Command 'javac' not found, but can be installed with:
apt install default-jdk
apt install openjdk-11-jdk-headless
apt install ecj
apt install openjdk-8-jdk-headless 

Reading package lists... Done
Building dependency tree
Reading state information... Done
The following additional packages will be installed: ca-certificates-java java-common libasound2 libasound2-data libgraphite2-3 libharfbuzz0b liblcms2-2 libpcsclite1 openjdk-11-jre-headless
Suggested packages: default-jre libasound2-plugins alsa-utils liblcms2-utils pcscd openjdk-11-demo openjdk-11-source libnss-mdns fonts-dejavu-extra fonts-ipafont-gothic fonts-ipafont-mincho fonts-wqy-microhei | fonts-wqy-zenhei fonts-indic
The following NEW packages will be installed: ca-certificates-java java-common libasound2 libasound2-data libgraphite2-3 libharfbuzz0b liblcms2-2 libpcsclite1 openjdk-11-jdk-headless openjdk-11-jre-headless
0 upgraded, 10 newly installed, 0 to remove and 31 not upgraded.
Need to get 259 MB of archives.
After this operation, 404 MB of additional disk space will be used.
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export JAVA_HOME=/usr/lib/jvm/java-11-openjdk-amd64 #jdk的安装路径
export JRE_HOME=${JAVA_HOME}/jre
export CLASSPATH=.:${JAVA_HOME}/lib:${JRE_HOME}/lib
export PATH=${JAVA_HOME}/bin:$PATH

Setting up openjdk-11-jre-headless:amd64 (11.0.11+9-0ubuntu2~18.04) ...
update-alternatives: using /usr/lib/jvm/java-11-openjdk-amd64/bin/java to provide /usr/bin
/java (java) in auto mode
update-alternatives: using /usr/lib/jvm/java-11-openjdk-amd64/bin/jjs to provide /usr/bin/
jjs (jjs) in auto mode
update-alternatives: using /usr/lib/jvm/java-11-openjdk-amd64/bin/keytool to provide /usr/
bin/keytool (keytool) in auto mode
update-alternatives: using /usr/lib/jvm/java-11-openjdk-amd64/bin/rmid to provide /usr/bin
/rmid (rmid) in auto mode

Usage: javac <options> <source files>
where possible options include: @<filename> Read options and filenames from file -Akey[=value] Options to pass to annotation processors --add-modules <module>(,<module>)* Root modules to resolve in addition to the initial modules, or all modules on the module path if <module> is ALL-MODULE-PATH. --boot-class-path <path>, -bootclasspath <path> Override location of bootstrap class files --class-path <path>, -classpath <path>, -cp <path> Specify where to find user class files and annotation processors -d <directory> Specify where to place generated class files -deprecation Output source locations where deprecated APIs are used --enable-preview Enable preview language features. To be used in conjunction with either -source or --release. -encoding <encoding> Specify character encoding used by source files -endorseddirs <dirs> Override location of endorsed standards path -extdirs <dirs> Override location of installed extensions -g Generate all debugging info -g:{lines,vars,source} Generate only some debugging info -g:none Generate no debugging info -h <directory> Specify where to place generated native header files --help, -help, -? Print this help message --help-extra, -X Print help on extra options -implicit:{none,class} Specify whether or not to generate class files for implicitly referenced files -J<flag> Pass <flag> directly to the runtime system --limit-modules <module>(,<module>)* Limit the universe of observable modules --module <module-name>, -m <module-name> Compile only the specified module, check timestamps --module-path <path>, -p <path> Specify where to find application modules --module-source-path <module-source-path> Specify where to find input source files for multiple modules --module-version <version> Specify version of modules that are being compiled -nowarn Generate no warnings -parameters Generate metadata for reflection on method parameters -proc:{none,only} Control whether annotation processing and/or compilation is done. -processor <class1>[,<class2>,<class3>...] Names of the annotation processors to run; bypasses default discovery process --processor-module-path <path> Specify a module path where to find annotation processors --processor-path <path>, -processorpath <path> Specify where to find annotation processors -profile <profile> Check that API used is available in the specified profile --release <release> Compile for a specific VM version. Supported targets: 6, 7, 8, 9, 10, 11 -s <directory> Specify where to place generated source files -source <release> Provide source compatibility with specified release --source-path <path>, -sourcepath <path> Specify where to find input source files --system <jdk>|none Override location of system modules -target <release> Generate class files for specific VM version --upgrade-module-path <path> Override location of upgradeable modules -verbose Output messages about what the compiler is doing --version, -version Version information -Werror Terminate compilation if warnings occur