javac — Java compiler

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Compiles Java sources into JVM bytecode (.class files).

Basic Information

What is this command?

javac converts .java files into .class bytecode executed by the JVM. The output is platform independent, but running it requires a JRE/JDK of the same major version or newer.

Directory layout must match package declarations. Using -d creates package directories automatically. When files reference each other, pass them together.

What problems does it solve?

• Compiling Java sources into bytecode
• Keeping package structure while separating output
• Targeting a specific release for compatibility
• Compiling several sources at once

Key features

Bytecode output

Compiles sources into .class files.

• When to use: Testing a simple class
• What you get: You get output the JVM can run.
• Limitations: Execution is a separate step with java.

Output directory

The -d option writes into package directories.

• When to use: Building projects that use packages
• What you get: Classpath setup becomes simpler.
• Limitations: Stale output can cause confusion.

Target release

The --release option pins API and bytecode version.

• When to use: When compatibility with older JVMs is required
• What you get: Prevents accidental use of newer APIs.
• Limitations: APIs missing from that release become errors.

Installation and usage

Check whether it is installed

javac -version

sudo apt install openjdk-21-jdk-headless # Debian/Ubuntu sudo dnf install java-21-openjdk-devel # Fedora

Real-world examples

Compile one file

Produce a class file.

javac Hello.java
java Hello

Packaged project

Specify an output directory to create the package layout.

javac -d out $(find src -name '*.java')
java -cp out com.example.Main

Pin the release

Target an older release for compatibility.

javac --release 17 -d out src/Main.java