macOS系统在终端执行Java项目时报如下错：
具体为：
1 org.openqa.selenium.SessionNotCreatedException: Unable to create a new remote session. 2 Please check the server log for more details. 3 Original error: An unknown server-side error occurred while processing the command. 4 Original error: Cannot verify the signature of '/var/folders/sw/b411gncn4cq2f16rvkst35kc0000gn/T/2021125-26019-1gzi67z.7852/ 5 appium-uiautomator2-server-v4.19.0.apk'. 6 Original error: Error: A JNI error has occurred, please check your installation and try again 7 Exception in thread "main" java.lang.UnsupportedClassVersionError: 8 com/android/apksigner/ApkSignerTool has been compiled by a more recent version of the Java Runtime (class file version 53.0), 9 this version of the Java Runtime only recognizes class file versions up to 52.0 10 at java.lang.ClassLoader.defineClass1(Native Method) 11 at java.lang.ClassLoader.defineClass(ClassLoader.java:763) 12 at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142) 13 at java.net.URLClassLoader.defineClass(URLClassLoader.java:468) 14 at java.net.URLClassLoader.access$100(URLClassLoader.java:74) 15 at java.net.URLClassLoader$1.run(URLClassLoader.java:369) 16 at java.net.URLClassLoader$1.run(URLClassLoader.java:363) 17 at java.security.AccessController.doPrivileged(Native Method) 18 at java.net.URLClassLoader.findClass(URLClassLoader.java:362) 19 at java.lang.ClassLoader.loadClass(ClassLoader.java:424) 20 at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:349) 21 at java.lang.ClassLoader.loadClass(ClassLoader.java:357) 22 at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:495)
核心的报错信息为：
Exception in thread "main" java.lang.UnsupportedClassVersionError: com/android/apksigner/ApkSignerTool has been compiled by a more recent version of the Java Runtime (class file version 53.0), this version of the Java Runtime only recognizes class file versions up to 52.0
…… 也是各种百度、各种查找原因，最后查看此文得到解决思路：【jar】Java Runtime recognizes class file versions up to 52.0
1、52.0和53.0 是什么意思？
—— 具体版本以及对应的java版本：
49 = Java 5 50 = Java 6 51 = Java 7 52 = Java 8 53 = Java 9 54 = Java 10 55 = Java 11 56 = Java 12 57 = Java 13 58 = Java 14
因此，上述报错的意思是：使用了Java版本9（53.0）编译了代码，而当前的版本是Java8（52.0）
所以，需要降版本，那么，怎么降呢？……
2、继续往上看第4行的报错：
Original error: Cannot verify the signature of '/var/folders/sw/b411gncn4cq2f16rvkst35kc0000gn/T/2021125-26019-1gzi67z.7852/appium-uiautomator2-server-v4.19.0.apk'.
—— 即android 的com/android/apksigner/ApkSignerTool进行 appium-uiautomator2-server-v4.16.0.apk 包signer处理时提示版本问题
根据参考文章的提示，
“进入文件/opt/androidsdk/android-sdk-linux/build-tools/， 依赖的是 30.0.0-preview 此时安装的android-sdk的版本是30，通过FQ重新下载了版本29”
我也前往Android SDK目录查看文件及其路径，发现我的也是 30.0.0-preview ！所以需要更换低一点的 build-tools ！！！ 因此，重装一个低版本的Android SDK。
3、 重装了一个低版本的Android SDK（android-sdk-r23-macosx，原本是r24）之后，又报新的错：
1 org.openqa.selenium.SessionNotCreatedException: Unable to create a new remote session. Please check the server log for more details. 2 Original error: An unknown server-side error occurred while processing the command. 3 Original error: Cannot verify the signature of '/var/folders/sw/b411gncn4cq2f16rvkst35kc0000gn/T/2021126-94555-107z0f9.k27k/
appium-uiautomator2-server-v4.19.0.apk'.
Original error: Could not find 'apksigner.jar' in
["/Library/android-sdk-r23-macosx/platform-tools/apksigner.jar",
"/Library/android-sdk-r23-macosx/emulator/apksigner.jar",
"/Library/android-sdk-r23-macosx/cmdline-tools/latest/bin/apksigner.jar",
"/Library/android-sdk-r23-macosx/tools/apksigner.jar",
"/Library/android-sdk-r23-macosx/tools/bin/apksigner.jar",
"/Library/android-sdk-r23-macosx/apksigner.jar",
"/Library/android-sdk-r23-macosx/build-tools/23.0.1/apksigner.jar",
"/Library/android-sdk-r23-macosx/build-tools/23.0.1/lib/apksigner.jar"].
Do you have Android Build Tools installed at '/Library/android-sdk-r23-macosx'? 4 Build info: version: 'unknown', revision: 'unknown', time: 'unknown' 5 System info: host: 'crgtiOSCI.local', ip: 'fe80:0:0:0:1455:c69a:4de1:af22%en0', os.name: 'Mac OS X', os.arch: 'x86_64', os.version: '10.16', java.version: '1.8.0_201' 6 Driver info: driver.version: AndroidDriver 7 remote stacktrace: UnknownError: An unknown server-side error occurred while processing the command.
