background

Through maven's GAV, you can avoid using the same component in the project ===", that is, the component that is packaged out and eventually the same GA must have only one

But in addition to this GA component only, we often encounter the same scene in the java project. For example, the typical is mail.

referenceThe maven project that the mail could not be sent introduces a new dependency problem

There are also a few problems with class names that cause class duplication.

How to solve this problem? ? ?

Countermeasures

In fact, it is difficult to say that it is not easy ~ what we need to do is to check the corresponding class to check whether there is a duplicate!

Abstract, that is to say, in a bunch of folders we have to find the file with the exact same file path.

plan 1

All jars are traversed inside the class file for output and sorted.

For example, we can have some third-party scriptsshow-duplicate-java-classes

After the corresponding directory is executed, the output is as follows

11 (1@3): ./woden-impl-dom-1.0M8.jar ./stax-api-1.0.1.jar ./xml-apis-1.3.04.jar
    1   javax/xml/namespace/QName.class
12 (223@2): ./quartz-all-1.6.1.jar ./quartz-1.6.1.jar
    1   org/quartz/impl/jdbcjobstore/JobStoreSupport$VoidTransactionCallback.class
    2   org/quartz/impl/jdbcjobstore/JobStoreSupport$25.class
    3   org/quartz/impl/StdJobRunShellFactory.class
    4   org/quartz/impl/QuartzServer.class

In this case, it can be corrected according to the corresponding problem.

Scenario 2

Use the maven plugin to find problems during compile time [of course I will ignore some jars provided by the container] but most of them are OK, as follows

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-enforcer-plugin</artifactId>
    <version>1.4.1</version>
    <executions>
        <execution>
            <id>enforce-ban-duplicate-classes</id>
            <goals>
                <goal>enforce</goal>
            </goals>
            <configuration>
                <rules>
                    <banDuplicateClasses>
                        <ignoreClasses>
                            <ignoreClass>javax.*</ignoreClass>
                            <ignoreClass>org.junit.*</ignoreClass>
                            <ingoreClass>org.aspectj.*</ingoreClass>
                            <ingoreClass>org.jboss.netty.*</ingoreClass>
                            <ingoreClass>org.apache.juli.*</ingoreClass>
                            <ingoreClass>org.apache.commons.logging.*</ingoreClass>
                            <ingoreClass>org.apache.log4j.*</ingoreClass>
                            <ingoreClass>org.objectweb.asm.*</ingoreClass>
                            <ingoreClass>org.parboiled.*</ingoreClass>
                            <ingoreClass>org.apache.xmlbeans.xml.stream.*</ingoreClass>
                            <ingoreClass>org.json.JSONString</ingoreClass>
                        </ignoreClasses>
                        <findAllDuplicates>true</findAllDuplicates>
                    </banDuplicateClasses>
                </rules>
                <fail>true</fail>
            </configuration>
        </execution>
    </executions>
    <dependencies>
        <dependency>
            <groupId>org.codehaus.mojo</groupId>
            <artifactId>extra-enforcer-rules</artifactId>
            <version>1.0-beta-6</version>
        </dependency>
    </dependencies>
</plugin>

For example, if a classmate introduces a new dependency, some new classes appear in the system and the original is repeated. Then, in the process of maven compilation, an error will be reported.

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-enforcer-plugin:1.4.1:enforce (enforce-ban-duplicate-classes) on project web: Some Enforcer rules have failed. Look above for specific messages explaining why the rule failed. -> [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/MojoExecutionException
[ERROR]
[ERROR] After correcting the problems, you can resume the build with the command
[ERROR]   mvn <goals> -rf :web

Can view detailed error information

[INFO] --- maven-enforcer-plugin:1.4.1:enforce (enforce-ban-duplicate-classes) @ web ---
[INFO] Adding ignore: javax.*
[INFO] Adding ignore: org.junit.*
[INFO] Adding ignore: org.aspectj.*
[INFO] Adding ignore: org.jboss.netty.*
[INFO] Adding ignore: org.apache.juli.*
[INFO] Adding ignore: org.apache.commons.logging.*
[INFO] Adding ignore: org.apache.log4j.*
[INFO] Adding ignore: org.objectweb.asm.*
[INFO] Adding ignore: org.parboiled.*
[INFO] Adding ignore: org.apache.xmlbeans.xml.stream.*
[WARNING] Rule 0: org.apache.maven.plugins.enforcer.BanDuplicateClasses failed with message:
Duplicate classes found:
  Found in:
    org.skyscreamer:jsonassert:jar:1.4.0:compile
    org.json:json:jar:20140107:compile
  Duplicate classes:
    org/json/JSONString.class

We can know that the org.json.JSONString class has appeared in both jars~ need to pay attention!