Compile:
./gradlew build
or, download: https://github.com/izgzhen/java2smali/releases
Run (in Unix system):
./java2smali example/Example.java
Then you should be able to find *.smali files in the example/ directory.
Another example run with imports:
APP_CLASSPATH=. ./java2smali example2/Example2.java
Note that Example2.java import example.Example, so you must specify its class path directory.
https://github.com/ollide/intellij-java2smali is a IntelliJ plugin.
https://r8.googlesource.com/r8/+/1.0.21/tools/java2smali.sh is a bash script that doesn't work off-the-shelf.
The following steps might work as well, as long as the *.java file
does not have a package namespace.
javac HelloWorld.java
dx --dex --output=classes.dex HelloWorld.class
baksmali classes.dex