:/Users/prabath/.m2/repository/commons-cli/commons-cli/1.0/commons-cli-1.0.jar:/Users/prabath/.m2/repository/commons-codec/commons-codec/1.2/commons-codec-1.2.jar:/Users/prabath/.m2/repository/commons-collections/commons-collections/3.1/commons-collections-3.1.jar