Spring Boot有很多功能特性值得借鑑和學習，很多玩Spring Boot的人知道不需要安裝Tomcat很方便，其實並沒有發現Maven也是不需要提前安裝。它這樣做的好處在於解決了開發環境maven版本不一致導致的各種問題，spring boot中集成了maven-wrapper的確比較務實。
它是什麼
相信大家都用到spring的腳手架：https://start.spring.io/ 來生成Spring Boot項目，而項目的根目錄中會多幾個文件：
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解決了2個問題：
1.可以爲某個Java工程指定某個特定Maven版本，避免因爲版本差異引起的詭異錯誤，這樣就統一該項目的開發環境；
2.不再需要提前安裝Maven，簡化了開發環境的配置；
玩法及原理
1.當前項目(spring boot)
在項目目錄下執行mvnw clean，其實就是將之前你熟悉的mvn替換爲mvnw命令即可，一點也不復雜。
mvnw第一次運行會檢測$USER_HOME/.m2/wrapper/dists 目錄下是否有maven-wrapper.properties中指定的Maven版本，如果沒有就自動下載。
此時你會問下載後的maven會在哪裏？
一般會在${user.home}\.m2\wrapper\dists目錄，我的機器是在：C:\Users\alex\.m2\wrapper\dists\apache-maven-3.6.2-bin\795eh28tki48bv3l67maojf0ra
如何調整版本呢？
具體可參與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邏輯也不復雜，先將maven-wrapper.jar添加到classpath，再運行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
還可以mvn -N io.takari:maven:wrapper -Dmaven=3.3.3表示我們期望使用的Maven的版本爲3.3.3，將mvnw提升爲全局命令，個人感覺意義不大，它最大的貢獻在於保留及堅持個性化。
3.idea插件maven-wrapper-support
它需要結合當前項目（找maven-wrapper.properties配置）來使用。
maven-wrapper-support的邏輯也不復雜：會監測項目下的.mvn/wrapper/maven-wrapper.properties中的distributionUrl屬性值，且自動下載maven版本到用戶目錄.m2/wrapper目錄中，並且改變setting->build->build Tools ->maven-> maven home directory的值，但執行的命令是原生mvn的命令，而不是項目中下的mvnw命令。
原理：具體驗證可查看plugin安裝目錄一般爲：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代碼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;
}
}
注意事項
1.distributionUrl下載會比較慢
建議替換爲：http://www-us.apache.org/dist/maven/maven-3/3.5.4/binaries/apache-maven-3.5.4-bin.zip
2.mvnw.bat中執行老版本的maven可能會報錯："Error: M2_HOME is set to an invalid directory"
Maven早期版本不叫mvn.cmd，而是叫mvn.bat，找到代碼替換掉即可
