Maven 学习笔记

mvn archetype:generate \
    -DgroupId=com.example \
    -DartifactId=my-maven-demo \
    -DarchetypeArtifactId=maven-archetype-quickstart \
    -DinteractiveMode=false

...
[INFO] ----------------------------------------------------------------------------
[INFO] Using following parameters for creating project from Old (1.x) Archetype: maven-archetype-quickstart:1.0
[INFO] ----------------------------------------------------------------------------
[INFO] Parameter: basedir, Value: /tmp
[INFO] Parameter: package, Value: com.example
[INFO] Parameter: groupId, Value: com.example
[INFO] Parameter: artifactId, Value: my-maven-demo
[INFO] Parameter: packageName, Value: com.example
[INFO] Parameter: version, Value: 1.0-SNAPSHOT
[INFO] project created from Old (1.x) Archetype in dir: /tmp/my-maven-demo
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  3.908 s
[INFO] Finished at: 2025-01-17T23:21:31+08:00
[INFO] ------------------------------------------------------------------------

$ tree my-maven-demo
maven-demo
├── pom.xml
└── src
    ├── main
    │   └── java
    │       └── com
    │           └── example
    │               └── App.java
    └── test
        └── java
            └── com
                └── example
                    └── AppTest.java

10 directories, 3 files

生成的内容如下, 仅包含一个 jUnit 依赖项.

<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/maven-v4_0_0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.example</groupId>
  <artifactId>my-maven-demo</artifactId>
  <packaging>jar</packaging>
  <version>1.0-SNAPSHOT</version>
  <name>maven-demo</name>
  <url>http://maven.apache.org</url>
  <dependencies>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>3.8.1</version>
      <scope>test</scope>
    </dependency>
  </dependencies>
</project>

4.1 增加编译器属性，告诉 Maven 使用指定的 JDK 版本来编译源码

<properties>
  <!-- 指定项目构建时的源文件编码格式, 防止源文件中的非 ASCII 字符出现乱码 -->
  <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
  <!-- 指定项目源代码所使用的 Java 版本 -->
  <maven.compiler.source>1.8</maven.compiler.source>
  <!-- 指定编译后的字节码目标版本 -->
  <maven.compiler.target>1.8</maven.compiler.target>
</properties>

4.2 升级到 jUnit4

<dependency>
  <groupId>junit</groupId>
  <artifactId>junit</artifactId>
  <version>4.13.2</version>
  <scope>test</scope>
</dependency>

为更好的演示打包的效果, 增加一个依赖项.

<dependency>
  <groupId>commons-codec</groupId>
  <artifactId>commons-codec</artifactId>
  <version>1.17.2</version>
</dependency>

2.1 编写代码 src/main/java/com/example/App.java

package com.example;

import org.apache.commons.codec.digest.DigestUtils;

public class App {
    public static void main(String[] args) {

        if (args.length < 1) {
            System.err.println("请输入任意字符串!");
            System.exit(0);
        }
        System.out.println("SHA-256哈希值为: " + sha256hex(args[0]));
    }

    public static String sha256hex(String input) {
        return DigestUtils.sha256Hex(input);
    }
}

2.2 编写单元测试 com/example/AppTest.java

package com.example;

import org.junit.Test;

import static org.junit.Assert.*;

public class AppTest {

    @Test
    public void sha256hex() {
        assertEquals("8d969eef6ecad3c29a3a629280e686cf0c3f5d5a86aff3ca12020c923adc6c92", App.sha256hex("123456"));
    }
}

执行 mvn package 完成构建

$ mvn package
[INFO] Scanning for projects...
[INFO]
[INFO] ---------------------< com.example:my-maven-demo >----------------------
[INFO] Building my-maven-demo 1.0-SNAPSHOT
[INFO]   from pom.xml
[INFO] --------------------------------[ jar ]---------------------------------
[INFO]
[INFO] --- resources:3.3.1:resources (default-resources) @ my-maven-demo ---
[INFO] skip non existing resourceDirectory /tmp/my-maven-demo/src/main/resources
[INFO]
[INFO] --- compiler:3.13.0:compile (default-compile) @ my-maven-demo ---
[INFO] Recompiling the module because of changed source code.
[INFO] Compiling 1 source file with javac [debug target 1.8] to target/classes
[INFO]
[INFO] --- resources:3.3.1:testResources (default-testResources) @ my-maven-demo ---
[INFO] skip non existing resourceDirectory /tmp/my-maven-demo/src/test/resources
[INFO]
[INFO] --- compiler:3.13.0:testCompile (default-testCompile) @ my-maven-demo ---
[INFO] Recompiling the module because of changed dependency.
[INFO] Compiling 1 source file with javac [debug target 1.8] to target/test-classes
[INFO]
[INFO] --- surefire:3.2.5:test (default-test) @ my-maven-demo ---
[INFO] Using auto detected provider org.apache.maven.surefire.junit4.JUnit4Provider
[INFO]
[INFO] -------------------------------------------------------
[INFO]  T E S T S
[INFO] -------------------------------------------------------
[INFO] Running com.example.AppTest
[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.111 s -- in com.example.AppTest
[INFO]
[INFO] Results:
[INFO]
[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0
[INFO]
[INFO]
[INFO] --- jar:3.4.1:jar (default-jar) @ my-maven-demo ---
[INFO] Building jar: /tmp/my-maven-demo/target/my-maven-demo-1.0-SNAPSHOT.jar
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  4.145 s
[INFO] Finished at: 2025-01-16T23:40:30+08:00
[INFO] ------------------------------------------------------------------------

