jar --create --main-class: create an executable JAR and run it with java -jar

jar --create --file app.jar --main-class com.example.App -C out . packs the compiled classes in out/ into app.jar and writes a manifest with Main-Class: com.example.App. Then java -jar app.jar starts that class. The short, older form of the same command is jar cfe app.jar com.example.App -C out ..

Build and run an executable JAR

mkdir -p src/com/example
cat > src/com/example/App.java <<'EOF'
package com.example;

public class App {
    public static void main(String[] args) {
        System.out.println("running from " + App.class.getProtectionDomain().getCodeSource().getLocation().getPath().replaceAll(".*/", ""));
        System.out.println("args: " + String.join(" ", args));
    }
}
EOF
javac -d out src/com/example/App.java
jar --create --file app.jar --main-class com.example.App -C out .
jar --list --file app.jar
mkdir check && cd check && jar --extract --file ../app.jar META-INF/MANIFEST.MF && cd ..
grep Main-Class check/META-INF/MANIFEST.MF | tr -d '
'
java -jar app.jar one two

META-INF/
META-INF/MANIFEST.MF
com/
com/example/
com/example/App.class
Main-Class: com.example.App
running from app.jar
args: one two

-C out . changes into out/ and adds everything there, so the JAR holds com/example/App.class at the right path. Arguments after the JAR file name go to main. Manifest lines end with CR LF, which is why tr strips the carriage return before printing.

Without a Main-Class

A JAR made without --main-class cannot be started with java -jar:

mkdir -p out/com/example
echo 'package com.example; public class App { public static void main(String[] a) { System.out.println("ok"); } }' > App.java
javac -d out App.java
jar --create --file plain.jar -C out .
java -cp plain.jar com.example.App
java -jar plain.jar

ok
no main manifest attribute, in plain.jar

Running the class explicitly with -cp still works; -jar needs the manifest entry and otherwise stops with no main manifest attribute, in plain.jar and exit status 1.

jar options

Dependencies with Class-Path

When the program needs other JARs, add a Class-Path line to the manifest. The entries are relative URLs separated by spaces, resolved from the folder of the JAR:

Class-Path: lib/greeter.jar lib/json.jar

Create the JAR with --manifest manifest.txt (the file must end with a newline, or the last line is ignored) and ship the lib/ folder next to it.

Pitfalls

• Could not find or load main class from java -jar means the Main-Class value does not match a class inside the JAR, often because -C was missing and the classes were stored under out/com/....
• UnsupportedClassVersionError on another machine means the JAR was compiled for a newer Java; compile with javac --release N for the oldest Java you support.
• -cp is ignored when you use -jar; put dependencies in the manifest instead.

Confirm it worked

jar --list --file app.jar shows the class files at their package paths and META-INF/MANIFEST.MF; java -jar app.jar runs without extra arguments.

FAQ