Original error: Cannot verify the signature of '/var/folders/sw/b411gncn4cq2f16rvkst35kc0000gn/T/2021126-94555-107z0f9.k27k/
appium-uiautomator2-server-v4.19.0.apk'.
Original error: Could not find 'apksigner.jar' in
["/Library/android-sdk-r23-macosx/platform-tools/apksigner.jar",
"/Library/android-sdk-r23-macosx/emulator/apksigner.jar",
"/Library/android-sdk-r23-macosx/cmdline-tools/latest/bin/apksigner.jar",
"/Library/android-sdk-r23-macosx/tools/apksigner.jar",
"/Library/android-sdk-r23-macosx/tools/bin/apksigner.jar",
"/Library/android-sdk-r23-macosx/apksigner.jar",
"/Library/android-sdk-r23-macosx/build-tools/23.0.1/apksigner.jar",
"/Library/android-sdk-r23-macosx/build-tools/23.0.1/lib/apksigner.jar"].
Do you have Android Build Tools installed at '/Library/android-sdk-r23-macosx'? 8 at getResponseForW3CError (/usr/local/lib/node_modules/appium/node_modules/_appium-base-driver@7.4.1@appium-base-driver/lib/protocol/errors.js:804:9) 9 at asyncHandler (/usr/local/lib/node_modules/appium/node_modules/_appium-base-driver@7.4.1@appium-base-driver/lib/protocol/protocol.js:380:37) 10 Build info: version: 'unknown', revision: 'unknown', time: 'unknown' 11 System info: host: 'crgtiOSCI.local', ip: 'fe80:0:0:0:1455:c69a:4de1:af22%en0', os.name: 'Mac OS X', os.arch: 'x86_64', os.version: '10.16', java.version: '1.8.0_201'
——这个问题 有搜到是因为AndroidSDK有缺失引起的，于是更新SDK。
更新方法：
到Android SDK目录下的tools文件夹下（/Library/android-sdk-r23-macosx/tools），
找到 android.exe，双击打开可视化界面，选择项目进行install。（install的时间挺久的……）
更新了之后，build-tools出现了一系列的的版本号？有些里面有'apksigner.jar'，有些没有。
再次运行还是说：the Java Runtime only recognizes class file versions up to 52.0 这个错。
它可能会自动匹配到最高的那个的'apksigner.jar'
于是手动删除：最高的那个30.0.0版本：
4、重新跑，又报新的错……
org.openqa.selenium.SessionNotCreatedException: Unable to create a new remote session.
Please check the server log for more details.
Original error: An unknown server-side error occurred while processing the command.
Original error: Error executing adbExec. Original error: 'Command '/Library/android-sdk-r23-macosx/platform-tools/adb -P 5037 -s 66J0218C21003454 install -r /var/folders/sw/b411gncn4cq2f16rvkst35kc0000gn/T/2021126-86279-1u3qq29.yylv/appium-uiautomator2-server-v4.19.0.apk' timed out after 20000ms'. Try to increase the 20000ms adb execution timeout represented by 'uiautomator2ServerInstallTimeout' capability Build info: version: 'unknown', revision: 'unknown', time: 'unknown' System info: host: 'crgtiOSCI.local', ip: 'fe80:0:0:0:1455:c69a:4de1:af22%en0', os.name: 'Mac OS X', os.arch: 'x86_64', os.version: '10.16', java.version: '1.8.0_201' Driver info: driver.version: AndroidDriver remote stacktrace: UnknownError: An unknown server-side error occurred while processing the command. Original error: Error executing adbExec. Original error: 'Command '/Library/android-sdk-r23-macosx/platform-tools/adb -P 5037 -s 66J0218C21003454 install -r /var/folders/sw/b411gncn4cq2f16rvkst35kc0000gn/T/2021126-86279-1u3qq29.yylv/appium-uiautomator2-server-v4.19.0.apk' timed out after 20000ms'. Try to increase the 20000ms adb execution timeout represented by 'uiautomator2ServerInstallTimeout' capability at getResponseForW3CError (/usr/local/lib/node_modules/appium/node_modules/_appium-base-driver@7.4.1@appium-base-driver/lib/protocol/errors.js:804:9)
——有查到一个原因说是 appium版本太高导致的，不知道是不是。试了再说。
5、更换 appium版本 （从1.20.2降到1.17.1）
使用命令：sudo cnpm install -g appium@1.17.1 --force （很快哦！）
如果不加 --force 命令的话，可能会失败，如下图中红框上面的ERR报错，提示需要手动去删除原来appium版本的安装目录文件夹，所以加 --force比较方便啦。
安装成功：
6、最后再次终端执行命令：java -jar target/XXXX-1.0-SNAPSHOT.jar ，可终于成功运行起来啦！！！
【Reference】
1、【jar】Java Runtime recognizes class file versions up to 52.0