4.1 尝试执行

java -cp target/my-maven-demo-1.0-SNAPSHOT.jar com.example.App 123456

发现报错了, 报错原因是根据当前的 pom 文件配置, Maven 不会将项目中的依赖项 commons-codec 添加到 jar 文件中。

Exception in thread "main" java.lang.NoClassDefFoundError: org/apache/commons/codec/digest/DigestUtils
        at com.example.App.sha256hex(App.java:16)
        at com.example.App.main(App.java:12)
Caused by: java.lang.ClassNotFoundException: org.apache.commons.codec.digest.DigestUtils
        at java.net.URLClassLoader.findClass(URLClassLoader.java:387)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:418)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:352)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:351)
        ... 2 more

4.2 使用 maven-shade-plugin² 插件来生成 fat-jar - 将所有依赖项打包进同一个 jar 文件中

更新 pom.xml

<build>
  <plugins>
    <plugin>
      <groupId>org.apache.maven.plugins</groupId>
      <artifactId>maven-shade-plugin</artifactId>
      <version>3.6.0</version>
      <executions>
        <!-- 指定该插件的 shade 目标在 Maven 的 package 阶段执行 -->
        <!-- 也就是当运行 mvn package 命令时，该插件的 shade 目标将被自动触发 -->
        <execution>
          <phase>package</phase>
          <goals>
            <goal>shade</goal>
          </goals>
        </execution>
      </executions>
    </plugin>
  </plugins>
</build>

4.3 重新构建 mvn clean package

...
[INFO] --- jar:3.4.1:jar (default-jar) @ my-maven-demo ---
[INFO] Building jar: /tmp/my-maven-demo/target/my-maven-demo-1.0-SNAPSHOT.jar
[INFO]
[INFO] --- shade:3.6.0:shade (default) @ my-maven-demo ---
[INFO] Including commons-codec:commons-codec:jar:1.17.2 in the shaded jar.
[INFO] Dependency-reduced POM written at: /tmp/my-maven-demo/dependency-reduced-pom.xml
[WARNING] commons-codec-1.17.2.jar, my-maven-demo-1.0-SNAPSHOT.jar define 1 overlapping resource:
[WARNING]   - META-INF/MANIFEST.MF
[WARNING] maven-shade-plugin has detected that some files are
[WARNING] present in two or more JARs. When this happens, only one
[WARNING] single version of the file is copied to the uber jar.
[WARNING] Usually this is not harmful and you can skip these warnings,
[WARNING] otherwise try to manually exclude artifacts based on
[WARNING] mvn dependency:tree -Ddetail=true and the above output.
[WARNING] See https://maven.apache.org/plugins/maven-shade-plugin/
[INFO] Replacing original artifact with shaded artifact.
[INFO] Replacing /tmp/my-maven-demo/target/my-maven-demo-1.0-SNAPSHOT.jar with /tmp/my-maven-demo/target/my-maven-demo-1.0-SNAPSHOT-shaded.jar
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  4.268 s
[INFO] Finished at: 2025-01-16T23:58:36+08:00
[INFO] ------------------------------------------------------------------------

可以看到生成了两个 jar 包, 查看一下它们的文件大小

$ ls -lh target
...
-rw-r--r-- 1 demo demo 365K Jan 16 23:58 my-maven-demo-1.0-SNAPSHOT.jar
-rw-r--r-- 1 demo demo 2.9K Jan 16 23:58 original-my-maven-demo-1.0-SNAPSHOT.jar
...

5.1 尝试运行

$ java -cp target/my-maven-demo-1.0-SNAPSHOT.jar com.example.App 123456
SHA-256哈希值为: 8d969eef6ecad3c29a3a629280e686cf0c3f5d5a86aff3ca12020c923adc6c92

5.2 很好, 现在执行成功了。但是能不能直接作为 jar 包执行而不用指定主类呢

➜ java -jar target/my-maven-demo-1.0-SNAPSHOT.jar
no main manifest attribute, in target/my-maven-demo-1.0-SNAPSHOT.jar

5.3 很遗憾，还不可以，可以通过配置 maven-shade-plugin 插件来指定主类

<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-shade-plugin</artifactId>
  <version>3.6.0</version>
  <executions>
    <execution>
      <phase>package</phase>
      <goals>
        <goal>shade</goal>
      </goals>
      <configuration>
        <transformers>
          <transformer implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
            <mainClass>com.example.App</mainClass>
          </transformer>
        </transformers>
      </configuration>
    </execution>
  </executions>
</plugin>

6.1 重新打包

mvn clean package

6.2 作为 jar 包执行

➜ java -jar target/my-maven-demo-1.0-SNAPSHOT.jar
请输入任意字符串!

➜ java -jar target/my-maven-demo-1.0-SNAPSHOT.jar 123456
SHA-256哈希值为: 8d969eef6ecad3c29a3a629280e686cf0c3f5d5a86aff3ca12020c923adc6c92