错误: 找不到或无法加载主类 org.codehaus.plexus.classworlds.launcher.Launcher 问题排查
1. mac 下面是用homebrew 安装maven，安装成功，设置完M2_HOME ,使用mvn －version进行检测
houenxundeMacBook-Pro:3.3.9 houenxun$ mvn －version 错误: 找不到或无法加载主类 org.codehaus.plexus.classworlds.launcher.Launcher
2. 网上看了一堆解决方案，不明所以，于是自己尝试解决，首先，先定位一下问题的脚本
houenxundeMacBook-Pro:3.3.9 houenxun$ which mvn /usr/local/bin/mvn houenxundeMacBook-Pro:3.3.9 houenxun$ ls -l /usr/local/bin/mvn lrwxr-xr-x 1 houenxun admin 29 4 7 15:52 /usr/local/bin/mvn -> ../Cellar/maven/3.3.9/bin/mvn
打开mvn文件
#!/bin/bash
JAVA_HOME="${JAVA_HOME:-$(/usr/libexec/java_home)}" exec "/usr/local/Cellar/maven/3.3.9/libexec/bin/mvn" "$@"
可以看到真正调用的是
/usr/local/Cellar/maven/3.3.9/libexec/bin/mvn
打开该文件,文件比较长，定位到最后报错的地方
exec "$JAVACMD" \
$MAVEN_OPTS \
$MAVEN_DEBUG_OPTS \
-classpath "${M2_HOME}"/boot/plexus-classworlds-*.jar \
"-Dclassworlds.conf=${M2_HOME}/bin/m2.conf" \
"-Dmaven.home=${M2_HOME}" "-Dmaven.multiModuleProjectDirectory=${MAVEN_PROJECTBASEDIR}" \
${CLASSWORLDS_LAUNCHER} "$@"
使用echo打印真正的语句
exec /Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/bin/java -classpath /usr/local/Cellar/maven/3.3.9/boot/plexus-classworlds-*.jar -Dclassworlds.conf=/usr/local/Cellar/maven/3.3.9/bin/m2.conf -Dmaven.home=/usr/local/Cellar/maven/3.3.9 -Dmaven.multiModuleProjectDirectory=/usr/local/Cellar/maven/3.3.9/libexec/bin org.codehaus.plexus.classworlds.launcher.Launcher
3. 很奇怪，在/usr/local/Cellar/maven/3.3.9目录下boot和bin都是不存在的，而却存在/usr/local/Cellar/maven/3.3.9/libexec目录下
houenxundeMacBook-Pro:3.3.9 houenxun$ pwd /usr/local/Cellar/maven/3.3.9 houenxundeMacBook-Pro:3.3.9 houenxun$ ls INSTALL_RECEIPT.json NOTICE bin LICENSE README.txt libexec houenxundeMacBook-Pro:3.3.9 houenxun$ houenxundeMacBook-Pro:3.3.9 houenxun$ cd libexec/ houenxundeMacBook-Pro:libexec houenxun$ ls bin boot conf lib houenxundeMacBook-Pro:libexec houenxun$
4. 尝试将他们拷贝出去，的确OK了
houenxundeMacBook-Pro:3.3.9 houenxun$ mvn -version exec /Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/bin/java -classpath /usr/local/Cellar/maven/3.3.9/boot/plexus-classworlds-2.5.2.jar -Dclassworlds.conf=/usr/local/Cellar/maven/3.3.9/bin/m2.conf -Dmaven.home=/usr/local/Cellar/maven/3.3.9 -Dmaven.multiModuleProjectDirectory=/usr/local/Cellar/maven/3.3.9 org.codehaus.plexus.classworlds.launcher.Launcher -version Apache Maven 3.3.9 (bb52d8502b132ec0a5a3f4c09453c07478323dc5; 2015-11-11T00:41:47+08:00) Maven home: /usr/local/Cellar/maven/3.3.9 Java version: 1.8.0_77, vendor: Oracle Corporation Java home: /Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre Default locale: zh_CN, platform encoding: UTF-8 OS name: "mac os x", version: "10.11.4", arch: "x86_64", family: "mac"
5. 问题似乎解决了，但不禁奇怪，难道homebrew安装maven有bug？？？
我习惯性的设置M2_HOME,为啥要设置，是要把maven的目录设置成执行环节目录，但根据第二步骤
/usr/bin/mvn是mvn命令的符号链接，有了这个符号链接M2_HOME也就不起作用了。我们尝试禁用M2_HOME环境变量
houenxundeMacBook-Pro:~ houenxun$ mvn -version exec /Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/bin/java -classpath /usr/local/Cellar/maven/3.3.9/libexec/boot/plexus-classworlds-2.5.2.jar -Dclassworlds.conf=/usr/local/Cellar/maven/3.3.9/libexec/bin/m2.conf -Dmaven.home=/usr/local/Cellar/maven/3.3.9/libexec -Dmaven.multiModuleProjectDirectory=/Users/houenxun org.codehaus.plexus.classworlds.launcher.Launcher -version Apache Maven 3.3.9 (bb52d8502b132ec0a5a3f4c09453c07478323dc5; 2015-11-11T00:41:47+08:00) Maven home: /usr/local/Cellar/maven/3.3.9/libexec Java version: 1.8.0_77, vendor: Oracle Corporation Java home: /Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre Default locale: zh_CN, platform encoding: UTF-8 OS name: "mac os x", version: "10.11.4", arch: "x86_64", family: "mac"
依然可以执行，而且boot和lib的路径也发生变化了！
原来新版本的maven已经不需要配置M2_HOME 环境变量，并且目录层次结构也发生了变化，但是为了兼容原来的老版本，脚本中通过M2_HOME来选择新旧目录层次，哎，自己画蛇添足了！！！