spring boot之maven-wrapper

落日映苍穹つ 2023-06-16 01:50 157阅读 0赞

Spring Boot有很多功能特性值得借鉴和学习，很多玩Spring Boot的人知道不需要安装Tomcat很方便，其实并没有发现Maven也是不需要提前安装。它这样做的好处在于解决了开发环境maven版本不一致导致的各种问题，spring boot中集成了maven-wrapper的确比较务实。

### **它是什么** ###

相信大家都用到spring的脚手架：[https://start.spring.io/][https_start.spring.io] 来生成Spring Boot项目，而项目的根目录中会多几个文件：

> cmd > C:\\Users\\alex\\Desktop\\demo> tree /f  
> │  mvnw            //linux-shell  
> │  mvnw.cmd        //window-cmd  
> │  pom.xml  
> ├─.mvn  
> │  └─wrapper  
> │          maven-wrapper.jar  
> │          maven-wrapper.properties  
> │          MavenWrapperDownloader.java  
> ├─src

maven-wrapper解决了2个问题：  
1.可以为某个Java工程指定某个特定Maven版本，避免因为版本差异引起的诡异错误，这样就统一该项目的开发环境；  
2.不再需要提前安装Maven，简化了开发环境的配置；

### 玩法及原理 ###

**1.当前项目(spring boot)**

在项目目录下执行mvnw clean，其实就是将之前你熟悉的mvn替换为mvnw命令即可，一点也不复杂。

mvnw第一次运行会检测$USER\_HOME/.m2/wrapper/dists 目录下是否有maven-wrapper.properties中指定的Maven版本，如果没有就自动下载。

> 此时你会问下载后的maven会在哪里？
> 
> 一般会在$\{user.home\}\\.m2\\wrapper\\dists目录，我的机器是在：C:\\Users\\alex\\.m2\\wrapper\\dists\\apache-maven-3.6.2-bin\\795eh28tki48bv3l67maojf0ra

> 如何调整版本呢？
> 
> 具体可参与maven-wrapper.properties中配置：  
> distributionUrl=https://repo.maven.apache.org/maven2/org/apache/maven/apache-maven/3.6.2/apache-maven-3.6.2-bin.zip  
> wrapperUrl=https://repo.maven.apache.org/maven2/io/takari/maven-wrapper/0.5.5/maven-wrapper-0.5.5.jar

原理：mvnw.cmd逻辑也不复杂，先将maven-wrapper.jar添加到classpath，再运行MavenWrapperDownloader\#main。

public class MavenWrapperDownloader {
        private static final String WRAPPER_VERSION = "0.5.5";
        /**
         * Default URL to download the maven-wrapper.jar from, if no 'downloadUrl' is provided.
         */
        private static final String DEFAULT_DOWNLOAD_URL = "https://repo.maven.apache.org/maven2/io/takari/maven-wrapper/"
            + WRAPPER_VERSION + "/maven-wrapper-" + WRAPPER_VERSION + ".jar";
    
        /**
         * Path to the maven-wrapper.properties file, which might contain a downloadUrl property to
         * use instead of the default one.
         */
        private static final String MAVEN_WRAPPER_PROPERTIES_PATH =
                ".mvn/wrapper/maven-wrapper.properties";
    
        /**
         * Path where the maven-wrapper.jar will be saved to.
         */
        private static final String MAVEN_WRAPPER_JAR_PATH =
                ".mvn/wrapper/maven-wrapper.jar";
    
        /**
         * Name of the property which should be used to override the default download url for the wrapper.
         */
        private static final String PROPERTY_NAME_WRAPPER_URL = "wrapperUrl";
    
        public static void main(String args[]) {
            System.out.println("- Downloader started");
            File baseDirectory = new File(args[0]);
            System.out.println("- Using base directory: " + baseDirectory.getAbsolutePath());
    
            // If the maven-wrapper.properties exists, read it and check if it contains a custom
            // wrapperUrl parameter.
            File mavenWrapperPropertyFile = new File(baseDirectory, MAVEN_WRAPPER_PROPERTIES_PATH);
            String url = DEFAULT_DOWNLOAD_URL;
            if(mavenWrapperPropertyFile.exists()) {
                FileInputStream mavenWrapperPropertyFileInputStream = null;
                try {
                    mavenWrapperPropertyFileInputStream = new FileInputStream(mavenWrapperPropertyFile);
                    Properties mavenWrapperProperties = new Properties();
                    mavenWrapperProperties.load(mavenWrapperPropertyFileInputStream);
                    url = mavenWrapperProperties.getProperty(PROPERTY_NAME_WRAPPER_URL, url);
                } catch (IOException e) {
                    System.out.println("- ERROR loading '" + MAVEN_WRAPPER_PROPERTIES_PATH + "'");
                } finally {
                    try {
                        if(mavenWrapperPropertyFileInputStream != null) {
                            mavenWrapperPropertyFileInputStream.close();
                        }
                    } catch (IOException e) {
                        // Ignore ...
                    }
                }
            }
            System.out.println("- Downloading from: " + url);
    
            File outputFile = new File(baseDirectory.getAbsolutePath(), MAVEN_WRAPPER_JAR_PATH);
            if(!outputFile.getParentFile().exists()) {
                if(!outputFile.getParentFile().mkdirs()) {
                    System.out.println(
                            "- ERROR creating output directory '" + outputFile.getParentFile().getAbsolutePath() + "'");
                }
            }
            System.out.println("- Downloading to: " + outputFile.getAbsolutePath());
            try {
                downloadFileFromURL(url, outputFile);
                System.out.println("Done");
                System.exit(0);
            } catch (Throwable e) {
                System.out.println("- Error downloading");
                e.printStackTrace();
                System.exit(1);
            }
        }
    
        private static void downloadFileFromURL(String urlString, File destination) throws Exception {
            if (System.getenv("MVNW_USERNAME") != null && System.getenv("MVNW_PASSWORD") != null) {
                String username = System.getenv("MVNW_USERNAME");
                char[] password = System.getenv("MVNW_PASSWORD").toCharArray();
                Authenticator.setDefault(new Authenticator() {
                    @Override
                    protected PasswordAuthentication getPasswordAuthentication() {
                        return new PasswordAuthentication(username, password);
                    }
                });
            }
            URL website = new URL(urlString);
            ReadableByteChannel rbc;
            rbc = Channels.newChannel(website.openStream());
            FileOutputStream fos = new FileOutputStream(destination);
            fos.getChannel().transferFrom(rbc, 0, Long.MAX_VALUE);
            fos.close();
            rbc.close();
        }
    
    }

**2.全局命令**

还可以mvn -N io.takari:maven:wrapper -Dmaven=3.3.3表示我们期望使用的Maven的版本为3.3.3，将mvnw提升为全局命令，个人感觉意义不大，它最大的贡献在于保留及坚持个性化。

**3.idea插件maven-wrapper-support**

它需要结合当前项目（找maven-wrapper.properties配置）来使用。

maven-wrapper-support的逻辑也不复杂：会监测项目下的.mvn/wrapper/maven-wrapper.properties中的distributionUrl属性值，且自动下载maven版本到用户目录.m2/wrapper目录中，并且改变setting->build->build Tools ->maven-> maven home directory的值，但执行的命令是原生mvn的命令，而不是项目中下的mvnw命令。

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2l0c29mdGNoZW5mZWk_size_16_color_FFFFFF_t_70][]

原理：具体验证可查看plugin安装目录一般为：idea.config.path=$\{user.home\}/.IntelliJIdea/config，比如我的：C:\\Users\\alex\\.IntelliJIdea2018.3\\config\\plugins\\maven-wrapper-support\\lib中的maven-wrapper-support-0.5.1.jar代码MavenWrapperProjectComponent.class。

public class MavenWrapperProjectComponent extends AbstractProjectComponent {
        private VirtualFile wrapperSettings;
        private Logger log = Logger.getInstance(this.getClass());
    
        public MavenWrapperProjectComponent(Project project) {
            super(project);
        }
    
        private void applyWrapper() {
            if (this.wrapperSettings != null) {
                StringBuilder output = new StringBuilder();
                WrapperExecutor wrapperExecutor = WrapperExecutor.forWrapperPropertiesFile(new File(this.wrapperSettings.getPath()), output);
                File mavenUserHome = new File(System.getProperty("user.home") + "/.m2");
                Installer installer = new Installer(new DefaultDownloader("mvnw", "0.4.0"), new PathAssembler(mavenUserHome));
    
                try {
                    File mavenHome = installer.createDist(wrapperExecutor.getConfiguration());
                    this.changeMavenHomeTo(mavenHome.getAbsolutePath(), "maven wrapper defined in " + this.wrapperSettings.getPath());
                } catch (Exception var6) {
                    this.log.error(var6);
                }
    
            }
        }
    
        private void changeMavenHomeTo(String mavenPath, String message) {
            MavenGeneralSettings generalSettings = MavenProjectsManager.getInstance(this.myProject).getGeneralSettings();
            if (generalSettings != null) {
                String oldMavenHome = generalSettings.getMavenHome();
                if (!mavenPath.equals(oldMavenHome)) {
                    generalSettings.setMavenHome(mavenPath);
                    this.log.info("Maven changed to " + message);
                    Bus.notify(new Notification("maven-wrapper", "Maven changed", "Maven changed to " + message, NotificationType.INFORMATION));
                }
            }
        }
    
        public void projectOpened() {
            VirtualFileManager.getInstance().addVirtualFileListener(new com.blackbuild.intellij.wavenwrappersupport.MavenWrapperProjectComponent.ChangeListener(this));
            this.wrapperSettings = this.myProject.getBaseDir().findFileByRelativePath(".mvn/wrapper/maven-wrapper.properties");
            this.applyWrapper();
        }
    
        @NotNull
        public String getComponentName() {
            String var10000 = this.getClass().getName();
            if (var10000 == null) {
                $$$reportNull$$$0(0);
            }
    
            return var10000;
        }
    }

**注意事项**

**1.distributionUrl下载会比较慢**

建议替换为：[http://www-us.apache.org/dist/maven/maven-3/3.5.4/binaries/apache-maven-3.5.4-bin.zip][http_www-us.apache.org_dist_maven_maven-3_3.5.4_binaries_apache-maven-3.5.4-bin.zip]

**2.mvnw.bat中执行老版本的maven可能会报错："Error: M2\_HOME is set to an invalid directory"**

Maven早期版本不叫mvn.cmd，而是叫mvn.bat，找到代码替换掉即可![94594375][]

[https_start.spring.io]: https://start.spring.io/
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2l0c29mdGNoZW5mZWk_size_16_color_FFFFFF_t_70]: https://img-blog.csdnimg.cn/20191123142704358.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2l0c29mdGNoZW5mZWk=,size_16,color_FFFFFF,t_70
[http_www-us.apache.org_dist_maven_maven-3_3.5.4_binaries_apache-maven-3.5.4-bin.zip]: http://www-us.apache.org/dist/maven/maven-3/3.5.4/binaries/apache-maven-3.5.4-bin.zip
[94594375]: https://blog.csdn.net/alex_xfboy/article/details/94594375